An Investigation of Transient Two-Phase Flow Patterns in a Horizontal Pipeline

Abstract

ABSTRACT The successful exploitation of much of the remaining oil and gas reserves will involve their transportation as multiphase flows. While significant work has been carried out to design and operate these processes under steady state conditions the actual processes are normally transient in nature. This can be due to changes in demand, seabed topography or thermodynamics. Only limited transient operational and experimental information has been collected and as a result transient computer simulations have not yet been adequately validated. CALtec through its Multi-phase Pipelines and Equipment project (MPE) recently performed a series of transient twophase air/water experiments on a 0.0508 m (2") diameter, 367 m long horizontal pipeline. The objective of the work was to evaluate existing transient prediction methods and computer simulations by comparing them against experimentally supplied data involved changing either the air or water flow rate of a two-phase mixture from one equilibrium condition to another. Data was collected for both steady state and transient conditions, observations of flow pattern and measurements of liquid hold-up and sweep-out, pressure drop and slug characteristics were made. This data was used to evaluate the performance of a number of transient prediction methods. The work has shown the limited range of applicability of a number of prediction methods. This paper concentrates on the flow patterns observed during the transient phases of the experiments and compares them against predictive methods available in the open literature. 1. INTRODUCTION The transportation of oil and gas simultaneously in a single pipeline as a multi-phase flow is one of many design scenarios that must be evaluated before an offshore field can be developed. Such practices are increasingly needed to exploit natural gas and crude oil produced in deep water or marginal fields where installation of platforms, high performance separators and the laying of two pipelines for gas and liquid is not economical. This is in part due to the continuing depletion of North Sea oil and gas reserves which is forcing the North Sea operators to seek cost effective exploitation of the more marginal and deep water fields. Similar economic pressures are also resulting in the development of multi-phase systems in the Gulf of Mexico, off the coast of Brazil and in Indonesia. To do this requires new and improved design methods and hence a greater understanding of the multi-phase flow phenomena such as flow pattern, pressure drop and liquid hold-up. Many studies have been conducted in the area of steady state two-phase flow in pipelines. Various empirical correlations have been proposed for both the pressure loss and liquid holdup calculations. Several steady state twophase flow models and practical computer codes have also been developed to help engineers design two-phase flow pipelines. In fact, there are some reliable models now to predict the steady state two-phase flow phenomena with acceptable accuracy in horizontal and near horizontal pipelines.