As the moored floating structures are highly influenced by the slowly-varying horizontal motions, the research on the second-order wave forces and moments acting on the vessels is steadily in progress. Since the second-order wave forces and moments are mostly induced by the interaction of wave frequency components, a lot of computational resources are required to estimate such second-order quantities accurately. In regards to computational time, even a potential-based method is not mature enough to be used practically. Therefore, a guideline is beneficial that is able to minimize the number of meshes constituting the vessels. In this study, second-order wave forces and moments are estimated using surface meshes on a vessel generated by different distributing logics. The quadratic transfer functions are computed by the commercial program, WADAM, which solves the boundary value problems in the frequency domain. Through the analyses, a mesh refinement method is suggested that effectively assists to constitute the surface meshes to be used in the second-order computations.
The second-order wave forces and moments, having long periods, affect responses on the moored floating structures as the different types with typical responses of the wave excitation periods. These second-order wave forces are induced by the interaction of wave frequency components, and a lot of computational costs are required to assess the second-order quantities. Even if numerical programs based on potential theory in the frequency domain are used, calculating these second-order properties takes a lot of time, and has limitations to the amount of computation. Thus, it is required to find a relatively accurate solution by efficiently utilizing the number of meshes that make up the numerical model of the vessel.
For computing the mean drift forces, three types of solving methods are applied, which are near-, middle-, and far-field methods, to horizontal responses, especially. The far-field method is based on the principle of momentum conservation and uses the potential solution on a vessel (Maruo, 1960; Newman, 1967), and thus the convergence is better than the near-field method, which assesses the second-order forces through the direct computation of the pressures and velocity vectors on the surface of the vessel (Pinkster and van Oortmerssen, 1977). The middle-field method was developed by combining the near- and far-field methods, with the advantages of both ones (Chen, 2006; Rezende et al., 2007). For vertical components of mean drift forces, and difference frequency wave forces, near- and middle-field methods are only applied because the direct computation on second-order potentials is needed with free surface integral.
