Seismic Response of Wind Turbines: Time domain simulations, including SSI
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In this study, a numerical model of a 5MW offshore wind turbine on a monopile foundation was created in order to calculate the dynamic response of the structure including soil-structure interaction. The main focus was to develop a reliable numerical model of the coupled system - including the tower, monopile foundation and the surrounding soil. The wind turbine was subjected to earthquake load in the time-domain. The global response of the wind turbine was compared for the two prevalent methods of analyzing soil-structure interaction: The direct method and the substructure approach (later referred to as the three-step method). Through several validation steps, the author has gained a thorough insight of the problem and confidence that the numerical model is able to represent the correct dynamic response of an offshore wind turbine.First, a numerical analysis of a real scale 65kW wind turbine subjected to earthquake load, was conducted. Data from the real scale shake table experiment were used to verify the numerical results of a wind turbine with the same structural properties and earthquake loading. The response-histories from the numerical model matched the corresponding response-histories of the experiment. Secondly, the amplification function of a soil layer with depth, density and shear wave velocity were found in Abaqus. The numerical solutions reflected a near perfect correlation of the theoretical transfer functions. which implies that the correct boundary conditions are applied at the soil.Important findings from this study are:With the appropriate boundary conditions, Abaqus matches the theoretical amplification function perfectly.The analysis of a 65kW reference turbine showed that the simplication of the tower, blades and rotor as a lumped mass in the tower top yield satisfactory results.There are some weaknesses of the numerical model: 1) The pile and soil are assumed to be in full contact during the dynamic analysis. In reality the contact is nonlinear, i.e. gapping and relative sliding will occur between the soil and the steel elements. 2) There are also some uncertainties related to the boundaries of the soil volume. If the soil volume is to small, the waves propagating through the soil may be reflected at the boundaries and cause disturbance in the soil and give non-physical results. Modeling the tower with beam elements (with the cross sectional properties of the 5MW tower) has proved to represent the more detailed model of the turbine tower with linear shell elements very well, both in terms of displacements, acceleration, section forces and moments. The static stiffness of monopile in Eurocode 8 correlates well with the findings from this study. The maximum deviation of 20 % is acceptable.The full three-step method predicts the response compared to the direct method very well: The fundamental eigenfrequency is similar for the two methods. The response in terms of base shear and moment, as well as the displacements and accelerations are slightly overestimated in the three-step method. This implies that the method yields conservative results.The seismic response to the simplified three-step method (with no rotational input motion in the base) also correlates well with the direct method. However, the response is too low compared to the direct method. The computational demand is significantly reduced for the simplified three-step method, compared to the full three step method. From a parameter study of the properties of the monopile it has been shown that reducing the diameter or the wall thickness may lead to significant SSI effects on the global response of the tower, especially in terms of base shear and moment.