Experiments with various drag reducing additives in natural gas dense phase flow have been carried out in a 40 m (131 feet) 1-inch flow loop at Statoils Metering and Technology laboratory (K-lab). The main objective of the study was to examine if commercially available drag reducers for hydrocarbon liquids might have a potential for use in long distance dense phase gas transmission pipelines, in terms of reducing pressure drop and hence increase flow capacity. Additionally, some chemicals specially made for these testing purposes were tested. Another objective was to investigate the mechanism behind a possible drag reduction. Published field tests have shown that drag reducing additives (DRA) and corrosion inhibitors may have an effect on the pressure loss in gas pipelines. This has basically been explained by a smoothening of the pipeline wall surface when additives deposit on the wall surface.
Drag Reducing Agents, DRA, have for several decades been used in the petroleum industry for increasing capacity or reducing the pressure friction loss. Tests have shown that drag-reducing additives may give a reduction in the pressure friction loss by up to 80-85 % in turbulent single-phase liquid flow systems. DRA in liquids works to reduce frictional pressure loss. It works in turbulent flows only and is influencing the structure of the turbulent boundary layer. Field tests have shown that DRA in oil transmission lines tend to have better efficiency the lower the density, or the viscosity. Gas in dense phase, i.e. the pressure is higher than the cricondenbar, will behave more similar to liquids, and there is no sharp phase transition between gas and liquid. The main difference is expected to be the density, which is lower than for the lightest hydrocarbon liquids. It has in this context been questioned whether injection of DRA in rich gas in dense phase may have an effect or not. Statoil operated in autumn 2001, 3 long distance pipelines transporting rich gas with a total capacity of about 66 BCM/year (2.3 Tcf/year), Figure 1, including 2 long distance pipelines carrying wet gas. The total distance was about 1300 km (800 mile). Additionally; Statoil operated pipelines transporting processed gas in dense phase to Europe, with a capacity of about 81 BCM/year (2.9 Tcf/year) and a total length of about 3700 km (2300 mile), although this length also includes some pipelines that are not landed in central Europe. In 2002 the operator ship for the gas transmission network was transferred from Statoil to Gassco. Statoil started some years ago an R&D programme on flow improver applications. The programme was in the start focusing on oil pipelines and several test facilities were built for testing purposes. One of the tasks in the R&D project was, however, to evaluate effects of drag reducer agents in rich gas dense phase flow. The scope was to investigate the feasibility of obtaining reduction in the frictional pressure loss in pipelines transporting rich gas in dense phase, by injection of commercially available DRA.
