Nonlinear Dynamic Analysis of Ship Capsizing in Random Waves

In this paper, the large amplitude nonlinear rolling motion and capsizing behavior of an offshore supply vessel hull-form is analyzed. In order to investigate the importance of damping, a dynamical perturbation technique (Vishnubhotla, Falzarano and Vakakis, 1998) is applied to the hull using various approximations to the roll damping. The offshore supply vessel (OSV) is probably one of the most common seagoing hull forms. The specific offshore supply vessel hull-form we study herein is the US Navy’s T-AGOS. This small vessel was required to operate in the severe waters of the North Atlantic during all weather conditions. The severity of the North Atlantic environment has suggested that a new analysis methodology be considered in lieu of traditional ship static stability analysis. This is due to the occurrence of extreme wind and waves and the resulting large amplitude dynamics response. As a result of non-linearities inherent in extreme response, a dynamics based analysis procedure must be used In this work, our previously developed dynamical perturbation technique (Vishnubhotla, Falzarano and Vakakis, 1998) is applied to study the large amplitude nonlinear rolling motion of an offshore supply vessel (OSV). This approach makes use of a closed form analytic solution which is exact up to the first order of randomness, and takes into account the unperturbed (no forcing or damping) global dynamics. The result of this is that very large amplitude nonlinear vessel motion in a random seaway can be analyzed with similar techniques used to analyze nonlinear vessel motions in a regular (periodic) seaway. The practical result is that dynamic capsizing studies can be undertaken considering the true randomness of the design seaway. The capsize risk associated with operation in a given sea spectra can be evaluated during the design stage or when an operating area change is being considered. Moreover, this technique can also be used to guide physical model tests or computer simulation studies to focus on critical vessel and environmental conditions which may result in dangerously large motion amplitudes.