In pipeline transportation, pigging is a practice used to perform maintenance operations on pipelines without stopping the flow of the product. This is done by using devices called pigs or scrapers that are inserted into the pipeline. Pigs can be used for cleaning, inspecting, and separating different products in the pipeline. They can also be used to detect leaks and clear slugs of liquid from multiphase gas/liquid pipelines. Pigging requires the pipeline to be designed for pigging from the beginning. There are different types of pigs, including foam pigs, intelligent pigs, and inspection pigs.
https://en.wikipedia.org/wiki/Pigging