pg3039e

8/20/2019 pg3039e

http://slidepdf.com/reader/full/pg3039e 1/6

Copyright © 2001 Scientific Surveys Ltd. All rights reserved. 1

Pigging multi-diameter linesand other specialist techniquesby C Kershaw

McAlpine Kershaw, UK

Contents of this Paper:

• Introduction

• Multi-diameter pig

• Pressure by-pass pig

• Magnetic cleaning pig

• Pin-wheel pig• Brush pig

• Shunting pig

• Easy-loading pig

• Summary

8/20/2019 pg3039e


Pipeline Pigging and Integrity Monitoring Conference: Houston, 1990

2 Copyright© 2001 Scientific Surveys Ltd. All rights reserved.

Introduction

Whilst the majority of pipelines can be successfully pigged using standard proprietary products, there areoccasions where a specialist 'one-off' type of pig is required. Due to the individual nature of such pigs, it is notreasonable to expect the manufacturers of standard pigs to produce them, and in any case they often do not havethe necessary operational experience to design such a specialist pig.

In 1979 McAlpine Kershaw was established for the specific purpose of designing and producing specialist pigs tocope with unusual circumstances. Our initial thoughts were to produce a range of various specialist pigs, but wequickly learnt that it was better to wait for a pipeline operator to approach us with a specific problem and then todesign a pig to solve the problem.

During the 11 years of our existence, we have designed and developed many specialist pigs to solve specificproblems which are described in this paper.

Specialist pigs

Multi-diameter pig

This was the first development project which we undertook on behalf of a Middle-Eastern client who required toclean a water-injection ring main having diameters of pipe ranging from 20in to 26in. At the time this project wasundertaken, there were no other suitable multi-diameter pigs on the market, but it is fair to say that since that timeSUN Engineering has introduced a full range of multi-diameter pigs for most applications. Our own multi-diameter pig is based on a different principle of construction to that of SUN, in that we utilize over-size butterflydiscs which are protected by overlapping thin spring-steel plates. These plates protect the urethane butterfly discsfrom abrasion, assist with the cleaning operation and give added support to the pig whilst it is in the pipeline. Asmulti-diameter pipelines in the oil, gas and petrochemical industry are few and far between, our involvement withmulti-diameter pigs has been limited.

Pressure by-pass pig

One of the most notable new pig designs to emerge in recent years has been the pressure by-pass pig produced byourselves. It was specifically developed for pre on-line inspection pigging operations, and is used for both provingand cleaning. The front of the pig is fitted with what is effectively a pressure-relief valve having a diameter of around 40% of the internal bore of the pipeline, the valve being pre-set to open at a chosen differential pressure.If, during a proving or cleaning run, the pig builds up a large accumulation of debris ahead of it, the differentialpressure across the pig will automatically rise as the pig begins to work harder. If a conventional cleaning pig wasbeing used, the accumulation of debris ahead of it might well increase until the pig became stuck or substantiallydamaged.

This cannot happen with a pressure by-pass pig, since once the pre-set differential pressure is reached, the by-passvalve opens thereby allowing a substantial volume of fluid or gas to flow through the pig body. This results in thedebris being jetted or blown away from the front of the pig, after which time the differential required to run thepig will drop, the by-pass valve will close, and the pig will move on. This sequence may take place manyhundreds of times during a run in a particularly dirty pipeline before the pig reaches the receiver.

Also, the by-pass pig cannot ever block the pipeline in the unlikely event that it becomes totally stuck, since theby-pass facility allows continuous by-pass of product. To date we have designed and supplied many by-pass pigsranging from 6-in to 42-in diameter.

8/20/2019 pg3039e


Pigging multi-diameter lines and other specialist techniques


Magnetic cleaning pig

Whilst the presence of ferrous debris, such as welding rods and the like, does not generally present a majorproblem in an operational pipeline it is essential that such debris is removed if on-line inspection is to take place.Most major pig manufacturers offer magnetic-cleaning pigs which are generally standard swabbing pigs withpermanent magnets attached. Under normal circumstances, such pigs are adequate and will generally remove thedebris during three or four runs through the pipeline. However, if the presence of ferrous debris is particularly

high, then a more aggressive approach is required, so that the debris can be removed more efficiently andtherefore more quickly. We are aware of one Middle East pipeline which was so heavily contaminated withferrous debris that the pipeline operator carried out a total of 43 separate pigging runs using a standard magnetic-cleaning pig before all debris was finally removed from the pipeline.Following an investigation into the varioustypes of magnet available, our first improvement has been to mount and orientate the magnets for maximumefficiency and performance. The second major improvement is the addition of magnetic brushes which closelyresemble the brushes of an on-line inspection pig (working on the magnetic flux leakage principle). Theadvantage of using magnetic brushes is that they can be arranged in close proximity to, or even touching, theinside wall of the pipe, due to their ability to flex when traversing bends or other restrictions. Permanent magnets,on the other hand, have to be at least 3in away from the pipe wall to avoid the pig fouling or becoming stuck in abend. We have also found that for optimum magnetic cleaning it is better to run two separate pigs coupledtogether using a universal joint, with the front section carrying the magnetic brushes and the rear section carryingpermanent magnets. In some situations, we will add a third unit if circumstances demand it.

It is recommended that pipeline operators carry out a magnetic cleaning programme well in advance of any formof on-line inspection operations, as it is never known how much magnetic debris is present in a particular pipelineuntil magnetic cleaning operations have commenced. If, for instance, it is planned to carry out on-line inspectionin perhaps two or three years time, then it would not be too soon to commence magnetic cleaning immediately,since once the line has been successfully cleaned, further contamination is not likely to take place as ferrousdebris is generally the result of construction operations.

The magnetic-cleaning pigs which we have developed are generally hired to a pipeline operator on the basis thatonce the ferrous cleaning operation is completed he has little or no use for the pigs thereafter. Additionally, wesupply magnetic cleaning kits, which are simply bolted around the body of an existing swabbing pig.

Pin-wheel pig

This pig has been specifically designed and developed for the removal of hard wax and scale adhering to theinside wall of the pipe. Although this wax or scale is usually at its worst in the 4 to 8 o'clock position, the pin-wheel pig, through its cleaning discs, will give a 360° circumferential cleaning action.

Each disc, which is up to 2in thick, is manufactured from heavy-duty polyurethane, having an outside diameter inthe order of 4in less than the inside diameter of the pipeline. Protruding radially from the circumferential edge of each disc are a number of steel fingers which are screwed into threaded housings anchored into the disc. Thelength of the fingers is such that the diameter across any two opposite fingers is greater than the inside diameter of the pipeline by up to 1in, depending on line size. This means that when the disc is travelling through the pipeline

the fingers are bent back at a slight angle which both assists in the cleaning action and also compensates for wear.The fingers have hardened inserts to reduce wear to a minimum.

Two or more discs are attached to a purpose-built steel body using appropriate retaining bolts with the pig beingeither self-propelled through the pipeline (by the fitting of cups or discs) or alternatively towed behind aconventional swabbing pig. The method of propulsion adopted will, of course, depend upon the circumstances. Itis normal to use four cleaning discs per pig body with each disc being arranged to ensure that the cleaning fingerson each disc are suitably offset. As the fingers are some 2in apart, this offset ensuring that the total surface area of the pipeline is cleaned. The use of removable fingers enables many options for wax and scale removal andcleaning to be adopted.

8/20/2019 pg3039e




Also, different types of finger shapes can be utilized to ensure the efficiency of the operation, and on completionof each run, any worn or damaged fingers can be simply replaced with new ones. Additionally, by increasing thehardness of the polyurethane disc and the length of the cleaning fingers, increased aggressiveness is achieved.

We always recommend a progressive approach when cleaning a pipeline, in order to reduce the risk of a blockagewhich can occur when too much material is removed from the pipe wall. It is preferred that during the initialpigging runs less than the entire internal surface of the pipe will be cleaned, as it is better to remove wax or scalefrom the pipe wall progressively during a number of pigging runs rather than trying to remove it all during onerun. The design of the pin-wheel pig is such that little of the wax or scale removed from the pipe wall will actuallybe pushed forward by the pig itself; it will be left behind in the line. For actual removal of this loosened scalefrom the pipeline, we use the pressure by-pass pig. Therefore, following each cleaning run using a pin-wheel pig,the next run would be a pressure by-pass pig to remove the wax and scale from the pipeline.

Brush pig

This pig was developed for a client operating 8-in aviation spirit pipelines, where cleanliness is extremelyimportant. The pipelines were being cleaned using standard articulated pigs carrying steel wire brushes whichwere relatively successful in removing larger dirt particles. However, attempts to improve the cleaning action by

using stiffer brushes merely removed the protection of the corrosion inhibitor from the pipe wall, which wasunacceptable. We designed and produced a unique brush pig using nylon brushes impregnated with carborundumgrit. During trials, it was found that the brush pig was extremely efficient in removing very fine debris from thepipeline, thereby considerably increasing the times between filter changes at the airfield due to the increasedcleanliness of the product.

Due to superior cleaning ability, far in excess of a conventional cleaning pig, we now use the brush pig in ourservice operations for clients requiring as clean a pipeline as it is possible to achieve. However, due to theefficiency, we generally adopt a progressive cleaning approach, starting off with conventional cleaning pigs andonly using the brush pig for final cleaning once the majority of debris has been removed from the pipeline. Forsizes of 16in and above, a slightly-different design has been adopted, which enables us to produce pigs up to andincluding 48in diameter.

Shunting pig

This pig is basically a three-section articulated pig which has been specifically developed for the removal of stuck or lost pigs from pipelines. Our experience has taught us that if a pig does become stuck or lost in the pipeline,there is little point in running a second pig of similar or identical design, since this pig is likely to succumb to thesame problem as the first pig and also become stuck or lost. What generally happens to a pig which is required topush a stuck or lost pig (usually in pieces) is that the additional effort to remove the debris causes the second pigto become damaged itself. Using a three-section articulated pig, we recognize that the first section will becomedamaged to a considerable extent as it pushes the debris ahead of it, but drive will be maintained because of thesecond and third sections which never come into contact with the debris being pushed out. Additionally, the

shunting pig is deliberately made to be extremely heavy, to give increased momentum, since light-weight pigs areof little or no use in removing stuck or lost pigs from pipelines. Much attention is paid to the design of a shuntingpig, such that there is no metal-to-metal contact between the shunting pig and the debris being pushed out; this isachieved by fitting a hard polyurethane bumper ahead of both the pig body and front cup. The shunting pig is alsoequipped with permanent magnets for tracking purposes, together with a battery-operated electro-magnetic devicefor positive location when stationary.

A further use for the shunting pig is in pipelines which are particularly hostile to pigs, thereby requiring a muchstronger construction of pig. Also, the extended length and increased number of cups substantially improves itsperformance in difficult conditions.

8/20/2019 pg3039e


Pigging multi-diameter lines and other specialist techniques


Easy-loading pig

This pig has been developed by our sister company ITAC specifically for offshore use during the final tie-inbetween a subsea pipeline and the platform riser. Prior to the tie-in being carried out, the pipeline itself will havebeen successfully pigged and gauged, as will have the riser. Once the two are tied-in, it is generally necessary torun a final gauging pig so that the tie-in spool will also have been gauged. As it is virtually impossible to back-load a cupped or bi-directional gauging pig into the open end of a subsea pipeline prior to tying-in, it is usually

necessary to run a gauging pig from the very start of the pipeline through to the platform to gauge the tie-in spool.This is costly and time-consuming, since the only relevant piece of pipe which needs to be gauged is the short tie-in spool between the pipeline and riser.

The easy-loading pig is effectively a bi-directional pig using split discs which initially are undersized to thepipeline bore. This allows it to be easily inserted by a diver into the open end of the pipeline prior to the tie-inoperations. Once inserted, the discs are increased in size to form a tight seal with the pipe wall by activating aspring mechanism within the pig body. Following tie-in operations, the easy-loading pig is then run through thetie-in spoolpiece, up the riser and into the pig trap on the platform. This obviously saves tremendous time andmoney, especially so where the pipeline is of considerable length.

SummaryThe art of pigging a pipeline is not an exact science, especially in respect of pipelines which do not conform tonormal parameters. It is hoped that this paper will give pipeline operators food for thought, and let them know thathelp can be on hand in situations where conventional pigs are not appropriate. It is fair to say that nothing isimpossible, providing time, effort, expertise and money are available to solve the problem.

8/20/2019 pg3039e




Documents

pg3039e