Time to capsize for damaged passenger ships in adverse weather conditions. A Multi-modal analysis

After an accident in open seas, the final fate for a damaged ship could be the loss of stability/floatability and consequently capsize/sinkage. The latter may occur even in calm water, but is more critical and probable in adverse weather conditions, i.e., in irregular waves. Identifying a possible capsize event and determining the time that it takes for the ship to capsize is extremely important for safety and risk assessment, meaning whether it would be possible or not to evacuate the ship in a specific scenario and the possibility of loss of life. In this respect, and notwithstanding the impact of many other factors, model tests or time-domain simulations could be used to provide answers to this question. However, even in this case, dealing with irregular waves, entails that both approaches are affected by the random nature of phase spectral components, which leads to a different time to capsize determination at each run/simulation or to the identification of cases where the vessel is not capsizing in the given time window. Here, a dedicated study is provided to describe the time to capsize in irregular waves for critical damages. Simulations performed on a passenger ship at different wave heights and different metacentric heights highlights the appearance of more than one capsize mode for the same damage case. A model based on Mixed-Weibull distributions has been developed to describe the multi-modal behaviour of the time to capsize distributions for the analysed damage cases.