Recent studies have demonstrated the significance of the vertical seismic acceleration component for offshore wind turbines, as their low natural period in this direction can result in significant excitation, potentially making this load case design-driving. Unexpectedly high vertical ground accelerations, well exceeding their horizontal counterparts, have also been recorded in a number of recent seismic events. This study explores the concept of resonance between the vertical seismic component and the natural frequency for compressional waves of fully saturated soil deposits, which can aggravate further the vertical accelerations at the top of structures of interest, using numerical analysis and monitoring data. The site response at a strong motion station that registered the second highest peak ground vertical acceleration during the 2011 Mw 6.2 Christchurch earthquake in New Zealand is modelled in finite element analyses. Two different depths are also considered: the first one is truncated at the interface of the softer surficial deposits with the stiff gravel horizon. This has been shown to be adequate for S-wave propagation modelling. Conversely, the second one models the full depth to bedrock. Despite the number of uncertainties involved, the results demonstrate the concept of resonance in compression against field measurements and highlight the importance of the modelled depth in the case of vertical site response analysis. They further demonstrate the potential relevance of the findings to the seismic design of offshore wind turbines.
Vasiliki Tsaparli, Stavroula Kontoe, David M. G. Taborda and David M. Potts
Keywords: vertical seismic component, resonance, natural frequency, field measurements, offshore wind turbines
Publisher: DFI