One of the stranger conversations you can overhear at a pipeline facility is someone saying that they are going to “launch a pig.”
No animals are involved.
In the oil and gas industry, a pig is a device that travels through the inside of a pipeline.
Some pigs clean the pipe.
Some remove accumulated liquids.
Some separate different products.
Others contain sophisticated electronics capable of inspecting hundreds of kilometres of pipeline from the inside.
Pipeline pigging sounds simple at first. Put something into one end of a pipe, allow the flowing product to push it through, then remove it at the other end.
In reality, pigging can be an important part of pipeline maintenance, production operations, corrosion management, and integrity programs.
Understanding why requires looking at what gradually happens inside a pipeline while it is operating.
Why Do Pipelines Need Cleaning?
A pipeline may look like an empty steel tube before it enters service.
It does not necessarily stay that way.
Depending on what the pipeline transports, the inside can accumulate:
Wax
Water
Sand
Scale
Corrosion products
Debris
Condensate
Other solids or liquids
Those materials can reduce the effective diameter of the pipeline and interfere with normal operation.
They can also contribute to corrosion or create problems at downstream facilities.
A pig provides a way to remove some of that material without dismantling the pipeline.
What Does a Pipeline Pig Look Like?
There is no single design.
A simple pig may consist of polyurethane components that fit closely against the internal wall of the pipeline.
Other pigs contain discs.
Some use cups.
Some have brushes.
Some contain magnets.
Some are essentially foam cylinders.
Inspection pigs can be much more complicated and may contain sensors, batteries, data storage, odometers, and other electronics.
The correct pig depends on what the operator is trying to accomplish.
Cleaning a waxy crude oil pipeline and inspecting a high pressure natural gas transmission pipeline are very different jobs.
How Does the Pig Move?
In many operations, the pipeline product itself moves the pig.
Pressure is applied behind the pig.
The pressure in front of it is lower.
That pressure difference creates a force that pushes the pig through the line.
If it is a natural gas pipeline, gas may push the pig.
If it is an oil pipeline, liquid may push it.
The pig therefore travels with the help of the pipeline’s existing pressure and flow.
That does not mean it moves at exactly the same velocity as the fluid.
Pig speed is something operators need to consider and control.
What Is a Pig Launcher?
You cannot normally open a pressurized pipeline and push a pig inside.
A special piece of equipment is used.
It is commonly called a pig launcher.
A launcher is a section of piping designed so the pig can be loaded while the main pipeline remains part of an operating system.
It normally has a closure that provides access to the barrel.
Valves allow the launcher to be isolated, depressurized, loaded, repressurized, and connected to pipeline flow in a controlled sequence.
The details vary significantly between facilities.
The important point is that the launcher allows the pig to enter the pipeline safely.
What Is a Pig Receiver?
Eventually the pig needs to come back out.
That happens at a pig receiver.
The receiver is another enlarged section of piping equipped with isolation and pressure control arrangements.
The arriving pig enters the receiver barrel.
The receiver can then be isolated from the operating pipeline.
Pressure is safely removed.
Only after the equipment has been verified in the required condition can the closure be opened and the pig removed.
Pig receivers often collect much more than the pig itself.
Everything pushed ahead of the pig may arrive there too.
Opening a Pig Trap Is Not Like Opening a Door
This is one of the most important things for anyone new to pipeline operations to understand.
A pig launcher or receiver can contain pipeline pressure.
Depending on the system, that could mean hundreds or even thousands of psi.
It may also contain flammable hydrocarbons.
Hydrogen sulfide may be possible in some services.
Liquids can remain trapped.
Pressure can remain even when a gauge appears to show zero.
For those reasons, pig launching and receiving follow specific operating procedures.
Isolation, depressurization, draining, venting, verification, and closure operation all matter.
A large pipeline closure is not something to open because someone thinks the pressure is probably gone.
Why Is It Called a Pig?
There are several stories about the origin of the term.
One common explanation claims that early cleaning devices made a squealing sound as they travelled through pipelines.
Another interpretation treats PIG as an acronym for various phrases.
The historical origin is less certain than people sometimes make it sound.
What matters operationally is that “pig” became the standard industry term.
The equipment used to launch and receive pigs is consequently often called a pig trap.
What Is a Cleaning Pig?
A cleaning pig removes deposits or debris from the inside of a pipeline.
Different cleaning pigs can perform different levels of cleaning.
Some mainly push loose material.
Others scrape deposits.
Brush equipped pigs can provide more aggressive mechanical cleaning.
Magnets may collect ferrous debris.
The pig is selected according to the pipeline condition and what needs to be removed.
An operator usually does not begin with the most aggressive pig available.
If a pipeline contains years of accumulated deposits, suddenly removing everything at once can create a new problem downstream.
Why Is Wax Such a Problem in Crude Oil Pipelines?
Many crude oils contain paraffin components.
At sufficiently high temperature, those components remain dissolved in the oil.
As the crude cools, some can precipitate.
Wax can then deposit on the internal wall of the pipeline.
The process can be gradual.
A thin layer forms.
Then another.
The effective internal diameter slowly decreases.
That creates additional resistance to flow.
Upstream pressure may have to increase to maintain the same production rate.
Eventually pipeline capacity can be affected.
Regular pigging can help remove wax before it becomes a major restriction.
What Happens to the Wax?
The pig pushes it forward.
That means the wax does not magically disappear.
Imagine scraping material from kilometres of pipeline and moving it toward one location.
The receiving facility has to be ready for it.
The arriving material may include oil, water, wax, sand, corrosion products, and other debris.
Pig receivers and associated equipment therefore need systems for handling what comes out of the pipeline.
A successful cleaning operation can create a substantial cleanup job at the other end.
Can a Pig Get Stuck?
Yes.
A stuck pig is one of the problems pipeline operators try hard to avoid.
Potential causes include excessive deposits, pipeline damage, incorrect pig selection, internal restrictions, partially closed valves, geometry problems, or other obstructions.
The consequences can range from inconvenience to a major operational problem.
If the pig blocks the pipeline, flow can be reduced or stopped.
Pressure may build behind it.
Recovering the pig can become complicated.
This is why operators need good information about the pipeline before sending a pig through it.
A Valve Can Stop a Pig
A pig needs a clear path.
A valve that is not fully open may create an internal restriction.
Some valve designs are more suitable for piggable pipelines than others.
Full bore valves are commonly used where pigs need to pass.
Valve lineup is therefore a critical part of pigging preparation.
A valve being technically “open” does not necessarily guarantee that the internal path is suitable for a particular pig.
The entire route has to be checked.
Bends Matter Too
Pipelines are not perfectly straight.
They contain bends.
A pig needs enough flexibility to travel through those bends without becoming stuck or damaged.
Bend radius therefore matters.
So do changes in pipeline diameter.
Tees, valves, fittings, and other internal features must be considered.
This is why pipeline piggability is something engineers think about during design.
Trying to make an existing pipeline piggable later can be much more difficult.
What Is a Foam Pig?
A foam pig is one of the simpler pig designs.
It is made primarily from flexible foam and may have an external coating or abrasive material depending on the application.
Its flexibility allows it to pass through certain pipeline configurations that might be difficult for more rigid pigs.
Foam pigs can be useful for cleaning, drying, or proving that a pipeline is passable.
They also have limitations.
A foam pig is not automatically the right solution for every pipeline simply because it is flexible.
What Is a Cup Pig?
Some pigs use flexible sealing cups.
The cups contact the pipe wall and create a pressure differential across the pig.
Pipeline pressure behind the pig then pushes it forward.
Cup pigs can be configured for different cleaning and displacement duties.
The cups need to maintain enough contact to move the pig effectively while remaining flexible enough to pass through the pipeline geometry.
What Is a Disc Pig?
Disc pigs use circular polyurethane discs that contact the pipeline wall.
Different combinations of sealing and guiding discs can be used depending on the application.
Like cup pigs, they can be used for cleaning and displacement.
The details become surprisingly specialized.
Pipeline pigs are not simply one universal plastic plug available in different diameters.
Their design is matched to the job.
Why Are Gas Pipelines Pigged?
Natural gas pipelines may appear to have less need for cleaning because the product is gas.
But gas systems can still accumulate liquids.
Water may enter or condense.
Hydrocarbon condensate may accumulate.
Corrosion products and debris may also be present.
In gathering systems, gas can arrive from producing wells containing substantial quantities of liquids.
Those liquids tend to collect in low points.
Pigging can push accumulated liquid toward a receiving facility.
Why Is Liquid in a Gas Pipeline a Problem?
A gas pipeline is usually intended to transport gas efficiently.
Liquid occupying part of the pipe reduces the available flow area.
It can increase pressure loss.
It can contribute to unstable flow.
Water can also create corrosion problems.
Under suitable conditions, water and natural gas can form hydrates.
Removing accumulated liquids can therefore improve both hydraulic performance and integrity management.
What Happens When a Gas Pipeline Pig Arrives?
Potentially, a lot of liquid arrives with it.
This is important.
Suppose liquid has accumulated throughout a long gas gathering line.
The pig sweeps that liquid forward.
The receiving facility that normally sees a relatively modest liquid rate can suddenly receive a large slug.
Separators and liquid handling equipment need enough capacity to manage it.
Operators may reduce gas flow or make other operating adjustments during a pig run to manage the arrival.
The pipeline has been cleaned.
Now the facility has to deal with everything that was cleaned out.
What Is a Slug Catcher?
A slug catcher is equipment designed to handle large temporary quantities of liquid arriving from a pipeline.
This is particularly useful in multiphase and gas gathering systems where liquid slugs can be significant.
Some slug catchers use large vessels.
Others use multiple sections of large diameter piping.
Their purpose is to provide enough volume to separate and temporarily hold liquid without overwhelming downstream equipment.
Pigging is one event that can create a large liquid arrival.
Terrain induced slugging can create another.
Can Pigs Separate Different Products?
Yes.
Some pipelines transport different products sequentially.
A pipeline might carry one refined product followed by another.
Pigs can sometimes be used to separate batches and reduce mixing between them.
This is called batching or product separation.
Not every multiproduct pipeline uses pigs between every batch, and some amount of interface mixing can still occur.
But separation pigs can be useful in particular systems.
Pigs Can Help Remove Water After Construction
Before a new pipeline enters hydrocarbon service, it may undergo hydrostatic testing.
The line is filled with water and pressurized to verify its integrity.
After testing, that water has to be removed.
Pigs can be used to push the water out.
Additional pigs may then help dry the line.
This is particularly important for gas pipelines.
Leaving significant water inside a gas system can create corrosion and hydrate concerns.
Pipeline commissioning therefore often involves a sequence of pigging operations before normal product ever enters the line.
What Is a Gauging Pig?
Before sending expensive inspection equipment through a pipeline, an operator may want to know whether the internal bore is clear.
A gauging pig can help.
One common design includes a metal gauge plate with a diameter slightly smaller than the expected internal pipeline diameter.
The pig travels through the line.
If the plate returns significantly bent or damaged, that indicates an internal restriction or deformation may exist.
The gauge plate does not provide a detailed map of the problem.
It provides evidence that the expected clearance may not be available.
What Is a Smart Pig?
This is where pipeline pigging becomes much more technologically interesting.
A smart pig is an instrumented inspection device.
Instead of merely cleaning the pipe, it collects information about the condition of the pipeline as it travels.
The industry often calls this inline inspection.
A smart pig may contain many sensors along with onboard electronics and data storage.
After the run, engineers analyze the collected data to identify areas that may require further investigation.
How Can a Pig Detect Corrosion?
One common inspection technology is magnetic flux leakage.
The inspection tool magnetizes the steel pipe wall.
If the steel has a consistent thickness, the magnetic field behaves in a relatively predictable way.
Where metal has been lost because of corrosion or another defect, the magnetic field changes.
Sensors detect those changes.
Software and analysts then use the data to estimate the location and characteristics of metal loss.
The pig is effectively examining the steel while travelling inside the operating pipeline.
Can Smart Pigs Find Cracks?
Specialized inline inspection tools can be designed to identify crack like features.
Different technologies may be used depending on the pipeline material, product, and type of defect being investigated.
Ultrasonic inspection is one approach used in suitable applications.
Electromagnetic methods can also be used for particular crack detection tasks.
Crack inspection is technically challenging because a narrow crack can be very different from broad corrosion metal loss.
This is why operators select inspection technology based on the threats expected for that pipeline.
How Does the Pig Know Where a Defect Is?
Finding a defect is not useful if nobody knows where it is.
Inspection tools therefore track distance as they travel.
Odometer wheels can measure movement through the pipeline.
Above ground markers and other reference information can help improve location accuracy.
The inspection results can then identify a feature by distance along the line.
Crews use that information to locate the corresponding area from the surface.
The next step may involve excavation and direct examination.
Does a Smart Pig Prove the Pipeline Is Safe?
No single inspection tool can identify every possible defect with perfect accuracy.
Inline inspection is one part of an integrity management program.
Engineers consider the type of inspection technology, its detection capabilities, pipeline history, operating conditions, corrosion mechanisms, previous inspections, and other information.
Features identified by the tool may require further evaluation.
Some are harmless.
Some need monitoring.
Some require repair.
The inspection gives engineers information needed to make those decisions.
What Happens After a Serious Defect Is Found?
Suppose inspection data indicates substantial metal loss at a particular location.
The operator may excavate the pipeline.
The coating is removed locally.
The pipe can then be examined directly.
Measurements determine the actual condition.
Engineers evaluate whether the remaining wall is acceptable for continued operation.
Depending on the result, the operator may repair the section, replace pipe, install an engineered reinforcement, reduce operating pressure, or take another appropriate action.
The smart pig finds potential problems.
People still have to decide what to do about them.
What Is a Pipeline Dent?
Pipelines can experience mechanical deformation.
A dent may result from external forces acting on the pipe.
Construction activity is one possible source.
Ground movement is another.
The significance of a dent depends on its shape, depth, location, associated stress, and whether other damage such as cracking or metal loss is present.
Geometry inspection tools can measure changes in the internal shape of the pipeline.
This helps operators identify deformations that deserve further evaluation.
Why Run Different Pigs Through the Same Pipeline?
Because different pigs answer different questions.
A cleaning pig asks:
Is there material we can remove?
A gauging pig asks:
Is the internal bore clear enough?
A geometry tool asks:
Has the pipe changed shape?
A metal loss tool asks:
Is steel missing?
A crack detection tool asks:
Are crack like features present?
No single tool necessarily provides the best answer to all of those questions.
An integrity program may therefore use several technologies over time.
Why Clean Before Running a Smart Pig?
Inspection sensors work best when they can interact with the pipeline as intended.
Heavy wax, debris, or other deposits can interfere with the inspection.
There is also a more basic concern.
An advanced inspection tool can be extremely expensive.
Before sending it through hundreds of kilometres of pipe, operators want reasonable confidence that it can make the trip.
Cleaning and gauging runs may therefore occur before the inspection tool is launched.
How Fast Does a Pig Travel?
There is no universal speed.
Pig velocity depends on the pipeline, product, flow rate, pig design, and operation.
For inspection tools, speed can be particularly important.
Sensors need to collect useful data.
If the pig moves outside its intended velocity range, data quality can be affected.
Operators may therefore adjust pipeline flow to help control pig speed.
In complicated gas systems, specialized speed control features can also be used.
How Do Operators Know Where the Pig Is?
Several methods can be used.
Pig signallers installed on the pipeline can indicate when a pig passes a particular location.
Tracking equipment can also detect transmitters carried by the pig.
Operators may use expected travel time and flow information as additional clues.
Knowing where the pig is matters.
If it stops moving, crews need to determine approximately where it became stuck before deciding what to do next.
What Is a Pig Signaller?
A pig signaller is a device that detects or indicates that a pig has passed a certain point.
Depending on the design, it may operate mechanically, magnetically, electronically, or through another detection method.
Signallers are commonly installed near launchers and receivers.
Additional tracking points may exist along the pipeline.
A signaller can provide simple but valuable information:
The pig left.
The pig passed this location.
The pig arrived.
What Happens If the Pig Arrives Too Fast?
A pig has mass.
A large inspection tool can be very heavy.
If it enters the receiver at excessive velocity, significant mechanical forces can result.
The receiving system is designed to safely capture the pig, but speed still needs to be controlled.
This is another reason pigging is an operating procedure rather than simply dropping a device into a pipeline and waiting.
Pressure, flow, timing, and receiving conditions all matter.
Why Is There Sometimes Pressure Behind a Stuck Pig?
The product continues trying to move.
If the pig stops and forms a substantial restriction, pressure can build upstream.
That stored pressure creates additional risk.
The obvious temptation may be to keep increasing pressure until the pig moves.
Whether that is safe depends on the pipeline and situation.
A suddenly released pig can accelerate.
The blockage ahead may create further complications.
Pipeline pressure limits still apply.
A stuck pig requires controlled troubleshooting, not brute force.
What Is an Unpiggable Pipeline?
Some pipelines cannot accept conventional pigs because of their design.
Possible reasons include sharp bends, diameter changes, restrictive valves, complex connections, lack of launchers and receivers, or other geometry.
These are often called unpiggable pipelines.
That does not mean they cannot be inspected or cleaned by any method.
Specialized technologies and alternative inspection approaches exist.
But conventional pigging becomes much more difficult.
Modern pipeline design often considers future inspection requirements from the beginning.
Why Would Anyone Build a Pipeline That Cannot Be Pigged?
Many older systems were built when current inspection expectations and technology did not exist.
Other pipelines were designed for specific process needs where conventional pigging was not considered necessary.
Facilities also evolve.
A pipeline that was originally simple may later receive new branches, valves, or modifications.
What seems like an obvious design requirement today may not have been obvious several decades ago.
Long pipeline lives make these historical design decisions important.
Pigging Frequency Is Not the Same Everywhere
Some pipelines may be cleaned frequently.
Others much less often.
The correct frequency depends on what the pipeline carries and what problems develop inside it.
A waxy crude pipeline may need regular cleaning.
A wet gas gathering line may need liquid removal.
A dry transmission line may have different requirements.
Inspection intervals are determined using integrity considerations rather than the same schedule used for routine cleaning.
There is no industry rule saying every pipeline should receive one pig every month.
How Do Operators Know When Cleaning Is Needed?
Pressure trends can provide clues.
If maintaining the same flow rate requires progressively higher upstream pressure, internal deposits may be one possibility.
Liquid behavior can provide another clue.
Historical experience matters too.
If a particular crude begins accumulating problematic wax after a predictable period, operators can establish a cleaning program before performance deteriorates significantly.
Pigging can therefore be preventive rather than reactive.
Waiting until the pipeline is nearly plugged makes the operation more difficult.
Why Not Pig Constantly?
Pigging has costs and risks.
Production conditions may need to be changed.
Personnel are required.
Waste and liquids have to be handled.
Equipment experiences wear.
Each launcher and receiver operation introduces tasks involving pressurized hydrocarbon equipment.
Pigging only makes sense when the operational or integrity benefit justifies doing it.
The objective is not to run the maximum number of pigs.
It is to maintain the pipeline safely and efficiently.
Cleaning Pigs and Smart Pigs Solve Different Problems
This distinction is worth remembering.
A cleaning pig changes the condition inside the pipeline by removing material.
A smart pig measures the condition of the pipeline.
One performs maintenance.
The other performs inspection.
Some tools combine capabilities, but the objectives remain different.
If pressure drop is increasing because wax is accumulating, a sophisticated inspection tool is not necessarily the immediate solution.
If the concern is external corrosion reducing wall thickness, a basic cleaning pig cannot tell you how much steel remains.
Use the tool that answers the question you actually have.
Pigging Is Really About Keeping the Pipeline Useful
Pipelines are valuable because they can move enormous quantities of hydrocarbons continuously.
But they are also difficult to access.
Much of a pipeline may be buried underground.
Offshore pipelines can sit on the seabed.
Opening the line every few kilometres to inspect or clean it would defeat much of the reason for having a pipeline in the first place.
A pig takes advantage of the pipeline itself as a transportation route.
The tool enters at one accessible location.
It travels through kilometres of otherwise inaccessible pipe.
Then it emerges somewhere else.
A simple cleaning pig can push out material that would otherwise reduce capacity.
A sophisticated inspection pig can return with a detailed record of conditions inside steel that nobody can directly see.
That is why an oddly named piece of pipeline equipment has become such an important part of operating modern oil and gas infrastructure.
Frequently Asked Questions
What is a pig in oil and gas?
A pipeline pig is a device designed to travel through the inside of a pipeline. Pigs can be used for cleaning, liquid removal, product separation, pipeline gauging, and internal inspection.
What pushes a pig through a pipeline?
The pipeline fluid commonly provides the driving force. Higher pressure behind the pig than in front of it pushes the pig through the line.
Why are oil pipelines pigged?
Oil pipelines may be pigged to remove wax, water, scale, debris, and other accumulated material. Pigging can help maintain flow capacity and prepare pipelines for inspection.
Why are natural gas pipelines pigged?
Gas pipelines can accumulate water, condensate, debris, and corrosion products. Pigging can remove those materials and can also be used for internal integrity inspection.
What is a smart pig?
A smart pig is an instrumented inline inspection tool that travels through a pipeline collecting information about its condition. Different tools can detect or characterize metal loss, deformation, cracks, and other features.
Can a pipeline pig get stuck?
Yes. Excessive deposits, internal restrictions, damaged pipe, valve positions, unsuitable geometry, or incorrect pig selection can cause a pig to stop inside a pipeline.
How do operators know where a pig is?
Pig signallers, electronic tracking systems, transmitters, above ground monitoring, travel time, and other methods can be used to track a pig’s progress.
What is the difference between a pig launcher and a pig receiver?
A launcher allows a pig to be inserted into the pipeline. A receiver allows it to be safely removed after completing the run. Both are designed so they can be isolated and depressurized according to the operating procedure.
What does a cleaning pig remove?
Depending on the pipeline, cleaning pigs can remove wax, liquids, loose scale, corrosion products, sand, and other debris.
Can smart pigs detect leaks?
Inline inspection tools primarily identify pipeline features and defects that may contribute to future failure, such as corrosion or cracks. Leak detection during pipeline operation can involve separate monitoring systems using pressure, flow, acoustic, computational, or other methods.
Why is a pipeline cleaned before a smart pig inspection?
Cleaning helps provide suitable internal conditions for the inspection sensors and reduces the risk that deposits or debris interfere with or stop the inspection tool.
Are all pipelines piggable?
No. Some pipelines contain geometry, valves, diameter changes, branches, or other features that prevent conventional pigging. Specialized inspection or cleaning methods may be required for those systems.