Top-of-line corrosion (TLC) is aninherent problem in high carbondioxide, wet and multiphase gas

gathering or gas transmission pipelines dueto the difficulties of adequately deliveringcorrosion inhibitors to the uppermost areaof the inner pipe wall. Subsea pipelines canbe especially susceptible to TLC and thusare at high risk for premature failure if cor-rosion develops; offshore lines in SoutheastAsia have, in some instances, suffered fromserious TLC-induced degradation. Millionsof dollars have already gone into repairingor even replacing subsea lines compro-mised by corrosion. Regardless of the linelocation, unchecked corrosion can easilycause large-scale production losses. Shouldthrough-wall failure occur, environmentaldamage could be severe.

Until recently, the standard response tothe threat of line corrosion was batch treat-ment using a solid chemical slug or col-umn held between two batching pigs. Thegoal was to keep the chemical intact andcontacting all surfaces of the inner pipe –including the upper wall – long enough todeposit a corrosion-deterring protectivefilm. In many cases, batch treatment canstill be a viable option. However, when youfactor in manpower and equipmentrequirements, chemical expense and pro-duction loss, batch treatment is not cheap.Even more worrisome for some operatorsis the fact that inline inspections haveshown that, in many instances, batch treat-ment is not effective in offshore lines. Onereason for this is that it is very difficult tomaintain the integrity of a chemical slug

through the vertical drop that results whensending batch treatment from a platformdown into a subsea line.

In response to both the TLC phenome-non and the realization that batch treat-ment is not always effective in combatingcorrosion, many pipeline operators havesought out other chemical treatmentoptions, such as the patented V-Jet “spraypig” technology developed by Rick D.Pruett and others of T.D. Williamson, Inc.(TDW), with assistance and support fromYves Gunaltun and his former team at Total(France). Ongoing developments of thistool have included incorporation of a data-logging system, the brainchild of Pruett’sinitiated at the request of consultant Robert

J. Waterhouse. This system has been co-developed with Pruett by TDW engineersTyler Lloyd and Robert Strong. As embod-ied by TDW’s V-Jet Corrosion Inhibitor Pigand the recent Semi-Smart V-Jet (with data-logger) version, spray pig technology relieson a series of nozzles mounted on the frontof a pig to disperse chemical inhibitor(introduced into the line in conjunctionwith the pig) onto the “hard to reach”upper regions of the inner pipe wall.

Spray pig primerThough the use of spray nozzles to dis-perse inhibitor upward is innovative, partof what makes the V-Jet pig attractive is itsbasis in two simple, standard pigging con-

Recently developed “spray pig” helps offshore operators gauge the success of their corrosion inhibitor applications.

Improving corrosion inhibition Eric N. Freeman, P.E.,

T.D. Williamson, Inc., Tulsa, Oklahoma

Pigging Update

April 2009 www.pipelineandgastechnology.com

T.D. Williamson’s Semi-Smart V-Jet Pig just prior to an offshore run in Southeast Asia.

As seen in the April 2009 issue of Construction • Operations • Maintenance • Management

cepts: differential pressure and bypass flow.A typical pipeline pig is pushed by theflowing pipeline product. As pressurebuilding up behind the pig exceeds thepressure in front of the pig, the pig movesforward. This difference in the pressurefrom the front to the back of the pig isknown as the “differential pressure,” alsocommonly called Delta P or simply DP. Theconcept of “bypass flow” is used in manytypes of pigs to vent pressure from the backto the front of the pig, allowing movementof product (liquid and/or gas) through thepig as it advances downstream. Bypass flowand differential pressure are the onlydynamics required for the pig to work.

The special pigging device allows thehigher pressure to bypass through itspatented body and spray head design. Asthe pig advances, residual inhibitor fluid,mixed with liquid that has pooled on thepipe bottom, is drawn up and sprayed ontothe top area of the inner pipe wall. Bypassacts as the accelerant to transfer and vapor-ize this fluid by creating a low-pressure area in the spray head as the bypass flowsthrough (due to what’s known in physics asthe Venturi Effect). This drop in pressureproduces the relative vacuum at the front of the pig that draws up the idle inhibitorfluid to be redeployed. The inhibitor-richspray is not only directed toward the top of the pipe, but it also creates a denseinhibitor “cloud” that grows out ahead of the pig, extending the contact time and overall effectiveness of the inhibitor.Additionally, the drive cups on the pig tendto “press” the inhibitor against the pipewall as the pig moves along the line.

A counterweight system is used toensure proper orientation of the spray head so that the pig projects inhibitor fluidupward at an approximately 45-degreeangle from parallel to the centerline of thepipe. The fluid fans out through the top120 degrees perpendicular to the centerlineof the pipe. Multiple nozzles are used onall pig sizes, with the number of nozzlesincreasing as the pig size goes up in orderto increase the delivery and coverage areafor larger diameter pipelines. For liquids

with physical properties similar to water,each nozzle typically delivers about twoquarts of fluid per minute at 15 psi.Inhibitor delivery rates do vary dependingon differential pressure, fluid viscosity andspecific gravity. Higher efficiency nozzlesare employed on the larger diameter pigs.

Another feature designed into the pig is its short-term reservoir. Line pressureforces fluid that is immediately behind thepig into this reservoir area, and it is thendirectly jetted through the nozzles ontothe upper pipe wall. As this fluid is jettedforward, the excess gathers in front of thepig, where it is drawn back into the pigand redeployed. This circulation of theinhibitor fluid continues throughout thepig run until the fluid has completely dis-sipated onto the pipe wall.

Usage methodsThough it offers simplicity of design, the V-Jet pig can be used in a variety of ways.The simplest method of using the spray pigcan be applied when continuous injectionis the primary means of introducing corro-sion inhibitor fluid into the pipeline. Thismethod is effective on relatively level linesand pipelines with a continuous upgrade,such as those associated with offshorewells. The spray pig has proven to be avery effective dewatering pig while it dis-tributes inhibitor-containing fluids to thetop of the pipe. In this manner, a densevapor cloud is created in front of the pig asit splashes through and jets the liquidsfrom the bottom of the pipe up to the top.

Another method of using the spray pigin a single pig application is what is known

as “lock and load,” so named because thepig is inserted into the launcher, which isthen locked. Inhibitor fluid is loaded intothe launcher through a fill valve prior tolaunching of the pig. This technique is par-ticularly useful when applied to relativelyshort pipeline runs that may not use con-tinuous injection as the primary means of corrosion control. Adding a slug ofinhibitor in the launcher behind the spraypig permits a vapor cloud to be formed atthe very beginning of the run.

A third option is to run the spray pig inbatching mode, which can be useful if noinhibitor fluid is present in the pipeline, ifthe line is too long to load an adequatesupply of inhibitor in the launcher, if thereis a long, steep downhill section in theline, or if there’s a need to reduce pigvelocities associated with speed excur-sions. The spray pig is designed to be used as the front pig and/or back pig (pre-ferred) in batching mode. If the spray pigis run behind a standard batching pigwith a slug of corrosion inhibitor betweenthem, the spray pig develops a higher dif-ferential pressure across the spray nozzles,resulting in higher jetting intensity as thefront pig surges ahead.

Also, the bypass flow through the pig atthe rear of the slug is normally gas, whichdoes a better job of energizing the spraythan liquid bypass, which would be promi-nent if the spray pig were at the front.Batching may be advantageous in largerdiameter applications (to help make sureinhibitor is making it onto the top walladequately) and on lines with multiple ele-vation changes. It’s also worth noting that,

www.pipelineandgastechnology.com April 2009

Performance data collected by the Semi-Smart V-Jet Pig can be downloaded by atechnician following a run.

when using the spray pig as a batching pigin front, especially, it is not necessary tohave a solid slug of inhibitor fluid betweenthe pigs. Rather, a “bow-wave” effect willtend to keep the fluid level well above thesuction ports at the bottom of the sprayhead since gas flow rate (and thus, pigspeed) is generally faster than the liquidflow rate.

Quality control In order for a spray pig run to be trulysuccessful, several conditions must bemet. First and foremost, the pipeline mustbe piggable, meaning valves are open,obstructions are removed, and bend radiiare adequate (at least three times the pipediameter in most cases). Assuming theseperquisites are met, the line should alsobe clean, both to prevent debris frombuilding up in front of the pig and dis-rupting nozzle operation and to makesure that the inhibitor fluid is actuallycontacting the pipe wall (rather than sim-ply coating scale, black powder or othercompounds attached to the wall). Paraffinbuildup, for example, can affect a tool’sorientation, the amount of inhibitor-to-wall contact, pig speed and jetting action.Finally, a corrosion inhibitor compatiblewith product and other constituents inthe pipeline must be utilized.

Assuming an adequately preparedpipeline and proper inhibitor and spray pigusage, the challenge then becomes howbest to gauge the effectiveness of the

run(s). To gauge effectiveness, it is neces-sary to know if inhibitor fluid is being suc-cessfully delivered to the desired pipe sur-face and at the desired film thickness.Further, there is a need to verify that thisdelivery is happening at the right frequencyto maintain the chemical film intended toinhibit corrosion and thus preserve thepipe wall’s integrity.

Attempts to gauge effectiveness are hin-dered by a number of variables at play inany inhibitor application program, includ-ing chemical performance, changes influid composition and changes in operat-ing conditions (such as temperature, pres-sure and flow rate). All of these variablesneed to be considered when planning aspray pig regimen and when attempting togauge a regimen’s success. For onshorepipelines, use of top-line corrosioncoupons and frequent, regular monitoring(perhaps monthly) is a good way to vali-date success. Due to the lack of access tooffshore pipelines, the best method forgauging inhibitor application effectivenesshas been through the use of inline inspec-tion (ILI) tools. Though they are capable ofmapping the total length of a line and pro-viding detailed information on wall defects,these tools are costly and, therefore, infre-quently run (typically bi-annually). Thisinherent time gap between when a chemi-cal treatment plan is undertaken and whenthe inspection tools can map changes cre-ates a potential delay in detecting problemsin inhibitor application.

Data acquisition To eliminate this gap and provide moreimmediate feedback on the performance ofa spray pig in any given application, engi-neers at TDW have developed a “Semi-Smart V-Jet Pig,” so named for its incorpo-ration of some, but not all, existing inlineinspection technologies into the body ofthe pig itself as an on-board datalogger. Itis, essentially, a spray pig with a brain.Capable of operating under normal spraypig conditions, this pig is equipped to col-lect a variety of information during everyrun. This includes data on differential pres-sure, which is helpful because, as previ-ously noted, Delta P is of key importanceto achieving proper jetting action. Thesemi-smart spray pig gauges pressure atboth the front and back of the pig.

In order to be effective, a spray pigmust deliver inhibitor to the top of thepipe. To verify that this is indeed happen-ing, the semi-smart design also monitorsrotation/orientation of the pig. The dataacquired by the device offers a completethree-dimensional profile of how the pigmoved throughout the run, including toolrotation, spray nozzle orientation and portorientation. Should an improper orienta-tion occur, the pig’s sensors can tellwhether this was temporary or perma-nent, and where it took place in thepipeline (for example, relative to knowncorrosion areas).

To be optimally effective, a spray pigshould stay within speed limits that allowfor proper contact time between theinhibitor and the pipe wall. The semi-smartdesign includes an odometer to generate aspeed profile on the run, as well as accu-rate location of events recorded during therun. It may be possible to calculate theminimum inhibitor contact time for spe-cific pipe segments within the run andcross-check this with vendor suggestions.Contact time is generally longer than what

April 2009 www.pipelineandgastechnology.com

Development of the Semi-Smart V-Jet Pig wasprompted by needs to verify tool perform-ance required for successful application ofcorrosion inhibitor fluids inside pipelines.

would be calculated due to the extendedcloud of inhibitor that forms in front of thepig. This information can also be used tohelp guide increases and decreases in thefrequency of spray pig usage.

The semi-smart spray pig design is also capable of measuring temperature.Gauging temperature is helpful because itis believed that TLC occurs in specifictemperature ranges; accurately gaugingline temperature throughout a run canspotlight areas that may be most suscepti-ble to corrosion. Or, when used in con-junction with other inline inspectiondevices, this temperature data may offerinsights into the conditions that are mostconducive to corrosion development.

Last but not least, the semi-smartdesign allows for acceleration logging. Inother words, for the measuring of speedexcursions during a pig run. Speed excur-sions are typical when running in gaspipelines but are not conducive to consis-tent, manageable corrosion inhibitorapplication. Monitoring these excursions,and matching them to specific line loca-tions based on odometer readings, canoffer further valuable insight into overalljetting performance during a given run.

Looking ahead The Semi-Smart V-Jet pig is but one exam-ple of newly emerging tools that seek toaccurately gauge and treat top-of-line cor-rosion in hopes of avoiding or at leastminimizing the instances of pipeline fail-ure due to this phenomenon. Certainlymore work remains to be done before thecorrosion mechanisms are fully under-stood or fully controllable. While devicessuch as the semi-smart spray pig are sig-nificant steps forward, it is anticipatedthat future developments may includestand-alone sensor and data loggingdevices capable of being attached to, ordelivered by, pigging tools rather than sen-sor combinations built into the body of aspecific pig. The versatility of such stand-alone sensor devices would facilitate evenwider use of this technology and a conse-quent broader understanding of corrosiondevelopment and mitigation in a variety ofenvironments. ■

Acknowledgment Based on a paper presented at Clarion’s20th International Pipeline Pigging &Integrity Management Conference, held inHouston, Texas.

The author Eric N. Freeman, PE, is the EngineeringManager for the Pigging Products Division of T.D. Williamson, Inc., in Tulsa, Oklahoma.

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