A new method to predict vessel/platform critical dynamics in a realistic seaway

In this paper, a recently developed approach is used that makes use of a closed form analytic solution, which is exact up to the first order of randomness, and takes into account exactly 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 analysed with techniques similar to those used to analyse nonlinear vessel motions in a regular (periodic) seaway; the practical result being 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 spectrum 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. In order to demonstrate the practical usefulness of this approach, extensive comparative results are included. The results are in the form of solutions which lie in the stable or unstable manifolds and are then projected onto the phase plane.