Seismic analysis of offshore wind turbines on bottom fixed support structuree: coupled and uncoupled analyis

Offshore wind energy is one of the most import renewable energy resources worldwide. In the last year, the growth of energy demand and the great potential of wind resources in the sea have encouraged the realization of offshore wind turbines (OWTs). The present thesis investigates the seismic response of a horizontal axis wind turbine on two bottom-fixed support structures for transitional water depths (30-60 m), a tripod and a jacket, both resting on pile foundations. Fully-coupled, non-linear time-domain simulations on full system models are carried out with the software BLADED© under combined wind-wave-earthquake loadings, for different load cases, considering fixed and flexible foundation models. A comparison with some typical design load cases given by international guidelines is implemented. Although fully-coupled non-linear time domain simulations provide the “exact” numerical solutions, they involve some significant disadvantages: (i) a dedicated software package is needed, capable of accounting for inherent interactions between aerodynamic, hydrodynamic and seismic responses; (ii) computational costs are significant, almost prohibitive when several analyses have to be implemented for different environmental states and system parameters, as in the early stages of design. For these reasons, in this thesis, the results of uncoupled analysis, obtained as linear superposition of separate wind, wave and earthquake responses with an additional damping referred to as aerodynamic damping, will be compared with fully-coupled non-linear time domain simulations. Important conclusions will be drawn on the reliability of uncoupled analyses for seismic assessment of OWTs.