First Surveys Run With Electro-magnetic Acoustic TransducersBy Dr. Joerg Damaschke and Thomas Beuker, Rosen Technology & Research Center, Germany

runs of its EMAT crack detec-tion and coating disbondment tool,RoCD2, one in a gas pipeline and

another in an oil pipeline, both owned bySaudi Aramco. The inline inspection tool forthe detection of stress corrosion cracking(SCC) and coating disbondment based onelectro-magnetic acoustic transducers was setLIp on the basis of a high-resolution approach.Rosen was invited by Saudi Aramco to test thetechnology in the Saudi Ararnco transmissionpipeline network. Rosen previously had suc-cessfully tested its 16-inch RoCD 2 thoroughlyin sample pipes containing real SCCs and vari-OLIs types of artificial defects.

This article introduces some parts of the dataevaluation process used and presents first resultsobtained from these two field tests. An extensivefollow-up validation program is in progress.

Commonly, non-destructive ILl tools arebased on technologies such as magnetic fluxleakage (MFL), ultrasonic testing (UT) or eddyCurrent systems. However, none of these tech-niques is applicable to the detection of SCC,especially in gas pipelines. Rosen has devel-oped a new type of ultrasonic sensor that isbased on an electro-magnetic acoustic transduc-er (EMAT) (Klann and Beuker, 2006). By uti-lizing physical effects such as the Lorentz forceand magnetostriction, this technology allows,unlike conventional UT, contact free-generationand observation of ultrasonic signals. Since thepipeline serves as its own transducer, this newapproach works independently of a couplingmedium between the sensors and the pipeline tobe inspected. A 16-inch tool was manufacturedand equipped with EMAT sensors (Fig. 1).

rlg, i. i. uz,ýl ii U-ilUi I MIM uILACLU;I UUMLULIUHIand coating disbondment tool RoCD 2

EMAT Module ArrangementFollowing a high-resolution approach (Fig.

2), numerous EMAT modules were arrangedon the inline inspection tool.

Fig. 2: Low-resolution approach (left) andhigh-resolution approach (right)

Fig. 3 shows the basic arrangement of theEMAT modules used to inspect a distinct area(pixel) of the pipeline. The ultrasonic wavesdo not travel around the whole circumferenceof the pipeline before they are observed by areceiver. Rather, the acoustic waves only travela short distance between the EMAT senderand the receiver thereby allowing compara-tively simple data evaluation and avoiding falsealarms. The sensor arrangement required toinspect one pixel of the pipeline consists of oneEMAT sender (left hand side) and two EMATreceivers (one on the left hand side, one on theright hand side). The EMAT sender generatesa tailored shear horizontal wave which is char-acterized by distinct frequencies and thereforeespecially sensitive to near-surface defects.

Pixel

Sender

Echo

//Crack ToRun Direction

Fig. 3: Basic arrangement of threules comprising one sender motwo receiver modules (one left, o

Crack DetectionThe generated wave propag

EMAT sender on the left-hanthe EMAT receiver on the righino cracks are present, this wa)receiver and is recorded as a smission signal. However, if thelike defect between the EMATopposite EMAT receiver, partsenergy are reflected in the diiEMAT sender. There, this signala so-called echo signal by thereceiver. This means that twochannels exist for each pixel, natand one transmission channel.

From these data channels, nuparameters can be extracted, equencies, signal amplitude, tra)

the acoustic wave and so on. Unlike an MFLmeasurement, not only one value (magnetiza-tion level) is recorded at one particular pipelineposition, but several vectors (time signals), thusproviding much more information.

Additional information, e.g. lift-offbetween the EMAT modules and the pipe-line, is stored in separate data channels. Thisindependent data storage ensures that echoand transmission data can unambiguously beevaluated with respect to the success of thephysical measurement.

TransmissionSignal Evaluation

The transmission channel contains infor-mation about the wave that directly propa-gates from the EMAT sender to the transmis-sion receiver. The overall amplitude of thiswave depends on the amount of lift-off, thepresence of a defect, and the existence (andtype) of external coating. The latter depen-dency can be used to detect coating disbond-ment, since a coating generally damps theacoustic wave. Hence, if the damping effectis missing due to a reduction in the bondingquality of the coating, a significant increasein the signal amplitude can be observed.Distinct examples for several cases areshown below.

I axs Echo Signal EvaluationFrom Fig. 3 it can be seen that an echo signal

will only be recorded if a significant amount ofenergy is reflected into the EMAT echo receiver.Since the echo receiver is active for a short time

Transmission interval, only signals that are reflected from aspecific position relative to the sensor within

ee EMAT mod- the pipeline are detected. Consequently, otherdule (left) and signals emitted from adjacent EMAT senders orne right), late reflections from other positions within the

pipeline can easily be excluded during the dataevaluation process.

ates from the Due to the arrangement of the EMAT mod-d side toward ules, the system is especially suitable for thet-hand side. If detection of features with an axial dimension.e reaches the A detailed analysis of significant echo signals

o-called trans- signal amplitude, arrival time and frequen-re is a crack- cy content - provides valuable information

sender and the about the type of defect detected.of the signal

rection of the First Surveysis recorded as First inspections employing the RoCD 2 16-

second EMAT inch tool were performed in May 2006, one in aacoustic data gas pipeline and another in an oil pipeline. Both

mely one echo runs were evaluated using automated algorithmsdeveloped during the project. For example,

merous signal girth welds can be detected easily since they.g. signal ftc- cause typical signal characteristics in differ-eling time of ent data channels (transmission channel, echo

Pipeline & Gas Journal / October 2006 / www.pgjonline.com 55

Figure 4: poC-scan of the echo channels of about 16 meters ofthe gas pipeline (circumferential angle as a functionof the log distance). EMAT echo signals were inte-grated to obtain one single value for each particularpipeline position. At several positions, echo signalsclearly stand out from the background noise, e.g.at the end of the joint (girth weld) or at the linearanomaly around 70 degrees. The three insets showsketches of the recorded wave signals at three par-ticular pipeline positions: no-anomaly signal (left),strong linear anomaly signal (center), weaker linearanomaly signal.

Z5

o0

logdistance in rn

channel, lift-off channel). Similarly, long seamscan be observed in echo channels (increase) andtransmission channels (decrease).

Fig. 4 shows a C-scan view of one partof the gas pipeline. For this plot, the EMATecho signals obtained from each particularpipeline position were integrated to form onesingle value. A C-scan can thereby be gener-ated. Recorded wave signals obtained fromthree particular positions within the pipelineare shown as examples in the insets of Fig.4. The high-resolution approach covers thefull circumference with 36 channels. As canbe seen from Fig. 4, several significant signalincreases can be observed. For example, sincegirth welds are good reflectors of acousticwaves, they can easily be identified in the echodata. At the end of the joint shown in Fig. 4(lower right corner), another significant echosignal can be observed. This echo is generatedby a linear anomaly in the pipeline.

091

0)

0 s

Iogdistance in m

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4 Figure 5: Details ofthe anomaly shown inthe lower right cornerof Fig. 4. The upper twoillustrations representC-scan views of theintegrated echo signals(left) and transmissionsignals (right). The twolower illustrations shownon-integrated echodata as a function ofthe log distance of thechannel at 75 degrees.The left illustrationshows the time signalswhile the right onedepicts the correspond-ing spectra.

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56 Pipeline & Gas Journal / October 2006 / www.pgjonline.com

Fig. 5 consists of four individual illustra-tions which provide additional informationon the particular anomaly shown in Fig. 4.While the two upper graphic representa-tions show integrated data of the echo (left)and the transmission data (right) as a func-tion of the circumferential position and thelog distance, the two lower images shownon-integrated vector data of one specificchannel (the channel at 75 degrees). The

lower left illustration shows the echo timesignals as a function of the log distance,whereas the lower right image representsthe corresponding signal spectra.

Time domain signal analysis allows col-lection of information about the orientationof the defect in relation to the pipe axis.This means that the echo channels aresensitive to defects in both the axial andcircumferential direction.

logdistance in mA Figure 6: C-scan view of the transmission channel of one particular joint of the gas pipe-line (circumferential angle as a function of the log distance; represents a different part of thepipeline from that shown in Fig. 4). The regular shape (stripes) of the signals is due to thetape coating. Significant decreases in amplitude can be observed at the beginning and endof the joint owing to lift-off effects. At the long seam, a large amount of the signal energy isreflected into the echo channels. As a result, the transmission signal amplitude is decreased.In contrast, increased transmission signal amplitudes indicate areas of weaker or even loosecoating as shown at the beginning and end of the joint.

Fig. 6 shows transmission data of anotherjoint as a C-scan view. Decreased signalamplitudes can be observed at the girthwelds and the long seam, since ultrasonicenergy is reflected at these positions. As aresult of the reflection, this energy does notreach the transmission receiver. The regularshape (stripes) in the transmission signals isgenerated by the tape coating. Red-coloredareas, e.g. at the beginning and end of thejoint, indicate a weaker or even loose coat-ing. This means that the transmission chan-nels are on the one hand - sensitive tolarger reflectors (signal decrease) and onthe other hand - to different coating quali-ties (signal increase if coating is weaker).

ConclusionA novel high-resolution EMAT technology

has been developed and implemented on anILI-tool by Rosen. The intelligent inspectiontool has been run successfully in two different16-inch pipelines, a gas pipeline and an oilpipeline, both operated by Saudi Aramco. Thepromising results of the first inspection surveyare being further validated. The data evaluationcan rely on multi-dimensional data sets, whichallows a continuous improvement, on the basisof the scheduled validation program.P&GJ

REFERENCES:

Klann and Beuker. 2006, "Pipeline Inspection With TheHigh Resolution EMAT ILl Tool: Report On Full-ScaleTesting And Field Trials," IPC2006-10156, in: Proceedingsof IPC 2006, 6th International Pipeline Conference, Sept.25-29, 2006, Calgary, Alberta, Canada

Pipeline & Gas Journal / October 2006 / www.pgjonline.com 57

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TITLE: First Surveys Run With Electromagnetic AcousticTransducers

SOURCE: Pipeline & Gas Journal 233 no10 O 2006PAGE(S): 55-7

WN: 0627401199009

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