Influence of scour on the natural frequency of offshore wind turbines with tripod bucket foundation in sand

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4. Influence of scour on the natural frequency of offshore wind turbines with tripod bucket foundation in sand

The extent to which scour affects the natural frequency of an Offshore Wind Turbine (OWT) is crucial for its long-term safety, as the SoilFoundationStructure Interaction (SFSI) can significantly influence its dynamic behaviour. However, obtaining a reliable estimate of the natural frequency hinges on sophisticated foundation modelling that incorporates SoilStructure Interaction (SSI). In this study, a finite element (FE) model that considered SSI was developed to examine the effect of scour on the natural frequency of 4.2 MW OWT supported by a tripod suction bucket foundation designed in South Korea. First, the FE model was validated by comparing the natural frequency computed using a theoretical equation and the design value determined by another FE model without SSI. Then, the ratio of scour depth to bucket skirt length (S/L) was varied from 0 to 0.5 for different bucket dimensions (L/D = 1, 1.16, 1.5 and 2.0). The tower and tripod were modelled using beam elements and subjected to static loads by the actual design wind and wave conditions, with the nacelle's self-weight applied at the apex. The foundation was constructed using plate elements with discrete springs, whose stiffness was determined based on soil reactions obtained from PLAXIS. The soil was modelled utilizing HS-Small model with parameters representing loose to dense sand. The findings indicated that the natural frequency decreased by 1.3% to 12.0% as S/L increased from 0 to 0.5. The rate of decrease in natural frequency associated with bucket dimensions (L/D) and relative density was comparatively small.

5th International Symposium on Frontiers in Offshore Geotechnics (ISFOG2025)

11 - Suction installed foundations and anchors