Stochastic Dynamic Analysis of an Offshore Wind Turbine Considering Frequency-Dependent Soil-Structure Interaction Parameters

This study investigates the dynamic response of a 5 MW offshore wind turbine with monopile foundation subjected to wind and wave actions under parked condition. It includes dynamic interaction between the monopile and the underlying soil subjected to stochastic wind and wave loading. The offshore wind turbine tower has been modeled using the finite element software ANSYS 14 as a line structure and it comprises a rotor blade system, a nacelle, and a flexible tower under parked condition. The mass of the rotor, blade, and nacelle are lumped at the top of the tower for simplicity. Stochastic wind and wave loadings are simulated using the Kaimal spectrum and the Pierson-Moskowitz spectrum correlating wind and wave forces, respectively. The soil-structure interaction (SSI) effect at the foundation level is taken into consideration by including rotational as well as lateral spring constants derived from Wolf's double cone model for embedded foundations. The results are studied in the frequency domain for both wind and wave loadings in the form of power spectral density functions, which show that the response of the structure depends not only on the external forces but also on the soil-structure interaction effect. Under very soft soil conditions, the displacement response is amplified to a very high value under wind loading when compared with that under wave loading at lower frequencies. Incorporation of soil-structure interaction also modified the peak value of displacement and its subsequent frequency when compared with that for the fixed base structure which does not consider soil-structure interaction.