Pigging of Subsea Pipelines

Abstract

AN ART?....A SCIENCE?? As offshore developments move into greater water depths the problems of slugging, associated with condensate drop out and two-phase flow in gas pipelines becomes particularly acute at the pipeline riser. These vertical sections of pipe, some of which are over 350 m high, present some interesting problems in relation to slug formation and removal as the pressure varies due to the head of liquid which may build-up during some operational phases. Factors Affecting Two-Phase Flow Gas-Liquid Ratio The basic behavior depends on whether or not both the liquid and gas phases are present as a continuous phase. In bubble flow, one has a series of bubbles of gas in the liquid, and gas N not in a continuous phase. At the other end of the scale--spray or mist flow, small amounts of liquid are suspended in the gas. Now, liquid is now the discontinuous phase and the gas is the continuous phase. In systems where both the gas and liquid are continuous, the character of the flow is more difficult and de ends on such variables as phase densities, viscosities, and velocities of each phase. Volume/Flow Under certain conditions, constant flow can be maintained without the use of spheres, if the flow velocity is sufficient to keep the liquid moving. However, under most conditions, periodic pigging must be maintained to provide the maximum efficiency. Operating Pressure/Design Pressure As liquid columns increase in size in each valley, the total pipeline pressure differential increases to the point where up-stream pressure may be increased in excess of the MAOP (maximum allowable operating pressure). The importance of periodic pigging is now obvious. Allowable Pressure Drop Liquids that sag in valleys cause pressure differentials as the gas pushes the liquid level to the point where the gas has to bubble through the liquid column in order to maintain flow. Velocity of the Gas-Liquid Due to the interaction between discrete phases, the gas moves more rapidly than liquid and can cause problems down-stream as the column of fast-moving liquid comes to the slug catcher. During the life of a pipeline, depending on the transported fluid and its service conditions, the pipe wall will deteriorate, possibly through corrosion or as a result of scale or other solid deposits adhering to it. This will cause the deterioration in the economics of the pipeline operation due to increased pressure drop in one of two ways, Firstly, the deposits may effectively reduce the pipeline diameter and thus its throughput capacity and secondly, the roughness of the pipe wall may be increased with its attendant adverse effect on pressure drop. For example, the presence of wax in crude oil can lead to the precipitation of wax deposits on the pipeline wall. This wax deposition is time dependant but a thin layer can build up quickly to produce, on larger throughputs, significant reductions in line capacity. Why Pig? There are basically four reasons for pigging a pipeline.