The Influence of Unstable Wind Models on the Motions and Loads of a Semi-Submersible Floating Wind Turbine

Abstract

As the wind turbine industry continues to play a part in the production of energy, the need for larger and more powerful wind turbines keeps increasing. The movement towards offshore wind turbine structures provides opportunities to increase power production, but there are still adjustments that need to come to compensate with the challenges of moving into deeper waters. Floating offshore wind turbines have different structural and environmental factors when compared to onshore or even bottom-fixed offshore wind turbines. The two recommended models for generating wind profiles come from research on onshore wind turbines, with neutral atmospheric stability. In deeper waters, it is much more harmful to ignore the effects of unstable atmospheric stability conditions on the loading of a floating offshore wind turbine (FOWT). This master’s thesis will examine the influence of unstable atmospheric conditions on a FOWT. The OC4-DeepCwind semi-submersible FOWT is the model that has been chosen for this work. To generate the wind profiles, the Pointed-Blunt Model and Højstrup’s Unstable Spectra Model will be used to examine the influence of unstable wind on a FOWT. The turbulent wind fields are generated in MATLAB before analysis in SIMA. In general, it is noted that Højstup’s Unstable Spectra Model provides greater turbulence intensity values, greater energy profiles, and greater damage equivalent loads. As the atmospheric stability grows increasingly unstable, Højstrup’s Model produces a notable change in the response of the FOWT. The Pointed-Blunt Model has limited change from neutral to unstable to very unstable conditions and generates smaller responses and loads. The tower base fore-aft moment produces the greatest damage equivalent loads in both cases and is affected by both increasing wind speed and the shift from neutral to more unstable conditions. However, there can be improvements to the models and expanding of the simulations that could contribute to a more extensive study.