Seismic response of offshore wind monopiles in cohesionless soils

Abstract

Development of offshore wind farms in seismic zones requires consideration of monopile foundation design subjected to dynamic earthquake-induced loads. This paper presents the results of a dynamic centrifuge test, investigating the monopile response to seismic loads in dry cohesionless soils. The test involved two identical instrumented monopiles, driven into a layered soil sample, with a loose layer overlying a very dense layer. A servo-hydraulic earthquake actuator was used to generate sine sweep and sinusoidal bedrock motions, during which pile and soil accelerations, dynamic bending moments and pile rotation were assessed. A monotonic push was performed on one of the piles prior to the earthquakes to assess initial capacity, while the second pile was pushed after completion of the dynamic load sequence to establish the post-seismic capacity. The results show strong acceleration amplification for earthquake loading frequencies close to the first mode of frequency of the pile-soil system. The monotonic tests demonstrate that, for the layered sand configuration considered here, the monotonic behaviour of the pile is strongly affected by the dynamic loading history.