Impact of surge motion on wake characteristics, power performance and fatigue loads of two tandem floating wind turbines

Understanding how platform surge motion influences wake dynamics, downstream power production, and structural loads of floating offshore wind turbines (FOWTs) is essential for improving array-level energy efficiency. This study performs high-fidelity large-eddy simulations coupled with a validated elastic actuator line model to investigate two tandem FOWTs, both subjected to surge motions, operating in an atmospheric boundary layer. Surge motions with two nondimensional amplitudes and three reduced frequencies are examined to assess their impact on wake recovery, power output, and blade-root loads. The results reveal a pronounced frequency-dependent influence of surge motion on wake recovery and power production of the tandem configuration. Surge motion enhances wake mixing and accelerates recovery behind the upstream turbine, with the intermediate dimensionless frequency St=0.39 producing the most effective reduction in velocity deficit. Downstream of the second turbine, the wake response differs markedly from that of the upstream turbine, with the lowest-frequency case (St=0.26) yielding the most effective far wake recovery. Spectral analyses show that lower-frequency surge motions promote larger-scale coherent wake structures that persist farther downstream, whereas higher-frequency motions generate smaller, rapidly dissipating structures. Under identical surge-motion conditions, the relative increase in mean power output of the downstream turbine is markedly larger than that of the upstream turbine, indicating that the reduction of the upstream wake velocity deficit plays a dominant role in enhancing downstream power output, rather than the direct effect of platform motion itself. Moreover, surge motion amplifies blade load fluctuations and fatigue for both turbines, with fatigue increasing monotonically with surge intensity and being governed primarily by motion-amplified load oscillations rather than mean load levels. These findings provide physical insight into the interplay between platform motion, wake dynamics, and array performance, offering guidance for motion-aware design and assessment of floating wind turbine arrays.