Mechanics of Local Scour Around Submarine Pipelines

TLDR

The study aims to clarify how unidirectional currents cause scour around submarine pipelines. Local scour develops due to a pressure drop between upstream stagnation and downstream wake, creating a hydraulic gradient that initiates scour. Scour initiation is driven by piping and stagnation eddies, with a critical hydraulic gradient equal to the bed sediment flotation gradient; scour is avoided when flow depth/pipe diameter exceeds 3.5, for half‑buried pipes, or by installing an impermeable membrane.

Abstract

The primary objective of this study is to improve understanding of the mechanism causing scour in unidirectional current. Experiments have shown how local scour develops around submarine pipelines in noncohesive sediments. The study shows that piping is the dominant cause of the initiation of scour. Piping and the stagnation eddy combine to undermine the pipeline, and mark the onset of scour. The critical hydraulic gradient associated with the initiation of scour is equal to the flotation gradient of the bed sediment. The pressure drop between the stagnation pressure upstream and wake pressure downstream of the pipe induces this hydraulic gradient. When a pipe is just embedded, the onset of scour does not occur if the ratio of the flow depth to pipe diameter exceeds 3.5. Similarly, the onset of scour does not occur for half‐buried pipes. The reduction in pressure gradient across the pipeline for these flow/pipe combinations accounts for the lack of scour. The onset of scour can be prevented by placing an impermeable membrane underneath the pipeline.