LOCAL SCOUR AROUND HORIZONTAL CIRCULAR CYLINDERS

Abstract

In the accelerated exploration of offshore resources there is a growing need to place objects and structures on the seabed. Interaction between a submarine pipeline and an erodible bed has attracted much attention because of its importance in offshore engineering. Scour underneath the pipeline may expose part of the pipe causing it to suspend in the water, if the free span of the pipe is long enough, the pipe may experience resonant flow-induced oscillation, leading to structural failure. two Things have to be discussed in order to understand scour and to evaluate its problem, which is an impact effect. The first point is the correct estimation of the maximum scour depth and the second point is the mechanism of local scour around pipelines. The main objective of this study is to elaborate more on the second point. experiments have shown how local scour develops around a horizontal circular cylinder in non-cohesive sediments. It has been found that piping is the dominant cause of scour initiation. Piping and the stagnation eddy combine to undermine the cylinder 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 drops between the stagnation pressure upstream and wake pressure downstream of the cylinder induces the hydraulic gradient. When the cylinder is just embedded in the sediment, the onset of scour starts immediately under all flow conditions considered; while when there is any embedment in the sediment, the onset of scour will depend on the pressure gradient level with respect to the critical pressure gradient