Methods of Numerical Analysis of Monopiles under Pseudo-Random Lateral Loading-Paper

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The paper examines numerical methods for predicting the long-term response of offshore wind turbine monopiles under realistic, multidirectional pseudo-random loading. Unlike regular laboratory cycles, wind and wave loads vary in amplitude and frequency. Design models must capture changes in stiffness and hysteresis, accumulated displacement, and the pile’s post-cyclic response, including after millions of cycles.<br /><br />The authors review existing approaches. Simple factoring methods modify monotonic design curves but cannot represent realistic loading paths or changes in stiffness and energy loss. Extrinsic methods add cycle-related effects to a baseline response and can be calibrated against tests, but often rely on rainflow counting and can be difficult to integrate consistently into numerical analyses. Intrinsic models track the response to each load increment and handle complex histories, but may be hard to calibrate and computationally expensive.<br /><br />The paper presents HARM (Hyperplastic Accelerated Ratcheting Model), which combines these approaches. Built on multisurface plasticity, it models nonlinear hysteresis and multidirectional loading. A separately calibrated ratcheting mechanism captures progressive displacement accumulation, while an acceleration technique represents very large numbers of cycles with reduced computation. The model can be applied as a foundation-level macro-model, a generalized Winkler model, or a soil constitutive model.<br /><br />Comparisons with model pile tests, PISA-related analyses, and cyclic clay testing show that HARM can reproduce observed responses, including complex load paths and accumulated rotation. The authors conclude that it offers a practical framework for extending monopile design methods from monotonic loading to realistic cyclic conditions, potentially reducing unnecessary conservatism in offshore wind foundation design.

Keywords

offshore wind turbines

monopile design

cyclic loading

pseudo-random loading

hysteresis

stiffness degradation

ratcheting model

multisurface plasticity

HARM model

soil-structure interaction