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 61400-1 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.