Hydromorphology of bidirectional vibration-induced local scour around tandem pipelines

Dual tandem pipelines are highly susceptible to simultaneous vortex-induced vibrations and seabed scour, posing challenges in ocean engineering. A computationally efficient framework is developed to simulate the influence of diverse bidirectional vibration patterns of tandem pipelines on seabed morphology under controlled conditions, which are difficult to replicate in full free-motion simulations. This framework employs the vibration–trajectory description approach to integrate the pipeline dynamics into the numerical model, while Reynolds-averaged Navier–Stokes equations and sediment transport models describe the hydrodynamics and seabed evolution. The effects of 16 vibration modes, varying horizontal gap ratios, different vibration frequencies, and downstream phase lags are systematically analysed. The results demonstrate that bidirectional vibrations are a dominant mechanism driving seabed evolution, producing deeper scour than fixed or unidirectionally oscillating pipelines. Among the vibration trajectories, the figure-eight mode induces the most pronounced scour. Two alternating hydrodynamic mechanisms were found to dominate seabed erosion: the nozzle effect during downward motion and the collision effect during horizontal motion. Gap ratio, phase lag, and vibration frequency collectively shape the scour pattern, with small gaps forming merged depressions and higher frequencies increasing scour depth by 23.4%, accompanied by a shift in vortex shedding from 2S to 2P.