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The lack of a necessary rational framework for assessing ship stability was the main concern of this research. The aim was to develop a rational philosophy and a logical procedure of assessing intact stability in order to ensure a consistent and unified approach to design for operation and for survival. The method uniquely brings together a linearised analysis for assessing a potentially dangerous roll motion with a probabilistic assessment of ship performance in rough seas on a standard test-track. This represents a significant advance on previous research. A novel feature of the analysis was that prediction of the extreme capsize roll motion was not attempted per se. Instead a reduced level of roll response termed "potentially dangerous" roll motion was selected (based on discussions with seagoing personnel) beyond which there was evidence that loss of the vessel is likely. Validation of the linear spectral analysis used in the simulations was performed using full scale trial results of a fisheries protection vessel. Provided that measured values of roll damping coefficient were used, the predicted values of extreme roll closely matched the maximum values experienced on sea trials up to the chosen value of critical roll angle of 30 degrees. Particular attention was paid to the realistic modelling of total system behaviour in rough seas. Families of wave spectra were used to represent the complete range of wave conditions encountered in nature. Avoidance and pacifying seamanship were incorporated based on the results of available trials data and discussions with serving masters. Independent (Bernoulli) trials procedures were used to calculate the cumulative probability of a critical roll motion being exceeded at least once during the vessel's passage through the test-track. The value of critical motion exceedance obtained was 5x10ˉ² for the fisheries protection vessel which has a large metacentic height and is reported to have good seakeeping characteristics.

Parametric roll resonance is a dangerous resonance phenomenon affecting several types of ships, such as destroyers, RO-RO paxes, cruise ships, fishing vessels and especially container ships. Worst case, parametric roll is capable of causing roll angles of at least 50 degrees, and damage in the tens of millions of US dollars. Empirical and mathematical investigations have concluded that parametric roll occurs due to periodic changes in the waterplane area of the ship. If the vessel is sailing in longitudinal seas, with waves of approximately the same length as the ship, and encounter frequency of about twice the natural roll frequency, then parametric resonance can occur. While there is a significant amount of literature on the hydrodynamics of parametric roll, there is less on controlling and stopping the phenomenon through active control. The main goal of this thesis has been to develop controllers capable of stopping parametric roll. Two main results on control are presented. To derive, analyze and simulate the controllers, it proved necessary to develop novel models. The thesis thus contains four major contributions on modeling. The main results are (presented in order of appearance in the thesis): Six-DOF computer model for parametric roll One-DOF model of parametric roll for non-constant velocity Three-DOF model of parametric roll Seven-DOF model for ships with u-tanks of arbitrary shape Frequency detuning controller Active u-tank based controller for parametric roll

Small open-deck roll-on/roll-off passenger ferries in Indonesia have a poor safety record. The Indonesian Government is interested in means by which the safety of these vessels can be improved, and this was the main catalyst for commencing research in this area. Any solution should be capable of being retrofitted to both existing vessels and new designs to improve their stability and, hence, their safety. The research therefore focused on the intact stability of the bare hulls, and with addition of side casings, for the vessels for which data was made available by the Indonesian Government. The research covered both quasi-static analysis, based on the objective of meeting the IMO intact stability criteria, and a dynamic approach using time-domain simulation in regular beam waves. A parametric study of the stability parameters of the twenty vessels demonstrated that, without the presence of side casings, the vessels had difficulties in complying with the IMO intact stability criteria. The problems were solved by introducing side casings (watertight spaces above the vehicle deck) either inboard of the vessel’s side-shell plating, or partially inboard and partially outboard of the side shell. The minimum extent (breadth) of side casings required was determined by iteration on each of the twenty vessels, incorporating variations in the height of the centre of gravity and loading conditions. The implementation of the minimum side casings showed that each vessel then met the IMO intact stability criteria. However, the assessment of the vessels’ dynamic stability characteristics using time-domain simulation provided inconsistent results for these vessels with side casings which met the IMO intact stability criteria. For some particular conditions, the existence and the different forms of side casings could decrease vessel survivability by increasing the roll motion amplitudes for both inside and outside casings and could lead the vessel to capsize. The results of the dynamic stability analysis also confirmed the vulnerability of small vessels with small stability parameters to large waves, and the different roll seakeeping behavior of the different vessel stability parameters.

Small open-deck roll-on/roll-off passenger ferries in Indonesia have a poor safety record. The Indonesian Government is interested in means by which the safety of these vessels can be improved, and this was the main catalyst for commencing research in this area. Any solution should be capable of being retrofitted to both existing vessels and new designs to improve their stability and, hence, their safety. The research therefore focused on the intact stability of the bare hulls, and with addition of side casings, for the vessels for which data was made available by the Indonesian Government. The research covered both quasi-static analysis, based on the objective of meeting the IMO intact stability criteria, and a dynamic approach using time-domain simulation in regular beam waves. A parametric study of the stability parameters of the twenty vessels demonstrated that, without the presence of side casings, the vessels had difficulties in complying with the IMO intact stability criteria. The problems were solved by introducing side casings (watertight spaces above the vehicle deck) either inboard of the vessel’s side-shell plating, or partially inboard and partially outboard of the side shell. The minimum extent (breadth) of side casings required was determined by iteration on each of the twenty vessels, incorporating variations in the height of the centre of gravity and loading conditions. The implementation of the minimum side casings showed that each vessel then met the IMO intact stability criteria. However, the assessment of the vessels’ dynamic stability characteristics using time-domain simulation provided inconsistent results for these vessels with side casings which met the IMO intact stability criteria. For some particular conditions, the existence and the different forms of side casings could decrease vessel survivability by increasing the roll motion amplitudes for both inside and outside casings and could lead the vessel to capsize. The results of the dynamic stability analysis also confirmed the vulnerability of small vessels with small stability parameters to large waves, and the different roll seakeeping behavior of the different vessel stability parameters.

In this dissertation, the extended Melnikov's method has been applied to several nonlinear ship dynamics models, which are related to the new generation of stability criteria in the International Maritime Organization (IMO). The advantage of this extended Melnikov's method is it overcomes the limitation of small damping that is intrinsic to the implementation of the standard Melnikov's method. The extended Melnikv's method is first applied to two published roll motion models. One is a simple roll model with nonlinear damping and cubic restoring moment. The other is a model with a biased restoring moment. Numerical simulations are investigated for both models. The effectiveness and accuracy of the extended Melnikov's method is demonstrated. Then this method is used to predict more accurately the threshold of global surf-riding for a ship operating in steep following seas. A reference ITTC ship is used here by way of example and the result is compared to that obtained from previously published standard analysis as well as numerical simulations. Because the primary drawback of the extended Melnikov's method is the inability to arrive at a closed form equation, a 'best fit'approximation is given for the extended Melnikov numerically predicted result. The extended Melnikov's method for slowly varying system is applied to a roll-heave-sway coupled ship model. The Melnikov's functions are calculated based on a fishing boat model. And the results are compared with those from standard Melnikov's method. This work is a preliminary research on the application of Melnikov's method to multi-degree-of-freedom ship dynamics. In the last part of the dissertation, the method of manufactured solution is applied to systems with chaotic behavior. The purpose is to identify points with potential numerical discrepancies, and to improve computational efficiency. The numerical discrepancies may be due to the selection of error tolerances, precisions, etc. Two classical chaotic models and two ship capsize models are examined. The current approach overlaps entrainment in chaotic control theory. Here entrainment means two dynamical systems have the same period, phase and amplitude. The convergent region from control theory is used to give a rough guideline on identifying numerical discrepancies for the classical chaotic models. The effectiveness of this method in improving computational efficiency is demonstrated for the ship capsize models. / Ph. D.

Les critères de stabilité à l’état intact de seconde génération sont en cours de finalisation par l’Organisation Maritime Internationale. Ils doivent compléter les critères actuels en apportant une sécurité accrue dans les vagues. Ils sont organisés en cinq modes de défaillance et trois niveaux d’évaluation dans chaque mode de défaillance. Le premier niveau est basé sur une approche déterministe simplifiée des phénomènes et assure des marges de sécurité importantes. Le second niveau requiert des calculs plus complexes basés sur des considérations hydrostatiques dans les vagues. Il est supposé assurer des marges de sécurité réduites. Le troisième niveau, actuellement en cours de développement, devrait consister en des simulations numériques du comportement du navire sur des états de mer réels réalisés par des instituts spécialisés. Les deux premiers niveaux des modes de défaillance perte pure de stabilité et roulis paramétrique ont été implémentés dans un code de stabilité. Les courbes de KGmax associées à ces critères sont calculées pour une sélection de navires civils et militaires de différents types ayant des comportements connus ou supposés différents vis-à-vis de ces modes de défaillance. Les exigences et la pertinence des critères sont analysées. La seconde vérification du critère de niveau deux en roulis paramétrique est étudiée en détail. Une méthode simplifiée de calcul de l’angle maximum de roulis paramétrique supposant un GZ linéaire est proposée et implémentée dans le critère correspondant. / The second generation intact stability criteria are currently under finalization by the International Maritime Organization. They are intended to improve the current intact stability rules by adding safety in waves. They are structured in five failure modes and three levels of assessment in each failure mode. The first level is based on a simplified deterministic approach of the phenomena and ensures high safety margins. The second level requires more complex computations based on hydrostatic considerations with regard to static waves and is expected to provide reduced safety margins. The third level, currently under development, would consist of numerical simulations of the ship’s behavior in real sea states performed by specialized institutes. Level-one and level-two criteria of both pure loss of stability and parametric roll failure modes have been implemented in a stability code. The KGmax curves associated with these future criteria are computed for a selection of different ships of different types, both civilian and military, expected or known to have different behaviors with regard to the considered failure modes. The requirement and the relevance of the criteria are analyzed. The second check of parametric roll level-two criterion is thoroughly analyzed. A simplified method providing the maximum parametric roll angle assuming a linear GZ is developed and implemented in the corresponding criterion.