Study on dynamic slope angle of sandy seabed around the submarine piggyback pipeline in steady flow

Abstract

Submarine piggyback pipeline is an important tool for oil and gas transport, especially, in the marginal oil field. When the scour happens in the seabed, the pipeline slides easily, which may cause the pipeline failure. The dynamic slope angle is one of the most important characteristics of the scour. In this work, the dynamic slope angles of the sandy seabed under piggyback pipelines in steady flow are investigated experimentally and numerically in detail. The physical experiments are conducted in an annular flume and the numerical simulations are carried out using a two-phase flow model which is resolved by the finite volume method (FVM). In the numerical model, the free surface is tracked by the volume of fluid (VOF), and both bed and suspended loads are considered in the scour module. Depending on the comparison and convergence analysis, the numerical results are in good agreement with the experimental results. The results indicate that the dynamic slope angle is significantly affected by the incoming flow velocity, gap-ratio, spacing-ratio and pipe diameter. With the reduction of gap-ratio and space-ratio, the upstream dynamic slope angle α increases slightly, but the downstream dynamic slope angle β decreases. With the increase of the grain Reynolds number, the angle α increases slightly, but the angle β decreases severely. The conclusions drawn from this paper could provide the reference for the design of submarine piggyback pipeline.