Evaluating Soil Reaction Models for Offshore Wind Turbine Monopile Foundations: Implications for ULS and FLS Design

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4. Evaluating Soil Reaction Models for Offshore Wind Turbine Monopile Foundations: Implications for ULS and FLS Design

To meet crucial Net Zero goals, increasingly large Offshore Wind Turbines (OWTs) are being deployed in deeper waters. This trend has heightened the dynamic sensitivity to environmental loads, making monopile foundations vulnerable to large strain cycles. Time domain simulations using linearised beam-spring models are commonly employed to facilitate ULS and FLS analysis under representative wind and wave loads during an OWTs operational life. However, the dynamic response of OWT-monopile systems is dependent on the unloading behaviour of the spring elements. This paper examines the effects of three distinct soil reaction models on the dynamic response of a monopile. These are: i) Linear, ii) Nonlinear elastic, and iii) Hysteretic. For FLS analysis, a random wave load model is employed, and the damage after 25 years is estimated using the Damage Equivalent Moment (DEM). For ULS, extreme wave events are simulated by imposing a constrained wave onto a random wave and calculating the location and value of the maximum moment along the monopile. Results suggest that the hysteresis model reduces the expected damage after 25 years due to material damping, and nonlinear spring models in general experience larger maximum bending moments at lower depths. The hysteresis model is shown to retain the plastic strains due to the extreme event.

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

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