Seismic excitation of offshore wind turbines and transition piece response

Christopher M Hamilton, Christelle N Abadie

OAI: oai:www.repository.cam.ac.uk:1810/348130 • DOI: 10.17863/CAM.95546

Expansion 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 8MW 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 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 TPless 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.

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