Combined effect of wind and sea excitations on the internal resonance response of floating offshore wind turbines

The sea excitation is added into a reduced nonlinear model of floating offshore wind turbine, in order to assess its effect on the system dynamics. To this aim, the sea wave motion is modelled as heave displacement of the turbine tower, which results in a parametric excitation to the along-wind displacement. The occurrence of the 1:1 internal resonance between along-wind and cross-wind oscillations is investigated by means of an asymptotic approach based on the multiple scale method. It is shown that the coupled solutions arising as a result of the wind forcing undergo a significant modification in terms of stability and quality of the response precisely because of the additional presence of the excitation due to the sea waves. A behavior chart is obtained to describe the model response as a function of the forcing parameters. The analytical results are verified by comparison with numerical simulations in terms of bifurcation diagrams and reconstructed solutions.