Abstract

The design of crude oil production systems involving the pipelining of unseparated reservoir fluids requires a detailed understanding of multiphase flow phenomena. Slug flow in particular may cause undesirable process upsets if the separation and processing process upsets if the separation and processing facilities are not designed to handle the resultant gas and liquid flowrate variations.

Slug characteristics, primarily length, holdup and frequency, cannot be adequately predicted by existing empirical correlations. predicted by existing empirical correlations. Mechanistic models are more satisfactory if the measured slug frequency is used as an input. However, existing methods for predicting slug frequencies are also unsatisfactory.

BP have collected a large database on slug flow characteristics in both low pressure test rigs and field production flowlines. From this database BP have developed an improved method for the prediction of slug frequency. This method correlates slug frequency against the two parameters which appear from test rig parameters which appear from test rig observations to be the most important in the initial formation of slugs from a stratified flow. These parameters are the liquid holdup in the stratified film at the start of the line, and the gas-liquid slip velocity. The results indicate a much improved comparison with the data. The phenomenological approach to the correlation development gives increased confidence of its applicability to conditions outside the range of data from which it was developed.

Introduction

In current and future oilfield developments there is an increasing interest in production systems in which unseparated reservoir fluids are transported significant distances by pipeline. If an adequate wellhead pressure is pipeline. If an adequate wellhead pressure is available this can be achieved by natural flow, otherwise multiphase pumps can be used. The implementation of such systems can result in significant capital savings over more conventional schemes. Current producing examples include BP's South East Forties and Endicott developments.

The detailed design of the systems requires a thorough understanding of the behaviour of multiphase flow in a pipeline, and of the resultant potential effects on processing facilities. In order to reduce any overdesign, which has especially significant implications for offshore topsides weight, and avoid underdesign, which may result in inefficient separation or overloading, it is necessary to ensure that design methods are both accurate and reliable.

One phenomenon occurring in a multiphase pipeline, which can have a significant influence pipeline, which can have a significant influence on both the operability of process plant and on the mechanical construction of the pipeline system, is known as slug flow (Figure 1). This flow regime can cause large variations in gas and oil flowrates entering a separator (possibly resulting in oil carryover, gas carryunder, or significant level excursions resulting in shutdown), and can cause mechanical damage to pipeline connections and supports. pipeline connections and supports. P. 141

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