PROBABILISTIC ANALYSIS OF ROLL PARAMETRIC RESONANCE IN HEAD SEAS

The paper presents some background for an analysis of the risk of severe parametric roll motion for a ship operating in head seas. This background includes a consideration of basic probabilistic qualities of parametric roll in head seas: ergodic qualities and distributions, since these results are necessary to establish a method of prediction of extreme values. The ship motions that generate parametric excitation, heave and pitch, have also been studied in this analysis. The post-Panamax C11 class container carrier was chosen for analysis, since a vessel of this type is known to have suffered significant damage in an incident attributed to severe parametric roll. It was shown that despite large-amplitude of motion, pitch and heave retain their ergodic qualities and normal character of distribution, while the roll motions are clearly non-ergodic and do not have a normal distribution. The analysis is built upon the numerical simulation of ship motion in head seas using Large Amplitude Motion Program (LAMP). The paper also considers the effectiveness of anti-rolling devices in mitigating parametric roll by suppressing the parametric excitation. It was shown with numerical simulation that correctly tuned U-tube type of anti-rolling tank has to potential to reduce the occurrence of parametric roll and significantly increase the stability and safety of large modern container carriers