Nonlinear dynamical behaviour of jacket supported offshore wind turbines in loose sand

Soil-structure interaction (SSI) is a vital aspect in the performance of bottom fixed offshore wind turbine (OWT) structures. This paper investigates the response of a jacket supporting a 5-MW OWT in intermediate water depths of 70 m, under the influence of hydrodynamic and aerodynamic loading. SSI is incorporated by means of a Winkler-spring formulation and sands of varying densities are considered. A spectrum of wind speeds are considered, increasing from the cut-in value of 3 m/s to an extreme case of 45 m/s. Response charts have been proposed in terms of rotations at various levels of the jacket, to serve as design-aids for similar structures. The effect of SSI becomes predominant for OWT in loose sands (with friction angle less than 30∘). Parametric resonant responses have been observed for the jacket in loose sands, in the region bounded by the rated and cut-out wind speeds.