Pigging Pipelines — An Overview

1 Introduction to Pigging Pipelines

The practice of inserting a device into a pipeline without interrupting a product’s transmission is called pigging. The backronym ‘pipeline inspection gauge’ is sometimes used to describe these devices, whose name originates from the squealing sound emitted from primitive in-line maintenance and inspection devices. These models were made from simple materials wrapped in barbed wire, leather, or other materials to clean and scrape the interior pipeline. Modern in-line pigging devices scarcely resemble these early ad-hoc creations.

In-line inspection pigs are versatile in their designs. Ranging from humble polymers, gels, or sophisticated detection rigs, these devices boast remarkable variability. Employing pigging devices for several operations is standard industry practice to maintain the integrity and efficiency of a functioning pipeline.

A true understanding of why a pipeline requires one type of pigging schedule as opposed to another makes all the difference between an effective, efficient pipeline and one that imposes exorbitant costs on its operators. Proper planning and understanding upfront are essential to pipeline operations.

2 What Does Pigging Do

If a pipeline problem suddenly appears on the horizon, it is simply too late. A once efficient method of transporting valuable resources has been halted. An investment of millions or even billions of USD is jeopardized. Assets are lost. Environmental damage is incurred. Avoiding catastrophes and minor disruptions is paramount.

Properly selected pigs and an ideal pigging pipeline program have a tremendous impact on the integrity of pipeline systems. Pipelines are the most cost-effective and efficient method of transporting a wide variety of fluid and gaseous materials. Though cost-effective, the capital and operating costs are significant (often increasing with pipe diameter and length). Other pipelines are constructed for their ability to traverse terrain strategically, facilitate international trade, or foster political growth between countries.

Regardless of its role, a pipeline must maintain two operating principles: the continuous flow of products and minimal overall cost. Pigs are critical to both fundamental principles of pipeline operation, though they are used throughout the lifetime of a pipeline.

They maintain a constant flow of products by:

• Removing damaging debris
• Preventing corrosion cell formation
• Inform developing problems
• Provide data regarding potential defects
• Allow for continuous operation during lawfully required testing

Pigs also address the second principle of pipeline operation, efficiency, by:

• Removing build-up
• Removing flow-restricting deposits
• Providing data for line integrity and flow efficiency

The correct pig and pigging schedule varies significantly from one pipeline to another. A singular pigging solution for all pipelines does not exist. Each pipeline must be evaluated for the unique circumstances, constraints, and variables acting upon it.

A basic way of envisioning the effects of deposits is to consider throughput loss resulting from common deposits. A reduction of no more than 5% in diameter can reduce throughput by 10-15%. The resultant pressure increase necessary to generate original throughput levels would be nearly half (150%) of the original operating pressure. It is commonplace for friction to increase by several percentage points daily in a line if pigging is not conducted.

Primitive pigs of the early application were utilized solely to remove large deposits as needed. Modern pigging schedules can be considered when planning a system, as the process plays an essential role in all phases of the modern pipeline. Some of the most common uses for pigs occur during these phases:

Construction

• Removing initial construction debris
• Initial acceptance testing
• Commissioning

Operating

• Batching
• Cleaning
• Condensate removal
• Inhibitor application

Inspection

• Detecting leaks
• Detecting corrosion
• Physical damage inspection
• Sampling
• Line cover and spanning

Maintenance

• Decommissioning
• Recommissioning
• Corrosion prevention
• Isolation
• Preparatory inspection cleaning

Rehabilitation

• Application of coatings
• Chemical application
• Scale removal
• Product conversion preparation
• Gel pigging

Decommissioning

• Inspecting
• Cleaning
• Testing
• Rendering inert

The way each of these operations is conducted will vary from one system to another. The physical variances of a pipeline system, compliance codes, statutes, and the construction and operating procedures must be considered when establishing pigging schedules and pigs. Without these crucial considerations, the effectiveness of pigging operations is far from guaranteed.

3 Designing a Pipeline for Pigging

Developing a pipeline entirely around pigging is ludicrous. The form and dimensions of a pipeline ideal for pigging operations resemble a straight line, precludes valves and outlets, and would require several other optimizations. Designing a pipeline to accommodate several dozen types of pigs makes more sense. Accommodating pigs through pipeline design is required in many parts of the world, though it varies from country to country. Hydrocarbon pipelines are typically designed using the largest and heaviest of pigs in mind, the non-destructive test (NDT) oriented in-line inspection devices known as smart pigs. Among the many elements that play a significant role in the effectiveness of pipeline design in pigging are:

• Conditions of operation
• Pipeline materials
• Pipeline dimensions and Bends
• Offtakes and Valves
• Diverters and Junctions
• Bundles and Flexibles
• Feature interactions

3.1 Pipeline Materials

The pipeline construction and design materials are classified as primary materials, internal linings, and external coatings. These materials are fundamental considerations that impact the necessary accommodations for continuous product flow and efficient operation.

Primary materials for pipelines vary widely. Cement, concrete, plastic, metals, and other materials are routinely used in systems around the world. The most common primary material for pipelines is steel, typically carbon steel, for lengthy pipeline construction. Considering the product, the type of steel required for a given line is always chosen. Customized solutions involving combinations of materials are used to achieve maximal effectiveness and efficiency.

Interior linings are designed for dual purposes. They protect the line from interactions with the product while reducing flow resistance. Frequently coated at the factory, linings may also be applied after the system has been constructed. The materials that comprise these linings are usually epoxy-based, so choosing the correct pig for cleaning is required.

A pipeline system’s exterior is just as crucial as the interior. Proper coatings greatly aid the prevention of corrosion from the soil and other materials. Failure to properly plan or execute external coatings results in corrosion that can require significant investment or temporary shutdown, in worst-case scenarios, to account for. Precisely controlled conditions and execution are required to avoid unnecessary delays, costly expenses, and environmental and site safety.

3.2 Pipeline Dimensions and Bends

When considering pipeline design, the elements of diameter, the length between traps, and wall thickness are among the most essential and costly. Bends in the line are often necessary to traverse certain terrains. Though necessary, these bends should operate within constraints designed to accommodate the use of pigs.

The pipeline diameter is determined by the system’s required throughput. Wall thickness is an interrelated factor in this consideration; both are balanced by the cost of compressors and pumps. Generally, the efficiency of throughput in each pipeline increases with pipeline diameter.

Local conditions concerning transportation infrastructure, population density, and other factors may require adjustments to wall thickness. Lines are commonly constructed to retain the external diameter and vary the internal dimensions, a less-than-ideal configuration for pigging. Customized pipelines can be constructed with externally varying diameters, increasing the effectiveness of pigging operations (though often requiring specialized equipment to construct and modify).

Another aspect of pipeline dimensions is the length or approximate distance between pig traps and stations. Variations are the pipeline rule, as conditional requirements can change dramatically for any system. The product a pipeline carries is often a determining factor in the length a pig is expected to travel. Materials and coatings within a pipeline are chosen as a result of product requirements. The materials in gas pipelines tend to require traps within 100 miles, approximately 150 miles apart, and nearly 200 miles apart for crude oil pipelines. Exceptions to these general rules include placing traps up to 600 or more miles apart.

Finally, as pipelines are established along the desired path, it is sometimes necessary to follow the lay of the land. Benders or bending machines are familiar with on-site machining operations, as are the services of expert onsite welders who may properly fit joints of the mating pipe section. Bends that maintain a short radius are often factory-forged to standard radii. The dimensions of all bends must be supplied to providers of pigging services to accommodate proper procedures and pig selection.

3.3 Junctions and Diverters

Junctions are Y-shaped divergences that allow products and pigs to travel through and converge several connected branches of a pipeline system as needed. Essential to the operations of many modern pipeline systems, junctions require considering convergence angles, bores, and diverters to function properly with necessary pigging schedules and pig types.

Typical convergence angles are between 22 and 30 degrees, with 30 degrees being the primary convergence angle. Several methods exist for traversing pigs through wye junctures without causing damage to either the pipeline or the pig. Creating a shallower angle or a parallel bore produces variations in the navigation method and can alter the cost of creating junctures. Caution should be taken to consider the variables best suited to pig a pipeline system effectively.

Subsea junctions are common and exceedingly necessary phenomena in the conditions native to that environment. These junctions are typically equipped with a diverter, a mechanism that guides the pig through a junction. Many of these are mechanical designs, acting as a non-sealing gate, while others magnetically divert the pig to the desired branch.

3.4 Outlets and Valves

Outlets and valves, whether installed before the operation or during operations via hot tapping, are known as potential problem spots for pigs.

Outlets (offtakes) are installed using tees. These tees are specially forged or fabricated to fit around a pipeline, creating a valid point of entry for tapping. Outlets must be placed at an appropriate distance from one another to avoid certain feature interactions. A minimum of three diameters of the continuous line is required between fittings. Additionally, a ratio of pipeline diameter to outlet size should guide the placement of guide bars parallel to the axis of the pigging run.

It is recommended that off-takes greater than 12” in diameter add perpendicular reinforcing bars to the above arrangement. Typically, parallel guiding bars and right-angled reinforcement bars may be installed with the same hot-tapping machine after the tap has been successfully created.

Lateral intakes or outlets are defined as intersecting a pipeline at any angle other than a perpendicular one. In this instance, the opening is always greater in length than the diameter of the lateral. Preventing the pig’s entry into these offtakes requires the seals to exceed the length of this opening.

Valve systems are central to many operations in a pipeline system. Despite their essential nature, valves maintain perhaps the greatest likelihood of a pig run’s disruption. Many types of valves are used on lines. All valves used on pipelines must aim to mitigate features capable of interfering with pig runs. Often, this means full-bore ball or gate valves equal in diameter to the pipeline itself.

In cases where valves are difficult for pigs to navigate, a sacrifice of performance may be required to navigate the line unless steps are taken to remove this difficulty. Beginning a pigging operation when mainline valves are not fully open can cause a disastrous scenario. Extensive damage to the equipment, line, and valve could result.

3.5 Pigging Pipelines: Bundled and Flexible Pipelines

Prevalent in offshore operations, bundled pipeline configurations and flexible pipelines serve as versatile means for drilling and product transportation endeavours. Given their significantly differing nature from standard pipelines, these subsea-specialized systems require pigging considerations suited to their environment.

Pipeline bundles appear initially complex, though they bear some striking similarities to bundled utility or electric wires. This type of pipeline configuration is widely used because it effectively combines complex elements into a convenient and efficient package. Furthermore, onshore assembly and ease of towing make for remarkably versatile installations.

Bundled pipelines are often accessible to the same variety of pigging approaches as ground-based pipeline systems. The most notable exception is inspection using Magnetic Flux Leakage testing (MFL). MFL is a type of nondestructive examination conducted by an inline inspection (ILI) tool known as a smart pig. This testing method examines magnetic fields that spacers within the bundled pipeline may affect if they are made of similar materials to the carbon steel pipeline being tested.

Flexible pipelines are commonly employed in offshore operations. They are commonly found for floating production systems (FSO, FPSO, TLPS, etc.) and dynamic risers from a subsea pipeline end manifold (PLEM). Flexible pipelines are well established for deepwater operations, serving over a dozen operations.

Depending on whether the internal finish is rough or smooth, it may or may not be pigged. Manufacturers generally suggest avoiding metal-on-metal contact between the pluggable and rough bore varieties. Therefore, ILI, metal-brush-equipped pigs and similar metal pigs are discouraged. Pigging procedures should never proceed on rough bore pipe if the damage is suspected. Due to the construction of these pipelines, a pigging run after the damage is incurred could have disastrous consequences. Pigging a damaged flexible pipeline would likely unravel the line, resulting in a massive loss of product and functionality.

3.6 Feature Interactions

This element of pipeline design is entirely preventable if enough consideration is given to pigging operations. However, given the requirements of some line systems, these feature interactions are occasionally unavoidable. Certain pipeline features, such as successive welded bends or two tees installed equidistant to pig seals, can stall or potentially damage themselves and the pipeline.

Where possible, consideration should be given to the distance of a straight pipe between two tees and similar scenarios. If these scenarios are unavoidable in a system, they must be adequately described to pigging operators to mitigate risks.

3.7 Conditions of Operation

Several key conditions of operation demand scrutiny when selecting the correct pig for a pipeline:

• Product and flow rate
• Pressure
• Temperature

Each of these components offers significant operational challenges. Addressing them accordingly is the only viable approach to consistently effective pigging operations.

The product itself is likely to determine the type of pig used. If a type of pig is necessary for the desired operation (e.g. ILI MFL testing), the materials comprising various parts or seals may require adjustment. Refined hydrocarbon pipeline systems frequently adapt or select pigs to function properly with detergent or other additives. Failing to adapt the type of pig used can permanently damage the equipment or contribute to hazardous storage scenarios.

Pigging runs often proceed at the native flow rate of a pipeline. If the flow rate is steadily maintained, pigs produce or exceed desired levels of effectiveness. Loss of flow rate, stalling, hydroplaning, and high-velocity accelerations negatively impact pigging operations. While these speed changes disrupt cleaning and maintenance operations, they severely limit ILI pigs. ILI tools utilize highly sensitive equipment and display significant decreases in data collection when negatively impacted by flow rate.

The majority of pigs are largely unimpeded by native pipeline pressures. ILI instruments are, however, adversely affected by pressure within pipelines. Smart pigs use pressure and product-sensitive equipment, which must be properly protected. The primary concern of operating pressures revolves around low-pressure pipelines. These pipelines can stall on minor obstacles, lacking the necessary differential pressure to continue. In such cases, pressure may build to extremely high levels, causing a sudden, rapid acceleration. Speed excursions such as these can reach nearly 200 mph in less than 250 feet. Downstream venting and pressurizing the pipeline can mitigate the occurrence of this dangerous scenario.

Temperature ranges most impact ILI devices as with the above operating conditions. The electronic components inside ILI tools are sensitive compared to other pigging materials. The polyurethane seals present on pigs are capable of enduring operating temperature ranges found inside most pipelines.

4 Pig Pipeline Stations: Launchers and Receivers

Keeping with the primary operating goals of a pipeline, pig stations use valves and ‘traps’ to maintain continuous product flow. Pig stations may have launchers or receivers for beginning or ending a pigging run. Pig traps, or barrels, are oversized loading or receiving sections which taper to the pipeline diameter. Barrels are sealed by trap isolation valves designed to capture or release a pig.

Pig launching traps are designed to function solely with utility pigs or for ILI smart pigs and utility pigs. Barrel designs for launchers using only utility pigs are typically one and a half times the length of the longest utility pig. Barrel designs for launchers using ILI devices are considerably longer. Typical ILI tools are comprised of several jointed sections and require additional length. ILI-friendly traps require at least one pipeline diameter of extra length.

Pig-receiving traps are designed with additional concerns for those launching traps. The barrel length of receiving traps is of primary concern. Occasionally, a pig may get stuck on the run. When this occurs, a second pig may be sent downline to retrieve the initial pig. Receiving two pigs requires a minimum barrel length of two and a half times the longest pig length. As with launching traps, pig receiving traps require additional room for ILI devices employed in NDT exercises.

5 Pig Design

Pig designs vary widely according to circumstances. Utility pigs are often simple and durable, while ILI devices are more complex and more likely to suffer difficulties navigating a pipeline. Early pigs were often constructed without due consideration to their specific task. Modern materials are chosen for their durability, weight, and ability to perform a task or function most effectively during a pigging run.
Rather than cobbling together at-hand materials, pig designers look at each aspect of a pig and optimize it for usability in a specific environment. Among the hundreds of pig designs used for pigging programs, each has an ideal application. The utility pig is perhaps the most widely used variety and remains among the simplest in design.

5.1 Body

Initial utility pigs evolved rapidly from their original form. Designers experimented with creating lightweight and even hollow bodies. Hollow bodies proved largely ineffective over lightweight bodies. Along with other design elements, the weight of a pig determines how concentric it remains during a run. Remaining close to absolute centricity is an important performance metric for pigs. While travelling within pipelines, forces act on the nose of the pig, turning it slightly downwards. Pigs commonly display more wear at the southern-most 6’oclock position, primarily on the driving or front seal.

The body of pigs is often distinctive in appearance. The material, shape, seals, and cleaning elements are indicators of a pig’s operating environment. Utility pigs take several common forms; some are modular and easily altered for task specialization. Other pigs may be cast or inflatable to suit their operating conditions better. Among the most common utility pigs is the mandrel pig.

Mandrel pig bodies are simple, cylindrical steel tubes. Their straightforward design makes them a versatile mounting platform for seals, brushes, and other equipment. These components can be bolted, clamped, or threaded snugly onto the pig. The design of this body allows for another ingenious advantage: optional bypass flow.

Bypasses allow pipeline flow to continue through the pig and may be used to aid cleaning efforts significantly. Though operators vary in their approach, bypasses are generally avoided with sealing pigs. Depending on the differential pressure a cleaning pig generates, the flow volume can vary significantly. The flow must be carefully determined in low-pressure environments to avoid stalling the pig. Maintaining centricity can be aided by allowing bypass through rear seals. This arrangement forces propulsion onto the front seal, centring the pig and improving performance.

5.2 Seals

Seals are the components of a pig that induce effective action and propulsion within a pipeline. They are designed for functionality and durability, as they can remain in contact with the pipe wall for hundreds of miles. Materials vary, but they are primarily polyurethane. Several common seal shapes exist, including cups, conical, and disc shapes.

Cups are perhaps the most used seal shape. Their development originated from rapid wear, which was expressed in early shapes. These designs pressed seals harder against the pipe wall, resulting in rapid deterioration. Early seals were also made from inflexible materials, further contributing to their lack of durability. Initial variants on seal shape caused the seals to adhere strongly to the pipe wall, causing the pig to tear through and continue. Seal design shifted to shapes and materials, promoting necessary contact while avoiding excessive contact.

Designing seals for pigging relies primarily on two polyols to achieve diverse properties. Polyether and polyester have distinct operating advantages in certain conditions. Polyethers are typified by their resistance to water, while polyesters have greater degrees of physical and hydrocarbon resistance. These polyols can perform exceptionally and remain cost-effective under most circumstances. They can be further engineered at greater cost to serve in subsea or other specialized environments when necessary.

5.3 Cleaning Elements

Whether built into seals or attached, cleaning elements are essential to maintaining pipelines’ continuous flow and cost-efficiency. These elements are most often made of steel or polyurethane. Consideration must be given to the placement of the cleaning devices on the pig. They must be arrayed in a formation to cover the pipe wall completely during pigging operations.

Traditional cleaning elements were sloping steel blades that resembled ploughshares. Tremendous variations in cleaning element designs are now widely available. As with the above elements of pig design, modern materials and engineering produce more consistent results with little risk of pipeline damage. Blade elements are often made with polyurethane or other plastics for modern pipelines.

Brush elements are similarly constructed from metals such as hardened or stainless steel, though they are frequently manufactured with polyurethane. Brushes of different shapes are made to accommodate pigs limited to mounting one or two rows of brushes. Configurations limited to one row may utilize wedge-shaped or trapezoidal brushes so that the overlapping dimensions ensure complete wall coverage. Pigs capable of two or more rows may simply stagger rectangular brushes to ensure overlap in the brush arrangement.

In some instances, cleaning elements built into disc seals are used. These elements may be moulded into the sealing edge of seals.

6 Pig Types

Discussed below are select pigs designed and optimized for specific tasks. Choosing the correct pig is essential to successful pigging operations.

6.1 Utility Pigs

Utility pigs are often subdivided into cleaning pigs and sealing pigs. Cleaning pigs are outfitted to meticulously remove debris or solid deposits while sealing pigs are equipped to remove unwanted or accumulated liquids. Regardless of category, each type of utility pig is outfitted to handle the unique environment of the specific pipeline designated for the operation.

6.1.1 Mandrel

As alluded to above, mandrel pigs can fulfil the role of cleaning or sealing pigs. Their modular design allows operators to plan and assemble a custom pig relatively easily and carefully. This variability makes the mandrel pig such a widely used pigging device. Virtually every aspect of a mandrel pig is customizable and may be modified to accommodate pipeline needs.

6.1.2 Foam

Durable, inexpensive, and requiring little to no maintenance, foam pigs boast several appealing advantages. Foam pigs are primarily employed to rehabilitate pipelines and can be tailored to dozens of specialized applications. Comprised of open-cell polyurethane foam, they can maintain their form when confronted with operating pipeline pressures. Due to their lightweight, foam pigs are easy to handle, but they absorb product during a run. This requires caution when handling after pipeline pigging is completed. Foam pigs vary in density and coating, so they may be designated for cleaning or sealing operations. These pigs may also be moulded to form a conical fore-shape and a flat or concave rear shape. Specialized descaling and decoking foam pigs are also manufactured. These heavy-duty foam pigs are ultra-dense and moulded to accommodate the insertion of hardened, threaded steel pins to aid in their cleaning operations.

6.1.3 Spheres

Sphere pigs are categorically used as sealing pigs, with some exceptions. The use of spherical pigs is often relegated to pipelines void of potential stall points. Exceptions include pipelines that install flow tees to mitigate the risk of spheres losing their seal. Despite the initial drawbacks of using sphere pigs, they maintain significant pipeline handling advantages. Their ability to navigate short radius bends nets sphere pigs a niche advantage in complex pipeline systems. Pipelines that require very frequent pigging can automate sphere pigging. These automatic systems are frequently employed to remove condensates from gas-gathering lines or to meet similar pipeline requirements. Small sphere pigs used to navigate these lines can be gathered by releasing a larger sphere pig and returning to their original trap.

6.1.4 Cast

Cast pigs or solid cast pigs are typically used for sealing (sometimes referred to as batching or swabbing) operations, though they can be manufactured for cleaning runs. As with other elements of pigging operations, the use of cast pigs arose from cost-efficiency considerations. Smaller diameter pipelines traditionally used mandrel or other pig types. The drawbacks of using a mandrel pig for small pipelines rest in the cost of assembly and parts replacement. Small mandrel pigs from two to eight inches require similar amounts of time to assemble and repair compared to medium-sized mandrel pigs. Often, it is cheaper to replace the mandrel pig than to spend resources on maintaining it.
This dynamic results in cast pigs being very common in smaller sizes. Most cast pigs are sealing pigs, regularly employed at process plant facilities. Cast pigs designed for cleaning may be moulded with integral discs or have allowances for adding cleaning elements. Though not as common, larger designs are also available for some applications.

6.2 In-line Inspection

Intelligent pigs or smart pigs are inline inspection devices that perform non-destructive examinations (NDE) of pipeline systems’ interior and exterior. Several methods of NDE or NDT have been steadily pursued since the mid-20th century. Prominent methods feature NDT methods such as magnetic flux leakage (MFL) testing, ultrasonic testing (UT), geometry measurement, electromagnetic acoustic transducer technology (EMAT), and eddy currents. Smart pigs are, regardless of the NDT method used, more delicate instruments due to the sensor arrays, data collection, and data storage devices on board. Precisely tracking intelligent pigs (detailed below) is essential for properly mapping defects within a pipeline system. Often, relying on one method of tracking virtually guarantees inaccuracy. Small errors in tracking result in large errors later as the smart pig travels dozens or hundreds of miles.

6.2.1 Magnetic Flux Leakage

MFL tools have an extensive history of detecting corrosion, cracks, and other anomalies found in various industrial settings. Applying the MFL principle to pipelines for these purposes is an area of intense, sophisticated, and continual research in modern pigging.

Summarized below, MFL testing is addressed in detail in our Magnetic Flux Leakage article.

In-line inspection tools that use the MFL principle magnetically saturate the pipeline walls and record the magnetic field’s flux lines generated in this process. Anomalies are detected when defects—usually small quantities of missing material—disrupt the field, creating flux leakage. Properly discerning the type of damage detected during MFL testing is a highly developed scientific endeavour. Complex models, statistical analysis, and artificial intelligence are applied with experienced technical experts to better articulate and detect pipeline defects. Though MFL testing began as a low-resolution, unrefined process, it has steadily evolved into an essential aspect of pipeline maintenance.

ILI devices also use ultrasonic inspection devices. UT tools use a transducer to emit ultrasonic pulses that travel at predetermined speeds into the pipeline wall. The reflected echo creates a direct reading of wall thickness, allowing precise mapping of pipelines. Limitations to UT operations include pipeline product; signal deflect due to dents or other features, false calls, and transducer alignment. As with MFL techniques, consistent development of technologies related to UT tools has significantly mitigated early limitations.

EMAT tools rely on electromagnetic acoustic transducers to detect cracks in a pipeline wall. Steel pipelines may be excited through electromagnetic waves to generate an ultrasonic pulse. The main advantage of ILI conducted with an EMAT device is that it can detect cracks in a non-liquid environment. Consequently, gas pipelines benefit significantly from using smart pigs equipped with EMAT technology.

Several types of ILI tools are designed to determine pipeline geometry accurately. This pigging process usually occurs before the metal loss or crack detection devices are employed. The benefit of using geometry pigs before metal loss detection rests in data triangulation. From cleaning and maintenance pigs, geometry pigs create an initial ‘rough mapping’ of pipeline systems. Operators can analyze the initial data to compare with later high-resolution ‘maps’ such as those created by MFL, UT, EMAT, or other ILI devices.

Gauging pigs are often the first low-resolution geometry pigs used to determine buckle, denting, ovality, or other deformations. Generally, the dimensions of a gauging pig are designed within 95% of a pipeline’s inside diameter. Operators can rule out deformations over anticipated amounts if this gauging pig successfully navigates a pipeline. Gauging plates attached to this type of pig frequently return damaged, allowing operators to determine the location and expected severity of defects. Further procedures use electro-mechanical geometry detection tools or induce alternating magnetic fields to inspect pipeline deformations.

Electro-mechanical pigs use seal-covered, spring-loaded arms to maintain pipe wall contact throughout the run. As the arms make contact, the mechanical movement generates dimensional data, which is recorded on a data storage device within the pig. Recorded diameter changes generate a higher-resolution map than simple gauging pigs.

Some geometry pigs are designed to detect electrical currents generated by inducing alternating magnetic fields into the pipe wall. Eddy current pigs can efficiently map several types of pipeline defects and have long been able to generate analyses in the field immediately following a pigging operation.

6.3 Gel Pigs

Gel pigs are versatile elements of pigging operations. Usually water-based, these gels originate from oil field uses, including hydraulic fracturing operations (see our articles on Hydraulic Fracturing and Horizontal Drilling for more information). Though composition varies, these gels may be used for batching or separating, sealing, cleaning, applying inhibitors, and flushing debris from pipelines.

Batching or separator gel pigs are often water-based and gelled with cross-linked polymer. These gels are designed to flow around objects and reform. They are also designed to be biodegradable and easily destroyed with chemical breakers if necessary. Circumstantially, these pigs may be used alone or with mechanical pigs, though they require consideration based on pipeline product.

Pick-up and cleaning gels are typical of a water-based variety. Designed to suspend debris within the gel, this pig type’s viscosity demands the accompaniment of mechanical pigs to function optimally. Batching gels are often placed on either end of cleaning gel pigs to prevent dispersal by water.

Carrier and inhibitor gel pigs are created when hydrocarbon-based fluids are gelled. These pigs are ideally suited to removing water or debris from pipelines and boast similar sealing properties to batching gel pigs. An added benefit of these pigs is that they may be used as applicators for corrosion inhibitors. Other forms of carrier pigs may be alcohol or methanol-based gels designed for dewatering and drying operations.

Aside from these broad categories, many specialized gel pigs can serve in several capacities. Plugging gels, valve-sealing pigs, and even foamed fluids are among the more specialized varieties.

7 Pig Detection and Tracking

Among the top priorities of pigging operators (particularly with ILI devices) is tracking or detection. Though many technological and manual methods exist for pig detection, most of these methods take place outside of the pipeline. Pigs are typically equipped with odometer wheels, subject to various forms of disruption. Many of the methods discussed below are intrinsically flawed and unreliable as single-source detection methods. Fortunately, these methods infrequently interfere with one another. Combining several available methods of pig tracking is a highly recommended practice. Available methods include:

Acoustic detectors

• Magnetic detectors
• Isotopes
• Pingers
• Human and trained canine tracking
• Drone tracking
• GPS assisted tracking
• Pressure pulses
• Extremely Low-Frequency systems (ELF)
• Odometer Wheels
• Noisemakers

Pig detection and tracking methods always vary with the unique constraints of a given pipeline, such that geography, subsea, or other conditions often determine possible tracking methods.

8 Standard Operating Procedures and Pigging

The primary goal of pigging is to maintain operating efficiency and minimize pipeline capital and operating costs, so all pigging procedures must contribute to these ends.

Achieving ideal operating pipeline efficiency begins with correctly assessing and executing a pipeline’s pigging needs. To achieve this goal, operators are encouraged to meticulously document and record data regarding cost, power consumption, throughput, and effective pigging operations. Scrupulously analyzing this data allows pigging operators to develop ideal pigging procedures and schedules for each pipeline system.

Data collected on these metrics are often extensive. Combining the knowledge of expert operators, experienced technicians, complex statistics, and modern computational technology is perhaps the most effective means of consistently producing maximal results for pipeline operators.

9 Future Developments: Pigging Pipelines

As an industry practice, pigging has only grown in relevance to pipeline operators since its inception. The need for more effective and efficient pigging solutions has increased with time.

Not surprisingly, cutting-edge technologies are continually applied in academic and practical settings to advance every aspect of pigging operations. Materials sciences, ILI technologies, data modelling, and data analyses are among the most concentrated areas of improvement.

Case studies, research reports, and white papers are routinely published regarding the technologies employed and data collected in pigging operations. Pigging operators are highly encouraged to seek out and become acquainted with relevant journals and researchers to produce the best results on an ongoing basis.

10 Conclusion

Developing correct pigging schedules is essential for maintaining pipeline efficiency while reducing capital and operation costs associated with pipeline systems. To this end, operators must accurately evaluate each pipeline’s requirements.

Choosing the right pig and developing an effective pigging schedule is of primary concern. Maintaining successful pipeline efficiency and cost-effectiveness hinges on meticulous record-keeping and scrupulous data analysis. Additionally, pigging operators should maintain knowledge of and continually integrate the latest methods and technologies which enhance their operations.

11 Further Reading

EPCM Holdings Knowledge

Clarion Publishing

Pipeline and Gas Journal — Pigging

12 Resources

Cordell, J.; Vanzant, H., The Pipeline Pigging Handbook. 2003

Hiltscher G.; Muhlthaler, J.; Smits, J., Industrial Pigging Technology. 2003

Adegboye, M.A.; Fung, W.; Karnik A., Recent Advances in Pipeline Monitoring and Oil Leakage Detection Technologies. 2019