Dynamic response analysis of an offshore wind turbine under seismic loads and vibration-induced shutdown

There are versatile design scenarios for load assessment of an offshore wind turbine. Under an accidental scenario of earthquakes, extreme structural loads may occur during an emergency shutdown characterized by fast blade pitching. The present study investigates the effect of sensor thresholds on dynamic responses during emergency shutdowns. Nacelle accelerometers in fore-aft and side-side directions are assumed to be the sensors triggering shutdown and two thresholds (1 m/s2 and 2 m/s2) are considered. Fully coupled numerical simulations are carried out for a 5-megawatt offshore wind turbine under wind, waves, and an earthquake. Dynamic characteristics of the wind turbine are compared for the shutdown scenario and the reference scenario of normal operation during the earthquake. Observations are discussed using spectral plots and load ratios evaluated for time segments before threshold crossing (detection delay) and shutdown duration. Results demonstrate that code guidelines for implementing a shutdown during the strong ground motion part of the earthquake may not be physically pragmatic and consistent across possible shutdown scenarios. The tower-top loads under the shutdown scenario can be approximately 1.7 times higher than those under the reference scenario. Results indicate that the nacelle fore-aft sensor with the lower threshold has the lowest structural loads. It is recommended to use the sensor in the side-side direction to trigger shutdown for early detection of seismic behavior for adequate decoupling of maximum seismic loads and the shutdown procedure.