Accurate evaluation of second-order wave loads in direct time-domain simulations

Accurate and efficient calculation of all second-order wave load components in six degrees of freedom (6DoF) remains a challenging task, in particular for structures with sharp edges. Based on Gauss theorem, we have tailor-made an efficient method for direct time-domain solvers utilizing, for instance, boundary element methods. Unlike other methods based on momentum conservation or Gauss theorem that require control surfaces, this method only involves integrals on the structure and free surfaces. The accuracy and efficiency of the method are firstly verified for the mean and sum-frequency wave forces on a hemisphere and a truncated vertical cylinder. Subsequently, an application to study the motion responses of a semi-submersible floating offshore wind turbine (FOWT) in focused waves is presented, where experimental results are also available. The results indicate that the proposed method can greatly improve the computational efficiency in accurately predicting second-order loads on offshore structures with sharp edges in direct time-domain solvers. For the FOWT in focused waves, this alternative method is essential in achieving accurate prediction of hydrodynamic responses near the focus time.