Extension of the p-y Curves Framework for Cyclic Loading of Offshore Wind Turbines Monopiles in Soft Rock-Paper

Paper

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The paper extends the conventional P–y curve method to model cyclic lateral loading of offshore wind turbine monopiles in soft rock. Because monopiles must meet strict limits on long-term displacement and rotation, the method aims to represent accumulated movement without explicitly simulating every cycle.<br /><br />The approach combines nonlinear P–y curves, which describe the pile’s immediate soil response, with Kelvin–Voigt viscoelastic models in series. These models represent time-dependent accumulation that eventually stabilises. Unloading paths account for irreversible movement and possible gaps between the pile and surrounding ground. For driven piles, two Kelvin–Voigt models distinguish accumulation in the compacted crushed-rock annulus from that in the underlying rock; drilled-and-grouted piles are modelled with one model for the rock response.<br /><br />Parameters calibrated against field tests on a driven pile were used to predict cyclic responses of other driven and drilled-and-grouted piles installed in weak limestone. The predictions generally agreed well with measured displacement accumulation, including for cycle periods of 8–25 seconds.<br /><br />The framework is intended for stabilising cyclic response; it does not yet capture ratcheting or unstable accumulation leading to failure. Its assumptions, including uniform properties with depth and simplified model behaviour, may need revision for full-scale monopiles. The authors also note that cyclic loading produced greater accumulation than sustained loading at the same level in some tests, a difference the current model does not represent. Further work could incorporate nonlinear or damage-based rheological models and improve the estimation of ground parameters at prototype scale.

Keywords

cyclic lateral loading

offshore wind turbine monopiles

soft rock

P-y curve method

Kelvin-Voigt model

displacement accumulation

driven piles

drilled-and-grouted piles

weak limestone

cyclic response prediction