..Technical Paper 33

Ultrasonic Guided Wave Phased Array Inspectionof Pipelines

S. Palit Sagar, Jia Jerry Hua and Jo eph L Rose"

'Scientist Ell, Materials Science & Technology Divi ion, National Metallurgical Laboratory, Jamshedpur 831007, India"Paul Morrow Profe or, Department of Engineering Science & Mechanics, The Penn ylvania State University, PA l6802,USA

ABSTRACTPiping systems are often inspected ultrasonically to ensure afety. This can be accomplished by a series of point to point testsfrom the out ide surface of the pipe. If coating cover the pipe, as is often the case, access LO the outside surface requires removalof the coating to perform the test, and then re-installation when testing is complete. Removal and reinstallation of coating isnot only time consuming but in most cases it is prohibitively expensive too. Ultrasonic guided waves provide an attractivealternative solution to the basic bulk wave ultrasonic test. Using Guided Waves, a probe can be applied to the pipe at a singlelocation and several meters of the pipe can be in pected. The coating is only removed where the probe is applied. This paperdescribes the visco-elastic properties of two different coating materials: Bitumen tape and wax tape is considered, an FEMsimulation of the propagation ofaxi ymmetric guided wave through bare and coated pipes having different coating thicknesses.The ABAQUS FEM code is used and validation of FEM simulation re ult through experiment in bare and coated pipes usingguided wave inspection sy tern is reported.

1. INTRODUCTION

The oil, gas, chernical and petro-chernical industries operatehundreds of ki lometres of pipelines for transportingchernicals, oil, water, and other necessities. Testing ofthese large structures using conventional bulk ultrasonicwave techniques is slow because the test region is Iirnitedto the area immediately surrounding the transducer.Therefore, scanning is required if the entire structure is tobe tested. Moreover, a high proportion of industrialpipelines are insulated using visco-elastic coatings, so thateven external corrosion cannot readily be detected withoutthe removal of the coating, which in most cases isprohibitively expensive. Ultrasonic guided waves providean attractive alternative solution to this problem [1-7]."Guided Waves" are ultrasonic waves guided by thegeometry of the object in which they propagate. Due todecreased attenuation loss. these waves transrnit along thewhole circumferential ection of the pipe while propagatingin the axial direction. These waves travel across the straightstretches of pipe to several meters from a single pointusing a pulse-echo transducer bracelet wrapped around apipe [6]. Current long-range guided wave techniques forpipeline inspection include axisymmetric and non-axisymmetric waves with partial loading and phased arrayfocusing [5, 6]. It has been shown by Li and Ro e [5]that among the e two techniques, the focusing techniquecan increase energy impingement, locate defects andgreatly enhance the inspection sensitivity and propagationdistance of guided waves, thus consequently reducingin pection cost. Viscoelastic coatings, such as bitumenand wax tape are commonly used for protection againstcorrosion in the pipeline industry. The presence ofviscoelastic coating results in changes of guided wavepropagation characteristics. Because of a variation ofcoating material and the complexity of the wave mechanicsin a viscoelastic coated multilayered structure, many

Vol. 9, Is ue 2 September 2010

aspects and questions on guided wave inspection in coatedpipe are still untouched and remain quite challenging. Inthis work, a three-dimensional finite element method isstudied for modeling guided waves in a bare pipe and alsofor Bitumen (BT) and Wax tape (WT) coated pipes. Visco-elastic properties of the BT and WT materials were tudiedand the experiment were performed on 4inch bare andBT and WT coated steel pipes using 8 channelaxysymrnetric torsional guided waves to validate the FEMresults for practical applications.

2. SELECTION OF APPROPRIATE GUIDEDWAVE MODE

Guided waves in the axial direction of a hollow cylinderinclude longitudinal and torsional waves with bothaxisymmetric and nonaxi ymmetric modes. Typicaldi per ion curve in a bare pipe are plotted in Fig. I,showing various wave modes in a pipe.

O+---~----~----~---T----~----~--~o os 0.8 0.70.1 0.2 03 0..4F"~ncv (MHz)

Fig. 1 Phase velocity di persion curves ofaxysymmetric andflexural mode in lO-in. chedule 40 steel pipes [5]

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34 Technical Paper

Table I: Elastic Properties of coating materials

Material Cs(mm/ms}

aL(w)l~(1/mm)

Youngs Modulus Poissons's(GPa) Ratio

CL( mm/rn s}

as

35Technical Paper--------------------------------------------------~--------------------~4.1.1 FE modeling of bare and 8T and WT coated pipe with

torsional guided waves

A 40 kHz Hanning windowed 7 cycles tone burst signalwas applied in the circumferential direction at one end ofthe test pipe and the signal was received at the other endof the pipe as depicted in Fig. 3.

Fig. 3 : Simulated coated pipe with axial load applied at one end

yoO.231689122>< o

2 2.5 31.5

Coating Thickness (mm)

Fig. 4 Variation of wave attenuation with coating thickness forBT and WT coated teel pipes

Fig. 5 Contracted view of the pipe showing energy leakage fromthe pipe wall into the wax coating

From the received signals, wave attenuation was determinedfor bare and BT and WT coated pipes with Imm, 2mmand 3 mm coating thicknesses and plotted in Fig. 4.

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3.5

Fig. 6 Photographs of tested bare and BT and WT coated pipes

Result shows that the wave attenuation increases withcoating thickness and the attenuation is higher in WTcoated pipe compared to the BT coated pipe. A contractedview of Von-Mises stress wave propagation throughsimulated 3mm WT coated 3.2m long steel pipe is presentedin Fig. 5 to show the stress leakage from the pipe into thecoating.

300_Bare Pipe

2000 000 600 8000 000 1200 14000

TIme (sec)

200

! 100"'C"""Q.~ -100

-200

-300

200,_----------------------------------,-BTCoated

100

Time (see)

2000 000 6000 8000 10000 12000 14000

-100

-200 '------------------------------------'

~,-----------------------------------~

100

_WTCoaled

;;-.§..,'C

.~Q.E«

o t-----tt.....-ooo--------+

2000,.... 6000 8000 1()()()() 12000 14000

Time (see)-100

-200 .'-- -'

Fig. 7 Received signals from bare, BT and WT coaled 3.2m longpipe using low frequency torsional mode guided wave

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36 Technical Paper

4.1.2 Experimental validation of FEM simulation

Experiments were performed on 3.2m long, 41n.Schedule40 bare, 1.8 mm bitumen tape coated and 2.2nun wax tape coated steel pipes using the 4 channel lowfrequency ultrasonic guided wave system. Figure 6 showsthe photographs of the test pipes and the probe assemblyof the test system.

The guided wave in a torsional mode was used and thetest results of bare, BT coated and WT coated pipes at afrequency of 40 kHz are shown in Fig. 7.

Wave attenuation as determined from the received signalsfor Bare, BT and WT coated pipes were O.ldB/m, 1.01dB/m and 1.87dB/m respectively. Table 4 gives the comparisonof the wave attenuation as determined from FEM simulationand experimental data. The experimentally determinedattenuation (dB/m) values were incorporated in the graphas shown in Fig. 4.

Table 3: Comparison of FEM simulation and experimentaldata of wave attenuations in bare and BT and WT'coated pipe

Material Coating Attenuation dBlmthickness

(mm) FEM Experimentalsimulation

0 0.25 0.1

1.8 0.92 i.o 1

2.2 1.71 1.87

Bare

BT Coated

WT Coated

5. CONCLUSIONS

Torsional mode guided wave ultrasonic was used to inspectthe 3.2m long 4inch Schedule40 steel pipe with bitumenand wax tape coating of various thicknesses. FEM

Journal of Non destructive Testing & Evaluation

simulation shows that the single probe positioned guidedwave technique can inspect the long coated pipes withoutremoval of coatings, which is otherwise not possible bypoint to point inspection using bulk ultrasonic measurementtechnique. Experimental results successfuUy validate theapplicability of guided wave technique in long range pipeinspection with visco-elastic coatings.

ACKNOWLEDGEMENTS

This work is a part of the Raman Research Fellowship ofthe principal author, spon ored by Council of Scientific &Industrial Research (CSIR) , Govt. of India. Authors alsoacknowledge the help from the graduate students ofUltrasonic group, Dept. of Engg. Science & Mechanics,The Pennsylvania State University and staffs of FBS Inc.,Pennsylvania for carrying out guided wave experiments.

REFERENCES

1. Joseph L Rose, Ultrasonic Waves in Solid Media, CambridgeUniversity Press, Book

2. J. L. Rose, IEEE Ultrasonics Symposium, (1995) 761

3. Wei Luo and J. L. Rose, 1. Acoust. Soc. Am. 121 (2007) 1945

4. H. Kwun, S.Y. Kim, M.S. Choi, S.M. Walker, NDT&EInternational 37 (2004) 663

5. J. Li and J. L. Rose, 1. Acoust. Soc. Am. 109 (2001) 457

6. 1. Li and J. L. Rose, IEEE Trans. Ultrason. Ferroelectr. Freq.Control, 48 (2002)761

7. M.J.S. Lowe, D.N. Alleyne, P. Cawley, Ultrasonics. 36 (1998)147

8. Mu Jing, "Guided Wave Propagation and Focusing in ViscoelasticMultilayered Hollow Cylinders", Ph. D thesis, EngineeringMechanics, The Pennsylvania State University, 2008

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