ACTUAL SHIPS STABILITY PROBLEMS AND THE INFLUENCE ON SAFETY OF NAVIGATION
Copyright Cristian Andrei
Publicata de Cristian Andrei la Editura Digitala
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The opportunity of this book is given by highlighting of some aspects that are insufficiently approached in practical assessment of ships stability as well as the demonstrated usefulness of theoretical approaches documented, analysed and proposed.
The book is important because is approaching the problems of ships stability loss through the study of operational aspects as well as dynamic ship behaviour in severe sea conditions offering a picture of some stability failure modes that presently are not covered by any regulations or criteria.
The motivation for the idea driving this approach and the necessity to address the issue of new approaching of assessment the ship stability comes from a number of observations from the field of ships stability losses and capsizing. Despite of the regulations in force, referring to intact ship stability, many ships continued to loss their intact stability and/or capsize.
The synopsis of current stability criteria presented in the book is illustrating the details at which stability requirements are determined, yet will also reveal the shortcomings of the criteria and the fact that the current stability criteria are not covering actual demand of safety for intact stability in certain situations. References of the present regulations are critically discussed and some conceptually approaches are proposed.
The subjects treated in this book are parts of the works related to importance of ship stability assessment. It fits into a very complex system of research concerning the intact stability of ships, more exactly to modes of ship stability loss and the possibility of assessment the ship’s stability to prevent such losses.
The importance of ship’s stability in maritime transport
1.1 Actual stage regarding ships stability loss problems
In recent years, the number of accidents involving loss of ship stability was on the rise leading to damage of goods, loss of ships and lives. With the increasing of commodity transported and delivered to destinations as fast as possible, the speed of the ships increased which required major changes in ships design and construction in terms of deadweight capacity as well as hydrodynamic forms.
Presently are occurring modes of ship stability failure that a long period was treated only experimental. Moreover, a number of factors that in the past were only suppositions amplified the already known modes of ship stability failure.
If 25-30 years ago, at different international conferences, in the field of maritime transport, were presented general aspects related to ship stability failure modes, those were considered simple opinions about how the ship’s intact stability could be affected, in different situations, from operational as well as environmental point of view. Practically, those opinions were treated with less importance to practical application. Moreover, there were no signs of encouragement from the competent authorities to implement the opinions in practice.
In the last ten years, intact ship stability loss has become a significant problem. Stability failure modes in severe sea conditions, large amplitude rolling generated by phenomena like parametric rolling, pure loss of stability broaching or surf-riding, appeared as a major problem especially with relevance to new ship designs, such as large container vessels. The starting point, which involved major claims of over $ 100 million, was the incident of container ship “APL China”, and has proven that such phenomena exists beyond theory and can carry major risk for safety of ship and safety to navigation.
Although the theoretical existence of such dangerous phenomena has been known from before, the attention, from prevention and regulatory point of view, was paid only recently. Reality has proven that the mentioned casualty was not an isolated one and a number of vessels were involved in such situations, became a real danger for safety of navigation, with the result of loss the cargo and ship and potential risk for loss of lives.
The prediction of causes that lead to loss of intact stability and its impact on safety of navigation has attracted recently a huge interest documented in scientific publications and international conferences. In the same time, the subject has presented a significant interest of national and international regulatory authorities (Classification societies, International Maritime Organization), because of the risks involved that could lead to loss of lives, cargo and ships.
The Classification Society American Bureau of Shipping was the first international authority that issued rules, in form of a guide, for assessment the ship’s stability in order to prevent the occurrence of one of the most dangerous stability failure mode in extreme seas, like parametric rolling.
Despite the fact that the American Bureau of Shipping Guide is issued in a form of criteria used to determine if a particular vessel is vulnerable to parametric roll (susceptibility criteria) and how large the roll motions might be (severity criteria), the methodology of assessment is based on the calculations that are not accessible for ship’s officers on board vessel. In order to be implemented as a practical tool, it is necessary a detailed computer software and thus is needed an additional cost for ships’ owner. As a result, the assessment methodology remains for the moment just as guidance and more important with less applicability in practice.
1.2 Ship stability as part of seaworthiness
A classical field of ship safety and safety to navigation is without any doubt the intact stability of the vessel. With regard to the safety of the vessel, the stability of a ship is of paramount concern. The earliest regulatory recognition of this can be traced to before Samuel Plimsoll in the 1860’s. Despite the fact that times have moved on and stability regulations have come on a long way, the concern remains high.
Vessel’s intact stability is a fundamental component of seaworthiness so it is in the interest of all owners, operators, charterers and naval architects to learn about this topic and ensure that their vessel posses a satisfactory level of stability in order to ensure its safety as well as that of the people on board the ship. Understanding ship’s stability, trim, stress, and the basics of ship’s construction is a key to keeping a ship seaworthy.
A vessel can be considered in a seaworthy condition, from the intact stability point of view, if she fulfills the following two essential conditions :
1. The ship shall never be unstable when upright at the beginning or at the end of any given voyage while at sea.
2. The worst conjunction of wind and waves that can possible be encountered during the given voyage shall not cause the ship to roll beyond the safe effective range of heel.
There are many reasons that the initial stability at sea is in danger but two of them are of relevant importance:
1. When under the action of wave and wind pressure, the ship is very easily heeled.
2. When a certain course is maintained and is a great tendency for cargo to shift so that the angle of heel is continuously increasing and tends to become greater.
In order that any losses of stability that can occur during the sea passage to be covered and to maintain the limiting degree of safety, the vessel must sail with sufficient margin of metacentric height.
However, in practice, prior commencement of the voyage, there are a lot of situations when the metacentric height is known with a certain degree of accuracy. Moreover, in some situations, even if the metacentric height is ascertained with a higher degree of accuracy, it cannot be said wether the stability is and will remain satisfactory during the voyage
The condition of the ship throughout the intended voyage should be examined; it may often happen that it is necessary to commence voyage with a greater metacentric height than is immediately called for, in order to provide against conditions arising at later stages .
The safe effective range of ship’s intact stability depends mainly on two very important factors:
1. The nature of the cargo transported. The safe effective range is limited by the proprieties of cargo (such as angle of response in case of liquefaction and shifting of bulk cargoes).
2. The environmental factors encountered.
Among the environmental factors that can affect the intact stability of a ship, extreme weather conditions are dominant. In particular, the combined effect of wind and waves can lead to an excessive roll angle, water on deck, or motion of cargo. Thus, one of the contributory factors to safety at sea is the weather. Wind and sea forces may dominate the ship behavior or even overwhelm her physically, although a well designed, strongly built ship has a good chance of surviving the worst ravages of the open sea. However, the heavily rolling of modern ships in rough seas is a problem that has not been yet adequately addresses in international regulations.
Another important contributory factor to safety of ship is human factor. Proper decisions erroneously applied by master may result in a failure of equal consequences so that of bad design. A good ship can be easily turned in an unseaworthy one by an incompetent ship’s officer, but a badly designed and unseaworthy ship cannot be turned into a safe ship even by the most experienced, prudent and vigilant officer.
The overlap between these basic factors contributing to ship safety in a seaway is schematically shown in figure 1.1.
Fig 1.1 Overlap between factors contributing to safety of ship
Following the logic of figure 1.1, the problems of seaworthiness are discussed from three different aspects:
1. Fundamental design features which determine a ship’s behavior in extreme weather conditions and her seaworthiness in terms of intact ship stability failure. Although there seem to be a large number of different ship stability failure situations with different mechanisms involved, their number can be reduced to a few basic modes.
2. The dynamic aspects of wind and wave forces, particularly relevant to capsize situation.
3. The human decisions.
Good seamanship in heavy weather is not a quality which can be learned from reading books alone. It is, however, reasonable to expect that a man who learns much from his own experience can also gain a little from the experience of others, or from knowledge acquired through scientific experiments. This may help him to make rational decisions regarding seakeeping strategy in particular sea conditions.
Ship stability system is rather complicated and in most cases, it could be considered as consisting in four basic elements: ship, environment, cargo and operations, as shown in Figure 1.2 .
Fig. 1.2 Four-fold Venn diagram for ship stability system
The Venn diagram in figure 1.2 illustrates the strong interactions between the four elements. Analysis of the ship stability casualties reveals that the causes of the casualties may be attributed to :
1. Functional aspects resulting from stability characteristics of the ship,
2. Operational aspects resulting from action of the personnel handling the system (crew members, ship management, cargo handling, marine administration and owners company organizations),
3. External causes, like environment, resulting from factors independent from designers, builders and operators of the technical system.
All elements that create the stability system have to be taken into account in order to achieve sufficient level of safety with respect of ship stability. Considering the fact, that part of the maritime casualties are caused by faulty or bad design of the ship, the existing safety requirements can not guarantee a sufficient level of safety, especially for new designed ships.
The whole issue of seaworthiness is unlikely to be resolved through opinion polls. The only promising path to follow is through sensible, logical analysis of the dynamic aspects of ship behavior in heavy seas. The ultimate object of such an exercise supported, when necessary, by experimental evidence, would be to find a clean-cut answer to two questions:
1. What are the forces and mechanisms which capsize the ship?
2. What are the resistive design characteristics that a ship must posses in order to increase the probability of survival in extreme conditions?
If we succeed in this effort then we shall advance our understanding of seagoing condition qualities in a major way. Subsequently, the designer’s as objective would be to minimize the content of luck and maximize the effect of intelligent design.
This vicious circle of chain events can eventually drive the ship to capsize because unfortunately the capsize mechanism has not yet be fully understood due to underlying complex dynamics and parameters . Despite today’s advanced technology, it is not yet feasible to design and construct capsizing-resistant ships and the reason lies in the fact that it is not possible to model and simulate nature mathematically with all its aspects . Hence, the unpredictable character of the environment, especially ocean waves, can be considered the main factor responsible for capsizing.
The character and consequences of ship motion in a seaway largely depend on her three fundamental components, namely: the amount and disposition of weight (displacement, mass inertia), damping and stability. Taken together, the relative magnitudes of these components and the way they are blended into a unified whole, determine what might be called the dynamical personality of the ship, so that different ships have different dynamical personalities and, as such, they respond different to the same wave and wind forces.
It is impossible to design a ship that would be immune from all perils of the weather and so be fit and safe in all statistically probable conditions. We may agree that, by enlightened choice of the three fundamental components listed above, the probability of disaster can be reduced. Of course, with respect to ship stability is important to identify the causes that leads to stability failure. Thus, the possible hazards that may affect the ship stability can be identified by:
1. Analysis of loss of stability data casualties;
2. Statistical analysis of cause of casualties available in various sources;
3. Detailed description of ship stability casualties.
Ship stability casualty has to be analyzed as a distinct category, based on the causes that are at the origin of the event. A listing or capsize casualty caused by intact ship stability loss can be attributed to a multitude of factors, defined as potential hazards to threaten the ship stability. The possible hazards that can be defined as a potential situation to threaten the ship stability are shown in figure 1.3.
In the figure 1.3, critical stability is defined as a stability characteristic that do not meet IMO requirements. Forces of the sea include action of wind and waves, and may be the most difficult to evaluate because of the complex hydrodynamic structural model of behaviour of the ship in seaway. External heeling moments comprise different heeling moments apart of heeling moments caused by forces of the sea and shifting of cargo. In this category is the heeling moment caused by water on deck, by centrifugal force when turning or by tow rope forces. Although fire looks to be an atypical hazard in our context, it is important because fire fighting water can reduce stability and cause capsizing.
The problem is complex because a strong interconnection between above listed hazards may exist and two or more hazards may be present at the same time. Moreover, hazards are different for different types of ships and for different types of operation. For example, icing need not to be considered as hazards for ships operation in Mediterranean, and requires high ranking for ships operating at high latitudes.
Fig. 1.3 Possible hazards that can threaten the ship stability
Studying the causes of ship stability failures in more detail, understanding the nature of waves and wind blowing over them, and finding forces and moments that these conditions apply on the ship will contribute to a better understanding of this problem.
1.3 Ship stability casualties a serious threat to safety of navigation
As the earth is almost covered by seas and oceans, shipping can be considered as the fundamental as well as the dominant means of transport for the world trade. Huge number of vessels of various sizes and types, specialized on cargo and passenger trade, serve for humanity. Billions of tons of raw materials and finished goods are carried onboard ships, between ports, cleanly and without mishap everyday.
In the age of electronic navigation and the satellite era, casualties continue to occur at sea. Maritime transport safety is being enhanced by introducing numerous technical measures, by building safer ships, developing new and more efficient methods of transportation, investing in human resources, increasing traffic surveillance and control, issuing new regulations, etc. Nevertheless, accident statistics show that these measures are not sufficient and sometimes unable to halt shipping casualties. Casualties with catastrophic consequences still happen.
Any shipping casualty, whatever in nature, is every seafarer’s nightmare Should it occur in a confined area, like a channel or a strait where the traffic is heavy, several as well as serious risks are likely to be faced. On the other hand, a serious shipping casualty becomes even more critical by way, say, shifting of cargo or flooding thus possibly affecting the ship’s stability if exacerbated by heavy weather . In some other casualties, however the issue becomes environmental due to oil spillage involved.
The reasons leading to shipping casualties are many and complex. One of the primary reasons is the continuously building of bigger size ships that corresponds to increased cargo capacity. Hence, when an accident or a casualty occurs, the risk of loss becomes higher. On the other hand, large ships are characterized by reduced maneuverability which is a function of increased risk and contributing factor in marine casualties
In order to find out the laws of occurrence of shipping casualties and to achieve an estimate of maritime transport safety measures success, statistical study of shipping casualties is an important and effective tool, which has been often used by governmental bodies and experts worldwide. This is an important factor as to provide references for the maritime safety administrations, to adopt reasonable safety decisions – making, and for seafarers, to take relative precautionary measures.
Shipping casualties by types are various and their impact on safety to navigation differs from one another. Capsize, listing, foundering, grounding and stranding, structural failure, machinery damage, fire or explosion, collision or contact are the best examples.
This chapter presents the statistics and study data analysis of the ship stability casualties. The emphasis is put on the causes that influenced the stability of considered ships.
As the ship stability failure is the type of casualty, which in many cases ended with capsizing of ships that caused many deaths, ships and cargo losses, the detailed analysis for every type of ship have been made.
The study is focused only on the cargo ships (fishing vessels are not included) and the correlation between types of vessels and the listing/capsizing statistics presented. Besides the statistics, causes of casualties were analyzed. Correlations between factors involved are explained by comparing the ships with the casualties in certain periods.
The main purpose of this research is to establish ship stability failures classification criteria, which are helpful for constructing information, related to causes and modes of stability failures for enhancing navigation safety system, so it is important and basic research for improving navigation safety. The results of this study can make seaman to realize the frequency and cause of casualty and strengthen the seaman’s ability to deal with an emergency accident.
1.4 Analysis of statistics of ship stability casualties issued by maritime organizations
Statistics are indispensable factors for the amelioration of safety. Through the reconciliation of accidents that may appear isolated to interested parties, they permit tracking of typical causes of accidents; conversely, they can prevent, after a serious accident due to some fortuitous cause, the taking of incautious measures under the pressure of public opinion, which always inclines to gauge the gravity of the causes by that of the results. Lastly, they permit appraisal of the efficacy of rules in force.
In the world of shipping, various maritime organizations issued statistics of maritime casualties. The fact that the number of casualties related to pure intact ship stability failure (where the first event is loss of intact stability) is at relative low rates every year, led that most of maritime organizations not to include this type of casualty as a criteria in the statistics of maritime casualties.
Casualty statistics provided by IUMI
Maritime Insurance companies, like International Union of Maritime Insurance – IUMI, prepare statistics for different types of casualty but separated in two categories like “total loses” (fig. 1.4) and “serious loses” (fig. 1.5) based on the cause of the partial or total loss of the ship or loss of human lives .
Fig. 1.4 Total losses of ships 1994 – 2014 presented by IUMI
Fig. 1.5 Serious losses of ships 1994 – 2014 presented by IUMI
In the graphs presented above, the intact stability casualties are included in sub-categories like “weather” or “other”. Moreover, despite the fact that the statistics are separated for gross tonnage of ships involved in casualty, i.e. greater and/or lower than 500 gross tons, in these statistics are included also the fishing vessels. As the number of casualties for fishing vessels, for any type of casualty, is bigger than for cargo ships, a clear picture of one type of casualty, as in our situation is intact ship stability failure, cannot be compiled.
Casualty statistics presented by MAIB
Other maritime agencies, like Maritime Accident Investigation Board – MAIB, prepare statistics for maritime casualties only for UK register vessels that suffered casualties (fig. 1.6) or non-UK vessels that suffered casualties only in UK waters (fig. 1.7) .
Fig. 1.6 Casualties of UK merchant vessels 2002-2014
Fig. 1.7 Casualties of Non-UK vessels in UK Waters for 2014
Despite de fact that casualties like listing and capsizing are distinct from the other types of casualties, they are counted together. Moreover, there is no evidence that these kinds of casualties occurred due to intact stability failure or other cause. It is to be mentioned that the number of casualties represents only the casualties reported to MAIB, having in view that non-UK vessels are not required to report accidents unless they are within a UK port or within UK 12 miles territorial waters and carrying passengers to or from UK port.
Casualty statistics presented by Lloyd’s Register of Shipping
According to the definition of the Lloyd’s Register of Shipping database, the categories of casualty (made on the first event reported) are foundering, missing, fire/explosion, collision, contact, wrecked/stranded and other. It is important to be mentioned that the above listed categories are of total losses.
From the database presented for the period between 2007–2014, the accident frequency by category of losses was as shown in figure 1.8.
Fig. 1.8 Accident by category of losses (2007-2014)
The graph reveals that the main cause of accidents is foundering, this event being observed in about half of the accidents, as shown in figure 1.9.
Fig. 1.9 Percentage of accidents by category of losses (2007-2014)
Foundering, according to definition from Lloyd’s Register of Shipping, represent the ships that sank as a result of heavy weather, springing of leaks, breaking in two, etc., but not as a consequence of the other categories of accidents.
Having in view the above definition, the records of the accidents pointed out that among the accidents for which the information on weather conditions is available, heavy weather is quite frequent.
Even in Lloyd’s Register of Shipping database there is not a definition of accidents like listing or capsizing related to ship stability failure. We can only assume that part of the foundering casualties related to heavy weather is associated with ship stability failure. However, even an approximate number of this kind of casualty remains hardly to be estimated. Thus, is not a clear source of identification for the number of casualties related to ship stability failure.
Casualty statistics presented by IMO-FSI
International Maritime Organization – IMO by the Sub-Committee on Flag State Implementation (FSI), using the data received from rescue Co-ordination Centres (RCCs) in the form of situation reports (SITREPs) and other sources, prepare one of the most appropriate statistics to maritime casualties ,9.10,11,12]. According to IMO, principal casualties are divided into listing, capsizing, grounding/stranding, collision, contacts hull/structural failure, fire/explosion and machinery damage. Is the only one statistic where listing and capsizing casualty are counted as separate categories.
The graphic from the figure 1.10 gives a general overview about the total casualties occurred in the whole reference period, for all types of ships.
Fig. 1.10 Total number of casualties for period 1999-2003 for all ship types
As our interest was only the listing and capsizing casualties, our attention was paid only for these types of casualties. The distribution of listing and capsizing casualty numbers, shown in figure 1.10, represent about 6% from the total number of casualties, as illustrated in figure 1.11.
Fig. 1.11 Percentage of total number of casualties for period 1999-2003
Even with a small proportion in comparison with other type of casualties, listing and capsizing casualties were present every year, figure 1.12.
Fig. 1.12 Listing/Capsizing casualties per year for all ship types
According to statistics by ship type, the predominant proportion of listing and capsizing casualties are general cargo ship, about 79%, then Ro-Ro ships about 7% and container ships about 5%, figure 1.13.
Fig. 1.13 Percentage of Listing/Capsizing casualties for all ship types
The graph from figure 1.14 shows the number of casualties of listing and capsizing for every type of ships for the reference period.
Fig. 1.14 Listing/Capsizing casualties for period 1999-2003 for all ship types
The IMO – FSI statistics presented above are prepared only for a period of 5 years and the last update was made in 2003. As in the last ten years the number of ships increased considerably the world fleet as well as the amount of cargo transported, it is obvious that also the percentage of casualties is different. Even in this statistics were casualties like listing and capsizing are presented as separate categories, there is no any evidence that in case of those ships involved in such casualties the first event was loss of stability.
In the end of this chapter it can be conclude that there is still not a clear and actual picture of the casualties related for intact ship stability reasons.
In order that the casualties of intact stability to be assessed correctly, regularities noticed in the course of statistics analysis, regarding for example the influence of some hydrodynamic phenomena as well as types of cargoes carried on board ships, must allow formulating the causes of failures.
Like in any accident type, for the purpose to adopt the relevant safety measures and regulations to prevent such an accident in the future or to minimize the losses, in ship stability casualties it is essentially to keep a strictly statistic of those casualties. Hence, intact ship stability failure casualties have to be taken into consideration like separate events in the statistics.
The history and analysis of intact ship stability regulations
2.1 Development of intact ship stability regulations
The stability problem of the floating bodies, which can be traced back to Arhimedes himself, has never ceased to interest scientists and engineers, and has become an important part of academic studies .
Intact ship stability has been known for a very long time in terms of positive righting moments. Since 1747 Bouguer define in his work “Traite du navire, de sa construction et de ses mouvements” the metacentre as the intersection of two vertical axes passing through the centre of buoyancy at two slightly different angles of heel . Euler in “Scientia navalis sea Tratatus de Construendis ac Dirigendis Navibus”  describe the stability in a form of a general criterion, based on the restoring moment: the ship remains stable as far as the couple between the weight and the buoyancy forces creates a restoring moment.
In 1757, Bernoulli, established the relationship between rolling period of ships and the metacentric height (GM), whilst Moesley, introduced, later on, the dynamic approach with respect to the area under level arm curves . Around 1900, the problem of ships stability was considered as solved based on knowledge to evaluate the dynamic stability of existing ships. In fact, only the theoretically considerations were solved, but the main problem was to apply these fundamentals as a practical calculations of ships stability related to righting levers, having in view the complex geometry of ships hulls. This problem remained up until last decades of 20th century, several methods, based on approximations, were invented to overcome this problem, but the final solutions came with the appearance of computers. Ship stability was judged mainly on the calculated value of metacentric height which also in nowadays is still wrongly viewed as a main factor.
In 1939 Rahola carried out extensive statistical investigations into ship stability . Various still water lever arm curves of capsized ships were analyzed and he concluded that a large number of ships had righting levers below the minimum values of righting levers recommended by experts at that time. He identified that the ships had various values of righting levers, from too small, according to maritime board, to “critical” levers and sufficiently large lever arms. His investigations resulted finally in the definition of a “standard” lever arm curve defined by minimum levers at 20 and 30 degrees heel, the maximum lever being at 35 degrees heel and the angle of vanishing stability at 60 degrees. All lever arm curves are accepted as equivalent when the enclosed area up to 40 degrees is of the same amount or larger as the standard curve.
Rahola’s investigation was a success and proved, later on, to be the base of minimum stability criteria adopted over the years. Even the present intact ship stability criterion, issued by International Maritime Organization through Resolution MSC.267 (85) , is based on Rahola’s conclusions. From his investigation, it is important to note, that ships that capsized due to dynamic effects like resonant rolling or shifting of cargo has been categorized as safe ships with sufficient large still water lever arms in most of the situations. Thus, dynamic influences were neither considered directly, nor indirectly, in Rahola’s minimum requirements.
Intact ship stability provisions were been introduced at a later stage in international regulations of ship safety. The necessity of intact stability rules was uncertain until SOLAS’48, as stated in Recommendations contained in Annex D, recommends to the Administrations a more detailed examination of intact ship stability. The first international intact ship stability rule at IMO was originated by a recommendation contained in the conclusions of SOLAS’60, when for the first time was recommended to initiate studies on the basis of information referred by ships types, as intact stability for passenger ships, cargo ships and fishing vessels as well as standards of stability information .
As a result, in 1968, the General Stability Criteria based on righting arm characteristics was adopted through IMO Resolution A.167 . This recommendation, known as “statistical criterion” is originated from the studies of Rahola and was developed in terms of global quantities related to initial metacentric height, static and dynamic stability arms satisfying a set of standards obtained empirically from statistics of casualties. The requirements in A.167 consisted of a minimum GM value, a minimum lever arm value at 30 degrees heel and three minimum areas below the lever arm curve. However, dynamic effects were not taken into account. When A.167 was developed, data in sufficient quantity was only available for smaller ships, thus the resolution was applicable only to ships smaller than 100 meters in length. On one hand, it was simply to use but on the other hand it was difficult to improve as has no physical modeling and no mention to sea state and the level of safety was unknown. The introduction of A.167 constituted a tremendous improvement of previous state of art regarding stability at international level but practically…was nothing. In comparison, all subsequent changes and new introduction can be considered only as smooth changes ,22.
Again, as an answer to recommendations given in the conclusions of SOLAS’74, where was recommended the improvement of international standards on intact stability of ships taking into account the external forces affecting ships in seaway which may lead to capsize or unacceptable angles of heel, the Weather Criterion was adopted in 1985 by IMO Resolution A.562 . The main aim of this criterion was to assure that ships are able to withstand heeling moments due to incoming waves and wind without exceeding certain roll angles . The structure of the criterion was prescriptive as well, whereas the threshold values were based on statistical long-term evaluations of accidents made since the first formulation in the stability requirements of the Soviet Register of Shipping from 1947 . The critical KG value was adjusted to fit the mean of all KG values of ships in the statistics, which were considered safe in operation.
Shortly after its introduction, from the first time in 1985, the weather criterion was criticized. The main point of critics was, beside the partly unrealistic simplifications regarding the constant heeling lever due to wind and wave induced roll motion, that the criterion is calibrated for old ship types with traditional hull forms, moderate to small lateral areas and small B/T –ratios.
The general outcome of resolution A.167 and A.562 is typically in the form of limiting curve for GM and KG as a function of ship draft .
Families of ships of same typology have been compared with statistical and weather criteria requirements 28, resulting in the fact that the second one is more severe. Another comparison has been made between A.167, A.562 and SOLAS’90, for particular types of ships, like the modern large passenger cruise ships, which revealed that the weather criterion more severe than SOLAS’ 90 3031. If the A.167 is subject only to criticism the other two instruments are severely criticized.
All regulations and recommendations with regard to ship intact stability and safety against capsizing issued by IMO were consolidated in Code on Intact Stability of All Types of Ships Covered by IMO Instruments, adopted by resolution A.749(18) on 4th November 1993 .
With the adoption of Res. A749, which incorporates A.167 the upper limitation in ship length was lost apparently without prejudice and both criteria, the general criteria and weather criteria, were considered for ships of 24 m in length and over. The general intact stability criteria regarding lever arm curves proprieties are almost unchanged from those stated in resolution A.167. The ‘severe wind and rolling criterion”, originally introduced by IMO resolution A.562(14) in 1985, was also part of the IMO resolution A.749(18).
All the general intact stability requirements were applicable to ships equal or larger than of 24 meters in length. The requirements also applied to ships of more than 100 meters in length, although the majority of the ships represented in the statistical example on which the requirements were based, has lengths of less than 60 meters. This being in contrast to the old regulation A.167,
It was widely accepted that the general intact stability criteria, in the form of A.749 (18), do neither provide a sufficient level for large ships, nor do they assure a uniform safety level for ships of different size or type . The main reason for these problems was the fact that the minimum requirements were not correlated with the ship size. In practice, this leads to the situations that a large container vessel, with a length of 320 meters, is allowed to sail with the same minimum GM of 15 centimeters as a small vessel with a length less than 100 meters. If by the same principle is applied the Froude’s similarity law, it revealed that lever arms increase with a geometrical scale with increasing ship size. Thereafter, in order to resist in front of heeling moments in the above mentioned example, the GM value would have to be increased for the large ships by the factor 320m/100m = 3.20.
In figure 2.1 it is revealed the scale of magnitude between a value of righting lever of 0.2 metres and a new generation of post-panamax container ship. Because that the righting lever is small compared with the ship dimensions it lies in the same scale of magnitude as the uncertainities in the hydrostatic calculations. If the of lever arms variations is taken further into account, as a result of waves for example, it becomes relevant that the small limit values are not sufficient to provide an adequate safety level for large ships.
Because of the container ship development in the early 1980’s, a clear trend could be noted to ship designs with increasing beam of ships, without similar increase of the depth. This is simply, because this type of ship typically carries large amounts of deck cargo resulting in relative large vertical centers of gravity. Under the pressure to optimize the ship’s economically, designers usually try to maximize the number of containers carried on deck. To fulfill the minimum stability requirements it is an appropriate measure to increase the ship’s beam and thus, to maximize the waterline area of the vessel. This results in larger initial stability, but reduced form stability, whereas an increase in depth is always unfavorable for the initial stability of the vessel (not for the form stability). Large initial stability, low additional form stability and a relative small range of positive righting levers characterize the lever arm curves of the resulting ship designers.
Fig. 2.1 Illustration of a post panamax container ship righting lever of 0.2m
Having recognized that containerships had hull forms characteristics significantly different from traditional designs were mainly the statistical basis used for the general development of the general IMO intact stability criteria, Wagner  concluded that new criteria should be developed, in order to compensate the more unfavorable beam-to-depth ratio of the modern designs by increased stability . The absolute magnitude of righting lever is extremely small with respect to the inaccuracies in calculations or other effects such as alteration of those righting levers in waves. In 1986, Soding and Tonguc  demonstrated already that the righting lever is about 1% of the vessels beam, which they concluded is to small with respect to possible inaccuracies in calculation . For the 2010-vessel the same required minimum righting lever is only 0.35% of the vessels breadth and thus lies within the range of typical calculation accuracy.
Thus, model test were conducted by Blume and Wagner for several container vessels to demonstrate that such a vessel could easily capsize in case the stability was adjusted according to the relevant minimum standard 38. In order to establish a criterion for the minimum stability of a vessel in rough weather, Blume 38 tried to develop a statistical criterion based on the residual area under still water righting lever curve in order to distinguish a vessel from clearly being safe or unsafe in a specific condition. The aim of tests was to find a correlation between characteristics of the righting arm curve, derived from the Rahola’s parameters, and the statistical collective divided into safe and unsafe samples, and he found out the following :
1. The only parameters which showed significant correlation with capsizing events occurring or not, were the maximum righting lever of the still water righting lever curve and the area under righting lever curve.
2. No other Rahola’s parameters, such as initial GM or position of the maximum righting lever, showed any influence on the safety against capsizing.
Blume, Wagner and Hattendorff also tried to find a connection between Rahola’s parameters and the righting levers of the ship situated in wave crest condition for a wave equaling the ship’s length 3738. They failed to succeed because the results were almost the same as obtained for the correlation with still water lever arm curve.
In order to keep their criterion simple they calibrated their form factor, called C-factor, based on the still water levers. In this respect, the C-factor was taken into consideration as an addition requirement to the minimum requirements of the Rahola’s parameters, but in accordance with the calculation procedures already in use. The concept of the resulting criterion was to adjust the constant independent threshold values according to the IMO A.167 with specific C-factor. This mean that, for example the minimum still water righting lever at 30 degrees should take a constant value divided by C.not the constant value of 0.2 m. The so-called C-factor is to be determined by a regression formula taking into account the draft, a modified depth including hatches, the centre of gravity above base line, block and waterline coefficient, respectively:
The aim of C-factor is to define different minimum requirements for the righting levers with respect to the following design features of container vessel, which were according to the model tests related to ship capsizing :
1. Hull forms having large values B/T and B/D need larger righting levers than conventional hull forms. This may be explained by the reduced form stability combined with larger alteration on righting lever in waves.
2. It was found that in bow or stern quartering seas, those vessels were endangered where the centre of gravity was significantly higher than the still water line. Therefore, the C-factor decreases with increasing T/KG-ratio. The explanation might be that the difference between the alternating restoring moment due to the wave action and heeling moment takes larger values.
3. Hull forms having large ratio of waterline coefficient over block coefficient are suspected to have large righting lever alterations in waves and therefore more vulnerable rolling.
The C-factor criterion is part of the IMO Resolution MSC.267(85), the International Code on Intact Stability 2008, as an alternative assessment of intact stability for container vessels above 100m in length, but as the whole code it is not mandatory so far. Therefore, as there is no measure to force the use of C-factor concept, it is revealed the fact that container vessels are usually designed according to the weaker general intact stability criteria,.
As explicitly, intended for container vessels above 100m, the C-factor concept is restricted to those types of ship and to the related phenomena, accordingly. Investigations of Kruger  and Hass , carried out later, emphasized that besides the parameters covered by C-factor, also the L/T-ratio plays an important role, which has the same importance as the B/T-ratio or B/D-ratio.
Because the larger stability loss on the crest due to barge type aft bodies the C-factor actually fails for vessels like Ro-Ro or Ro-Pax ferries due to the fact that is not correctly represented by the CB/CWP-ratio. This confirms that the C-factor concept may not be generalized for other ship types than those the C-factor was intended for.
2.2 Analysis of actual intact ship stability criteria recommended by International Maritime Organization (IMO)
The revision of Intact Stability Code started in 2001 and completed in 2006. The Code was put in discussion as a consequence of some inconsistency in the application of existing regulations to the design of large passenger vessels, continuously growing in size; also the spectacular accident of the containership APL China in the Pacific Ocean revealed that parametric rolling of ships in waves may be a much more dangerous issue than expected.
The first step consisted in an important structural reorganization and in the development of an alternative way on experimental basis to fulfill the requirements of weather criterion for ships having parameters outside the original range.
All recommendations and regulations relating to ship intact stability and safety against capsizing issued by the International Maritime Organizations (IMO) are consolidated nowadays in the International Code on Intact Stability (2008 IS Code) adopted by resolution MSC.267(85) on 4th December 2008 . Compliance with the new Code was required under changes to the SOLAS and Load Line Conventions, for ships whose keels are laid on or after July 1st, 2010, to which these Conventions apply.
The stability criteria as included in the revised Code are virtually the same as the original IMO resolution A.167 adopted in 1968 (statistical criteria) and in resolution A.562 adopted in 1985 (severe wind and rolling criterion) with small amendments and some relaxations. As a first main difference from the previous regulations, the new Code, which is referred to as the 2008 IS Code, has two parts: Part A which is mandatory, contains general intact stability criteria for cargo and passenger ships and Part B, which is recommendatory, contains intact stability criteria for certain types of ships as recommendations and additional guidelines.
What has changed? There are two significant changes. The first is the requirement for all ships to demonstrate compliance with wind and wave criteria. If the standard criteria are not applicable to the vessel, due to the vessel dimensions falling outside those relevant for the formulae given, model tests may be used to derive a value for the angle of roll. Model tests may also be used to identify the wind heeling lever for all vessels.
The second significant change is the requirement for flag administration approval of stability instruments, in cases where an instrument is proved to supplement the stability book. However, is necessary the development of guidelines for the approval of stability instruments, defining acceptable tolerances.
Trimmed hydrostatic data must also be provided in the stability book covering the expected trim range, and any limiting curves should also cover the expected trim range. In other words, is the request to take into account trim effect in stability calculations in view of the quite large variations of waterplane with trim and/or waves. The fixed trim calculations accepted up to now were not on the side of safety with respect to the more realistic calculations.
Additionally, the text covers dynamic stability phenomena and non-mandatory requirements for stability instruments, like the use of anti-heeling measures, which must not adversely affect the stability when they fail.
Other changes have been made to different aspects of IS Code:
1. The cancellation of the method based on the use of factor k and relative table for the evaluation of the correction to the righting arm due to the presence of liquids with free surface on board;
2. The simplification of the regulation requiring that the maximum of the righting arm should be at an angle preferably greater than 30°. But in any case not less than 25 deg, which, following the common practice of many Administrations, now simply requires that the angle should be not less than 25deg;
3. Warning was added to the alternative assessment of stability for containerships greater than 100 m. Since the regulation was obtained empirically using data from ships not greater than 200 m, it is advised to apply this criterion with special care to ships outside this range.
The following items have been removed from the Code:
1. The requirements for dynamically supported craft (these requirements are now included in HSC 2000 Code),
2. The references to rolling period tests, as these tests were considered to be applicable only to vessels under 24 meters in length. Therefore, as the 2008 IS Code was implemented under SOLAS and Load Line Conventions, these ships are not covered.
3. The formula and associated table for calculating the free surface moment.
2.2.1 General Stability Criteria (Criteria regarding righting lever curve proprieties)
The IS Code 2008 inherits the general stability criteria almost unchanged from A.167. These are (figure 2.2):
Integrated area below the lever arm curves has to reach the following minimum values:
1. The area under righting lever curve (GZ curve) shall not be less than 0.055 meter-radians up to 30 degrees angle of heel
2. the area under righting lever curve (GZ curve) shall not be less than 0.090 meter-radians up to 40 degrees angle of heel or the angle of down-flooding if this is less than 40 degrees
3. The area under the righting lever arm curve (GZ curve) between the angles of heel of 30 degrees and 40 degrees or between 30 degrees and the angle of down-flooding, if this is less than 40 degrees, shall not be less than 0.03 meter-radians
The righting lever GZ shall be at least 0.2 m at an angle of heel equal to or grater than 30°:
The maximum righting lever shall occur at an angle of heel not less than 25°:
The initial metacentric height GM0 shall not be less than 0.15 m:
Fig. 2.2 Righting lever curve
The Code is still based on the same assumptions, according to which the ship indicator of stability safety is the righting arm curve on calm water.
2.2.2 Alternative stability criteria for container ships of more than 100 meters in length
Having in view the explosive demand in containers transport in the last decade, was obvious that the containers ships buildings will have a huge growth. From the economically point of view this fact was reflected in the new trends of containers ships design, in order to maximize the number of containers carried, especially on deck, reflected in increasing beam of ships but unfortunately without similar increase of the depth.
The alternative stability criteria for container ships of more than 100 meters in length established by IMO are:
Integrated area below the lever arm curves has to reach the following minimum values:
1. The area under righting lever curve (GZ curve) shall not be less than 0.009/C meter-radians up to 30 degrees angle of heel
2. The area under righting lever curve (GZ curve) shall not be less than 0.016/C meter-radians up to 40 degrees angle of heel or the angle of down-flooding if this is less than 40 degrees
3. The area under the righting lever arm curve (GZ curve) between the angles of heel of 30 degrees and 40 degrees or between 30 degrees and the angle of down-flooding, if this is less than 40 degrees, shall not be less than 0.006/C meter-radians
The righting lever GZ shall be at least 0.033/C m at an angle of heel equal to or grater than 30°
GZ (φ = 30°) ≥ 0.033/C.
The maximum righting lever shall should be at least 0.042/C
GZmax = 0.042/C.
The total area under the righting lever curve up the angle of flooding should not be less than 0.029/C meter-radians
In the above criteria the factor C is a factor influenced by the ship’s forms and is expressed empirically by the following formula:
The above mentioned stability criteria were empirically developed with the data of containerships less than 200 meters in length, while for the ships beyond such limits, also with considerably flares and large water plane areas, they should be applied with special care.
2.2.3 Severe Wind and Rolling Criterion (Weather Criterion)
Hazards posed by the forces of sea are of the most serious hazards to the loss of stability casualties and the safety against capsizing. The present set of stability criteria, that are inherent part of the IS Code 2008, includes consideration of the forces of the sea in the form of “weather criterion” which was originally adopted by Resolution A.562.
The ability of a ship to withstand the combined effects of beam and wind rolling should be demonstrated for each standard condition of loading, with reference to the figure 2.3.
Fig. 2.3 Severe Wind and Rolling Criterion
The scenario that the ship has to survive according to this criterion illustrated in figure 5.3, is as follows :
The ship faces an initial heeling moment, estimated by the steady heeling wind arm (), due to a steady beam wind, which is given by a static pressure. The steady wind velocity is 26m/s, a value that presents a kind of “average” condition between the center of typhoon and the following steady wind zone. The criterion assumes that the resulting lever arm is constant over all heeling moments. The heeling angle of equilibrium ( ) should be limited to a certain value: 16° or 80% of the angle of deck immersion, whichever is less . From this initial floating condition ( ) the ship performed a wave induced roll motion to the opposite, to windward, reaching to a maximum healing angle (). The absolute value of this roll angle () is determined by a formula, which contains correction coefficients , and related with the draft to beam ratio, the roll period and the roll damping in a simplified way and the ship’s block coefficient. Respective tables derived from a series of ships estimate these coefficients. As an improvement to resolution A.562, IMO proceeded through the SLF Sub – Committee sessions, to a modification of the procedure of calculating rolling amplitude regarding the tables related with factors and , as they were quite unfavorable for new Ro-Ro and Ro-Pax ship designs .
Having reached the leeward turning point of this roll motion then is suggested that the ship is subjected to a wind gust pressure, resulting in a total heeling lever of 1.5 times the value of the initial static lever, = 1.5 . The criterion proposes that letting the ship roll freely from position with zero roll velocity, the potential energy should be sufficient in order to prevent the ship to roll leeward beyond the limiting angle . Considering the area under lever arm curve as a measure for the amount of potential energy stored in the inclined system, the criterion requires that the area under the lever arm curve beyond the new static equilibrium heeling angle must be at least of the same size as the area under the curve integrated from windward turning point up to the new equilibrium. The integration limit on the leeward side is given by the angle of vanishing stability, the angle where progressive flooding occurs or a maximum angle of 50 degrees, whichever is less. In fact, the ship is assumed to be released in still water from the off equilibrium position as wave action is not taken into account at the leeward motion, thus only potential energies are accounted in the energy balance131.
2.2.4 Analysis of actual ships intact stability criteria
The assumed weather criteria is simply to use, it is based on physical phenomena / modeling but was adjusted with capsizing casualties in the form of the wind velocity. In other words, the wind velocity in the weather criteria does not represent the actual sea state and has rather empirical meanings. Since the weather criteria involve such an empirical factor, it is not easy to improve the criteria. However, the simplified modeling takes into account only beam waves and wind, why no internal degree of freedom, like shifting of cargo or water on deck, was introduced . In fact, it concerns only one mode of ships loss and the level of safety is largely unknown.
Adoption of the described prescriptive requirement and structural model was possible only because it was based on statistical evaluation of data for ships capsized and operated safely during the long time of the first formulation of criteria in 1948 in Register of Shipping of USSR. The crucial element of the criterion, the value of wind pressure was adopted in such a way that the resulting critical KG value would correspond to average KG values of the population of vessels existing at the time of development of the criterion that were considered safe in operation (trial calculations did show that the majority of existing at that time ships satisfied adopted weather criterion) .
The weather criterion is the only amongst all of the IMO criteria taking into account the influence of waves and only one being based on a balance between heeling and righting levers. Although it considers the dynamics of ship roll motions, at least in a simplified way, this prescriptive scenario is not suitable to assess phenomena endangering ships in head, following and quartering waves and it also never was intended to be used in such a way.
There were still some other pending issues, connected with the possible consequences of a mandatory IS Code 2008 making impossible the adoption of some alternatives currently used by Administrations. The most important is connected again with the required minimum value for the angle of maximum righting lever.
The safety level guaranteed to the ships by the compliance with stability criteria, however, is in general unknown and it is still a big open problem. It is indeed typical to open the way to alternatives by stating that “a level of safety has to be guaranteed, as a minimum, by any alternative assessment” . Statements like this are often used to try to avoid excessive relaxation of safety standards, but in fact are less meaningful than they could appear. Of course, ship safety at sea was greatly improved by the development and implementation of present stability criteria, as contained in IS Code 2008, and other measures (for example the assignment of freeboard), although being these measures recommendatory in nature or not so widely adopted.
In addition, it is clear that the safety level is unequally distributed among different ship typologies and, even inside a given ship typology, it appears to be strongly dependent on ship size. This is particularly true for the General Criterion, which is the result of a global re-active approach. It mixes indeed in the same pan good and bad designs in a set of standards most of which not having a clear physical relation with the phenomena they are trying to avoid. Also the present version of Weather Criterion, due to its relatively poor, although physical, modeling spreads unevenly the safety level among ship types.
From point of view of ship safety this is however, not the final solution. From time to time, stability casualties happen in spite of the fact that the particular ship meets all existing IMO criteria. The existing criteria may also be not applicable to some type of modern ships incorporating novel design features especially because original criteria as resolution A.167 developed more than forty years ago were based on casualty statistics that included mainly vessels under 100 m in length . With many modern ships there is no previous experience in relation to safety and stability and satisfying existing criteria may not assure required level of safety .
In order to achieve sufficient level of safety with respect to stability, all elements creating stability system have to be taken into account . Taking into account the fact, that only part of the casualties are caused by faulty or bad design of the ship, the safety requirements that refer mainly to design features of the ship cannot ensure sufficient level of safety, in particular with regard to ships having design features .
A black box is constituted by the sentence “to the satisfaction of the Administration” that often accompanies these alternative measures. Actually, these sentences should be accompanied by some guidelines or codes of practice.
Presently, the assessment of intact ship stability is confined to the fulfillment of empirical criteria related to the static lever arm curve for still water condition only. The IMO stability criteria are prescriptive rules based on practical experiences quite many years ago.
Even if model tests or dynamic simulations show a clear improvement, innovative measures for improving the dynamic intact stability of a design are scarcely rewarded or even punished by current rules. Ship designs change very rapidly, due to market demands, and the current criteria (based on static lever arm curve for still water condition) are easy to handle neglecting the (dynamical) physical characteristics of modern vessels. Geometries of novel ship designs have become considerably different from conventional forms. These considerable differences in geometry may invoke dynamic behavior that is drastically different from historical experience. A good example of this phenomenon is pure loss of stability and parametric roll of containerships. Capsizing still seems unlikely for these large ships. However, large roll angles and accelerations may pose on obvious danger for both cargo and crew. Such behavior is outside of historical expectations for a ship; therefore, dynamic stability concerns are well grounded.
An increased number of container ships recently suffered from parametric excitation with results in loosing and / or damaging the cargo, implied also the risk of capsizing. Not only the new designed container ships are susceptible to parametric excitation, but also Ro-Ro, Ro-Pax, Ro-Ferry and Cruise ships vessels.
The fact that most of them have not yet encountered such a dangerous situation, can be considered only a matter of luck. Moreover, quite a few vessels were endangered by pure loss of stability or combinations of parametric excitation as well as loss of stability. All these dangerous mechanisms as well as other dangerous and unfavorable seakeeping characteristics are not covered by currently intact stability rules. Very short roll periods leading to high accelerations (especially if combined with insufficient roll damping as well as insufficient course keeping capabilities in rough weather) for the Ro-Ro, Ro-Pax, Ferry, cruise and container ships made the vessels operators of such vessels to complain.
Operational ships intact stability failure modes
Transport is the most important link in economic relations. It is involved in creating of products and delivering it to consumers, provides the link between production and consumption, between different industries, between countries and regions. It affects the development of the economy as a consumer of petroleum products, grains, metal, timber, and many other products.
Sea transport is used for external relations. This transport carries more than 80% foreign trade goods and is the cheapest mode of transport because the exploitation of maritime requires greater network costs, as in other types of transport. Modern ships can carry cargo of any size and weight.
Most of cargo of the maritime transport accounted for liquid cargo – oil and petroleum products. In their basic structure bulk is dominated, then – general and secondary mass. Dry-cargo vessels have an important share.
The structure of marine transport is dominated by specialized vessels – tankers, bulk carriers, container vessels, timber carriers, reefers and dry cargo vessels. Most of these ships are designed under certain type of cargo.
Every cargo transported has particular proprieties for loading, stowage and transport. These cargo proprieties can influence the nautical qualities and the seakeeping behavior of ship. One of the nautical qualities, which is influenced, and in many cases affected by cargo proprieties, is the ship intact stability. Inability of acknowledge the proprieties of the cargo to be loaded, in many cases, is the result of dangerous situations, even to capsize.
Safe transport of cargo is not just a matter of safe stowage and securing of cargo, solely, but is strongly related with the design and construction of the ship, her outfit, as well as with the way the ship is being operated at sea in different environmental conditions.
Minimum stability requirements by authorities do not show clearly on which operational conditions they have been based and do not include any risk possible. They are still seen as guidelines for ship operator. Further information on the actual ship behavior to be expected in extreme conditions as well as measures for prevention and survival in these conditions are necessary to be provided to ship master.
As cargo loaded on board ship represents the most part of the total ship mass, its feedback to ship behavior is of paramount importance. In relation to ship stability, ship mater is concerned about safety from capsizing and about low motion accelerations on the cargo. Seakeeping theory and the results of modern ship motions should be transferred aboard the ship to the operator in a comprehensive fashion. Information provided on board vessel must include not only measures on cargo stowage and securing, but also operational measures to reduce ship motions. Maybe naval architects should reconsider the designing of ships with good motion behavior. Furthermore, the ship design should account more fore a feedback from the practical experience of the ship operator.
Operational stability represents the picture of actual ship stability during the voyage. This kind of stability varies in time due to changes of two important factors: environmental conditions at sea and cargo and/or ballast on board the ship.
In order to ensure safety of the ship from capsizing, the actual stability status is compared with the minimum stability requirements. This is so called “regulatory” stability, while the operational stability is influencing the ship motions and the resulting loads that acts on cargo. Therefore, the operational stability of a ship and the minimum stability requirements are very clearly distinguished.
Ship’s operational stability is often seen as guidelines for ship masters. A minimum stability requirement alone lacks the needed information on its background, such as at which severe conditions the ship will survive. Thus, the master finds himself alone, although he has to take decisions in the daily life operations, very often at the border-line of safety.
In order to allow the ship to sail, those minimum stability requirements constitute the minimum set standard, but they cannot include all possible risks from any extreme and severe events. Hence, if the minimum levels are set to high, in order to cope with extreme events, transportation may feel drawbacks such as reduced economy, worse ship behavior in seaway, cargo experiences unnecessarily higher motion accelerations, which requires more securing and lashing. On the other side, if the minimum stability levels are set too low, this may be dangerously understood by operators and the ship may not be able to resist a severe environment.
Apart from the casualties related to heavy weather conditions, the study continued for the assessment of other factors involved in ship stability failure. The study was performed on a number of ships involved in loss of stability casualties, due to operational factors.
In order to identify the causes, a study of a selection of casualties was performed for which the analysis was carried out and a presentation is given here below.
3.1 Proprieties of cargo loaded and the influence on ship stability
3.1.1 Timber cargoes
Casualty of ship related to stability failures due to proprieties of timber
ABDULKADER F – 5,209 GRT, general cargo vessel
Vessel completed loading operations, on 16th December 2009, and shortly upon developed a list to port which produced a first collapse of deck stow and lost of timber packages overboard. The lost of timber packages from port side developed a rolling of the ship to starboard side, thus leading to a second lost overboard of a bigger number of timber packages. Then the phenomenon repeated in opposite side and induced a violent rolling motion to the ship. Finally, the vessel remained listed about one degree to starboard side (fig. 3.1).
The investigation concluded that one of the causes of collapse the timber deck stow was the fact that packages of timber was wet and covered by snow/ice, thereafter the weight of timber packages was more than declared and the friction between packages was lower than in dry conditions.
Fig. 3.1 MV Abdulkader F, collapse of timber deck stow
FJORD PEARL – 6,780 dwt, general cargo vessel
The vessel, loaded with 8,446 m³ of timber, encounter heavy weather conditions, with winds gusts up to 27 m/s, south of the Aland Islands, on 1st January 2002. The wind heeling was of about 5 degrees and the vessel rolling was 10-15 degrees to both sides. During the change of heading the vessel, first listed 15 degrees to port and immediately after 30 degrees to starboard at which point was noticed the shifting of deck cargo of about 1.5 m. Furthermore, the vessel remained listed about 22 degrees to starboard and in few hours the list increased to 26 degrees due to water ingress (fig. 3.2)42.
Fig. 3.2 MV Fjord Pearl, listed to starboard due to shifting of timber deck cargo 
The chains of the events led to the shifting of the deck cargo. The investigation revealed that the friction limit was exceeded due to list and accelerations and the cargo started to move. The friction coefficients were lower than expected due to the weather during loading, the timber deck cargo was covered by ice/snow at the time of loading, and during the voyage, sea water was sprayed over deck cargo in low atmospheric temperatures.
Proprieties of timber that affects intact ship stability
Timber is loaded on board vessels in various forms with different weights. According to the definition used in maritime literature, timber means any sawn wood, or lumber, cants, logs, poles, pulpwood and all other types of timber in loose or packaged forms except wood pulp or similar cargo . Timber products in the form of sawn timber in pre-slung bundles or logs can be stowed above or below decks.
A timber cargo exposed to environmental factors changes its physical proprieties but, to a certain degree, retains its quality, due to which it is possible to transport it on weather decks of ships.
Among various proprieties of timber, the ones who have influence on transport process are specific volume, apparent density, humidity, stowage compactness, shrinkage, and swelling .
Specific volume is defined as the volume occupied by one metric tone of timber material. Is the factor on which can be appreciated the weight of cargo to be loaded. The volume of one metric tone of timber depends of the nature of timber and varies between 1.0 and 3.5m³.
The apparent density depends on the tree species and humidity contained by timber at the time of assessment of weight and volume, and represents the mass of timber in a given volume. It varies in broad range during transportation from logging grounds to shipboard (rafting by rivers, carriage by road or rail, etc.).
Humidity of timber is the amount of water contained in relation with the weight of timber in completely dry condition and can be calculate with the following formula:
– is the weight of timber in wet condition [t],
– is the weight of timber in absolute dry condition.
Stowage compactness depends on wooden material type and composition (averaged volume of cargo transport unit), cargo space shape (curvature of cargo hold lines), deck openings, size and structural adaptation to timber loading technology. It increases somewhat for shorter wooden materials and significantly grows for materials with large cross section.
Shrinkage and swelling . Timber will shrink when its moisture content falls below fiber saturation point. The degree of shrinkage will depend on the final moisture content that the timber reaches and varies considerably between timber species. Across the width of a flat sawn board (the tangential direction), shrinkage can be twice as much as across a quarter-sawn board (the radial direction). As a rough estimate, tangential shrinkage can be as 1% for every 3% reduction in moisture content. For example a change in moisture content from 19% to 13% can result in a 250mm wide flat sawn board shrinking to 245mm, as this would result in a gap of 5mm.
Timber, like many natural materials, is hygroscopic, it takes on moisture from the surrounding environment, it picks up or gives off moisture to equalize with the relative humidity and temperature in the atmosphere. That means by changing of weather conditions is changing also the amount of water contained; the timber humidity is increasing as the relative humidity of air is increasing. Once the timber humidity is increasing, his weight is also increasing, this being an important factor that has to be taken into consideration when loading timber cargoes. Moisture exchange between timber and air depends on the relative humidity and temperature of the air and the current amount of water in the wood. At 100% relative humidity and 20° temperature of the air, the timber humidity is 30% that means the timber is saturated .
When the IMO Code of Safe Practice for Ships Carrying Timber Deck Cargoes  was drawn up, it may have been assumed that all timber, including logs and packaged timber would have densities less than 1,000kg/m³. Following loses of packaged timber deck cargo from Brazil, investigations revealed that the density of the timber was greater than 1,000kg/m³: in other words the timber as a whole and as loaded dry, was heavier than water. Samples were cut for 14 timbers, each clearly different from the others, but shipped collectively as “sawn Brazilian hardwood”. Scientific analysis of each sample revealed that 78% of the cargo by weight, had specific gravities between 1.0 and 1.4 and that the remaining 22% had an average specific gravity of 0.93. The overall average of specific gravity was 1.080 as compared with the specific gravity of oceanic salt water, which is 1.033 .
The average specific gravity of a normal packaged timber deck cargo is about 0.6. The data for timber conditions in most standard ship stability books indicates a specific gravity of 0.4 where “condition volume” is set against “condition weight” 46. This tends to underline the technical philosophy of the Code, namely, that a timber deck cargo should float and if it shifts and causes a severe transverse list it will provide buoyancy to prevent the ship listing further towards capsize .
As the timber reaches high stowage factors, possible up to 1.4m³/t, this indicates that a ship whose holds are full loaded with forestry products will often not be down to her load line marks. In this case, the most common method used in practice is to split the timber cargo below and above decks.
Timber is usually loaded on board vessels in packages or bundles of planks of various lengths and sizes, secured with steel or plastic strapping bands. Timber packages are delivered for loading in two forms: protected (by plastic wrapping) or unprotected (when has been kiln dried).
The cargo of timber carried on an uncovered part of a deck or superstructure is defined as a timber deck cargo .
Vessels that carry timber on deck are allowed for less freeboard than other cargo vessels whilst certain vessels are assigned timber freeboards but have to be complied with certain additional construction conditions .
An important factor for the assessment ship stability is the correct weight of the deck cargo. Normally, the weight of timber should not exceed the maximum permissible loading of weather decks and hatch covers. Instances have occurred, because the standard stability booklet has indicated a given height of cargo as representing a given weight, the master have assumed that any cargo of the same height will have the same weight and this assumption has proved to be wrongly based and had serious consequences .
The fact that timber deck cargoes absorb great quantities of water during the voyage is well known 4546. Even if the ship was loaded with adequate GM and sufficient positive stability, the absorption of water, especially in the events of shipping heavy seas, correlated with burning off tones of fuel from low-situated tanks could dramatically affecting the ship’s GM and destroy the positive stability of the vessel. Same results for ship stability are common in the case of accumulation of ice and snow on timber deck cargo.
Timber cargoes carried on weather deck of ships affect their seaworthiness in two ways: first, deck cargo increase buoyancy and stability at large heeling angles, second, they reduce initial stability due to higher position of vessel’s center of gravity and the timber propriety to absorb water and increase weight. The degree of timber weight increase during the voyage has been little investigated and no provision of water absorption by wooden materials, carried on weather deck, was made in the earlier regulations. In accordance with IMO recommendations, the currently effective rules make provisions for a 10% increase of timber deck cargo weight, where reliable data on water absorption degree is lacking .
Fig. 3.3 Bundles of timber, covered by snow, stored at open premises prior loading on board vessel
Of course, the percentage is relative, having in view that the degree of water absorption by wood during the voyage is influenced by many factors like, temperature, air humidity, atmospheric precipitation, waves splashing the timber stow, type of wood, dimension of wood, time of exposure and duration of voyage. A correlation between all these factors probably will bring the solution to a more accurate information for a particularly type of wood carried on a particularly voyage. For the moment, the only one solution available for deck officers is that deck timber weight to be ascertained by variation of vessel’s drafts. However, this method to determine water absorption degree is approximate and lacks accuracy.
3.1.2 Cargoes that may liquefy
Casualties of ships related to ship stability failure due to liquefaction of cargo
Over the last years, an increased number of vessels lost their stability due to bulk cargo liquefaction. Part of them developed large angles of list whilst others unfortunately capsized. The below Table 3.1 illustrates the casualties that involved ship stability failure due to liquefaction of cargo .
Table 3.1 Casualties of ship stability failure related to liquefaction of cargo 
The liquefaction process of cargoes and associated problems
The problems associated with liquefying cargoes whilst on board vessels are nothing new and is well known in the bulk trade 495051525354, There have been a number of very serious incidents occurred over recent years, where vessels have experienced liquefaction of cargo, leading to loss of stability and capsize. The areas with high profile problem are India (for iron ore fines), Philippines, Indonesia and New Caledonia (for nickel ore), affected by the monsoons and were many casualties occurred. However, the problem can arise anywhere where mineral cargoes (with fine particles) are extracted and stored in open areas.
Usually, the cargo is extracted and stored in open premises that are not protected from the adverse weather conditions (frequent and large rainfalls). A particular case is India, where in summer months the monsoon is affecting a large part of the country. However, the hot humid climate is present in all the countries where this problem is prevalent.
The cargo may be wet when extracted from the mines or becomes wet when left in open storage areas (fig. 3.4).
Fig 3.4 Exposed stockpiles of iron ore surrounded by water 
The cargoes that are containing at least some fine particles and some moisture, although they need not be visibly wet in appearance, are those with risk of liquefaction.
Cargoes such us iron ore fines, iron ore concentrates, nickel ore, fluorspar, certain grades of coal, pyrites, sinter/pellet feed and others have all given rise to liquefaction associated problems 5052. Moreover, any cargo containing fine material and moisture has the potential to liquefy and the proprieties of such cargoes being indeed at risk from this issue, although they look dry in appearance at time of loading.
The liquefaction process appears when in a fine grained cargo the spaces between cargo grains are filled with both air and water. The problem occurs in mineral cargoes of predominantly fine particles, mined and stored in conditions which allow the soaking up of large amounts of water which is then retained, with minimal drainage or evaporation occurring.
During loading on board vessels, the cargoes are usually in their solid state, the particles are in direct contact with each other and, therefore there is a physical strength of resistance to shear strains (fig. 3.5).
Fig. 3.5 Iron ore prior liquefaction 
If the amount of absorbed moisture is too high, the mineral cargoes can turn into muddy slush. Whilst at sea the cargo is subject to forces due to the engine vibration and motions of the vessel as well as waves impact. This aspects leads the forces that causes the inter-grain spaces to contract resulting in compaction of cargo. If compaction is such that there is more water inside the cargo than there are spaces between the particles, the water in the spaces between particles is subject to a compressive force but as it is a liquid, it cannot be compressed 53.
The water pressure inside the cargo can rise sharply and press the particles apart and where enough moisture is present, the reduction in inter-grain friction due to the ship’s motion and vibration can be sufficient to cause the cargo flow like a liquid, i.e. to liquefy (fig. 3.6) 53.
Fig.3.6 Nickel ore with large quantity of moisture (up) and Iron ore with large quantity of moisture (down), inside cargo holds 50
Shifting of cargo is the most significant consequence for the vessel resulted from liquefaction. As the ship is rolling, the cargo flows to one side of the hold but not completely return. Thus, the accumulation of cargo to one side is progressively leading to loss of stability, resulted in dangerous angles of list (figure 3.7) that in some instances can be disastrous such that the vessel and the lives of those onboard are lost.
Fig. 3.7: Developing of list due to cargo liquefaction
Based on the modality and speed of loading, the geometry of cargo inside the cargo holds can have multiple peaks and plateaus. The mechanics of wet loaded cargo inside the cargo holds is rather complex. Thus, mixing process with the cargo appears when the water drains into the hold bilges, due to vibration of the ship. However, part of the water migrates separately to the surface of the cargo and forms scattered puddles. This result in compaction of cargo at lower part that increase, in the same time, the draining resistance and loose surface cargo with presence of water.
Liquefaction of bulk cargoes is a special characteristic of cargo because it happens throught the depth of stow not at surface. This is a very big inconvenient because a preventive measure for shifting can not be taken (like for example in case of grain cargo when in order to prevent the shifting it is restrained the cargo surface by strapping or by loading bagged grains on top). In case of iron ore ore nickel ore, the cargo may flow in the depth of stow even if the surface if restrained.
By their nature, bulk carriers are not designed to carry liquid or semi-liquid cargoes. When process of bulk cargo liquefaction happens, it can cause stability problems that in many cases have led to vessel’s capsizing and sinking. Small lists, due to de-ballasting, create surface water flow, truncating peaks / collapsing cargo and thus can result in huge sudden dangerous movements of the ship. Large liquefaction can happen in mere minutes and adversely affects the shipment, operationally as well as commercially, and can lead to severe casualty beyond ship’s control. Moreover, as large liquefaction is initiated at loading port it gets aggravated during the voyage and may persist during discharging.
Same aspects may be encountered in case of multi-purpose ships with cargo hatches covered by pontoons. Despite the fact that vessel completed loading in upright position, during maneuvering the pontoons the vessel develop a small list which correlated with flowing of cargo can result in sudden increased listing angle with dangerous consequences.
The main reference for any ship operator or ship master when considering whether a cargo is likely to liquefy is the IMO International Maritime Solid Bulk Cargoes Code, named IMSBC Code . Within the Code Group A cargoes, the dangers associated with cargoes that are likely to liquefy are listed. In accordance with the provisions of the IMSBC Code, ny cargo listed as Group A, should be shipped and carried strictly. The definitions, tests, and precautions in the Code for cargoes that may liquefy are widely associated with metal ore concentrates, for which their application is relatively straightforward.
The general framework for the carriage of all cargoes is contained in the SOLAS Convention, Chapter VI-Carriage of Cargoes. However, the Code provide in Section 1.2.1 that schedules for individual cargoes are not exhaustive and it may be that some cargoes, which can liquefy, that are not included in the Code.
There are two important proprieties of such kind of cargoes that have to be taken in consideration when loading on board vessels: Flow Moisture Point (FMP) and Transportable Moisture Limit (TML).
Flow moisture point is the maximum water content, expressed as a percentage, at which a sample of cargo will begin to lose shear strength . Cargoes with moisture content beyond FMP may be liable to liquefy. Transportable moisture limit is defined as 90% of the FMP . The major factor that increases the moisture content in the cargo, above TML, is the rain during storage in open space or during transit from the mines to the port, in open top train wagons, barges, or conveyors.
The IMSBC Code, in Appendix 2, describes the sampling and test procedures that should be carried out for the cargoes that are subject to liquefaction.
The TML and its actual moisture content, of the representative sample for the cargo to be loaded, are the most important figures from the ship operator’s and master’s perspective. The laboratory must first determine the FMP (in order to find the TML) and then the moisture content of the cargo is obtained by drying samples of the cargo in accordance with section 4.6.4 of the Code. The cargo should be safe to load only if the moisture content of the cargo sampled is below the TML. Loading a cargo above or close to its FMP represents an unacceptable high risk for vessels.
Liquefaction process may occur unpredictably at any time during the voyage in cargoes loaded with moisture content in excess of the FMP point. In some situations, cargo have liquefied and caused catastrophic cargo shift almost immediately on departure from the loading port, in other situations liquefaction occurred several weeks after uneventful sailing . While the risk of liquefaction is greater during heavy weather, in high seas, and while under full power, there are no safe sailing conditions for a cargo with unsafe moisture content. Even in relatively calm conditions on a vessel at anchorage or proceeding at low speed, liquefaction can occur unpredictably.
The problem is how reliable is the information received by vessel’s Master from the local shippers and authorities as the cargo presented for shipment is compliant with IMSBC Code. It is not unusual for shippers to present incorrect or, at best, inadequate cargo documentation.
Many alerts have been issued regarding Brazilian sinter feed cargoes  whereby Brazilian shippers have been wrongly declaring these cargoes as non-hazardous cargoes or namely Group C cargoes (i.e. neither liquefy nor possess chemical hazards) under the IMSBC Code.
In other cases, certificates were presented stating only that a cargo of ore bulk material was tested “in accordance with the IMBSC Code, and passed successfully” . The cargo’s alleged average moisture content was recorded without the corresponding TML and FMP values, and of course, unless those values are known, the moisture content is meaningless.
The fact that current testing and certification regime for these cargoes may be inadequate was revealed by the casualties happened in this respect and thus, reliance only on shipper’s certificates is not a good measure and should be avoided. Given such lapses, it is unsurprising that cases of liquefied cargoes resulting in capsized ships are still regular.
The poor compliance of some shippers with testing and certification requirements that are required under IMSBC Code and designed to ensure that cargoes are loaded only if the moisture content is sufficiently low to avoid liquefaction occurring during the voyage, looks to be one of the main cause of the casualties.
However, the causes of casualties is probably a mixture of understanding of the problem, and inadvertent or, occasionally, deliberate misrepresentation of the true nature of the cargo by shippers or by others.
From the investigation reports of the casualties, resulted also that not every master or chief officer was aware of the problem or of the simple “can” or “shake” test that can be performed to check the risk of cargo liquefaction, despite the fact that the test is described in IMBSC Code. The facts revealed that masters are often not aware of what information they are entitled to receive from shippers under the IMBSC Code; nor are they fully aware of their rights under international carriage of goods conventions to reject or land unsafe cargo.
Only in few cases, masters seem to abrogate all responsibility for checking whether or not the cargo being loaded on board is safe.
It is very important that all the ships’s deck officers to have adequate knowledge and to be made aware of the nature of the cargo to be loaded and of any risks which may arise. A detailed risk assessment can be a guarantee for the safe carriage of cargo with potential risk of liquefaction.
The vessel’s crew should remain vigilant during the entire process of loading and should note any signs of the cargo (like moisture content) that can affect the safe carriage.
If the Master is in doubt about the characteristics and the condition of the cargo, instead of doing a mistake with later catastrophic consequences, a good solution is to call an expert surveyor to attend for sampling and testing the cargo.
However, despite all the precautions taken, there are many situations when the cargo which is unsafe for transport is loaded on board vessels. Masters has to bear in mind all the time that carrying such kind of cargo is contrary first of all to SOLAS and if any dangerous situation arise during the voyage (like for example liquefaction of cargo) the crew and the vessel are exposed to a potential danger.
In this respect, detailed stability calculations have to be carried out not only prior departure but also during entire loading operations in order that if any abnormal situations occur to be eliminated in time and prior vessel’s departure from the port.
It is very important that the results of the stability calculations to be analised with attention and accuracy and the corrective measures regarding loading of cargo, if necessary, to be applied with caution.
In the past, where difficulties have arisen in persuading shippers or loadport authorities to discharge unsafe cargo, various sugestions have been made as to steps which can be taken to minimize the risk of liquefaction. These includes sailing with reduced speed, staying close to shore, and avoiding heavy weather. While such measures may reduce the amount of movement to which the cargo is subjected, there are not supported by SOLAS. There is no guarantee of their effectiveness and it is not possible to quantity the risk in any meaningful way, as vessels have been known to sink suddenly in calm conditions as a result of liquefaction of the cargo. It is recommended that Masters should not, therefore use such measures as a substitute for talking all required steps to ensure that the cargo is safe for transport .
3.2 Shifting of cargo in severe sea conditions
Casualties of ships related to shifting of cargo
TRADEN – 8,188 GRT, Ro-Ro cargo ship
The vessel en route from Valencia to Norkoping, loaded with containers, pallets and roll trailers, occurred in emergency on 19th October 2001, due to a severe cargo shift. The cargo shifted when the ship got into storm for more than two days and into exceptionally troublesome confused seas .
KARIN KAT – 1,501 GRT, general cargo vessel
Vessel loaded with various stainless steel LNG cargo handling equipment was sailing from Antwerp bound for Ras Laffan. On 17th February 2003, the vessel encountered bad weather with winds up to gale force 8. The vessel was rolling 15-20 degrees and also pitching and heaving resulted in shifting of cargo inside vessel’s holds. The investigations concluded that the rough weather and heavy swells, which caused the rolling, pitching and heaving of the vessel, has been a contributory cause of the foundering. Moreover, the cargo shifted because it was insufficiently secured to withstand the movement of the vessel during the prevailing rough sea .
ANSAC ORIENT – 16,715 GRT, bulk carrier
Vessel encountered, on 2nd February 2004, a swell on its starboard bow and rolled violently up to 20 degrees to each side. Logs fell from the after end on no.3 hatch allowing the lashing to slacken. Cargo then shifted athwartships and about 45 logs were lost overboard. Investigation revealed that the vessel’s GM was of 3.38 m, the ship was very stiff and had a short period of roll .
SUNSHINE – 1,214 GRT, general cargo ship
The vessel loaded with marble in blocks sailed, on 22nd April 2007, in bad weather conditions, with heavy seas, north of Libyan coast. During Master’s effort to maintain a reasonable course under the given circumstances, the vessel took, suddenly, excessive list and capsized within 10 minutes. The shifting of a single heavy block of marble has identified primary cause of failure .
RIVERDANCE – 6,000 GRT Ro-Ro trailer ferry
This vessel encountered, on 31st January 2008, stability problems in heavy weather during the course of a voyage across the Irish Sea towards the UK, and ultimately foundered on a beach on the UK coast. Among other factors that contributed to loss of ship stability, the investigations pointed out that shifting of cargo was one of the causes .
Associated problems related to cargo shifting
According to the statistics, 10% of serious stability accidents of ships were caused by the shift of cargo. These kinds of accidents are quite common for general cargo vessels, container vessels, Ro-Ro vessels and bulk carriers.
Cargo shifting is a complex phenomenon. Usually it is caused by the motions of the ship and is largely influenced by the proprieties of cargo, lashings as well as operational aspects. During the voyage, a various number of different conditions is encountered by the ship influenced by sea state, ship’s speed and heading towards the waves, and loading condition. Forces acting on the cargo and ships motions will be different in each such condition. The waves will have specific proprieties and in order to response to these waves the ship will move in a certain way. These ship motions will induce forces on the cargo, and if these forces are larger than what the cargo and its lashings can withstand the cargo will shift. Thus, the probability of cargo shifting will differ in the various conditions.
Hua  has presented a concept of an equivalent roll angle and probabilistic approach to the problem of cargo shifting. He discussed the effect of linearization of the model on the predicted results.
The dynamic forces acting on a piece of cargo
The diagram of the forces acting on the cargo piece is presented in figure 3.8, below.
Fig. 3.8 The diagram of the forces acting on the cargo piece
and are the components of the total force F in the global inertial coordinate system. These components can be derived from the Lagrange’s equation as follows:
As can be seen from equations (3.1) and (3.2) force F has inertia, Coriolis and gravitational force components. The total force F is given by the components in the moving frame as
The equations (3.3) and (3.4) can be used for determining the required strength of the cargo lashing system. In this case, cargo shift is set to zero, i.e. in equations (3.3) and (3.4), and the ship motions govern the forces acting on the cargo only.
Effect of cargo shifting on ship intact stability
The effect of transversally cargo shifting is the moving of ship’s center of gravity from to , as illustrated in figure 3.9.
Fig. 3.9 The effect of shifting of cargo on G
In this way, the ship’s center of gravity has two components of movement: y- vertical component and x-horizontal component. Both of components can be calculated by the formulas
, , (3.5)
x – horizontal movement of G;
y – vertical movement of G;
Δ – displacement of ship;
w – weight of cargo shifted;
a – horizontal movement of centre of gravity of w;
b – vertical movement of centre of gravity w.
Thereafter, the new righting lever GZ will be (fig. 3.10),
Fig. 3.10 Reduction of righting lever due to shifting of G
In the situation when only the horizontal movement is taken into consideration the loss of righting lever will decrease as the angle of list increases. In other words, the greatest loss of righting lever will be when the ship is upright. The ship will come to rest at the angle of list in still water condition and the horizontal movement of ship’s centre of gravity will represent a negative value of righting lever when the ship is upright (fig. 3.11).
The effect of horizontal movement of G (when the vertical movement of G is ignored) is as follows :
1. The initial GM remain unchanged (since only the vertical movement of G will cause this to change).
2. All values of righting lever across the range of stability are reduces, particularly at the smallest angles of heel. When the ship is upright, the horizontal movement of G acts as a capsizing lever causing the ship to heel over to the angle of list.
3. A reduction of dynamical stability (area under the curve). Since the ship is already listed, less work is required by the external forces to heel the ship over to dangerous angles of heel on the listed side.
4. Range of stability is reduces (at both ends of the range for a ship having an initial range of stability less than 90°).
5. The angle of heel at which the deck edge immersion occurs remains unchanged but there is less work required by the external forces to reach it on the listed side.
Fig. 3.11 The effect of horizontal movement of G on stability curve
In the cases where the vertical movement of G is not ignored, the effects of cargo shifting is illustrated in Figure 3.12.
This situation involves a decreasing of GM and this effect is reflected as follows :
1. All values of righting lever across the range of stability are reduced, particularly at the larger angles of heel.
2. Dynamical stability (area under the curve) is reduced making the ship less able to resist heeling by external forces.
3. Range of stability is reduced.
4.The angle at which deck edge immersion occurs remains unchanged as freeboard has not changed.
Fig. 3.12 Effect of vertical shifting of G on stability curve
In both situations presented above, the effect of cargo shifting revealed that most aspects of stability are worsened.
Shifting of cargo on board vessels can lead to dangerous listing and ultimately capsizing. The majority of the casualties involved the consequences of the transverse movement of cargo in heavy weather.
Cargo movement can be attributed to contributory factors like:
1. Movement caused by waves hitting the stow (for deck cargo), and exacerbated by movement of the vessel in rough sea.
2. Failures in the methods of stowing and lashing the cargo.
Cargo is not always stowed in compact stows and large void spaces remained. These gaps allowed the stow to move especially when vessel was rolling in heavy seas. Lashing materials (wire rods, turnbuckles, rings, etc.) are in many cases old and poor maintained. Moreover, many are not certified and not adequate to be used like lashing materials in particular cases. Provisions stated in regulations issued by ship’s administration (such as Cargo Securing Manual) or international regulations (such as IMO Code of Safe Practice for Cargo Stowage and Securing) are often ignored with intention or by rashness. In many cases, ship’s Cargo Securing Manual did not contain any instructions on stowage and securing of particular types of cargoes, or any information or guidance for the crew related to stowage and securing of cargo on deck/hatch covers.
3. Inadequate friction between the cargo and ship steel structures (decks, hatch covers, etc.).
The decks and hatch covers are made from steel and usually are covered by ordinary paint coating which does not provide a non-slip surface, especially when wet. In this way, would be of great benefit in reducing the risk of cargo shift if a proprietary high friction coating is applied on top of these surfaces where the cargo is to be loaded. An alternative method can be the mixing of paint with sand which can increase considerably the friction. A particular and very often case found in practice is that steel bands strap the bundles of sawn timber. These steel bands are the contact points at the lower part of the bundle with steel tops of the hatch covers. Steel-on-steel provides a very low level of friction.
3.3. Overweight and miss-declared weight of containers
Casualties of ships stability failure related to overweight and miss-declared weight of containers
MSC NAPOLI – 53,504 GRT Container vessel
Vessel encountered bad weather condition and developed large rolling angles (Figure 3.13). The vessel has broken in two sections with total loss of ship. The investigation into the accident (August 2000) , revealed what had long been suspected in the industry that many of the containers, which managed to be salvaged intact from the deck, the manifested details, did not always match up with the actual contents and weights. The casualty gave investigators a unique opportunity to compare cargo details supplied by shippers with a container’s actual contents. The industry of containers transport was able to get some idea of the true extent of certain problems. The findings were quite alarming, with the weight of 137 of the 660 containers on deck (that means 20%) found to differ from the declared figure by more than three tones, resulting in a total deviation of 312 tones from the cargo manifested.
Fig. 3.13 MV MSC Napoli, listed to starboard side, 
LIMARI – Container ship
Vessel was under loading operations of containers at Damietta port, Egypt, in February 2007. During loading operations, the container stack collapsed in vessel’s Bay no.52 (Figure 3.14). Investigations revealed  that the container weights exceeded the declared weights by 362% (Row 08), 393% (Row 06), 407% (Row 04) and 209% (Row 02) in Bay 52 where the collapse occurred.
Fig. 3.14 Collapse of container stack on board MV Limari, 
REPUBLICA DI GENOA – Ro-Ro vessel
After a sudden and progressive listing, March 2008, vessel capsized alongside in Antwerp port, under loading containers and cars (fig. 3.15). The incident, as per investigation report , most likely occurred as a result of a stability problem, which may have been caused by the actual weight of containers not equating to the given weights in the cargo manifest.
Fig. 3.15 Ro-Ro vessel Republica di Genoa, capsized alonside berth 
HUSKY RACER – 10,000 GRT Container vessel
The container feeder vessel whilst alongside and working at Bremenhaven, October 2009, suffered a deck stow collapse on bay 26, resulting in the loss of 18 containers overboard (Figure 3.16). It was subsequently found  that the top tier containers in 7 of the 9 stows which were shown on the bay plan as empty, were in fact full, with weights ranging from 15 to 30 tons.
Fig. 3.16 Collapse of container stack on board containership Husky Racer 
DENEB – Container ship
The 508 TEU containership suffered a significant stability incident and capsized alongside berth at Algeciras port, during loading containers (fig. 3.17)
Fig. 3.17 Containership Deneb capsized at berth, in Algesiras port,63
The investigation  found that out of 168 containers on the load list, 16 containers (almost 10%) had actual weights in excess of the declared weights. The range of exceeded weights was between 1.9 times to 6.7 times. The actual weight of the these 16 containers was more than 278 tons above their total, declared weight of about 93 tons or 4 times higher than their declared weight.
Problems that may arise from overweight containers
The container shipping industry has made gigantic strides since, the first overseas container ship Encounter Bay was berthed at Sydney, on 3rd of April 1969, to inaugurate a full container service from the UK/Europe to Australia/New Zealand. It was a revolution. It was not just a gradual progression from one system to another such as sail to steam, it was a revolution that spelt the death knell to conventional shipping, as containerization rapidly took over.
Once upon a time it was cube cutting that was the fraud of choice for the world’s freight forwarders but with the burgeoning trade in full load containers (FCL’s) over the past few decades, often stuffed with consolidated cargo from different consignors, a new and potentially much more dangerous problem has arisen: overweight .
Containerization has become the dominant method of carriage of goods in international trade, by sea and by land, almost all types of cargo being transported. The intermodal method of transportation has proved to be a very efficient and in the same time a cost effective way of transporting goods over long distances.
The issue of overweight containers is struggling the freight transport system, despite the rapid progress of containerization, and is a subject of concern for maritime industry, insurance companies and governments as the incidents involving overweight boxes continue to be present.
The overweight of containers, have dogged the industry since its first appearance and unfortunately this practice is still widespread, over the world, in all countries even in the more developed.
This major problem are said to interfere not only with the sea transport but also with rail transport or road transport and generate excessive wear on roads, rails or bridges. It is a more complicated problem as involves a wide range of participants from the chain of trade, from transport to logistics, from shippers to trucking companies, and from shipping lines to ports.
There are varying contexts or definitions of “overweight” . An overweight container is the one who exceeds not only the container carrying capacity limits or its weight as declared by the shipper but also the road weight limits, rail weight limits or crane lifting limits. Each of these overweight situations are involving risks of casualties and presents operational and safety problems. In maritime industry, but not limited to that, the most common overweight situation is when the actual container weight exceeds the shipper’s declared weight. Based on the actual weight of a container would enable a carrier and a terminal operator to knowledgeably address all the various container weight issues and requirements.
Overweight containers are becoming a very serious problem. It is estimated that as many as 20% of containers are overweight or miss-declared weight 67.
The graph illustrated in figure 3.18, presented by Lloyd’s List Magazine in August 2013, show the types and number of incidents on board container ships, during the first half of the year 2013, involving containers.
Figure 3.18: Types of containers incidents (Jan – Jul 2013)
From the above figure, it can be noted that the incidents related to overweight containers were present every month and moreover, at high level, i.e. 19% compared with other dangerous incidents like fire (4%) and explosion (1%)(fig. 3.19).
Figure 3.19: Percentage of types of containers incidents (Ian – Jul 2013)
As containers are stacked higher to keep up with the growth of world trade, overweight and miss-declared weights can :
1. Lead to vessels being improperly stowed, which can adversely affect vessel stability and stress.
2. Produce collapsing of container stacks on board ships.
3. Damage the ships.
4. Cause containers lost overboard (both the overweight and containers that were not overweight).
5. Be a risk of pollution damage due to contents of containers spilling into the sea.
6. Impair of vessel’s optimal trim and draft, thus causing impaired vessel efficiency, suboptimal fuel usage, and greater vessel air emissions.
7. Produce huge cargo liability claims.
8. Many incidents has occurred when the bottom of the container has fallen off during the lifting of the same due to the cargo weight in container exceeding the declared weight.
9. Lead to last minute shut-outs of confirmed, booked and available loads when the actual weight on board exceeds what is declared, and the total cargo weight exceeds the vessel limit or port draft limit.
Shippers are continuously trying to maximize the space in the container and this fact can be regarded as the main reason for the overweight of containers and miss-declared weights. This fact correlated with incorrect loading of the deck stows (according to the container weights and as per the ship’s standard instructions), is the way to disaster. All the handling and loading equipments (gantries, fork lifts, trucks) may not be up to the strain and, in worst case scenarios, ships can be incorrectly loaded leading to instability.
On board the ship, the containers are generally stowed with the heavy containers at the bottom, either on deck or under deck of the ship, and the lighter containers on top of these containers. This is done in order to maintain the stability of the ship and achieve an even spread on board. If this elementary rule is not respected the ship stability can be very serious affected with unpleasant consequences.
For example, a client has some containers that weigh about 24 tons each but in order to save costs on overweight, if he declared the containers as 18 tons each (this fact happens frequently) the shipping line will take the word of the client and advise the ship that these containers are 18 tons each. When the ship’s planner does the planning, following the weight categories, he is going to place the 18 tons containers on top of 20 tons containers. Now if these 20 ton containers are actually the right weight, we now have a 24 ton container (miss-declared as 18 tons) sitting on top of a 20 ton container. If we imagine many such miss-declarations as this, the chief officer of the ship has a problem now to try and adjust the stability. According to his calculations, the ship should be stable when the cargo is planned to be loaded in a certain way, but in reality because of the miss-declared weights, he is not able to get the right stability of the ship. On many occasions the shipping lines give reasons such as “due to stability constraints we had to short ship your cargo” .
As the loading is planned in accordance with the weights provided by the shippers, the weight miss-declarations are one of the major reasons for such instances. Thus, in order that the ship’s officers to compute accurately the ship’s stability, various hull stresses and to ensure the ship is within the safe parameters for the voyage, the information provided fir the correct weights of containers are essential.
Despite the fact that the location of standard size containers may seem to be more precise indicated in the loading plan the ship’s officers are unlikely to have any knowledge of the centre of gravity of each container. This will, almost in all situations, be lower than the container’s mid height, as they are unlikely to be filled right up to the top, or contain heavy items overlying lighter ones. However, there are exceptions like in the case of containerized suspended meat carcasses where their weight acts at the container top. Thus, exceptions do occur and, though this particular example should be readily identifiable, others may not be. Even though the shipping lines are taking many precautions and deterrents such as Over Weight Surcharge, Miss-declaration penalties etc, these situations continue to be present. As mentioned above, this can seriously affect the lives of the innocent crew on board the ship if anything should happen to them due to such negligence.
When under this circumstances, heavy containers are stowed on top of light or empty containers, not only the stability is then compromised, the additional dynamic forces created could also be the trigger for a stow collapse
Because of false declarations of weight, the potential dangers that might lurk within any of the hundreds of those steel boxes onboard container ships, is indeed a situation that no shipmaster should have to contend with.
The continuous growing number of incidents, due to miss-declared weights of containers, either small feeder vessels capsizing or deck stows collapsing in large container ships, must be a cause for raise the alarm.
For many of the shipping companies, the problem of miss-declared container weight information unfortunately seems to be in a “too big basket”. Despite the concerns from organizations such as the Classification Societies, Government Agencies, P& I Clubs, marine surveyors and other parties associated with marine transport, little action seems that have been succeeded to address this issue.
In late 2008, the Australian Senate heard testimony from Customs and other officials there that “approximately 80% of consignments from developing countries are miss-declared” and that “the evasion of duty through undeclared cargo is considered to be another incentive for more container inspections” .
No accurate data exist to indicate how many containers are overloaded or have miss-declared weights, but the problem is significant and arises in almost every trade to some extent. In some geographic trade lanes, the problem is common and, at times, rampant. Ship lines have reported that in severe cases, the overweight or incorrectly declared weights reaches 20% of the total cargo on board vessel. Some carriers report that it is not uncommon for actual total cargo weight aboard ship to be 3 to 7 percent more than declared weight .
The fact that there is no mandatory requirement for containers to be weighted prior shipment on board, in many ports in the world, is unacceptable and against any safety policy. The operator acts with abuse and in a unscrupulous manner based on the fact that all the containers weights are as per shipper’s declaration. Moreover, many of the container weights, even if the contents are supplied accurate, do not always take into account the tare weight of the container.
By a simple example if considering a container ship of 4,200 TEU capacity in a typical full load condition, as described in vessel’s loading manual, as follows: Total 20 TEU containers loaded on deck : 2,603. Total 20 TEU containers loaded in hold : 1,575. Total 20 TEU containers loaded on board: 4,178. The average container weight is considered to be 9.1 mt.
Vessel commences the voyage with 96% from the capacity of fuel oil, diesel oil, and fresh water, and arrives at destination port with 10% of those consumable liquids. The stability calculations carried out for the departure and arrival conditions revealed that the vessel complies with all stability criteria as per IMO Res. 749 (18). The calculated initial metacentric height , corrected for free surface effects, for the arrival condition was found to be 0.408 m.
Let’s consider now that 10 % of the containers, loaded on upper tiers on deck, are overweighed with 1 tone each That means a number of 260 containers (with a declared weight of 2,366 tons) are overweighed 1 tone and results in an increased weight on deck of 260 tons.
For the arrival condition, the metacentric height , corrected for free surface effects, will decrease with about 6 to 7 cm, which represents 17% of the metacentric height. This fact associated with decreasing of stability when the vessel is on the wave crest may lead to dangerous situations.
Too many under declarations of weight can expose the ship to a higher risk for incidents. While the container key industry players attest that safety is of paramount concern, evidence suggests that, in reality, the safety of ships, crews and the environment is being compromised by the overriding desire to maintain established schedules or optimize port turn round times.
3.4 Miss-declared weight of cargo
Casualties of ship stability failure related to miss-declared weight of cargo
RIVERDANCE – Ro-Ro Trailer Ferry
This vessel encountered stability problems in heavy weather during the course of a voyage across the Irish Sea towards the UK, September 2008, and ultimately foundered on a beach on the UK coast (fig. 3.20). The investigations 73 revealed that the true weights of loaded trailers were unknown.
Fig. 3.20 Ro-Ro ship Riverdance foundered 
MEDY – 4,120 GRT General cargo vessel
The ship loaded a cargo of scrap metal in bulk, September 2010 at Constanta port. Vessel cast off the lines around midnight, shortly upon pilot disembarked, altered the heading to port start “flopping” from side to side, and finally developed a list to portside. Even after ballasting of the tanks in opposite side, i.e. starboard side, the list increased more to portside. As the list continued to increase master decided to drop the anchor (fig. 3.21).
Fig. 3.21 Vessel Medy at anchor prior sinking 
When it became evident that the situation is not under control, why the list is continuously increasing, the crew abandoned the ship. After few hours, the vessel capsized and finally sunk.
The cargo loaded on board vessel consisted in pieces of metal of various shapes, types and weight. As this pieces were mixed and loaded in the same time, it was difficult to obtain a stowage factor for such a cargo and moreover hard to estimate when and in what position inside vessel’s cargo holds the heavy cargo or light cargo/pieces was loaded.
On completion of loading operation the vessel has a large angle of list to starboard side. To adjust the list, vessel was ballasted in a “randomly manner”, port and starboard tanks, without filling completely any of the tanks. As soon as ballasted completed in one side, vessel starts lolling to opposite side.
The fact that vessel start “flopping” from side to side and finally developed a list means that the vessel starts to loose gradually the stability, probably being in a unstable equilibrium condition. This may be the evidence that the cargo’s centre of gravity was considerably higher than had been assumed, so high, that the vessel’s initial metacentric height was negative. This leading to the idea that the scrap metal that was loaded initially was a lot lighter than that which was loaded later on top.
Miss-declared weight of cargoes and associated problems
The miss-declared weight of cargoes is not a common practice only for containers. The problem occurred also for other packaged homogenous cargoes or project cargoes. In this situation, the cargo arrived at loading premises with the weight declared by shippers. Moreover, the centers of gravity as well as lifting positions of such cargoes are indicated by painted marks, by the manufacturer. In many situations, that information proved to be wrong. Despite the fact that ship’s cranes were certified to lift such declared weights, were unable to do it. In other situations, the trim or list of the vessel, upon loading of such cargoes, was more than expected.
In the last years, the problem of miss-declared weights of cargoes was present in bulk trades, especially in scrap metal trades. The maritime transport of scrap metal in bulk is growing up every year. Scrap is seen increasingly as a global commodity. It is a cargo that apparently is easy to load, stow and transport by ships. The reality proved the contrary. In the last years a number of ships, loaded with scrap metal, loss their intact stability and furthermore most of them capsized.
As a normal rule, in practice, the weight of bulk cargoes is determined by dividing the volume of cargo hold to the stowage factor. Of course, this rule applied as most of the bulk cargoes are homogenous cargoes. But, what means homogenous cargoes? According to maritime publications , homogenous cargo is that stowed loose in the hold and is not enclosed in any container such as box, bale, bag, cask or the like.
Bulk cargoes consist entirely of one commodity and are usually shipped without packing. Specifically, bulk cargo is composed of either :
1. Free flowing articles such as oil, grain, coal, ore and the like which can be pumped or run through a chute or handled by dumping, or
2. Uniform cargo that stows as solidly as bulk cargo and requires mechanical handling for landing and discharging.
A very important aspect of bulk cargoes is fungibility (goods that are identical with others for the same nature). Under normal circumstances, break bulk cargoes means the cargo that is not containerized and thus cannot be classified as bulk cargo under the above definition. It is important to note that the difference between bulk and break bulk is based not only on the type of cargo, but also on the way in which the cargo is stowed or loaded.
The first problem which arises is where can be categorized the cargo of scrap metal in bulk: homogenous cargo or break bulk cargo. As per above mentioned definitions we can state that this type of cargo is neither homogenous nor break bulk cargo. However, such cargo should never be considered as homogenous cargoes.
From the commercial point of view, such cargo is described as bulk freight. As a matter of loading on board vessel, being a relative light weight cargo, in all situations, the total weight of cargo to be loaded is determined by vessel’s masters in such a way, using in calculations the entire volume of cargo spaces, cargo being loaded by volume occupied. A very important aspect is what volume of cargo holds have to be used in case where structural members, such as frames, are exposed inside the hold, with reference to bale or grain capacity.
Scrap metal in bulk, loaded on board vessels, usually consists in pieces of metal of various sizes, shapes and weights (fig 3.22) and it is difficult to be stowed compactly and moreover to occupy the entire space between various structural members exposed inside cargo hold.
Fig. 3.22: Scrap metal in bulk of various shapes and weights, presented for loading on board vessels
Of course, the commercial parties are using the volume of cargo holds that fulfils their interests in order to load cargo as maximum vessel’s capacity. In such cases, vessel’s master is between hammer and anvil and have to satisfy the commercial interest as well as vessel’s safety (which in many cases was on the second plan).
Another problem, which arises, is related to cargo stowage factor. According to International Maritime Solid Bulk (IMSB) Code, shippers must provide the master with accurate stowage factors of different parcels of cargo loaded. Most of the shippers involved in such transports evaluate the stowage factor of the scrap metal between 60 and 90 cubic feet/ton, or these values are extremely wide. It is hard to ascertain exactly the total weight of cargo to be loaded on board vessel when this kind of information is provided to ship’s master.
Such type of cargo is usually loaded by shore cranes using grabs, cargo being freely dropped into vessel’s cargo holds. During loading, from time to time, master requests that the cargo to be pressed, in order to compact the cargo and increase the remaining volume of cargo hold. This kind of operation consists in dropping of a “weight” from a high point above cargo hold. In many situations, this procedure proved to be dangerous for ship. During pressing of cargo, hard pieces of scrap metal penetrated the ship’s structure resulted in damage to structural members, ballast tanks and even side shell plating.
In normal conditions, these kinds of damage are difficult to be ascertained, having in view that inspection inside cargo holds is practically impossible, particularly if such damage occurs over the night. In the worst case scenario, we can presume, for example, that during pressing of cargo the side shell plating was penetrated at location below water line and water ingresses into cargo hold. The vessel could leave the port, without any knowledge about the damage, and the final result can be catastrophic. Such situations are often happen.
The inaccurate determination of cargo weight is reflected in an erroneous assessment of the position of the cargo’s vertical centre of gravity. Hence, as the weight of the cargo is the main element influencing the ship’s stability, it is very clear that the calculation and assessment of ship’s intact stability is affected from the beginning. The problem is more complicated because scrap metal, stored at loading places, are a mixture of metal pieces with different densities, i.e. stowage factors, so a proper estimation of weights and positions of centre of gravity it is very difficult. Incorrect estimates can have serious consequences as the voyage progresses with reducing stability margins. Again vessel’s master is in a difficult situation.
Normally, shippers must advice vessel’s masters when and where are loaded on board vessel cargoes of different densities. This is practically impossible, because since from the collecting and then discharging at storage places, cargo is not separated by weight due to insufficient storage place or time, facilities and expenses involved.
3.6 Human error
Casualties of ships stability failure related to human error
TORM ALEXANDRIA – 270 TEU feeder containership – July 2001
The vessel was under loading and in the same time discharging operations of containers at the port of Monrovia, Liberia. While lifting a containers from the berth, using the ship’s own crane, the vessel suddenly heeled over to port towards the quayside. Then, the master attempted to control the list using the ballast but without the desired effect and the vessel continued to list further until she capsized (fig. 3.23).
The subsequent official enquiry  revealed that the proximate cause of capsize was the attempted lift of a heavy container from the berth at a time when the ship had little or no residual stability. The cause of the large and sudden heel to port was the suspension of the heavy container from the ship’s crane jib, which took the vessel to an angle beyond which all righting levers were negative. Nothing could be done to remedy the situation as was also exacerbated by the shifting of containers across the deck and sea water flooding the engine room through an open trap hatch on main deck.
Fig. 3.23 Feeder vessel “Torm Alexandria” capsized along berth 
The unstable condition of the vessel and her subsequent capsize was a result of the following factors:
1. The ship’s staff not paid the importance of preparing the curve of righting levers for the vessel’s actual loading condition.
2. Too much reliance by the ship’s staff was placed upon initial GM as a criterion for the vessel’s stability in all conditions of loading and at all angles off heel.
3. When the GM of the vessel was calculated, no allowance for free surface effect was made, despite that clearly instructions were stated in the stability booklet in this respect.
4. Despite the fact that a written letter issued by the owners indicated that a Deck Cargo Licence had been granted for 740 tonnes, corresponding with loading condition no.7 from ship’s stability booklet, no attention was paid for this issue. At the time of the accident the vessel had 1150 tonnes on deck, that means a 55% in excess and the limit was already breached with the first tier of containers on deck.
COUGAR ACE – 55,328 GRT Car Carrier – July 2006
After the vessel departed from Yokohama port, loaded with 4,703 cars, Chief Officer planned the ballast exchange. Vessel’s Master advised Chief Officer to conduct the ballast exchange in one pair of tanks at the time. Chief Officer informed him that in the worst case scenario, if four tanks were to be deballasted together till they were emptied, the vessel’s GM would be 0.50 m positive. In the next four days, the deballasting and ballasting operations went as planed by Chief Officer. However, during this period of time, a couple of tanks, that were not planed for deballast operations as per ballast water exchange plan drawn by Chief Officer, were deballasted in order to correct the vessel’s list in the same time with the planned ballast tanks. On 24th July, during ballast exchange process, the vessel started to list to port and within few minutes was lying on its portside about 80° list (fig. 3.24).
Fig. 3.24 MV Cougar Ace lying on its portside 
As per investigation report  , the sequential exchange of water ballast would result in the ship having four of its nine water ballast tanks empty. This aspect, together with additional water ballast being pumped out for the adjustment of list, and correlated with the consumption of fuel from double bottom tanks, resulted in the ship becoming unstable and developing an angle of loll to port side of about 80°.
There were some important inadequacies in the ship’s ballast water exchange operation that came out from the investigation report 78:
1. Improper planning and execution of ballast water exchange operations. This fact resulted in insufficient weights in the water ballast tanks below the ship’s waterline;
2. Ship’s staff in charge with the ballast exchange operations, i.e. Chief Officer and Master, failed to ensure that the ship stability is maintained throughout the operations, at various stages of ballast water exchange;
3.Failure of Chief Officer and Master as to clearly understood and complied with the IMO recommendations related to safe operations of the ballast water exchange procedure.
GULIZAR ANA – 1,500 GRT, general cargo vessel
Vessel was on ballast voyage from Turkey to Romania, one port on Danube River, for loading steel coils. Before entering the river, vessel de-ballasted all ballast tanks, double bottom and side tanks as well as the fore peak tank. During loading of the 5th steel coil, vessel began to lean on starboard side till the vessel’s accommodation has propped on main deck of the floating crane berthed alongside vessel. The list of the vessel was 51 degrees (fig 3.25). Engine room and starboard side crew quarter have been flooded.
Notwithstanding the list was a result of shifting of coils inside the hold, the main cause of the accident was that the vessel, after de-ballasting all ballast tanks, remained with insufficient weight below waterline thus leading to insufficient stability of ship. This fact was the result of improper planning of de-ballasting and failure of ship’s staff to maintain the ship stability .
Figure 3.25 Vessel “Gulizar Ana” listed at starboard side, alongside berth 
Human factor and ship stability
Over the last 25 years or so, the main target of shipping industry was the improving the ships structure and the reliability of ship systems in order to minimize accidents and increase efficiency and economy. The ship systems became technologically advanced and highly reliable due to important improvements that have been made in hull designs, propulsion systems and navigational equipments.
With a growing awareness of safety issues, public tolerance of accidents in shipping has decreased. So even nowadays, despite all of the improvements and advanced technologies, maritime casualties continued to happen, moreover with a high rate. This happen because the ship structure and system reliability are a relatively small part of the safety equation. People are part of the maritime system and thus human errors are predominant in casualty situations. Since first man went to sea, the human error has been blamed as the cause of accidents. Current wisdom is that human error, attributable to the master and /or crew of a ship, has been responsible for approximately 80% of maritime accidents, at least in part .
The response to incidents can be towards regulation and enforcement and these developments may suggest that the sheer volume of safety related measures, in the field of legislation and other rules or standards might be defeating the objective, and that maybe an entirely new approach might in consequence be called for.
Human error is often described as being an incorrect decision, an improperly performed action, or an improper lack of action . In considering the human contribution to marine accidents, it is important to distinguish two kinds of error: active errors, whose effect are felt immediately and latent errors whose adverse effects may lie within the system for a long time .
Active errors can be seen to be the actions of the ship’s crew leading up to and at the time of the incident. Latent errors may have their origins many years before, at the building of a ship or in some high level or low level management decisions.
Stability of ship, as part of the maritime safety, is a problem which depend only on two main aspects: the designed features of the ship and the action of its crew that determines weather the ship will survive in critical conditions. The border between the survival and disaster is decided by the correct reaction in dangerous situations and the skills of the crew. Thus, in the overall analysis of safety against capsizing, the human factor should be always taken into consideration.
The changing attitude for the connection between stability of ships and human factor is well reflected in the work of the international research community, in particular IMO. In Code of Intact Stability of Ships, the stability requirements and standards have been supplemented by some paragraphs related to human factor in operational aspects of stability safety, as for instance:
“Compliance with the stability criteria does not ensure immunity against capsizing regardless of the circumstances, or absolve the master from his responsibilities. Masters should therefore exercise prudence and good seamanship having regards to the season of the year, weather forecasts and the navigational zone, and should take the appropriate action as to speed and course warranted by the prevailing circumstances.”
Human factor still remains a decisive factor for the ship stability and safety at sea, despite the growing process of automation to ships. The casualties presented shown that human factor is one of the most important elements of the ship safety system. The loss of control over a ship, not only in a critical situation, can be a main cause of casualties.
It is of paramount importance that stability (refresher) courses should be given at regular intervals for those working in the field (i.e. ship designers, crew, operators, managers etc.)
3.7 Inaccuracies of information provided by ship’s documentation
Casualties of ships stability failure due to inaccuracies of information provided by ship’s documentation
This kind of ship stability failure is somehow atypical. It refers to inaccuracy and sometimes lack of the information provided by ship’s documentations, such as Ship Stability Booklet, Ship Loading Manual, Cargo Securing Manual, or Loading & Stability Computer, necessary for ship’s officers to ascertain the stability of ship and securing of cargo.
There are some casualties of ships, which lost their stability based on inaccuracies or lack of information provided by ship’s documentation or computer software, out of which three are representative and are presented here.
SUN BREEZE – 7,816 GRT General cargo ship
The vessel loaded, in August 1999, packs of timber of various sizes in holds and on deck, at port of Bunbury, Australia. After pilot disembarked, the 3rd Mate changed the steering from manual to autopilot, at which time the ship started turning to starboard on its own accord. Immediately the steering was changed back to manual in order to bring the vessel back on course. The vessel initially listed to port then flopped over to starboard and listed up to 25°. Some packs of timber were lost over board from hatch top in starboard side. A distress message was sent by Master thereafter vessel was anchored. After three days, the vessel entered to port of Bunbury and berthed safely.
Although at departure from the berth the ship complied with all stability criteria recommended by IMO Res. 749(18) Code of Intact Stability for All Types of Ships as well as with Code of Safe Practice for Ships Carrying Timber Deck Cargoes, she still lost her stability.
Among other factors that have contributed to the incident, the investigation  revealed two of them which where related to inaccuracies of ship’s documents, as follows:
In ship stability booklet were two figures for free surface moments for no.1 fuel oil tank. The tabulated maximum moment of inertia () was 439.1, but a worked stability example in the booklet illustrating the departure ballast condition figure for the free surface moment () of 1,288.39 tm. This suggested that the correct figure for the value should be around 1,356. The investigation of the ship’s plans revealed that the moment of inertia, for no.1 fuel oil tank, should be more than that of no.2 fuel oil tank, because the first one showed to have larger width. Moreover, the tabulated value of for fuel oil no.2 tank was 973.
The matter was brought to the attention of the vessel’s classification society, Class NK, which confirmed that the value of indicated as 439.1 was erroneously tabulated and the correct value was 1,356.2.
With the correct value of for no.1 fuel oil tank used in calculation of the free surface correction, the metacentric height of the vessel decreased from 0.36 m (as initially calculated by master) to 0.21 m, indicates a considerable difference of 0.15 m.
From the point of view of initial stability, the vertical position of centre of gravity in the lightship condition it is very important and it is necessary that this position to be known with considerable accuracy. The position of the centre of gravity of the lightship as well as lightship weight is determined by inclining experiment as prescribed in a separate chapter by International Code of Intact Stability 2008 (known as IS Code 2008) , issued by IMO.
The experiment consists of causing the ship to heel to a small angle to the vertical by moving known weights through known distances across the deck and noting the shift of pendulums of known length. The angle of inclination can be calculated from the deflection of the pendulums. Several shifts of weights are made to obtain a mean angle of inclination. Calculations are then carried out to determine the position of the centre of gravity of the ship. The draughts at which the ship is floating are observed and the lightweight of the ship is determined. The accuracy of calculations depends on the magnitude of the weight shifted, the distance through which it moves, the length of the pendulum and its deflection.
There are described the standard conditions and the instruments that have to be used in order to carry out the experiment and to ascertain the accurate data. Among these, IMO recommends:
1. The total weight used to incline the ship should be preferably sufficient to provide a minimum inclination of one degree and maximum of four degree of heel at each side,
2. The pendulums should be long enough to give a measured deflection to each side of upright of at least 15 cm and that the pendulum lengths of 4-6 m should be used. A pendulum shift of 150 mm or more is recommended since, if the pendulum does not settle the judgement has to be made as to the central position, the fluctuation is only a small percentage of the reading.
3. All tanks should be empty and clean or completely full and the number of slack tanks be kept to an absolute minimum.
The report of the inclining experiment for the Sun Breeze, carried out by Class NK, indicating that at the time of experiment the conditions were as follows:
4. Vessel had liquid in 26 tanks, 10 of which had free surfaces and the rest were pressed up. The total mass of liquids was 1,927.45 tons or 52.6% of the lightship mass.
5. The mean angle of heel obtained from the shift of weights was only 0.36496° (instead of 4°) using o forward plummet of 2.183 m and an after plummet of 2.197 m (instead of 4-6 m). So the lengths of the plummets were about half of the recommended length.
6. The mean deflection of the plummets measured aboard vessel was only 14.11 mm for the fore plummet and 13.80 mm for the aft plummet (instead of 150 mm). So, the deflections measured were less than one tenth of the recommended deflections.
7. Only five weight movements were shown in the report of inclining experiment, whilst the standard test recommended by IMO employs eight distinct weight movements.
Given that there were known inaccuracies in the ship’s approved stability book and that the ship’s light KG was not calculated in accordance with IMO criteria, the vessel’s stability calculations can not have confidence in accuracy.
Having in view the above two mentioned inaccuracies, the investigation report stated that a possible scenario that would explain the shift of cargo on board vessel was that the vessel sailed with negative GM, resulting in an angle of loll. At the best, the vessel’s stability at sailing was marginal and substantially less than that calculated by Master and this was due in part of erroneous data in the ship’s approved stability book. Both, the safety of the ship and its crew were compromised by the unreliable nature of the approved stability data.
ANNABELLA – 9,981 GRT container ship
On passage in the Baltic Sea, on 25th February 2007, vessel encountered heavy seas, rolling and pitching heavily, that cause a collapse of a seven containers stack of 30ft onto vessel’s hold no.3. The collapse of containers was a result of :
1. Mis-declared cargoes in containers,
2. Not enough support of the lower containers of the stack,
3. Shortcomings in the flow of information relating to container stowage between the shippers, the loading terminal and the vessel,
4. Inaccurate information provided by ship’s loading computer program.
Two important aspects have to be analyzed in this case: shortcomings in the flow of information between parties involved in planning the loading and the ship’s loading software.
First, is the shortcomings in the flow of the information relating to container stowage between the shippers, the loading terminal and the vessel. Thus, there were three main parties involved in loading of the ship: shippers, loading terminal and ship’s personnel.
The shipper planned the stowage plan for the cargo to be loaded on board vessel. The stowage plan was prepared in his office by his planners from the operations department. The planners were aware of ship’s stowage capabilities and also held some details of her stability. However, they ultimately relied on the vessel’s staff to alert them on any errors in the stowage plan, and expected the Chief Officer to critically check every aspect of the stowage plan before the vessel began loading.
In the previous instances, where their plans to load stacks of 30 foot containers onto vessels of the same class had exceed the maximum permitted stack weight limit, the planners had been alerted by the vessel’s staff before the cargo was loaded. Even so, no corrective action had been taken by the shipper as a result of the earlier planning errors.
The shipper’s planners used a computer planning software programme, developed by a specialist company, and was created to use the stability and stowage information received from the shipbuilder for the class of vessels similar to Annabella. During planning, no alarm was triggered by the programme when the stack of seven 30 foot containers, with a total of 225 tons, was planned to be loaded in ship’s number 3 hold, bay 12.
A simulation of the collapsed container stack was carried out later on after the accident and revealed that the planning software had not been programmed to recognize 30 foot containers. The programme automatically changed the 30 foot containers to 40 foot containers without any alert being given to the operator.
The software programme used by shipper’s planners was not the same as the loading programme used on board vessel, and has no information regarding longitudinal and transversal stability of the vessel. Moreover, there were no requirements for the planners’ programme to be checked or approved by an external authority.
Once the stowage plan was ready, it was sent in electronic format to the vessel and to the loading terminal.
The loading terminal planners loaded the information onto their own computer programme. They checked the plan in order to ensure that the planned containers are available for loading. The computer programme used by terminal planners did not hold any information for ship stability. After all containers scheduled to be loaded were confirmed that are available for loading, the terminal planners checked that the Chief Officer agreed the final stowage plan, thereafter the stowage plan was sent to operational staff to commence loading.
Despite the fact that all containers were checked and confirmed available for loading, during loading one of the seven 30 foot container planned to be loaded in hold no.3 was found unavailable and the terminal planners replaced it with a container of similar dimensions and cargo but with a difference in weight of 0.4 tonnes more.
Second aspect is the loading software of the vessel. The vessel’s loading software was fitted to provide information on the stability, stress conditions of the ship, cargo securing arrangements for containers stowed on deck, dangerous goods and segregation requirements as well as with alarm designated to operate if a number of parameters, including individual stack weight is exceeded. At the time of the accident, the loading computer had not been approved by the vessel’s classification society.
As in case of shipper’s planners software, the ship’s software had not been correctly programmed and did not recognize the 30 foot containers, converted them automatically to 40 foot containers. Therefore, no alarm was raised when the stack of seven containers intended to be loaded in hold number 3, bay 12, was introduced into the programme, even though weight of the stack exceeded the limit.
Chief Officer checked onto vessel’s loading computer the stowage plan and ensured that the bending moments and shear forces are acceptable as well as if the planned locations of containers were suitable for the various containers that held hazardous cargoes and were in accordance with the vessel’s cargo securing manual. The investigation of this case concluded that were many discrepancies involved in this accident, but the main important were:
1. Vessel’s loading computer contributed to the programming error, not being detected and providing no assistance or guidance to chief officer for the stowage and securing of 30 foot containers,
2. The special requirements for the carriage and securing of 30 foot containers were not readily apparent in the vessel’s cargo securing manual.
MIMASAKA – 9,807 GRT multipurpose ship
Vessel left the port of Burnie (Tasamania) on 1st October 2010 loaded with timber veneer on deck. During the voyage up to the New South Wales coast, deterioration of weather and sea conditions occurred. Vessel rolled heavily and about 600 packs of timber were lost overboard when lashing system failed. The ship developed a list of 20° to starboard. Among the contributing factors, the investigation report  revealed that:
1. Vessel’s cargo securing manual did not contain any information relating to the stowage and securing of timber veneer.
2. The Operation Manual for Loading and Lashing of Tasmanian/Malaysian Dry Veneer, developed by vessel’s operator (charterer) for use by their ships carrying timber veneer, did not contain any information relating to stowage and securing of timber veneer cargo on deck.
3. The loading instructions provided to ship’s master by the charterer did not provide the crew with proper guidance about how to stow and secure the packs of timber veneer on deck.
Problems that may arise from the inaccuracies of information provided by ship’s documentation
The question that arises is how reliable are the ship’s manuals and computer software for ascertain the ship stability. As previously mentioned, the ship’s officers calculate and asses the ship stability based on the stability criteria, represented by righting lever curve. The GZ curve accuracy depends, among others, on the accuracy of following factors:
1. Accuracy of ship’s hydrostatics data – like KM value and the KN curves;
2. Accuracy of lightship weight and her center of gravity;
3. Accuracy of ship’s constant weights;
4. Accuracy of cargo weights and the position of their centers of gravity;
5. Accuracy of weights based on tank soundings.
Naval architects prepare the hydrostatic data in the design stage of the ship. The ship built hull does not always conform exactly to the plans, thus the upright KM and KN values should be within a margin of error, most likely 0.5%.
The Load Line regulations require a ship to carry an “Approved Stability Book” giving all the hydrostatic data necessary for calculating the vessel’s loaded draft, stability and trim, though port or flag state authorities do not usually check for compliance with the stability code.
The ship’s approved stability book includes a range of sample loading conditions at both, at the beginning and the end of the voyage. The range of sample conditions will include the lightship state, which usually fails to meet the minimum stability criteria. The lightship condition is the basis of all other conditions and can be required for special situations, such as dry-docking the vessel.
The calculated ship’s KG is invariably less accurate than the hydrostatic data, as it depends upon the precision of the lightship KG, which is measured by the inclining experiment and the accuracy of cargo weights provided by shipper.
The inclining experiment for measure the lightship KG is not always carried out as precisely as it should be (see mentioned example of vessel Sun Breeze). Furthermore, the lightship weight and its distribution tends to change over the years through alterations to the ship structure, accumulation of the paint, stores and steel wastage. The amount of weight accumulated over the years is not easy to be assessed on an older vessel, particularly if the inclining experiment was carried out twenty years previously whilst the weights of stores and spare items is, quite frankly, a bit of guess.
The investigation of a 29 years old sunk bulk carrier  revealed that the difference between lightship weight according to ship’s documentation and the lightship weight assessed during the investigation was of 116 tons, which means 3%. The difference in lightship weight was attributed not only to vessel operational lifetime but also to structural modifications ( few years before the accident, the ship’s tweendeck structure has been cut off in all four holds and partial repairs have been implemented in erroneous way by replacing sections of main deck and double bottom with plates thinner than the recommended reduction). The above mentioned reduction in weight was not stated in ship’s documentation and the ship’s stability calculation was made with the lightship figure as calculated by the shipyard at the time of ship’s construction.
Most of the Chief Officers usually try to adjust the weights of stores, spare parts etc. as much as plausible in stability calculations, to reconcile the calculated draft with the observed values that typically indicate an extra 0.5 to 1% of weight to the calculated displacement.
The weight of stores, including engine room spare parts and tools, are particularly problematic in ships where they can make up a significant proportion of the vessel’s displacement.
Cargo weights rely on the accuracy of the information provided by the shippers, which can be very variable, whilst the location of these weights within the hold spaces of a general cargo ship depends to a considerable extent of judgment of the ship’s staff.
The loaded KG and KM values for most of the ships are typically about 60% of the hull’s depth. The minimum allowed GM of 15 cm implies that the upright KM and KG are known to within 0.5 cm, which gives a margin of error of less than 0.1% for the two values. A more realistic accuracy of 0.5% for the KM and 1% for the KG give an error margin of 9 cm, which is a considerable fraction of the minimum allowable GM. If we consider a larger ship, such as 70,000 tons container ship with hull depth of 24 meters, then the error margin could be as large as 21 cm .
The location of standard size containers may seem to be more precisely but the ship’s officers are unlikely to have any knowledge of the centre of gravity for each container. In most of the situations, this will be lower than the container mid height, as they are unlikely to be filled right up to the top, or contain heavy items overlying lighter ones, so its common practice to assume the centre of gravity is 45% above the base of the container. However, the weight of the containerized suspended meat carcasses acts at the container top, so exceptions do occur and though this particular example should be readily identifiable, others may not .
Many ships have an approved and dedicated stability computer with the hydrostatic information stored as a database in the software. This makes stability assessment considerably easier and quicker than longhand calculations but the margin of error in the weight distribution input still applies.
Based on the casualties presented and analysis carried out, important lessons can be learned from the problems identified. It is important that this lessons to be used in order to develop appropriate corrective measures that can be taken in order that such situations to be avoided in the future.
Among safety measures, which can used for best practice safety, can be inter alia:
1. Cargo loading and securing manuals to be issued in an easy access format which can provide ready and easy access to relevant information regarding loading and securing.
2. Loading computer software accuracy to be regularly checked by the ship’s officers in order to ensure that information provided are full reliable.
3. Deck officers need an appropriate experience to use the ship’s loading documentation and loading software in order to ensure that ship safety considerations are fully recognised.
4. Deck officers need to undergo an appropriate and extensive training in order to be able to identify the errors provided by the loading manuals and software. It is very important that ship’s officers to analyse first the results and data provided and only when these results are reliable and according to the actual condition of loading, to be used.
5. One of the most important aspects to be mentioned is that every loading software is governed by the people working with them. Deck officers shall never forget that the computers provide the results based on the data introduced by them. A very old but very true proverb says: “Wrong input means wrong output”.
Ships stability failure modes in severe sea conditions
4.1 Pure loss of stability in waves
First mode of ship stability failure by physical phenomena is related to the variation of restoring lever in waves; the restoring moment becomes larger on the wave trough and smaller on the wave crest, thus, the result is the occurrence of very large roll angles under certain circumstances. The reason is the changes of ship’s stability while the wave passing by. It is mainly the result of the changing underwater hull geometry.
This phenomenon can be illustrated in figure 4.1., where a wave is almost equal with the ship length. Just the forces of hydrostatical nature, however, do not limit physical reasons of stability changes in waves; pressure in waves is distributed differently than in calm water .
Fig.4.1 Ship in a situation of a wave with .The figure illustrates the wave crest (green) and wave trough (blue)
Changing of stability in waves invokes two physical mechanisms of stability failure, pure loss of stability and parametric resonance . As the waterplane shape is mainly influenced by the encountered, radiant and reflected waves the calculation of the stability change presents certain challenges, especially at high speed.
Although this phenomenon has been known for more than forty years, it becomes a serious problem once the first ships, with large aft body and V-shaped fore body correlated with large flare, were introduced.
Pure loss of stability in waves could happen if a vessel encounters a single large wave in following seas and spends a considerable amount of time on the wave crest . If occurs a large deterioration of stability of this vessel on wave crest the vessel may develop a very large heel angle or even capsize. If the nominal speed of the ship is lower than the critical speed of surf-riding, the ship experiences a periodic motion . Due to the non-linear nature of periodic surging motion with potential surf-riding equilibrium the vessel spends a longer duration on the wave crest than in the wave trough . During the duration on the wave crest the righting lever of a ship could decrease significantly. In the case of pure following sea, e.g. heading angle of zero degrees, a ship could capsize simply due to loss of static balance by such a reduction of transverse stability. This phenomenon is known as pure loss of stability .
The alterations of the righting lever, which can be expressed by the differences of the righting levers at the trough and crest condition, are always related to a specific hull form, and resulting in significant changes of hull’s wetted surface and waterline area (fig. 4.2 and fig. 4.3) .
Fig.4.2 Change of ship’s waterplane area on wave crest 
Fig.4.3 Change of ship’s waterplane area on wave trough 
The energy introduced into a specific hull form in a sea state may be expressed by the alterations of the area below the righting levers at crest and trough condition . The calculation and representation of righting lever curves for a ship, with large aft body and V-shaped forward with large flare, in the wave in both situations, where the wave crest is situated amidship and where the ship’s midship section is situated in a wave trough is illustrated in figure 4.4 .
Fig.4.4 Resulting righting lever arm curves due to position of ship on wave 
It is evident, the significant changes in ship’s stability due to alterations of righting levers and thus the ship loose the stability almost completely when on wave crest .
In the crest condition, the fore and aft part of the ship are significantly raised out of the water that result in decreasing of waterplane area and significant loss of stability. In the trough condition, the ship has excessive stability due to increasing of waterplane area by submersion of ship’s fore and aft sections. During this cycle of waterplane variations, the righting levers oscillate periodically between the extreme values.
When the stability loss on the crest exceeds a certain limit, the vessel may have no stability left at all and capsize if it rest in this crest condition for a certain time (this is a typical situation of pure loss of stability) . If the stability remains at a low level, for a sufficient long time, there is a danger that external loading, such as gusty winds or a shift of cargo, may make the ship to capsize. The dynamics of pure loss of stability are close related to the severity and duration of waterplane changes .
A possible scenario for the development of stability failure caused by pure loss of stability is presented in  and is illustrated below:
First the ship sailing with relative high speed in following waves, figure 4.5, and a large wave is approaching from the stern. If the speed of the large wave is just slightly above the ship speed, the time duration for the large wave to pass the ship may be long.
Fig.4.5 Vessel sailing in following waves, large wave approaching from stern
There is a typical changes of stability caused by relative small waves (fig. 4.6).
Fig.4.6 Changes of stability caused by small relative waves
Then, the large wave is overtaking the ship (fig. 4.7). Once the crest of the large wave is near the midship section of the ship and if the time exposure to the crest of the large wave is long enough, the stability may be significantly decreased.
Fig.4.7 Large wave is overtaking the ship
In this situation there is a large decrease of the instantaneous GZ curve, caused by the crest of the large wave (fig. 4.8).
Fig.4.8 Decrease of stability caused by the wave crest
Because the wave speed is just slightly more than ship speed, the condition of decreased stability may exist long enough for the ship to develop large heel angle, or even capsize.
As the large wave has passed the ship (fig. 4.9), her stability is regained and the ship will eventually return to the upright position, if she did not already heel too far. Otherwise, typical changes of stability, caused by relatively small waves, are encountered.
Fig.4.9: Large wave passed over the ship and the stability is regained
Thus, to occur a dangerous situation of stability failure caused by pure loss of stability, there are two conditions to be fulfilled.
The first one is how large are the stability changes on the wave. If stability changes are small because the hull is wall-sided, then this mode of stability failure is impossible. So, the first condition is related to the hull form, which must be such (for instance pram type stern) that heaving and pitching motion in following seas is associated with large variation of the waterplane area.
The second condition is the likelihood of encountering a large steep wave, which can cause a decrease of stability for a sufficient long time so large heel will develop. This condition is associated with wave length and ship’s length and speed, as follows:
1. Length of the ship has influence on the time of exposure, as the longer waves are faster (e.g., a wave with a length of 250 meters has a celerity of 38 knots in deep water).
2. Speed of the ship has influence on the time of exposure. Large waves are usually faster than typical ship speed. However, faster ships may potentially have large durations of exposure.
3. Length of the ship has influence over length of the wave. For a large ship, a longer wave is needed to cause significant changes of stability.
The stability of the ship may be lost if the wave length – – is about the same as the ship length – , and if the wave velocity (so called phase velocity) is close to the ship speed – . In other words, in following seas the encounter frequency tends to zero.
Besides the danger of reduction of stability when the ship is riding on the wave crest for a long time there is also an exciting effect of waves in head/stern sea when the waves are travelling along the ships hull periodically – this will yield potential for parametric rolling . This leads to extreme dangerous situation when several high waves will trigger the ship coming as a group.
This phenomenon is presently addressed by the MSC.1/Circ.1228 but does not provide any ship-dependent information concerning the considered failure mode. IMO has given in this guidelines, a diagram (fig. 4.10) highlighting the potential occurrence of high wave group encounters; however, the information is given in a dimensionless format only by a ratio of ships speed and wave period , is the encounter angle.
Fig.4.11 Diagram Indicating Dangerous Zone due to High wave group encounters
The difficulty of assessment stability of ship in case of pure loss of stability is not limited to the calculation of the curves in waves, whilst a probabilistic assessment of the stability changes is also dificult, due to nonlinear nature of stability changes in waves. Bulian & Francescutto ,  considered statistical characteristics of GM and other elements for the righting moment as a stochastic process.
Because pure loss of stability is a one wave event, it is straightforward to separate the dynamical problem from the probabilistic problem . In the first instance are searched the parameters that are relevant to the waves capable of causing pure loss of stability, then is assessed the probability of encountering such waves. A new approach was made by Bulian , where the effective wave approach was combined with upcrossing theory. In this case the pure loss of stability is associated with an upcrossing event for the amplitude of the effective wave that allows the time of exposure to be considered directly and produces a time dependent probability of stability failure .
Due to very stochastic nature of the phenomenon, a method for assessment the pure loss of stability of ships are likely to be probabilistic . For a future criteria related to this mode of stability failure, could be included the time duration while the stability is reduced on a wave. The methods for assessment pure loss of stability may include the effective wave, the narrow-banded waves assumption, the critical wave approach or upcrossing theory .
4.2 Parametric rolling
Another physical mechanism of stability failure caused by restoring lever variation in waves is parametric roll resonance.
Parametric roll behavior may lead to sudden increase in large roll amplitude angles experienced by the ship typically in longitudinal waves, caused by parametric roll resonance (the encounter frequency of waves of length similar or larger than the ships length is comparable to twice the ship’s roll natural frequency) .
Model tests and full-scale observations have shown that parametric rolling can occur not only in head and following seas, but also at slightly oblique headings, and the physical phenomenon is based on successive alterations of the restoring lever between crests and troughs, exhibited by many ships in steep longitudinal waves .
When a ship is sailing through waves, the submerged part of the hull changes. These changes may become especially significant if the length of the wave is comparable to the length of the ship.
New design trends of the ships revealed wide bow section at the upper part as a result of large bow flare. This aspect was taken into considerations from two points of view: the possibility of increasing the deck stowage area in that section and protection against shipping sea water on deck. From stability point of view, this aspect results in increasing the waterplane area as the bow submerges.
For modern container ships, the upper part of the aft section of the hull is considerably increased and this is due to increasing the stowage area. In this respect, when the upper part of the aft is lowered down on the wave trough, the waterplane in that section is increased.
However, despite the hull shape at bow and aft sections, most of the ships are with midship section in form of wall-sided and thus, the changes in the width of the waterplane area are very small, taking in consideration the variations of draft. Even if the draft at the midship section is low and the wave trough is amidships, it is a small change in waterplane area because the hull has a wall-sided shape in that region. The result is a substantially overall increasing of waterplane area around the midship section when the wave trough is near the midship section of the ship unlike the situation when the waterplane decreasing substantially at the time of wave crest located near amidships.
One of the most important aspects taken in consideration when the ships are built is the resistance and this aspect is reflected in the shape of the underwater part of the bow section, especially around the waterline, which is usually quite narrow. Even in the case of ships provided with bulbous bow, the underwater part is still narrower than for the section with bow flare. Consequently, is the decreasing of the waterplane in this region in the situation when the wave length is equal with the ship length (if the wave crest is amidships then the wave trough is around the bow section and the draft at the bow will decrease considerably).
For the new ships, especially for container carriers, hull forms were substantially changed also in the aft part. The main aspect taken into consideration by ship designers was the energy efficiency, to provide the sufficient inflow for the propulsion system. The result was buttock flow stern design. This aspect makes the waterplane in aft part to be very narrow when the wave crest is located amidships and the wave trough is located at aft part of the ship because the draft at the stern decreases. As the ship speed is increased and propeller must deliver more power, the aspect mentioned above (i.e. narrowing of waterplane in the aft part) is considerably increasing .
The most important aspect that have to be mentioned is the considerably effect which the waterplane area has on ship stability (this aspect is very well reflected in ship’s hydrostatics). Consequently, if the waterplane area is reduced, then so is the area under GZ curve.
The researches on parametric roll phenomenon started with the studies on a 1-DOF model based on Mathieu equation. France  and Shin  pointed out that a Mathieu type one degree of freedom model can easily be used to show when ships encounter parametric resonance used the same approach later. In 2006, Bulian  came up with a 1.5 DOF model where the assumption of quasi-static heave and pitch leaded to an analytical description of the GZ curve. This model is valid for moderate ships speed in head seas and gives reasonable results for the prediction of parametric roll resonance.
To get an extensive analyze and understanding of the ship behavior, Neves  derived a 3-DOF model where the motions of heave, roll and pitch were coupled. Thus, the restoring forces and moments in heave, roll and pitch motions were described by using Taylor expansion up to second order. However, compared with experimental results, the roll angle predicted by this model was too large. Therefore, in 2005 Neves and Rodriguez  expanded the model found in 2002 by using Taylor expansion to third order. In this model, the nonlinear coupling coefficients are derived as functions of the characteristics of the hull shape. Later on, using the third order model from Neves and Rodriguez, Holden  made predictions of the roll angle of container vessels.
Parametric rolling on board ships
A ship is a rigid body that moves in a 6-DOF, therefore its motion is completely determined by six coordinates: three of them to define the translations and the other three to define the orientation , as illustrated in figure 4.12.
Fig.4.12 Illustration of ship motions, 
In calm water, ships can be externally excited by wind. If due to the wind the ship develops a roll motion, than the roll motion decrease to zero after a few periods of time, due to the roll damping of the ship  (fig. 4.13).
Fig.4.13 Rolling motion of ship in calm sea, 
In the situation when the sea is not calm, the instability is caused due to large variation in a model parameter and the ship will encounter the parametric roll resonance. Then, the ship starts to roll until capsize or stabilizes up to a certain roll angle, as illustrated in figure 4.14 .
Fig.4.14 Illustration of parametric roll resonance 
As was previously mentioned, ship stability depends on the waterline of the ship. Periodic stability variations, occurring with certain frequency (about twice the roll frequency) are the cause of development the parametric rolling. This was very well explained in  and is illustrated in figure 4.15.
Fig.4.15 Development of parametric roll 
If the ship is rolled on the wave trough, due to a wide waterline, the restoring moment is increased over its magnitude in still water. There is more stability than in still water (this means that is need a bigger force to push the ship away from it’s equilibrium point) and the ships rolls to the other side with an increasing roll angle with time. If the wave crest is amidship at that time, due to the greater speed of rolling and less resistance of heeling, the stability is decreased and the ship will roll further the opposite side. So, the push back force is smaller and the roll speed increases.
Finally, the vessel comes again with midship section on the wave trough, where the stability is again large. This situation leads to a large push back force and the ship roll more over (because the roll speed was increased in previous step) which leads to a larger roll angle and the ship reaches its maximum amplitude roll. The scenarios repeats until the ship capsize or stabilize up to a certain roll angle.
In this scenario, was one half of the roll cycle associated with the passing of an entire wave. So, there are two waves that pass during each roll period. That means the roll period is about twice that of the wave period (fig. 4.16)
This time variation in ship stability might give rise to a parametric roll excitation that is only limited by the non-linear restoring characteristic and the roll damping. The parametric excitation can be diminished if the critical wave elevation dies out in the train of irregular waves.
In contrast with pure loss of stability, parametric roll is generated by a series of waves of certain frequency, therefore parametric roll cannot be considered as a one wave event.
Fig.4.16 Development of parametric roll 
One of the main reasons for parametric rolling is the variation in GM and damping values from trough to crest conditions associated with bow flare immersion and pitch resonance in head sea conditions.
The occurrence of parametric roll phenomenon is related to the hull forms that experience large volumetric changes in the submerged portion during the wave passage, predominantly the ships with large bow and stern flares and fine underwater hull, such as new generation of containerships.
Today’s Post-Panamax container ship designs feature wide beam and large bow flares in order to carry more containers on deck while still minimize the resistance with the stream lined underwater hull . As wave travels down along hull, the stability varies as the wave crests travel along the hull. When the bow is down due to moderate pitching couple with slight roll, the large flare suddenly immersed in the wave crest. The restoring buoyancy force plus the wave excitation force would “push” the ship to the other side whilst similar action will happen on the other side as the bow pitch down in the next cycle. These coupled synchronous motions could lead to large roll angles with short period in few cycles even with moderately high waves .
Many model tests carried out  highlighted the major influence of two factors related to parametric rolling: the wave heading and the ship speed on the roll angle. Particularly, the wave heading influenced the amplitude of roll oscillations through speed reduction and through the variation of metacentric height, which in turn determined a roll damping degrease .
Thus, to cause the parametric roll resonance, is need simultaneously to be involved environmental and physical factors [*:*]
1. Parametric roll occurs when natural roll period is between 1.8 to 2.1 times the encounter period (normally associated with the pitching period):
2. Shape of ship’s hull. This condition needs some more attention. The hull geometry is critical for occurrence of parametric roll resonance. Large flare the more likely is the parametric roll angle and wider range of resonance. These type of hull designs are based on years of investigation of what is the optimal design in respect of economical aspects, like for example maximum cargo and reduced water resistance. The result is a hull shape like a box in the middle and with large geometry gradients at the head and stern of the ship, which involves large difference in water plane area. This type of hull designs are common in container vessels and due to this shape these ships are susceptible to encounter parametric roll resonance.
1. To be initiated the parametric roll, it requires a group of waves above the threshold or critical height: . The threshold depends on size and shape of the hull.
2. The wave length is approximately equal to the ship length: . In this condition the ship needs to sail in head or stern seas. Only waves in such direction can reach such a length, especially for large container vessels.
3. The ship’s roll damping is low.
These conditions are represented by the graph in brown color in the figure 4.17, where the amplitudes for parametric resonance are brown, dotted, and red for the synchronous roll resonance.
Fig.4.17 Ratio a of rolling and exiting wave amplitudes versus ratios between ships rolling period Tr and wave encounter 
While the phenomenon of parametric rolling of ships in regular waves has been extensively investigated and solid knowledge about this interesting phenomenon gained in recent years, the particular conditions for the occurrence of such resonance in case of irregular waves, which represent more realistic sailing conditions, are less clear .
Due to unexpected nature of the motion as compared with synchronous roll in following or beam seas on smaller and finer ships, parametric roll can be very dangerous and may induce costly ship operations problems.
In the last decades serious accidents of parametric rolling were reported that have resulted in loss and damage of cargo . Apart from these, possible consequences may include machinery failure, structural damage, and even capsize.
It can be concluded that changes of waterplane areas between crest and trough (which has direct influence on variations of righting levers) are the primary cause of the appearance of phenomena that leading to large rolling angles in severe sea conditions. A possible solution to reduce or to avoid large rolling angles is to minimize the variations during crest-trough situations or to increase the minimum stability required values to increase the ship stability during those variations. A method for assessment the parametric roll may be related to the size of the instability area in regular seas and a group wave approach seems to be a solution.
4.3 Dead ship condition
The third mode of ship stability failure by physics phenomena that may be considered is ship stability under dead ship condition as defined by SOLAS regulation II-1/3-8.
As stated by the Classification Societies, such as American Bureau of Shipping or Det Norske Veritas, dead ship condition is the scenario when the entire machinery installation, including the power supply, is out of operation and the auxiliary services for bringing the main propulsion into operation and for restoration of the main power supply are not available.
The beam seas considerations (currently used in the weather criterion) came from the steam era, when most vessels had the superstructure in the middle and the upper and underwater body configuration was more or less symmetrical relative to midship section . For such a vessel, in case of power loss in storm, aerodynamic wind moment and wave drifting moment will turn it into beam seas, so it made seas subject to the action of gusty wind and severe waves . In this respect, as the ships built in the last decades are characterized by new and various design configurations, the beam sea assumption is not universally applicable.
As a result, dead ship conditions could be more complex than just beam seas so that it is necessary to estimate drifting attitude of ships without power by solving an equation of equilibrium of a surge-sway-roll-yaw motion .
IMO considered that the dead ship condition scenario is one of main scenarios for stability loss at sea . It is assumed dangerous especially for ships with large lateral areas of the portion of the ships above waterline, e.g. with big superstructures like Ro-Pax ships, or high stacks of cargo over waterline, e.g. loaded container ships.
The Weather Criterion which was adopted by IMO in 1985 through Res. A.562(14) was the first physics-based severe wind-and-roll criterion related to dead ship condition as a mode of stability failure. The present Intact Stability Code , as well as its predecessor, IMO Res. A749, contains the “Severe Wind And Rolling Criterion (Weather Criterion).”
The scenario of the weather criterion is shown in figure 4.18 as explained in .This scenario is based on the assumption that a ship has lost its power and has changed the course into beam seas, where it is rolling under action of waves and is heeling and drifting under action of wind (situation 1).
Fig.4.18 Scenario of stability failure in dead ship condition 
The action of two forces: wind aerodynamic force and hydrodynamic reaction caused by transverse motion of the ship, is the result of the drift and related heel.
In the next step (situation 2), a sudden and long gust of wind occurs. In this situation, when the ship is rolling at the maximum windward angle, it is the worst possible moment because to the action of waves is added the action of wind.
Then, in the situation 3, the ship starts to roll back under combined wave and wind action. Velocity of drift and drift reaction start to increase.
In the situation 4, the ship continues to roll leeward, while drift velocity and drift reaction continue to increase, providing additional heeling moment. The increase of drift velocity leads to increase of the hydrodynamic reaction and therefore, to the increase of the heeling moment by the pair of aerodynamic and hydrodynamic forces.
Finally, situation 5, the ship has reached a maximum roll angle on leeward side. this is most instant for stability failure. The gust is assumed to last long enough so the ship can roll to the other side completely; the achieved leeward roll angle is the criterion. If too large, or some openings may be flooded, the stability of the ship is considered insufficient.
This criterion contains a simple mathematical model for ship motion under the action of beam wind and waves but some parameters of this model are based on empirical data .
Therefore it may not be completely adequate for unconventional vessels. In this way MSC.1/Circ.1227  was implemented to enable alternative methodologies to be used for the assessment of the Weather Criterion on experimental basis.
The design of the modern ships implies, in most of the situations, an un-symmetrical windage area. Therefore, the ship is heeled at a certain angle relative to wind and wave direction and the motions taken into consideration may not be limited just to the transverse plane.
Even if the problem is significantly simplified assuming the action of beam seas, it includes forces of different characteristics and is based on three degrees of freedom (sway/drift, roll and heave).
The hydrostatic and Froude-Krylov forces include changes of normal pressures from water onto submerged portion of the hull because of the waves but not include changes caused by the radiation and diffraction of waves from moving ship . Many numerical models and procedures are established for calculation of these types of forces although additional care made be necessary for large amplitude radiation and diffraction forces . The problem associated with these forces is that the results cannot be separated from hydrostatic forces. Because of the fact that the dynamical system cannot be expressed as an ordinary differential equation is the fact that excitation and restoring are expressed in one term.
The influence of the ship on the local pressure field is reflected in the diffraction and radiation forces. The incoming waves (which are diffracting and reflecting from the ship’s hull, as they would from any other body in the fluid) are distorted by the ship-generated waves that interfere with them. In this way, the wave force is different from Froude-Krylov component because the pressure field changes . As a system of partial differential equations must be solved numerically, calculation of this type of forces is more involved. Moreover, the problem is still within potential flow hydrodynamic theory because the viscosity of water does not have much of an influence in this situation. The value of the Froude-Krylov forces are relatively high than the value of this forces.
Usually, the damping attributed to the oscillatory part of motions. The result of the dispersion of kinetic energy of oscillatory ship motions are the damping forces which are applied to all three degrees of freedom .
The way in which the energy is lost may be illustrated by different mechanisms. In the first instance, the waves produced by the ship take away the energy. This component is calculated within the framework of potential flow hydrodynamics and it is dominant for heave motions. The second component is related with generating and shedding vortexes, while the third component is related to skin friction. The calculation of these two components leads to the consideration of fluid volume, as potential flow hydrodynamics is no longer applicable. This takes the problem into real of Computational Fluid Dynamics (CFD), and increases computational cost by several orders of magnitude. Inertial hydrodynamic forces, commonly presented as added masses are calculated within framework of potential flow hydrodynamic theory, and their evaluation is not difficult.
Aerodynamic forces also have a vortex nature and the common way to evaluate these forces is a model test in a wind tunnel . This conventional way does not account for two circumstances that may be significant for stability assessment of dead ship conditions. When the waves are large, they influence the air pressure field: in a trough the waves can shield the ship from wind and decrease aerodynamic forces. Also, when a ship is rolled, the decks also work as an aerodynamic surface to produce force. Some experimental data show that deck-generated forces may exist for small angles as well, and that these forces may be significant . Another possible deviation from the conventional scheme is the case of very large passenger vessels, where the spatial variability of the air flow may be not so significant .
Hydrodynamic drift reaction forces are somewhat similar of nature to aerodynamic forces; generation and shedding vortexes play important role. However, there is no established mature technology for how to measure these forces. Again some limited experiment-based information is available from .
Another important aspect, which plays a significant role in the dynamics of ship in dead ship condition, is the forces related to the entrance of the deck into the water. A rather comprehensive review of this subject is available from .
It is important to be mentioned that there is a difference between two situations: deck in water and water on deck. The deck in water situation is where the water outside the hull and on deck can be considered as one fluid domain and lead to development of hydrodynamic forces on deck surfaces. The water trapped on deck, without interference of the sea water with the outside fluid domain, acts like a moving mass when the ships rolls. These effects were known since the late 1960s, and have been discussed previously at IMO (IMCO STAB/INF.27, 1966) and in succeeding session.
The result of the deck entering water is severely difference in dynamics between ships with low and high freeboard, under the action of a similar sudden gust of wind .
A particular aspect of stability failure in dead ship condition is that the forces of different physical nature may dominate the dynamics, depending on the geometry of the hull.
Heave motion may invoke another mechanism of stability failure in dead ship conditions. As the underwater geometry changes with heave motion, the curve may lead to parametric roll even if a vessel is in beam seas; as the dead ship condition is not limited to beam sea. Parametric roll may be a concern in dead ship condition as well .
Another phenomenon that became important is when large angles of heel are developed in following seas. Broaching can appear as heeling during an uncontrollable, tight turn during which the stability failure caused may be “partial” or “total” .
The dynamics of broaching is probably the most dynamically complex phenomenon of ship instability . Among many of the theoretical studies on broaching include Spyrou . As experimental methods, broaching has also been studied by test with scaled radio-controlled physical models that have taken place in large ship square model basins, by Umeda , , .
Broaching is a violent uncontrollable turn of a vessel, occurring despite maximum steering effort in the opposite direction, which lead to violent yaw motions resulting in rapidly change of ship’s course, when the waves approaching the ship from astern or from stern quartering directions .
During this high turn, when are produced very high centrifugal forces, a very large heel angle may be developed that may eventually lead to capsizing or represent a partial stability failure. In other words, broaching is a phenomenon in which a ship cannot maintain a constant course despite the maximum steering effort of her helmsman , . As a result, the loss of keeping the ship’s course is considered, among others, as one of the elements defining the broaching phenomenon.
Broaching is an unintentional change in the horizontal-plane kinematics of a ship and it may be described as the “loss of heading” by an actively steered ship that is accompanied by an uncontrollable build-up of large deviation from desired course . The situation of broaching occurs mainly in waves which come from behind and propagate in a direction forming a angle of 180 degrees with the longitudinal axis of the ship .
Steering control can be lost if a wave crest is very slow passing the stern and, consequently, the rudder is in reduced flow conditions for a protracted period. The ship starts veering off course during a prolonged period of poor rudder control and the rate of yaw increase with increasing wave angle to the stern. The hydrodynamic force increases with the rate of yaw so, as the ship comes beam onto the sea, the combined heeling moments of and the wind may be sufficient to capsize the vessel .
A following wave crest impairs a ship’s steering as it passes the rudder because the fore and aft component of the water particle velocity is in the same direction as the ship velocity so water flow over rudder is reduced by water particle velocity.
Rudder effectiveness reduces with increasing of water particle velocity and the control decreases with increasing time taken by the wave crest to pass the rudder, so steerage is lost if the stern sits on a following large crest for long enough .
The peak period in the wave spectrum of a Beaufort force 9 gale is typically about 10 seconds, which gives a wavelength of 150 m with a phase velocity of 15.3 m/s or 29.7 knots. If the average wave height is 5% of the wavelength (i.e. 7.5 m) , then the surface water flow over the rudder is reduced by about 5 knots as an average wave crest passes the stern, though this could occasionally increase up to 9 knots for an extra large wave . However, the reduction in the water flow over the entire depth of the rudder will be less than this, as the water particle velocity also decreases exponentially with depth to become zero at the depth beneath the surface equal to about 40% of the wavelength. Nevertheless, the slow passing of large following waves has a considerable effect on the ship’s steering .
Although the inception of broaching represents a problem of instability on the horizontal plane, capsize may be incurred at the post-critical stage due to development of large heel as energy is transferred into roll direction .
Fig.4.19 Example of broaching behavior in following waves
Broaching could happen to small as well as to larger ships . More frequent the broaching phenomenon is manifested as a sudden divergent yaw, which peaks within a single wave length whilst the control is lost when the middle of the ship lies somewhere on the down-slope and nearer to a trough. In other cases there is a gradual, oscillatory type build-up of yaw as successive waves came on the ship from behind . In moderate sea states a ship is more likely to broach-to if it runs with a high speed and is slowly overtaken by the waves. Broaching may also occur at lower speed if the waves are very steep .
IMO, through MSC Circ. 1228, considers a ship to be liable to broach when running ahead of the following or quartering seas at speed equal to or greater than the critical speed. The circular also advices that the ship may broach at less than the critical speed but gives a minimum speed for risk of broaching. For speeds higher than this and for running course ahead of a large following sea, the helm actions should be made with caution.
Broaching occurs mainly in following waves of lengths between 75% and 150% of the ship’s length. The oblique velocity of the shorter waves in this range equals knots. Broaching is highly possible to occur when such waves overtake a ship at less than about 20% of the ship’s speed, so MSC Circ. 1228 strongly recommends that a ship’s speed is kept below the critical speed of knots when running before heavy seas coming from within 45° of right astern.
However, having in view his nature, the broaching phenomenon is not a classical problem of intact ship stability. As mentioned by , neither the simple increase of up-righting stability, nor the alteration of certain design parameters, in order to reduce restoring arm variations, will diminish the probability of occurrence of broaching and also does not attenuate the consequences substantially. In the situations of broaching, large rolling angles can be developed and can occur at low values of initial GM. As broaching phenomena are related to course keeping problems in heavy weather, broaching can hardly be avoided by modifying the GM value of the ship. Therefore, broaching has to be considered as a problem of maneuvering, and, hence, of operating and operator guidance, and must be treated accordingly.
Surf-riding is another phenomenon of following / quartering seas and is often associated with broaching and can be described as the “catching and dragging”  of ship by a wave approaching from stern and accelerates her to the wave celerity. Physically, surf-riding is caused by the equilibrium created by the longitudinal wave force, thrust and resistance .
During surf-riding, the vessel sails with speed equal to the wave celerity. The vessel stay longer in the crest region and then passes quickly from the trough. In the literature  this referred as asymmetric large-amplitude surging, or surfing on a crest . A stationary behavior of ship starts to coexist in parallel owed to the fact that the resistance force that opposes the forward motion of the ship in water, can be balanced by the sum of the thrust produced by the propeller and the wave force along the ship’s longitudinal axis. This is known as surf-riding and the main feature is that the ship is forced to advance with a constant speed that equals to the wave celerity. Such mode of motion may become directionally unstable if a ship is captured at wave down slope; as a result, a ship may experience the sudden uncontrollable turn and unexpected large heeling . Hence, surf-ridding can be defined as a prerequisite to broaching.
In order for surf-ridding to occur, the wave length must be comparable with ship length and the wave celerity must be comparable with the ship speed. This is the reason way large ships cannot surf-ride, as waves of the necessary lengths are simply too fast compared to the ship speed .
To consider an example of surf-riding have to be assumed a wave with a length comparable to ship length has a celerity of 35 kts, while ship’s speed is 25 kts in calm water .
The thrust produced by the propulsor, when sailing in calm water with a speed of 25 kts, is same as the resistance at the speed of 25 kts. At the moment of wave overtaking the ship, the ship will surge because the axial forces push back and forth the ship and the speed of the ship become equal with the speed of the wave. The thrust provided by the ship is not sufficient to move the ship with the same speed of the wave (35 kts) and there is a difference between the resistance of the ship at the higher speed of wave and the ship’s trust for 25 kts.
If the thrust setting in calm water is slightly increased, for example at 27 knots, then the difference between the resistance and produced thrust is sufficient to compensate for this difference and theoretically, if the ship is at a certain position on the wave, the surf-riding becomes possible.
In fact, surf-riding is a stable equilibrium realized when a ship is situated on the front slope of the wave, close to the trough.
There are two possible modes of motions, surging and surf-riding, and the result are influenced by the ship’s instantaneous speed and ship’s location along the wave length. If the ship has sufficient instantaneous speed and is near the equilibrium then the surf-riding can occur.
There are two thresholds to enables surf-riding to be possible under certain conditions :
1. The 1st threshold corresponds to the thrust setting. If the ship’s thrust in calm water is further increased then the difference between the resistance at the wave speed and the ship’s thrust has further decreased. In order to occur the surf-riding of a ship located anywhere along the front of the wave, the axial surge force is enough. In this condition, surging is not possible and surf-riding remain the only mode of motion.
2. The 2nd threshold corresponds to the speed setting. The situation is applying to certain value where the surging does not exist anymore and the surf-riding becomes inevitable at any instantaneous speed and at every position along the wave.
The wave steepness and wave length has influence over both speed thresholds. The likelihood of exceeding one of these thresholds can be used as a criteria because in a realistic seaway they are random figures. The 1st threshold is easy to exceed, but in order to experience surf-riding, when the ship is located at a particular position on the wave the instantaneous speed must be increased considerably. However, the probability of such a coincidence is quite low and therefore, the second threshold should be used as a criteria. The surf-riding for the particular wave is guaranteed once the threshold is exceeded.
Based on the analysis of the possible situations of ship stability failure modes in severe sea conditions presented, it can be concluded that those situations can have a great impact on safety of navigations with unpleasant results regarding loss of ships and cargoes.
The possible stability failure modes presented in this chapter may indicate that appart from the fact that the stability of the vessel is put in danger, the vessel intself is exposed to large stresses in structure and abnormal dynamic load conditions generating shifting of cargo (especially for larger container ships where the slamming effect can be more accentuated in the aft part which result in the container stacks to collapse or the containers to “jump” from their locks).
The main goal in the field of intact stability is the fact that stability failures due to severe sea conditions (due to phenomena like pure loss of stability, parametric rolling, broaching or surf-ridding presented in this chapter) are presently not offering any viable measure to be avoided. This aspect is sustained by the present intact stability regulations that were described and analysed in this book.
Future steps for improvement of ships stability assessment
The studies and analysis presented in this book revealed that ship’s stability is a very important nautical quality with a huge impact on safety of ship and safety of navigation. A vessel in an unstable condition will be all the time in an unseaworthiness condition. Loss of ship stability has a great share and is an important cause of the maritime casualties with a huge impact on safety of navigation, being a serious threat in this respect. Thus, the connection between intact ship stability and safety of navigation is obvious. Any loss of intact stability, due to one of the causes presented in this book, will lead to a dangerous situation that will directly affect the safety of navigation.
One more time it was revealed the fact that the continuous learning and training as well as the experience of deck officers in this respect is a very important factor for avoiding dangerous situations in which the crew, vessel and cargo can be put .
Human factor still remains a decisive factor for the assessment of ship safety, despite the growing process of automation to ships. The casualties presented shown that human factor is one of the most important elements of the ship safety system. The loss of control over a ship, not only in a critical situation, can be a main cause of casualties.
With a growing awareness of safety issues, public tolerance of accidents in shipping has decreased. So even nowadays, despite all of the improvements and advanced technologies, maritime casualties continued to happen, moreover with a high rate.
Human error include inter-alia, a lack of adequate knowledge and experience, technical inability, not paying proper attention to procedures and rules, carelessness in commanding a ship but also misinterpretation of information provided by documentation or computer software.
An important lesson can be learned from the presented casualties, that the ship safety loss is in many cases a chain of events, especially on those related to operational factors. The events must be analyzed separately and ideally would be that the solutions to be integrated into a single framework.
The aspects presented in this book can be valuable and can be taken into consideration by the officers working on board ships, because the accident data presented is reliable data from official resources. But it is also suggested that the findings presented should be properly referenced and used by consideration practical conditions and individual experiences.
The occurrence of ships stability casualties in the last years has its statistical laws and has different relationship with conditions of ships and many other operational influent factors.
In the future, measures that are relatively easy to implement, in aim to increase safety of maritime transport, should be based on already proved measures, i.e. with improving existing safety measures it is the simplest and fastest way to reduce possibility of accidents
A better education of ship officers is needed. Better guidelines should be given to ship officers regarding the choice of the right time to apply the right decisions.
It is of paramount importance that refresher courses should be given at regular intervals for those working in the field (i.e. crew, ship designers, operators, managers etc.) in order to give an actual situations of the errors that can be present in the loading documents or loading software.
The problems identified in investigations related to containers transport revealed the necessity for a “Code of Safe Practice for Containers Transport”.
The “Code” can be a useful tool and guidance for all the interested parties working in container shipping industry (shippers, owners, charterers and port operators) providing them with the minimum standards necessary to operate in this branch of maritime transport. Among many important issues to be stated in the “Code”, it will be very important to mention that ship’s officers are provided with sufficient time to verify/approve proposed stowage plans received from the shore planners. This important issue is borne from the reality of today as the new generation of containerships (up to 20,000 TEU capacity) and very rapidly container operations in ports is such that it is very difficult, and sometimes impossible, for ships’ officers to maintain control of the loading plan.
A very important aspects in containers trading (as well as a very usefull help for deck officers working on board container ships) is that the shore planners (the persons who prepare the preliminary stowage plans onshore) to have maritime experience or background; the best people to plan the load on shore ideally are former deck officers with experience on board containerships. Unfortunately, in many planning companies are employed shore planners with inssuficient knowledge in this aspects. Moreover, some of the planners are not aware of what huge responsibility is on their shoulders in respect of the ship’s safety for the intended voyage.
The loading programs should be provided with warnings that alert planners and ship’s officers to any errors and abnormal situations. The entire chain of communications and procedures between all parties involved in the planning and delivery of containers to be efficient and to ensure the ship’s officers that has the necessarily time and means to safely oversee the loading operations and securing of cargo.
Apart from the fact that the most common causes that the container stacks collapse are due to positioning heavy containers on top of light or empty ones as well as overweight and miss-declared weight of containers, another very impostant cause, which in many situations can not be controlled by ship’s officers, is the commercial pressure on vessel’s masters, ussualy from charterers, to maintain voyage schedule and not to deviate or slow down even for the situations when bad weather (rough seas) conditions are encounted.
Moreover, a common practice in container shipping industry encountered almost in evey port is to put pressure on vessel’s master to make all the necessary arrangements and to have everything ready for departure as the last container has been loaded and all the shore personel left the vessel. By the overwhelming desire to maintain schedules in those situations and in situations of fast turnaround times and commercial pressure, the ship’s officers have not enough control as they should have and the safety is compromised.
After many years of struggle regarding the correct declaration of weight contained in containers, it seems that the solution was finally found. In June 2014 International Maritime Organization issued the MSC.1/Circ 1475 (that will entry in force on 1st of June 2016) representing the “Guidelines regarding the verified gross mass of container carrying cargo”. It is a huge step forward in containers transport but what is much more important is the fact that those guidelines are establishing an approach for an enforcement of the requirements regarding the verification of the gross mass of packed containers contained and reglemented through SOLAS.
If the containers will be delayed from being loaded on board vessels due to overloading it will be a constrain action against shippers to correctly declare the actual weight, and moves the action forward to educate those who stuff the containers.
However, despite the fact that the proposed concept of weighting containers seems to be apparently simple, for maritime community have to be a question mark why it is only now ready after almost half of the century after first container was loaded on board ship.
Of course, the problem will prove that is more complex than at first look, because there are aspects that remained to be clarified like: Who exactly the “shipper” is? What happen with containers that arrive at terminal without a verified gross mass certificate? There will be demurage issues till the documentation for such containers will be issued?
Most probably the concerns in repsect of the containers weight will still remain, because international standards for accuracy of weighing are to be established. It will be practically impossible to determine with accuracy the weight and in this respect most probably a tolerance of, let say up to 1%, will be acceptable. However, the national administrations are the most appropriate to assess this matter.
Presently, the prevention measures to avoid dynamic instability of ships, generated by phenomena like parametric rolling or pure loss of stability, are the simply alteration of ship course and speed and are chosen based on the guidance provided in the references and on a subjective „seamanship practice feel“ for how the ship is reacting to external forces. The information to guide the master when encountering severe sea conditions is very little. Moreover, the information and guidance for assessment ship’s stability in severe sea conditions is almost absent.
Ship designs change very rapidly due to market demands, and the current intact stability criteria (based on static lever arm curve for still water condition) are easy to handle neglecting the (dynamical) physical characteristics of modern vessels. Geometries of novel ship designs have become considerably different from conventional forms. These considerable differences in geometry may invoke dynamic behavior that is drastically different from historical experience.
Capsizing still seems unlikely for these large ships. However, large roll angles and accelerations may become on obvious danger for ship, cargo and crew. Such behavior is outside of historical expectations for a ship; therefore, dynamic stability concerns are well grounded.
An increased number of containerships recently suffered from parametric rolling with results in loosing and / or damaging the cargo, implied also the risk of capsizing. Not only the new designed containerships are susceptible to parametric excitation, but also Ro-Ro, Ro-Pax and Pure Carr Carrier ships (as the design of these ships presents large flares fore and aft resulting in a considerably reduced waterplane area, especially when the ships are on the wave crest).
The fact that most of them have not yet encountered such a dangerous situation, can be considered only a matter of luck. However, a number of vessels were put in danger by development of excessive roll excitation and the results were catastrophically, leading not only to loose a considerably amount of cargo/containers overboard but also to to large cracks in ship’s structures or even broke the ship in separate parts (as possible in the case of the containership “MOL Comfort” in 2013 during sailing in severe sea conditions in Indian Ocean).
Very short roll periods leading to high accelerations (especially if combined with insufficient roll damping as well as insufficient course keeping capabilities in severe sea conditions) for the Ro-Ro, Ro-Pax, PCC and containerships made the vessels operators of such vessels to complain.
For managing the hazard of ship stability loss, the traditional method was to adopt conservative methods. These include use of prescriptive stability criteria, based on data from past loses and hurricane avoidance techniques that may not work well under operational conditions.
In order to allow time for changes to be made, it is necessary to establish methods to help ship’s officers in identifying the onset of threatening ship motion conditions, thus altering the commanded response to environmental conditions and possibly reducing exposure to devastating ship motions or capsize. Therefore, it is of benefit to all sectors of the marine industry to develop methods, which could provide ship’s officers with indicators of the onset of inclement ship motions.
All these dangerous mechanisms as well as other dangerous and unfavorable seakeeping characteristics are not covered by currently intact stability rules. The general feeling is that still, there is not a fully reliable method for ship intact stability assessment taking in considerations the main factors (like ship design, actual loading condition and environmental factors) that are influencing the stability loss.
The studies and analysis presented in this book are leading to the following important tasks that have to be settled by international maritime organizations as soon as possible:
1. The necessity of establishing a new set of stability criteria, to show the vulnerability and behavior of the vessels in severe sea conditions, especially for situations of parametric rolling or pure loss of stability.
2. The validation of the stability criteria through calculations on various types of ships.
3. The implementation on board vessel of the procedure for calculation the dynamic stability as a useful and accessible computational tool for ship’s officers.
Based on these observations can be considered that there is a necessity of rethinking the stability problems (arising from actual modes of ship stability loss and new design of ships) generating new requirements with the positive impact on safety of navigation that will protect lives, environment, and proprieties.
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The opportunity of this book is given by highlighting of some aspects that are insufficiently approached in practical assessment of ships stability as well as the demonstrated usefulness of theoretical approaches documented, analysed and proposed. The synopsis of current stability criteria presented in the book is illustrating the details at which stability requirements are determined, yet will also reveal the shortcomings of the criteria and the fact that the current stability criteria are not covering actual demand of safety for intact stability in certain situations. References of the present regulations are critically discussed and some conceptually approaches are proposed. The subjects treated in this book are parts of the works related to importance of ship stability assessment. It fits into a very complex system of research concerning the intact stability of ships, more exactly to modes of ship stability loss and the possibility of assessment the ship’s stability to prevent such losses.