Engineering Failure Analysis 57 (2015) 553–561

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Engineering Failure Analysis

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Integrity assessment of a corroded API X70 pipe with a singledefect by burst pressure analysis

Kyu Jung Yeoma,b, Young-Kwang Lee c, Kyu Hwan Oha, Woo Sik Kimb,⁎a Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Koreab Research and Development Center, Korea Gas Corporation, Koreac Research and Development Center, POSCO Engineering & Construction, Korea

a r t i c l e i n f o

⁎ Corresponding author.E-mail address: wskim@kogas.or.kr (W.S. Kim).

http://dx.doi.org/10.1016/j.engfailanal.2015.07.0241350-6307/© 2015 Elsevier Ltd. All rights reserved.

a b s t r a c t

Article history:Received 7 April 2015Received in revised form 10 July 2015Accepted 11 July 2015Available online 26 July 2015

Gas pipes invariably develop defects. Assessment and repairing methods vary depending on thedefect type. When evaluating corrosion in the base metal of a pipe, the burst pressure is usuallycalculated using conventional evaluation equations, and the possibility of operating the pipe is de-termined. These equations are applied differently depending on the pipe's physical properties andenvironment. Therefore, in this study, the failure behavior and burst pressure of a pipe are deter-mined through a full scale hydrostatic burst test; the results are used to simulate the lengths anddepths of defects through the finite element method for the shapes and physical properties iden-tical to those of a full-scale pipe. Thus, the corrosion in the base metal of an API X70 pipe wasassessed by a stress-based method, and an evaluation equation suitable for use was proposed.

© 2015 Elsevier Ltd. All rights reserved.

Keywords:API X70Corrosion defectFinite element methodFull-scale hydrostatic burst test

1. Introduction

Onshore and offshore pipelines have been recognized as an economic method of transporting oil and gas as the consumptionof natural gas has increased sharply. Accordingly, storage and transportation of gas have become very important issues. Pipe-lines of gas are used to supply natural gas, which is the fundamental source of energy in South Korea, and as the gas pipe networkexpands, a stable supply of natural gas and safe pipe operation become essential. Because gas pipes are exposed to the generalenvironment, defects are inevitable. Defects are assessed based on the predicted burst pressure of a corroded pipe. The AmericanSociety of Mechanical Engineers (ASME) B31G standard [1] is the most widely used equation for assessing pipe corrosion. Inorder to obtain a more accurate assessment of corrosion, a modified B31G [2] is used with a bulging factor and flow stress.Det Norske Veritas (DNV) and BG Technology developed a corrosion assessment guideline, DNV-RP-F101 [3], based on a full-scale experiment and finite element method (FEM). The Pipe CORRosion Criterion (PCORRC) method was developed usingFEM and full-scale experiments [4]. The numerous experiments and numerical simulations have been performed recently. A sig-nificant amount of experimental and numerical works have been performed recently. The limit load solution was developed forcorroded API X65 pipelines [5]. An external corrosion defect was studiedwith small-scale tests and a nonlinear numerical model[6]. Longitudinal metal loss was studied to compare burst tests and regression equations [7]. The assessment of defects withinpipelines presented the best practice for assessing corrosion [8]. The assessment of a corroded pipeline was conducted withFEM [9]. Nonlinear FEM was applied to assess a corroded pipeline [10]. Defects in a pipeline were studied with changes in op-erating conditions [11]. A regression equation was developed using Ramberg–Osgood hardening stress–strain relation [12].

Nomenclature

D outside diameter of the piped depth of the defectpf burst pressure or maximum pressureL length of the pipeR radius of the pipet wall thickness of the pipeσy yield strengthσSMYS specified minimum yield strengthσUTS ultimate tensile strength

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Nevertheless, these studies did not consider high strength pipelines. The failure pressure is an important factor in the predictionof corroded pipelines. In order to provide an appropriate assessment of corroded pipelines, this study investigated the failurebehavior and failure pressure of an API X70 pipe using a full-scale hydrostatic burst test and FEM.

2. Evaluation equations for corrosion defects

When a pipe defect is detected, an integrity assessment must be carried out to determine whether to change or repair the pipedepending on the type and degree of damage. The criteria of integrity assessment are based on the method of measuring burst pres-sure according to the size and shape of pipe damage and residual thickness of the pipe. The ASME B31G [1], Modified ASME B31G [2],DNV [3], and PCORRC [4] equations are used as the burst pressure evaluation equations. Explanations of these equations are providedin the following.

The most fundamental evaluation equation of burst pressure was developed at Battelle Columbus Laboratory for a project by theAmerican Gas Association [13]. This equationwas developed using the toughness facture equation of a pressurized pipewith a length-wise defect. The pipe materials of Grade B and X52 were used for assessment, and the equation is given by Eq. (1). The variables inEq. (1) are described in Eq. (2) and Eq. (3).

Table 1Geomet

Mater

API X

σ ¼ σ1−

dt

1−1M

dt

� �2664

3775 ð1Þ

In the case of flow stress, the relationship between yield strength and ultimate tensile strength is shown in Eq. (2) asfollows.

σ ¼ σy þ σUTS

2ð2Þ

Eq. (3) is used to determine the bulging correction factor.

M ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1þ 0:52

LffiffiffiffiffiffiDt

p� �2

sð3Þ

where M is the Folias factor, and it indicates the stress concentration of a pipe under internal pressure.In addition,many evaluationmethods such asModified ASME B31G and PCORRC predict the burst pressure of a pipe by using var-

ious bulging factors.The US standard ASME B31G is the most well-known equation for evaluating burst pressure. The ASME B31G equation, de-

veloped approximately 30 years ago, was derived by using the burst test results of 47 full-scale pipes. ASME B31G defines thecorrosion defect depth d and corrosion defect length L for a parabolic corrosion defect. Irregular defect shapes are simplified

ry and results of the full-scale hydrostatic burst test for the corroded API X70 pipe containing a single defect in API X70.

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

70 Base metal Length 300 × Width 50 × 0.5 (d/t) 21.5

Fig. 1. The full-scale hydrostatic burst test of a corroded API X70 pipe containing a single defect.

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into parabolic shapes. The defect depth is (2/3)dL, and the longest length is used as the defect length. In the assessment, a longdefect is assumed to be rectangular.

P f ¼2� 1:1σSMYS

Dt

� � � Rs ð4Þ

Rs ¼1−

23

dt

� �

1−23

dt

� �1ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

1þ 0:8 LffiffiffiffiDt

p� �2r for

dt

� �≤ 0:8;

LffiffiffiffiffiffiDt

p ≤ 4:479 ð5Þ

Rs ¼ 1−dt

� �for

dt

� �≤0:8;

LffiffiffiffiffiffiDt

p N4:479 ð6Þ

Fig. 2. The true stress–strain curve of the API X70 pipe.

Table 2Mechanical properties of the API X70 pipe.

Young's modulus Poisson's ratio Yield strength Tensile strength

E (MPa) ν σy σuts

207000 0.3 532.2 626.8

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Pf in Eq. (4) represents the burst pressure, and RS in Eqs. (5) and (6) represents the Folias factor, which is applied accordingto the defect length, defect depth, pipe thickness, and pipe diameter. As shown in Eqs. (5) and (6), the respective evaluationequations are applied differently according to the defect condition, and they yield non-continuous results at a certain length.Furthermore, the results of burst pressure assessment using ASME B31G were too conservative compared to the results of anactual pipe burst experiment [14]. Research has been carried out to reduce the conservativeness of ASME B31G, and majorchanges were made to the flow stress and Folias factor. A different model for the defect type with a different flow stress hasbeen proposed. At the Battelle Memorial Institute (BMI), a conservative and realistic evaluation equation was developedthrough a burst test of 86 full-scale pipes: the Modified ASME B31 evaluation equation. This evaluation equation assumes a par-abolic shape, but does not allow slightly corroded parts. Eq. (7) represents the burst pressure, and similar to Eqs. (8) and (9), theevaluation equation was derived by applying changes in flow stress and Folias factor. By modifying ASME B31G, the burst pres-sure could be assessed more accurately and less conservatively.

Table 3Geomet

Mater

API X

P f ¼2� σSMYS þ 68;9MPa

Dt

� � � Rs ð7Þ

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 ð8Þ

Rs ¼1−0:85

dt

� �

1−0:85dt

� �1

3:3þ 0:032 LDt

� �2h i fordt

� �≤ 0:8;

LffiffiffiffiffiffiDt

p N7:071 ð9Þ

In the case of DNV–RP-F101, assessmentwas carried out by DNV as a Linepipe Corrosion Project, and the burst test was conductedfor 12 corrosion defects, which were mechanically processed at a constant depth. For the burst test, pipes of API X52 with 323.9 mmdiameter were used. Furthermore, a nonlinear FEA was carried out for various geometric shapes of corrosion defects. BG Technologyconducted large-scale research on corrosion defects, and provided equations for the failure of a single corrosion defect, as shown inEqs. (10) and (11).

P f ¼2t

D−tð ÞσUTS

1−dt

� �

1−dt

� �Q−1

2664

3775 ð10Þ

ry of the API X70 pipe with a single defect and of the defects in the FEA.

ial Geometry of pipe Geometry of defects

D (mm) t (mm) L (mm) d (mm)

70 762 15.9 50 3.98100 5.96150 7.95200 9.94300 11.93500 13.91900

Fig. 3. The corroded API X70 pipe with a single defect: confirmation between full-scale hydrostatic burst test results and FEA results.

557K.J. Yeom et al. / Engineering Failure Analysis 57 (2015) 553–561

Q ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1þ 0:31

LDt

� �2s

f ordt

� �≤ 0:85 ð11Þ

Using the PCORRCequation, BMI conducted research on thebasicmechanismof a failure occurring in the corrosion defect of a pipe,and it was found that the burst pressure of a damaged pipe is governed by the tensile strength rather than flow stress. Further, thisequation can beused to predict the residual stress for high-toughness pipes, inwhich failures occur by the plastic collapsemechanism.Based on the results of burst tests, FEA was identically carried out, and the developed equation is shown in Eq. (12).

P f ¼ σUTS2tD

1−dt

1− exp −0:157Lffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

R t−dð Þp

! ! !ð12Þ

The evaluation equations of ASME B31G andModified ASME B31G are suitable for pipes with low toughness and strength, but areknown to overestimate the stress and toughness when assessing pipeline [8,15]. In the case of PCORRC and DNV F101 evaluationequations, a full-scale pipe burst test and FEA were carried out to evaluate the blunt defect of a pipe with high stress and toughness.It is common to apply a suitable evaluation equation according to the material and characteristics of an actual pipe. In this study, theresults of FEA according to corrosion depth and length were comparatively analyzed with those of full-scale pipe burst tests for thecorrosion evaluation equation and high-toughness pipes. Based on the results, an evaluation equation suitable for the gas pipes inSouth Korea is proposed.

3. Full-scale pipe burst test

A corrosion defect assessment method is established by linking a full-scale pipe burst test and FEA. In this test, the burst pressurefor a defect artificially produced throughmechanical processing is measured.When a pipe bursts, the regionwhere failure occurs andthe shape of burst can be identified, and the starting point of burst can be deduced. These are the important elements that provide

Fig. 4. FEM-based stress distribution of the corroded API X70 pipe with a single defect.

Fig. 5. FEM-based burst pressure results of the API X70 pipe with different defect depths and lengths.

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basic data for predicting the starting point of failure when conducting the FEA. In the case of a full-scale pipe burst test, a defect is ar-tificially produced by usingmechanical equipment after cutting a pipe to a segment of 2.3m length. In the case of a defect produced atthe base metal of the pipe, the defect depth to thickness ratio was 50% of the pipe thickness, which is 7.95 mm, and the defect lengthwas 300mm. Both ends of the pipe were closed bywelding caps, and then, the burst test was conducted by usingwater until the pipeburst [5,9,11,16]. The details of the defect shape and burst pressure of the testing pipe are presented in Table 1. Fig. 1(a) shows theshape of the artificially processed defect in the full-scale pipe. Fig. 1(b) shows the defect failure part of the pipe after the burst test,and Fig. 1(c) shows the burst test equipment. Through the test, the burst pressure, failure, and deformation behavior could be inves-tigated according to the type and size of the corrosion defect.

4. Finite element analysis of corrosion

Full-scale pipe burst tests under various conditions are limited by time and cost. Therefore, the burst pressures of various defectsare measured by pipe modeling and analysis through FEA. By comparing the burst trend and burst pressures obtained from the FEAresults and from the full-scale pipe burst tests, an integrity evaluation equation for the corrosion-damaged part was derived. To thisend, the burst trend and the burst pressure were analyzed by checking themaximum stress part according to the length and depth ofthe corroded pipe. By changing the stress–strain curve, which was obtained through a tensile test using a pipe made of API X70 basemetal at the Korea Gas Corporation, to the true stress–strain curve, the elastic–plastic failure theory was applied. The test results pro-vided a curve as shown in Fig. 2, and the applied pipe properties are listed in Table 2. The simulated corrosion defects and pipes wereidentical to the actual defects and full-scale pipes, respectively. A pipe of 762mm diameter, 15.9mm thickness, 1500mm length, and¼ scale was used after considering the pipe shape and stress conditions. For modeling and FEA of the pipe, a commercial analysis pro-gram, ABAQUS 6.10 [17], was used. The number of elements used in the pipe varied depending on the length and depth of the corrod-ed part, but 20,000–25,000 eight-node hexagonal (C3D8) elements were used. The stress in the pipe was measured using the vonMises yield criterion. The corrosion thickness of the pipe was changed from 25% to 87.5% of the pipe thickness, and the corrosionlength was changed from 50mm to 500mm, as presented in Table 3. As in the full-scale pipe burst test, the internal pressure was in-creased across the entire internal diameter, and the point at which the burst pressure reached 95% of the ultimate tensile strength byincreasing the internal pressure was set as the burst initiation point, and FEA was conducted [18].

Fig. 6. Comparison burst pressure of FEA results and results obtained using equations with different depth to thickness ratios ((a) Defect depth to thickness: 25%,(b) Defect depth to thickness: 50%, and (c) Defect depth to thickness: 75%).

Fig. 7. The obtained PCORRC equation for API X70 pipe ((a) 100% UTS of PCORRC, b) 95% UTS of PCORRC and (c) 90% UTS of PCORRC).

559K.J.Yeom

etal./EngineeringFailure

Analysis

57(2015)

553–561

Fig. 8. Comparison between original PCORRC equation and modified PCORRC for a API X70 pipe.

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5. Results and discussions

Based on the burst pressure results obtained from the full-scale pipe burst test, the burst trend and burst pressurewere investigat-ed according to the depths and lengths of various pipe defects by FEA. The experimental burst test results are given in Table 1, and theresult of comparing the FEA results is shown in Fig. 3. The failure location identified by the full-scale burst test was found to be iden-tical to that by the FEA. Thus, the agreement between the full-scale pipe burst test and the FEAwas confirmed. Accordingly, the failurebehavior and burst pressure were analyzed according to depth and length by FEA.

As the internal pressure increases, the circumferential stress in each part of the pipe increases; the stress increases as the pipethickness decreases, and the shape changes rapidly in the damaged part. Further, as the depth of a single corrosion defect increases,it was confirmed through Fig. 4 that themaximum stress concentration area forms the boundary of the damaged part. In the damagedpart of the pipe, stress concentration arises because its thickness is the least in the entire pipe, and it can be predicted that the burstwill occur first in this part. As the depth of the damaged part increases, the burst pressure of the pipe also decreases, and when thedefect length is very long, i.e., more than 500mm, the burst pressure of the pipe is proportional to the residual thickness. The behaviorof burst pressure according to the length and depth of the defect is shown in Fig. 5; when the depth and length of the defect increase,the overall burst pressure decreases. When the defect depth is the largest, the burst pressure is the lowest. As the depth of the dam-aged part increases, the burst pressure decreases because the residual pipe thickness of the actual damaged part decreases relativelyas well.

Fig. 6 shows a comparison of results between the FEA and the aforementioned corrosion defect evaluation equations—ASMEB31G,DNV, and PCORRC equations—to determine the suitable corrodedpipeline assessment. Here, the graphswere plotted for 25%, 50%, and75% of the defect depth to thickness ratio. In the case of relatively low defect depth to thickness ratio, as shown in Fig. 6(a), the burstpressurewas overestimated in order to compare the burst test results [5,10]. The defect shape and stress concentration of the pipelineare expected to influence the burst pressure depending on the defect depth to thickness ratio; however, further study is needed toprove this behavior. The burst pressure exhibits the behavior predicted by the PCORRC equation, as shown in Fig. 6(b) and (c).When a full-scale pipe is assessed, the equation that assesses for low values can be applied for the safety of pipe. ASME B31G providesa conservative assessment compared to the other equations. However, because it is too conservative and because the burst pressurecurve changes rapidly as the length of corrosion increases, there will be difficulties in applying it to an actual pipe. On the other hand,as the depth and length of the defect corrosion increase, the PCORRC equation, which is suitable for high toughness pipes, is deter-mined to show a very similar behavior to the decreasing trend of pipe burst pressure found in FEA. Furthermore, when the strengthis sufficient, the PCORRC standard is shown to yield reliable and less conservative predictions compared to the other standards be-cause the maximum depth and maximum length data are used for the blunt, high-toughness corrosion defect. Therefore, thePCORRC equation was modified and applied according to the gas pipe environment of South Korea. Further, changing C1, which isthe ultimate tensile strength, and C2 value, which is 0.157, in the conventional PCORRC, Eq. (13) was used, and a conservative assess-ment was derived.

P f ¼ C1σUTS2tD

1−dt

1− exp −C2Lffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

R t−dð Þp

! ! !ð13Þ

The nonlinear curvefitting obtained by changing two variables, C1 and C2, is shown in Fig. 7. For the ultimate tensile strength usingPCORRC, 100% was applied to obtain Fig. 7(a), 95% to obtain Fig. 7(b), and 90% to obtain Fig. 7(c). As shown in Fig. 7(b), when 95%ultimate tensile strength was applied, some results were non-conservative. Because a conservative judgment is required for gaspipe integrity assessment, 90% ultimate tensile strength was applied to the modified PCORRC equation (Eq. (13)), as shown inFig. 7(c), and the burst pressure of the corrosion-damaged pipe was expressed. C2 was shown to change in the range of0.142–0.224 with depth. However, for the conservative assessment for all ranges of defect depths and lengths, the highest value of

561K.J. Yeom et al. / Engineering Failure Analysis 57 (2015) 553–561

0.224 was selected for C2 in Eq (13). A conservative evaluation equation for single corrosion defect obtained through this isEq. (14).

P f ¼ 0:90σUTS2tD

1−dt

1− exp −0:224Lffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

R t−dð Þp ! ! !

ð14Þ

The estimated burst pressure was compared between the original PCORRC equation with a C2 value of 0.224 in Eq. (12) and themodified PCORRCequationwith a C2 value of 0.157 in Eq. (14), as shown in Fig. 8. The result obtained by Eq. (14) for the burst pressurewas more conservative than that of Eq. (12) in order to secure the integrity of the corroded pipeline.

6. Conclusion

It is known that pipelines are very effective for the transportation of oil and gas. During the operation of a pipeline, defects arisebecause of environment conditions. The measurement of failure pressure is an important factor in the maintenance and operationof a pipeline. In this paper, failure pressure was predicted using nonlinear FEM with a single corrosion defect of various depths andlengths. Based on the results of the FEM and full-scale hydrostatic burst tests, the assessment of a pipelinewith single corrosion defectwas conducted. The results of the full-scale hydrostatic burst test were used for analysis of failure pressure using the FEM, and resultsobtained using regression equationswere comparedwith the FEM results. The proposed equation can be used in engineering practiceto obtain accurate predictions of failure pressure for pipes with a single corrosion defect.

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