Cost-based mooring designs and a parametric study of bridles for a 15 MW spar-type floating offshore wind turbine

This paper highlights a cost-based design method for feasible and cost-effective mooring configurations. A parametric study of mooring designs considering bridle impact is performed for a 15 MW spar-type floating offshore wind turbine (FOWT). This site-specific FOWT is based on the preliminary model from EU funded H2020 Project COREWIND. Without running numerical simulations, the cost-based design method generates 1443 mooring configurations with normalized costs ranging from 1.00 to 1.667. Six configurations with the lowest normalized costs are selected for OpenFAST simulations. The ultimate test results verify the structural strength of mooring configurations. Bridle designs cause significant impact on the ultimate mooring tensions and floater motions in surge, sway and yaw, which is clearly influenced by the mass ratio of a bridle to a mooring line. When the mass ratio decreases from 13% to 5%, peak mooring tensions increase by up to 17%. The influence of bridle designs on mooring tension fatigue is less prominent and the effect of mass ratio on mooring fatigue varies with wind speed. Near the rated wind speed, deviations in mooring fatigue loads are within 5% for all six configurations. This cost-based method promotes feasible and economical mooring designs. The parametric study of bridle designs provides solid support for optimal mooring designs of spar-type FOWTs.