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

摘要

This paper presents an analytical study on the risk andvulnerability assessment of an Offshore Wind Turbine (OWT)subjected to coupled hydrodynamic and aerodynamic loads. TheComputer 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 nonlinearitycoupled with both hydro and aero dynamic effectsresulting from wind-and-wave loading scenarios. Sitecharacteristics of the OWT are considered based on NantucketSound, Massachusetts, the United States, which is an ideal sitefor a future U.S. wind farm. The target site that belongs to theeast coast is regarded to be a more hurricane-prone region; thus,this paper utilizes an extreme turbulent model (ETM) coupledwith irregular waves determined based on Pierson-Moskowitzspectrum. The OWT supported by a fixed-bottom foundation ismodeled with multi-degree-of-freedom modules enabling thetime-domain coupled analysis. The OWT is simulated,considering the extreme loading scenarios specified by theInternational Electrotechnical Commission (IEC 61400-3)design standard that takes variability of both wind and wavesinto consideration. Structural responses of the OWT subjected tocoupled wind and wave loads are captured at various criticallocations across the overall system, and the flexural demands ofthe OWT at the mudline are found to be critical in evaluating itsfailure mechanism. Peak flexural demand quantities are thenemployed for the development of vulnerability functions forvariations in wind and wave characteristics, including windspeed and wave height. The limit state function pertaining toflexural failure mode used for the vulnerability determination isbased on First Order Reliability Method (FORM). The analysisof the resulting vulnerability data reveals that the exceedingprobability increases due to increase in both wind speed andwave height, especially beyond 12m/s, while the wave heighthas less impact on the probability than the wind speed until thewave height of 10 m is reached.