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 surge frequency (St=0.39) producing the most effective reduction in velocity deficit. In contrast, the downstream wake exhibits a distinct frequency dependence, with lower-frequency surge motion leading to more effective far-wake recovery. Spectral analyses show that lower-frequency surge motion generates larger-scale coherent wake structures that persist farther downstream, whereas higher-frequency motions generate smaller, rapidly dissipating structures. The power response reflects the combined effects of surge-induced velocity modulation and wake recovery within the array. At the array level, the surge frequency that maximizes the upstream turbine power does not correspond to the best overall array performance; instead, St=0.39 yields the most favorable overall performance, with an approximately 20% increase in downstream turbine power. This behaviour is associated with the persistence of large-scale coherent wake structures, which enhance wake recovery and improve the inflow conditions for the downstream turbine. 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.