Study of the response characteristics of a spread moored FPSO in the bidirectional sea state

Abstract

The difference frequency wave drift force may excite large resonant responses for a moored FPSO in horizontal motions, so the moored FPSO responds in both low frequency motion and wave frequency motion generally. In this study, the parameter sensitivity analysis is carried out to investigate the motion response characteristics of a spread moored FPSO in the bidirectional sea state comprised of wind sea and swell. The effects of water depth, loading condition, existence of mooring system, numerical method, sea state type, significant wave height and wave direction on the motion responses are studied in numerical simulation. The research shows that the wave frequency motion responses of FPSO are dominated by swell, and the low frequency motion responses are contributed from both wind sea and swell in the bidirectional sea state. The motion responses are proved to be affected indistinctively by the change of loading condition. The change of water depth affects the motion responses in some degrees. The responses of FPSO in roll can be enlarged by the spread mooring system, and the enlargement becomes more significant if the spread mooring system is arranged in deep water with large pretension force for the mooring lines.