Abstract
Successful design and maintenance of mooring systems for floating wind turbines requires a good understanding of mooring line loads. Previous work on 5-10 MW rotors suggests that the spatial coherence in the wind field is particularly important for these loads. As offshore wind turbine rotors continue to increase in size, spatial and temporal wind variations across larger scales become relevant. A preliminary sensitivity analysis is carried out based on the INO OptiFLEX 22MW semi-submersible with the IEA 22MW rotor. A semi-taut chain-rope-chain mooring system is used. Non-linear time domain simulations are performed with two standard turbulent wind fields with different spatial coherence: the Kaimal spectrum with exponential coherence, and the Mann uniform shear turbulence model fitted to the Kaimal spectrum. Mooring chain fatigue is shown to be dominated by low-frequency wind-induced platform motions, and the wind field with exponential coherence generates stronger fairlead inline motions, resulting in higher short-term fatigue damage compared to Mann turbulence. These results are consistent with those observed for smaller floating wind turbines.