Turbulent wind at offshore sites is known as the main cause for fatigue on offshore wind turbine components. Numerical simulations are commonly used to predict the loads and motions of floating offshore wind turbines; however, the definition of representative wind input conditions is necessary. In this study, the load and motion responses of a spar-type Offshore Code Comparison Collaboration (OC3) wind turbine under different turbulent wind conditions is studied and investigated by using SIMO-Riflex in Simulation Workbench for Marine Applications (SIMA) workbench. Using the two spectral models given in the International Electrotechnical Commission (IEC) standards, it is found that a lower wind lateral coherence under neutral atmospheric stability conditions results in an up to 27% higher tower base side–side bending moment and a 20% higher tower top torsional moment. Comparing different atmospheric stability conditions simulated using a spectral model based on FINO1 wind data measurement, the highest turbulent energy content under very unstable conditions yields a 26% higher tower base side–side bending moment and a 27% higher tower top torsional moment than neutral conditions, which have the lowest turbulent energy content and turbulent intensity. The yaw-mode of the OC3 wind turbine is found to be the most influenced component by assessing variations in both the lateral coherence and the atmospheric stability conditions.

Even though it is widely known that unstable atmospheric stability conditions can lead to higher turbulence, the use of proper turbulent wind models considering unstable conditions are not often used in the simulation of loads and motions of offshore wind turbines. For this reason, the Højstrup model, which was specifically developed for unstable conditions, is used to simulate a spar-buoy offshore wind turbine (OWT) and investigate the importance of unstable conditions in the design of floating offshore wind turbines. It is found that fatigue damage of a spar-buoy OWT is strongly influenced by unstable conditions, where very unstable condition gives 65% higher fatigue damage than neutral conditions for the tower top torsion, followed by 37% higher for tower base side-side bending and 24% higher for blade root flap-wise mode.

Abstract. Turbulence spectral characteristics for various atmospheric stratifications are studied using the observations from an offshore mast at Vindeby wind farm. Measurement data at 6 m, 18 m and 45 m above the mean sea level are considered. At the lowest height, the normalized power spectral densities of the velocity components show deviations from Monin-Obukhov similarity theory (MOST). A significant co-coherence at the wave spectral peak frequency between the vertical velocity component and the velocity of the sea surface is observed, but only when the significant wave heights exceed 0.9 m. The turbulence spectra at 18 m generally follow MOST and are consistent with the empirical spectra established on the FINO1 offshore platform from an earlier study. The data at 45 m is associated with a high-frequency measurement noise which limits its analysis to strong wind conditions only. The estimated co-coherence of the along-wind component under near-neutral atmosphere matches remarkably well with those at FINO1. The turbulence characteristics estimated from the present dataset are valuable to better understand the structure of turbulence in the marine atmospheric boundary layer and are relevant for load estimations of offshore wind turbines. Yet, a direct application of the results to other offshore or coastal sites should be exercised with caution, since the dataset is collected in shallow waters and at heights lower than the hub height of the current and the future state-of-the-art offshore wind turbines.

Previous research on the OC3 spar floating offshore wind turbine (FOWT) has shown the sensitivity of the yaw and side-side modes’ load and motion responses to different atmospheric conditions. Using the same baseline turbine of the OC3 spar wind turbine for a semisubmersible floater (OC4), this study investigates the load and motion responses of such offshore wind turbine for neutral and unstable atmospheric conditions. The effect of different levels of wind spatial coherence associated with two different wind spectral models for neutral conditions (Kaimal and Mann) are studied for the same turbulence intensity levels. An increase of 18% in the tower torsional moment fatigue damage equivalent load (DEL) is observed for the wind inflow with the weakest coherence (Mann spectral model), compared to the DELs under turbulent wind inflow with the highest coherence (Kaimal spectral model). Unstable atmospheric conditions are also simulated based on the Pointed-Blunt spectral model derived from FINO1 wind measurement. The yaw mode of the semisubmersible wind turbine is found to be the response component most affected by the variation in atmospheric stability conditions. A 28% higher fatigue DEL for the tower torsional moment is observed for very unstable atmosphere than the DELs under neutral atmospheric conditions.

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