Modelling the impact of gapping behaviour on monopile mounted offshore wind turbine dynamics

Increased demand for renewable energy production has stimulated interest in the offshore wind turbine (OWT) industry as a viable solution, and with OWTs growing larger in scale, further research is required into the dynamics of these newer structures. The majority of installed OWTs to date are built upon monopile foundations, and it is widely acknowledged that the current design methods for offshore piles are not appropriate for the large diameter piles required. This paper uses a novel pile-soil gapping algorithm to simulate the effects of degradation to the soil conditions in the sea bed. Using a 1D Winkler beam-spring approach, a dynamic model is validated for prediction of the natural frequencies of several OWT case studies, and the gapping algorithm is shown to reproduce well the reduction in natural frequency likely attributed to soil degradation measured from an OWT in Kentish Flats wind farm. It is found through the simulation of rotor-stop tests that the presence of gapping decreases the measured natural frequency, and this effect is greater for the monopile foundations with a smaller slenderness ratio.