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Unique Pipeline Tool

58 Pipeline & Gas Journal/June 2002/pipelineandgasjournalonline.com

Non-Invasive Inspection MethodFor Unpiggable Pipeline Sections

ew pipeline integrity regula-tions allow operators toassess pipeline sections thatpreviously could not beinspected by conventional in-

line-inspection processes, i.e., smart pigs.The new rules allow other technology to beused to assess the condition of these lines orsections of lines. Hydrostatic testing may bean option in these cases but can be verycostly. Clearly, other technology would bedesirable if it produced results and gave theoperator assurance the lines in questioncould meet operational requirements.

One of the emerging technologies toaddress this need is long-range ultrasonicinspection. Long-range ultrasonic testing orguided wave ultrasonic testing was com-mercially introduced in early 1998 for in-service monitoring of pipes and pipelines.The oil, gas and chemical process industriesnow use it for detection of corrosion andother metal loss defects and it is gainingacceptance as a valid means of assessing thecondition of pipes and pipelines whereinspection preparation or access is difficultor expensive. The rise of the technology isespecially significant in view of the govern-ment’s emphasis on pipeline integrityassessment and the lack of a proven tech-nique to inspect pipelines that cannot beevaluated with “smart pigs.”

The technique (as represented by theTeletest® system) has now been exten-sively used in the field for evaluating pipesfrom 2 to 48 inches in diameter and hasperformed well in identifying corrosion inpipes in a variety of situations. As with anynew technology, a crucial stage in gainingindustry acceptance as a proven inspec-tion and monitoring tool is the perform-ance achieved in real field situations.

This article describes how this process isnow being used to inspect buried andabove ground pipeline sections and com-pares actual field data with the equipmentmanufactures performance claims.

The Teletest® low frequency, longrange, guided wave ultrasonic techniquehas been developed for the rapid surveyof pipes, for the detection of both internaland external corrosion. The principaladvantage is that long lengths, i.e., 75 to100 feet in each direction on buried pipeand 300 feet plus above ground (also in

each direction) is examined from a singletest point. The benefits are:

Reduction in the costs of gaining access topipes for inspection, eliminating removaland reinstallation of insulation, except in thearea where the transducers are mounted;

Direct assessment of unpiggablepipeline sections in lieu of hydrotesting orpig launcher-receiver installation;

The ability to inspect inaccessible areas,such as under clamps, sleeves or buriedpipes; 100% of the pipe wall is tested; and

Site trials have demonstrated that thismethod is capable of detecting corrosion<30% wall thickness depth and <25% cir-cumferential width.

Corrosion and other conditions resultingin metal loss in piping is generally unpre-dictable. Once in service, it becomes a chal-lenge to find these conditions and monitor orrepair them. Other than direct measurementwith conventional ultrasonic thickness equip-ment or expensive radiographic procedures,global survey tools leave much to be desiredin terms of sensitivity to all but the mostsevere conditions. Access to some pipe canpresent a significant problem, and in manycases, exceed the cost of the inspection oreven total replacement of the pipe itself.

The benefit of using long-range ultrasonictesting (LRUT) to obtain information on pipeconditions in areas where access is restrictedbecomes obvious. The experience gained byPetroChem Inspection Services through theuse of LRUT over the last 18 months has ver-ified the developmental data and field expe-rience of Plant Integrity Ltd. (PI).

Development Developed by The Welding Institute

(TWI) in the UK, the tool was commercial-

ized as Teletest®. The equipment is sold,distributed and serviced through PI. TWIbegan development of the LRUT method inMarch 1992. UK and European agenciesand a group of industrial companies fund-ed the initial development projects.Pipeline Research Council International Inc.(PRCI) funded the final phase through itsAdvisory Committee, with the aim of deter-mining the applicability to large diametergas transmission pipelines. The final devel-opmental phase concluded with a reportfrom TWI to the NDT SupervisoryCommittee of PRCI in September 1997 (3).That report summarized a successful per-formance trial conducted at the GRI WestJefferson Pipeline Simulation Facility nearColumbus, OH. The test was conducted ona 24-inch diameter pipe section that con-tained 128 known and documented metalloss defects. The test section was coatedwith FBE and was located above ground.

Subsequent to September 1997, PI begancommercial services with the Teletest® systemin the UK, Alaska, The Netherlands, andhoned the equipment into a field-hardenedunit capable of producing consistent resultsunder difficult field operating conditions. Overthe past four years, the PI team spent severalmonths in Alaska each summer successfullytesting road crossings on the North Slope.

North America Introduction In March 2000, PetroChem Inspection

Services brought the Teletest to the U.S.for two months of field-testing with theirrefinery and chemical plant clientele.Ultimately, those trials led to the commer-cial introduction of the Teletest® byPetroChem Inspection Services in theLower 48 states, Figure 1.

by Scott Lebsack, PetroChemInspection Services, Houston, TX

Figure 1


LRUT Vs. ConventionalSystems

LRUT using guided waves is different fromconventional ultrasonic inspection. Straightbeam UT measures pipe thickness at a smallspot under the transducer, Figure 2. In orderto cover a large area, the transducer must bescrubbed over the pipe surface. TheTeletest® system uses a transducer collar thatproduces a 360° soundwave moving axiallydown the pipe. The pipe itself becomes theguide for the wave – thus “guided wave.”

Guided waves propagating in the pipewall are similar in nature to Lamb waves inplates. They are capable of propagationover hundreds of feet in plain pipe. Alarge number of guided wave modes arepresent in a GWUT shot; however, thesignals are simplified and only selectedwave modes are utilized by the system.This is necessary for signals from anom-alies and pipe features to be interpreted.

Other wave modes that are present arecanceled out by the transducer arrangement

and spacing,Figures 3 and 4.T r a n s d u c e ra r r a n g emen tand spacing iscritical to a-chieve the high-est signal tonoise possibleand allow forevaluation ofthe signals fromdiscontinuities.The Teletest®system employstwo other wavemodes in addi-tion to the pri-mary longitudi-n a l m o d e .These are theonly two flex-

ural modes that are axisymmetric. Theyallow the operator to discern whether ananomaly is in the top/bottom of the pipeor along the sidewalls and to distinguish

Figure 2

Figure 3

Figure 4


Pipeline & Gas Journal/June 2002/pipelineandgasjournalonline.com 61

pipe features such as welds from actualmetal loss anomalies.

Performance FactorsField experience over the last 18 months

has verified the manufacturer’s claims forboth sensitivity and sound transmissioncapability. PetroChem’s experience duringthe last six months has focused on inspect-ing buried pipelines accessed from a “bellhole” dig. On average, inspection dis-tances of between 70 and 100 feet havebeen achieved. Many times greater dis-tances could be recorded but the techni-cian elected to cut the examination dis-tance due to difficulties in interpretationbeyond a specific distance. This is a rou-tine decision made by the technician dur-ing each test.

In situations where pipe is above-ground, inspection distances have rangedwell in excess of 200 feet and at timesmore than 300 feet. However, in these sit-uations the operator must use good judg-ment in determining at what point the datais no longer useful. This is a condition ofdeteriorating signal to noise ratio over dis-tance and one of the more importantaspects in using guided waves.

Other factors that affect sound attenua-tion are liquids in the pipe. The higher theliquid viscosity is, the greater the attenua-tion. Where pipe is severely corroded or pit-ted, the signal will drop dramatically imme-

diately beyond the problem area. Pipe thathas numerous elbows, bends, or smallerdiameter branches will not yield the samedistances as a straight section of pipe. It isimportant to mention that pipe geometrydoes not normally restrict inspection withTeletest,® but the distances obtained will bereduced as the complexity of the pipegeometry increases.

SensitivityTeletest® is a screening tool. Its use

yields valuable information about a pipethat is not otherwise accessible or access iscost prohibitive. In pipeline “bell hole”digs, the information envelope is expand-ed from the pipe exposed in the dig, nor-mally 10 to 15 feet, to 165 feet assuming75 feet as an average distance for sound

Figure 5


transmission in buried pipe sections. Animportant point to note is that LRUT tech-niques do not provide a direct measure-ment of pipe wall thickness, but are sensi-tive to a combination of the depth and cir-cumferential extent of any metal loss. Thisis due to the transmission of a 360° circu-lar wave front traveling axially along thepipe. The wave front interacts withchanges in the annular cross-section of thepipe. A signal response is produced wherethe cross-section is reduced (metal loss) orchanges character (at a weld or other pipefeature). This is shown schematically inFigures 5 and 7.

Detectable flaw areas are calculated asa percentage of the pipe wall cross-sec-tion. Teletest® is equally sensitive to inter-nal and external wall loss but cannot dis-tinguish between the two. The effect of

multiple flaws is additive. Under normalfield conditions the unit can detect metalloss anomalies that are approximately 9%of the annular cross section of the pipe.Tests during development of the Teletest®proved that this level of sensitivity wasachievable with 95% probability whereas3% anomalies could be detected with 75%probability. This would apply to buriedpipe sections, but under more favorableconditions, such as gas filled, above-ground piping has shown higher sensitivi -ty levels, especially as the diameter of thepipe increases.

Understanding anomaly size can beconfusing since anomalies are expressedin terms of the percentage of the pipecross-section area. Therefore, a 9% metalloss area in a 10-inch (STD 0.375-inch)diameter pipe would represent 0.560

square inch reduction in the annular crosssection of the pipe. This area configured at30% through wall would be 0.11-inchdeep by 4.97-inch long. Since LRUT is avolumetric exam the probability of detec-tion would not change if reconfigured tofive areas whose sum equaled the originalarea of 0.560 square inches. As mentionedpreviously, detectable anomaly sizes maybe reduced by as much as 50% in pipediameters greater than 20-inch.

ApplicationsThe initial commercial use of the system

by PetroChem concentrated on processpiping in refineries and chemical plants.The primary applications were assessingthe soil to air interface of berm penetra-tions and road crossings. Typically, thesepipe sections had never been checked dueto the difficulty and expense of exposingthem, yet had been suspect due to thenature of their environmental conditions.

The following list summarizes the appli-cations that were tried and found withinthe capabilities of the LRUT techniqueusing the Teletest® system:

n Berm penetrations of uninsulatedpipe;

n Road crossings and berm penetra-tions of sleeved and insulated pipe;

n Above ground insulated piping;n Spirally welded pipe (gas collec-

tion field);


Table 2:

Pipeline & Gas Journal/June 2002/pipelineandgasjournalonline.com 63

n Coiled heater tubes (4'' O.D. X 40’);n Insulated pipe—pinpointing weld

locations under insulation, (Figure 5);n Wall penetrations of piping in compres-

sor stations (sleeved and encased pipe);n Locating smart pig anomalies in a

buried pipe section; and n Corrosion assessment of buried pipe

sections (Unpiggable).During the past 18 months, PetroChem

has performed services encompassing allthe applications listed above. In the fol-lowing sections, the operating circum-stances, limitations and unexpected find-ings are reviewed.

Process Piping ApplicationsWork on process piping has taken two

basic directions. The first is corrosionunder insulation programs. The LRUTmethod is adept for these surveys because

it reduces pipe access costs, especially inoverhead piping and pipe racks. The sec-ond is miscellaneous piping running onthe ground usually to and from storagetanks. These piping systems are typicallyinsulated and make numerous road cross-ings passing through sleeves. Storagetanks have emergency retaining bermssurrounding them and the pipe passesthrough these as well. In both cases, roadcrossings and berm penetrations, piping issusceptible to corrosion and difficult toinspect. PetroChem performed an exten-sive in plant survey for a major U.S. chem-ical producer. Table 1 represents theresults of that survey. Conventional UTwas used to verify anomalies that could beaccessed. The data in Table 1 is a tabula-tion of the anomalies that were verified.Anomalies under roads, in berms or pipesleeves are not included in the tabulation

unless they were verified. When compared with the results of the

Rach Project(2), Figure 6 results wereroughly similar but better than thosereported by PI. The trials were conducted‘blind’ without knowledge of the defectspresent and the results were evaluated byan independent team from Bureau Veritas,Paris, France. This figure shows the resultsfrom the Teletest® technique on 36 indi-vidual defects. The plot is in terms ofdepth and circumferential extent of thedefects and indicates whether each wasdetected or not. The figure shows thatunder blind trial conditions the Teletest®technique performs as expected from thedevelopment work. (1)

Pipeline InspectionsDeveloping proficiency on pipeline sec-

tions with the Teletest® equipment requireddifferent techniques than originallyemployed on above ground piping. Buriedpipelines accessed through “bell holes” pre-sented variables in their condition that ele-vated the task of evaluation. Moisture andsoil type along with the condition and ageof the bitumastic wrapping, (see cover)were three factors noted that affected soundtransmission in buried pipelines. These con-ditions required the technicians to be dili-gent in selecting the correct inspection fre-quency and only interpret data with accept-able signal to noise ratios.

In order to inspect buried pipe, numerous

Table 1


“shots” must be made to determine the frequency that yields thebest signal to noise ratio. In addition, ghost signals resulting frommode conversions can appear to look like anomalies. These usu-ally are verified by checking the response from other data sets atfrequency ranges above and below that of the suspected anom-aly. A real anomaly will usually be present at more than one fre-quency while reverberations and ghost signals will disappear. Afeature to allow more rapid “tuning” during operations is beingincorporated into the next generation of the equipment.

Field Test On Condemned PipelineA test was conducted by a major southeastern gas pipeline

operator. The line had been condemned and was out of service.Smart pig data was available on the 300-foot section of a 16-inchOD Schedule 10 (0.250-inch) inspected by PetroChem with theTeletest® system. Table 2 summarizes the results of the test.

Field Trial On Buried PipelineThe following Teletest® graphic data set is from a line operat-

ed by a large Northeast refiner/pipeline operator. The section ofline from 42.6 feet to 82 feet was previously known to have exten-sive corrosion. The condition was detected over and extended dis-tance in the last portion of the test length. A pipe clamp and thearea of severe corrosion were correctly recorded (blind) duringthis test. Note that the area of severe corrosion is more than 75feet from the setup point. The graphic data demonstrate the corro-sion impact, Figure 7. The signal strength drops rapidly beyond the

From left to right these lines represent the 3%, 9%, 14% and 100% ref-erence levels.

ConclusionPipeline inspections using LRUT were more problematic than

those conducted on aboveground piping whether insulated ornot. However, after understanding that soil, moisture, coating ageand actual pipe condition are variables that can affect test results;test procedures were initiated to minimize their effects.PetroChem has verified the performance claims of PI. In actualpractice, the Teletest® system will detect conditions that arebelow the stated sensitivity levels. However, the ability to exceedthe stated performance levels of the equipment is based on thetechnician’s skill and experience.

The use and ultimate success of LRUT techniques, irrespec-tive of equipment manufacturer, is largely dependent on thetraining and field experience of the technician. Ultrasonic sig-nal recognition skills are paramount. PI has taken a responsi-ble position in marketing their equipment by requiring that endusers take at least 40 hours of instruction from their field per-sonnel. PetroChem has also recognized the importance of oper-ator skill in delivering a reliable service to industry. PetroChemtechnicians are required to have a strong background in shear-wave inspection. Many have passed the Electric PowerResearch Institute (EPRI) test for detection and sizing inter-granular stress corrosion cracking and passed the ASME SectionXI PDI initiative for ultrasonic operators inspecting piping inAmerican nuclear power stations. They are also required towork with an experienced Teletest® crew for a minimum ofthree months before being allowed to conduct examinationsindependently.

LRUT technology is significant to pipeline operators at thispoint in time. There are more than 2 million miles of pipelinesin the United States. The Office of Pipeline Safety has statedthat 30% of these lines cannot be inspected with smart pigs.With the current legislation now pending, pipeline operatorswill be required to assess the integrity of lines that are unpig-gable. This may require the use of direct assessment or hydro-testing. Guided Wave UT technology offers a cost-effectivemethod of compliance when used with the direct assessmentmethod. While there is no one technology that can cover allinspection situations, GWUT coupled with CIS and otheremerging technologies, will be important tools in maintainingany pipeline integrity program. P&GJ

Author’s Note: Scott Lebsack has over 25 years working in thefield of Non-Destructive Inspection and Testing. He is executivevice president for PetroChem Inspection Services and responsiblefor developing new business and services related to pipeline inspec-tion. Prior to joining PetroChem, he was the NDT Division VicePresident for Society General de Surveillance (SGS) in Geneva,Switzerland. He joined PetroChem after it acquired the U.S.Division of SGS, SGS Industrial Services. For the last 15 years hehas focused on identifying inspection technologies that could beapplied to specific industry problems.

Figure 6Unique Pipeline Tool

Figure 6:

Figure 7:

pipe clamp as indicated by the steep decline of the DAC referencecurves. The dotted black line on the left, the green trace, turquoisetrace and solid black trace on the far right show these references.

REFERENCESMudge, P J, Teletest® Long Range Ultrasonic Testing Technique –

Performance Details, Document Reference: TTP/01, May 2001.Reliability Assessment for Containment of Hazardous Materials, RACH

European Commission Project OG 112/FR/UK, Final Report, 1999.Lank, A M and Mudge, P.J., ‘Development of Long Range Ultrasonic

Methods of Assessing Pipeline Condition’ Final (Phase 4) Report for the NDTSupervisory Committee of PRC International, September 1997.

BIBLIOGRAPHYMudge, P J and Lank, A M, ‘Detection of corrosion in pipes and pipelines’, ASNT

International Chemical and Petroleum Industry Inspection TechnologyTopical Conference V, Houston, Texas, 16-19 June 1997.

Koenig, M J, Bubenik, T A, Rust, S W, Nestleroth, J B: ‘Topical ReportGRI-94/0381: GRI Pipeline Simulation Facility Metal Loss Defect Set’, GasResearch Institute, April 1995.

Crouch, A E, ‘Assessment of NDT Needs for Pipeline Integrity Assurance II’, R-15-9507:Final Report to NDT Supervisory Committee of PRC International, April 1996.

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