A. Introduction

A large proportion of hydrocarbon production pipelines operate in two-phase flow (natural gas and liquid hydrocarbons) or three-phase flow (natural gas, hydrocarbon liquid and water). There is thus, within the oil industry, an interest in drag reduction in such pipelines. Manfield et al. (1999) review the earlier work on drag reduction in multiphase flow systems. They note that the first experiments with drag reducing solutions in two-phase flow were by Oliver and Young Hoon (1968) who used the solution of 1.3% polyethylene oxide (PEO) in water in a two-phase flow with air. They studied both slug and annular flows and noted a reduction of pressure gradients; however, they did not use the term "drag reduction" or refer to PEO as a drag reducing agent (DRA). The first publication to explicitly mention drag reduction with additives in two-phase gas-liquid flow was by Greskovich and Shrier (1971) who reported a small number of tests, mostly in the slug flow regime. They obtained drag reductions up to 40%.

Though drag reduction in multiphase flows has been studied far less than that in single phase flows, there has been a burgeoning of work in the area over recent years. Reflecting the focus on hydrocarbon transportation, most of the work has been on horizontal gas-liquid flows (Section 236B) though there have been studies of vertical gas-liquid flows (Section 236C). Most of the work has focussed on polymeric DRA’s but there have been a limited number of studies on surfactant systems (see Section 236D). Other systems studies include three-phase flows (Section 236E) and solid-liquid flows (Section 236F).

B. Horizontal gas-liquid flows

The most important feature of gas-liquid flows is that of flow pattern. Thus, in horizontal tubes, the gas and liquid can flow in separated layers (stratified flow), in an intermittent fashion with slugs of liquid separated by stratified regions (slug flow) or in the form of a flow with a continuous gas core (often carrying entrained liquid droplets) surrounded by a film on the tube wall (annular flow). The incidence of DRA’s in a given flow regime can either be to change the characteristics of the flow in that regime or to change the regime itself.