Experimental Investigation of Scour Around Tripod Foundations Under Steady Current

Scour around a tripod foundation under a steady current is investigated experimentally. Experiments are conducted on three tripod foundation models and three flow-angle-of-attack conditions. The three tripod foundations differ only in the size of the top braces. The mechanisms of scour are discovered both experimentally by scanning the scour profiles at various stages of scour progression and conducting numerical simulations of flow around the foundations. The scour is found to be a combination of global and local scour, with three local maximum scour depths occurring at each of the three legs. Although the scour below the main column is shallower than the three legs, its contribution is significant to global scour: the scour below the main column of the tripods ranged from 38–72% of the maximum scour depth. Compared with the case of three truncated piles only, the global scour caused by the main column and the braces dramatically increases the volume of eroded sand. Scour below the main column develops more slowly than the three legs because of its greater diameter. The top brace size has a significant effect on scour; although the braces are located well above the sand surface, their effect is greatest at a 30° angle of attack. In addition to horseshoe vortices, other scour mechanisms, i.e., flow amplification and weak vortex shedding, are also identified.