A slugcatcher is the part of a pipeline system that dissipates the energy of liquid slugs that intermittently occur and temporarily stores any excess material for later processing. Slugs are large volumes of liquid or gas the 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 buffer all input or brought online only when a slug is expected.
Pipelines that allow two-phase flow, or the simultaneous transportation of both liquid and gas, are subject to slug formation. Petroleum from a drill site carries pockets of natural gas and water. Gas pipelines often experience hydrocarbon condensation and accumulations of water or processing chemicals. When the flow of liquid and gas in a pipeline is not constant, plugs consisting 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 gas flow. The gas is compressed behind the blockage until the pressure overcomes the weight of the accumulation and dislodges it. A liquid slug is created that is pushed along in front of the expanding gas. This process is responsible for both terrain slugging and the riser-based slugging common to offshore drilling facilities.
Under certain conditions, gas flowing over a slower moving liquid will cause a wave to form. If the height of the wave reaches the top of the pipe, slug formation begins. Pushed along by the flow of the gas, the slug accumulates volume in the direction of flow and looses volume in the opposite direction. The relative rates of accumulation and loss determine if a sustained slug results. This formation process is called hydrodynamic slugging.
Pigging involves inserting devices into a pipeline for inspection, maintenance or cleaning. Typically, pipeline flow is not halted during this process. Consequently, pigging slugs will often form as either the flow is blocked or the pipe's content is push out ahead of the device. Unlike other instances of slug formation, which are periodic, this type is induced.
Slugcatcher designs vary according to the needs of the facility, however, there are three basic categories. The vessel type slugcatcher is generally a simple storage vessel that also allows for separation of the gas and liquid phases. A multiple-pipe slugcatcher can be implemented from standard pipeline material and has great flexibility in dealing with a wide range of flows. In the parking-loop slugcatcher, separation and storage are distinct elements. The separator might be a vessel type while the storage part is a pipeline loop located at a distance.
