Resonance as the Source of High Vertical Accelerations: Field Demonstration and Impact on Offshore Wind Turbines-Paper

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The study investigates whether resonance between vertical seismic waves and the natural compressional-wave frequency of fully saturated soil deposits can produce unusually high ground accelerations, with implications for offshore wind turbines (OWTs).<br /><br />Researchers modelled the Pages Road Pumping Station site in Christchurch, New Zealand, which recorded a vertical peak ground acceleration of 1.63g during the 2011 Mw 6.2 earthquake. Effective-stress finite-element analyses used vertical records from two nearby stations as input. They compared a shallow model terminating at 28 m, above the stiff Riccarton Gravel, with a deeper model extending to bedrock.<br /><br />The shallow model had a fundamental frequency of about 14.6 Hz and amplified frequencies that did not match the main peaks in the recorded motion. Extending the model to bedrock lowered the fundamental frequency to approximately 8 Hz. This brought the predicted amplification into the 6–10 Hz range and produced a much closer match to the observed ground-surface spectrum. The results support resonance as a plausible explanation for the site’s high vertical accelerations and show that modelling the full soil depth is important for vertical, even where shallower profiles may suffice for horizontal response. Predictions also depended strongly on the chosen input motion.<br /><br />The authors then considered a theoretical offshore profile comprising 60 m of saturated sand beneath a 20 m water column. The soil–water system showed nearly fourfold amplification near 5.6 Hz, within the typical 4–7 Hz vertical natural-frequency range of OWTs. Such amplification could increase structural demands. However, the water column may reduce motion at certain frequencies through wave interference, and additional damping mechanisms require investigation.<br /><br />The study concludes that vertical seismic loading and site-specific resonance should be considered in OWT design, particularly in seismic regions. Offshore assessments should represent the full soil profile and water column and account for both possible amplification and de-amplification.

Keywords

vertical seismic waves

resonance

ground acceleration

fully saturated soil

offshore wind turbines

effective-stress finite-element analysis

Christchurch earthquake

Pages Road Pumping Station

site amplification

soil-water interaction