Abstract
As offshore wind turbine rotor designs continue to increase in size, spatial and temporal wind variations across larger scales have increasing importance for dynamic responses. For floating wind, the platform weight is also being reduced as designers optimize their substructures to save costs. Successful design, installation and maintenance of future mooring systems require understanding how the combination of lighter floaters and larger rotors impacts mooring lines loads. Previous work on smaller rotors suggests that the spatial coherence in the wind field is particularly important for mooring line responses.
A preliminary sensitivity analysis answering the above challenge is thus proposed, based on the INO Optiflex 22MW semi-submersible with the IEA 22MW rotor. The model can consider the platform (hull) as either flexible or rigid. A semi-taut mooring system is used with polyester rope between top chain and bottom chain sections. Non-linear time domain simulation is performed with two IEC standard turbulent wind loads - Kaimal spectrum with exponential coherence, and Mann uniform shear turbulence. Results show how turbulent wind input affects the fatigue life along the mooring lines. Frequency content of the platform motions is investigated to understand what key factors are driving mooring system response, both at low frequencies where differences between these models are well-documented for smaller rotors, and also at higher frequencies related to 3p excitation, tower vibration, and platform vibration.