Influence of aspect ratio on the dynamic performance of suction caisson supported wind turbines: insights from wind tunnel tests

The aspect ratio of suction caissons strongly influences the dynamic performance of offshore wind turbines, whereas excessively large ratios increase foundation cost. This paper presents wind tunnel tests on scaled wind turbine models supported by suction caissons with aspect ratios of 0.50, 0.75, and 1.00, covering conditions from cut-in to failure. The results show that increasing the aspect ratio enhances ultimate capacity and natural frequency while reducing nacelle displacement. However, it also increases tower-top acceleration and decreases the global damping ratio. These trends indicate a stiffer but less damped overall wind turbine system. The test results are converted to prototype scale using similitude relationships. A fitted relationship between foundation rotation angle and aspect ratio is established, enabling estimation of the corresponding ultimate wind speed. In addition, fitted equations are proposed to predict the prototype-scale dynamic performance of suction caisson-supported wind turbines, including tower-bottom bending moment, natural frequency, and damping ratio. The ultimate capacity and failure modes are also discussed. Based on a multi-criteria evaluation of ultimate capacity, natural frequency, and damping ratio, aspect ratios lower than 0.75 appear unfavorable within the range investigated, where ultimate capacity is identified as the governing factor in aspect-ratio design.