Contemporary Ideas on Ship Stability

In this introductory chapter of the book “Contemporary Ideas on Ship Stability: From Dynamics to Criteria” are summarised the 42 research contributions that appear as individual chapters of the book. They are classified in 13 sections covering the following topics: Development of second generation IMO intact stability criteria; history of stability criteria; improvements on current methods of probabilistic assessment of ship stability; evaluation of probabilistic methods and interpretation of results; parametric roll, operational measures and stability monitoring; surf-riding and broaching-to; roll damping; damaged stability; model experiments; accident investigation; cargo liquefaction; offshopre structures; special craft.

The paper summarises background and current status of the development of the second generation intact stability criteria at the International Maritime Organization (IMO). The decisions at the IMO so far together with the implementation scheme for the vulnerability criteria, the guidelines for the direct assessment procedures and for operational measures. The remaining issues, mostly related to the finalization of the explanatory notes are presented.

The safety level of a criterion is defined as a probability that a failure will occur, if the criterion is met exactly, i.e. without any surplus. This chapter considers how the safety level can be evaluated, in principle, for the vulnerability assessment included in the Second Generation Intact Stability Criteria (SGISC). The chapter also provides a review of the background literature for the SGISC and considers the alignment of SGISC with Goal Based Standards and Formal Safety Assessment.

The formulation of the Second Generation of Intact Stability Criteria was finalized by the International Maritime Organization (IMO) in 2020. The criteria have been developed for a future incorporation into the 2008 IS Code, however they require testing before using them as a mandatory criterion. Member states are by IMO invited to use the Interim Guidelines and report back the experience. The criteria are formulated for five failure modes, each of which is analyzed by two vulnerability levels and, if needed, a direct numerical simulation. The present paper summarizes results testing the vulnerability levels in these new stability criteria. The calculations are carried out for 17 ships using the full matrix of operational draughts, trims and GM values. Each failure mode criterion is examined individually regarding construction of a GM limit curve for the full range of operational draughts. The consistency of the outcomes has been analyzed, and finally examined whether the new criteria tend to be more or less conservative compared to the present rules by evaluating approved loading conditions. The analyses were performed in 2016 and based on criteria developed in 2015 and 2016 and amended by the Sub-Committee on Ship Design and Construction of IMO. Work performed in IMO up to Spring 2020 relevant for the analysis is described.

The chapter examines different aspects of consistency between the levels 1 and 2 of the vulnerability assessment within the IMO second generation intact stability criteria (SGISC). Both the dead ship condition and the pure loss of stability failure modes are considered. The most important aspect of consistency for the dead ship condition is its possible influence on the integrity of the existing mandatory stability regulations since the consistency between the levels of vulnerability criteria is, in fact, representative of consistency between the 2008 IS Code and the SGISC. The chapter describes a possible solution for the between-the-levels consistency of the pure loss of stability failure mode. The main idea is to assess the safety level of the deterministic criterion for the level 1 vulnerability criteria. Then, the standard for the probabilistic level 2 vulnerability criteria must be set to a higher level than the assessed level 1 safety level (as the level 2 criterion is meant to be less conservative than the level 1 criterion). For this approach to work, both levels 1 and 2 should use the same mathematical models of the stability failure or, at least, the model for level 1 should be inherently more conservative compared to the level 2.

Direct stability assessment requires a significant computational time since stability failures, which are very rare in practically relevant cases, should be encountered in numerical simulations. The problem can be simplified if stability failures can be assumed independent and thus described as a Poisson process, which requires neutralization of self-repetition, transient effects and autocorrelation of big roll motions, solutions for which are proposed in the paper. Two other simplifications considered here are the extrapolation of the stability failure rate over significant wave height and reduction of the assessment to few design situations. In the former method, the failure rate is defined from numerical simulations at large significant wave heights, where the failure rate is large, and extrapolated to lower significant wave heights. In the design situations method, the assessment is performed for few selected combinations of the significant wave height, mean wave period, wave direction and ship speed. Both methods are applied to five ships (a cruise and a RoPax vessels and three container ships), each in six loading conditions, and demonstrate a significant reduction in the required simulation time. Recommendations for practical application of these methods are provided.

This chapter is focused on the physical background of the second level vulnerability criterion for surf-riding /broaching-to as a part of the IMO second generation intact stability criteria. The criterion is based on Nonlinear Dynamics, homoclinic bifurcation, in particular, and uses the Melnikov method for calculations. While well understood in the scientific community, these concepts may present a challenge for regulatory use since most practicing naval architects are not familiar with these concepts. The paper presents an explanation of the criterion background using conventional Naval Architecture physical concepts, and gives an overview of the dynamical aspects of the calculation procedure.

The Criterion for Intact Ship Stability proposed by Rahola (The judging of the stability of ships and the determination of the minimum amount of stability—especially considering the vessels navigating Finnish waters, Doctoral thesis 1939) spread around different countries after the World War. It constituted the basis for the first international provision on intact stability adopted in 1968 in the frame of the recently created International Maritime Organization. This Criterion, although heavily criticized since the beginning for its semi-empirical nature, was included in both the Intact Stability Code, IMO Res. A. 749 and, with some modifications, got mandatory status in the International Intact Stability Code 2008. It is quite easy to foresee that it will survive in the near future too, at least until the Second Generation Intact Stability Criteria, recently approved as Interim Guidelines, will undergo thorough testing and tuning.

The formula to determine the roll angle for structural strength assessment in ClassNK’s Technical Rule and Guidance gives a value based upon maximum roll amplitude at probability of exceedance Q = 10–8 within the design life of ship on long-term prediction of roll amplitude. The long-term prediction is obtained from combining short-term prediction of roll amplitude and a probability of occurrence of short-term irregular sea in long term (design life of ship). In the current rule, non-linearity of roll is included as some correction coefficients obtained from model experiments and empirical knowledge at the time of development. However, the ships have considerably changed since then, and the coefficients are not always suitable for the novel vessels. The purpose of this study is to propose a rational short-term prediction method considering nonlinearity of roll. In this paper, applicability of a non-Gaussian PDF (Probability Density Function) for PDF of roll angle is investigated.

The paper presents a simplified setup of the “critical wave groups” method, suitable for swift probabilistic evaluations of ship capsize tendency due to beam-sea resonance. The simplifications proposed herein are twofold and aim at reducing the computational cost associated with the identification of the critical, for ship stability, wave episodes when these are represented by the “expected” wave groups for the ambient sea state. The first simplification concerns the initial conditions of the vessel at the moment of a wave group encounter which, according to the exact “critical wave groups” formulation, should be probabilistically treated. Instead, the simplified approach pursues reliable estimates by examining only the upright equilibrium state. Moreover, by focusing on sea states being highly probable to provoke resonance, fewer simulations need to be performed since, among all critical wave group candidates, the main probability contribution essentially comes from those having periods close to the natural period of the vessel in question. Considering these wave groups only constitutes the second simplification. Within this framework, regular wave trains are also tried to investigate the possibility of eliminating the computational burden due to the generation of the “expected” wave groups. The accuracy of both schemes in calculating the probability of extreme responses is assessed through comparisons with Monte Carlo simulations of roll motion.

When modeling a random phenomenon (e.g. ship motions in irregular seas), data are often available from multiple sources, or models, of varying fidelity, those with higher fidelity carrying higher costs. Multifidelity uncertainty quantification (UQ) offers tools that allow using lower-fidelity and lower-cost models to inform decisions being made about high-fidelity models. With a few exceptions though, much of the focus of the multifidelity UQ literature has been on characterizing uncertainty related to averages, in the context of non-rare problems where data are available to estimate these averages directly. In this work, we extend some multifidelity UQ methods to estimation of probabilities of rare events, possibly those that have not been observed in high-fidelity data. The suggested approach is based on bivariate extreme value theory, applied to simultaneously large observations from low-fidelity and high-fidelity models. The ideas are illustrated on simulated data associated with ship motions. It is not assumed that the reader is familiar with multifidelity UQ, with the discussion focusing on the most basic setting and building naturally from the recalled methods for non-rare problems.

This paper focused on reviewing some statistical methods based on large-sample-size time-domain simulations to characterize dynamic stability failure (for example large roll angle or large acceleration). In order to analyze the assumptions behind these methods and to identify the link between them, these statistical methodologies have been tested in two datasets obtained by numerical simulations. The first dataset represents a nonlinear process obtained for a ship in parametric roll condition and the second dataset represents a linear process. Both processes are obtained from a very long simulation 3000 h (3 h × 1000) in order to insure a better statistical convergence of the sampling. In addition, when possible, a Pearson chi-square test goodness of fit is performed to determine whether there is a significant difference between the predictions of the discussed methodologies and the observed data.

Verification, Validation and Accreditation (VV &A) are introduced in the context of IMO’s Second Generation Intact Stability Criteria (SGISC). IMO’s implementation of the SGISC has put in place a multitiered process by which the adequacy of a vessel’s stability can be assessed. The application of Verification and Validation (V &V) to the Level 1, Level 2 and Direct Assessment stages of the SGISC are discussed. From the perspective of Level 1 and Level 2 V &V, the user’s only responsibility is to verify that the algorithms for assessing vulnerability to stability failure contained in IMO documentation are implemented correctly. The developers of the algorithms for the Level 1 and Level 2 vulnerability assessments need to validate that their algorithms are consistent across a large range of vessel types and sizes. The most stringent criteria of SGISC is Direct Assessment where a vessel is assessed using a physics-based simulation tool. For direct assessment using ship dynamics software for predicting motions in extreme seas, existing well established and documented VV &A processes apply. To be applied to stability assessment, these tools should undergo a formal VV &A to assure that they perform adequately. Before the VV &A can be performed, the problem for which the simulation tool is to be assessed must be defined. This use—the objectives of the simulation are defined by the establishment of Specific Intended Uses (SIUs). SIUs are characterized and the way in which they are used are defined.

The application of a statistical validation procedure for estimating the probability of capsizing with the split-time method is described. The method is a numerical-extrapolation scheme incorporating motion perturbation simulations to evaluate a critical roll rate leading to capsizing following an up-crossing event. Fast volume-based numerical simulations create a sample of capsizing events in realistic irregular wave conditions that serves as a “true” value. Subsets of this data are used with the split-time method to estimate the capsizing probability. The split-time estimates are compared to the “true” value to judge the validity of the estimate. A short description of volume-based numerical simulation, review of the essence of the split-time methods, and the statistical validation and performance assessment of the estimation of the probability of capsizing is contained in the chapter.

The paper investigates the effectiveness of the generalized Pareto distribution (GPD) for modelling the tail of the distribution of ship rolling motions, and particularly for calculating the probability of capsize in beam seas. To this end, large-scale Monte Carlo numerical experiments were performed for an ocean surveillance ship assumed to operate in two qualitatively different, in terms of the observed frequency of stability failures, sea states; one where capsizes are realized quite often and another where they are extremely rare. For both sea conditions, GPD models were fitted to datasets containing roll angle exceedances above a pre-defined threshold and their reliability is tested herein against the rough Monte Carlo estimates, obtained by direct counting. Aiming at establishing links between dynamics and probability, in this study, the roll angle distribution is parametrized through successive GPDs and the idea of associating threshold selection with the shape of the GZ curve is developed. In this setting, a new framework is proposed for addressing the problem of ship capsize which, formally, is outside the scope of the classical GPD implementations. To evaluate the rumoured “extrapolation” character of the GPD beyond the largest observation used in the fitting procedure, a comparison with the predictions of the “critical wave groups” method is presented for the second (mild) sea state.

A statistical validation of the EPOT method for estimating the probability of large roll angle for a ship in irregular ocean waves is described in this chapter. EPOT is a numerical extrapolation scheme based on modelling the tail of the roll distribution while accounting for the nonlinearity of the roll response. Fast volume-based numerical simulations create a very large sample of roll events in realistic random irregular wave conditions, which establish a “true” value of the frequency of large-roll events. The EPOT method is applied to a number of small subsets of this data to estimate the probability of a large roll angle encounter. The EPOT estimates are compared to the “true” value in order to judge the validity of the estimate. This chapter describes a set of “true” value simulations, reviews the essence of the EPOT method, and presents the statistical validation and performance assessment of EPOT with two models for the tail of roll distribution.

Running a numerical simulation of motions in waves is in and of itself of little significance. The results of the simulation—the motion time histories—must be processed to produce statistical quantities if they are to be of any practical use. Techniques for dealing with time histories of non-rare and rare events are presented. In the realm of non-rare statistics, the techniques are further divided into statistics for the linear and nonlinear motion regimes. The focus of this paper is on non-rare events but predicting rare event statistics is discussed.

Within the framework of the second-generation intact stability criteria finalised at the International Maritime Organization in 2020, ships that fail to pass either vulnerability criteria or direct stability assessment can still be operated by taking operational measures as a risk control option. The introduction of such operational measures is a new attempt to secure the safety of the ships at sea. Therefore, careful examinations and discussions based on various case studies are indispensable for their implementation in actual use. Therefore, a case study was conducted on the operational measures to avoid parametric rolling, which is one of five stability failure modes. In addition, an example of operational limitations related to significant wave height and simplified operational guidance for a C11 class container ship are presented. Finally, the voyage simulation was performed considering the operational limitations using a rather small maximum significant wave height. Consequently, the practicality of operational measures in terms of their influence on the navigation route and time is discussed.

The authors executed measurements of the encounter waves by an X band wave radar and the roll angles by a gyro sensor onboard for a Ropax ship. By using the measured wave spectrum, the roll amplitude is estimated by using the simplified method for parametric rolling, which is used for the draft IMO vulnerability criteria. The estimated roll angle shows reasonably good agreement with the measured roll angle. Therefore, the wave-radar-assisted simplified operational guidance could be promising for practical uses onboard.

In this paper, the practical implementation methodology of an artificial neural network (ANN) based parametric roll prediction system, is studied. In order to avoid expensive scale tests, an uncoupled nonlinear roll model is applied to tune the system. The capability of this model to accurately simulate the phenomenon of parametric roll resonance is validated using towing tank tests. Finally, the behavior of the ANN system for forecasting roll motion in a realistic sailing condition has been investigated, obtaining very promising results.

This work presents a comparative study of two signal processing methods for the estimation of the roll natural frequency towards the real-time transverse stability monitoring of fishing vessels. The first method is based on sequential application of the Fast Fourier Transform (FFT); the second method combines the Empirical Mode Decomposition (EMD) and the Hilbert-Huang Transform (HHT). The performance of the two methods is analysed using roll motion data of a stern trawler. Simulated time series from a one degree-of-freedom nonlinear model, and experimental time series obtained from towing tank tests are utilized for the evaluation. In both cases, beam waves are considered but, while irregular waves are adopted in the simulated data, the towing tank tests are made in regular waves. Based on the available data the performance of both estimation methods is comparable, but the EMD-HHT method turns out slightly better than the sequential FFT. Finally, the use of a statistical change detector, together with the EMD-HHT methodology, is proposed as a possible approach for the practical implementation of an onboard stability monitoring system.

Stability failures are known to be one of the major causes of accidents involving fishing vessels. The use of guidance systems, focused on complementing the capabilities of the crews for dealing with the assessment of their vessel stability, have been proposed by many authors as a possible way for reducing the frequency of this type of incidents. Initially, these systems were basically color-coded posters or were relying on subjective data introduced by the crew. However, the use of approaches which operate in real time with no need of interaction could overcome the problems identified for these “first generation” methods. This work presents a methodology based on the analysis of roll spectrum for estimating in real time the metacentric height of the vessel. The integration of the presented methodology within a first generation guidance system could increase its capabilities for providing stability guidance. The performance of the proposed methodology is analysed using the simulated roll motion of a mid-sized fishing vessel in irregular beam waves and lateral gusty wind, computed by a one degree of freedom nonlinear model. Obtained results are promising in most of the analysed conditions, although some open issues regarding the implementation of the methodology still require further analysis.

The concept of surf-riding in irregular seas is investigated and two calculation schemes are implemented in order to establish upper and lower bounds of the probability of surf-riding. The first scheme extrapolates from the concept of surf-riding in regular seas and is product of analysis of system dynamics. Points of the phase-space that could be considered as the counterparts, for irregular sea, of the conventional surf-riding equilibria are targeted. Due to the time-varying nature of the considered phase-space however, these points exist only temporarily, emerging and later vanishing at random time instants. The second scheme is more empirical, targeting time intervals of ship motion where the speed remains above the expected range. We call such behavior a “high run”. The probability values obtained by the two schemes are compared against each other and conclusions are drawn.

Steps are taken towards extending the theory of surf-riding in order to deal with multi-chromatic waves. New bifurcation phenomena are identified and classified which are intrinsic to the presence of extra frequencies in the excitation. Alternative types of surf-riding are discovered. Chaotic transients seem to be quite a common feature of ship surge motion in extreme following seas.

The formulation of a metric for the likelihood of surf-riding in irregular waves is addressed in this chapter. This metric measures how close a ship is to surf-riding at a given time and is computed through a series of perturbation simulations. This approach allows the physics of severe ship motions to be included in the statistical extrapolation of the response by the split-time method. The candidate metric is the distance, in the surge phase plane, between the ship’s position (location and velocity) at an instant of a random seas simulation and the instantaneous boundary between surging and surf-riding (if the latter exists). The distance is measured along the line connecting the position of the dynamical system and the stable surf-riding pseudo-equilibrium at that time. The instance of surf-riding is defined when the surging velocity exceeds the value of the instantaneous celerity, computed at the position of a stable pseudo-equilibrium just behind the ship.

Recent research on the application of extreme value theory is reviewed for stability failures associated with qualitative physical changes in the dynamic system: broaching-to and capsizing associated with the change of stability in large waves. As these events are very rare, evaluating their occurance in ocean waves through direct numerical simulation is not practical with a code of reasonable fidelity. Probability must, therefore, be assessed without direct observation. This is done in the split-time framework, which introduces a metric of the likelihood of failure at selected times of an irregular wave response. The metric is computed by perturbing the dynamical system, in phase space, towards the failure state, and computing the perturbed response by numerical simulations that account for the changing physics of the extreme motions. Extreme value theory is applied to this metric in order to extrapolate a rate of failure.

A finite-volume method (FVM) is used to simulate the roll motion of an ellipsoid equipped with wall-bounded flat plates with and without forward speed. Due to the circular form and a fixed roll axis of the simulated ellipsoid, only normal forces act on the plates. The normal force component in phase with the roll velocity over a harmonic roll period is estimated. The roll period, amplitude and the plate dimension are varied. The simulation results are compared with results of different model test techniques. The focus is set on modeling a simple definition for the normal force coefficient based on the Keulegan-Carpenter number (KC). To transfer roll damping results from model scale into full scale, the frictional roll damping component of different ships is investigated. FVM simulations of the roll motion for full and model scale are carried out. A simple extrapolation procedure based on Kato’s approach is developed.

Accurate prediction of roll damping moments of ships is important to predict roll responses accurately. A method for estimating the roll damping moment in the time domain is necessary for estimating parametric rolling and other phenomena in irregular waves. In this paper, we investigate the characteristics of the roll damping moments acting on bilge-keels by utilizing CFD computation to propose a time-domain prediction method for the bilge-keel component. The roll damping moment acting on the bilge keel depends on the current and previous rolling of the hull. The reason for this is that the vortices generated at the bilge keel are rapidly strengthened by the vortices generated by the previous rolling. A method for predicting the roll damping moment of the bilge keel is proposed considering this effect.

Proper estimation of roll damping moment is of paramount importance for adequate assessment of dynamic stability of ships. However, experimental data on roll damping of inland vessels are scarce and unreliable. Thus, the applicability of the classic Ikeda’s method and its simplified version on typical European inland vessels is investigated, with specific focus on eddy making component. It is found that the simplified Ikeda’s method, in comparison to the classic method, may considerably underestimate the eddy making component of damping of full hull forms, or even return negative values, although the block coefficient is within the limits of method applicability. This deficiency of the simplified Ikeda’s method does not affect inland vessels only, but it is equally relevant for seagoing ships with high block coefficients. Hence, the paper explores the possibilities of adjusting the simplified Ikeda’s method in order to improve the observed shortcoming, as well as to extend its application to stability analysis of inland ships.

Roll damping is probably the most intriguing of the components of hydrodynamic reaction in ship dynamics. It is also a problematic one—small, nonlinear, difficult to measure or predict and crucially, a key determinant of ship stability. Undoubtedly, some of the problems in computing or predicting roll damping are intrinsic. It can be argued, however, that most of the difficulties do not originate from physical anomalies of energy dissipation in roll but are due to fundamental flaws in the approach to roll damping estimation or measurement. It appears that the root causes of these flaws stem from three concepts central to analysis of hydrodynamic reaction in roll: decomposition of the hydrodynamic reaction moment to added moment of inertia and roll damping moment, the assumption of small-amplitude motions, and the inevitable coupling of roll with other modes of motion. In this paper, the authors present a pragmatic approach to these fundamental concepts and discuss the implication of incorrect assumptions, pertaining to definition, measurement, calculation and use of roll damping in intact and damaged ship dynamics.

This paper describes the background and provides the rationale and the framework to embrace the whole spectrum of measures (regulatory, design, operational and emergency response) for improving the damage survivability of existing RoRo passenger vessels. The damage stability workshop elaborated here is the first step of a process initiated by INTERFERRY Europe to assess impact on/options for existing ships of increasing the required subdivision index R should IMO decide to apply new damage stability requirements retrospectively. This, in turn, would provide the motivation for instigating and establishing a framework and propose an approach for alternative compliance to account for the contribution made to damage survivability by operational and active damage control measures that could be undertaken in case of a flooding accident. This represents a step change both in the mind-set of naval architects and in safety legislation but the impact will be immense and mostly positive.

The Stability in Waves committee of the 27th ITTC investigated how to deal with the inertia due to floodwater mass from three points of view: (1) floodwater domain, (2) floodwater inertia itself, (3) floodwater entering ship. The committee suggested three criteria indicating the concept of how to deal with floodwater and providing clues on what to consider as floodwater when examining damage ships: (1) whether the water is moving with the ship or not, and amount of that water, (2) whether there is a significant pressure jump across the compartment boundary or not, (3) whether the dynamics of water can be solved separately or not. For floodwater inertia, the committee divided this into the partially flooded case and fully flooded case, and investigated the properties and showed how to deal with floodwater inertia for each case. For the case of the floodwater entering ship, the treatment of inertia change due to floodwater was made clear using the momentum change principle. The related procedure was updated reflecting this work.

The Stability in Waves committee of the 27th ITTC has investigated the significance of scale effects in air pressure on flooding model tests under atmospheric conditions. For this purpose, the committee classified the flooding cases into the trapped air case and vented air case and investigated the flooding process for a simple geometry, using the state equation of air and orifice equation. As a result, the committee concluded that the scale effect is large for the case of trapped air and small vent area. For the other cases, the effect is small and can therefore be neglected in the model test of a damaged ship. In addition, the committee proposed some directions that can be used to reduce the scale effect of air pressure.

This contribution provides an overview of the framework implemented in the joint industry project “eSAFE—enhanced Stability After a Flooding Event” for probabilistic damage stability assessment of passenger ships. The framework takes into account collision, bottom grounding and side grounding/contact accidents, by providing specific corresponding attained subdivision indices. Damage cases and associated flooding probabilities are determined through a common automatic non-zonal approach, while the post-damage survivability metric is based on the SOLAS “s-factor”. The framework is intended for practical application and is generally consistent with existing SOLAS probabilistic damage stability regulations. To support designers in the application of the framework, a specific software functionality has been developed, tested and applied. Some example applications of the framework are reported.

Recent developments in damage stability legislation have drawn from ships with simple internal architecture such as RoPax and cargo ships. However, ships with complex internal architecture, such as cruise ships, have been rather neglected. In a regulatory context, cruise ships are currently grouped with RoPax and other passenger ships and this can be misleading. Moreover, it is well known that cruise ships vary significantly in their behaviour post-flooding incidents in comparison to RoPax ships. This problem has been acknowledged by the Cruise Ship Safety Forum Steering Committee who consequently funded the Joint Industry Project eSAFE to undertake cruise ship-focused research on damage stability. This entails analysis of pertinent simplifications embedded in SOLAS, the development of a methodology to combine consequences from collision and grounding accidents, the establishment of new survival criteria for cruise ships and finally the development of guidelines to use numerical flooding simulation in seaways as an alternative approach to assessing ship damage survivability. The findings of this research are presented in this paper, based on a full set of time-domain numerical simulations along with static calculations for a number of cruise ships. A new s-factor is derived catering specifically for cruise ships that accounts more accurately for survivability in a wave environment. A number of simulations are undertaken on varying size cruise ships with the view to deriving a relationship between the critical significant wave height and the residual stability properties of such vessels. The results provide the requisite evidence for comparison between SOLAS 2009 A-Index and the ensuing Damage Survivability Index.

For facilitating the development of the guidelines for direct stability assessment as a part of the second generation intact stability criteria at the International Maritime Organization (IMO), this paper provides examples of comparison between model experiments and numerical simulations for stability under dead ship condition and pure loss of stability in astern waves. As a result, some essential elements for proper validation were identified. For dead ship stability, a good selection of representative wind velocity generated by wind fans is crucial. For pure loss of stability, accurate Fourier transformation and reverse transformation of incident irregular waves are essential. These remarks were partly utilised in the interim guidelines as finalised in 2020.

At the IMO (International Maritime Organization), the second generation intact stability criteria for pure loss of stability was developed in 2020. In its interim guidelines (IMO in “Interim Guidelines on the Second Generation Intact Stability Criteria”, MSC.1/Circ. 1627, 2020 [2]), vessels with extended low weather deck such as offshore supply vessels (OSVs) are exempted from this application, but its background has not yet been explained other than a sample calculation resulting in inconsistencies between different criteria levels. To solve this problem, the authors executed model experiments for a typical OSV in astern waves. The test results demonstrated that the phenomenon assumed by the pure loss of stability criteria is not pertinent to the OSV. Instead, it is the phenomenon which occurs due to trapped water on deck which seems to be of greater relevance for the stability of the OSV in astern seas. The effect of low weather deck length was also investigated by systematically modifying hull forms with the help of CAD software. Further, the on-deck trapped water behaviour was also studied in a separate series of model experiments involving direct measurement of the changing deck water level. The tests confirmed that the trapped water can induce both static and dynamic moments depending on the speed of the vessel and the wave conditions.

The second generation intact stability criteria have been finalized at International Maritime Organization. The criteria are developed for 5 stability failure modes, those are pure loss of stability, broaching-to, dead ship condition, parametric rolling and excessive acceleration. The authors have carried out free running capsizing model experiments in following and quartering seas for more than 18 fishing vessels. The series of experiments demonstrated pure loss of stability, broaching-to and bow-diving are major phenomena resulted in capsizing for fishing vessels while parametric rolling is not.

This paper outlines the experimental investigations into serious accidents of a purse seiner, which capsized and foundered during lying to with a parachute sea anchor in the North Pacific Ocean on 23 June 2008, and a stern trawler of pair trawling, which foundered on the way to a fishing ground in the East China Sea on 12 January 2010. In order to clarify the sequence and mechanism of each accident, model experiments in waves were carried out individually and further consideration with stability calculation so on were made.

Liquefaction of granular materials in a rectangular container has been experimentally investigated using NTUA’s School of Naval Architecture and Marine Engineering “shaking table” facility. Two different materials (sand and olive pomace) in several moisture content scenarios were tested. Harmonic forcing in a range of frequencies and amplitudes has been applied. The intention was to develop some qualitative understanding on how liquefaction comes about for materials of different properties; and also how the phenomenon relates with the duration and intensity of the excitation. The two materials presented substantially different behaviour, interpreted to be due to differences in moisture’s diffusion in material’s body and in their specific gravity. In a parallel study, was investigated the impact of liquefaction to a bulk carrier’s stability by using commercial design software. Different cases of cargo stowage and distribution in the holds were examined. This study confirms that homogeneous cargo loading can lead to substantial loss of stability after cargo liquefaction and that alternating or suitable inhomogeneous loading is often preferable. The current paper is an updated and improved version of a paper presented in the International Ship Stability Workshop, held in Brest in 2013 [16].

The paper shows results from a comprehensive experimental investigation on a mono-column in regular and irregular waves. Focus is centered on improving the understanding on the occurrence of resonant motions associated with Mathieu instabilities for cylindrical floating platforms. Experimental results with the mono-column showed both roll and pitch parametric amplifications. It is concluded that the instabilities observed in the mono-column experiments were very much influenced by the mooring system configuration. A numerical algorithm is used as a relevant tool for discriminating the role of the different nonlinear contributions to parametric amplifications arising from hydrostatics, Froude-Krylov and mooring loads within the observed diverse patterns of roll and pitch responses.

The paper describes model test experiments representing a Rigid Hulled Inflatable Boat (RHIB) in heavy seas. A numerical simulation tool is briefly described. Simulation and experimental results are compared in a deterministic way. The cases that are compared include regular and irregular waves from various directions.

This paper describes the use of two potential flow simulation tools, of varying degrees of non-linearity, for predicting landing craft motions, impulsive loads and water ingress. A comparison between experimental and simulation results for a landing craft hull form operating in irregular seas is provided. During the experiments, severe wave impacts against the bow door were recorded, with water ingress occurring through the bow door. Simulation results for these phenomena are compared with corresponding experimental results. The results from both non-linear and semi-linear versions of the simulation tool are discussed, together with measures adopted in the semi-linear method to yield results that approach the more representative non-linear results.