Pipeline Pigging

Design and Application Manual

PIPELINE PIGS & PIGGING

Copyright 1999 Section 1Pipeline Engineering & Supply Co. Ltd

CONTENTS - SECTION 1.0

1.1 Tools and Techniques

1.1.1 Description (What are Pigs?)

1.1.2 Function (What do pigs do?)

1.1.3 Description of Pigging Functions

1.1.4 Method (How do pigs work?)

1.1.5 Selection (How do I choose a pig?)

1.2 Pipeline Design Factors

1.2.1 Internal Conditions (Conditions inside the pipe)

1.2.2 Pipeline Specifications (Dimensions & materials)

1.2.3 Fittings & Equipment (Components fitted to the pipe)




TOOLS AND TECHNIQUES

1.1.1 DESCRIPTION (“What are Pigs?”)

1.1.1.1 OVERVIEW

Pipeline Engineering’s formidable reputation is based on the high specification andquality of its pigs, traps and related equipment – which have been used to maintainpipelines world-wide for many years.

But, what is a ‘pig’. The name refers to a mechanical device that can be inserted intoa pipeline and used to carry out pre-defined tasks (depending upon its construction)at critical points, or along the full length of the pipeline.

Pigs are driven by pressurising the pipeline behind them or, in most cases, insertingthem into the existing product flow, using specialised laundering equipment.

Whilst the potential applications for pigs are limited only by the imagination of thespecifier and the manufacturer, most tasks have now been defined and accepteddesign characteristics are used by all the major suppliers.

Pigs and spheres must be suitably equipped to carry out a function or ‘task’. Thesetasks are given names (cleaning, gauging, separation, drying, etc.) but the demandsof any individual task may require very different qualities from the pig depending uponthe stage in the development of the pipeline (pipeline development stage) at whichthe pig is to be used (construction, pre-commissioning, commissioning, etc.)

Pipeline design factors influence the overall design of the pig and the types of sealsand fittings to be used.

1.1.1.2 PIG TYPES

The chart in fig. 1 shows the way in which pigs are generally classified:

Fig. 1 Pipeline Pigs

Utility Pigs In-Line Inspection (ILI) Tools

Metal-Bodied Pigs Solid Cast Pigs Foam Pigs Spheres Special Pigs (Mandrel Pigs)

Bi-Directional Uni-Directional Hard Medium Soft Solid Inflatable (Disc) (Cup)

Standard Conical




There are two types of pigs at the highest level:

1. In-line inspection tools (‘Intelligent Pigs’)2. Utility pigs

1. In-Line Inspection Tools (I.L.I)

Because of the complexity of these pigs (which make extensive use of on-boardelectronics, they fall outside the scope of this manual.

2. Utility Pigs

Utility pigs include all categories except intelligent pigs. They can be divided into 5sub-groups based on their construction characteristics:

1. Metal bodied pigs (often referred to as ‘mandrel’ pigs) – sealing componentsaxially mounted on a tubular or solid metal body.

2. Solid cast pigs – single piece polyurethane casting – usually ‘dumb-bell’ shaped3. Foam pigs – moulded polyurethane foam, usually in 3 ‘densities’: soft; medium;

hard.4. Spheres – manufactured from cast polyurethane and various rubbers according

to task5. Special pigs – construction is similar to metal bodied pigs.

Specialised categories are allocated according to function (the task, or type of workthat the pigs will be required to perform). Although functions are referred tocollectively (cleaning, gauging, etc.) operating conditions will vary according to thepipeline development stage. This will influence parameters such as component ratesof wear which will, in turn, effect the quantity of spares or complete pigs required tocomplete the task.

1.1.1.3 CONSTRUCTION CHARACTERISTICS

Overview

The length of metal-bodies, solid cast and foam pigs is approximately 1½ - 2 xnominal diameter of the pipeline in which they will be operating. Special pigs may notconfirm to this formula due to other factors that may effect their design. Spheres are,obviously, spherical.

There are 3 elements to be considered with regard to pig construction:

• The pig body• The type of seal• The fittings

Only metal-bodied and special pigs have independent bodies onto which seals andfittings can be attached. These bodies are usually made from carbon steel, althoughpolyurethane can be specified in some types of pig for improved flexibility.




Solid cast pigs are constructed entirely from polyurethane whilst spheres can be madfrom several types of elastometric material:

• Polyurethanes• Rubbers: Neoprene; Nitrile; Silicon; Viton Seals

There are 3 types of detachable seal:

• Discs• Standard Cups• Conical Cups

Seals made from polyurethane are specified for most pigging functions – it possessesexcellent physical characteristics which can be formulated for specific applications.However, there are good reasons for specifying some rubbers, which are given later.

Careful formulation of urethanes allows characteristics such as harness, tearstrength, abrasion resistance, hydrocarbon and hydrolysis resistance, flex life andcompression set to be modified.

There are 2 types of polyurethane:

• Ethers (Good hydrolysis resistance; high resilience; low compression set).• Esters (Better physical properties than ethers).

As a general rule, it is not always possible to specify and formulate a particularurethane, unless all the operating conditions are taken into account. Althoughurethanes work in applications where rubbers could not, they must be carefullyselected, especially for very critical tasks.

Pipeline Engineering was one of the first companies to use polyurethane for pig sealsin the late 1960’s. The company developed its own polyurethane, known as‘Omnithane®’ that was very successful and is still being specified by clients.However, the drive for technical excellence led to the development of ‘SuperOmnithane®’ an ester/MDI/diol system and ‘Hyper Omnithane®’ a TDI/amine system.These formulations are used for the seals of some of the most durable pigs in usetoday.

Fittings

Fittings are those items other than seals which can be attached and removed fromthe pig body; these include: brushes; scraper blades; plough blades; transmitterhousings; gauging plates; magnets and any item not usually found on a standard pig.

1.1.1.4 PIG CHARACTERISTICS BY TYPE

Metal-Bodied Pigs

This is the only pig type in which the 3 construction elements can be identifiedseparately. This form of construction allows the specifier to develop a pig thataccurately meets the demands of the task.




The pig body is manufactured from steel tube in pigs 8” and above and a steel bar inpigs below 8”. Other materials, including aluminium and polyurethane, are also used.Flanges are welded to the body at both ends of the pig for seals and spacers; studsand pads are provided for attachment of fittings.

These pigs incorporate discs, standard cups and conical cups, manufacturedprimarily from polyurethane, although discs can also be supplied in several types ofrubber, as previously described. The quantity and type of seal depends upon thepigging function and operating conditions.

Metal-bodies pigs allow the greatest range of fittings to be attached. This rangecovers fittings of all types and includes brushes, scrapers, plough blades, transmittedhousings, gauging plates, magnets and many other types necessary for specifictasks.

Solid Cast Pigs

These pigs are made from a single polyurethane casting. Therefore, the sealingelements are not interchangeable or replaceable. All seals are disc-type andcomprise 4 sealing and 2 support discs. They are designed, primarily, for small-borepipelines of approximately 2” to 12” nominal diameter.

The one-piece construction method limits the choice of fittings. However, brushes,gauging plates and magnets (for tracking and location purposes) can be fitted to thistype of pig.

Foam Pigs

Foam pigs are often specified in similar circumstances to metal-bodies pigs.However, they employ very different design characteristics. The pigs are normallybullet shaped and moulded from open cell polyurethane foam which is available inseveral densities (designated soft; medium; hard). External coats are applied toincrease the pigs’ effectiveness in specified operating conditions.

They have no independent sealing elements. The pig is compressed in the pipelineby the pressure and forced against the pipe wall to form a seal.

The density of the pig is matched to the pigging function and operating conditions.‘Hard’ pigs are tougher and last longer; ‘soft’ pigs are more commonly known as‘swabs’ and are used for drying and other specialised tasks.

A thin polyurethane coating is often applied to bare pigs to help increase theefficiency of the pig with respect to its function: ‘cross-cross’, spiral or fully coated. Acoating of polyurethane is also applied to the base (the flat end) of most bar pigs.

Although there are many options for foam pigs, there are relatively few fittings.Specialisation is controlled at the moulding stage when options are ‘built-in’: grittedbands, brushes, jetting holes, magnet inserts (for tracking and location), transmittedcavities, ropes, studs for gauging plates, stud inserts for scraping are some of theoptions available. Some fittings are removable and include magnets, gauging plates,transmitters and studs for scraping.




Spheres

The spherical elastomeric body of a sphere forms a single-line seal in the pipe.Therefore, there are no independent sealing elements to consider. Spheres do notaccept fittings (with the exception of some spheres which have magnets cast intothem for tracking and location purposes).

There are 2 types of sphere (with typical uses given):

• Solid – automated pigging programmes; batching; condensate removal• Inflatable – meter proving; batching; condensate removal

Once the type has been decided, the characteristics of the pig are dependent uponthe formulation of the material from which the spheres are constructed. This willinfluence properties such as sealing efficiency, rate of wear, operating temperaturelimits and chemical resistance.

Inflatable spheres have a removable valve and are usually filled with water or awater/glycol mixture. Meter proving spheres are produced in hardnesses of 65-70°Shore A while, for most other operational purposes, spheres tend to be specified withhardnesses of approximately 65 – 75° Shore A.

Special Pigs

‘Special pigs define those pigs – usually metal-bodies – which have to be significantlymodified (relative to the ‘ideal’ standard in terms of dimensions and sealconfigurations) in order to pass through the limitations imposed by the pipeline designfactors which exist for that specific pigging application.

Special pigs include:

• Dual diameter pigs• Articulated pigs• High differential pigs

By definition, special pigs must cope with increased problems imposed by thepipeline design factors. Therefore, their design is much more critical. Body, sealsand fittings may use standard components, but it is much more likely that they willinclude characteristics which will require detailed information on all aspects of thepipeline and its design factors.




1.1.2 FUNCTION – (What Do Pigs Do?)

1.1.2.1 OVERVIEW

Pigs must first be selected to carry out a specific function (task). This results in a pigwhich can generally be described by its function type. e.g cleaning pig, gauging pig,de-watering pig (section 1.4.0 explains this principle in more detail). For now, thescope of pigging functions is set out as follows:

1.1.2.2 PIGGING FUNCTIONS

As a result of the experience gained in pigging operations world-wide, most functionsare now pre-defined and can be found somewhere within the following list:

• Debris removal• Cleaning• Gauging• Filling• De-watering• Drying• Separation (batching)• Condensate removal• Meter proving• Product displacement• Product conversion• Gel pigging• Coating application

These pre-defined functions all rely on one (or a combination of) the followingaspects of the pig’s design characteristics (the ability(s) required to complete the tasksuccessfully):

• Ability to SEAL• Ability to CLEAN• Ability to GAUGE• Ability to ABSORB• Ability to RETAIN PRODUCTS

Some of the characteristics may need to be enhanced in order to provide the best pigfor the task (e.g more – or improved – seals and fittings may be required). All will beinfluenced by the pipeline development stage.




1.1.2.3 PIPELINE DEVELOPMENT STAGE

Pigging function depends upon the stage in the operational life of a pipeline at whichthe pig must carry out its task. Most stages in the development of a pipeline can befound in the following list:

• Construction• Pre-commissioning (including hydrostatic testing)• Commissioning• On-line operations• Inspection• Maintenance and repair• Renovation and rehabilitation• Decommissioning

Some pre-defined functions can take place at several stages in the development ofthe pipeline. However, pigs designed to carry out tasks at one stage of pipelinedevelopment may not possess features that are suitable to enable them to carry outthe same task at other stages. This is due to a combination of the following factors:

• Pigging distance• Frictional resistance of the materials in contact• Lubrication• Pigging speeds

These factors are dealt with later when we consider pipeline design factors and theoperating conditions that apply. However, for now we can consider what is involvedin each of the pigging functions previously listed.

1.1.2.4 DESCRIPTIONS OF PIGGING FUNCTIONS

1.1.3.1 DEBRIS REMOVAL

Debris removal is generally the first operation to be considered. It occurs at theconstruction stage and involved the clearance of the constructor’s waste, rocks, sand,dead animals, etc. These items must be removed prior to cleaning and gaugingoperations at the pre-commissioning stage. This particular task is extremelydemanding and requires a robust, well specified pig. Bi-directional pigs – either foamor metal-bodied – are required as it may be necessary to reverse them if theybecome stuck.

The nature of the operation varies depending on whether it is carried our on land ofoffshore/sub-sea. Sub-sea operations are more complex and may involve the use of‘lay-down heads’ with the pigs being pre-loaded for operations after the line is ‘tied-in’. Debris removal onshore usually involves pigging with compressed air between‘test-ends’ (temporary pig traps).




Compressed air operations impose several conditions on the pig – no lubrication isavailable and pigging occurs in a series of high speed ‘excursions’ between localisedrestrictions. The pig stops and the pressure builds until there is sufficient energy tolaunch the pig past the obstacle. As the pig moves forward rapidly, the pressure isdissipated until the pig reaches the next restriction, causing the process to berepeated.

1.1.3.2 CLEANING

The specification of cleaning pigs depends upon the pipeline development stage atwhich cleaning is required – usually at one of the following:

• Pre-commissioning• On-line operations• Inspection• Renovation and rehabilitation• Decommissioning

Cleaning at each stage imposes a different set of demands on the pig. This could,but would not automatically, require variations in pig design. There are 2 mainrequirements for cleaning – irrespective of the type of pig.

1. Pigs must be fitted with cleaning devices – brushes (circular or spring mounted;scrapers; ploughs.

2. ‘By-pass’ must be introduced across the pig and, in particular, the brushes.

Cleaning operations are usually carried out by either metal-bodied or foam pigs. Bothhave advantages, although metal-bodied pigs can be more readily adapted to suitspecific conditions.

Liquid flows improve cleaning efficiency by allowing the pig to maintain a constantvelocity. Unfortunately, a liquid flow is not always available (at the constructionstage, for example). Cleaning then has to be done with compressed air, where theefficiency is diminished as the pig speed cannot be easily controlled.

Cleaning included the removal of ‘scale’ (mill scale, silica, coke, calcium) which oftenrequires a much more aggressive pig. Conversely, pigs can be fitted with nylonbrushes for cleaning pipes that are internally lined. Foam pigs are often used for thistask – ‘hard’ foam pigs have inserts for studs integrally moulded into the pig body.These studs are made from steel and are available in several types to suit theoperating conditions. Metal-bodied pigs are available with steel scraped blades andspecial pigs (such as the pin-wheel pig) have also been developed for this purpose.

Old pipelines, or those which have never previously peen pigged, require a specialapproach. A progressive pigging programme must be carried out to avoid the risk ofa more robust pig becoming stuck in a pipeline which cannot be accurately evaluated.This approach involves the step-by-step use of progressively harder, moreaggressive foam pigs, run until the pipeline is considered acceptable for metal-bodiespigs to be used.




1.1.3.3 GAUGING

Gauging is commonly carried out at the following stages:

• Construction• Maintenance and repair• Decommissioning

It is necessary for 2 reasons:

• To check that there are no unintended intrusions into the Pipeline Engineering• To confirm that the ovality of the pipeline is within acceptable limits (tolerances)

Metal-bodied pigs (uni-directional or bi-directional) are fitted with a circular aluminiumgauging plate (steel plates can also be used), which is usually machines to 95% ofthe smallest internal pipeline diameter. Foam pigs can also be adapted to acceptgauging plates but they are not often used for this purpose.

At the construction stage, gauging operations are usually conducted with compressedair. As with all other operations using compressed air as the pigging medium, it isextremely dangerous and less effective.

1.1.3.4 FILLING

Filling is carried out at the following stages:

1. Pre-commissioning2. Commissioning

1. Pre-Commissioning

Filling is carried out in order to evacuate air and fill the line with a solid column ofwater prior to hydrostatic testing. The most important characteristic required is theability of the pig to maintain an effective seal against the pipe wall in order tominimise leakage. For this reason metal-bodied bi-directional pigs using multiplesealing discs are specified.

Pigs are run in front of the water column, evacuating the air from the pipeline that isvented at the receiver. It is extremely important that all pipeline design factors thatapply are considered in order to produce a pig which is capable of travelling throughthe pipeline system successfully.

2. Commissioning

Filling is carried our in order to bring the pipeline to the point of operation – when ithas been filled with product. Several metal-bodied, disc-type pigs are required asfilling involves ‘slugs’ of other liquids e.g methanol, glycol, held between pigspreceding the pipeline product. Commissioning is complex and requiredconsiderable expertise.




1.1.3.5 DE-WATERING

After hydrostatic testing the water must be removed from the pipeline prior to dryingand subsequent commissioning. This is done by a series of pigs pushed through thepipeline using compressed air.

The same qualities are required for de-watering as for filling – the most importantcharacteristic again being the ability to maintain an effective seal in order to minimiseleakage. All relevant pipeline design factors should be considered.

Soft foam pigs (swabs) are often run after de-watering to swab away any waterremaining on the pipe walls, or which has accumulated at low points in the pipeline.

1.1.3.6 DRYING

In on-shore pipelines, foam swabs are pigged with extremely dry compressed air (-90°F atmospheric dew point temperature). Together with pigs and the air pick up theremaining water leaving behind a dry pipeline. However, it takes more than oneswab to do this and is, of course, dependent on the length of the pipeline. Severalhundred swabs may be required to fully dry a pipeline.

After the pipeline is dry, brush pigs must be run to remove the rust, mill scale and anydeposits that have settled out from the hydrostatic test. Again, several hundredswabs may be required to remove the material loosened by the brush pigs. Theseswabs will then be run until the air reaches a specified dewpoint.

1.1.3.7 SEPARATION (BATCHING)

Batching is a technique used during on-line operations, primarily on multi-productpipelines that transport different products in the same line at the same time.

In order to minimise ‘interface mixing’ (caused when two products being transportedin the same line come into contact with each other), the pig has to be inserted into thepipeline at exactly the right time. This task can be automated by using spheres pre-loaded into magazines. They can then be automatically inserted into the pipelinewhen required.

1.1.3.8 CONDENSATE REMOVAL

Condensate removal is a constant activity during on-line operations in untreated gaslines. It settles out in low spots in the pipeline, reducing efficiency. As it builds up,the condensate is pushed along the line by the flow until it reaches the receiver. Theresulting slug can be so large that it frequently exceeds the capacity of the slugcatcher set up to receive it.

Sphering provides the ideal solution. Automatic launching from pre-loadedmagazines allows un-manned pigging activity over extended periods. Spheres do nothave to seal in the pipe – their purpose is to move the condensate into the slugcatcher at the receiver.




1.1.3.9 METER PROVING

Meter proving describes the procedure for calibrating flow meters (e.g turbine andpositive displacement meters) in which pigs are used to batch a known volume ofproduct through the meter. This volume is then compared against the volumeindicated by the meter.

Early meter provers were simply a ‘measured mile’ of pipe in which batching pigswere used to push the product through the meter in one direction only. When thepigs had completed the task they were transported back to the launcher to be usedagain. To save on transportation and handling costs, bi-directional pigs began to beused as these pigs could be returned simply by reversing the flow of the pipeline.

The next development saw spheres used in a continuous pipeline loop with aninterchange valve closing the top from the bottom. These interchange valvessubsequently proved to be inadequate and bi-directional meter provers becamestandard.

In their simplest forms, meter provers consist of an internally coated pipe containingan inflatable sphere. Electrical pig signallers and a scraper trap (for launching andreceiving spheres) are installed at each end of the pipe. The volume of productcontained in the pipe between the detectors is accurately calibrated against certifiedcolumetric tanks and the meter to be proved s connected in series with the meterprover system.

Outputs from both signallers and the flow meter are connected to a counter. Whenthe pig trips signaller 1, pulses from the flow meter are counted. When the pig tripscounter 2, counting stops and the count is recorded. The signal count is comparedagainst the known volume to obtain a factor which can then be used to calibrate themeter.

Only inflatable spheres with a hardness of between 65° - 70° Shore A are specifiedfor use in meter prover loops.

1.1.3.10 PLUGGING

Plugging is carried out during the maintenance and repair phase of a pipeline’s life.The plug is most commonly a modified metal-bodied, bi-directional, disc-type pig inwhich the sealing elements not only seal but cause the pig to be an extremely tight fitin the pipeline.

Depending upon the diameter of the pig, pressures of up to 8 bar may be required tomove it in the pipeline – higher pressures are required to reverse the pig and ‘flip’ thediscs.

Plugging is often carried out in ‘trains’ where the pressure that a single pig canwithstand s cumulative and which therefore allows a series of pigs to withstandgreater differential pressures.




1.1.3.11 PRODUCT DISPLACEMENT

The same qualities are required for displacement as for filling and de-watering duties.Product displacement is carried out whenever the pipeline contents have to beevacuated, particularly at the decommissioning stage.

1.1.3.12 PRODUCT CONVERSION

This involves ‘sterilising’ the pipeline prior to running other products which maybecome contaminated. Pigs used for this purpose require similar quantities to pigsused for filling and de-watering duties.

1.1.3.13 GEL PIGGING

This technique does not involve utility pigs. A jelly-like substance is either injecteddirectly into the pipeline or moulded into cylindrical ‘pigs’. The jelly picks up andholds within it deposits found in the pipeline. It is bio-degradable and can be brokenup under pressure.

1.1.3.14 COATING APPLICATION

There are 2 pigging phases involved in the application of ‘in situ’ pipeline coatings:

1 Cleaning2 Coating application

1. Cleaning

Cleaning prior to coating application requires as much debris to be removed from thepipe wall as possible. Therefore, it is usually done using methods other than cleaningpigs. e.g: grit blasting techniques). Pigs are then used to remove loosened depositsfrom the pipeline.




1.1.4 METHOD – (“How Do Pigs Work?”)

1.1.4.1 PROPULSION METHODS

Pigs travel through the pipeline under pressure from the pigging medium beingpumped behind them. This medium takes on of two forms:

1 Gas2 Liquid

Whatever the medium is – gas or liquid – has a dramatic effect on operational piggingcharacteristics, especially in terms of the pig’s velocity profile.

Irrespective of the type of pigging medium, pig velocity has a profound effect onpigging efficiency. Pigs are designed to work most efficiently within a limited velocityrange which may be difficult to achieve, depending upon the pigging medium or otherfactors, such as the ability to regulate existing product flow rates.

1.1.4.2 PIGGING WITH A GAS

A stationary pig requires increased pressure to start it moving. Once this pressure isreached, the pig moves forward very quickly as the pressure begins to decreasebehind it. Eventually the pig comes into contact with an obstruction, such as a weldbead, the pressure is insufficient to keep it moving forward and the pig stops. Thecycle then begins again.

This velocity profile is typical when pigging with gasses, due to their ability to becompressed. Once the pig begins to move this energy is released very rapidly. Pigshave been measured at speeds in excess of 100mph and have been know to burstout of thin walled pipes at bends in the pipeline.

Aside from the safety aspects, pigging in this manner is inefficient, increases wear onthe pig seals and brushes (if used) and is generally more destructive to the pig.

1.1.4.3 PIGGING WITH A LIQUID

This method is much more efficient. Velocities can be maintained, within limits. Theyare lower and controllable at the pump. Seal wear rates are also reduced as theproduct or pigging medium acts as a lubricant.

The pressure required to move a pig depends upon a number of factors:

• The pressure in front of the pig• Condition of the pipe wall surface• Pressure exerted by the seals on the pipe wall (the ‘fit’ of the pig in the pipe)• The presence of a lubricant

Pigs move in the pipeline under the influence of the ‘differential ‘pressure acrossthem. That is to say, the pressure behind the pig (P1) minus the pressure in front ofthe pig (P2). This differential pressure (‘delta P) gives the pig a velocity. (i.e. It givesthe pig both speed and direction). Obviously if P1 is less than P2 the pig will movebackwards – which is only of benefit when using bi-directional pigs (refer toillustrations on page 14).




The differential pressure contributes to the efficiency of pigging operations in thefollowing ways:

• It improves sealing efficiency by forcing the seals against the pipe wall, makingthem act like non-return valves.

• It is used as ‘by-pass’ in cleaning operations

1.1.4.4 BENEFITS OF BY-PASS

‘By-Pass’ is the controlled flow of pigging medium, gas or liquid, through and/or pastthe pig. It has two effects:

• It reduces the pressure (P1) behind the pig and hence the differential pressure(delta P) across the pig – variations in the differential pressure effects the speedof the pig. The pig slows down as the differential pressure decreases.

• It provides a very necessary jetting action which increases the efficiency ofcleaning pigs by preventing brushes from becoming blocked with looseneddeposits and holding these deposits in suspension in front of the pig rather thanallowing them to form into a solid ‘slug’. These slugs are difficult to deal with atthe receiver but, more importantly, may cause the pig to become stuck.

Studies have shown that it is better to drive the pig in the pipe using the front sealsonly, the rear seals act as support. Pluggable by-pass ports are built into the pigbody for this purpose and allow the pressure to flow through the pig body and bedirected into the space between the two sets of seals.

More ports can be added to create flowpath right through the pig. This reduces thepressure P1 behind the pig and the differential pressure (delta P) across the pig.Consequently, the pig can be driven at slower speeds in pipelines with fixed flowrates.

The relative importance of by-pass depends on the task which the pig has beendesigned to perform. These tasks can be divided into 2 groups:

1. Sealing2. Cleaning

1. Sealing

If the pig is used for operations where its ability to seal in the pipe is most important(e.g filling, de-watering) then the principle benefit of by-pass is to allow the pig to bedriven from the front set of seals. However, in addition, it is thought that by-pass mayhelp prevent the formation of slugs of solids building up in the liquid which the pigwould then tend to ride over and leave behind in the pipeline.




2. Cleaning

During cleaning operations, especially was removal, the deposits on the pipe wallvery quickly build up in the brush reducing its effectiveness. To prevent thishappening, limited by-pass can be used to create a flow path of pipeline productthrough and around the brushes. This tends to prevent the deposits removed fromthe pipe wall from building up in the brush by holding them in suspension within theproduct flow created by the by-pass.

Pigs are often fitted with jetting nozzles (also known as ‘spider noses’) which arepositioned in front of the leading set of seals. The nozzle jets are directed backtowards the leading discs or brushes (depending on the pig type) and again are usedto create a flow of product that is designed to hold in suspension any depositsremoved from the pipe wall.




1.1.5 SELECTION – (“How Do I Chose A Pig?”)

1.1.5.1 OVERVIEW

There are 3 fundamental questions to ask when selecting the appropriate type of pig:

1. What is the function, of task of, the pig?2. At what state in pipeline development will the task be performed3. Which pipeline design factors are relevant and to what extent will they influence

pig design

This approach will establish:

1. The most suitable type of pig and consequent arrangement of body, seals andfittings

2. The extent of any modifications and additions to the body, seals and fitting ofstandard pigs (resulting in the basis of a design or a special pig).

All pre-defined tasks will be carried our by one of the following 5 types of pig:

1. Metal-bodied pigs2. Solid cast pigs3. Foam pigs4. Spheres5. Special pigs

The construction characteristics of each type of pig influences its suitability for aparticular task. Some types (metal-bodied and foam) include within their range pigssuitable for most tasks while others (solid cast and spheres) are themselvesspecialised and are limited in the number of options they can offer. Special pigs arethose pigs which have had their body and seal configurations significantly modified asa result of pipeline design factors or which belong to no pre-defined category.

In order to carry out the function for which they were designed, many pigs areequipped with fittings of some type. The range of fittings, and hence the range oftasks, are limited by the type of pig specified.

Metal-bodied pigs, other than those specified solely for their sealing qualities, mayincorporate any of the fittings listed below:

• Blades• Brushes• Scrapers• Magnets• Gauging plates• Jetting nozzles• Transmitters




The way in which some fittings are mounted will influence the pigging direction. Thefollowing table refers to those fittings which do influence the pigging direction:

Uni-Directional Bi-Directional

Brushes ‘U’-spring mounted CircularCantilever Mounted Arch-spring mounted

Blades ‘U’-spring mounted Arch-spring mountedCantilever Mounted

Scrapers ‘U’-spring mounted Arch-spring mountedCantilever Mounted

Each pig type incorporated very different design characteristics which are examinedin more detail below:

1.1.5.2 METAL-BODIED PIGS

Metal-bodied pigs are divided into 2 groups according to their directional ability withinthe pipeline. This can be selected or, more likely, will be imposed by the operatingconditions;

• Uni-directional pigs travel through the pipeline in one direction only• Bi-directional pigs travel through the pipeline in either direction

The directional ability of a metal-bodied pig depends upon 2 factors

• Seal type• Fittings

Seal Type

There are 3 types of seal which are further classified by their suitability for use withregard to pigging direction:

• Discs – bi-directional• Standard cups – uni-directional• Conical cups – uni-directional

Discs

Discs increase the scope of the pig to be used for a more diverse range ofapplications. They allow it to be reversed, they provide a scraping/swabbing actionand are lighter and cheaper. However, they wear more quickly, are more prone todamage and are less able to pass obstructions and reductions in the pipeline thanconical cups.




Cups

• Standard – cost effective design• Conical – greater flexibility of use

Selection depends on the pipeline design factors detailed in section 2.0.0. However,conical cups do offer one particular advantage over standard cups – an increasedability to pass restrictions in the pipeline (up to 20% of minimum pipeline internaldiameter compared to 5% with standard cups).

Other Factors

After ensuring that the pig will travel through the pipeline, selection is based on thepigging function, particularly when the function relies on the qualities of the seal (e.g.de-watering, filling, pigging long distances)

Unless operating conditions dictate otherwise, the pig body will be constructedrelative to the diameter and length of the pig. This ratio is as follows and variesbetween pig types – it does not include spheres:

Length of pig = nominal pipe diameter x ‘n’(The value of n varies between approximately 1.4 – 2.0 according to pig type)

Fittings will then be sized and configured to match the pigging function. All of thefittings listed earlier in this section are available for selection for use with metal-bodied pigs.

1.1.5.3 SOLID CAST PIGS

These pigs have limited application as they cannot be readily adapted to suit complexoperating conditions. They are often used in the food process industry because theycan be manufactured from a single piece of polyurethane that complies with foodhygiene standards. They are typically supplied in diameters from 2” to 12” and theintegrally moulded disc seals offer low leakage and high scraping efficiencycompared with spheres. They are bi-directional and can have a limited range ofbrushes, gauging plates and magnets fitted (for tracking and location).




1.1.5.4 FOAM PIGS

Foam pigs are initially selected according to pigging functions – the following typesare commonly available for the task specified:

• Bare – swabbing displacement• Fully polyurethane coated – displacement• Criss-cross polyurethane coated – displacement/light cleaning• Wire brush – spiral pattern – cleaning• Wire brush – fully covered – cleaning (long run)• Silicon gritted – heavy cleaning/scale removal

The characteristics of foam pigs are more closely matched to the task by specifyingthe density of the open cell polyurethane foam:

• Soft – approximately 2lb/ft3• Medium – approximately 5lb/ft3• Hard – approximately 8lb/ft3

‘Hard’ foam pigs assist aggressive cleaning whilst ‘Soft’ foam pigs improve swabbingcharacteristics.

Foam pigs are often specified as an alternative to metal-bodies pigs as they haveseveral advantages.

• They are relatively cheap• They can pass through tight bends (ID depending on density and type)• They are easier to handle than comparable metal-bodied pigs• Softer, non-brush types do not require specialised launching facilities• Swabs can be used for testing blockages in a pipeline (if they become stuck they

will disintegrate under increased pressure)• As a product group, they form the basis of a progressive pigging programme ( a

vital approach for pipelines which have never been pigged previously).

Not all foam pigs are bi-directional but most can be suitably adapted at the designstage. Several types of fitting are available:

• Jetting holes• Magnet inserts• Transmitter cavities (for tracking and location)• Ropes for pulling (or being pulled)• Studs for gauging plates• Stud inserts for scraping




1.1.5.5. SPHERES

Spheres play a vital role in pigging operation due, particularly, to their shape. Itallows them to be pre-loaded into launcher magazines and used to carry outautomated pigging operations (e.g. batching, condensate removal in untreated gaslines) where the launching facilities are un-manned for long periods.

There are 2 types of sphere:

• Solid• Inflatable

Solid spheres can be made from various elastomer materials:

• Polyurethane• Nitrile rubber• Neoprene rubber

Solid spheres are used for batching and condensate removal from unmannedlaunching stations

Inflatable spheres – made from polyurethane – are always specified for use in meterprover loops because the sphere can be precisely inflated to give a constant seal asthe sphere wears with use. Meter prover spheres are softer than standard spheres(65-70° Shore A compared to 70 – 75° Shore A for standard spheres). Making thesphere softer improves the quality of the seal.

Rubbers such at nitrile and neoprene allow spheres made of the materials to be usedwithin a much greater temperature range than is available with spheres made fromthe standard formulations of polyurethane. However, polyurethane formulations doexist which greatly improve the sphere’s ability to operate in temperatures up to150°C

The best polyurethane spheres are of seamless construction which, although it ismore technically difficult to achieve and requires significant investment by themanufacturer, results in a sphere of much higher quality.

Due to their construction and sealing characteristics, spheres cannot accept fittingshowever, some spheres do have magnets incorporated during the moulding processfor tracking and location purposes.




1.1.5.5 SPECIAL PIGS

If standard pigs must be modified to such an extent that they no longer comply withan ‘ideal’ standard model in order to perform a function, then a ‘special’ pig must bedesigned.

This usually involves changing the dimensions of a pig relative to the nominal pipelinediameter. The body dimensions may be changed or constructed in two modules(articulated), and the fittings and seals may also be suitably adapted. In someinstances, the dimensions of the pig remain unchanged with modifications beingconfined to altering the specification of the materials used in the construction of thepig.

Pigs designed for one-off tasks (tasks for which there is unlikely to be any furtherdemand) can also be classified as special pigs.




PIPELINE DESIGN FACTORS

1.2.1 INTERNAL CONDITIONS (“What Are Conditions Inside the Pipe?”)

This section considers the potential problems posed by factors such as the chemicalcompatibility of the pipeline product with the materials used in the construction of thepig. It also considers the contributory effects of temperature, pressure, flow andpipeline deposits on the pig and its components.

1.2.1.1. PIPELINE PRODUCTS

Most pipelines carry one or more of the products listed below:

• Hydrocarbons• Petrochemicals• Chemicals• Water• Food Products• Gases

Each of these product groups has an effect on polyurethanes and rubbers to agreater or lesser extent. (Pig body materials are also effected by pipeline product).

The typical effect of chemical incompatibility with polyurethanes and rubber leads toblistering, cracking and chemical breakdown (Material is described as having a‘cheese-like’ consistency).

Whilst it is extremely important to select a material which has good compatibility withthe pipeline product, a compromise will almost certainly have to be made becausethe most compatible material with the product is unlikely to have the necessaryphysical characteristics.

Most rubbers can be moulded into spheres or supplied in sheet form (hence can besupplied as discs) but they cannot be economically moulded into more complex cupshapes. Therefore, the choice of seal materials may have great effect on the overallpig design.

1.2.1.2 TEMPERATURE

Internal pipe temperatures primarily effect elastomeric components and, in particular,pig seals. Temperature increases the effect that product incompatibility has onelastomeric components. Elastomers, in this instance rubbers and polyurethanes,operate most effectively within specified limits. These limits are much wider inrubbers (Nitrile, Silicon, Neoprene, Viton) than they are in polyurethanes, which haverelatively low upper operating limits.

In almost all instances, rubbers withstand high temperatures better thanpolyurethanes but have significantly inferior physical characteristics, resulting ininferior wear rates, material strength, etc.




1.2.1.3 PRESSURE

Pressure does not have a significant effect on the construction of utility pigs whichcontain no sealed units. In-Line (I.L.I) pigs do have sealed units and pipelinepressure has a much more significant effect on this type of equipment.

The general effect of pressure is to move the pig in the pipeline. If this movement issmooth and constant then pigging will be much more efficient. In pipelinestransporting liquids, this characteristic movement is not difficult to achieve. However,it is much more difficult in gas pipelines or where pigging is being carried out usingcompressed air.

Minimum pressure levels need to be achieved in order to keep pigs moving pastobstacles (typically weld beads) without stopping. IF a pig does stop, pressure willincrease until it is sufficient to launch the pig past the obstacle. Initial acceleration willbe rapid and overall velocity will be high with consequent safety implications, possibledamage to pipelines and fittings and much lower pigging efficiency. These problemscan be relieved by pressurising the pipeline ahead of the pig and venting at thereceiver.

Recommended minimum pigging pressures are often requested but are not easy tosupply as they are a compound of many factors: function, pipeline, condition,deposit, lubricant, seal material and type, liquid or gas pigging.

1.2.1.4 PRODUCT FLOW

Maximum pigging efficiency id dependent upon a constant product flow that, bydefinition, is usually only available in on-stream applications. There are guidelinesgiving approximate pigging speeds for different types of pig however, the speed isdependent on the velocity of the product flow and may not be changeable.

The main consideration is t move the pig smoothly at a constant speed through thepipeline. Pigging in gas lines is less efficient and moor likely to result in ‘speedexcursions’. This is more destructive to the pigs as the higher speeds and moreabrasive conditions cause greater wear on the pig components. Increased speedalso causes a decreased pressure differential across the pig which, in turn, results indecreased sealing efficiency.

1.2.1.5 DEPOSITS

The main types of deposit are as follows:

• Condensate (untreated gas lines)• Water (most lines)• Waxes (oil lines)• Scale• Mineral deposits• Rust

The type of deposit to be removed influences the type of seal and cleaning elementsrequired for carrying out the work most effectively.




Condensates and water are swabbed our using pigs with seal discs, spheres or foampigs.

Polyurethane ploughs and scrapers are recommended for removing soft and hardwaxes respectively. Scale and mineral deposits often have to be removed usingfoam stud pigs to break up the hard scale initially then followed using cleaning pigsequipped with brushes.

Rust (corrosion) can be removed using brush-equipped cleaning pigs, possibly withmagnets fitted to pick up the ferrous debris. Foam pigs gritted with silicon carbidecan be used to burnish the pipeline, which will improve frictional losses and which, inturn, lead to increased pipeline output.




1.2.2 PIPELINE SPECIFICATIONS (“What Are the Pipeline Dimensions andMaterials?”)

1.2.2.1 PIPELINE LENGTH

It is not the overall pipeline length that is of interest but the maximum piggingdistance – the distance between launching and receiving stations. This factorinfluences the wear rate of the seals along with several other factors:

• Pipe wall condition• Pipe wall lining (if any)• Product compatibility• Seal material• Pigging medium• Presence of lubricant• Pigging speed

Throughout the world, pigging distances range from a few metres to hundreds ofkilometres. Whilst the pig design for the two extremes may possibly involve similarcharacteristics, it is essential that, along with the factors mentioned above, themaximum pigging distance is specified.

1.2.2.2 PIPELINE DIAMETER

Most pipelines have a constant external diameter while their wall thickness changesto accommodate design pressure, etc. This variation in wall thickness can greatlyeffect the function of the pig, whether it is batching, cleaning or gauging. At theextremes, the variation in wall thickness is equivalent to specifying a dual diameterpig and many ‘standard’ pigs would possibly become stuck in the narrower bores.The other aspect relating to pipeline diameter is the tolerance on ovality, particularlyimportant to consider when gauging.

1.2.2.3 MULTIPLE PIPELINE DIAMETERS

Pipeline systems can include pipes of two or more nominal diameters (say 8” and 10”for example). This can result in a very significant narrowing of the internal diameterof the pipe. In order to overcome this problem, dual-diameter pigs are availablewhich are usually designed to cope with two changes in pipe diameter (e.g. 10” to8”; 12” to 10”).

Ideally, pigging should be carried out from the large diameter into the small diameterpipe. To assist the pig, the transition from diameter to diameter should be smoothand gradual. Reducers should be concentric with a taper no steeper than 1:5.




1.2.2.4 PIPELINE MATERIALS

Pipelines are made from many different materials:

• Steel, including stainless steel.• Cast iron• Wrought iron• Cement• Reinforced concrete• Plastic

Whilst there are many reasons for selection of the line pipe, compatibility with theproposed product is extremely important. If the product is particularly aggressive, thepipe can be lined (this also improves the flow characteristics of the product in thepipe.

1.2.2.5 INTERNAL LININGS

Linings minimise corrosion of the pipe walls and improve the flow characteristics ofthe product. They can be applied when the pipe is constructed or after it has beenlaid.

There are several types of lining:

• Epoxy linings• Concrete linings• Plastic lining (sleeve)

Pigs must be carefully chosen for use in lined pipe – all points of contact with the pipe(brushes, seals) should be constructed from non-metallic materials (polyurethane,nylon, etc.).

1.2.2.6 EXTERNAL COATINGS

External coatings do not directly influence pig design but can influence the receptionof signals from pig mounted transmitters fitted for tracking and location purposes.

Any external coating and the depth and nature of ground cover should be madeknown to the pig manufacturer who can then offer advice on suitably powerfultransmitters, should they be required.

1.2.2.7 FLEXIBLES

1. Smooth bore – plastic lined for chemical/water injection systems2. Rough bore – for flowlines, jumpers, risers.

Smooth bore pipelines are not suitable for pigging and any metal-to-metal contactwith pigs should be avoided in rough bore pipelines.




1.2.2.8 BUNDLES

Bundles allow several pipes to be laid together. The operational pipelines arecontained within a large conduit that is laid as a single pipeline in the normal manner.Bundled pipelines do not present any problems to utility pigging.

1.2.2.9 RELATIVE POSITION FEATURES

This refers to the minimum distance between any two significant pipeline featuresthat might influence the efficiency of a pig or cause it to become stuck.

These features are as previously listed and are particularly relevant to the positioningof:

• Valves• Offtakes (tees and laterals)• Bends

The general rule is that a minimum distance of 3 x pipeline diameters should beallowed between any two features.




1.2.3 FITTINGS AND EQUIPMENT (“What Components Are Fitted to thePipe?”)

1.2.3.1 VALVES

Valves present major obstacles to successful pipeline pigging. There are severaltypes:

• Ball valves• Gate vales• Check valves• Butterfly valves• Plug type valves

Some types cannot be pigged but others can. Certain features can be included in thevalve design to assist pigging. Only those valves which can be pigged are illustrated,with relevant features noted.

There are two types of valve that cannot be pigged:

• Butterfly/Plug type

There are three types of valve that can be pigged:

1. Ball valves2. Gate vales3. Check valves

Valve design should include the following features:

1. Ball valve:• Solid ball• Full bore• Concentric bore• Smooth, shallow transitions between valve bore and pipe

2. Gate valve (Through-conduit only)• Full bore• No gaps or intrusions• Concentric bores• Smooth, shallow transitions between valve bore and pipe

3. Check Valve• Full bore• Concentric bores• Smooth, shallow transitions between valve bore and pipe• Contoured (shaped) valve clapper• Side hinged valve clapper improves piggability




Possibly the single most important factor applies to all valves, irrespective of type.They must all be OPEN. Appropriate mechanisms or checking procedures must bein place to ensure that this happens as the consequences of not doing so involvegreat expense.

Valves should always be fully specified as there will usually be some compromisewith regard to the most effective pig design for the function. Specialised componentsmay need to be added to the pig to improve its ability to pass safely through anyvalves.

1.2.3.2 OFFTAKES

All offtakes present a potential hazard to pigging. It is easy for pigs, especiallyspheres, to be diverted into an offtake and become stuck or continue their journeydown the offtake with obvious and expensive implications. Alternatively, if the sizeand relative positioning of offtakes are not offered for consideration at the pig designstage, it is possible that the pig may stall at the offtake as a flowpath opens up forproduct/pressure to bypass the pig. Spheres are particularly vulnerable as theyincorporate a ‘single line seal’ (the circumference of the sphere I contact with thepipe).

Offtakes are sub-divided into 2 groups:

1. Offtakes which connect to the main pipeline at 90° - ‘Tees’2. Offtakes which connect to the main pipeline at other anges – ‘Laterals’

1. Tees

There are three types of tee:• Unbarred• Barred• Sphere (or flow)

Unbarred Tees

Most pigs can pass tees where the inside diameter (ID) is less then 70% of the mainpipeline ID. However, spheres would undoubtedly be lost at this point. It is always agood practice, therefore, to install guide bars when designing pipelines for pigging.Always make the presence of unbarred offtakes – tees or laterals – known to the pigdesigner.

Barred Tees

This is the preferred condition of all tees in a pigging system (except sphere tees).

Sphere (Flow) Tees

Sphere tees have been designed for use in pipelines that are regularly pigged withspheres. If spheres do not disappear into an offtake, they are highly likely to stallopposite the offtake. This is because their spherical shape results in a single lineseal in the pipe. When the pig reaches an offtake, the seal is broken and flowbypasses the sphere.




Sphere Tees allow product flow into and out of the offtake through slots around theinner sleeve preventing the sphere from pushing into the offtake.

2. Laterals

Laterals are offtakes that connect to the main pipeline at angles other than 90°. Theyare either barred or unbarred although, when designing pipelines for pigging, theyshould always be barred. Laterals present an increased opening when compared toTees. Therefore, pig design must take account of this. Pigs have to be made longenough to span the opening to prevent stalling.

1.2.3.3 WYES

For the purpose of pigging, wyes join two lines together, both of which must bepigged. The pig emerges from a branch pipeline into the main pipeline (‘main’ and‘branch’ do not imply any difference in pipe diameter) where it changes direction bybouncing off the opposite wall of the pipeline into which it has emerged. Theconvergence angle between the two pipelines is between 22° - 30°.

Wyes can be pigged on one direction only and there is therefore no requirement forthe pigs to be bi-directional although disc-type pigs offer the most popular solutions.

There are two types of wye, each of which requires a pig possessing different designcharacteristics:

1. Wyes with parallel bores2. Wyes with over-size bores

1. Wyes with Parallel Bores

The pig must be long enough to emerge into the main pipeline whilst still being drivenin the old pipeline. As with laterals, the convergence angle extends the opening atthe junction of the two pipes which allows product/pressure to bypass the pig before itseals in the new pipe. Articulated pigs are usually specified to overcome thisproblem.

2. Wyes with Over-size Bores

Standard size pigs are used and are intended to ’float’ into the main pipeline. The pigis driven out of the branch pipeline under pressure and into the over-size bore whereproduct/pressure bypasses the seals. The pig then drifts on the flow until it hits thewall of the main pipeline where it seals again and continues its journey.

In both examples, pigs are equipped with special polyurethane bumper noses toassist with shock absorption and to help locate the pig correctly in the main pipeline.




1.2.3.4 DIVERTERS

Diverters are designed to guide pigs from a main pipeline into a branch (lateral) – theopposite of wye pigging. The pipeline product is not diverted – only the pig.

Most diverters use mechanical methods but some divert pigs by means of magnetswhich attract the metal bodied pigs.

1.2.3.5 BENDS

There are three types of bend:

1. Forged (or factory) bends2. Field bends3. Mitred bends

1. Forged (Factory) Bends

Forged bends are classified according to their ‘bend radius’ which is expressed interms of ‘diameters or ‘D’. Standard sizes are produced as follows:

• ID or ‘short radius bend’ should not be included in a system designed for pigging.However, if they are present, it may be possible to manufacture special pigs tocope.

• 1½D or ‘long radius bend – suitable for: spheres, foam pigs, specified metalbodied pigs

• 3D – most utility pigs are suitable• 5D – all pigs are suitable• 10D – all pigs are suitable

2. Field Bends

These are ‘cold’ bends made by machine before laying. Bend radii are extremelylarge and will not cause problems for pigs – localised pipeline deformation as a resultof bending/laying is a much greater problem.

3. Mitred Bends

Mitred bends are sections of pipe cut and welded at an angle in order to change thedirection of a pipeline. They are not recommended as part of a piggable pipelinesystem. However, it may be possible to design a pig for some applications butdetailed specifications of the bend would need to be supplied to the pig manufacturer.

Bends have other characteristics that influence pig design:

• Bend angle – 22°; 45°; 90°.• Increased wall thickness in forged bends• Ovality in forged bends• Distance between bends in a pipeline system (Minimum 3 x pipeline diameter).




The minimum bend radius is almost always the first piece of information requested bythe pig manufacturer after establishing the pipeline diameter and pigging function. Itis important that as much relevant information as possible is made available.

1.2.3.6 TRAPS AND PIG HANDLING EQUIPMENT

To some extend, pig design is dependent upon the type of launching and receivingequipment installed. Some systems will only accept their own specially designedpigs and some systems will allow pig design within the limits of the trap dimensions.These systems may be specified in applications where complex industrial piggingstrategies are necessary or where trap size must be restricted because of spacelimitations.

Where there are no such restrictions, the following points should be considered:

• Are handling facilities adequate for pig types and weights?• Are the traps sufficiently oversize to allow the pigs to be properly

launched/received?• Are the traps large enough to meet the requirements of the pigging programme?• Do the traps have the necessary characteristics for launching and receiving?• Are the traps suitably equipped for automated pigging programmes?

Pipeline Pigging

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