A Study on the Effect of Parametric Rolling on Heave and Pitch Motions in Head Seas

The methods to be used for direct stability assessment of parametric rolling are now under development by the International Maritime Organization (IMO) in the second generation intact stability criteria. In order to provide a reliable numerical method for predicting parametric rolling, the effects of parametric rolling on heave and pitch motions in regular head seas are investigated by simulations and experiments. Firstly, a coupled heave-roll-pitch mathematical model based on a nonlinear strip theory is used to calculate heave and pitch motions in regular head seas with parametric rolling taken into account. Secondly, model experiments are conducted to investigate the effect of parametric rolling on heave and pitch motions where an optical tracker is used to measure ship motions. Finally, time-domain heave and pitch motions are analyzed in the frequency-domain by the Fourier transformation. The both results of experiments and simulations show that heave and pitch motions are obviously affected by parametric rolling and one distinct phenomenon is newly observed that the pitch motions in experiment include subharmonic component when parametric rolling occurs. However the distinct phenomenon does not clearly reproduced in numerical simulation, therefore the effects of parametric rolling on heave and pitch motions should be further investigated in the numerical method for direct stability assessment of parametric rolling.