Investigation of Coupling Effects of Wave, Current, and Wind on a Pile Foundation

To investigate the coupling effects of wave–wind, wave–current, and wave–current–wind on a pile foundation of a marine structure, harbor basin tests on loads induced by single and combined action of wave, current, and wind were conducted. The time histories, power spectrums, and characteristic values of drag forces in the test conditions were compared. Then, the coupling coefficients were calculated based on the characteristic values to quantitively evaluate the coupling effects. The influence of natural vibration of the pile on the characteristic values and coupling effects were studied by comparing the test loads between rigid and elastic models. The results show that the shapes of power spectrums of drag forces and their peak frequencies in the wave-involved conditions are similar to the spectrum of incident wave, which means that the steady current and uniform wind cannot change the frequency distribution of incident random wave. Although the drag forces of rigid and elastic model under wave–wind, wave–current, and wave–current–wind are not the linear superposition of corresponding single field, the coupling effects among them are quite weak as the coupling coefficients are small. It is speculated that the weak wave–wind coupling effect may be because, firstly, the interaction of wave and wind is limited to the zone near the water surface, which is far less than the height of the model; secondly, the wave-induced load is dominant compared to wind and current. The loads in test conditions of elastic model are similar to the rigid model; for that, the model’s natural frequency is far away from the peak spectral frequency of incident wave, having little influence on the drag force.