Abstract
While most existing modeling and analysis of floating wind turbines (FWTs) considers isolated systems, interactions among multiple FWTs arranged in an array have received little attention. In this study, two 10 MW semi-submersible FWTs, separated by 8 rotor diameters (D) in the wind direction, are simulated with an ambient wind speed of 10 m/s and in moderate wave conditions using FAST. Farm to investigate the effects of wakes on global responses. Synthetic inflow is generated using three methods: the Kaimal turbulence model, 1) without and 2) with spatial coherence in the lateral and vertical velocity components, and 3) the Mann turbulence model (where spatial coherence in all three dimensions is inherent to the model). The first method results in negligible wake meandering, a relatively uniform wake deficit, while the second and third methods result in meandering of the upstream turbine’s lateral wake center at the downstream turbine’s rotor plane of up to approximately 1D and 1.5D, respectively. The slow meandering behavior of the upstream turbine’s wake resulted in increased low-frequency platform motions for the downstream turbine. Yaw motions were especially susceptible to wake meandering as the standard deviation of the downstream turbine’s yaw motion increased by 28.0 % for the second method and 11.3 % for the third method. Increased low-frequency response in structural loading was also observed. Wake effects led to between 2 % and 30 % greater fatigue damage at the top of the tower for all three methods and at the base of the tower for the second method. However, other results were found to be sensitive to the blade-passing frequency.
Highlights
Floating wind turbines (FWTs) represent the generation of offshore wind turbines
The standard deviation in surge and pitch increases for FWT2 compared to FWT1 between 5 % and 8 % for the low-frequency movement in the meandering wake results in increased lowfrequency response in the platform motions
The second method, Kaimal - Coh u, v, w, has spatial coherence in the longitudinal, lateral, and vertical velocity components which induces meandering of the lateral wake center with a standard deviation of 0.6D at the downstream turbine’s rotor plane (8D)
Summary
Floating wind turbines (FWTs) represent the generation of offshore wind turbines. Since bottom-fixed offshore wind turbines are only economically viable in shallow to intermediate water depths, FWTs will be used to harness the wind resource in deep water. The modeling of FWT concepts, such as semi-submersibles, spars, and tension-leg platforms, has progressed significantly in recent years. Most state-of-the-art analysis has been conducted for single systems without consideration of the wake interaction from nearby FWTs. The first floating wind farm, Hywind Scotland, began producing electricity in October 2017 [1] and as more floating wind farms are being developed, understanding the effects of wakes on the motions and loading of FWTs may have a significant impact on farm layout and both wind turbine and farm control systems. Wind turbine operation induces a downstream decrease in wind speed and increase in turbulence intensity. Wind turbines that are subjected to the wake of upstream wind turbines
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