Numerical investigation of local scour below a sagging pipeline in current

Abstract

Subsea pipelines are an integral part of energy supply. Pipelines run in long distances and become subject to various loads in the harsh submarine environment. A pipeline sags due to its weight when a span is created by the scour of the sediments under the pipeline caused by the flow. In this numerical study, the extent of pipeline sagging and the duration of a complete sagging where the pipeline is laid down in the scour hole have been investigated. Sagging of pipeline is followed by a backfilling process where as the result, pipeline eventually is buried in the seabed and becomes protected against hydrodynamic forces. The duration of sagging is important from a design point of view since it determines the time needed for a storm to create the scour hole leading to selfburial. The equations pertinent to a sagging pipeline are derived in a normalised format to allow extending the findings to a broader range of applications. In this computational study, the Reynolds Averaged Navier-Stokes (RANS) equations dominating the incompressible flow are solved by an Upstream Galerkin Finite Element scheme. The flow results are simultaneously coupled with a morphological model to assess the transportation of sediment on the seabed. Based on the results of simulations, efforts have been made to obtain empirical relations to predict the maximum scour depth, sagging time and a time history of scouring development under a sagging pipe. In this study, various case scenarios are considered to factor the effects of pipeline diameter, pipeline unit weight, flow strength and initial embedment depth on the final results. The results show that the scour hole depth that is developed under the pipe, is about one pipeline diameter. The scour time of a sagging pipe, however, varies considerably by the intensity of flow, pipeline weight, and pipeline initial embedment depth.

Date of Award	2018
Original language	English