Dynamic Response Of Monopile Offshore Wind Turbine Subjected To Wave And Earthquake Joint Action

While the wind energy is exploited in the whole world, offshore wind turbines have entered their fast development track since the new century. Compared with land-based wind turbines, offshore wind turbines suffer different environment loads such as wind, waves, currents, sea ice, tide and even earthquakes. Therefore, the operating condition of offshore wind turbines in their lifetime is more severe. Wind turbines in offshore regions of active seismicity are under a potential threat caused by the joint action of a strong earthquake and a moderate wave condition. So far, there have been only very limited research works addressing the behavior of offshore wind turbines under this action, and the works based on experimental studies are rarely carried out. In this paper, the NREL-5MW monopile offshore wind turbine is used as the prototype to do the following studies:(1) Theories of seismic response of single DOF system and muti-DOFs system are briefly reviewed. The response-spectrum method and numerical integration methods in earthquake engineering are also briefed.(2) Wave theories and applicability of each theory are addressed in this paper. The-wave load on slender monopole foundation structures adopts the known Morison equation. The governing equations for rigid and flexible piles under the joint action of waves and earthquakes are established.(3) The similitude laws for the shake table tests are presented. Based on the Elasticity-Froude similitude law, and taking NREL-5MW monopile offshore wind turbine as the prototype, a structural model with scale 1:30 is designed and built. A series of model tests are carried out and the acceleration, stress and displacement data are measured.(4) Based on the theory of response-spectrum, particular interest has been in the nacelle acceleration and the important structural dynamic amplification factor (DAF). Results demonstrate that, because of the coupling effect of wave and earthquake, the structural response subject to the joint action is not equal to the linear superposition of the response to each single action. The joint earthquake and wave action is important for the proper evaluation of structural response. Ignoring the effect of wave action in seismic analysis will underestimate the stochastic structural response. (5) ANSYS software is used to establish the numerical model of the scaled NREL-5MW offshore wind turbine and for verification purpose the structural response is calculated. The numerical results are compared well with the experimental results.