Wake Effects on Multiline Anchor Loads for Floating Offshore Wind Turbines

Abstract

Abstract This paper studies the influence of wake effects on multiline anchor mooring line forces for floating offshore wind turbines for different environmental scenarios. Multiline anchoring is a concept in which seabed anchors are shared among three adjacent turbines through their mooring lines. In this scenario, one or more adjacent turbines could be in the downstream wind direction. In an offshore wind farm, the turbines in the downstream position experience lower wind speed and higher turbulence intensity because of the wake disturbances caused by the upstream turbines. Therefore, the multiline anchor system must be evaluated for each location for a given wind farm configuration. This paper studies five NREL 5MW turbines supported by OC3 Hywind spar buoys in the Hywind Scotland wind farm layout. Wind wake effects are predominant when the turbines are operating. Therefore, the operational design load case DLC1.6 is chosen for the analysis with three different environmental loading directions. NREL open-source tool FAST is used to perform the simulations. Jensen’s wake model and Frandsen’s Turbulence intensity (TI) model are employed to calculate the waked downstream turbine parameters such as reduced wind velocity and turbulence intensity. For each of the wind wave current directions, a case with wake and without wake has been studied. The result from each study is observed as peak multiline anchor force, which is the average of the maxima from six one-hour simulations. In the case of 30° and 120° load directions, one of the downstream turbines in the multiline system is experiencing wake effects, whereas in the case of 90°, two of the turbines are experiencing wake effects. Results show that the multiline system exhibited less tension with waked wind, resulting in decreased mooring line forces compared to cases without wake.