Nonlinear Numerical Simulation Of Large Amplitude Motion Of Mooring Floating Body

There are abundant oil resources in the ocean.With the continuous development of oil exploration to the deep ocean,the marine environment is even worse,and the nonlinear effects of interactions between large waves and structures are significantly enhanced.This is very detrimental to the safety of offshore engineering structures.Therefore,the nonlinear interactions between waves and structures cannot be ignored.Based on the fully nonlinear time-domain potential flow theory,a three-dimensional fully nonlinear numerical model for the interactions between waves and structures is established.And a coupled motion model of structures and mooring systems is also established.The high-order boundary element method(HOBEM)is used to solve the mixed boundary value problem in the Euler format at each moment,and the fluid particles are tracked in the Lagrange format and the free water surface boundary conditions are updated.The velocity potential separation technique is used to decompose the total velocity potential and total wave height into two parts.In order to calculate the wave force on the structures,some auxiliary functions are introduced instead of directly using the difference method to predict the time derivative of the velocity potential.Through these auxiliary functions,not only the wave force can be calculated more accurately,but also the fluid-structure coupling model can be decoupled,so that the calculation of the motion of the structures has nothing to do with the wave force.In order to eliminate the adverse effects caused by wave reflection,an artificial damping layer is placed on the far field boundary of the model.At the same time,such processing is also helpful for long-term calculations.Since the numerical model established in this paper is fully nonlinear,the shape of the free water surface and the wet surface of the structures change with time,so the spring approximation method is used to update and reconstruct the free water surface mesh,and the background mesh interpolation method is used to update and reconstruct the wet surface mesh of the structures to avoid possible numerical instability.In the establishment of the soft rigid arm mooring device model,the structural form and dynamic response characteristics of the soft rigid arm mooring device are considered.This paper firstly uses the established fully nonlinear numerical model to simulate the interactions between nonlinear waves and fixed and floating cylinders,and compares the simulation results with published literature results to verify the effectiveness and accuracy of the model.Then the effects of draught,motion status and size on the hydrodynamic characteristics of the truncated cylinder are explored.Subsequently,the hydrodynamic characteristics of a FPSO model are simulated and finds that the nonlinear effect has little effect on heave motion and greater impact on pitch motion.The amplitude of pitch motion response of FPSO in the resonance section increases significantly and the bandwidth becomes wider.Finally,the dynamic coupling motion model of the FPSO and soft rigid arm mooring device is established,and the hydrodynamic characteristics of the FPSO under the influence of the soft rigid arm mooring system are explored.It is found that when the incident wave frequency is close to the natural frequency of the hull movement,the dynamic coupling between the soft rigid arm and the FPSO can aggravate the hull movement,and when the incident wave frequency is far from the natural frequency of the hull movement,the soft rigid arm mooring device can suppress the pitching motion,but the impact on heave movement is small.