Scour Beneath and Adjacent to Submarine Pipelines with Spoilers on a Cohesive Seabed: Case Study of Hangzhou Bay, China

This study focused on scour beneath and adjacent to submarine pipelines with spoilers on a cohesive seabed. Field data about the seabed bathymetry and seabed subbottom imagery along the pipeline length direction in the Hangzhou Bay, China were collected between 2005 and 2016 for analysis. Results showed that during the last 12 years, self-buried pipelines increased from 40 to 90%; while distributions of scour pits changed randomly with time. Statistical parameters (depth and length of free span) of the scour pits beneath the pipeline in this study were roughly consistent with the values published in previous studies. However, scour adjacent to pipelines was different from that beneath pipelines in that it was much larger, and the formation of it was related to the erosion of the silt loam of the seabed. Three factors (decreased sediment supply, seabed scouring, and installation of spoilers), which probably led to the scouring, were investigated. Sediment supply to the Hangzhou Bay was reduced remarkably during the last 30years, and the decrease of sediment load in the Yangtze River was the primary cause. In the Hangzhou Bay, the seabed near deep channels was continuously eroded, and the extent of erosion near the pipelines was the largest. Spoilers on pipelines could increase the degree of local turbulence and stimulate the suspension and transportation of sediment. Under the measured spring tidal current condition, the dimensionless scour depth calculated by Goncharov's equation was relatively large; for example, the 273.1-mm (10.75-in.) diameter pipeline as an example, it was in the range of 0.33-2.49. According to the easily erodible property of silt loam, a hypothesis about the random distribution of scour pits was presented.