Analysis of Hurricane Gusts and Shear to Improve Offshore Wind Turbine Design

Tropical cyclones subject wind turbines to environmental conditions that may significantly affect their loading, durability, and operation. The strongest class of wind turbines prescribed in the International Electrotechnical Commission wind turbine design standards (i.e., IEC Class I) can withstand reference wind speeds of 50 m s^-1 with 3-second gusts up to 70 m s^-1 at hub height (~100 m). These extreme wind speed limits correspond to a weak Category 2 hurricane on the Saffir-Simpson hurricane wind scale. However, the standards do not encompass Category 3, 4 and 5 hurricanes that occur in the Gulf of Mexico and south of the Carolinas.  In addition, the standards do not specifically address the unique qualities of the hurricane boundary layer (HBL), such as frequent gusts, swift directionality changes, and shear and veer across the rotor layer. Because of the paucity of observational data in these conditions, we use the Cloud Model Version I (CM1) large-eddy simulation model to simulate high spatial- (10 m) and temporal- (0.1 s) resolution data across a theoretical turbine in the HBL. This unique dataset is used to answer the following questions: do severe gusts that exceed design standards occur?; how does the gust factor vary with distance from the eye?; and lastly, how does wind direction change across the rotor layer? We find that mean winds and gusts near the eyewall can exceed current turbine design thresholds of 50 m s^-1 and 70 m s^-1, respectively. Gust factors are greatest at the eye-eyewall interface just inward of the peak gust location and can exceed the 1.4 value used to convert a 50 m s^-1 reference wind speed to a 50-year 3-second gust. Strong veer (15-30 degrees) across a ~120 m-layer suggests that veer should be assessed against standard design prescriptions. Lastly, wind directions can shift 10-25 degrees in durations shorter than 10 minutes, which can lead to turbine misalignment if the yaw system is not able to respond fast enough. The answers to these questions could help manufactures assess necessary design modifications for turbines to survive a major hurricane passage.