Applicability of the Shields Number for Prediction of Particle Movements in Oil Transmission Pipelines

Garcia, Alfonso (Enbridge) | Place, Trevor (Enbridge) | Corredor, Fabio Rodriguez (Enbridge)

OnePetro 

ABSTRACT

Corrosive conditions can develop where pipeline sediments accumulate in crude transmission pipelines. The accumulation of sediment at pipeline over bends occurs when inertial forces in the pipe flow cause a thickening in the boundary layer at the pipe floor, which decreases the shear stresses responsible of mobilizing solids. The Shields method has been proposed to predict the accumulation of solid particles taking into account the critical role that shear stress plays in transportation of solids.

The Shields method was originally developed as a river model, a simple dimensionless diagram that can be used to forecast solid particle bed formation in open channels and Newtonian fluid. The model predicts the required minimum viscous shear stress to initiate movement of a particle bed. The dimensionless Shields number provides a ratio of the hydrodynamic drag forces (viscous shear stress) to the net submerged forces of gravity and buoyancy acting on a particle in a loose sediment bed. This meaningful physical number provides useful information about the onset of bed particle movement in pipe flow. The current work explores this dimensionless number, including the applicability domain, to predict particle movement in oil transmission pipelines. Multiple scenarios are considered to compare the model’s prediction to actual pipeline experience.

INTRODUCTION

Particle movement by turbulently flowing fluid is of great importance in a varied range of engineering processes1. During the design of hydraulic transport systems, it is important to estimate the pressure velocity and pressure loss. Commonly the design velocity is selected to minimize pressure losses1. This design velocity might be close to the limit deposit velocity of suspended particles. If the transport velocity is close to the limit deposit velocity, it can cause not only a variety of transport and product quality problems1 but also internal corrosion due to solids accumulation2.

Oil transmission pipelines normally transport petroleum compounds that remain in an incompressible liquid state under normal operating conditions, with solids and water contaminants representing less than 5% by volume2. Two important parameters need to be considered while determining the transport velocity to avoid internal corrosion problems. These parameters are water accumulation and solids accumulation2. Internal corrosion in oil transmission pipelines typically occurs only where water drops out from the hydrocarbon phase and wets the pipe surface2. Therefore, by using flow modelling estimations, it is necessary to analyze critical parameters to predict water drop out and accumulation2. In addition, suspended solids that might accumulate on the pipe surface can contribute to increased internal corrosion issues in locations were waters accumulate2. Thus, transport velocity should be capable not only of preventing water drop out from the liquid phase, but also moving particles to avoid solids deposition. This paper will be focused on studying the particles’ movement and determining critical velocities to ensure particles are transported.