Soil-monopile interactions for offshore wind turbines

Many offshore wind turbines are supported by large-diameter piles (known as monopiles) and are subjected to a large number of cyclic and dynamic loads. There are evidences suggesting that foundation stiffness are changing with cycles of loading and this may lead to changes in the natural frequency of the system with the potential for unplanned system resonances. There are other consequences such as excessive tilt leading to expensive repair or even complete shutdown. Therefore, it is vital to understand the long-term response of wind turbine foundation so that a method to predict the change in frequency and long-term tilt could be established. This paper aims to present the experimental work of small-scale physical modelling and discrete element modelling of the interactions between a monopile and the surrounding soil. Changes in soil stiffness under cyclic loading of various strain amplitudes were examined for both physical modelling and discrete element modelling. Micromechanics of soils underlying the soil stiffness change was investigated using discrete element method. Variation of force distribution along the monopile under cyclic loading was analysed to show the influence of monopile stability.