A Winkler model of monopile ratcheting under long-term dynamic cyclic loading

Abstract

Offshore wind turbines (OWTs) experience a range of two-way and partial one-way cyclic load conditions during their operational lifetime. Cyclic loading with a non-zero mean can lead to ratcheting in the soil, which causes the structure to accumulate displacement and rotation over repeated loading, which can compromise the design limit requirements. In this paper a novel ratcheting model is proposed using a Winkler approach with Masing rules and controlled with just one parameter. The model is validated against 5 experimental setups and 17 long term load tests up to 105 cycles, and predicts very well the experimental response, performing comparably with other existing ratcheting models. An OWT from North Hoyle wind farm is also modelled and it is found that the closer the driving frequency is to the resonant frequency, the greater the ratcheting response. This means that for soft-stiff type OWTs, where there is little tolerance between the resonant frequency and driving frequencies, it is crucial to consider the potential of ratcheting during the design phase. The model presented here provides a simple and effective method of predicting future ratcheting with a focus on ease of calibration and implementation into existing numerical analysis tools.