3D FE dynamic modelling of offshore wind turbines in sand: natural frequency evolution in the pre– to after-storm transition
Evangelos Kementzetzidis (TU Delft - Offshore Engineering)
Willem Geert Versteijlen (Siemens Gamesa Renewable Energy)
Axel Nernheim (Siemens Gamesa Renewable Energy)
F Pisano (TU Delft - Geo-engineering)
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Abstract
3D non-linear finite element analyses are proving increasingly beneficial to analyse the foundations of offshore wind turbines (OWTs) in combination with advanced soil modelling. For this purpose, the well-known SANISAND04 bounding surface plasticity model (Dafalias & Manzari 2004) is adopted in this work to incorporate key aspects of critical state soil mechanics into the analysis of monopile foundations in sand. The final 3D soil-foundation-OWT model is exploited to simulate the response of an 8 MW OWT to a long loading history of approximately 2 hours duration. The scope is to investigate/explain the drops in natural frequency observed in the field during storms, as well as its subsequent recovery. The numerical results point out a strong connection between transient frequency drops and pore pressure accumulation, whereas the original OWT natural frequency seems to be restored as a consequence of post-storm re-consolidation.