The Effect of Yaw Error on the Mooring Systems of Floating Offshore Wind Turbines in Extreme Weather Conditions

Abstract

As floating offshore wind turbines (FOWTs) become the most viable option for wind farms in deeper waters, it is important to investigate their dynamic response in inclement conditions when failures, such as yaw misalignment, are more likely to occur. This research uses hour-long simulations in FAST, software developed by The National Renewable Energy Lab (NREL), to analyze the effect of yaw error on anchor tensions and platform displacements in both a traditional single-line wind farm geometry, where each anchor is connected to one turbine, and an optimum multiline anchor geometry, where each anchor is connected to three turbines. NREL’s 5 MW reference turbine on a semi-submersible base is analyzed using six realizations of each combination of co-directional wind and waves, wind speed and yaw error; resulting in 2,484 simulations in total. The variability in platform displacements and mooring forces increases as wind speed increases, and as yaw errors approach critical values. The angle of incidence of the co-directional wind and waves dictates which anchor experiences the most tension for both the single-line and multiline concepts. In the multiline geometry, the greatest increases in anchor tension occurs when the downwind turbine has yaw error. Yaw error increases the maximum anchor tension by up to 43% in the single-line geometry and up to 37% in the multiline geometry. In the multiline geometry, yaw error causes the direction of the resultant anchor force to vary by up to 20°. These changes in anchor tension magnitudes and directions are governed by the platform displacements, and are a direct result of the differences in the tangential and normal coefficients of drag of the turbine blades. When designing floating offshore wind farms, the influence of yaw error on loading magnitudes and directions are to be considered when determining the necessary capacities and calculating the corresponding reliabilities for wind turbine components.