Advertisement

The Ultimate Guide to Pipeline Pigs

By Mohamed Adel Mohamadein | Reviewed by Raghvendra GopalCheckmark
Published: March 8, 2022 | Last updated: July 20, 2022 07:46:56
Key Takeaways

Pigging is one of the most suitable ways to assess working pipelines. There are many types of pigs on the market today; choosing the best one depends largely on the type of defect you're trying to detect.

Source: istockphoto.com

Pipelines are considered one of the safest and most feasible ways to carry hydrocarbons or gases from well heads to tankers or storage tanks.

Advertisement

However, after their installation, pipelines are prone to corrosion threat that might affect their integrity and operational safety. Corrosion threats affecting pipelines can be internal or external; therefore pipeline operators have always sought for techniques that can:

From this need came the importance of pipeline pigging. So, this article explores the various the types of pipeline pigs, the techniques they use to measure pipeline anomalies or flaws and each technique's strengths and weaknesses for measuring and detecting specific flaws.

Advertisement

What is Pipeline Pigging?

Essentially, a maintenance tool.

Pipeline pigs are put into the line via a pig trap, which has a launcher and receiver. Without interrupting flow, the pig is then made to go through it by product flow—or towed by another device or cable. Usually cylindrical or spherical, pigs sweep the line by scraping the sides of the pipeline and moving debris ahead. As they travel along the pipeline, pigs can perform tasks such as clearing the line to inspecting the interior.

There are two main hypotheses for why the process is called "pipeline pigging," but neither has been proven. The hypotheses are:

  1. "PIG" is actually an acronym standing for "Pipeline Intervention Gadget."
  2. A leather-bound pig was sent through a pipeline; and while it passed the leather squeaked against the sides of the pipe, sounding like a squealing pig.

What Types of Pipeline Pigs Are There?

Several types of pipeline pigs are available and each has unique functions. Some of the most common types are:

Advertisement

1. Cleaning Pigs

Cleaning pigs are typically for pipeline operators who are familiar with the amount debris usually found in their pipeline. That's because several types of cleaning pigs are available; and choosing one over the other depends on how much debris and scale you expect to find inside the pipeline. (For more on this topic, see: Why Pigging Out Is A-OK When It Comes to Cleaning Pipelines.)

(However, it should be noted that the frequency at which operators clean their pipelines has a great impact on the anticipated amount of debris. A pipeline that is frequently cleaned should have less debris than a pipeline that is seldom cleaned.)

Here are three common types of cleaning pigs:

Brush Cleaning Tools

Brush cleaning tools are used to clean the inside of pipelines using strong and flexible brushes, usually held to the inner pipe wall using strong springs. These pigs are usually equipped with polyurethane guiding and sealing discs to help seal the pigs inside the pipeline—which, in turn, helps propel the pig through the pipeline.

Magnetic Brush Cleaning Pigs

These pigs are used to perform a more thorough pipeline cleaning—for example, removing black powder from gas lines or ferrous debris.

Magnetic brush cleaning pics can travel long distances and pick up large quantities of debris thanks to their brushes and magnets which carries debris to a receiver.

Scrapper Pigs

Scrapper pigs are used for heavy-duty cleaning pipelines' inner surfacesnormally from scale or solid mineralized sediments. These pigs are equipped with scraper steel blades.

2. Geometry Pigs

Mechanical deformations could present a threat to pipeline integrity.

Accelerated cracking growth and corroded dents could present a threat to pipeline integrity that can hardly be estimated according to existing codes and standards.

Moreover, a significant reduction in the pipeline's circular shape (also known as "free through bore") can negatively affect product flow and cause excessive power consumption for product transportation. Even if a pipeline operator is successful in reducing Also free through bore, the reduction itself could pose a threat to the safe passage of an intelligent pig tool. This is a threat in itself; a pig getting stuck inside the pipeline can have serious consequences. (For more on this topic, see: Ice Pigging: The Benefits to Using This Icy Pipe Cleaning Method.)

Here are two common types of geometry pigs:

Gauge Plate Pigs

Gauge plate pigs consist of an aluminum disc installed over the pig body.

If a critical obstacle is found inside the pipelines—such asa a dent or a partially opened valvethe aluminum disc will bend. The pig operator then measures the amount of bending the aluminum disc sustains and, based on it, estimates the amount of free through bore on the pipeline. If the operator finds the free through bore is insufficient for an intelligent pig's safe passage, the inspection campaign may be postponed until the obstacle is cleared and a rerun using a gauge plate pig is done again.

Caliper (Geometry) Pigs

Caliper tools use mechanical measurement arms in contact with pipe walls and covering 360 degrees of pipe circumference. An on-board computing unit digitally records any deviation from the pipeline's perfect circular shape, which moves the pig's measurement arm.

Caliper units are typically multi-channel. This means every arm is equipped with an individual sensorindependent from each otherand with a separate signal channel. This will help:

  • Detect geometry anomalies.
  • Measure geometry anomalies' depth.
  • Identify geometry anomalies' shape
  • Categorize geometry anomalies.

3. XYZ Mapping Pigs

The basic component of the XYZ mapping pig system is an inertial measurement unit (IMU), which consists of a group of gyroscopes and accelerometers. Electronic systems and software control the sensor's operations to correct measurement errors caused by environmental variations. (For more on this topic, see: IoT for Corrosion Monitoring in the Oil and Gas Industry.)

Based on the data measured during the pig inspection, the trajectory of a tool movement is determined, providingon a time basis scalethe pig's X, Y and Z coordinates.

In order to improve the position accuracy, inertial data is corrected using known DGPS coordinates and/or above ground markers (AGMs) placed along the pipeline trail during the pig survey. Then, the pipeline operator converts the coordinates in accordance with the specified coordinate system.

This will help synchronize all pipeline data based on GPS mapping coordinates with submeter accuracy, giving the pipeline operator the ability to compare how the anomalies have grownbased on previous inspection dataand assess their future growth. This will help in corrosion growth studies and gives the pipeline operator a perfect opportunity to plan pipeline operating and maintenance scenarios.

4. Smart (Intelligent) Pigs

Metal Loss/Gain Detection Tools

There are two principal methods for detecting metal loss in pipe walls:

  1. The magnetic flux leakage (MFL) method.
  2. The ultrasonic testing (UT) method.

The MFL was the first method developed and has been the most widely used. A third method, called eddy current, has been developed; but is used only to detect defects on the inside of the pipe wall. (For more on this topic, see: Understanding the Causes and Cures for Corrosion Under Insulation.)

Let's take a look at each of these methods more in-depth:

1. Magnetic Flux Leakage (MFL) Tools

The basic MFL principle is to induce an axially oriented magnetic flux into the pipe wall between a magnet's two poles. A steel wall without defects creates an undisturbed and uniform distribution of magnetic flux. Metal loss or gain associated with the steel wall, however, causes a change in flux distributionwhich, in a magnetically saturated pipe wall, “leaks” out.

Sensors detect and measure this leakage field and hence detect the metal loss. The magnitude and shape of the measured leakage field is used to characterize the size and shape of the metal loss region. The leakage signals are passed through sophisticated microprocessors; and the resulting data is stored for detailed computer analysis and subsequent reporting.

MFL tools are commonly classified into two categories:

  • Standard-resolution (SR) (also called low- or conventional-resolution).
  • High-resolution (HR).

The differences between these categories are the number, size and orientation of MFL sensors, magnetic circuit design, magnetization levels and the type of analysis applied to recorded data supplied by each type of instrument. All types of tools use magnets to induce a magnetic field into the pipe wall; and either inductive search coils or solid-state (Hall-effect) sensors to detect flux leakage.

Standard-resolution tools have fewer MFL sensors (inductive coil sensors) for a given pipe size than do high- or extra-high-resolution tools. Each of these sensors covers a larger part of the pipe's circumference and gives an average of the flux leakage distribution in the area it covers. The much smallerand more advancedHall sensors (used on HR tools) can examine a smaller area of the pipe wall and reveal more detailed information. (For more on this topic, see: 8 Methods of Coating Thickness Measurement.)

Therefore, HR tools provide a much better characterization of anomalies in the pipeline. Accordingly, the amount of data is greater and the data processing procedures are more sophisticated.

Here are the general performance characteristics of MFL tools:

  • Indirect measurement, which allows limited quantification using complex interpretation techniques.
  • Discrimination between internal and external defects (with additional sensors). (For more on this topic, see: 21 Types of Pipe Corrosion & Failure.)
  • Limitation of maximum wall thickness due to magnetic saturation requirement.
  • Signal dependent on length-to-width ratio of defects; limited ability on narrow axial anomalies.
  • Results which may be affected by pipe steel characteristics and history.
  • Results which may be affected by pipe wall stress.
  • Performance which is not affected by the medium present in the pipeline; MFL tools are suitable for both gas and liquid pipelines.
  • Moderate pipeline cleaning required (compared to ultrasonic tools).
  • Tools available for pipelines 76 millimeters (three inches) and greater in diameter.

Here are the features MFL tools can detect:

  • External metal loss.
  • Internal metal loss
  • Welds—specifically, girth welds, longitudinal welds, spiral welds, coil welds and thermite welds (if ferromagnetic material present in the weld).
  • Hard spots.
  • Cold working.
  • Dents.
  • Bends.
  • Tee piece.
  • Flanges.
  • Valves.
  • Casings.
  • Location magnets.
  • Steel sleeves.
  • Clamps.
  • Patches.
  • Near-wall excess metal.

When discussing MFL tools, we should also briefly touch on transverse field inspection tools. The measurement principle of these tools is very similar to that of longitudinal field inspection tools, as it also applies magnetic flux leakage. However, transverse field inspection tools change the magnetic field's direction from axial to circumferential. Tools equipped with this measurement technology circumferentially magnetize the pipe and detect axial features. This makes them suitable for long-seam defect detection and narrow corrosion, channeling, crack-like features and preferential seam weld corrosion.

2. Ultrasonic Testing (UT) Tools

UT inspection tools directly measure pipe wall thickness as they travel through the pipeline. They are equipped with transducers emitting ultrasonic signals perpendicular to the pipe's surface. An echo is received from the pipe's internal and external surfaces of the pipe and, by timing these return signals and comparing them to the speed of ultrasound in pipe steel, the wall thickness can be determined. Transducers are deployed in a carrier to cover the circumference of the pipe wall uniformly. (For more on this topic, see: Ultrasonic Crack Inspection: How to Avoid Pipeline Failures.)

For efficient sound transmission from the ultrasonic transducer to the pipe wall and back, ultrasonic inspection procedures typically employ a liquid to “couple” the transducer to the pipe wall.

Many liquids commonly transported through pipelines provide sufficient coupling for UT. In gases, however, because of a mismatch in gas' and steel's acoustic properties, ultrasonic inspections are not possible without an additional couplant. Gas pipeline inspections can be performed by using the UT tool in a slug of liquid (e.g., water or diesel oil) between batching pigs.

Here are the general performance characteristics of UT tools:

  • Direct and linear wall thickness measurement, allowing for reliable depth sizing.
  • Ability to discriminate among internal, midwall and external defects.
  • Sensitivity to a larger number of features than MFL tools.
  • No upper limits to inspectable pipe-wall thickness.
  • Minimum wall thickness threshold, which limits the remaining measurable pipe wall thickness. If the wall thickness is too thin, UT tools cannot measure it because of the ultrasonic pulse's finite duration. (For more on this topic, see: 8 Methods of Coating Thickness Measurement.)
  • No dependence on changes in material properties.
  • Exclusive ability to run in homogeneous liquids (in a batch of homogeneous liquid in gas pipelines).
  • Generally, a higher degree of pipeline cleanliness than MFL tools.
  • Very accurate maximum allowable operating pressure (MAOP) calculation results, due to data accuracy (especially defect depth and length).
  • Easily comprehensible results due to dealing directly with measured wall thickness.

Here are the features UT tools can detect:

  • External metal loss.
  • Internal metal loss.
  • Welds—specifically, girth welds, longitudinal welds, spiral welds and coil welds
  • Dents and deformations.
  • Bends—specifically, field bends, forged bends and hot bends.
  • Welded attachments and sleeves (features under a sleeve are also detected).
  • Tee pieces.
  • Flanges.
  • Valves.
  • Laminations.
  • Hydrogen-induced cracking (HIC) and induced laminations.
  • Blisters.
  • Inclusions.
  • Longitudinal channeling.
  • Wall thickness variations of seamless pipe.

5. Crack Detection Tools

Pipeline cracks can develop at any stageduring manufacturing, fabrication, installation or throughout operational life.

There are many forms of crackingall caused by different mechanisms. Cracking morphology of cracking is highly variable; and there may be many anomalies present in a pipe body and seam weld that would not behave as cracks but create a crack-like indication in crack inspection data. (For more on this topic, see: QUIZ: Pipeline Crack Inspection (15 Questions to Test Your Knowledge).)

Two common crack detection tools are:

  1. Liquid coupled tools.
  2. Electromagnetic Acoustic Transducer (EMAT) tools.

Here's a brief excerpt about each:

1. Liquid Coupled Tools

Liquid coupled tools use shear waves generated in the pipe wall by angularly transmitting ultrasonic pulses through a liquid coupling medium (e.g., oil or water).

The angle of incidence is adjusted such that a propagation angle of 45 degrees is obtained in pipeline steel. This technique is appropriate for crack inspection; and it is established as one of the standard techniques in ultrasonic testing.

Here are the general performance characteristics of liquid coupled tools:

  • Can only be operated in liquid environments.
  • Can inspect gas pipelines by running the tool in a slug of liquid.
  • Full pipe body coverage—no exclusion zones.
  • Ability to discriminate based on defect type.
  • Ability to discriminate among internal, mid-wall and external defects.
  • Ability to measure actual wall thickness.

Here are the features liquid coupled tools can detect:

  • Longitudinally oriented cracks and crack-like defects.
  • Cracks.
  • Crack-like defects.
    • Notches.
    • Grooves.
    • Scratches.
    • Lack of fusion.
    • Longitudinal weld irregularities.
  • Geometry-related features.
    • Welds.
    • Dents.
  • Installations.
    • Valves
    • Tee pieces.
    • Welded attachments.
  • Mid-wall defects.
    • Inclusions.
    • Laminations.

2. Electromagnetic Acoustic Transducer (EMAT) Tools

An EMAT is a non-destructive inspection technique with many industry applications. It consists of a coil in a magnetic field at the pipe wall's internal surface. Alternating current (AC) placed through the coil induces a current in the pipe wall, causing Lorentz forces (forces acting on moving charges in magnetic fields)—which, in turn, generate ultrasound.

The transducer's type and the configuration define the types and modes of generated ultrasound and the characteristics of its propagation through the pipe wall. (For more on this topic, see: All About Electromagnetic Acoustic Transducers (EMATs).)

The EMAT inspection method is effectively used to detect cracks and stress corrosion cracking without the need for a liquid couplant. Thus it can be used readily in gas pipelines. A secondary benefit of the EMAT technology is that it may detect disbonded coating.

What Type of Pipeline Pig Should I Use?

The best type of inspection tool to use depends mainly on the type of anomaly that needs to be detected.

The following table extracted, from NACE SP0102-2010, could help determine the most suitable tool for pipeline inspection:


Anomaly

Imperfection/

Defect/Feature

Metal loss tools

Crack Detection tools

Deformation

tools

Magnetic Flux Leakage

Ultrasonic Compression wave

Ultrasonic Shear wave

Transverse MFL

Standard Resolution (SR)

High Resolution (HR)

Metal loss

External corrosion

Detection

Detection

Detection

Detection

Detection

No detection

Internal corrosion

Sizing

Sizing

Sizing

Sizing

Sizing

Gouging

No ID-OD discrimination

Crack-Like anomalies

Narrow Axial External corrosion

Detection

Detection

Detection , Sizing

Detection, Sizing

Detection, Sizing

No Detection

Stress Corrosion cracking

No Detection

No Detection

No Detection

Detection, Sizing

Limited Detection, Sizing

No Detection

Fatigue Cracks

No Detection

No Detection

No Detection

Detection, Sizing

Limited Detection, Sizing

No Detection

Long Seam Cracks, e.g., Toe cracks, Hook cracks, Incomplete Fusion, Preferential Seam Corrosion.

No Detection

No Detection

No Detection

Detection, Sizing

Detection, Sizing

No Detection

Circumferential Cracks

No Detection

Detection,Sizing

No Detection

Detection , Sizing

No Detection

No Detection

Hydrogen-Induced cracking (HIC)

No Detection

No Detection

Detection

Limited Detection

No Detection

No Detection

Deformation

Sharp Dents

Detection

Detection

Detection

Detection

Detection

Detection,Sizing

Flat Dents

Detection

Detection

Detection

Detection

Detection

Detection,Sizing

Buckles

Detection

Detection

Detection

Detection

Detection

Detection,Sizing

Wrinkles, Ripples

Detection

Detection

Detection

Detection

Detection

Detection,Sizing

Ovalities

No Detection

No Detection

No Detection

No Detection

No Detection

Detection,Sizing

Misc. Components

In-Line Valves and fitting

Detection

Detection

Detection

Detection

Detection

Detection

Casings (Concentric)

Detection

Detection

No Detection

No Detection

Detection

No Detection

Casings (Eccentric)

Detection

Detection

No Detection

No Detection

Detection

No Detection

Bends

Limited Detection

Limited Detection

Limited Detection

Limited Detection

Limited Detection

Detection,sizing

Branch appurtenances /Hot Taps

Detection

Detection

Detection

Detection

Detection

No Detection

Close Metal Objects

Detection

Detection

No Detection

No Detection

Detection

No Detection

Thermite Welds

No Detection

No Detection

No Detection

No Detection

No Detection

No Detection

Previous repairs

Type A repair sleeve

Detection

Detection

No Detection

No Detection

Detection

No Detection

Composite sleeve

Detection

Detection

No Detection

No Detection

Detection

No Detection

Type B repair sleeve

Detection

Detection

Detection

Detection

Detection

No Detection

Patches

Detection

Detection

Detection

Detection

Detection

No Detection

Puddle welds

Limited Detection

Limited Detection

Detection,Sizing

Limited Detection

Limited Detection

No Detection


Conclusion

Pipeline pigging is still considered one of the most feasible and suitable methods for assessing a working pipeline. It can also be used to perform a baseline study for a newly installed pipeline.

Today, several types of pigs are available and each type has its limitations and drawbacks. Pipeline operators must have a clear understanding of the type of defect they are trying to assess and should cooperate with pig operators to determine the most suitable pig for the job.



Advertisement

Share This Article

  • Facebook
  • LinkedIn
  • Twitter
Advertisement

Written by Mohamed Adel Mohamadein | Head of the Corrosion, Arab Petroleum Pipelines

Profile Picture of Mohamed Adel Mohamadein

Mohamed Mohamadein is the head of the corrosion department at Arab Petroleum Pipelines company (Sumed). He has 14 years of experience in the application of cathodic protection for pipelines as well as ASTs. He is also experienced in different coating applications and inspection. He is a NACE certified CP technologist #22607 and NACE CIP level 1 certified #076657. He holds a master's degree in electrical engineering from Alexandria University and a Master of Business Administration from the University of Northampton.

Related Articles

Go back to top