Numerical investigation of a semi-submersible platform interacting with steep waves using a fully nonlinear method

The nonlinear interaction of waves with multiple bodies is one of the important factors considered in the safety structural design of the semi-submersible platform. A fully nonlinear method is proposed in this paper for simulating nonlinear interaction between steep waves and a semi-submersible platform. The method involves constructing a definite solution for the disturbed potential by separating the incident and scattering potentials using 3D potential flow theory. Using the boundary element method, several auxiliary functions are introduced for decoupling motion of semi-submersible platform from wave motion and nonlinear boundary conditions on free surface are adopted. Two schemes for tracking instantaneous free surface were evaluated and a multidomain structured mesh automatically generation scheme was designed. The present method is validated for a cylinder and a semi-submersible platform, motion responses and wave loads of a semi-submersible are investigated for varying wave steepness. The evolution of the scattered wave field at typical moments and the location of local elevation peaks in each wave case are also discussed. Though viscous effect cannot be considered, the present method can simulate large deformation of free surface and solve nonlinear issues of multibody structures with complex shape in steep waves efficiently.