Capturing 3D Continuum Effects in a 1D Soil-Structure Interaction Model for Seismic Analysis of Offshore Wind Turbines

Abstract

Offshore wind energy is considered a necessary energy resource, that may stimulate the transition from fossil fuels. Following the successful development in Western Europe, offshore wind is quickly gaining momentum in the Asia-Pacific region. At variance with North-Sea based offshore wind turbines (OWTs), structures installed in the Asia-Pacific region are exposed to a high risk of strong earthquakes. To reasonably manage these risks a solid understanding of the physical process of seismic loading is required. Moreover, accurate and effective design procedures to account for this complicated type of loading, need to be developed. Considering that the response of OWTs to earthquakes is affected by the the interaction with the soil, the current thesis is aimed at providing a modelling method to accurately account for the effects of soil-structure interaction in the seismic design of offshore wind turbines. More specifically, the complicated load transferring mechanisms between the soil continuum and the most often applied monopile foundation are addressed. The accuracy of the currently applied design methods is questioned. The uncoupled lateral springs as used in these methods cannot capture the non-local reaction of the soil towards the rigid monopile. Moreover, these methods do not account for the effects of seismic wave diffraction as they use free-field ground motion to introduce the seismic action. For these reasons, the currently applied methods may provide inaccurate estimations of the seismic loads. Hence, more accurate modelling approaches are required. In establishing the modelling method, it is suggested to benefit from the accuracy of a 3D model as it automatically captures the complicated 3D soil-structure interaction mechanisms during earthquake loading. For this reason, a 3D finite element model is provided that simulates the seismic loading of a monopile-supported wind turbine. The 3D modelling approach is however computationally too expensive to replace the simple, 1D models used in the design of offshore wind turbine structures. Therefore, to combine the speed and simplicity of a 1D model with the accuracy of the 3D model, the current thesis presents a method to obtain a 1D effective model that mimics the 3D modelled response. In establishing the effective modelling approach, the 3D model is not only used as a target solution. The 3D model is directly employed to capture the 3D soil continuum reaction and the seismic excitation loads acting on the monopile. These components are incorporated into an effective model by making use of the substructuring method of analysis. To extract the 3D reactions of the soil, the non-local method of Versteijlen \cite{Versteijlen17} is used. The soil stiffness matrices obtained by this method are integrated into a 1D beam model. The ground motion required to introduce the seismic action into this 1D model is determined in a separate step; the ground response analysis. This analysis is performed using a 3D model of the soil subsystem, that incorporates an excavation at the location of the embedded pile. This cavity is included to account for the effects of wave diffraction.\\ To assess, the performance of the provided modelling approach, a comparative study is performed between the 3D soil-structure model and the 1D effective model. This study showed that the 1D pile response closely matches the response of the 3D model - for both horizontal and vertical earthquake motion. Hence, it is proven that the developed design method effectively combines the accuracy of a 3D model with the simplicity of a 1D model.\\ Furthermore, the effective modelling approach is applied to assess the influence of 3D continuum soil-structure interaction effects on the structural response to earthquakes. These analyses indicate that the diffracted component of the seismic wave field does not significantly affect the earthquake excitation load acting on the monopile. As a results, the seismic wave diffraction can safely be neglected. This makes it possible to use free-field ground motion to introduce the seismic action into the effective 1D model. Moreover, the frequency dependent characteristics of the soil are evaluated - which are associated with geometric damping and inertial effects. An initial study on the influence of this soil frequency dependence, showed that the geometric damping results in a considerably reduced structural response for high frequencies. The effect of the soil inertia forces on the response to earthquakes is limited.