Influence of scour pit depth on submerged conveyance of solidified soil and loading behavior of monopile

A three-dimensional (3D) numerical model based on an Eulerian two-phase model and a shear stress transport (SST) k–ω turbulence model is developed to investigate the effect of relative scour pit depth (Sd/D = 0.4–1.4, where D is the monopile diameter and Sd is the scour-hole depth) on solidified slurry transport for monopile scour repair. The results indicate that after the initial transient surge at the onset of slurry transport, the monopile experiences a relatively steady loading state, yet both the loading characteristics and the flow field features vary with scour-hole depth. At Sd/D = 0.4, the lift force exhibits pronounced periodic oscillations, and the pile surface achieves the highest coverage of solidified soil. As the scour-hole depth increases, the lift oscillations gradually decay. For Sd/D ≥ 0.8, the initial peak of the drag force increases considerably, followed by a slight rebound after the transient decay. Meanwhile, a strong recirculation zone emerges behind the delivery pipe, which enhances slurry entrainment and loss, resulting in reduced coverage on the pile surface. The wake region behind the pile exhibits a K-shaped distribution, which expands upward as the filling proceeds, influenced by wall confinement and the free surface of the solidified-soil slurry. The strong damping effect inherent to the non-Newtonian fluid rapidly stabilizes the wake after it passes through the recirculation zone, markedly suppressing the intensity of vortex shedding. The findings elucidate coupled pile–slurry-flow mechanisms, providing guidance for optimizing scour repair of offshore monopiles.