Evaluation of a 15 MW Turret Moored Floating Offshore Wind Turbine

Abstract Floating offshore wind is one of the most promising renewable energy sources over the coming decades and development of the floating substructure is of the most critical step for success of any floating system concept. A novel single point moored Floating Offshore Wind Turbine (FOWT) concept with the IEA 15 MW reference wind turbine is designed and evaluated in this paper. The FOWT consists of a traditional three-column semi-submersible with connecting truss beams and heave plates, A-frame airfoil shaped tower supporting structure, Rotor-Nacelle-Assembly (RNA), and a disconnectable turret system for stationkeeping. The platform is designed to weathervane freely around the turret to passively align the FOWT heading to the wind direction with yaw restoring function from a tailor-designed airfoil shaped A-frame tower. The passive alignment capability could potentially eliminate the active yaw system on top of the tower and the active ballasting system in the hull to improve the overall system reliability. The platform tilts, nacelle accelerations, tower bottom bending moment, mooring dynamics, vessel offset, self-alignment performance and efficiency of power generation under steady misalignment are investigated numerically using coupled aero-hydro-servo-elastic model in time domain for both operational and extreme conditions. Statistics of each design parameter are discussed for various design load conditions. A sensitivity study of floating tower natural frequency was also carried out and discussed in both frequency and time domain to understand its impact on the mooring system response. The yaw misalignments studies are presented for the normal operational DLC1.1 and DLC1.3 with both colinear and cross combinations of the wave, wind, and current conditions. Challenges and benefits of the single point moored FOWT concept are summarized and presented from the studies.