Pipeline Design and Construction


sion pipeline is constructed, a pig is run through the line to remove any remaining debris, such as tools, welding rods, and dead animals trapped in the line. Pigging will also remove mill scale in the line. The pipeline owner may also require a gauging pig or a geometry pig to verify the ovality of the pipeline.


After the pipeline has been cleaned, the next phase is acceptance testing


where pigs are used for filling the line with water for hydrostatic testing, de- watering (i.e., removing the water after testing), and drying. If the pipeline is a liquid line, a pig is used to fill the line with a product during the com- missioning and start-up of the line.


Once a liquid pipeline is in service, it will be necessary to pig the pipeline


to maintain its efficiency and to aid in the control of corrosion. For example, any liquids must be removed from wet gas pipe systems, accumulated water from product pipelines, and paraffin from crude oil pipelines. Pigs are also used to separate batches of corrosion inhibitors shipped through the pipeline. Lines used for clean products, such as liquefied petroleum gas, usually re- quire fewer pigging operations than lines used for more contaminated prod- ucts such as crude oil.


As time passes, special clean-


ing applications for the pipeline may rise. Under certain condi- tions a train of gel pigs may be used for chemical cleaning. Prior to running inline inspection tools, gauge plates and cleaning or scraper pigs may be used to remove the buildup of material on the internal pipe wall and en- sure passage of the instrumented smart pigs. Cleaning pigs are also used to remove accumulated water and other products from low spots in the pipeline. Pipelines that are to be aban- doned require cleaning before they are laid up or mothballed for possible future use.


Geometry pigs are used to


check for dents or buckles in the line. Other instrumented smart pigs are used to identify and de- termine the amount of corrosion or metal loss in the line and to identify cracks or gouges in the line. Additionally, separator pigs may be used to separate batches in liquid pipelines used for the batch shipment of multi- ple products, such as various grades of gasoline, heating oils, and jet fuels.


Figure 3.12 This Pig Receiver Station on a natural gas pipeline shows where the pig is trapped after arriving from the incoming pipeline.


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Figure 3.11 Cutaway of pipeline with pig inside.


Pig Receiver


Incoming Pipeline Outgoing Pipeline


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