Critical Wave Groups Versus Direct Monte-Carlo Simulations for Typical Stability Failure Modes of a Container Ship

Abstract

In the second Generation Intact Stability Criteria currently developed at IMO, the process of direct stability assessment (DSA) and providing operational guidance (OG) are interlaced with a requirement of performing a large number of numerical simulations . However, extreme roll motions that are generally behind stability failures are rare events as any extreme responses. An additional significant difficulty is that roll response as stochastic process is usually non-Gaussian, therefore, close-form expressions for the probability of extreme roll responses, based on spectral moments, are in general not applicable. A practical approach proposed recently exploits the idea that extreme events occur due to the encountering of extreme wave groups (critical wave episodes). This could alleviate the need for a large number of simulations by focusing on the systematic identification of those deterministic wave sequences that generate unacceptable roll responses. Taking a first step towards a systematic validation process of the wave groups method , the present study compares the exceedance probabilities of 40° roll angle and of g/2 lateral acceleration, computed by the critical wave groups method with Monte-Carlo simulations for a large containership. The nonlinear seakeeping code rolls is used as mathematical model of ship motion. Typical loading conditions where various stability failure modes can occur are examined.