Analysis of platform motions effect on the fatigue loads and aerodynamic unsteadiness in floating offshore wind turbines

Floating platform motions introduce an additional level of complexity to the analysis models for floating offshore wind turbines. A series of simulations of the NREL 5MW wind turbine on a semi-submersible platform, in constrained (fixed) and free-to-move configurations, were done in OpenFAST software. Simulated time histories show that the platform motions increase the force and bending moment at the blade root by 3% and 7.4% respectively, while increase the same quantities at the tower base by 61.3% and 59.4%. The rainflow counting method was used to calculate the cyclic load and fatigue at the blade root and the tower base. The results show that the platform motions result in higher fatigue on the tower base in both design load cases, power production and parked. Platform motions show a lesser effect on the fatigue at blades root while the wind turbine is operational and negligible effect while idling. Understanding the relation between platform motions and the differences in loads and consequently the fatigue can provide better tools for wind turbine design.