The combined flow of oil and gas in a pipeline presents many design and operational difficulties not present in single phase liquid or vapour flow. Frictional pressure drops are harder to estimate.

Liquid is likely to gather at low points in the pipeline and reduce the pipeline capacity to a point when slugs of liquid are pushed ahead by the gas.

The movement of large liquid slugs along the pipeline can cause additional pipeline stresses and the pipeline terminal facilities must be designed to receive such volumes of liquid by provision of large, specially designed vessels or energy absorbing pipework, known as slug-catchers.

The type of flow in a pipe is known as its flow regime.

Pipelines are seldom horizontal, as they have to follow the undulations of the seabed or the countryside, and often have vertical sections as they rise to join platforms or enter process streams.

In view of this, there can be complex flows regimes

The key difference between single-phase flow and two-phase flow is that it is much more difficult to determine pressure drops for two-phase flow. This is complicated if you consider that a difference in incline of several degrees, never mind 90º; can change entirely the nature of the flow regime.

Undulating terrain will generally not be a problem for single-phase pipelines; however, it can materially affect pressure drop in two-phase pipelines if there are a large number of. rises and falls, which the pipeline must cross.

Some two-phase regimes are caused by liquid condensation or fall-out from the gas due to reducing temperature and pressure along the length of the pipeline. For onshore gas lines liquid knock-outs can be provided at intervals such that liquids can be drained off by blow-down of the line.

Well flow lines often work in a two-phase regime, particularly because the well fluids usually contain both oil and gas and there may be no facility at the wellhead (E.g. at sub-sea wells) prior to the fluid reaching the gathering station (or platform).

Despite the problems associated with the prediction of two-phase estimates, more and more pipelines are being designed for such flow systems. For example when hydrocarbon condensate is separated from the gas at offshore platforms, it is invariably spiked back into the gas for transport to the shore in the pipeline. This is mainly because the economics would not support a separate line for condensate sales.