Investigation on Heave Motion and Load Response of a FPSO in Regular Waves by Fully Nonlinear Method

Abstract

Abstract In this paper, the full nonlinear method based on the three-dimensional potential flow theory and the dynamic analysis method of flexible components are combined to simulate the motion and load response of a FPSO in waves. On the boundary of the hull body, the coupled motions are considered in the impenetrable condition. An improved Eulerian method is adopted to trace strongly nonlinear 3-D free surface deformation. In the far field, artificial damping zone is applied to eliminate reflected wave. Rankine source method is adopted to solve the velocity potential in time domain. Hydrodynamic mesh on hull body is generated by using accumulative chord length cubic parameter spline function. After solving Poisson equation, the initial mesh on free surface becomes orthogonal. In each time step, elastic-mesh-technique (EMT) is used to optimize the mesh on free surface. Several auxiliary functions are introduced to decouple the motions and load, and then the fourth-order Runge-Kutta method is adopted to update the numerical model in time domain. For the forces of the mooring system on the hull at each time step, the dynamic equation of the flexible member is established by the dynamic analysis method based on the elastic slender rod mechanical model, and then the equation is discretized into matrix form by the finite element discretization method and solved. The coupled motion of FPSO hull and mooring system in regular waves of various frequencies in various directions is simulated, and the time domain solutions are obtained. RAO of heave motion in each wave direction is given. The load response at the midship section is analyzed.