Hydrodynamic mechanisms of artificial reefs for mitigating local scour around submarine pipelines

Submarine pipelines are prone to severe local scour under wave and current forcing, potentially leading to free spans, vortex-induced vibrations, and structural instability. This study presents a process-based numerical investigation of artificial reefs (ARs) as a near-bed flow control strategy for pipeline scour mitigation, with particular emphasis on the underlying hydrodynamic and sediment-transport mechanisms. A validated two-phase flow model is employed to explicitly resolve fluid–sediment–structure interactions, including seepage-induced sediment transport beneath the pipeline. Systematic parametric analyses are conducted to examine the influence of reef–pipeline spacing, reef width, height, and inter-plate spacing on vortex dynamics and scour evolution. The results reveal that ARs mitigate scour primarily by disrupting the formation and interaction of horseshoe and wake vortices, thereby redistributing near-bed momentum and reducing seepage gradients responsible for sediment entrainment. An optimal configuration is identified that achieves a maximum reduction in scour depth of 49.8% relative to the unprotected case. Each geometric parameter exhibits a distinct effectiveness threshold, beyond which adverse vortex reorganization and enhanced seepage may offset protective benefits. These findings provide new process-based mechanistic insight into reef-induced flow regulation and establish quantitative design guidance for integrating AR-based scour protection into submarine pipeline systems.