Analytical Study on Multi-Hazard Risk of Offshore Wind Turbine Subjected to Hydrodynamic and Aerodynamic Loads

Seo, Junwon (South Dakota State University) | Pokhrel, Jharna (South Dakota State University)



This paper presents an analytical study on the risk and vulnerability assessment of an Offshore Wind Turbine (OWT) subjected to coupled hydrodynamic and aerodynamic loads. The Computer Aided Engineering (CAE) tool FAST v8 simulator, developed by National Renewable Energy Laboratory (NREL), is used for the multi-hazard simulation of a “NREL offshore 5-MW baseline wind turbine”. FAST is able to incorporate non-linearity coupled with both hydro and aero dynamic effects resulting from wind-and-wave loading scenarios. Site characteristics of the OWT are considered based on Nantucket Sound, Massachusetts, the United States, which is an ideal site for a future U.S. wind farm. The target site that belongs to the east coast is regarded to be a more hurricane-prone region; thus, this paper utilizes an extreme turbulent model (ETM) coupled with irregular waves determined based on Pierson-Moskowitz spectrum. The OWT supported by a fixed-bottom foundation is modeled with multi-degree-of-freedom modules enabling the time-domain coupled analysis. The OWT is simulated, considering the extreme loading scenarios specified by the International Electrotechnical Commission (IEC 61400-3) design standard that takes variability of both wind and waves into consideration. Structural responses of the OWT subjected to coupled wind and wave loads are captured at various critical locations across the overall system, and the flexural demands of the OWT at the mudline are found to be critical in evaluating its failure mechanism. Peak flexural demand quantities are then employed for the development of vulnerability functions for variations in wind and wave characteristics, including wind speed and wave height. The limit state function pertaining to flexural failure mode used for the vulnerability determination is based on First Order Reliability Method (FORM). The analysis of the resulting vulnerability data reveals that the exceeding probability increases due to increase in both wind speed and wave height, especially beyond 12m/s, while the wave height has less impact on the probability than the wind speed until the wave height of 10 m is reached.