The effect of variations in soil stiffness on the dynamic response of an offshore wind turbine

Offshore wind is a growing energy sector that has potential to provide vast quantities of the energy required to meet renewable energy targets. The recent growth in this sector has stimulated renewed interest in the behaviour of large diameter monopiles. Currently, over 75% of offshore wind turbine generators are founded on monopile foundations. The future predictions for offshore wind indicate th at these systems will be constructed in deeper waters further from the coast. This increase in water depth will lead to longer monopile free lengths, a factor that could negatively impact on their dynamic stability. This paper presents the results of a num erical investigation in which a range of monopile diameters in different water depths are analysed with respect to the overall wind turbine system natural frequency. Three different sand stiffness profiles are generated corresponding to loose, medium dense and dense sand using the industry standard American Petroleum Institute lateral loading design code. These sand profiles are indicative of offshore conditions found at many sites. Results indicate that significant care is required to ensure that the globa l stiffness response of the structure is such that the system natural frequency does not coincide with resonant excitation bands from the rotor’s motion. The stiffness of the soil governs the diameter of the monopile that can be used in different design water depths.