Seismic response of large diameter monopiles for offshore wind turbines in liquefiable soils

Offshore wind energy is rapidly expanding to earthquake-prone regions, with design to seismic loading becoming a primary concern. This is especially relevant to monopiles, which are the preferred option in shallow to medium water depths at present. This paper presents centrifuge tests of a monopile foundation and turbine tower subjected to low- and high-frequency earthquakes. The experiment reproduces calm air conditions, meaning that no lateral wind load was considered. Low-frequency earthquakes were found to cause excitation of the topmost part of the superstructure, whereas high-frequency earthquakes generated strong resonance effects at the base. Further data analysis suggests that high-frequency earthquake shaking can trigger liquefaction of the soil surrounding the monopile, which may in turn lead to significant tilting of the foundation and superstructure. Technical guidelines recommend a maximum allowable rotation of 0.5 degrees for offshore wind turbines at present.