Application of nonlinear roll motion model for assessment of ship stability in waves

Ship capsizing could be a result of various unwanted situations such as accidents, flooding, fire, movement of cargo or hostile behavior of extreme sea conditions. Providing the vessel with enough statical stability would prevent capsizing from the first group of causes, however it might be ineffective against wave action and other environmental conditions. The present study aims at addressing the ship stability safety in beam waves to find answers to the second type of capsizing. For this reason, a nonlinear equation of roll motion was set up and solved by Duffing's method in order to implement ship stability qualities in waves realistically. Nonlinearities in damping and restoring moments were introduced into the equation of roll motion. A procedure associated with the nonlinear roll motion model was proposed in stability analysis of ships in waves. Certain environmental data related to the real sea conditions were included in the analysis. The most critical case, resonance, for rolling motion in beam waves was studied and roll responses were calculated for this phenomenon. A geometrically related trawler series was chosen to investigate the effects of main hull form parameters on ship stability and on the constituents of the nonlinear differential equation. The nonlinear roll motion model was applied to the series, and it was verified that there is a direct relationship between the ship responses and hull form parameters. Validity and accuracy of the proposed nonlinear roll motion analysis were confirmed on a destroyer model whose experimental results are available. Comparison between the experimental results and the nonlinear roll motion theory shows very good correlation.