here is what www.pipelineengineering.co.uk has to say; Pipeline pigging was developed in the fifties in the USA to clear debris from crude oil pipelines. They have now become widely used around the world in pipelines carrying anything from chocolate to shampoo but are most commonly used in the oil gas and petrochemical industries. A pig acts like a free moving piston inside the pipeline, sealing against the inside wall with a number of sealing elements. Pigs can perform a number of tasks including cleaning debris from the line, the removal of residual product in, and gauging the internal bore of, the pipeline. Pigs generally need specially designed launching and receiving vessels to introduce them into the pipeline. These vessels consist of a quick opening closure for access, an oversized barrel, a reducer and a neck pipe for connection to the pipeline. Pigs can be located using fixed signallers along the pipe or electronic tracking systems mounted inside the pig. The pigging of any pipeline will increase the lifetime, guarantee maximum throughput and minimum downtime due to blockages.