Abstract
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.
Highlights
Offshore wind energy is a steadily growing industry, reaching in 2019 a total worldwide offshore wind power capacity of 30 GW out of a total worldwide wind power capacity of 600 GW (GWEC, 2019)
Vestas Wind Systems A/S installed a multi-rotor demonstrator at the Technical University of Denmark, named 4R-V29, composed of four 225kW wind turbines mounted on a single structure and in operation between 2016 and 2019. van der Laan et al (2019) recently compared numerical results obtained from several Reynolds-Averaged Navier–Stokes equations (RANS) tools against field measurements of power performance and wake deficit, showing faster wake recovery and marginally higher power output at below-rated environmental conditions given by the rotors aerodynamic interaction
Where θ5 is the static pitch angle, Fthrust is the overall thrust force acting at the hubs, HB is the vertical distance from the hubs to the center of buoyancy (COB) of the spar-buoy platform, and C55 is the hydrostatic restoring pitch stiffness, which can be derived from metacentric height relationships (Faltinsen, 1993)
Summary
Offshore wind energy is a steadily growing industry, reaching in 2019 a total worldwide offshore wind power capacity of 30 GW out of a total worldwide wind power capacity of 600 GW (GWEC, 2019). The overall dynamic response of the floating system must be carefully studied, as well as the aerodynamic interaction of the rotors under operative and extreme environmental conditions. The dynamic response of a two-rotor wind turbine mounted on a spar floating platform is studied. The study relies upon a reduced aerodynamic model, simplified yet adequate to get the overall dynamic characteristics of the multi-rotor FOWT concept. The aerodynamic loads are computed by considering the relative velocity between the hub and wind transversal to the rotor plane and mapping the steady-state aerodynamic coefficients of the wind turbine This method is thought of as a simplified alternative to more complex beam-element/momentum (BEM) models, and previous work (El Beshbichi et al, 2021) showed how results obtained are accurate in terms of overall dynamic response in operative environmental conditions. Remarks about mooring lines dimensioning applied to the 2WT system are given
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