A simplified, novel approach for modelling cyclic degradation of soil properties around monopile foundations for offshore wind turbines

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4. A simplified, novel approach for modelling cyclic degradation of soil properties around monopile foundations for offshore wind turbines

The combined action of cyclic wind and wave loading on monopile foundations for offshore wind turbines often leads to degradation in soil shear strength and moduli, making cyclic soil degradation a critical issue to address in design. Since design standards lack specific methodologies for modelling this degradation along large monopiles, a novel and simplified approach is developed based on cyclic contour diagrams derived from laboratory testing. This method involves redistributing the cyclic load history along the pile and at the base, using depth-specific degradation factors determined through strain or pore pressure accumulation, accounting for partial drainage in non-cohesive soils. The resulting degradation factors are linked to soil utilisations for each soil unit. Unit-specific curves define these factors, capturing the effects of the contour diagram, equivalent number of cycles, and utilisation ratio. The load history and soil utilisation ratios along the pile are derived from a non-linear 1D beam model of the monopile across turbine locations. Ultimately, location-specific degradation factors are applied to soil reaction curves as cut-off factors in the ultimate limit state analysis. The approach has been validated through a benchmark numerical study and successfully applied by Ramboll in multiple projects.

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

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