Atmospheric Stability Effects on a Spar-Type Floating Wind Turbine

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Simulations of a spar-type floating wind turbine supporting the DTU 10 MW reference turbine under fitted atmospheric conditions are performed. Atmospheric conditions are modelled using the turbulent models recommended by the IEC: the Mann model and the IEC Kaimal and exponential coherence model. Model parameters are fitted to measurements at the FINO-1 platform to generate turbulent boxes that are as close as possible to measurements. The simulations are carried out using OpenFAST for three mean wind speeds: 7.5, 12 and 16 m/s and the power law wind profile is used with fitted shear exponent. The motions, wind, loads spectra are quantified using the WAFO toolbox and the loads are assessed with short-term damage-equivalent loads using MLife. Depending on the wind model, global motions differ by up to 52%. Generally, Kaimal resulted in higher surge and pitch motions, and the opposite was found for the yaw motions. The influence of atmospheric stability on global motions was found to be important as well, unstable conditions giving the largest motions and stable conditions the lowest. It was observed that the wind model influenced the fairlead tension and tower top loads. Mann resulted in up to 35% higher loads on the tower top yaw moment and Kaimal resulted in up to 30% higher fairlead tension loads. Tower base fore-aft bending moment and blade root out-of-plane moment were not very sensitive to the choice of wind model. Atmospheric stability had an influence on all loads by up to 30%. Unstable conditions led to the most damage while stable conditions generally led to the least. Whether the choice of the wind model or the change in atmospheric
stability has more influence on the global motions of the platform and the loads depended on the wind speed, the degree of freedom or the load considered. It was found that wind models should always consider atmospheric conditions, not only neutral. Finally, impact of Mann turbulence model parameter variations on the wind, global motions and loads was investigated. It was found that turbulence intensity is influenced by the three parameters and has a bigger impact than the differences in coherence.