Investigation On Fully Nonlinear Method For Simulating FPSO Platform Motion And Load Response

For FPSO working in deep sea,a more accurate method of calculating environmental load is needed to simulate the influence of frequent bad sea conditions on FPSO.In this paper,a numerical method for calculating the motion and load response of FPSO platform is studied.This method can simulate the wave load on FPSO platform,the tension response of mooring system and the motion response of FPSO under the combined action of wind,waves and currents.The main research contents of this paper are as follows:1)A fully nonlinear method based on three-dimensional potential flow theory is established for the wave load on the platform body.In this method,the incident potential and disturbance potential are separated.The incident potential is given by plane waves,and the disturbance potential is solved by Rankine source boundary element method by considering both the nonlinearity of free surface and the nonlinearity of object surface.The auxiliary function is introduced to decouple the motion and load in the rigid body motion equation,and the time domain update of the calculation model is realized by the fourth-order Runge Kutta method.2)The convergence of the fully nonlinear wave load calculation method in terms of calculation grid and time step is verified,and the accuracy is verified by comparing the motion and load results of this method with those of commercial software in regular waves.Two strong nonlinear effects,slamming load in large amplitude waves and wave exceeding deck,are analyzed by this method.Then the method is compared with commercial software in irregular waves,and the load characteristics of vertical bending moment on FPSO in irregular waves are analyzed.3)The mooring force on the hull is obtained by using catenary theory to simulate the change of mooring cable,which is combined into the motion equation by using asynchronous coupling method.Compared with experimental results in regular waves and commercial software in irregular waves,the accuracy of calculation is verified.4)The wind and current loads suffered by the platform are calculated according to the test data or API specifications and combined with the motion equation through asynchronous coupling.The motion and load response of FPSO platform under different wind,wave and current combination conditions are calculated and compared with commercial software.The characteristics of mooring cable tension under different severe sea conditions are analyzed,and then the heading analysis under different working conditions is made.