Seismic Response of Offshore Wind Monopiles in Cohesionless Soils-Paper

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This study investigates the seismic response of offshore wind monopile foundations in dry, cohesionless soil, addressing a gap in design guidance that largely focuses on quasi-static wind and wave loading.<br /><br />Researchers conducted a dynamic centrifuge test at 60g using two instrumented monopiles embedded in layered sand: a loose upper layer over a very dense lower layer. Bedrock motions included frequency sweeps and sinusoidal earthquakes. One pile was laterally pushed before the earthquakes to establish baseline behaviour; the other was pushed afterward to assess changes in monotonic response.<br /><br />The pile–soil system’s first-mode natural frequency was approximately 0.5 Hz at prototype scale. Earthquake motion at this frequency produced substantial pile-head acceleration amplification, reaching a factor of 5.2. Loading at 1 Hz produced much smaller amplification (0.8), indicating that low-frequency motions near the system’s resonance can be especially significant. The measured natural frequency remained broadly similar after the earthquake sequence.<br /><br />Post-earthquake monotonic behaviour differed from the initial response in both stiffness and strength. The authors attribute these changes primarily to grain rearrangement and densification in the loose upper soil, alongside degradation of the soil–pile interface. They conclude that seismic effects on monopile performance may not be adequately represented by simply degrading conventional monotonic p–y curves, and that further research is needed to inform design methods.

Keywords

offshore wind

monopile foundation

seismic response

dynamic centrifuge test

cohesionless soil

sand layers

natural frequency

pile-soil interaction

earthquake loading

p-y curves