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An experimental and numerical study of wind turbine seismic behavior


This dissertation presents an experimental and numerical investigation into the seismic response of modern wind turbines. Currently, no consensus exists in the industry and there is significant interest in improving prediction of the behavior of wind turbines simultaneously subjected to wind, earthquake, and operational excitation. To this end, an experimental program was planned in order to evaluate seismic loading of wind turbines. In 2004, a preliminary shake table test of a 65-kW utility scale wind turbine was conducted that provided an experimental basis to begin the work discussed herein. A monitoring campaign was performed at Oak Creek Energy Systems in Mojave, California to assess variability of in-situ dynamic characteristics of two modern wind turbines (900-kW and 1.5-MW rated power) under different operational states and wind conditions. A second shake table experiment with a more extensive test program and improved instrumentation was executed, in which orientation of shaking and operational state were found to significantly influence response. Using the finite element program OpenSees, beam- column models of the tested specimens were constructed and calibrated. Collected data provided a basis to show that such a model could reproduce salient characteristics including natural frequencies, mode shapes, and dynamic response time histories for a parked turbine. In-situ results were used to guide construction of full turbine- foundation-soil models that provided insight into soil- structure interaction phenomena. An existing tool to simulate turbine dynamics, the FAST code, was extended to include seismic loading to allow simulation of operational turbines subjected to base shaking and validated based on shake table results. Using a calibrated model of the tested 900-kW turbine it is shown that neglecting aerodynamics results in significant over estimation of the tower bending demand. An investigation of turbines ranging from 65-kW to 5-MW concluded that consideration of aerodynamics and operational state becomes increasingly important with size. The updated FAST code was demonstrated to accurately reproduce observed dynamics of operating turbines, providing a validated tool for seismic design of turbines. These contributions clarify that operational state and orientation of shaking are important considerations and enable the development of a new generation of turbines that appropriately consider seismic loads

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