Seakeeping Assessment for Ships in Rough Seas

Since St. Denis and Pierson (1953) pioneered the spectral analysis for ship motions in irregular seas, linear theories have been extensively applied in the assessment of seakeeping performance of ocean-going vessels. It implies that the motion amplitudes of a vessel are proportional to the amplitude of the ambient waves. In other words, the non-dimensional motions or response amplitude operators (RAOs) are independent of wave amplitudes. Hydrodynamic analysis can then be simplified by considering the case of small amplitude waves and motions and the vessel shape under calm water surface, the result of which is generalized to large-amplitude wave conditions based on the linear assumption. In the last 20 years or so, researchers have begun to realize that linear theories are not sufficient for accurate ship motion prediction. In fact, linear theories may lead to wrong directions of hull form design for optimal seakeeping performance, especially when motions in rough seas are concerned. This paper demonstrates theoretical and experimental motion results for conventional ships in large-amplitude waves. The concept of the motion response amplitude operator (RAO) is extended and the RAOs are estimated at different wave amplitudes to assess the motion characteristics in different sea states. Discussions are provided towards the utilization of non-linear hydrodynamic motion prediction tools in the development of a methodology for the assessment of overall seakeeping performance of a vessel in different wave conditions.