Two-Phase Flow Simulation of Tunnel and Lee-Wake Erosion of Scour below a Submarine Pipeline

Antoine Mathieu,Tim Nagel,Julien Chauchat,Cyrille Bonamy

doi:10.3390/w11081727

Abstract

This paper presents a numerical investigation of the scour phenomenon around a submarine pipeline. The numerical simulations are performed using SedFoam, a two-phase flow model for sediment transport implemented in the open source Computational Fluid Dynamics (CFD) toolbox OpenFOAM. The paper focuses on the sensitivity of the granular stress model and the turbulence model with respect to the predictive capability of the two-phase flow model. The quality of the simulation results is estimated using a statistical estimator: the Brier Skill Score. The numerical results show no sensitivity to the granular stress model. However, the results strongly depend on the choice of the turbulence model, especially through the different implementations of the cross-diffusion term in the dissipation equation between the k − ε and the k − ω 2006 models. The influence of the cross-diffusion term tends to indicate that the sediment transport layer behaves more as a shear layer than as a boundary layer, for which the k − ε model is more suitable.

Highlights

The scour phenomenon is a major cause of the rupture or self-burying of submarine pipelines.This phenomenon has to be accurately predicted during the design process of these submarine structures
The erosion under a submarine pipeline can be decomposed into three steps: (1) the onset; when the current around the cylinder is strong enough, it generates a pressure drop, which liquefies the sediments underneath the cylinder; (2) the tunneling stage; when a breach is formed between the cylinder and the sediment bed, it expands due to the strong current in the breach; (3) the lee-wake erosion stage; when the gap is large enough, vortices are shed in the wake of the cylinder, leading to erosion downstream of the scour hole [1]
It clearly appears that the shape of the sediment bed and its time evolution were not very sensitive to the granular stress model

Summary

Introduction

The scour phenomenon is a major cause of the rupture or self-burying of submarine pipelines. This phenomenon has to be accurately predicted during the design process of these submarine structures. The erosion under a submarine pipeline can be decomposed into three steps: (1) the onset; when the current around the cylinder is strong enough, it generates a pressure drop, which liquefies the sediments underneath the cylinder; (2) the tunneling stage; when a breach is formed between the cylinder and the sediment bed, it expands due to the strong current in the breach; (3) the lee-wake erosion stage; when the gap is large enough, vortices are shed in the wake of the cylinder, leading to erosion downstream of the scour hole [1]. The data collected by Mao (1986) [2] have been widely used as a benchmark for sediment transport models applied to the scour phenomenon

Methods

Results

Discussion

Conclusion