Numerical Study On Hydrodynamic Performance And Motion Response Of Shallow Water FPSO With Soft Yoke SPM System

Nowadays, the wide-spread application of FPSOs in shallow water and the booming LNG market have brought back the focus on the shallow water hydrodynamics of floating bodies. Through the filed observation or the model test, there are some phenomena should be taken attention to, including severe surge response, larger amplitude of vertical motion, touch down and the collision with the yoke of the FPSO or the LNG carrier in the shallow water, which make the hull unstable and mooring systems danger. However,the results of the simulation can't agree well with the model test's. So that, the motion response characteristic of the FPSO in shallow water and the coupled effect between it and the soft yoke mooring system should be researched.The FPSO of soft yoke single point mooring system is taken to be the research object in this paper. The hydrodynamic coefficients and the first-order wave transfer function are gained in the frequency domain. The results reflect that with the decrease of the ratio of water depth to draft, added mass and damping present to increase, and the natural period of the FPSO become larger. The concept of multi-body is applied to model the whole FPSO system, instead of simulating the soft yoke SPM with nonlinear stiffness. The investigations are conducted on the characteristics of FPSO surge, vertical plane motion response and large yaw displacement that in the shallow water. And the study of second-order difference wave force is one of main content of the dissertation.The second-order surge wave transfer function obtained through the method of Pinkster approximation will be greater in the shallow water compared to the deep water. The spectrum density of difference frequency wave force is conducted, the results show that the density become larger with the increasing of the spectrum peak period. Compare the QTF matrix got by the Newman approximation to the Full QTF matrix above, the great difference is founded in the condition of shallow water. The performance of Newman approximation to calculate the surge wave QTF is relates to the water depth, wave spectrum and the surge natural period. It could not work well when the water depth is ultra-small or the spectrum peak period is large.Based on the multi-body dynamic method, the coupled analysis of the FPSO and its soft yoke SPM system is carried out in the time domain. The characteristics of FPSO vertical plane motion is also been analyzed. Be different to the condition of deep water, the contribution of the low frequency wave force to the heave and pitch response could not be neglected. However, which one of the low-frequency and the wave-frequency force playing a leading role to the motion is related to the spectrum peak period, significant height and motion natural period. There is no universal law of the vertical plane motion, and it must be analyzed by case to case.The numerical static offset and free-decay tests of calm water, in current and in wind are executed. The surge motion of FPSO is generally weakly damped and has long natural period. The amplitude of surge motion increase as the ratio of water depth to draft does, and the growth of the second wave potential is the greatest reason of the phenomena. Sensitivity analysis of the yaw-rate drag force is also conducted. The results tell us the yaw-rate drag force has certain effect to the system motion only when the yaw motion amplitude reaches to 27°。It exists to provide system damping effect, make the system movement easer and mooring force reduction. It would have conventional results when the yaw-rate drag force is not considered.By the research of the second-order wave force, it is helpful to get the accurate results in the numerical analysis. And the study of the characteristics of the FPSO response motion in the shallow will be useful for the design and development of the FPSOs, the LNG terminals and their mooring system. Furthermore, the multi-body dynamic method to simulate the mooring system should be approved and improved, in order to become an effective means to predict the FPSO motion and mooring force. The theoretical and practical significance of this paper is great.