Seakeeping - A Macroscopic View of Fluid-Structure Interaction

Abstract: Seakeeping is the term used to describe the performance of ships in a seaway. However, it is usually thought of as a process involving small perturbations about an "ordered" mean speed and "ordered" heading. In extreme seas, there can be a significant decrease in the mean speed relative to the "ordered" speed and the ship's heading will vary significantly from the ordered heading, and in some cases, the ship's mean heading will not even be close to the "ordered" heading. This hydrodynamic problem is better characterized as the maneuvering in waves hydrodynamics problem rather than as the traditional seakeeping hydrodynamics problem. Statistically, one can characterize ship responses as composed of non-rare events and rare events. However, one must keep in mind that both rare and non-rare events are stochastic - the responses of a vessel to random excitation. As such, one is generally dealing with zero mean processes, where one wishes to characterize the magnitude of the responses, including a measure of the confidence bands of these magnitudes. This requires the use of statistical methods outside of the usual mean and standard deviation normally considered by scientists and engineers. For ship responses to wave excitation, the usual metric used to characterize the non-rare responses is the single significant amplitude of the motions. That is twice the usual standard deviation of the motions and the confidence bands are calculated based on the variance of the variance.