Dynamic analysis of a two-rotor wind turbine on spar-type floating platform

The dynamic response of a two-rotor wind turbine mounted on a spar-type floating platform is studied. The response is compared against the baseline OC3 single-rotor design. Structural design shows how the two-rotor design may lead to a mass saving of about 26% with respect to an equivalent single-rotor configuration. Simulations predict significant platform yaw response of the two-rotor floating wind turbine — about 6 deg standard deviation at the rated operating wind speed. It is shown how the platform yaw response is directly caused by the turbulence intensity at the hub coupled with the transversal distribution of thrust loads on the structure. A coupled control strategy for the rotor-collective blade pitch controller is proposed, in which a simple proportional control mitigating platform yaw motion is superimposed to the baseline OC3 PI controller. Numerical simulations show how platform yaw response is reduced by about 60%, at the cost of mean power loss at below-rated wind speeds of about 100 kW and maximum increase of the rotor-collective blade-pitch angles standard deviation of about 2 deg. Parametric analysis of mooring lines design shows how an equivalent mass density of the line of at least 190 kg/m is needed to avoid vertical loads at the anchors