Seismic excitation of offshore wind turbines and transition piece response

Abstract

AbstractExpansion of the offshore wind industry in seismically active areas has raised concerns regarding the structural integrity of offshore wind turbines under earthquake loading. This paper details a 3D finite element study to investigate the behaviour of the structure, and in particular the transition piece (TP), under seismic loads. The work focuses on equivalent grouted connection and TP‐less designs, selected as promising design solutions for seismic zones. The numerical model is validated against a medium‐scale 4‐point bending laboratory test, and scaled up to a representative 8 MW turbine. The results show that cracking of the grout occurs due to earthquake excitation at the top of the TP; a location that is typically undamaged during monotonic loading. This can lead to excessive settlement of the transition piece and loss of axial capacity caused by deterioration of the grout‐steel bond and water ingress. The residual hub displacement after earthquake excitation is around 0.1 m. The global monotonic response post‐earthquake excitation is not significantly altered, with apparent stiffness and ultimate strength maintained. An equivalent TP‐less design is shown to have a 5% lower natural frequency than an equivalent grouted connection design, suggesting a reduced global stiffness, with reduced structural damping due to the absence of grout. However, TP‐less design eliminates the risk of grout deterioration and settlement and may therefore be a safer design option for offshore wind turbines installed in seismic zones in the future.