Engineering Failure Analysis 70 (2016) 375–386

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Engineering Failure Analysis

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Integrity assessment of API X70 pipe with corroded girth andseam welds via numerical simulation and burst testexperiments

Kyu Jung Yeoma, Woo Sik Kimb, Kyu Hwan Oha,⁎a Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Republic of Koreab Gas Research Institute, Korea Gas Corporation, Republic of Korea

a r t i c l e i n f o

⁎ Corresponding author.E-mail address: kyuhwan@snu.ac.kr (K.H. Oh).

http://dx.doi.org/10.1016/j.engfailanal.2016.09.0081350-6307/© 2016 Elsevier Ltd. All rights reserved.

a b s t r a c t

Article history:Received 1 June 2016Received in revised form 6 September 2016Accepted 19 September 2016Available online 20 September 2016

Welded pipes containing corrosion may lead to catastrophic situations during transportation ofgas. Appropriate methods are needed to assess defective pipelines for integrity assessment.Corroded girth and seam welds were studied for the effect of corrosion length and depth onthe burst pressure of API X70 pipe. Full-scale hydrostatic burst pressure tests and finite ele-ment analysis (FEA) were performed to investigate the burst pressure resulting from the cor-roded girth and seam welds. The experimental results were used to validate the finiteelement model, which considered the 3D elastic–plastic behavior. The FEA predicted burstpressures were compared with the ASME B31G, Modified ASME B31G, DNV RP-F101, andPCORRC standards in order to derive a proper equation for evaluation of corroded seam andgirth welds.

© 2016 Elsevier Ltd. All rights reserved.

Keywords:Burst pressureFull-scale hydrostatic burst testCorroded weldFinite element analysis

1. Introduction

Pipelines are the most economical and safest mean of gas and fuel transportation. For successful operation of gas pipelines, it isnecessary to identify the existence of defects, assess the damaged pipe for gas supply reliability and safety management, and fol-low appropriate corresponding repair procedures. While rare, operational gas pipelines can suffer from corrosion, mechanicaldamage, internal pressure, soil loading, traffic loading, ground subsidence, thermal loading, or cracking. In general, corrosion isthe most frequent problem in welded joints. The corrosion of welded joints is assessed in terms of the residual strength of thepipeline. For this purpose, a burst test is performed using a full-scale pipeline and finite element analysis (FEA), which can accu-rately simulate and predict the results.

There have been numerous studies on the strength of corroded pipes. In order to evaluate the state of an API X52 pipeline,which is a relatively low grade of pipe, Chanyalew et al. [1] utilized FEA results to propose equations in terms of the depthand length of the corrosion. Ishikawa et al. [2] analyzed weld metal, heat affected zone (HAZ), and base metal through a wideplate tensile test in the seam weld to find that the lowest stress occurs in the HAZ. Bueno et al. [3] utilized assessment equationsto conduct burst tests with five tubular API X42 pipes that contained seam welds with corrosion. Chido and Ruggieri [4] proposeda stress-based criterion based on plastic instability analysis to examine the failure pressure of corroded pipelines with axial de-fects. Moreover, Otegui et al. [5] found that sleeve fracture occurred in the weld boundary of a seam weld in API X52 pipeowing to the lack of fusion, and Fazzini et al. [6] found that cracks occur in the unwelded region because of imperfect welding.

Nomenclature

D outside diameter of the piped depth of the defectPf burst (i.e., maximum) pressureL length of the pipeR radius of the pipet wall thickness of the pipeσSMYS specified minimum yield strengthσUTS ultimate tensile strength

376 K.J. Yeom et al. / Engineering Failure Analysis 70 (2016) 375–386

Meanwhile, for girth welds, Denys and Lefever [7] found that cracks propagate from the defects through HAZ or base metal in APIX52 pipe.

Since these works were performed using small-scale experiments with relatively low-grade pipes, it is difficult to apply theresults to high-strength, high-toughness pipes with weld corrosion. Moreover, pipeline material tends to have high toughnessand strength. Organizations and pipeline companies require verification of pipelines; however, this can be performed using un-necessarily conservative evaluations or non-conservative evaluations. In particular, the corrosion of buried pipelines most oftenoccurs near girth welds, as shown in Fig. 1. The procedures, the weld shapes, and heat input differ for each company/organization.Therefore, it is necessary to verify the particulars of the weld procedure. To assess actual defects, evaluation of actual pipes andappropriate standard equations for assessment are required. In order to achieve this, the dependability of a pipe is assessed byderiving an appropriate equation through burst tests and FEA of corrosion in girth welds and seam welds of full-scale pipes.

2. Review of assessment standards for corrosion residual strength of pipeline

The integrity assessment of a pipe is determined by the ability of the pipe to withstand the pressure of the gas flowing withinthe pipe. Pipes rupture when the stress in the material exceeds its limit as the internal pressure increases. Standard methods forassessment of welded joints include the American Society of Mechanical Engineers (ASME) B31G [8], Modified ASME B31G [9],Det Norske Veritas (DNV) [10], and Pipe Corrosion Failure Criterion (PCORRC) [11] standards. For ASME B31G, which is a standardfor the assessment of corrosion defects, the residual strength of a damaged pipe is evaluated based on the pipe diameter, thick-ness, minimum allowable yield strength, depth of the corrosion defect, and length:

P f ¼2� 1:1σSMYS

Dt

� � � Rs; ð1Þ

Fig. 1. Corroded girth weld in a pipeline.

Table 1Weld consumables and welding parameters of API X70 pipe.

Weld Layer no. Welding processFiller metal

Polarity Amperage (A) Voltage (V) Welding speed (cm/min)Class Diameter (mm)

Girth weld

1 GTAW ER70S-G 2.4 DCSP 100–170 12–20 6–122 GTAW ER70S-G 2.4 DCSP 170–240 15–24 8–143 SMAW E9016-G 3.2 DCRP 80–150 20–40 3–124 & 5 SMAW E9016-G 4 DCRP 100–180 20–42 3–12

Seam weld

1 GMAW ER70S-G 1.6 DCEN 520 28 17

2 SAWF8A4-EA2 4

DCEP 820 37105

AC 660 42

3 SAWDCEP 890 37

115AC 710 42

377K.J. Yeom et al. / Engineering Failure Analysis 70 (2016) 375–386

where

Rs ¼1−

23

dt

� �

1−23

dt

� �1ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

1þ 0:8 LffiffiffiffiDt

p� �2r for

dt

� �≤0:8;

LffiffiffiffiffiffiDt

p ≤4:479; ð2Þ

or

Rs ¼ 1−dt

� �for

dt

� �≤0:8;

LffiffiffiffiffiffiDt

p N4:479: ð3Þ

The results from ASME B31G display discontinuous intervals when compared to the burst test results of full-scale pipes of cer-tain length, and it appears to be excessively conservative. Modified ASME B31G Standard compensates for this disadvan-tage:

P f ¼2� σSMYS þ 68:9MPa

Dt

� � � Rs; ð4Þ

(a) Joint design of seam weld

(b) Joint design of girth weld

Fig. 2. Joint design of seam and girth welds of API X70 pipe.

(a) Seam weld in API X70 pipe (b) Girth weld in API X70 pipe

5mm(a) 5mm(b)

Fig. 3. Microstructures of API X70 pipes with seam and girth welds.

378 K.J. Yeom et al. / Engineering Failure Analysis 70 (2016) 375–386

where

Rs ¼1−0:85

dt

� �

1−0:85dt

� �1ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

1þ 0:6275 LffiffiffiffiDt

p� �2

−0:003375 LDt

� �4r fordt

� �≤0:8;

LffiffiffiffiffiffiDt

p ≤7:071; ð5Þ

or

Rs ¼1−0:85

dt

� �

1−0:85dt

� �1

3:3þ 0:032 LDt

� �2h i fordt

� �≤0:8;

LffiffiffiffiffiffiDt

p N7:071: ð6Þ

This equation is based on modifications that reflect various burst tests of pipes, and it is less conservative than ASME B31G.The DNV equation was proposed in DNV Recommended Practices (RP)-F101 after developing the results from the Linepipe

Corrosion Project into RP. For DNV, twelve burst tests of pipes containing defects were performed with varying internal pressuresand environmental loadings. Grade X52 pipe with a diameter of 323.9 mm was used for these tests. The corrosion defect geom-etry was based on the river-bottom profile. The DNV standard is as follows:

P f ¼2t

D−tð ÞσUTS

1−dt

� �

1−dt

� �Q−1

2664

3775; ð7Þ

(a) Before test (b) After test

(c) Equipment setup

Fig. 4. Full-scale hydrostatic burst test of API X70 pipe with girth and seam welds.

Table 2Geometry and results of the full-scale hydrostatic burst test for the girth and seam weld defects of API X70 pipe.

Defect location Defect geometry (mm) Burst pressure (MPa)

Girth weldLength 300 × Width 50 × 0.5 (d/t)

21.2Seam weld 17.7

379K.J. Yeom et al. / Engineering Failure Analysis 70 (2016) 375–386

where

Q ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1þ 0:31

LDt

� �2s

fordt

� �≤0:85 : ð8Þ

The PCORRC equation is an assessment standard developed by Battelle Memorial Institute based on burst tests of full-scalepipes and FEA that were conducted in response to a request from the Pipeline Research Council International. Comparing thetest results to the predictions, ASME B31G and Modified ASME B31G produce accurate predictions for low-toughness pipes, butthe PCORRC equation leads to more reliable predictions for high-toughness, high-strength pipes [12]:

P f ¼ σUTS2tD

1−dt

1− exp −0:157Lffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

R t−dð Þp

! ! !: ð9Þ

(a) Girth weld

(b) Seam weld

0.0 0.2 0.4 0.6 0.8 1.0 1.2400

600

800

1000

Tru

e St

ress

(MPa

)

True Strain (%)

Base metal of API X70 pipe Girth Weld of API X70 pipe HAZ of API X70 pipe

0.0 0.2 0.4 0.6 0.8 1.0 1.2400

600

800

1000

Tru

e St

ress

(MPa

)

True Strain (%)

Base metal of API X70 pipeSeam Weld of API X70 pipeHAZ of API X70 pipe

Fig. 5. True stress–strain curve of the base metal with API X70 pipe for girth and seam welds.

(a) Seam weld (b) Girth weld

Fig. 6. Diagrams of tensile specimen with seam and girth welds of API X70 pipe.

380 K.J. Yeom et al. / Engineering Failure Analysis 70 (2016) 375–386

3. Full-scale hydrostatic burst test and FEA

The burst pressure was identified by varying the pressure on girth and seam welded pipes using water. The welded regions inthe pipes were categorized in terms of welding direction into seam welds in the axial direction and girth welds in the circumfer-ential direction and in terms of welding into base metal, weld metal, and HAZ. This method was used to test corroded or devel-opment pipelines by the hydrostatic burst test [13–15].

Manual weldment was used for the HAZ for girth and seam welds. The weld consumables and welding parameters are shownfor API X70 pipe in Table 1. The joint designs of the girth and seam weld are shown in Fig. 2. The microstructures of the API X70pipe with seam and girth welds are shown as Fig. 3. In the case of domestic pipelines, the implement of radiographic testing isconducting after welding of the pipeline. Thus, the pipelines were assumed to initially have no defects.

Corroded regions were synthesized through mechanical processing of the girth and seam welded centers of pipes. The defectdepth was 7.95 mm with a defect depth to a thickness ratio of 50%, and the length was 300 mm. The pipes were manufacturedfrom two cuts of API X70 pipe that were 2.3 m in length, 762 mm in diameter, and 15.9 mm in thickness to measure their burstpressure with weld caps. During this process, the internal pressure was increased at a rate of 0.25 MPa/min up to 19.6 MPa and ata rate of 0.15 MPa/min up to 28.44 MPa. Fig. 4(a) shows the welded pipe with mechanically produced corrosion, Fig. 4(b) showsthe fracture in the corroded weld after burst test, and Fig. 4(c) shows the apparatus used for the burst test. The defect geometryand results of the burst test are presented in Table 2.

The residual thickness of the corrosion defect in the weld was measured an using ultrasonic thickness gauge. A slight deviationin thickness was observed, with the actual depth of corrosion in the seam weld metal being 45%, which is attributed to

(a) Girth weld (b) Seam weld

Fig. 7. FEA diagrams of girth and seam weld corrosion.

Table 3Geometry of girth and seam weld defects in the FEA.

MaterialGeometry of corrosion defects

L (mm) d (mm)

Girth weld

50 3.98100 5.96150 7.95200 9.94300 11.93500 13.91

Seam weld

50 3.98100 5.96150 7.95200 9.94300 11.93500 13.91

381K.J. Yeom et al. / Engineering Failure Analysis 70 (2016) 375–386

misalignment and welding beads. In order to compensate for the difference between the anticipated corrosion depth and the ac-tual corrosion depth, the burst pressure was deduced after normalizing the corrosion defect depth ratio to 50%.

Since it would have been both time and cost intensive to apply various defect conditions to the welded joint for burst tests offull-scale pipes, FEA was performed to consider various defect conditions.

Engineering stress–strain curves obtained from tensile tests of base metal, weld metal, and HAZ of API X70 pipe were con-verted to true stress–strain curves and applied in FEA, similar to previous research [16–19], in order to assess the failure of

(a) API X70 pipe with girth weld

(b) API X70 pipe with seam weld

Fig. 8. FEA-based burst pressure results of the API X70 pipe with different defect depths and lengths of girth and seam welds.

(a) Girth weld

(b) Seam weld

Fig. 9. Corroded pipe containing girth and seam weld defects in API X70 with confirmation between full-scale hydrostatic burst test and FEA results.

382 K.J. Yeom et al. / Engineering Failure Analysis 70 (2016) 375–386

pipelines. Fig. 5 shows the true stress–strain curves for base metal, weld metal, and HAZ. Rod-type subsize specimens (A:32 mm; D: 6.25 mm; R: 6 mm) were used for the tensile tests of base metal and weld metal. A plate-type subsize specimen(6.3 mm in width and 2.0 mm in thickness) with girth weld based on ASTM A370 [20] was used for the tensile test of HAZ.The tensile test specimens of base metal, weld metal, and HAZ were polished and etched, and their microstructures were ob-served, as shown Fig. 3; accordingly, the acquired tensile test specimens of 100% base metal, weld metal, and HAZ are shownin Fig. 6.

For comparison with the burst pressure obtained from actual burst test, the burst pressure of the pipe was predicted throughFEA using von Mises criterion, which is capable of producing the closest prediction to the actual burst [21]. PARTRAN 2008 [22]was used to model the pipe, and ABAQUS 6.10 [23] was used as the FEA software. During this process, C3D8R elements (i.e., 8-node linear brick, reduced integration, hourglass control) were used. The API X70 pipe used for analysis was 762 mm in diameter,15.9 mm in thickness, and 3000 mm in length. Considering the loads on the weld corrosion and boundary conditions, the modelwas built at 1/4-scale. The load condition considered applies pressure to the inside of the pipe with end forces enforcing theclosed-cap condition [24,25]. Diagrams of each welded region are shown in Fig. 7.

The depths of the girth and seam weld corrosion were varied from 25% of the pipe thickness to 75% in increments of 12.5%,and the length was varied from 50 mm to 500 mm. Table 3 shows the FEA parameters pertinent to girth and seam weldcorrosion.

Pipe fracture was assumed to occur when the stress in each defect reached the ultimate tensile strength (UTS) with the in-creasing internal pressure. Namely, the lowest pressure responsible for stress reaching the UTS in the base metal, weld metal,or HAZ was taken as the burst pressure, and the region that reached the tensile strength first was considered to fracture first.

The burst pressures of girth and seam weld corrosion compared to length and depth of defect are shown in Fig. 8. As the de-fect depth-to-thickness ratio increases, the weldment region of a girth weld decreases. The girth weld was formed using gas tung-sten arc welding (GTAW) from layer 1 to 2, which is different from the seam weld. Because the welding method is different, theburst pressures of the different defect depth-to-thickness ratios of 0.5 and 0.625 were observed to decrease rapidly. However, fur-ther study is needed to validate this behavior.

As the defect length increases, the burst pressures of both girth and seam welds decrease. Moreover, as the defect length in-creases, the burst pressure of the pipe also decreases. Fig. 9 shows a comparison of the FEA results with a defect depth-to-

(a) Girth weld of API X70: 25% defect depth

(c) Seam weld of API X70: 25% defect depth

Est

imat

edB

urst

Pre

ssur

e(M

Pa)

0

5

10

15

20

25

30

Est

imat

ed

0 200

Defect Length (mm)

400 600 800

AASMEPCORDNVFEM

E B31RRC

1000

G

(b) Girthweld of API X70: 75% defect depth

(d) Seam weld of API X70: 75% defect depth

1

1

2

2

3

Est

imat

ed B

urst

Pre

ssur

e (M

Pa)

Est

imat

ed B

urst

Pre

ssur

e (M

Pa)

00

5

10

15

20

25

30

00

5

10

15

20

25

30

0

200

2

0

D

200

40

Defect Length (mm)

De

00

400

efect

6

Len

600

60

gth (

00

(mm

800

ASMEPCORDNVFEM

8

)

E B31RRC

800

ASMEPCORDNVFEM

1000

G

E B31RRC

1000

1G

Fig. 10. Comparison of burst pressure from FEA results and results obtained using equations with different depth-to-thickness ratios with API X70 pipe.

383K.J. Yeom et al. / Engineering Failure Analysis 70 (2016) 375–386

thickness ratio of 50% and the results from full-scale hydrostatic burst test. These results validate the FEA; based on this finding,the weld corrosion assessment standards are compared to the FEA results for verification.

4. Comparison between assessment standards and FEA

Fig. 10 shows the ASME B31G, modified ASME B31G, DNV, and PCORRC assessment standards along with results from the FEAof corrosion defects in seam and girth welds of API X70 pipe.

The overall trends with change in defect depth are all similar. The burst pressures of the girth-welded and seam-welded pipesappear to be lower than that of base metal corrosion in identical pipes presented in a previous study [26], and the relative trendsin burst pressure with increases in length and depth are similar. It is believed that these trends are the consequence of relativelylower yield strength of HAZ compared to the base material. They are also affected by the FEA representation of the material prop-erties of the weld metal. While the pipe burst pressure decreased as defect length increased for all cases (FEA and assessmentequations), the PCORRC equation most accurately matched the FEA results.

To improve the PCORRC equation, the results of the present study were used to adjust its parameters:

Pf ¼ C1σUTS2tD

1−dt

1− exp −C2Lffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

R t−dð Þp

! ! !; ð10Þ

where C1 is used to scale the UTS and C2 replaces the original factor of 0.157. Nonlinear curve fitting was performed by varyingthe parameters for girth welds and seam welds, which are illustrated in Fig. 11 and Fig. 12, respectively.

In Fig. 11(a) and Fig. 12(a), it can be observed that applying 95% of the UTS leads to results that are not conservative in someparts. Since pipeline integrity assessment requires conservative evaluation, 90% of the UTS was applied to the modified PCORRCequation, which allows for conservative assessment, as shown in Fig. 11(b) and Fig. 12(b). Therefore, C1 was set as 0.90 in theassessment equation. Furthermore, C2 appeared to vary within the range of 0.142–0.224, depending on depth. However, for con-servative evaluation for all ranges of defect depths and lengths, 0.224 was used, as it is the highest. Therefore, the obtained

(a) 95% UTS of PCORRC

(b) 90% UTS of PCORRC

Fig. 11. Obtained PCORRC equation for API X70 pipe with a girth weld.

384 K.J. Yeom et al. / Engineering Failure Analysis 70 (2016) 375–386

conservative assessment equation for girth and seam weld corrosion is

P f ;Girth;Seam ¼ 0:902tDσUTS 1−

dt

1− exp −0:224Lffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

R t−dð Þp

! ! !: ð11Þ

5. Conclusion

The corroded girth and seam welds of API X70 pipe were studied using full-scale hydrostatic burst test and FEA for varyingcorrosion depths and lengths. The tests were performed with girth and seam welds by increasing internal pressure. The bursttests were used to validate the FEA, which was then used for comparison to assessment equations for corroded welds pipes:ASME B31G, Modified ASME B31G, DNV and PCORRC. Using the FEA results, the PCORRC equation, which offers the highest accu-racy, was modified to further improve its accuracy in predicting the burst pressure of corroded girth and seam welds in API X70pipe.

Acknowledgements

The authors are grateful for the support provided by a grant from the Korea Gas Corporation (KOGAS).

(a) 95% UTS of PCORRC

(b) 90% UTS of PCORRC

Fig. 12. Obtained PCORRC equation for API X70 pipe with a seam weld.

385K.J. Yeom et al. / Engineering Failure Analysis 70 (2016) 375–386

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