The present study carries out two-dimensional numerical simulations to investigate scour beneath a single pipeline and piggyback pipelines subjected to an oscillatory flow condition at a Keulegan–Carpenter ($KC$) number of 11 using SedFoam (an open-source, multi-dimensional Eulerian two-phase solver for sediment transport based on OpenFOAM). The turbulence flow is resolved using the two-phase modified $k-\omega$ 2006 model. The particle stresses due to the binary collisions and enduring contacts among the sediments are modeled using the rheology model of granular flow. The present numerical model is validated for the scour beneath a single pipeline, and the simulated sediment profiles are compared with published experimental data and numerical simulation results. The scour process beneath three different piggyback pipelines under the same flow condition are also considered, and the scour development and surrounding flow patterns are discussed in detail. Typical steady-streaming structures around the pipeline due to the oscillatory flow condition are captured. The scour depth during the initial development of the scour process for the piggyback pipeline with the small pipeline placed above the large one is the largest among all the investigated configurations. The phase-averaged flow fields show that the flow patterns are influenced by the additional small pipeline.

Pipe-in-pipe (PIP) flowlines have been widely arranged in offshore oil and gas fields for transportation, to achieve a significant thermal insulation capacity and outstanding resistance to external loads. However, the lateral indentation mechanisms of these multilayer pipelines are more complicated than those of single-layer pipelines. In this paper, the change and deformation laws of lateral indentation in the PIP structure were studied by experimental and numerical methods, and three stages of deformation behavior for PIP during lateral indentation were observed. The effects of the diameter and wall thickness of inner and outer pipes on the relationship between the indentation load and lateral indentation were also studied, which provided a reference for the design and analysis of PIP structures.

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.

In present study, two-dimensional numerical simulations have been carried out to investigate scour beneath a piggyback pipeline in the vicinity of the seabed subjected to a subsea current using SedFoam (an open-source multi-dimensional Eulerian two-phase solver for sediment transport based on the CFD toolbox, OpenFOAM). In the piggyback configuration, one or two small pipelines are

Numerical simulations are conducted to investigate the near-bed flow behaviors of submarine canyons with periodic ridge-valley topography on continental slopes. In the numerical simulation, two topographic parameters are defined: α represents the basal slope angle of the continental slope, and β denotes the side slope angle of the submarine canyon. The topographic modulation of these two parameters on near-bed flow velocity and pipeline on-bottom stability is explored. The results show that the ribbed slope exerts two distinct modulation mechanisms on near-bed flow fields, which are governed by the relative angle between incoming flow and ridge-valley structures. When the flow is perpendicular to the canyon axis (θ = 0°), local topographic contraction and expansion accelerate flow over ridge crests and reduce near-bed velocity in valleys as β increases. In contrast, when the flow is parallel to the canyon (θ = 90°), lateral confinement by bilateral ridges forms a topographic channel, leading to persistent flow acceleration throughout the entire valley area. Moreover, such directional velocity differences substantially alter the on-bottom stability of pipelines deployed within the canyon. Two pipe-soil interaction models are employed in the stability analysis: the Verley-Lund model for a flat clayey seabed and the Slope-Silt model for a sloping silty seabed. The sensitivity of their predictions to soil undrained shear strength is further evaluated.