When submarine pipelines are placed on erodible sandy beds, the interaction between the surrounding flow, the pipeline, and the sediment can induce local scour. While two-dimensional simulations have been extensively performed to capture the scour process in a two-dimensional cross-sectional plane, they cannot simulate the three-dimensional (3D) spanwise propagation of the scour hole, which leads to free-span development. This study presents 3D numerical simulations of current-induced scour propagation in the spanwise direction beneath a subsea pipeline using an open-source Eulerian two-phase flow solver. Systematic simulations are performed under four distinct Shields parameters. The results indicate a close spatial association between the evolving 3D scour hole and regions of localized shear stress amplification and sediment flux near the span shoulder. For low Shields parameters, the scour hole retains pronounced 3D features with steady spanwise propagation, which is also characterized by skewed wake flow structures and oblique downstream dunes. At higher Shields parameters, the scour front propagates rapidly across the domain, causing the morphology and flow streamlines to transition to a quasi-2D state. Furthermore, the scour slope at the span shoulder remains approximately constant across the investigated flow conditions. The spanwise propagation speed is approximately constant during the fitted propagation stage for the selected four simulated conditions.

Sediment transport beneath subsea pipelines is essentially a highly complex multiphase flow problem, involving strong coupling between hydrodynamics, structural response, and sediment transfer. Accurately capturing this process is of significant scientific and engineering importance for predicting seabed evolution and ensuring the structural safety and stability of subsea pipelines. In this study, the coupled vortex-induced vibration (VIV) and scour behaviour of subsea pipelines under steady current conditions is investigated using a computational fluid dynamics (CFD)-based numerical approach. The solver is developed within SedFoam by incorporating dynamic mesh motion to account for pipeline vibration. The pipeline is modelled as a single-degree-of-freedom system with a vertically mounted spring and a concentrated mass. The numerical results reveal a strong interaction between pipeline VIV and the evolving seabed morphology. Repeated pipeline motion toward the seabed significantly modifies the local scour profile and promotes the formation of a sand dune beneath the pipeline. The presence of the sand dune alters the near-bed flow field and suppresses vortex shedding beneath the pipeline. As the scour process develops and the fluid–structure–seabed interaction intensifies, the VIV response of the pipeline is progressively reduced. The present study highlights the important coupling mechanisms between pipeline vibration and seabed scour and provides insights into the dynamic evolution of pipeline–seabed interaction.