Estimation of Pressure Drop In Two-phase Oil-gas Looped Pipeline Systems
Authors: Gregory G.A. , Fogarasi Maria 
Published: March 2010

This paper presents a method for estimating the over-all pressure drop in a looped oil-gas pipeline. With this procedure, possible solutions are examined systematically in such a way that upper and lower bounds are established for the downstream pressure.

The use of the method is demonstrated with a number of examples that illustrate the effect of pipeline elevation profile, flow rate, pipe diameter for the side branch and the produced gas-oil ratio on the calculated results. It is shown that each of these parameters may affect not only the magnitude of the uncertainty in the calculated pressure drop, but also the nature of how it varies.

Introduction

When the capacity of an existing pipeline must be increased to handle large volumes of produced fluids it is common practice to install a loop, or parallel section of pipeline. The loop may extend over all or only part of the length of the existing line, and is usually connected to the original line at both ends through simple tee junctions. The effect of the loop is to increase the cross-sectional area available for flow.

This technique has been used for many years, particularly for single-phase gas pipelines, although many looped crude oil pipelines are also in service. Simplified techniques have been developed for performing flow calculations in these systems, usually based on the concept of a hypothetical equivalent pipeline which is hydraulically similar to the looped line.

The complexity of multiphase gas-liquid flow, with its dependence on phase behaviour, phase velocities, terrain effects, etc., precludes the use of a simplified equivalent single pipeline analysis, and each branch of the looped section must be considered separately. Unfortunately, however, a significant weakness in current multiphase flow technology is the lack of a method to accurately calculate the fractions of gas and liquid entering each branch when a two-phase mixture is split, say at a tee junction in the pipeline. Thus, when the flow in a pipeline carrying a multiphase mixture is split, the flow rates of the individual phases in each branch are unknown. The consequence is that the pressure drop across the looped section of the pipeline cannot be calculated accurately.

In this paper, a method is proposed by which the over-all pressure drop in a looped oil-gas pipeline can be estimated within reasonable limits. Using the suggested procedure, possible solutions are examined systematically, with the result that upper and lower bounds can be established for the over-all pressure drop.

The Proposed Method

The looped portion of a pipeline is shown schematically in Figure 1. At Point 1, the flow splits at a tee junction such that a fraction of the total flow continues in Line A, while the remainder flows through Line B. At Point 2, the two streams are recombined and continue to flow in a single line. In general, the flowing gas-oil ratio in Line A will not be equal to that in Line B due to the complex and unknown mechanisms associated with the splitting of a two-phase flow at a tee junction.