A Generalised Winkler Model for the Hysteretic and Ratcheting Behaviour of Monopiles in Clay and Sand-Paper

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The paper presents a method for modelling cyclic hysteresis and accumulated rotation (ratcheting) of offshore wind monopiles. It integrates the Hyperplastic Accelerated Ratcheting Model (HARM) into a generalised Winkler foundation using the PISA parameterised soil-reaction framework. Unlike conventional approaches that reduce static soil resistance by an empirical factor, the method represents cyclic response and rotation accumulation more explicitly.<br /><br />HARM is applied to the Winkler model’s distributed lateral and moment reactions, and its base shear and moment reactions. Soil-reaction backbone curves are first calibrated against 3D finite-element predictions, then adjusted to match field-test responses. HARM parameters are calibrated to reproduce initial loading and cyclic accumulation, with an accelerated procedure reducing the number of cycles needed in simulations.<br /><br />The method is evaluated against PISA field tests in glacial clay at Cowden and dense marine sand at Dunkirk. Calibrations to tests CM6 and DM2 reproduce observed accumulation reasonably well. Applying the CM6 ratcheting parameters to CM5 captures the trend of accumulation with cycle number, but overestimates its magnitude at higher load levels by a factor of two to three. Results are sensitive to the selected backbone curve.<br /><br />The approach is compatible with PISA design methods and could support more robust monopile design and assessment of lifetime rotation. Its use requires adequate calibration for the site’s soil profile; further work could improve predictions across different tests and develop alternative calibration methods.

Keywords

offshore wind monopiles

cyclic hysteresis

ratcheting

Hyperplastic Accelerated Ratcheting Model

HARM

Winkler foundation

PISA framework

soil-reaction curves

finite-element calibration

monopile rotation