Damage stability of passenger ships: a multi-modal analysis of the time to capsize

After an accident in open seas, the final fate for a damaged ship could be the loss of stability and consequently capsize. The latter may occur even in calm water, but it is more critical and probable in adverse weather conditions, i.e., irregular waves. Identifying a possible capsize event and determining the time that it takes for the ship to capsize is extremely important for safety assessment, meaning whether it would be possible to evacuate the ship for the scenarios considered. In this respect, time domain simulations or model tests should be performed to provide answers to this question. However, in dealing with irregular waves, both approaches are affected by the random nature of the phase spectral components, which leads to a different time to capsize determination at each calculation/run or to the identification of cases where the vessel will not capsize in the analysed time window. Here, a dedicated study is presented to describe the Time to Capsize (TTC) in irregular waves for critical damages. Simulations performed on a real passenger ship, highlight the appearance of more than one capsize mode for the same damage case. A model based on Weibull and Mixed-Weibull distributions has been developed to describe the multi-modal behaviour of the TTC distributions for the analysed damage cases.