Modelling the Response of Monopiles to Complex Cyclic Lateral Loading in Sand-Paper

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The paper investigates how monopile foundations in sand respond to complex cyclic lateral loading, including changes in load amplitude, direction and frequency. Such loading can cause permanent rotation (ratcheting) and alter foundation stiffness and dynamic behaviour, both important for offshore wind turbine design.<br /><br />The authors conducted 1g physical model tests using a monopile in dense, dry sand. Tests covered regular unidirectional and multidirectional loading, as well as an irregular storm-like load history. They then assessed a bidirectional macro-model based on the Hyperplastic Accelerated Ratcheting Model (HARM) framework.<br /><br />Experiments showed that rotation accumulated in the direction of the mean load, even when cyclic loading acted in another direction. Ratcheting generally followed a power-law trend with cycle number. Secant stiffness increased during most cyclic tests, and post-cyclic reloading capacity approached or exceeded the monotonic capacity. Loading asymmetry and the spread of loading directions affected ratcheting, although these effects were complex. Tests at different stress levels showed qualitatively similar behaviour, but ratcheting and stiffness evolution slowed as stress level increased.<br /><br />The HARM model reproduced many observed features, including multidirectional ratcheting, stiffness evolution, post-cyclic response and key aspects of the irregular storm response. However, it did not fully capture the effects of partial two-way loading or varying loading direction, and it underestimated permanent rotation under the irregular storm history.<br /><br />The findings support the potential of HARM models for predicting monopile response under realistic, continuously varying loads. Practical design use will require improved calibration methods and validation against full-scale or stress-level-appropriate data.

Keywords

monopile foundations

cyclic lateral loading

sand response

ratcheting

offshore wind turbines

foundation stiffness

multidirectional loading

HARM model

permanent rotation

physical model tests