Pipeline & Gas Journal / March 2003 / www.pipelineandgasjournal.com

luor Corp. has made a major break-through in a new application ofUltrasonic Time of Flight Diffraction(UT-TOFD) that enhances ultrason-

ic examination of thermoplastic, pipelinebutt welds. The capability to non-destruc-tively characterize, evaluate weld integrity,and detect defects, including porosity andlack of fusion (LOF) or cold welds of plasticpipe in operation, was needed by industry,but not previously available. Fluor, in con-junction with RTD, developed, calibratedand correlated a novel application of the UT-TOFD technique that proved to be highlyreliable at detecting, locating, and character-izing defects in high density polyethylene(HDPE) welds. For pipe wall thicknesses upto 4 inches (100 mm), UT-TOFD detecteddefects down to 4% of wall thickness.

A strong correlation was found betweenUT-TOFD scans and actual defects in a largesample of shop and field welds. The high-resolution examination of welds is suitablefor both shop and on-line field applications.This method has been tested and proveneffective in large-scale field pipeline applica-tions. Now, pipeline operators working in ademanding industry have a tool that pro-vides high-quality verification of weldintegrity for inspection, engineering evalua-tion, and comprehensive risk analysis.Avenues are now open for improvingpipeline construction and HDPE fusion prac-tices, and for monitoring integrity of operat-ing pipeline systems.

A novel application of Ultrasonic Timeof Flight Diffraction (UT-TOFD) in bothshop and “operating” field environmentsprovides immediate weld integrity assess-ments and offers the thermoplastic indus-try new opportunities to improve pipelineconstruction, fusion practices, and thecapability to monitor integrity of pipelinesystems during operation

BreakthroughThe breakthrough UT-TOFD method,

for inspection and assessment of weldintegrity on HDPE pipe, is capable of

accurately detecting and quantifying welddefects including LOF and porosity andproves that, unlike commonly held viewsin the industry, visual examination of com-pleted HDPE welds does not correlatepositively to weld integrity1. This high-res-olution UT-TOFD method provides anunprecedented, fast inspection and assess-ment method for shop and field and pro-vides HDPE pipeline operators withimmediate results for risk assessments ofweld integrity (Fig. 1 and Fig. 2).Specialized probes that combine propri-etary housing materials, low frequencieswith narrow bandwidths, and a wide-beam angle to provide the high-resolutionresponse were developed. In conjunctionwith proprietary software and hardware,this novel method has been tested andproven to achieve high signal-to-noiseratio with immediate, accurate results.

The UT-TOFD method allows 2-personcrews to set-up, inspect, analyze, and cate-gorize weld defects quickly (for example, 15

minutes per weld for 32 inches diameterpipe). For pipe wall thicknesses up to 4inches (100 mm), UT-TOFD detects defectdimensions of less than 4% of wall thick-ness. The method is capable of examining100% of fabricated welds and monitoringweld integrity on operating pipelines. UT-TOFD details data for inspection, engineer-ing evaluation, and comprehensive riskassessment. Patents are pending on thistechnology and its application.

BackgroundUntil Fluor, in conjunction with RTD,

found success with the novel application ofUT-TOFD method on HDPE pipe welds,visual examination of weld bead quality was

F

Fig. 1. UT-TOFD Pipe Scan

Fig. 2. Results On-Site

Novel High Resolution Defect DetectionFor Thermoplastic Butt-Welds

By: Barry Messer, Matthew Yarmuch, Fluor Corp., and Peter den Boer, RTD Quality Services Inc.

Messer Yarmuch den Boer

Order ReprintsPermission to Copy

Pipeline & Gas Journal / March 2003 / www.pipelineandgasjournal.com

When signals at specified frequenciesencounter anomalies, the ultrasonic com-

the major weld-integrity determinant forboth shop and field environments. Existingultrasonic and traditional tests applicable tothe steel industry have been applied to thethermoplastic industry’s unique HDPE pipe;unfortunately, previous ultrasonic tests havenot been able to evaluate weld integrity andensure long-term operation life for thermo-plastic welds as reliably as they do forsteel-fabricated welds.

Application of UT-TOFD for steel fabri-cation and construction has had success,and it has been approved by ASME Code,Case 2235-4. Quality control tests, such asvisual, bend-strap, and hydro, have beensuitable for steel-fabricated welds. Thephilosophy behind these tests has beencarried over to HDPE pipeline fabrication.However, due to HDPE pipe visco-elasticmaterial and mechanical properties, alongwith current HDPE fusion techniques,these tests are insufficient to ensure thelong-term operational life of thermoplas-tics2. Pulse echo tests have been attempt-ed on thermoplastics with limited success,and proved unreliable for identifyinganomalies on thick wall pipe. TOFD sur-faced in the late 1970s as a way to identi-fy locations of anomalies. Previous TOFDtechniques were in need of improvementsto signal enhancements and data interpre-tation, not able to identify LOF, and werelimited by capabilities to identify the indi-cation type and its extent 3,4. Lack of reli-

able ND tests has left visual examinationof weld bead quality at the forefront fortesting HDPE pipe weld integrity5.

Time of Flight Diffraction (TOFD) Method

The semi-automatic computer assisted UT-TOFD method uses specialized transmitter andreceiver probes attached to an encoder, collec-tively referred to as the scanner, with a cablelink between scanner and computer (Fig. 3).

Fig. 5. Illustration, Scanner — Butt Weld

Fig. 6. UT-TOFD Compression Waves

Fig. 4. Scanner

The scanner is portable with spring-loadedtransmitter and receiver probes to maintaincontact with the pipe surface (Fig. 4 and Fig. 5).

When the transmitter probe is posi-tioned over the weld area of investigation,the probe emits compression waves in apitch-catch arrangement (Fig. 6).

a = Lateral Waveb = Upper Tip of Flawc = Included Angled = Lower Tip of Flawe = Backwall Echot = Pipe Wall Thickness

Fig. 3 UT-TOFD Equipment

Pipeline & Gas Journal / March 2003 / www.pipelineandgasjournal.com

UT-TOFD method. Pipe areas containingdefects were clearly marked, and sampleswere then prepared for destructive exami-nation (DE) bend tests and macro exami-nation. A third-party materials engineeringtesting laboratory conducted the DE todetermine the presence or absence of welddefects on the prefabricated pipe.

Positions of potential defects were identifiedrelative to the top-dead center of the pipe,measured in the direction of the UT - TOFDscan. Relative positions of bend-sample defectswere superimposed over the UT-TOFD imagewith arrows to identify samples (Fig. 9).

Results showed that a loss of backwallecho or lateral wave signals correlated tosevere LOF defects. Point reflectors, or indi-cations, correlated to localized defects indi-cating poor overall fusion quality, and“clear” UT-TOFD scans correlated to prop-erly fused butt welds. Findings also revealeda surprising discrepancy between poor visu-al appearance and actual weld quality. Forexample, in Fig. 10, the weld profile dis-plays poor bead rollover and visual symme-

pression waves are diffracted. Dimensions ofthe anomaly are measured by calculating thetime of flight of the diffracted waves from thetransmitter to the receiver. The encoder

records and identifies the circumferentiallocations of defects. The new generation soft-ware developed by RTD interprets raw oscil-loscopic wave signals (Fig. 7) and instanta-neously displays real-time, cross-sectionalviews of the pipe circumference with defectssuperimposed on the pipe image (Fig. 8).

Preparation For FieldImplementation

The investigative team developed labo-ratory and field tests to verify that the UT-TOFD technique could be applied reliablyon operating pipelines. Development testsincluded equipment calibration, third-partyconstruction of prefabricated welds contain-ing defects, UT-TOFD examination of theprefabricated welds, third-party destructiveexamination (DE), and correlation of results.

The team calibrated equipment usingthree samples with various wall thicknessesand concluded that the maximum examina-tion range of pipe wall was up to 4 inches(100 mm). For all ranges of pipe size, cali-bration results also indicated that the UT-TOFD detectable flaw size limit wasapproximately 4% of wall thickness. Thedetectable flaw limit was not a function ofdepth or location of a given flaw. The sig-nal-to-noise ratio was >6 dB. Once UT-TOFD was calibrated, the team inspectedwelds on four prefabricated PE3408 pipespools of various diameters and DR ratingswith double blind testing. Prefabricatedwelds were examined by RTD using the

Fig. 7. Oscilloscope Signals

Fig. 8. Example Calibration Output

Fig. 9. Correlation UT-TOFD and DE

Pipeline & Gas Journal / March 2003 / www.pipelineandgasjournal.com

Fig. 11. (Far left) Weld A TOFD Sam-ples 1 and 1.1(CircumferentialPosition 0 mm to 700mm).

Fig. 12. (Left) Weld A UT-TOFDSamples 2 and 2.1(Circumferential Position 500 mm to 1000 mm)

try; yet, this weld contained no fusiondefects and was of sound quality.

This finding signaled a concern whenemploying visual inspection of HDPE pipe fordeterminations of butt-weld fusion quality.Knowledge of the discrepancy and subse-quent confirmation prompted a further exami-nation of the relationship between visual weldbead appearance and actual weld quality.

Field ImplementationA qualified third-party inspection agency

visually inspected all field welds; subse-quently, a two-person UT-TOFD crew exam-ined the field welds. During pipeline opera-tion, a total of 132 field welds were UT-TOFD examined, with pipe diameters rang-ing from 22 inches to 32 inches with a wallthickness range of 0.846 inches (21.5 mm) to

3.333 inches (85 mm). Within 15 minutes perfield weld, the crew setup, inspected, andcategorized butt weld defects.

Field ResultsAt the pipeline site, UT-TOFD scans detect-

ed welds that contained suspected severefusion defects. To ensure that the suspecteddefects did, in-fact, exist, the team removedtwo suspect welds from the pipeline labeledWeld A and Weld B. Samples were then pre-pared for destructive examination with char-acterization of fusion defects, followed by cor-relation of NDE and DE results.

UT-TOFD Detected FusionFlaws– Weld A

Weld A, sample 1 contained two dis-tinct point reflectors, but no loss of

Fig. 10. Poor Bead Rollover and Symmetry

Pipeline & Gas Journal / March 2003 / www.pipelineandgasjournal.com

backwall wave while sample 1.1 was“clear” of indications (Fig. 11). Samples 2

In sample 2.1 (Fig. 16), DE revealed a transition from brittle to ductile fracture sur-face, which was expected, based on the UT-TOFD examination. The results, shown inFig. 16, demonstrate the pinpoint accuracy of the UT-TOFD method to identify andlocate LOF defects in thermoplastic butt welds.

Table 1. Weld A—Destructive Examination Results

Weld A Sample Failure Mode Bend Test Observations and Remarks

1 Ductile Overload Failure near, but not on the weld interface. Dull fibrous fracture face and shear lips from final specimen failure.

1.1 Ductile Overload Failure near, but not on the weld interface. Dull fibrousfracture face and shear lips present from final fracture.

2 Brittle Fracture Failure on the weld interface. Brittle fracture face constant along entire through-thickness.

2.1 Brittle at onset Brittle fracture during onset of fracture on weldand ductile interface. Ductile fracture mode during final fracture.final fracture

Table 2: Weld A—Correlation Results

Weld A Sample UT-TOFD Fusion Flaws DE – Bend Strap Test

1 2 isolated point reflectors Failed via ductile overload (Fig. 13)

1.1 No indications, clear No fracture along weld interface, and exhibitedductile fracture surface (Fig. 14)

DE Fusion Defects - Weld A

Fig. 13. Weld A—Sample 1 Fig. 14. Weld A—Sample 1.1

Table 3. Weld A—Correlation Results (Continued)

Weld ASample UT-TOFD Fusion Flaws DE – Bend Strap Test

2 Areas with Loss of Backwall Sample exhibited brittle and LOF (Fig 15)and Lateral Wave

2.1 Areas with Loss of Backwall Failure initiated via brittle fracture, but final and Lateral Wave fracture was by ductile mechanisms (Fig. 16)

Fig. 15.(Far Left) Weld A Sample 2

Fig. 16.(Left) Weld A Sample 2.1

and 2.1 displayed areas with loss of thebackwall echo and lateral wave (Fig. 12).

Pipeline & Gas Journal / March 2003 / www.pipelineandgasjournal.com

Fig. 17. Weld B UT-TOFD Samples 1 to 4(Circumferential Position 0 mm to 500 mm)

Table 4. Weld B—Destructive Examination Results

Weld B Sample Failure Mode Bend Test Observations and Remarks

1 Brittle Fracture Failure on the weld interface without appreciable gross plastic deformation. The flat fracture face indicates brittle fracture

2 Ductile Fracture Failure not entirely on the weld interface. Dull fibrous fracture face and shear lips present fromfinal fracture. The presence of pores resultingfrom poor fusion is confirmed on fracture face.

3 Brittle Fracture Brittle fracture face on the weld interface from weld failure.

4 Brittle Fracture Brittle fracture along weld interface from weld failure.

Table 5. Weld B—Correlation Results—Sample 1 and 2

Weld B Sample UT-TOFD Fusion Flaws DE – Bend Strap Test

1 Multiple point reflectors Brittle fracture with porosity (Fig. 18)and disrupted backwall wave

2 Multiple point reflectors, Ductile fracture with excessive porosity (Fig 19)(bottom to top)

UT-TOFD Detected FusionFlaws —Weld B

The scan of Weld B (Fig. 17) revealedthe presence of multiple point reflectors,coupled with an interruption of the backwall echo, which indicated severe lack offusion, “cold weld” conditions andextreme weld porosity.

Table 6. Weld B—Correlation Results—Sample 3 and 4

Weld B Sample UT-TOFD Fusion Flaws DE – Bend Strap Test

3 Multiple point reflectors Brittle fracture with excessive porosity (Fig 20)with disrupted backwall wave

4 Multiple point reflectors Brittle fracture with porosity (Fig 21)with disrupted backwall wave

Weld B-sample 3 (Fig. 20) shows the excellent correlation of point reflectors on the UT-TOFD image with DE results. This result provides further evidence of high UT-TOFD accu-racy and reliability.

Fig. 18. Weld B Sample 1 Fig. 19. Weld B Sample 2

DE Fusion Defects—Weld B

Pipeline & Gas Journal / March 2003 / www.pipelineandgasjournal.com

REFERENCES:1. Fluor Confidential Study 2002.2. Technical Note 802 “Leak Testing” – CP

CHEM Performance Pipe EngineeringHandbook (2002). Chevron Phillips ChemicalCompany LP

3. J. Munns, G.A. Georgiou, Ultrasonic andRadiographic NDT of Butt Fusion Welds inPolyethylene Pipe (1995). The WeldingInstitute (TWI). Presented at Plastic Pipes IXHeriot-Watt University.

4. V.P. Rad’ko, V.A. Troitskij, Detection andLocalizing of Defects in Welded Joint of Itemsfrom Plastic Using Ultrasound. (2000) ElectricWelding Institute of the NAS of Ukraine.Presented at ROMA 2000

5. The Plastics Pipe Institute, Inc., Technical ReportTR33/2001, Appendix A, Generic Butt FusionJoining Procedure for Polyethylene Gas Pipe”.

Lessons LearnedWeld A UT-TOFD scan results verify

that an interruption of the backwall echoand lateral wave have a strong correlationto the severity of lack of fusion conditions.In Weld A (Fig. 12), the ultrasonic wavecan not readily travel through the weldinterface; and once the energy is scattered,the receiver probe cannot detect it. Theabsence of the arrival signals signifies non-homogeneity in the weld zone, indicativeof LOF conditions. The high reliability andresolution of the UT-TOFD method isshown in Fig. 16, where the LOF regionwas identified with pinpoint accuracy. Theresults of Weld A, samples 1 and 1.1 (Table2), also confirm that a “clear” TOFD scanis free of fusion defects and that isolatedpoint reflectors are not critical enough toinduce weld zone failure during bendtests. Isolated point reflectors are not aconcern for determining weld integrity;but as verified in Weld B (Tables 5 and 6),multiple point reflectors within close prox-imity of each other are definite cause forconcern. In Weld B, the significant disrup-tion of the backwall and lateral waves sig-nal a severe and serious LOF defect.

Complete joining of pipe ends to ensurehomogenous material properties acrossthe weld zone is an underlying assump-tion of proper fusion conditions. UT-TOFD scans at the weld interface (Fig. 22)demonstrate that a change in materialproperties results in a disruption of ultra-sonic signals (Fig 12).

The UT-TOFD image correlates to thechange in properties by reflection and/ordiffraction at the weld interface. The sam-ples established conclusively that UT-TOFD has sufficient sensitivity to easilyidentify LOF anomalies.

In Weld B, sample 2, the team foundthat weld failure via ductile mechanism, in

Fig. 20. Weld B Sample 3

Fig. 21. Weld B Sample 4

lieu of brittle fracture common to adjoin-ing material, could be the result of howthe cold weld was formed. For example,when a weld bevel cools significantly, acrystalline skin can form that does not per-mit proper fusion. It is possible that thislocal area in Weld B, sample 2, was notcooled to the same degree as the adjacentsamples. However, excessive weld porosi-ty (Tables 5 and 6) is adequate grounds forrejection of the weld.

For all samples, the excellent correla-tion between point reflectors and actualfusion porosity further verifies the sensitiv-ity level of UT-TOFD inspection method. Inaddition, the presence of multiple pointreflectors is a secondary indicator of possi-ble lack of fusion conditions. It is also clearthat the presence of multiple point reflec-tors, coupled with an interruption of thebackwall and lateral wave within the samearea of a weld, positively identifies an areaof extremely poor fusion. Of the initial 132field samples, it was found that there exist-ed a large “gray zone” including welds withmarginally poor or marginally acceptablevisual welds that did not correlate to theactual weld integrity based on UT-TOFDscans. Of these welds, 19% deemed visual-ly acceptable by project specifications weresubsequently found to contain various UT-TOFD flaw indications, with many indicat-ing severe fusion defects. In contrast, 63%of the welds identified as being visuallyunacceptable were found to be clear of anyserious flaw indications.

Future OpportunitiesField application of UT-TOFD verifies

laboratory findings and establishes thatUT-TOFD is highly reliable at detecting,locating, and characterizing defectsincluding porosity and LOF in HDPEwelds. The finding that there is a ques-tionable correlation between visual weldbead quality and actual weld integrity isalarming. A large disparity between visu-al weld bead quality and actual weldintegrity illustrates that current industryquality practices are often inaccurate andmay be inadequate.

Through actual field implementation,we have verified first-hand, using this newmethod that a number of welds, clearedfor operation, contain severe fusiondefects. It is known that LOF defects willcompromise pipeline integrity and presenta growing and urgent operational risk.This novel UT-TOFD method alleviatesthese concerns through detection of fusiondefects, both in fabrication and in opera-tion. UT-TOFD provides an unprecedent-ed understanding of defect details thatallows for optimizing fusion techniquesand for performing analysis to calculatecrack propagation susceptibility. We can

now conduct a more reliable risk assess-ment, verify operational integrity, andensure long-term operating life of HDPEPipeline systems. P&GJ

Authors: Barry Messer is the Groupleader of Welding and MetallurgicalEngineering for Fluor Corp., Calgary,Alberta, Canada. He is a PrincipalEngineer and has over 20 years experiencein metallurgical, welding and NDE devel-opment and selection. He is regularlyinvolved in the analysis and mitigation offabrication and in-service failures. EmailBarry.Messer@Fluor.com

Matthew Yarmuch is a Materials andWelding Engineering Specialist for FluorCorp., Calgary, Alberta, Canada. He isinvolved in material selection, welding,NDE testing and failure analysis.

Peter den Boer is Manager AdvancedNDE for RTD Quality Services Inc. He isresponsible for development, implemen-tation and management of NDE servic-es. He has over twenty years experienceproviding weld inspection services andnew product development.

Fig. 22 UT-TOFD Scan