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

Pipeline gel technology – applicationsfor commissioning and productionby A. Evett

Nowsco Well Service

Contents of this Paper:

• Summary

• Introduction to pipeline gel technology

• Types of gel

• Nowsco polymer gel pig

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Pipeline Pigging and Integrity Monitoring Conference: Houston, 1989

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

Summary

Pipeline gels have been developed and utilized for numerous applications where a pipeline has been required to becleaned to a high specification, either during initial commissioning or as part of a continuing maintenanceprogramme.

The original concepts of gel cleaning allowed lines to be cleaned where potentially large volumes of debris in theline may well have caused the pipeline pig to become stuck. This technique has actually enabled long pipelines tobe cleaned in a single operation. These tasks have been undertaken cost effectively, meeting the cleanlinessstandards specified.

Gel systems have many more applications and are used both in conjunction with mechanical pipeline pigs, andalso with other viscous polymer gel pigs. Simply by changing the characteristics of the gel it is possible to changetheir suitability for a large number of different applications in widely varying environments.

Introduction to pipeline gel technology

Nowsco has developed significant operational experience in gels which have been designed for use in verydifferent pipeline operations. These versatile fluids perform many of the functions of a conventional mechanicalpig and have the following characteristics:

1. They maintain a good seal over long distances of pipeline;

2. The gels are capable of passing through lines of changing diameter;

3. They pass through partial obstructions in the line without becoming stuck, and therefore can be used to locateobstructions in the line using pressure build-up calculations;

4. The gels can support a large volume of debris, without plugging or sticking or depositing their load in dynamicor static environments;

5. All of the gels can be chemically altered to affect the viscosity and adhesive nature of the pig for any particularapplication.

Gelled fluids can be pumped through any line capable of accepting liquids and can be used in conjunction withmechanical pigs to improve their performance.

Typical gelled fluid applications can be briefly summarized as follows:

1. Cleaning debris from the pipeline

Where long pipelines are required to be commissioned, and debris build-up ahead of the cleaning pigs isconsidered to be a problem, gels can be used to suspend and distribute the accumulated debris along the body of the cleaning train, allowing large volumes of material to be suspended and removed from the line in a single run.

In the past one has often had to rely upon a large number of pig runs, usually in combination with high velocityflushing, requiring, in many cases, high horsepower pumping capability to overcome friction and ensure particlesuspension.

2. Dewatering the pipeline

Gel pigs are also used to assist in the removal of water from the walls of a pipeline and can be manufactured to becompatible with, and be able to contain methanol or IPA between either high-viscosity polymer pigs ormechanical pigs. It has been found that certain type of gels are affected by these chemicals and care has to betaken in their selection; therefore full laboratory compatibility between the dewatering components and theproduct are recommended.

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Pipeline gel technology – applications for commissioning and production


Nowsco also usually proposes running a dewatering fluid and a hydrocarbon gel to leave the hydrocarbon pipelineoil-wet; the hydrocarbon gel can be altered to lay down an inhibitor coat if required at the same time.

3. Acting as product separation pigs

Gel can be used when two fluids are to be kept apart, e.g. water and oil. Here a viscous gel pig is placed in the linebetween the product. The gel system can be readily diverted on arrival and the lack of a mechanical pig may bepreferred in a production line. The type of gel and length of gel plug are specifically designed for a particular

operation depending on:

(a) type of fluid to be separated;(b) temperatures to be found in the line;(c) maximum line diameter and any changes in diameter;(d) optimum displacement velocity.

4. Displacing condensate from lines

Condensate, and other liquids can be removed from the system by the introduction of gel pigs into the line, whichat the same time can be designed to lay down inhibitors etc. on the pipe wall. The efficiency of the laydown canbe controlled by using a mechanical pig which is slightly undersized to sweep the gel forward.

5. Increasing the sealing efficiency of mechanical pigs

Sealing mechanical pigs can minimize fluid by-pass and therefore reduce pig wear. By using a gel with amechanical pig, pig wear can be reduced as the gels can be designed to lubricate the pipe wall, which may be of particular importance for long gas lines.

6. Aiding in the removal of stuck mechanical pigs

As mechanical pigs travel down a line, wear on the cups can increase the by-pass of the drive fluid. Movementwill stop when there is a lack of differential pressure across the pig, or when any debris ahead of the pig causesthe pig to stop. Conventionally another pig is launched to remove the first, but due to the wear or debris build-upthis may also become stuck.

A gel pig pumped down the line which, depending on the situation, can create a high differential pressure, wouldbe more than sufficient to move a stuck pig. If debris build-up has occurred, some of the gel will by-pass the pigand entrain the debris which will allow the pig to move forward.

7. Laying down coatings on the pipe wall

Where specifically required inhibitors, solvents and chemicals can be laid evenly down on the pipe wall to protectthe system. This can be undertaken at the beginning of the operational life, or during it, using gel systems whichare compatible to the line product.

8. De-oiling multi-diameter pipelines

In subsea applications, and other situations where multi-diameter pipelines occur in a system, gels have beensuccessfully used to separate solvents and to de-oil and remove hydrocarbons from the pipeline wall, allowinghigh-quality water injection to be undertaken through the system. In these cases a simple gel train has been usedand gel pigs separate the fluids. It should be noted that the actual gel pigs which are built for these jobs are built tobe compatible with the fluids used in the system.

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Types of gel

Three main types of gel pigs are commonly used for pipeline applications:

(1) High viscosity sealing gels

Sealant gels are based on the series of gels designed for downhole fracturing techniques. These gels are visco-

elastic and self-healing, with a strong cohesive attraction, and are typically used in situations where contaminationof the product or pipe wall is not important.

(2) Commissioning cleaning gel systems

Cleaning gel pigs are prepared from fresh water or seawater gelled with a biodegradable polymer. The gel hasvisco-elastic and plastic flow properties (pronounced yield-point and significant cohesive behaviour). The gelshave a high yield strength which ensures that the debris remains suspended even if the gel is static for longperiods.

(a) Debris pick-up mechanism

Debris pick-up gels are usually and most successfully run in conjunction with a following mechanical pig,

displaced at between 1 and 3 ft/sec to ensure that the gel is in plug flow during the pipeline transit. In this flowregime, the core volume of gel moves as a semi-solid plug at higher displacement velocity than gel on the wall;therefore there is little exchange with the material, with the almost-stationary gel near the pipe wall. Duringdisplacement the gel in this annular zone is removed from the pipe wall by the mechanical pig, and flows forwardinto the core zone, forming a 'convection system'.

The gel is very adhesive to either previously loose or newly pig-loosened debris. This debris is entrained andcarried forward into the core by the action of the following pig. In this system debris cannot accumulate in frontof the pig causing it to stick, but is distributed evenly throughout the gel body.

As some of the debris pick-up gels are readily water-dispersible, and if pig reliability is doubtful or a situationexists where mechanical pigs cannot be used due to diameter changes, or launching/landing difficulties, andpolymer pigs are used then the cleaning gel can be protected front and rear by a sealant preventing dilution by

entrained and by-passing water. Because of their very different characteristics, gel and sealant gels do not readilyintermix.

It should be noted at this time that there are two types of gel pig system used. The first type is used always inconjunction with a mechanical pig to prevent by-pass of displacing fluid; these have a lower viscosity than thesecond type of polymer gels which are premoulded and have a very high viscosity and can actually be used as amechanical pig.

(b) Train design

The amount of cleaning gel required is primarily dependent on the maximum amount of debris expected. In newpipelines, this is usually estimated at 0.05 kg/m2 of pipe wall (assuming the line has been gauged before) . Using4li of gel per kilogram of debris there is a more than adequate margin for such contingencies as gel dilution, ormore debris than expected. A typical gel pig will tolerate 100% dilution and still carry the total expected debris.Undiluted, it will carry several times this amount of debris with only a limited increase in flow resistance.

(c) Rinsability of gels

Following investigations into the success of the early gel treatments it became apparent that gels were capable of supporting large amounts of debris. It was, however, assumed that all of the removable debris had been carriedfrom the line by the gel. It was only at a later date, when subsequent flushing and pigging removed further debrisfrom the line, that the efficiency of the chosen gel system was questioned. Nowsco began an extensive researchprogramme into the gel systems that had been used on the operations.

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Pipeline gel technology – applications for commissioning and production


It was found that a thin layer of gel remained trapped on the pipewall and that the subsequent pig did not removeall of the gel. The gel layer left behind was found to vary from 1mm to 0.1 mm in thickness. This layer effect wasmore noticeable when the gels were not displaced by a pig and much larger volumes of gel were left behind.Subsequent flushing of the line did not remove the gel, and it was found that:

1. Remaining gel would become loose and entrain itself into the product if not fully removed prior to theintroduction of the product.

2. Debris with a conventional gel train design may be trapped below this film and remain in the line.

3. Any remaining gel would have an adverse effect on the efficiency of the drying process.

Nowsco has developed RPG (rinsable pipeline gel) as an alternative to the existing gels in certain applications.This gel is fully rinsable but does not break down on contact with water. It is, though, slowly diluted and itssuspension ability decreases with dilution. RPG is designed to be able to hold its full debris load after 100%dilution by water has occurred.

This trade-off between suspension and rinsability required the use of proven high-sealant pigs and, in most cases,a modified design of the gel train.

The gel which was used for cleaning the Fulmar line (290km, 20in) to a cleanliness level of 10 microns provedthat :

1. RPG was fully rinsable and no residue was left at the pipe wall.2. No effect on the drying period occurred.3. Subsequent pig runs found no debris in the line.4. The gel did not trap debris against the pipe wall.

(3) Hydrocarbon gels

Gelled hydrocarbons, such as diesel, kerosene or, in many cases, line product, can be mixed as the base fluid,giving the high sealing efficiency characteristic of gel pigs. They are used in operational oil or gas pipelineswhere aqueous systems are unacceptable, either run alone if displaced by liquids, or usually with a mechanical pig

when displaced by gas.

In gas pipelines, continuous injection of corrosion inhibitor may need to be supplemented with a periodic slugtreatment. Sticky diesel gels can be loaded with up to 20% of an inhibitor, and when injected ahead of a routinemechanical pig run, give a satisfactory laydown on the whole pipe circumference throughout its length, withinternal flow within the pig allowing continuous migration of fresh inhibitor to the pipe wall. When injected intothe line, the gel spreads along the pipe base, until launching of the mechanical pig bulldozes it into a diameter-filling 'gelly pig'. Gas transmission continues during gel injection, although the peak rate may have to betemporarily reduced.

An important additional benefit, if not the joint objective, of a diesel gel run is that it will flush out condensate, orwater that has dropped out and accumulated in the line. In a wet or rich-gas pipeline, especially if irregularlycontoured, even frequent conventional pigging can by-pass considerable quantities of such liquids.

The gelling chemicals contain no organo-chlorines and will not poison refinery catalysts, and are disposed of either by flaring or by dilution of the gel by an acceptable hydrocarbon.

The sealant and cleaning gels are usually aqueous systems, prepared from fresh water or seawater, and are bothbiodegradable and have no adverse environmental effects when discharged at sea.

It should be stressed that all the gel systems are designed for a specific application and that close liaison betweenthe engineers responsible for the design of the gel and the customer is required to ensure that a suitable system isutilized.

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Nowsco polymer gel pig

In addition to the cleaning gel systems, Nowsco has developed a viscous water-soluble pre-moulded pig which iscompressible and can be pumped through various diameters of pipework, and which has been used in place of conventional pigs as discussed earlier.

The polymer is precast in a steel canister for transportation and loading into the pig launcher; should the client

wish it can be colour coded for ease of identification.

Nowsco has found that these types of pigs have to be selectively used, and that in long gas lines breakthrough of gas and destruction of the pig may occur. They do, however, have applications where outlet restrictions are smallas the pigs can be broken up under pressure and discharged through small-diameter outlets.

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