Impact of Hurricane Wind/Wave Misalignment on the Analyses of Fixed-Bottom Jacket Type Offshore Wind Turbine

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Virginia Tech

A high risk of hurricane is threatening the development of offshore wind energy in the east coast of the United States. Hurricane loads on an offshore wind turbine, namely wind and waves, not only exert large demands but also have rapidly changing characteristics, especially wind and wave directions. Waves are, in general, inert to rapid changes, whereas wind can change its properties within very short time scales. Misalignment of local winds and propagating ocean waves occurs regularly in a hurricane environment. It is a common practice to design monopile support structures for offshore wind turbines (OWTs) under extreme conditions by the highest wind/wave loads when they are assumed to come from the same direction. However, this co-directional wind/wave assumption can be hazardous for non-axisymmetric fixed bottom support structures for deeper water such as jackets due to their sensitive capacity to loading directions. The goal of this work is to examine the impact of wind/wave misalignment on the extreme loads and structural response under hurricanes. We select a fixed-bottom jacket type offshore wind turbine located in a water depth of 50m as the example structure. The hurricane induced wind and wave loads on the structure system are calculated from a reduced set of 1000 simulated full-track hurricane events, selected from a database of 200,000 years of simulated hurricanes, to represent the hazard of Nantucket, Massachusetts. The meteorological ocean (met-ocean) conditions and wind/wave directions for each hurricane are identified from the track data by physical models. The wind direction, wave direction at different time and location and the orientation of structure are included to capture the misalignment impact on the structural analyses over a wide range of possible engineering designs and conditions. It will let us clearly understand the impact of wind/wave misalignment on the analyses of a jacket-type support structure. Summary : This work, (1) predicts environmental conditions and directions from a reduced set of synthetic hurricane catalogue based on the analytical models; (2) calculates wind and wave forces on a jacket-type offshore wind turbine example (3) studies impact of wind/wave misalignment on the structural analyses over a wide range of possible engineering designs and conditions

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