In this paper, the vortex-induced force characteristics of a flexible riser with buoyancy modules and strakes were experimentally investigated. The strain information caused by vortex-induced vibration (VIV) at different cross-sections on the flexible cylinder was measured in the model test. Combined with the modal superposition method and inverse analysis method for vortex-induced force, the vortex-induced forces on the flexible cylinder were inverted, including the excitation force in phase with velocity and added mass force in phase with acceleration. Then, the excitation coefficients and added mass coefficients at each time step were identified via a least-squares method. The results show that the added mass coefficient and excitation coefficient of the flexible riser covering buoyancy modules present a “sawtooth” distribution along the length of the pipe, while the total mass (sum of structural mass and added mass) is consistent along the pipe. The strake has little effect on the added mass coefficient of the buoyancy module and the bare pipe section, but when it covers the bare pipe section, the excitation coefficient of the bare pipe section decreases to a negative value, showing a damping effect. In addition, when the bare pipe section of the flexible riser is completely covered with helical strakes, only the vortex-shedding frequency component of the buoyancy modules remains in the VIV response of the pipe. The reason for this phenomenon is that helical strakes completely suppress the vortex shedding of the bare pipe section, resulting in the reduction in the VIV excitation energy of the original bare pipe section to 0.

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.

Unbonded flexible pipes are key components of deepwater high-temperature oil and gas transportation systems, and their bending performance directly affects in-place response and fatigue assessment. Interlayer contact and sliding of tensile armor layers govern bending hysteresis; under high-temperature service, incompatible thermal expansion of metallic and polymer layers changes contact pressure and the associated slip conditions. This study develops a thermo-mechanical bending hysteresis model in which thermally induced interlayer contact pressure links the radial temperature field to the bending response. A steady-state multilayer-cylinder heat-transfer model and a thermoelastic compatibility formulation are used to determine temperature distributions and interlayer contact pressures. The contact-pressure variation is then introduced into the tensile-armor slip criterion and the incremental moment-curvature relationship, covering non-slip, partial-slip, and full-slip stages. A sequentially coupled finite element model of a 2.5-inch unbonded flexible pipe is established for validation. The numerical model predicts hysteresis loop area and unloading/reverse-loading stiffness with relative deviations of 6.02% and 5.09% from the finite element results, respectively. Increasing internal temperature increases contact pressure and critical slip curvature, prolongs partial slip, and substantially increases hysteretic energy dissipation. The model provides a basis for high-temperature bending stiffness determination and fatigue-oriented analysis of unbonded flexible pipes.

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.