Effect of sand liquefaction on the first natural frequency of a monopile-supported offshore wind turbine using a nonlinear three-dimensional finite element model

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4. Effect of sand liquefaction on the first natural frequency of a monopile-supported offshore wind turbine using a nonlinear three-dimensional finite element model

Monopile-supported Offshore Wind Turbines (OWTs) are dynamically-sensitive structures and their foundations must be designed with special consideration for cyclic/dynamic loading conditions, especially in seismic regions. The most significant effect of earthquake loads on OWTs is the possible liquefaction at the construction site. Indeed, liquefaction may affect the dynamics of the OWT and particularly its first natural frequency (FNF). This paper aims to investigate the change in the FNF of an OWT-soil system during an earthquake and its subsequent evolution (recovery) using three-dimensional (3D) non-linear finite element (FE) simulations. A 3D FE model of a monopile-supported DTU 10 MW OWT was developed using the PLAXIS3D simulation platform, focusing on homogeneous saturated sands with varying densities (loose, medium and medium dense). The well-established SANISNAD-MS bounding surface plasticity model was adopted in this work to simulate the hydro-mechanical behavior of the sand. The build-up of excess pore water pressure in the soil surrounding the monopile and its effect on the FNF of the OWT-soil system was investigated. The numerical results have shown that the liquefaction in loose, medium, and medium dense sands significantly affects the FNF of an OWT-soil system and induces notable excitation in the soils natural frequency.

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

9 - Monopile design to cyclic loads: quasi-static, dynamic and seismic loads