COMPUTATION OF SECOND-ORDER WAVE LOADS ON FLOATING OFFSHORE WIND TURBINE PLATFORMS IN BI-CHROMATIC BI-DIRECTIONAL WAVES USING OPEN-SOURCE POTENTIAL FLOW SOLVER NEMOH

Second-order wave loads on a floating structure may excite resonance phenomena, such as slow-drift or springing, at the natural frequencies of the floating structure.  Therefore, taking into account the response of a floating offshore wind turbine (FOWT) platform under non-linear wave loads is of paramount importance for its design.This paper presents computation results of the quadratic transfer functions (QTFs) for sum- and difference-frequencies wave loads on FOWT platforms using open-source potential flow, boundary element solver NEMOH. The QTFs are composed of quadratic terms and second-order potential terms. The quadratic terms are computed based on the near-field formulation, while the computation of potential terms is based on the indirect method that uses the Green-formulation and an assisting function, which is the normalized radiation potential. In this paper, we show the NEMOH QTFs computation on two FOWT platforms, the OC4-DeepCwind submersible platform and the SOFTWIND spar-type platform. The results are compared with the reference commercial software HYDROSTAR. Excellent agreement between the two QTFs results is achieved. The focus is put on bi-directional difference-frequency of surge, heave, and pitch QTFs and on sum-frequency full QTFs.