The process of driving the pig through a pipeline by the fluid is called a pipeline pigging operation. The term pig was originally referred to as Go-Devil scrapers which were driven through the pipeline by the flowing fluid trailing spring-loaded rakes for removing the wax off the internal walls. People believe that “PIG” is the short form of the term “Pipeline Inspection Gauge”. During pipeline pigging a squealing noise that sounds like pig squealing arises, hence the name “PIG” is given.
Pipeline Pigging is used to perform various cleaning, clearing, dimensioning, maintenance, inspection, process, and pipeline testing operations on new and existing pipelines. A pipeline pigging system includes
- Pig Launcher.
- Pig Receiver.
Reasons for Pipeline Pigging
A pipeline Pigging operation is performed to
- Maintain a reliable and acceptable performance of the pipeline in terms of safety and product delivery at an economical cost.
- Provide a defined margin of safety for operating the pipeline at its designated pressure.
- Provide timely information on all pipeline defects such that repairs can be carried out according to a managed schedule.
There are several kinds of flaws and defects in pipelines that can lead to pipeline failure, the main families being:-
- Metal loss
- Cracks or crack-like defects
- Laminations and other mid-wall defects
- Geometric anomalies
There are basically three reasons to pig a pipeline:
- To separate dissimilar products.
- To remove undesirable materials.
- To perform internal cleaning, inspections, and maintenance.
Working of the Pipeline Pigging Process
Pipeline Pigs are inserted into a Pig Launcher and then the pressurized flow is applied to the rear of the device. The flow forces the pig to move into the pipeline. The force applied by a pig when it moves in a pipeline can be estimated by multiplying the cross-sectional area of the back of the pig by the pressure applied to the rear of the pig.
In General, the outside diameter of most pigs will be sized to be larger than the internal bore. This size difference creates a resultant ‘interference’ that enables the pig to scrape and remove debris as it moves through the pipeline. The type of pig employed determines the degree of effectiveness in cleaning a pipeline. Various other factors like flow rate, pig speed, length of the pipeline, number of pigging runs, pressure, temperature, the volume of debris to be removed, number and type of bends, pipeline elevations, pigging frequency, etc are also to be considered.
When the pig reaches the other end of the pipeline it is captured in a Pig Receiver which is isolated via a shut-off valve, allowing the pig to be safely removed.
Which Pipelines are ‘Piggable’?
Most of the pipelines constructed of materials like Carbon Steel, Stainless Steel, Duplex Stainless Steel, AC, GRP, HDPE, DICL, Cast Iron, Plastic, PVC, and others can all be pigged. Additional considerations for piggable pipelines are:
- number and type of bends,
- number and type of valves,
- flow rate,
- flow media,
- pressure rating,
- internal diameter(s),
- end connections,
- operating temperature,
- pipeline length,
- pipeline elevations,
- handling of flow and debris downstream.
The decision to pig any pipeline is considered by a thorough analysis of the line in conjunction with field-proven experience and advice offered by a reputable pigging specialist.
Pipeline Pig Types
Three categories of pigs are used to accomplish the above-mentioned tasks. They are:
- Utility Pigs, are used to perform functions such as cleaning, separating, or dewatering.
- In-Line Inspection Pigs: They provide information on the line condition along with the extent and location of any problems.
- Gel Pigs are used in conjunction with conventional pigs to optimize pipeline dewatering, cleaning, and drying tasks.
Based on the fundamental purpose, Utility pigs can be divided into two groups as listed below:
- Cleaning pigs
- Remove solid deposits.
- Remove Semi-solid deposits.
- Remove Debris.
- Sealing pigs
- Provide an interface between two dissimilar products.
- Provide a good seal in order to sweep liquids.
Types or forms of Utility pigs:
- Spherical Pigs (inflatable, solid, foam, and soluble).
- Foam Pigs (Polly-Pigs).
- Mandrel Pigs
- Solid Cast Pigs.
Spherical PIGs (Fig. 2):
There are four basic types of spheres:
Spherical PIGs are normally used for
- Removing liquids from wet gas systems.
- Serving to prove fluid meters.
- Controlling paraffin in crude oil pipelines.
- Flooding pipeline to conduct hydrostatic tests.
- Dewatering after pipeline rehabilitation or after new construction.
They should never be run in lines that do not have special flow tees installed.
Foam PIGs (Fig. 2):
- They are normally manufactured in bullet shape.
- Coated pigs may have a spiral coating of polyurethane, various brush materials, or silicon carbide coating.
- The standard dimension of foam pig length is twice the diameter.
- Foam pigs are lightweight, compressible, expandable, and flexible.
- They have the ability to travel through multiple diameter pipelines and go around mitered bends and short radius 90o bends.
- They also go through valves with as little as a 65% opening.
- Foam pigs are that they are a one-time-use product.
- High concentrations of some acids will shorten life.
- Foam pigs are inexpensive.
Foam pigs are used for pipelines for
- removal of thick soft deposits,
- condensate removal in wet gas pipelines,
- pigging multiple diameters
- scraping and mild abrasion of the pipeline.
- the shorter length of runs,
Mandrel PIGs (Fig. 2):
- The mandrel pig can be used for
- cleaning pig,
- sealing pig,
- combination of both.
- The seals and brushes can be replaced to make the pig reusable.
- These pigs are designed for long runs.
- The cost of redressing the pig is high, and larger pigs require special handling equipment to load and unload the pig.
- Occasionally the wire brush bristles break off and get into instrumentation and other unwanted places.
- Smaller size mandrel pigs do not negotiate 1.5D bends.
Solid cast PIGs (Fig. 2):
- Solid cast pigs are usually molded in one piece.
- The material used for the preparation of solid cast Pig are neoprene, nitrile, Viton, and polyurethane).
- Solid cast pigs are considered sealing pigs although some solid cast pigs are available with wraparound brushes and can be used for cleaning purposes.
- The solid cast pig is available in the cup, disc, or a combination cup/ disc design.
- Because of the cost to redress a mandrel pig, many companies use the solid cast pig up to 14 inches or 16 inches.
- Solid cast pigs are used
- In removing liquids from product pipelines.
- removing condensate and water from wet gas systems.
- Controlling paraffin build-up in crude oil systems.
Inline Inspection PIGs
In-line inspection PIG tools are used to carry out various types of tasks including:
- Measuring pipe diameter/geometry
- Monitoring pipeline curvature
- Determining the pipeline profile
- Recording temperature/pressure
- Measuring bend
- Detecting metal loss/corrosion
- Performing photographic inspection
- Detecting crack
- Measuring wax deposition
- Detecting leak
- Taking product samples, and
Types of Pipeline Defects
- Metal Loss
- Dents and Buckles
Metal Loss (Fig. 4):
- Pipeline operators typically use MFL to find metal loss due to corrosion or gouging.
- Metal loss is found when an MFL tool measures a local change in the magnetic field.
Gouging (Fig. 4):
- Gouging is the mechanical removal of metal from a local area on the surface of the pipe.
- The gouged area may contain shallow cracks, which further reduce the pressure-carrying capability of the pipe
Dents and Buckles (Fig. 4):
- Dents are depressions in the pipe surface, and buckles are a partial collapse of the pipe due to excessive bending or compression.
- MFL tools detect dents and buckles when the sensors lose contact with the wall.
Crack (Fig. 4) :
- Cracks are in pipelines are due to fatigue, stress corrosion, and weld defects.
- Cracks can sometimes be found when they are oriented in a direction that significantly affects the magnetic field around the defect.
Magnetic Flux Pigging
- Flux leakage techniques were used as early as 1868 by the Institute of Naval Architects in England.
- In 1918, magnetic particle inspection was accidentally discovered magnetic particles
- The first MFL in-line inspection tool for pipelines was introduced in 1965 by Tub scope.
Working Principle of MFL IN-LINE INSPECTION PIG (Fig. 5):
Ultrasonic Intelligent Pig
- The first-in-line inspection – 1980.
- Crack inspection tools – 1990.
- A new generation of ultrasonic tools – 2002
- They work in a pulse-echo mode with a rather high repetition frequency.
- Straight incidence of the ultrasonic pulses is used to measure the wall thickness and 45º incidence is used for the detection of cracks.
Working principle of Ultrasonic Inline inspection Pig (Fig. 6):
Types of Gel Pigs
- High-viscosity sealing gels
- Commissioning cleaning gel systems
- Polymer Gel Pig
- Debris pickup gel.
- Batching or separator gel.
- Hydrocarbon gel.
- Dehydrating gel.
The Principal Pipeline applications for gel pigs are as follows:
- Product separation.
- Debris removal.
- Line filling/ hydro testing.
- Dewatering and drying.
- Condensate removal from gas lines.
- Inhibitor and biocide laydown.
- Special chemical treatment.
- Removal of stuck pigs.
Pipeline Pigging Pressure
The following table gives the recommended pipeline pigging pressures and flow rates for pipeline pigging operation.
Applications of Pipeline Pigging
- During pipeline construction, pigging is used for debris removing, gauging, cleaning, flooding, and dewatering.
- During fluid production operations, pigging is utilized for removing wax in oil pipelines, and removing liquids in gas pipelines.
- pigging is widely employed for pipeline inspection purposes such as wall thickness measurement and detection of spanning and burial.
- Pigging is also run for coating the inside surface of the pipeline with inhibitors and providing pressure resistance during other pipeline maintenance operations.
In recent times, Smart PIG is used to travel through a pipeline which gathers important pipeline data like the presence and location of corrosion or other irregularities on the inner walls of the pipe. That’s why the pipeline pigging using smart pigs are often termed as Intelligent Pigging.
What is a PIG Launcher?
A Pig Launcher is a device, generally, a Y-shaped funnel section of a pipeline system from where the PIG is launched. It is basically a pressurized container that can be opened to insert the PIG. Refer to Fig. 8, Fig 9, and Fig. 11 for having a clear idea of what a Pig Launcher looks like.
What is a PIG Receiver?
A pig receiver is a container or device to receive a pipeline pig out of the pipeline without interrupting the flow. This forms a part of the pipeline pigging system. Both pig launcher and receiver are known as pig trap assemblies.
Both Pig launcher and receiver constitute of the following components:
- Major barrel
- Minor barrel
- Pig signaller
- End closure
- Kicker line connection
- Pressure gauge connection
- Utility connections
- Vent and drain connection and
Pig launcher and receiver systems are normally installed in a horizontal direction and designed based on pipeline design code.
Pigging is a vital tool that helps to achieve Pipeline integrity by providing
- Safety and product delivery at an economical cost.
- Reduces the failure
- Overall pressure drop.
- Pipeline flow efficiency.