Evaluation of Mooring Tension Reduction Ratio of Load Reduction Device Using HydroQus

Recently, various load reduction devices (LRDs) have been proposed as a means to reduce the mooring system costs of floating offshore wind turbines (FOWTs). The main feature of these devices is to implement peak shaving of mooring tension through a no-load stiffness region, and the purpose of this study is to evaluate the peak shaving capability of the LRD proposed by Company D. The FOWT used in the study was a 15MW FOWT consisting of a semi-submersible substructure with three catenary mooring lines. The study was divided into cases where the mooring load reduction device was modeled as a spring element with nonlinear stiffness and cases where the LRD structure was directly modeled. For both cases, Abaqus’s hydrodynamic plug-in HydroQus was used. Load analysis was performed for the design load case (DLC6.1) corresponding to the ultimate limit state to derive the tension history at the fairlead. A significant difference in tension reduction ratio was observed between the spring model and the direct LRD model. It was confirmed that the nonlinear stiffness of the spring model could have a large difference from the actual nonlinear stiffness of the LRD.