Objectives  In order to improve the safety performance of marine structures, fully nonlinear numerical simulation study is carried out on the problem of wave-structure interaction.

  Methods  Based on the three-dimensional fully nonlinear time-domain potential flow theory and the Higher-Order Boundary Element Method (HOBEM), an open water model for the interaction between waves and structures is established. The total velocity potential is decomposed into incident potential and scattering potential by the velocity potential separation technique. The Mixed Euler-Lagrange (MEL) method is used to track the particle in the instantaneous free surface, and the fourth-order Runge-Kutta method is used to update the instantaneous free surface. In order to calculate wave loads, a virtual function is introduced instead of directly predicting the time derivative of the velocity potential. To prevent waves from reflecting from the far field boundary, an artificial damping layer is placed on the outside of the free surface. In addition, the free surface mesh is generated only once at the initial time, and then the spring approximation method is used to reconstruct the instantaneous free surface mesh without changing the order of mesh nodes, so as to avoid the possible numerical instability.

  Results  On the basis of validating and verifying the present numerical model, a Floating Production, Storage and Offloading (FPSO) unit is numerically simulated to study its hydrodynamics. It is found that the motion response of FPSO increases obviously in the resonance section when considering the nonlinear effect, and it is proved that the prediction of the motion of FPSO by the traditional linear method is tend to be risk.

  Conclusions  The work of this paper will provide a more reliable prediction tool for the design of the offshore floating structures and also provide the necessary theoretical basis for the practical application of the offshore floating structures.