Pipeline Pig Launching and Receiving

Pipeline “pigging” is the practice of cleaning and/or inspecting pipelines. This process entails inserting (and later removing) a physical device – a “pig” – into the pipeline. During pigging operations, the pig is inserted into the pipeline and is forced through by pipeline pressure or by using compressed gas, such as nitrogen. When the pig gets to the end of the line, it is trapped in a receiver. Before this device can be inserted or removed through the hatch of a pig launcher or a pig receiver, the pipeline gas in the launcher or receiver barrel must be removed, leading to potential methane emissions.

Pigging operations are conducted on both oil and natural gas pipelines to clean debris and accumulated liquids, to conduct inspections, and to remove residual product from the pipeline between product transfer operations. In natural gas gathering systems, raw natural gas is transported from production wells to gathering and boosting stations and processing plants through networks of pipelines. Despite liquid separation processes that may occur at the well pad, much of the raw natural gas passing through the gathering pipelines is saturated with hydrocarbons other than methane and may contain other components such as water, carbon dioxide, and hydrogen sulfide. During the transport of this gas through gathering pipeline systems, the gas often experiences temperature drop and pressure change that cause the hydrocarbons and other components to condense to a liquid phase. These natural gas condensates can accumulate in low elevation segments of the gathering pipelines, impeding the flow of natural gas. To maintain gas flow and operational integrity of the gathering pipelines, operators mechanically push these condensates out of the low elevations and down the pipeline by an operation known as “pigging.” Pigging operations are also conducted on natural gas transmission pipelines. 

Pigging involves inserting a device (i.e., a pig) into a launcher upstream of the pipeline segment where condensates have accumulated. Pigs are typically made of materials such as plastic, urethane foams, and rubber. The gas flowing through the pipeline pushes the pig through the pipeline, allowing the pig to sweep along the accumulated condensates. The pig is removed from the pipeline segment when it is caught in a pig receiver. Figure 1 shows a typical pig receiver design with bypass valves that allow the gas to flow as normal until the pig approaches the pig receiver barrel, at which time the bypass valves are opened to route the pig into the barrel. Pig launchers are of similar design. The pipeline at the rear of the barrel is called a “kicker line” on pig launchers and a “bypass line” on pig receivers; these lines direct the main gas flow through the barrel to launch or receive a pig, respectively.

Before a pig can be inserted or removed through the hatch of a pig launcher or a pig receiver, the pipeline gas in the launcher or receiver barrel must be removed, leading to potential emissions. The barrel gas is under the same pressure as the pipeline and contains methane, ethane, volatile organic compounds (VOCs), and may also contain hazardous air pollutants (HAPs) including benzene, toluene, ethylbenzene, and xylenes (i.e., BTEX). Pig receivers can also contain collected condensate liquid that had accumulated in the pipeline. The barrel depressurization gases are often vented to the atmosphere but can be controlled or recovered to reduce emissions. When the hatch is opened, additional gas is released, and any liquids collected in a pig receiver that did not volatilize in the depressurization process may be spilled from the open hatch as the pig is withdrawn. These spilled liquids can fall to the ground or be collected in an open tub, either of which allow for further evaporation that contributes to emissions. 

The quantity of potential emissions from pigging operations depends on several factors including the launcher or receiver volume, pipeline pressure (i.e., higher pressure correlates with higher potential emissions), amount of liquid trapped in the pig receiver barrel prior to depressurization, frequency of pigging, and gas composition. 

Figure 1. Pig Receiver.

Crouch, A. E. (1993, May). In-line inspection of natural gas pipelines (Gas Research Institute Topical Report GRI-91/0365).

Emission Inventory Improvement Program, Point Sources Committee. (1999, September). Preferred and alternative methods for estimating air emissions from oil and gas field production and processing operations (Volume II: Chapter 10). https://www.epa.gov/sites/default/files/2015-08/documents/ii10.pdf

U.S. Environmental Protection Agency. (2019, September). Enforcement Alert: EPA observes air emissions from natural gas gathering operations in violation of the Clean Air Act. https://www.epa.gov/sites/default/files/2019-09/documents/naturalgasgatheringoperationinviolationcaa-enforcementalert0919.pdf