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

Pre on-line inspection piggingby C Kershaw

McAlpine Kershaw

Contents of this Paper:

• Introduction

• Pipeline proving

• Pipeline cleaning

• Monitoring during proving and cleaning runs

• Planning a pre on-line inspection pigging operation

• Specialist pigs

• Summary

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Pipeline Pigging and Integrity Monitoring Conference: Houston, 1989

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

Introduction

Pre on-line inspection pigging can be defined as any pigging operation carried out prior to running an on-lineinspection tool. Such tools will be inspecting the pipeline for metal loss, generally by the magnetic flux leakagetechnique. In order to ensure that the chosen inspection tool will give optimum performance it is vitally importantthat the pipeline is properly prepared by the removal of all extraneous matter. Such matter will include all or someof the following: ferrous debris (welding rods, brushes, spacers, etc.), coarse debris (stones, timber, etc.), finedebris (silt, sand, mill scale, etc.), wax(hard and soft) and corrosion products.

Where the amount of deposited matter is relatively light, the running of an on-line inspection dummy tool willoften suffice and properly prepare the pipeline for the live tool run. However in situations where heavy depositsoccur it will be necessary to carry out a progressive pigging programme, especially where the pipeline has notbeen previously pigged.

Finally, in addition to the removal of extraneous matter, a further consideration is pipeline geometry, sincereductions in pipeline bore due to dents, buckles, heavy-wall fittings and the like could well be detrimental to theinspection tool. Pipeline geometry is easily ascertained by the running of an internal geometry pig, a topic whichis covered by another author. However it plays a very important part in preparing the pipeline for on-lineinspection, and generally follows or is integrated into the pre on-line inspection pigging programme.

In preparing this paper the author has utilized over 20 years of experience in the proving and cleaning of pipelines, with sizes ranging from 2in to 48in diameter.

Pipeline proving

In order to effectively clear a pipeline of all extraneous matter, it will be necessary to run steel-bodied pigs fittedwith cups or discs or a combination of both onto which various cleaning devices (brushes, blades, scrapers,magnets, etc.) can be attached. However it would not be prudent to run such a pig in circumstances where thepipeline has never been pigged, and so the pipeline must first be proven to be piggable. Obviously pipelineswhich have been successfully pigged do not require proving, in which case this section is irrelevant. For the

purpose of this paper, the worst case will be assumed, this being a pipeline which has adequate pigging facilitiesbut which has never previously been pigged and therefore no knowledge exists regarding the presence of extraneous matter and reduction in internal bore. Discussed below are the various proving pigs and techniqueswhich can be used to minimize the risk of a blockage or a stuck pig.

Foam pigs

As an initial approach, and where absolutely nothing is known about the pipeline, it is prudent to run some formof foam pig, whether it be soft foam (with or without a polyurethane coating) or medium to high density foamsuch as a polly pig. As the running of such pigs is a specialist subject covered by another author, it is not intendedhere to go into any depth or detail regarding such pigs. However, if the pipeline operator has any doubts aboututilizing a steel-bodied proving pig, then some kind of foam pig is an ideal alternative.

Spheres

Spheres can be considered for initial proving operations, but it is almost impossible to determine any usefulinformation following a run through a pipeline. This is due to their ability to squeeze through obstructions andover debris which might well cause a pig to become stuck or damaged. Therefore a successful sphere run cannotguarantee that the running of a pig will also be successful. However, in many applications a sphere can and willperform a useful function in the proving of a pipeline.

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Pre on-line inspection pigging


Proving pig

In the author's experience the most suitable proving pig utilizes a slim steel body fitted with two or four conicalcups, such as the LBCC and SBCC types produced by T D Williamson, although other manufacturers offersimilar alternatives. Apart from the ability to successfully traverse obstructions as severe as 30% to 40% of thepipeline diameter, the design of the cup also ensures that only light cleaning is carried out due to its conical shape.It is therefore an ideal cup to fit to the proving pig, with the likelihood of it becoming stuck due to anaccumulation of debris or reduced pipeline bore being extremely low. Further valuable information can be gainedby fitting the proving pig with two or three aluminium gauging plates probably sized to between 75% and 80% of the internal bore. If the plates are substantially damaged during the first run, then it indicates that all is not welland further proving runs may well be necessary. If, on the other hand, the plates emerge relatively undamaged,then it proves that there are no serious obstructions which reduce the bore to below the plate diameter. In suchcircumstances the cleaning programme can commence.

During proving operations it may well be that a degree of pipeline cleaning is also being carried out, especially soin small-diameter lines. If this is the case, it would be prudent to continue running the proving pig until little or nodebris is removed.

Monitoring of proving run

To assist in evaluating the proving run or runs, the author has developed an accurate and sensitive pressure-monitoring system which is described in detail later in the paper. Additionally, in order to deal with the possibilityof a stuck pig, it is advisable to track some or all of the pigs at suitable intervals along the pipeline route and alsoto have the ability of accurately locating the position of a pig in the event that it becomes stuck. Both pig trackingand pig location are covered by another author.

Pipeline cleaning

There are two main types of extraneous matter present in a pipeline, namely ferrous and non-ferrous, with eachrequiring a different approach. The author generally favours dealing with the non-ferrous debris first, followed bythe ferrous debris, but this can be reversed or even combined to suit individual situations and requirements. The

main reason for commencing with removal of non-ferrous debris is that ferrous debris removal pigs (generallyreferred to as magnetic cleaning pigs) need to be substantially built to support the weight of the magnets andtherefore tend to be very aggressive in their cleaning action. This aggressiveness is totally the opposite of what isinitially required to remove the non-ferrous debris, which is best achieved by adopting a gradual approach. Thereason for this is that it is rarely, if ever, known just how much debris exists in the pipeline. It could vary from aslittle as a few pounds to several hundred tons, with the distinct possibility of an extremely aggressive pigbecoming stuck due to a massive accumulation ahead of the it if the quantity is particularly high. Therefore, acautious but progressive approach is recommended for the removal of non-ferrous debris, followed by magneticcleaning to remove ferrous debris

Pipeline cleaning - non-ferrous debris

Following initial proving of the line utilizing foam pigs and/or LBCC-type cups, the first cleaning run should be

carried out also using conical cups such as the Vantage IV type (produced by T D Williamson) or similar.Although less flexible than the LBCC cups, they still give a good degree of flexibility, and again will tend not toremove too much debris which might result in a blockage. It should be noted that Vantage IV cups are notavailable in sizes of 14in and below, although alternatives are available.

The first cleaning run should be carried out with a steel-bodied pig fitted with two or four Vantage IV-type cups.The decision as to whether two or four cups should be fitted will depend on circumstances such as pipeline length,diameter, aggressiveness, preference and the like. During all cleaning runs it would be prudent to continuouslymonitor and record the running pressure behind each pig.

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Once the pig has been successfully received, an assessment should be made as to how hard it has had to work,together with its cleaning performance. For instance, if the cups are particularly worn (especially the front cups) itcould indicate that a high degree of cleaning has been carried out, in which case a re-run should be consideredafter having fitted new cups. Additional information will be gleaned from a close examination of the pig receiverand any filters for signs of debris. If, on the other hand, it is considered that little or no cleaning has beenachieved, then a more aggressive approach is required. This can be achieved initially by fitting two discs, oneahead of the front cup and one ahead of the back cup. Careful thought should be given when deciding on thediameter of the disc and the Shore hardness. The author prefers to work up gradually regarding aggressiveness,with the first discs having an outside diameter equal to (or perhaps just under) the inside diameter of the pipelineand a hardness of 70 degrees Shore A. The object is to work up to fitting four discs to the pig, each having an ODequal to the pipe ID plus between 0.25 and 0.50 inches, and a hardness of 90 degrees Shore A. If a cautiousapproach is adopted, it might take as many as 10 or even 20 or more pig runs before the most aggressivecombination of cups and discs is reached. However, it is up to the pipeline operator or his advisors to decide howslowly or quickly aggressiveness should be increased, bearing in mind that an over-aggressive pig in a particularlydirty line might well become stuck.

At some time during the cleaning operation a change from Vantage IV-type cups to standard cups should beconsidered for two reasons. Firstly, the standard cups being of non-conical shape are more efficient in supportingthe pig and cleaning the line, and secondly they are considerably cheaper, especially so on large-diameter pigs.The precise time to change from Vantage IV cups to standard cups will depend on how the cleaning operation is

progressing. Again, the fitting of aluminium gauging plates of between 90% and 95% of the pipeline internal boreto the initial cleaning pigs will give valuable information regarding the state of the pipeline in relation to itsinternal bore.

The author has found that a combination of cups and hard discs is an extremely effective cleaning tool for theremoval of both loose debris and debris adhering to the pipe wall. However, it is recommended that additionalpipe wall cleaning is carried out at some stage during the cleaning operation by fitting brushes or scrapers to thepig body; it is likely that such cleaning runs will be carried out towards the end of the cleaning programme. Alsosome pipeline operators may prefer to use all-disc pigs (no cups fitted), generally referred to as 'bi-dis' due to theirbi-directional capabilities. However the choice of pigs must be left to the pipeline operator or his advisor.

Pipeline cleaning - ferrous debris

During the cleaning operation to remove non-ferrous debris it is likely, especially in fluid pipelines, that a certainamount of ferrous debris will be removed. However it is usual to complete cleaning operations by running amagnetic cleaning pig to lift and hold any remaining ferrous debris and bring it through to the receiver. This ismost important as the presence of ferrous debris, especially welding rods, can be detrimental to a successful on-line inspection run.

A magnetic cleaning pig in its simplest form will be a standard steel-bodied pig with permanent magnetspositioned around its body. More sophisticated pigs will utilize optimum design magnets which can be adjustedoutwards to put them closer to the pipe wall, thereby increasing their effectiveness. Finally, the addition of magnetized brushes (similar to those of some on-line inspection tools) at the front and rear of the pig will greatlyimprove its cleaning ability, since much of the smaller ferrous debris will be collected and held by the brushes. Inrelatively-clean pipelines perhaps two runs will be sufficient to remove all ferrous debris, whereas extremely dirty

lines may take as many as ten or more runs to remove all traces. As a rule of thumb it can be reasonablyconsidered that a line is clean if two consecutive pigs arrive at the receiver with little or no ferrous debrisattached. To increase cleaning capability, it is worth considering joining two pigs together using a flexiblecoupling. A combination of two close-coupled pigs is more efficient than running separate pigs, in that itconsiderably reduces pigging time. Furthermore it has been found that two close-coupled pigs are more effectivethan two separate pigs.

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Pre on-line inspection pigging


Monitoring during proving and cleaning runs

As previously mentioned, pig tracking during proving and cleaning operations together with the ability to locate astuck pig should be seriously considered. However an additional aid to assessing how the operation is progressingis to monitor and record running pressure behind the pig. This is best achieved by using a pressure transducer andstrip-chart recorder; a circular clockwork recorder is not suitable. A chart speed of between 6in and 24in per hourshould be selected to give good definition, and it is preferable that the entire pigging run be accommodated on a

single chart. As most charts are around 50ft in length, 6in/hr gives 100 hours of running time and 24in/hr gives 25hours.

As the purpose of recording the running pressure behind the pig is to enable trends to be assessed, it is likely thatlittle usable information will be obtained from the first run. However the second and subsequent runs can all becompared to previous runs to ascertain whether the running pressure is gradually reducing. For instance, the firstchart may indicate a particularly high running pressure due to the presence of high amounts of debris in the line.Also sudden drops in pressure might be observed where the pig has overrun the debris it was pushing. If, on thesecond and subsequent runs, the pressure chart shows a decline in running pressure and fewer sudden drops, it canbe assumed that the cleaning operation is progressing. Once two consecutive runs are identical from a pressurepoint of view, it generally indicates that whatever pig was being used is no longer aggressive enough to achieveuseful cleaning. Therefore a more aggressive action, such as the fitting of harder discs, is required. Whilst not aninfallible method of assessing cleaning progress, pressure monitoring is extremely useful in assisting a pipelineoperator during pre on-line inspection pigging.

The author has developed a pressure-monitoring system which works on the back-off technique, where runningpressure behind the pig is electronically processed to produce two outputs to a twin-pen chart recorder (or twosingle-pen recorders). The first output is direct pressure which will match the pressure transducer being used (e.g.0-500psi, 0-1000psi or 0-2000psi, etc.). The second output will be back-off pressure, which is running pressureplus/minus a preset level; normally a range of +/-50psi is ideal for most applications. The main advantage of theback-off facility over the direct pressure is that it is far more sensitive and enables even the smallest pressurefluctuations to be monitored and recorded. Any fluctuations in the running pressure behind the pig (assuming theydo not emanate from the pump or compressor) are due to the pig encountering some form of restriction orobstruction. This could be of a mechanical nature such as a valve, bend, tee, dent or buckle, or it may be non-mechanical such as debris. If it is debris and the pig is cleaning efficiently, the pressure fluctuations should

eventually disappear over subsequent runs. However mechanical features will remain for all time, since piggingwill not affect them. By careful study of the back-off pressure charts it is usually possible to identify mechanicalfeatures such as valves, tees and forged bends and assume that those pressure fluctuations (if any) remainingcould be due to dents or buckles. This is particularly relevant during the proving runs, and especially if thegauging plates or even the pig itself is suffering damage, since substantial fluctuation would indicate a significantreduction in pipeline geometry. In this situation a pipeline operator would probably have to run an internalgeometry pig to accurately assess the situation. It would not be prudent to progress cleaning operations with amore aggressive pig, as it could become stuck.

It can be seen that pressure monitoring is an extremely useful aid to pigging operations. However in situationswhere the input of fluid or gas to the pipeline is particularly erratic, or where pumping is continuously interrupteddue to breakdowns or stoppages, then pressure monitoring and recording becomes less clear. It does rely on flowinto the line being reasonably steady and continuous which is normally the case with most pipeline systems.

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Planning a pre on-line inspection pigging operation

It is most important for any pipeline operator who has decided to run an on-line inspection pig for the first time togive careful thought to the pigging operation to properly prepare the line. The object of pre on-line inspectionpigging is to remove all traces of debris, together with the assessment of reductions in pipeline bore, to ensure thatthe running of the live tool will be successful. Furthermore an inspection tool is a very expensive piece of equipment which is designed to inspect a pipeline and not clean it. Whilst it is often capable of performing anefficient cleaning function, it is at the expense of reducing its inspection potential or, worse still causing damage,both of which will necessitate an expensive re-run.

Therefore the pipeline operator must give careful thought to how he is going to achieve a clean pipeline. It maywell be that the line has been regularly pigged and that both internal geometry and cleanliness are assured. In suchcases it may only be necessary to run magnetic cleaning pigs prior to running a dummy tool (provided by theinspection company) and finally the live tool. On the other hand the line may never have been pigged sincecommissioning and could contain large quantities of debris. It must be stressed that pre on-line inspection piggingoperation, if carried out properly, can be very time consuming, and adequate time should therefore be given toenable the operation to be completed. It is not good engineering practice to rush or curtail an operation simply tofit in with a pre-chosen inspection date; far better to delay the inspection run or better still give adequate time.While most pipeline operators will have in-house expertise to plan and execute a pigging programme, some do

not. In this case, expert advice is available from independent consultants who have experience on all types andsizes of pipeline in many parts of the world. Also pig manufacturers themselves have a lot to offer, although theirexpertise can be limited to their own products and they rarely get involved with site operations. Much advice canbe gleaned from the operators of on-line inspection tools, and of course other pipeline operators are usually morethan willing to discuss their own experiences.

Specialist pigs

There will always be situations where a pipeline operator has a unique problem which has to be dealt with on aone-off basis. This can often be achieved by utilizing purpose-built pigs which may or may not have beenpreviously used. Such pigs can range from minor improvements to standard pigs to whole new concepts in

pigging technology.

The most notable new pig to emerge in recent times is the pressure by-pass pig developed by the author. It wasspecifically developed for pre on-line inspection pigging operations and is used for proving and cleaning. Thefront of the pig is fitted with a pressure-relief valve having a diameter of around 40% that of the internal bore of the line, and the valve is preset to open at a chosen differential pressure. If, during a proving or cleaning run, thepig builds up a large accumulation of debris ahead of it, the differential pressure across the pig will rise as the pigbegins to work harder. If a non pressure by-pass pig was being used the accumulation of debris ahead of it mightwell increase until the pig became stuck or substantially damaged. This cannot happen with a by-pass pig sinceonce the preset differential pressure is reached the by-pass valve opens thereby by-passing a substantial flow of fluid or gas through the pig. This results in the debris being jetted or blown away from the front of the pig, afterwhich time the differential required to run the pig will drop, the by-pass valve will close, and the pig will moveon. This sequence may take place many hundreds of times during a run in a particularly dirty pipeline.

Summary

It is hoped that this paper will give food for thought to pipeline operators contemplating on-line inspection. Bynecessity the paper is general in its approach, but the underlying recommendation is to proceed cautiously from aninitial proving run through to the running of an aggressive cleaning pig. Additionally pig tracking, pig locationand pressure monitoring are all recommended to further assist and support the pigging operation. Finally, neverforget that time and money spent in preparation leads to a better chance of achieving a successful on-lineinspection run.

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