Experimental investigation of the nearbed dynamics around a submarine pipeline laying on different types of seabed: the interaction between turbulent structures and particles

The role of sediment particle dynamics on the overall mechanics of flow-sediment-pipeline interactions has been investigated on the basis of dedicated laboratory experiments, of which one sample case has been analyzed in detail to provide a significant insight in the fundamental dynamics. Particle Tracking Velocimetry (PTV) has been used to study the seeded flow and the sediment particle dynamics. Turbulent structures around the pipeline have been characterized in terms of the spanwise component of the vorticity, of the Okubo-Weiss parameter and of the Reynolds stresses. It is shown that the vortical structures which dominate the flow around the pipeline generate sweeps and ejections associated to clockwise/anticlockwise rotating vortices. Clockwise vortices are observed downstream of the pipeline during flow acceleration phases (positive flow), anticlockwise vortices shed upstream of the pipe during flow deceleration phases (negative flow). These structures are seen to largely influence the sediment dynamics near the bed, in particular their entrainment from the bed with particles captured by ejections and, settling to the bed with particles captured by sweeps. The presented results highlight the important influence of turbulent structures in the erosion process around the pipeline.