Pigging

Pigging refers to the practice of using internal devices, commonly referred to as 'pigs' to perform various operations on the internal side of a pipeline without stopping the flow of the product in the pipeline.

Pigs are also referred to as scrapers (especially in the Middle East), go-devils (Old USAF term)or swabs.

These operations include but are not limited to cleaning and inspecting of the pipeline. This is accomplished by inserting the pig into a 'pig launcher' (or 'launching station' or 'pig trap') - a means of inserting a pig into an operating pipeline without requiring it to cease flow or de-pressurise. The launcher / launching station is then closed and the flow of the product in the pipeline is used to push it along down the pipe until it reaches the receiving trap - the 'pig catcher' (or receiving station). The pig trap main barrel is normally 2 pipe sizes bigger than the pipeline with a bottom flat eccentric reducer squezzing the pig into the pipeline. When lauching pigs, the pig must be firmly located into the reducer or "neck" of the trap so that when fluid is passed through the trap, the pig is forced along the pipeline. Pig traps are normally designed as part of the pipeline or piping and are notnormally calssified as pressure vessels, but commonly use pressure vessel quick opening enclosures rather than multi-bolt flanged ends.

If the pipeline contains butterfly valves, reducers, large un barred tees, internal fixed probes or injection tubes, reduced bore valves, control valves or other restrictions the pipeline cannot be pigged. Ball valves and Gate valves normally cause no problems because the inside diameter of the ball or gate can be specified to be the same as that of the pipe (assuming they are full bore valves).

Pigging has been used for many years to clean and maintain pipelines in the oil and gas industry. Today, however, the use of smaller diameter pigging systems is now increasing in many continuous and batch process plants as plant operators search for increased efficiencies and reduced costs.

Pigging can be used for almost any section of the transfer process between, for example, blending, storage or filling systems. Pigging systems are already installed in industries handling products as diverse as lubricating oils, paints, chemicals, toiletries, cosmetics and foodstuffs.

Pigs are used in lube oil or painting blending: they are used to clean the pipes to avoid cross-contamination, and to empty the pipes into the product tanks (or sometimes to send a component back to its tank). Usually pigging is done at the beginning and at the end of each batch, but sometimes it is done in the midst of a batch, e.g. when producing a premix that will be used as an intermediate component.

Pigs are primarily used in oil and gas pipelines: they are used to clean the pipes but also there are "smart pigs" or "inteligent pigs" used to measure things like pipe thickness and corrosion along the pipeline. They usually do not interrupt production, though a minor amount product can be lost when the pig is extracted, due to draining or venting of the reciving pig trap - usually equivalent to less than 10m of pipeline volume. They can also be used to separate different products in a multiproduct pipeline, but this practice - sometimes using rubber spheres - is now not normally undertaken as in reality the separation is not impacted and the operation of inserting and removing pigs or spheres adds to the risk of the pipeline operation, especially when trasnporting hazardous fluids and on-line continuous detectors for specific gravity or colour are used to detect the interface.

The use of intelligent pigs to inspect an entire pipeline in a matter of days for internal and external corrosion has revolutionised the pipeline world and the actions taken to repair, replace or monitor corrosion defects has prevented many leaks and ruptures and allowed pipelines to remain in service with a level of confidence previously not able to be used, especially for gas pipelines where annual pressure tests were not normally possible or permitted. Unfortunatley due to this technology not being commonly available until the 1980's, many gas pipelines in particualr were built in such a configuration that intelligent pigging is not possible (very tight bends, blind tees, diameter changes etc) Some pipelines can be inspected using a tethered pig where the pig has a cable which both transmits data and also physically pulls the pig back wards if flow cannot be reversed. The length of line able to be inspected this way is limited and normally it requires modifications to the pipework to allow normal in-line pigging to occur.

Etymology

Pigs are generally accepted to have got their name from the squealing sound they make while travelling through a pipeline. (Disputed: 'PIG' is an acronym or backronym derived from the initial letters of the term 'Pipeline Inspection Gauge' or possibly 'Pipeline Inspection Gizmo' or 'Pipeline Internal Geometry' or 'Pipeline Inspection Gadget'). The desire of people to believe that pig is actually an acroynm (P.I.G.)is not backed by any historical or reliable source. The term "scaper" has come into fashion to avoid disputes about this, especially in locations where the word "pig" could be mis construed

Pigging in production environments

Product and time saving

A major advantage of piggable systems is the potential resulting product savings. At the end of each product transfer, it is possible to clear out the entire line contents with the pig, either forwards towards the receipt point, or backwards to the source tank. There is no requirement for extensive line flushing.

Without the need for line flushing, pigging offers the additional advantage of a much more rapid and reliable product changeover. Product sampling at the receipt point becomes faster because the interface between products is very clear, and the old method of checking at intervals, until the product is on-specification, is considerably shortened.

Pigging can also be operated totally by a programmable logic controller (PLC).

Environmental issues

Pigging has a significant role to play in reducing the environmental impact of batch operations. Traditionally, the only way that an operator of a batch process could ensure a product was completely cleared from a line was to flush the line with a cleaning agent such as water or a solvent or even the next product. This cleaning agent then had to be subjected to effluent treatment or solvent recovery. If product was used to clear the line, the contaminated finished product was downgraded or dumped. In some cases, the finished product could contain polychlorinated biphenyl (PCB), which has been found to be carcinogenic. All of these problems can now be eliminated due to the very precise interface produced by modern pigging systems.

Safety considerations

Pigging systems are designed so that the pig is loaded into the launcher, which is pressured up to launch the pig into the pipeline through a kicker line. In some cases, the pig is removed from the pipeline via the receiver at the end of each run. All systems must allow for the receipt of pigs at the launcher, as blockages in the pipeline may require the pigs to be pushed back to the launcher. Most of the time, systems are designed to pig the pipeline in either direction.

The pig is pushed either with an inert gas or a liquid; if pushed by gas, some systems can be adapted in the gas inlet in order to ensure pig's constant speed, whatever the pressure drop is. The pigs must be removed, as many pigs are rented, pigs wear and must be replaced, and cleaning pigs push contaminants from the pipeline such as wax, foreign objects, hydrates, etc., which must be removed from the pipeline. There are inherent risks in opening the barrel to atmosphere and care must be taken to ensure that the barrel is depressured prior to opening. If the barrel is not completely depressured, the pig can be ejected from the barrel and operators have been severely injured when standing in front of an open pig door. When the product is sour, the barrel should be evacuated to a flare system where the sour gas is burnt. Operators should be wearing a self-contained breathing apparatus when working on sour systems.

A few pigging systems utilize a "captive pig", and the pipeline is only opened up very occasionally to check the condition of the pig. At all other times, the pig is shuttled up and down the pipeline at the end of each transfer, and the pipeline itself is never opened up during process operation. These systems are not common.

Intelligent pigging

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Inserting a pig into a natural gas pipeline

Modern intelligent pigs are highly sophisticated instruments that vary in technology and complexity by the intended use and by manufacturer. An intelligent pig, or smart pig, includes electronics and sensors that collects various forms of data during the trip through the pipeline.

The electronics are sealed to prevent leakage of the pipeline product into the electronics since products can range from highly basic to highly acidic and can be of extremely high temperature. Many pigs use specific materials according to the product in the pipeline. Power for the electronics is provided by onboard batteries which are also sealed. Data recording may be by various means ranging from analog tape, digital tape, or solid state memory in more modern digital units.

The technology used to accomplish the service varies by the service required and the design of the pig, each pigging service provider may have unique and proprietary technologies to accomplish the service. Surface pitting and corrosion, as well as cracks and weld defects in steel/ferrous pipelines are often detected using magnetic flux leakage (MFL) pigs. Other "smart" pigs use electromagnetic acoustic transducers to detect pipe defects. Caliper pigs can measure the "roundness" of the pipeline to determine areas of crushing or other deformations. Some smart pigs can combine technologies such as MFL and Caliper into a single tool. Recent trials of pigs using acoustic resonance technology have been reported.

During the pigging run the pig is unable to directly communicate with the outside world due to the distance underground or underwater and/or materials that the pipe is made of. For example, steel pipelines effectively prevent any reliable radio communications outside the pipe. It is therefore necessary that the pig use internal means to record its own movement during the trip. This may be done by gyroscope-assisted tilt sensors, odometers and other technologies. The pig will record this positional data so that the distance it moves along with any bends can be interpreted later to determine the exact path taken.

Location verification is often accomplished by surface instruments that record the pig’s passage by either audible or gravinometric (or other) means. The sensors will record when they detect passage of the pig; this is then compared to the internal record for verification or adjustment. The external sensors may have GPS capability to assist in their location or even to transmit the pig’s passage, but the pig itself usually cannot use GPS as it requires being able to receive the satellite signals.

After the pigging run has been completed, the positional data is combined with the pipeline evaluation data (corrosion, cracks, etc.) to provide a location-specific defect map and characterization. In other words, the combined data will tell the operator the location and type and size of each pipe defect. This is used to judge the severity of the defect and help repair crews locate and repair the defect quickly without having to dig up excessive amounts of pipeline. By evaluating the rate of change of a particular defect over several years, proactive plans can be made to repair the pipeline before any leakage or environmental damage occurs.

Pig

A pig on display in a section of cutaway pipe, from the Alaska Pipeline

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A "Pig" launcher/receiver, belonging to the natural gas pipeline in Switzerland.

A PIG in the pipeline industry is a tool that is sent down a pipeline and propelled by the flow of the product in the pipeline itself. There are four main uses for pigs:

1. physical separation between different liquids being transported in pipelines;
2. internal cleaning of pipelines;
3. inspection of the condition of pipeline walls (also known as an Inline Inspection (ILI) tool);
4. capturing and recording geometric information relating to pipelines (e.g. size, position).

The original pigs were made from straw wrapped in wire used for cleaning. They made a squealing noise while traveling through the pipe, sounding to some like a pig squealing. The term "pipeline inspection gauge" was later created as a backronym.

One kind of pig is a soft, bullet shaped polyurethane foam plug that is forced through pipelines to separate products to reduce mixing. There are several types of pigs for cleaning. Some have tungsten studs or abrasive wire mesh on the outside to cut rust, scale, or paraffin deposits off the inside of the pipe. Others are plain plastic covered polyurethane. a pig is usually at least double the diameter in length so that it is well supported and less likely to become jammed or wear excessively.

Many standard cleaing or separation pigs use a shaped soft cup with harder plastic discs to support the pig in its passage along the pipe. Many are uni-directional and in systems where the passage of a pig is not proven in a recent period or when filling or celaning a new or repaired pipeline, the use of bi-directional or "bi-di" pigs are recomended. All pigs need to seal tightly against the outer wall so that they stay in the flow of the product in the pipeline. In reality this is difficult over long distances due to bends, gradients, internal weld beads etc and a slippage of up to 1% of fluid is not unknown. In addition, the pressure accross a pig is normally in the range 0.3 to 1.0 bar, 5 to 15 psi. A greater differential presusre, e.g. if a pig becomes stuck or trapped, will normally blow past the sealing discs.

Inline inspection pigs use various methods for inspecting a pipeline. A guaging pig uses one (or more) notched round metal plates that are used as gauges. The notches allow different parts of the plate to bend when a bore restriction is encountered. More complex systems exist for inspecting various aspects of the pipeline. Intelligent pigs, also called smart pigs, are used to inspect the pipeline with sensors and record the data for later analysis. These pigs use technologies such as MFL and ultrasonics to inspect the pipeline. Intelligent pigs may also use calipers to measure the inside geometry of the pipeline.

In 1961, the first intelligent pig was run by Shell Development. It demonstrated that a self contained electronic instrument could traverse a pipe line while measuring and recording wall thickness. The instrument used electromagnetic fields to sense wall integrity. In 1964 Tuboscope ran the first commercial instrument. It used MFL technology to inspect the bottom portion of the pipeline. The system used a black box similar to those used on aircraft to record the information.

A pig has been used as a plot device in three James Bond films: Diamonds Are Forever, where Bond disabled a pig to escape from a pipeline, The Living Daylights, where a pig was modified to secretly transport a person through the Iron Curtain, and The World Is Not Enough, where a pig was used to move a nuclear weapon through a pipeline.

A pig was also used as a plot device in the Tony Hillerman book The Sinister Pig where an abandoned pipeline from Mexico to the United States was used with a pig to transport illegal drugs.

Capacitive sensor probes are used in the process of detecting defects in polyethylene pipe gas pipeline. These probes are attached to the pig in which the pig is sent through the polyethylene pipe that will detect any defects in the outside of the pipe wall. This is done by using a triple plate capacitive sensor in which the electrostatic waves are propagated outware through the pipe's wall. Any change in dielectric material will result in a change in capacitance. Testing was conducted by NETL DOE research lab at the Battelle West Jefferson’s Pipeline Simulation Facility (PSF) near Columbus, Ohio.

See also

• Hydraulically Activated Pipeline Pigging
• Pipeline video inspection

External links

• Magnetic Flux Leakage Information
• Inline Inspection Association