Experimental Study on the Extreme Wave Run-Up Phenomenon of a Semi-Braceless Floating Wind Turbine Foundation Under Focusing Waves

A series of physical model tests of a semi-braceless floating wind turbine foundation (FOWT) under freak waves was conducted in the present work. The extreme wave run-up phenomenon on the FOWT columns was reported and investigated. The back column exhibits a median wave run-up amplification factor of 1.65 (a 65% rise from the baseline condition), with the maximum recorded value hitting 3.18, marking a 218% amplification compared to the reference wave run-up. Four factors contribute to this phenomenon, including the wave-trapped mode between columns, the shoaling effect of the pontoons, the relative motion between the platform and the wave surface, and nonlinear wave run-up. The extreme wave run-up corresponds to violent wave dynamic pressures, showing nonlinear column pressure distributions. Steeper local freak waves can lead to higher wave dynamic pressures on the columns, and slamming pressures occur under breaking waves in some harsh wave conditions. The peak slamming pressure on the back column reaches 300% of that on other columns. The wave run-up and wave pressures on the columns are relevant to the structure’s load level, which is crucial to the safety of the engineering design.