Consideration of Hurricanes and Tropical Cyclones in the Design of Offshore Wind Turbines

Variability in extreme loads is greater in areas in which hurricanes or tropical cyclones are likely. Therefore, it is expected that design standards for offshore structures in such areas should prescribe different load amplification factors if a uniform structural reliability is to be achieved at all locations. In this paper we examine and evaluate the adaptation of methods within the American Petroleum Institutions Recommended Practice (API RP 2A) that considers loads from hurricane conditions for the design of offshore structures for possible use in the International Electrotechnical Commission’s standard, IEC 61400-3. Specifically, we address a method, termed the “Robustness Requirement” wherein load factors vary with both location and structural type and are calculated based on the ratio between a load effect calculated for design-level external environmental conditions and a load effect calculated for more extreme conditions. The impact of this method is assessed by comparing estimates for the weight of an offshore wind turbine designed based on this method with those based on the current edition of IEC 61400-3. Studies are carried out on the 5 MW offshore baseline wind turbine, developed by the National Renewable Energy Laboratory, and modeled using the United States National Renewable Energy Laboratory’s aeroelastic simulation code, FAST.