Numerical investigation of wake-induced lifetime fatigue load of two floating wind turbines in tandem with different spacings

Floating wind turbine (FWT) is a promising technology to utilize offshore wind resources. For offshore wind farms, wakes can result in 5%-15% increase in fatigue load on the downstream turbine rotor. Current offshore wind turbine design standard lacks accounting for the different wake characteristics and wake-induced fatigue load of the FWT from that of their fixed counterparts. Considering a scenario of two tandem FWTs, this study investigates the wake characteristics of a semi-submersible FWT accounting for multi-degree-of-freedom (DOF) dynamic motion characteristics and its wake effect on fatigue load of downstream unit with different spacing configurations. A comparison with the effective turbulence intensity method suggested in the current IEC 614001 standard is also carried out. With an interactive analysis framework composed of computational fluid dynamics (CFD) wake simulation and time domain aero-hydro-servo-elastic simulation for FWT, lifetime wake-induced fatigue loads are analyzed through a full range of turbine normal operational conditions. Results show that the wake characteristics of the FWT show a distinct difference between below-rated and above-rated conditions due to the feature of FWT's controller. Moreover, the Frandsen model adopted in the current IEC 61400-1 standard highly underestimates the FWT's wake turbulence, causing the predicted lifetime fatigue damage equivalent load at the tower base decreases up to about 10-15% for a spacing of 8D-10D under combined wind and wave conditions.