Comparative Analysis of Open-Source BEM Solvers for FOWT Towing Simulation in OpenFAST

This paper describes a methodology for simulating the towing of a fully assembled DeepCwind OC4 reference Floating Offshore Wind Turbine (FOWT) using OpenFAST. In OpenFAST, the wave diffraction and radiation forces are computed based on the output of an external boundary element method (BEM) solver. To that end, a comparative analysis is performed to assess the results of four BEM solvers: i) Capytaine, ii) HAMS, iii) Nemoh and iv) OrcaWave. The latter is a closedsource commercial software, whereas the others are open-source. Firstly, the zero-speed radiation coefficients and excitation forces computed by the four different BEM solvers are compared in the frequency-domain. Then, the resulting infinite added mass and impulse response function (IRF) matrices are compared, which will serve as inputs for OpenFAST. Finally, the response amplitude operators (RAOs) for heave and pitch motions are computed from the OpenFAST results and validated against experimental data. The impact of modeling second-order wave forces is also analyzed in this paper. This study highlights the limitations, ease of use, computational time, and accuracy of the BEM solvers in simulating FOWT towing in OpenFAST.