Numerical Investigation of Scour Beneath Pipelines Subjected to an Oscillatory Flow Condition

Jun Huang,Muk Chen Ong,Xu Jia,Dag Myrhaug,Guang Yin

doi:10.3390/jmse9101102

Abstract

The present study carries out two-dimensional numerical simulations to investigate scour beneath a single pipeline and piggyback pipelines subjected to an oscillatory flow condition at a Keulegan–Carpenter (KC) number of 11 using SedFoam (an open-source, multi-dimensional Eulerian two-phase solver for sediment transport based on OpenFOAM). The turbulence flow is resolved using the two-phase modified k−ω 2006 model. The particle stresses due to the binary collisions and enduring contacts among the sediments are modeled using the rheology model of granular flow. The present numerical model is validated for the scour beneath a single pipeline, and the simulated sediment profiles are compared with published experimental data and numerical simulation results. The scour process beneath three different piggyback pipelines under the same flow condition are also considered, and the scour development and surrounding flow patterns are discussed in detail. Typical steady-streaming structures around the pipeline due to the oscillatory flow condition are captured. The scour depth during the initial development of the scour process for the piggyback pipeline with the small pipeline placed above the large one is the largest among all the investigated configurations. The phase-averaged flow fields show that the flow patterns are influenced by the additional small pipeline.

Highlights

Offshore pipelines are commonly used to transport oil or gas in industries
It was found by Sumer and Fredsøe [2] using experimental measurement that the equilibrium scour depth mainly depends on the Keulegan–Carpenter number (KC) defined as KC = UmTw/D, where Um is the amplitude of the undisturbed near-bed orbital velocity, Tw is the wave period and D is the diameter of the pipeline, instead of the Shields parameter
The objective of the present study is to evaluate how the presence of the small pipeline and different configurations of the piggyback pipeline influence the scour under a sinusoidal oscillatory flow condition

Summary

Introduction

Offshore pipelines are commonly used to transport oil or gas in industries. When these pipelines are placed on the sandy seabed, the local scour will be induced by the surrounding complex flow. On the seabed in deep water, the scour is usually caused by currents, while in shallow water regions, the pipelines can be subjected to a wave-induced oscillatory flow, where a more complex scour process beneath the pipeline will happen. It was found by Sumer and Fredsøe [2] using experimental measurement that the equilibrium scour depth mainly depends on the Keulegan–Carpenter number (KC) defined as KC = UmTw/D, where Um is the amplitude of the undisturbed near-bed orbital velocity, Tw is the wave period and D is the diameter of the pipeline, instead of the Shields parameter. There is backfilling of the scour hole under an ever-changing wave and current climate according to Sumer et al [5]

Objectives

Results

Conclusion