Combined Earthquake & Wave action on Offshore Wind Turbine Monopile Foundation

Abstract

The offshore wind industry has been extended over the last years in areas of active seismicity, such as East Asia, where the design of offshore wind turbines becomes significantly challenging, because albeit aerodynamic and hydrodynamic loads mainly act on the offshore structures, earthquake could emerge as a potentially enormous threat. The demand for reliable and economical design of offshore wind turbine foundations has driven the research for analysis of the structural behaviour under the combined action of loads and the study of the parameters that could influence it. The present master thesis deals with the dynamic analysis of the response of an offshore wind turbine monopile, one of the most common types of foundations, subjected to the application of hydrodynamic and earthquake loads. This study focuses on the understanding of the dynamic properties contributing to the dissipation of energy experienced by the structure. More specifically, the sources of damping leading to reduction of the structural vibration in time are investigated, of which the numerical determination is considerably uncertain, while emphasizing on the hydrodynamic and the soil damping. A numerical approach for the estimation of the hydrodynamic viscous damping is presented based on the calculation of the drag coefficient CD and its dependency on the Reynolds number (Re), the Keulegan-Carpenter number (KC) and the surface roughness (k/D). The drag coefficient, and accordingly the hydrodynamic viscous damping, are derived over the length of the monopile where the waves act, highlighting also the consequences of the changes in diameter and depth. Furthermore, the soil radiation damping due to the seismic waves is studied by including the interaction of the soil with the structure. Particularly, the supporting soil is modelled around the monopile with frequency-dependent springs and dampers to represent the soil stiffness and damping, respectively. The estimation of the soil coefficients is accomplished by integrating in the model of the structure, an advanced soil model developed by Dr. J. De Oliveira Barbosa, which gives the dynamic impedance function for the desired band of frequencies. The analysis of the structural response is executed by examining three load cases for the hydrodynamic and earthquake loads. The overall outcome reveals that a noticeable amount of energy is dissipated because of the presence of the soil radiation damping, drawing also the conclusion that the soil-structure interaction should be considered as frequency-dependent during earthquake. Despite the fact that the approach for the estimation of the hydrodynamic viscous damping constitutes a more precise method, its participation in the specific tested cases is limited to the total amount of damping.