A practical two parameter model of pile-soil gapping for prediction of monopile offshore wind turbine dynamics

Monopile mounted offshore wind turbines (OWTs) are expected to experience a very large number of cyclic loads throughout their operational lifetime, and the existing p-y method of foundation modelling does not fully account for the effects of dynamic cyclic loading, such as pile-soil gapping. In this paper a dynamic model based on the beam on non-linear Winkler foundation scheme with a novel algorithm capable of capturing the effects of pile-soil gapping is presented. It can account for gap cave-in, and the resulting gap size can react dynamically to changing loading amplitudes, using only two calibration parameters. Static and dynamic cyclic loaded model validations are presented, and give very good agreement with experimental results, performing better than existing p-y curves for dynamic loading. The model is also applied to an OWT case study and predictions of natural frequency reduction due to soil erosion agree well with measured results. It is shown that the inclusion of gapping may result in a significant decrease to the natural frequency prediction of OWTs relative to the value predicted without gapping. As such, not to consider gapping could lead to unconservative predictions, and any additional soil degradation throughout the serviceable lifetime could therefore result in unwanted resonance. The method provided in this paper provides a simple and accurate model to predict this behaviour which is crucial to ascertain during the design phase.