Effects of Scour and Scour Protection on Monopiles for Offshore Wind Turbines Exposed to Wind-Wave Loading

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4. Effects of Scour and Scour Protection on Monopiles for Offshore Wind Turbines Exposed to Wind-Wave Loading

Scour and scour protection can change the foundation conditions of monopiles supporting Offshore Wind Turbines. Scour causes a reduction in the monopile embedment length and a loss in lateral stiffness, which induces a higher vulnerability to permanent and cumulative tilting. Scour protection, however, may fulfil a double purpose of preventing scour and potentially supplying additional stiffness to the foundation, which is often neglected in foundation design. The present study analyses and quantifies the effects of potential stiffness changes in monopile foundations caused by scour and scour protection under cyclic wind-wave loading. It presents a numerical Finite Element model of a representative 5MW Offshore Wind Turbine founded in Geba sand. A series of load combinations including wind and sea wave loads are applied to the Offshore Wind Turbine to simulate different environmental conditions. To reproduce the soils stress-strain response under cyclic loading, a hypoplastic constitutive model is implemented. This allows the simulation of the ratcheting phenomenon, the cycle-dependent strain accumulation, and the change of relative density induced by this type of loads. This study evaluates residual and maximum monopile rotations, and presents an application to a representative case of an Offshore Wind Turbine. The results highlight the significant risk that scour poses to monopiles regarding the accumulation of strains, which increases with a high number of cycles. It also evidences the potential of scour protection as a foundation stiffener under cyclic loading conditions. This study highlights the risk of scour and the foundation optimisations that may be achieved with scour protection.

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

8 - Monopile design to lateral monotonic loads