Design loads for strength analysis of semi-submersible floating offshore wind turbines

The design wave method is widely used in the strength assessment for ships and offshore structures. The present study aims to verify if the design wave method's design loads are conservative and if the design wave method can be used in the strength assessment of the floating offshore wind turbine platform in the preliminary design phase. An OC4 5MW semi-submersible platform is analysed in the frequency domain to determine the extreme design loads implied by the design wave method. Short-term responses of the integrated system are studied based on time-domain simulations for sea states identified by the environmental contour method for a specified return period. The long-term extreme loads are estimated and compared with those of the design wave method. It has been shown that the extreme responses of the support structure determined by the design wave methods are more conservative than those determined based on time-domain simulations.