Experimental Study Of An Overwater Soft Yoke Moored FPSO’s Hydrodynamic Response Characteristics

Numerous advantages of floating production system of storage and offloading (FPSO) make it become a mainstream facilities in ocean oil exploitation by far. An increasing number of FPSOs and FLNGs will be moored in shallow waters with the development of our country’s offshore oil and gas exploitation. Engineering practice shows that soft yoke mooring system has apparent advantages in the hydrologic environment of shallow sea water. Its hydrodynamic response characteristics have gained more and more importance from academics.Based on a physical model experimental study, This paper analyzes motion and mooring force response of a FPSO moored by an overwater soft yoke. Natural periods of each DOF’s motion are obtained by decay tests in still water. Results show that surge motion has obvious low frequency characteristics, and its damping is much smaller than vertical motions. Results of physical model test and numerical simulation show that horizontal stiffness of the soft yoke moored system is very sensitive to influential parameters such as heights of mooring tower, drifts of ship and weights of ballast tank. By using control variable method, Sensitive analysis of motion and mooring force’s influential parameters under the joint action of wind, wave and current is conducted, which indicates that horizontal mooring force is especially sensitive to the height of mooring tower. Furthermore, the effect of wave’s period, nonlinear and low frequency characteristics on motion and mooring force response are discussed.Wave and motion’s frequency spectrum are estimated by cross-correlation function method and FFT method. The characteristics of irregular waves in shallow water are analyzed. Combined with the shallow water wave theory, the mechanism of low frequency wave’s action on each DOF’s motion response is analyzed under collinear loads. The cause of surge resonant motion is analyzed in time domain. Motion characteristics of heave and pitch is achieved by using coherent analysis method. Moreover, a summary of each DOF’s motion response characteristics is given under non-colinear loads.The characteristics of extreme motion response such as grounding of ship bottom, green water and the crash between ship bow and ballast tank are analyzed. Safety evaluation of grounding is given by the analysis of smallest gap between ship bottom and seabed. The pattern of green water on ship bow and ship stern is concluded. The law of crash is also summarized.An overwater soft yoke moored FPSO’s hydrodynamic characteristics are concluded with the analysis motion and mooring force response of under various environment loads. Related results can provide some suggestions for engineering application.