What is a Slugcatcher?

Pipelines that allow two-phase flow, or the simultaneous transport of liquid and gas, are subject to slug formation.

A slugcatcher is the part of a piping system that dissipates energy from intermittently occurring liquid slugs and temporarily stores any excess material for further processing. Slugs are large volumes of liquid or gas that exit a pipeline as a unit. The slugcatcher is located at the end of a pipeline and acts as a buffer to protect processing equipment. It can be a permanent fixture designed to store all inputs or brought online only when a slug is expected.

Pipelines that allow two-phase flow, or the simultaneous transport of liquid and gas, are subject to slug formation. Oil from a drilling site carries pockets of natural gas and water. Pipelines often experience condensation from hydrocarbons and build-up of water or process chemicals. When the flow of liquid and gas in a pipeline is not constant, plugs that consist almost entirely of gas or liquid can form. These accumulations are called slugs.

Variations in the elevation of a pipeline due to terrain can cause liquid to accumulate at low points and block the flow of gas. The gas is compressed behind the blockage until the pressure overcomes the weight of the buildup and displaces it. A liquid mass is created and pushed in front of the expanding gas. This process is responsible for both ground slugging and riser-based slugging common in offshore drilling facilities.

Under certain conditions, gas flowing over a slower moving liquid will cause a wave to form. If the wave height reaches the top of the tube, packet formation begins. Pushed by the flow of gas, the slug accumulates volume in the direction of the flow and loses volume in the opposite direction. The relative rates of accumulation and loss determine whether the result is a sustained slug. This formation process is called hydrodynamic slugging.

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Pigging involves inserting devices into a pipeline for inspection, maintenance, or cleaning. The pipeline flow is not normally interrupted during this process. Consequently, pigging slugs will often form when the flow is blocked or the contents of the tube are pushed out of the device. Unlike other instances of slug formation, which are periodic, this type is induced.

Slugcatcher designs vary depending on the needs of the installation, however there are three basic categories. The container-type slugcatcher is generally a simple storage container that also allows separation of the gas and liquid phases. A multi-tube slugcatcher can be implemented from standard piping material and has great flexibility to handle a wide range of flows. In the parking loop slugcatcher, separation and storage are distinct elements. The separator can be a type of container, while the storage part is a pipeline circuit located at a distance.

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