Establishment of Prediction on Unstable Ship Motions under Extreme Sea States

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(1)Comparison of several numerical models from the world leading institutes and capsizing model experiments showed that qualitative prediction of capsizing for intact ships in following and quartering waves was possible but no numerical model could quantitatively predict capsizing.(2)They showed that the models could estimate critical wave height of capsizing of damaged ships in beam waves but none could explain roll angles before capsizing.(3)For broaching, e.g. one of the major capsizing modes of intact ships in following and quartering waves, a model to quantitatively predict capsizing due to broaching was newly developed and validated with the capsizing model experiments. The new model takes into account the wave effect on hydrodynamic derivatives with respect to ship motions and hydrodynamic forces due to large heel angles.(4)For bow-diving that could result in capsizing in following waves, a new calculation method to predict its onset was proposed and well validated with experiments.(5)For numerical modeling of damaged ships, a new theory was formulated to take the effect of progressive flooding into the hull on hydrodynamic forces acting on the hull into account and demonstrated that such effect is significant. Moreover, the difference between the numerical and physical modeling was explained as a kind of anti-rolling tank effect due to flooded compartments.(6)For a calculation method of capsizing probability based on a piece-wise linear approach, effect of wave statistics was investigated with numerical studies and then it was shown that calculated probability with operational effects could be comparable to actual casualty statistics.