Chong 2014

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Effects of wet H2S containing environment on mechanical

properties of NACE grade CMn steel pipeline girth welds

Tse-Ven Steven Chong a,b,, Shashi Bhushan Kumar b, Man On Lai a, Wai Lam Loh a

a Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singaporeb Det Norske Veritas Pte. Ltd., 16 Science Park Drive, Singapore 118227, Singapore

a r t i c l e i n f o

Article history:

Received 13 April 2014

Received in revised form 1 August 2014

Accepted 6 September 2014

Available online 16 September 2014

Keywords:

Sour service

Girth weld steel pipes

Hydrogen sulphide

Sulphide stress cracking

Hydrogen embrittlement

a b s t r a c t

Tensile and fracture specimens (pre-charged with H2S) were tested in solution using

customised rigs. When comparing with uncharged specimens, the tensile test results

showed significant reduction in ductility for base and weld metal. The fracture resistance

test results showed similar reduction of 0.3 times in base and weld metals and heat

affected zone. There was noticeable increase in yield and tensile strength in both base

and weld metals, due to the effect of hydrogen. When using partially coated specimens,

higher fracture resistance was obtained, with reduction of 0.4 times only. This was

mainly contributed by crack tip hydrogen and less so by the bulk hydrogen.

2014 Elsevier Ltd. All rights reserved.

1. Introduction

Subsea pipelines are designed and installed to transport oil and gas in large quantities continuously for offshore field pro-

duction. The pipeline which is typically S-layed or J-layed, is fabricated on-board a lay barge by joining several linepipes of

about 12.2-metre length using girth weld configuration. During production welding, it is inevitable that circumferentially

oriented weld imperfections such as lack of fusion and incomplete penetration, etc. may occur in the girth welds. These pipe-

lines are designed to safety considering the static and dynamic stresses and strains anticipated during laying (for example,

sag-bend, over-bend) and in-service including environmental effect, if any (i.e. sour well fluids, etc.).

To assess the structural integrity of girth welds in the pipeline, Engineering Critical Assessment (ECA) is commonly per-

formed considering mechanical properties, loading conditions and imperfection size using fracture mechanics principles.

In ECA, a series of fracture mechanics analyses is performed in accordance with the procedures stipulated in codes such as

DNV-OS-F101 [1], BS 7910:2005 [2] and DNV-RP-F108 [3], to establish alternative flawacceptance criteria for non-destructive

testing (NDT) inspection of the field production girth welds on-board the lay barge.

Often the flaw acceptance criteria (acceptance standard) for the pipeline girth weld defects are defined based on

mechanical property data and design parameters using fracture mechanics principles to meet adequate structural integrity

in the weld [4,5]. The mechanical properties of the base (parent) metal (BM), the weld metal (WM) and the heat affected zone

(HAZ) are required as part of the input parameters in conducting an ECA. Such data are derived from an established database

or tested to the specific environments representing the actual or more conservative situations than the pipeline will experi-

ence during installation and operation. The main mechanical properties required as input to the ECA are: tensile properties,

http://dx.doi.org/10.1016/j.engfracmech.2014.09.005

0013-7944/ 2014 Elsevier Ltd. All rights reserved.

Corresponding author at: Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore.

E-mail address: [email protected](T.-V.S Chong).

Engineering Fracture Mechanics 131 (2014) 485503

Contents lists available at ScienceDirect

Engineering Fracture Mechanics

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c at e / e n g f r a c m e c h
http://dx.doi.org/10.1016/j.engfracmech.2014.09.005mailto:[email protected]://dx.doi.org/10.1016/j.engfracmech.2014.09.005http://www.sciencedirect.com/science/journal/00137944http://www.elsevier.com/locate/engfracmechhttp://www.elsevier.com/locate/engfracmechhttp://www.sciencedirect.com/science/journal/00137944http://dx.doi.org/10.1016/j.engfracmech.2014.09.005mailto:[email protected]://dx.doi.org/10.1016/j.engfracmech.2014.09.005http://-/?-http://-/?-http://-/?-http://-/?-http://crossmark.crossref.org/dialog/?doi=10.1016/j.engfracmech.2014.09.005&domain=pdf

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stress strain curves, fracture toughness/resistance, fatigue crack growth rate (FCGR) parameters of all relevant material zones

(viz., BM, WMandHAZ). The design parameters requiredfor an ECAincludethe: peak strain/stress, fatigue histogram, welding

residual stresses, geometrical stress concentration factors and safety factor associated with the zone of operation.

The growing demand of oil and gas resources has inevitably led the industries to explore and produce such resources from

deeper and more complex reservoirs. Field developments with such wells pose a variety of technical challenges as these

complex deepwater wells are also typically associated with sour environments. NACE MR0175/ISO 15156[6]defines a sour

environment as one that contains sufficient H2S to cause cracking of materials by hydrogen induced cracking mechanisms.

Oil and gas fields can contain significant amounts of H2S and pipeline steels exposed to a sour environment can be suscep-

tible to a number of different cracking mechanisms. For example, hydrogen induced cracking (HIC) sometimes termed as

hydrogen pressure induced cracking (HPIC), stress oriented hydrogen induced cracking (SOHIC) and sulphide stress cracking

(SSC)[6,7](Fig. 1). The resultant fluid often containing H2S will place a significant demand on the pipeline mechanical prop-

erties such as ductility, fracture resistance and fatigue crack growth rate. The severity of such an environment depends onthe metallurgical characteristics of the materials, well fluid composition and operating conditions.

In order to develop such challenging offshore oil and gas fields, new design concepts and materials are required to be

developed and used. When designing and developing pipeline systems to transport highly corrosive oil/gas, NACE grade

CMn linepipe is selected and welded offshore using over-matching welding consumable wires. Recent research using con-

ventional CMn steel material has shown that mechanical properties like fatigue performance degrades in the presence of

H2S containing environment [8,9]. Although NACE grade CMn steels exhibit resistance to SSC, it is expected that properties

like tensile strength and fracture resistance will still be affected to some extent as a result of the high concentration of hydro-

gen present at the fracture process zone (FPZ). Subsequently, this leads to embrittlement. Fig. 2provides a schematic illus-

tration of the generation of hydrogen from the metal dissolution and hydrogen sulphide dissociation and Section 3.2

discusses the micro-mechanisms of hydrogen diffusion in detail.

The works performed by various authors [1016] suggest that the effect of hydrogen generated from various methods (i.e.

dry gas charging, electrochemical charging or metal dissolution) play an important role in influencing the mechanical

properties. However, there has been limited studies performed which give a holistic assessment of the effects of hydrogen

Nomenclature

A coefficient of power law fit equationa height for surface crackao initial height for surface crackAWTS all-weld tensile specimenB width of the specimen

BM base metalBMTS base metal tensile specimenCMOD crack mouth opening displacementECA Engineering Critical AssessmentEl elongationFf final load recorder as per load versus CMOD curveFL fusion lineFPZ fracture process zoneH Daylight between the grips of SENT specimenHAZ heat affected zoneJtot sum of elastic and plastic part of the J-integralJe elastic part of the J-integralJp plastic part of the J-integralK stress intensity factorLf gauge length after fractureLi initial gauge lengthRF reduction factorSEM scanning electron microscopeSENT single edge notch tensileUp the plastic part of the area under the load versus CMOD curveW wall thickness of the pipelineWM weld metalb exponent of power law fit equationgp dimensionless function of the geometryry yield strengthrT ultimate tensile strength(W-ao) the remaining ligament length

486 T.-V.S Chong et al./ Engineering Fracture Mechanics 131 (2014) 485503
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embrittlement on the tensile and fracture toughness/resistance of pipeline weldments exposed to wet H 2S environment

before and during testing. Furthermore, there has been little work performed to evaluate the effects of using coated versus

un-coated test specimen on the fracture resistance for the related weldments. A coated specimen here refers to specimens

coated with ceramic epoxy on all sides except the top face and the notch. This paper attempts to document experimental tech-

niques to quantify the changes in mechanical properties when welded test coupons are tested in wet hydrogen charged envi-

ronments. The paper also discusses the role of hydrogen on the fracture morphology of the test specimens used in this study.

2. Experimental procedures

This section describes the design of the test rigs, test materials, specimen preparation methods, testing procedures, data

processing and experimental observation made during the current investigation. Test coupons were prepared and tested

with reference to NACE, ASTM and DNV standards.

2.1. Material

NACE compliant 45.72 mm (18 inch) OD 14.3 mm WT DNV SMLS (seamless) 450 SFD grade linepipe girth welded with

Lincoln MD-SG2 (AWS Class: ER 70S-6/Spec No.: A5.18) were used in the current research. The linepipes were manufactured

by a plug mill seamless pipe fabrication process and the pipe sections were later joined by pulse gas metal arc welding

(p-GMAW) process in 5G downhill position. The chemical composition requirements of the pipe material (i.e. base metal)

and welding consumable are shown inTable 1a and b. A macro-section of the weld profile is shown in Fig. 3to illustrate

the important features of the girth weld. The different microstructures present in a welded connection such as the base

metal, weld metal and HAZ (also referred as fusion line i.e. FL) were investigated.

Sulphide stress

cracking

Fig. 1. Macrograph of a non-NACE grade welded coupon failed as a result of severe sulphide stress cracking after being strained in hydrogen sulphide

solution. Such alloys are non-cracking resistant and are susceptible to SSC. Source: DNV GL Materials Technology and Structural Integrity Laboratories,

Singapore.

Fig. 2. Schematic diagram of the hydrogen sulphide dissociation and atomic hydrogen generation process.

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2.2. Tensile test

The tensile tests were carried out in both an air environment and NACE Solution A (i.e. sour service environment). In the

latter case, the specimens were pre-charged and tested in a special purpose Perspex test cell containing H2S solution (NACE

Solution A). Round bar specimen was used to generate the tensile properties and the tensile stressstrain curves of the base

and weld metal.

2.2.1. Design of test rig

The test rig was designed and fabricated based on the research works performed by Yang et al. [17] on single edged notch

tension (SENT) specimens and with reference to NACE TM0177-2005[18]. The objective of the test cell, which made up the

rig, was to isolate the specimen in the intended test environment during the charging and testing phases. Test fixtures with

internal screw threads were used to hold the specimen and to increase the gripping area. It also allowed the specimen to be

encapsulated within the test cell and submerged in the test solution. The fixtures extended outside the test cell while the

design provided for sufficient freedom in the vertical extension during the test, yet with sufficient sealing capacity to prevent

leakage of the hazardous gas.

Considerations were made for the measurement of the elastic strain since the conventional extensometer could not be

used when the specimens were submerged in the solution during the test. Hence, the design accommodated for two (2)

water-proof strain gauges, installed on the reduced section of the specimen for the purpose of recording the stressstrain

data in the elastic regime.

2.2.2. Preparation and testing

Round bar specimens were machined from two locations: at the base metal and at the girth weld. The parent (base) metal

tensile specimen was extracted in the axial direction of the pipe while the weld metal tensile specimen was extracted from

the circumferential direction with reference to Appendix B of DNV-OS-F101. The testing was performed in accordance with

ASTM A370-13[19].Fig. 4illustrates the locations where the tensile specimens were extracted.

For the tests conducted in NACE Solution A, the test solution consisted of 5 wt% NaCl and 0.5 wt% CH3COOH in distilled

water with a pH of approximately 2.7, which was thoroughly deaerated with high purity nitrogen prior to the introduction of

99.5% (min) purity H2S at 1 bar. The specimens were pre-charged in the simulated sour service environment for 168 h (7-day

Table 1

Chemical composition requirements (by maximum weight%) of: (a) NACE grade pipeline steel (DNV SMLS 450 SFD, in compliance with DNV-OS-F101

specifications). (b) Lincoln weld filler metal (in compliance with AWS ER70S-6 specifications).

(a)

Element C Si Mn P S

Wt% 0.16 0.45 1.65 0.02 0.003

Element V Nb Ti Other

Wt% 0.09 0.05 0.06 Notes

(b)Element C Mn Si P S

Wt% 0.060.15 1.41.85 0.81.15 0.025 0.035

Element Ni Cr Mo V Cu

Wt% 0.15 0.15 0.15 0.03 0.50

Notes:The sum of niobium, vanadium and titanium contents shall be 60.015%. Cu6 0.5%, Ni6 0.5%, Cr6 0.45%, Mo6 0.35%, B6 0.0005%.

Heat Affected Zone (HAZ)

Weld cap

Weld root

Base Metal

Fig. 3. Macrograph of girth weld profile using J-prep bevel and p-GMAW process.

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duration) at ambient temperature of 24 C + 3 C and at atmospheric pressure prior to actual testing. The schematic sketch of

the test apparatus and setup for the pre-charging phase is illustrated inFig. 5.

Upon completion of pre-charging, the test cell was isolated by closing off all external valves and the test rig was trans-

ferred to a 25 ton Instron universal test machine. For pipeline in operation condition that is governed by displacement con-

trolled loading, the displacement rate in mm/s in more relevant. Henceforth, both the tensile and fracture tests used similar

displacement rate of 0.02 mm/min (3.3 104 mm/s) which is also within the range specified in ASTM G129-00[20]for

standard slow strain rate tests involving environmentally assisted cracking. The tests were conducted at ambient tempera-

ture of 24 C + 3 C using displacement/position control mode. During the rising load test, the test specimens were kept

immersed in the NACE Solution A with H2S at 1 bar.

The details of test specimens and setup are illustrated inFig. 6.After the completion of the tests, the test solution was

extracted and analysed to confirm that the pH and hydrogen sulphide concentration were maintained within range of

2.62.8 and 23003500 ppm respectively as per NACE Solution A test requirements.

For the tests conducted in an air environment, the specimens were directly mounted onto the grips of the machine. An

extensometer was attached to the reduced section to record the strain data at the elastic regime. The loading rate was main-

tained between 3 mm/min and 6 mm/min in displacement/position control mode. The test matrix for the tensile tests is

summarised inTable 2.

2.3. Fracture resistance test

The fracture resistance tests were carried out in an air environment and in NACE Solution A. In the latter case, the spec-

imens were pre-charged and tested in a customised Perspex test cell containing H2S solution, with reference to the design

proposed by Yang et al. [17]but with some modifications. SENT geometry specimens were employed to generate the J R-

curves for the base, weld metal and HAZ. DNV-OS-F101/Appendix A, allows SENT testing at operation environments since

SENT specimen is designed to give a crack tip constraint close to that of the pipe with a flaw. Furthermore, weld related

imperfections are controlled by the weld pass height which is relatively small in the typical range of 26 mm. Therefore

as per DNV-RP-F108, a SENT specimen with notch depth of a/W = 0.20.5 is considered to have a realistic crack depth. Here,

a represents the notch depth after fatigue pre-crack and W represents the wall thickness. If the cracks have realistic crack

depths, not only the crack tip fields in the pipe and specimen are similar, but also the deformation in the whole remaining

ligament is more accurately replicated. Moreover as the SENT specimen has been widely accepted for pipeline girth weld

fracture resistance tests, it was chosen to be used in the present investigation.

2.3.1. Preparation and testing

The specimens were machined from the steel pipes in the LR orientation according to ASTM E1823-13 [22], with the

notch running from the inner surface of the pipe coupon. The notches were machined at the base metal, weld metal centre

BMTSAWTS

Fig. 4. Extraction plan for the tensile tests (BMTS base metal tensile, AWTS all weld tensile).

Fig. 5. Schematic sketch of test apparatus and setup for the pre-charging phase.


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line and HAZ with an a/W ratio between 0.2 to 0.3 after fatigue pre-cracking. The specimen cross section was kept as B 2B

with a daylight between the grips of 10B (where B is the wall thickness of the pipe coupon), as recommended in DNV-OS-

F101.

The reference study was performed on specimens which were not coated to simulate the case of hydrogen ingress by bulk

charging. To further study the effects of bulk charging versus crack tip hydrogen charging, some of the specimens were sand-

blasted and subsequently coated with Belzona ceramic epoxy on all sides except the top face and the notch. This additional

study simulates one-sided diffusion to ensure that metal dissolution and hydrogen generation occurs only on the top face

and crack tip, which will also lead to ingress of hydrogen. This will be more representative in the case of fluid flowing inside

a pipeline.

(a)

(b)4-way valve

Tensile specimenTest cell

Pre-charging

circuitry

Tensile specimen

Test fixtures

Strain gauge

cables

(c)

Machine

Tensile specimen

Strain gauge

cables

Fig. 6. Tensile test specimens and setup. (a) Specimens fastened to test fixtures and with water proof strain gauge installed. (b) Pre-charging circuitry for

test cells in series. (c) Sour service tensile test setup, with dual strain gauges and gas cabinet.

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For the tests conducted in NACE Solution A, the pre-cracked SENT specimens were subsequently preloaded in the test rig

to longitudinal axial stresses of about 300 micro-strain (corresponding to typical pipeline operating pressures) to open the

crack mouth for exposure to the test solution. The specimens were subsequently pre-charged, similar to the procedures and

environmental set up as documented in Section 2.2.2for the tensile tests.

Upon completion of pre-charging process, the tests were conducted at ambient temperature of 24 C + 3 C using dis-

placement/position control mode with a slow displacement rate of 104 mm/s in accordance to ASTM G129-00[20]and

research works performed by Yuan et al. [21]. During the rising load test, the specimens were immersed in the NACE Solution

A with H2S at 1 bar. For the tests conducted in an air environment, the loading rate was maintained at 0.25 mm/min in dis-

placement/position control mode.TheJ R-curves were established by power law fit as per DNV-OS-F101/Appendix A, in which multiple specimen approach

(i.e. six specimens) was used. The first specimen in the test series was tested to a crack mouth opening displacement (CMOD)

value of about 120% of the CMOD value corresponding to maximum load. The 2nd, 3rd, 4th, 5th and 6th specimens for the

series were tested to CMOD values of 100%, 100%, 80%, 60% and 40%, respectively, of the CMOD value corresponding to max-

imum load from the first test.

The data were subsequently calibrated to compute for the J integral according to the formulation in DNV-RP-F108. The J

R-curves were calculated from the elastic and plastic component according to Eqs.(1)(4). Subsequently, a power law rela-

tion as per Eq. (5) was usedto fittheJ-Da data sets. Eq. (2) was used to derive theJcoordinate andDa coordinate was derived

from physical measurements of the fractured specimens.

Jtot Jel Jpl 1

Jtot K2

1 t2

E

gpUpBW ao

2

KF

ffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffiffiffiffiffiffiffi2 tan pa

2W

q 0:752 2:02 aW

0:37 1 sin pa2W

3BffiffiffiffiffiffiW

p cos pa

2W 3

gp 0:85 196:719eBW

64:642 a

W

5 493:511e BW 138:837

aW

4 463:503e BW 106:207

aW

3

201:862e BW 34:532 a

W

2 39:413e BW 4:525

aW

2:064e BW 1:039

4

JADab

5Here,Jeis the elastic part of theJ-integral, Jpis the plastic part of theJ-integral, gpis the dimensionless function of the geom-etry, Up is the plastic part of the area under the load versus CMOD curve, Kis the elastic stress intensity factor, Fis the force at

termination of the test, A is the coefficient of power law fit equation and b is the exponent of power law fit equation.

After the completion of the tests in the air and sour environment, each specimen was broken to measure the fatigue crack

front and the ductile crack extensionDa for computation ofJas per DNV-RP-F108. Fractography analysis was also performed

to study the effects of hydrogen ingress on the microstructure. For the tests performed in a sour environment, the test solu-

tion was extracted and measured to ensure that the pH and hydrogen sulphide concentration were maintained at pH 2.62.8

and 23003500 ppmrespectively as per NACE test requirements. Fig. 7 provides illustrations of the SENT specimen and Fig. 8

gives an overview of the test specimen geometry and setup.

A total of eight (8) sets of tests were performed: with three (3) sets performed at notch location at the base metal, weld

metal and HAZ in an air environment and the other three (3) sets conducted in a sour environment made up of NACE Solution

A at 24 C. Another two (2) sets with coating were performed at notch location at the weld metal and HAZ in a sour envi-

ronment. The test matrix for the fracture resistance tests is summarised in Table 3.

Table 2

Test matrix for the tensile tests.

Material No. of specimens Test environment

Base metal 3 Environment: air

Weld metal 3 Temperature: 24 C 3 C

Base metal 3 Solution: NACE TM0177 solution A

Gas: 99.5% min purity H2S at 1 bara

pH: 2.62.8

Weld metal 3 H2S concentration: 23003500 ppm

Temperature: 24 C 3 C

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3. Results and discussion

3.1. Effects of H2S exposure

3.1.1. Tensile propertiesThe tensile stress strain curves (engineering) and test results for the NACE grade CMn pipeline girth welds are summa-

rised inFigs. 9 and 10 andTable 4. It was observed that the weld metal is overmatching the base metal in terms of yield

strength and tensile strength when tested in an air and in a sour environment. There was no noticeable change in the Youngs

modulus for both base and weld materials in both environments. This was consistent as modulus should not change as a

result of H-embrittlement or any forms of strengthening; it changes as a result of temperature changes and microstructural

change.

The ductility parameter, elongation, is reported as follows:

El DLL Lf Li

Li6

Here,Elis the elongation, Liis the initial gauge length and Lfis the gauge length after fracture. The percentage changes in the

tensile properties i.e. yield strength, tensile strength and elongation when the material was tested in a sour environment

versus an air environment, are calculated as follows:

DEl% Elsour ElairElair

7

Dry% ry;sour ry;airry;air 8

DrT% rT;sour rT;airrT;air 9

Here, ryis the yield strength and rTis the ultimate tensile strength. When comparing the results due to the effects of a sourenvironment with that of an air environment, both the base metal and the weld metal coupons demonstrated a loss in duc-

tility as a result of hydrogen embrittlement. The embrittlement was most likely caused by atomic hydrogen that has diffused

into the metal lattice through the grain boundaries. The base metal exhibited less reduction in elongation to fracture of 35%

when compared to the weld metal of 51% (calculated with the air environment as the base case). Thus, when exposed to H2

S

pre-charging the weld metal was more susceptible to the effects of hydrogen embrittlement and retained less of its initial

Fig. 7. The clamped SENT (Single Edge Notched Tension) specimen geometry. Relationship between flaw orientation and height in the pipe and the crack

orientation and size in the specimen.

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(a)

SENT specimen

Pre-load frame

Test cell

(b)

(c)

SENT specimen

Epoxy coating

Gas containment

chamber

SENT specimen

H2S detector

Clip gauge

Fig. 8. Fracture test specimens and setup. (a) Pre-charging of SENT specimen for duration of 7 days. (b) Epoxy coating applied on all sides except notch side

to simulate one-sided diffusion. (c) SENT test setup in a sour environment with data logger and gas cabinet with exhaust venting.


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ductility when compared to the base metal. This was likely attributed to the difference in homogeneity of microstructure

arising from the welding process and the composition of the welding consumable.

Both the base metal and the weld metal exhibited an increase in strength as a result of a sour environment. The base

metal showed slight increase in yield strength (2.7%) and tensile strength (4.5%) when the material was tested in a sour envi-

ronment. However, the weld metal showed a more significant increase in yield strength (12.7%) and tensile strength (12.9%),

suggesting that the material is more sensitive to a sour service environment. The percentage changes in tensile properties

(yield strength, tensile strength, Youngs Modulus and elongation to fracture) of specimens tested in sour service environ-

ment with respect to the specimens tested in an air environment is summarised in Table 5.Fig. 11a and b further illustrate

the linear relationship between the tensile properties of the base metal and weld metal with the environment.

The fracture surfaces were examined using a scanning electron microscope (SEM). Fractographs of the specimens tested

in an air environment showed fracture characteristic of microvoid coalescence or dimple fracture. It was observed that some

of the small equiaxed dimples nucleate at the inclusions of the metal (Fig. 12a and b). The fractogaphs of specimens tested ina sour environment showed mix-mode fracture: localised areas of dimpled fracture surrounding relatively flat small facets,

which is characteristic of quasi-cleavage (Fig. 12c and d). The presence of such brittle features was likely due to hydrogen

ingress resulting in embrittlement. The fractographs of the weld metal tested in a sour environment showed reduced plas-

ticity as evidenced by a greater degree of cleavage facets when compared to that of the base metal. This was likely due to the

welding process in which brittle microstructures known as local brittle zones were formed resulting in reduced ductility.

The most evident trend presented by the stress strain curves and the fractographs was the mode of fracture for specimens

tested in an air and in a sour environment. Specimens tested in an air environment experienced large elongations to fracture

indicating that the material underwent a large plastic deformation before fracture which was representative of ductile fail-

ure of modern pipeline steel materials. However, when the material was tested in a sour environment, it fractured at a much

lower strain with a smaller degree of plastic deformation. Moreover, the H2S charged specimens displayed a slightly higher

strength when compared to non-charged specimens. These observations are consistent with the works by Zaleski [23]and

Wu and Kim[24]on testing hydrogen charged steels in dry environment.

3.1.2. Fracture resistance properties

3.1.2.1. Bulk charging. The fracture resistance J R-curves and test results for the uncoated SENT specimens machined from

NACE grade CMn pipeline girth welds are summarised in Fig. 13 and 14andTable 6. The reduction factor was calculated

by normalising the coefficient, A, of the power law fit Eq. (3)in a sour environment to the value in an air environment. The

reduction factor is defined as:

RF AsourAair

10

Here,RFis the reduction factor andAis the coefficient of power law fit equation. It was observed that the weld metal exhib-

ited the lowest fracture resistance followed by the HAZ and the base metal when tested in an air or in a sour environment.

Comparing the results due to the effects of a sour environment with those of an air environment, all the specimens with

notch location at the base metal, HAZ and weld metal demonstrated a reduction in fracture resistance. The slope of the JR-curves for all 3 materials had a reduction factor of about 0.280.33 times when pre-charged in a sour environment. This

indicates that fracture resistance in a sour environment is controlled by concentration of hydrogen.

It was also observed that the weld metal generally had higher tearing lengths (Da) at each load level followed by the HAZ

and base metal when tested in an air and in sour environment. Furthermore the results showed an increase in tearing length

(Da) associated with each load level by about 1.53 times for the base metal, HAZ and weld metal when tested in a sour

environment, which also contributed to the reduction in the slope of the J R-curve. Thus when exposed to H2S pre-charging

Table 4

Summary of tensile properties (mean of 3 tests) for NACE grade CMn pipeline tested in air and in sour environments.

Material Test environment Youngs modulus (GPa) Yield strengthRp0.2%(MPa) Tensile strengthRm (MPa) Elongation (%)

Base Metal Air 207 464 530 21.4

Sour 207 477 554 13.9

Weld Metal Air 207 584 648 16.8

Sour 207 658 732 8.2

Table 5

Summary of percentage change in tensile properties for NACE grade CMn pipeline tested in air and in sour environments.

Material Youngs modulus (%) Yield strengthRp0.2%(%) Tensile strengthRm (%) Elongation (%)

Base metal No significant change 2.7 4.5 35

Weld metal 12.7 12.9 51

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the base metal, HAZ and weld metal were more susceptible to the effects of hydrogen charging resulting in reduced tearing

resistance.

As per Section2.3.1, theJ R-curves were derived from the load versus CMOD curves for 6 specimens loaded to different

clip gauge opening displacements. When tested in a sour environment, the curves demonstrated a reduction in clip gauge

opening displacement associated with each load level for the base metal, HAZ and weld metal. Some representative graphsfor the base metal have been illustrated in Figs. 15 and 16. This was likely a result of the effects of hydrogen embrittlement

and reduced plasticity, consistent with the results from the tensile stress strain curves as per Section3.1.1.

Upon completion of the tests, the SENT specimens were broken apart following soaking in liquid nitrogen. Representative

macro-section examination of the fracture surface of the SENT specimens tested in a sour environment is shown in Fig. 17ac.

Fish eyes were most evident in the fracture surface of the weld metal specimen, which demonstrated that the material had

been saturated with hydrogen and that the fracture process involved hydrogen diffusion. For comparison, the fracture surface

of a SENT specimen testedin an air environment is shown in Fig. 17df. The fracture surface was relatively smooth with no fish

eyes present.

The fracture surfaces were further examined using SEM. The fractographs of SENT specimens tested in a sour environ-

ment exhibited predominantly quasi-cleavage features, which was more pronounced at the crack initiation area (Figs. 18a

and b). Dimples were also observed intermittently further away from the initiation site. The overall observation from the

fractographic study indicated that fracture propagation has been dominated by a semi-ductile tearing. This was more evi-

dent for the weld metal and could be attributed to its inhomogeneous microstructure. This also revealed that the material

Fig. 11. Linear relationshipbetween thetensile properties of thebase metal and weld metal with theenvironment, based on mean fitting. (a) Yield strength

and tensile strength. (b) Elongation.

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The role of bulk charging versus crack tip hydrogen has been investigated with a set of SENT specimens (with notch loca-

tion at the HAZ and weld metal) coated on all sides except the top face and around the crack tip. The fracture resistance J R-

curves and test results for the coated SENT specimens are summarised in Fig. 19.

Fig. 14. J R-curves determined for in a sour environment of base metal, weld metal and HAZ/FL at 24 C.

Table 6

Summary ofJ R-curve fracture resistance data for NACE grade CMn pipeline tested in air and in sour environments.

Material In air environment In sour environment Reduction factor

Base metal 970Da0.9 320Da0.85 0.33

Weld metal 720Da0.76 237Da0.9 0.33

HAZ/FL 885Da0.75 246Da0.9 0.28

Fig. 15. F versusV-curves determined for in an air environment of base metal at 24 C.

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There was a consistent reduction in fracture resistance due to the effects of the H2

S charged environment. It was further

observed that the reduction factor reduced from 0.28 to 0.33 times for non-coated specimens to 0.330.37 times for coated

specimens. While the fracture resistance was reduced in both the coated and uncoated specimens when tested in hydrogen

charged environment, the amount of reduction differed by about 16%.

It is generally accepted that fracture resistance of steels in a sour environment involves a hydrogen enhanced mechanism.

The present study demonstrated that for the case of SENT specimens tested in hydrogen charged environment, bulk charging

of hydrogen plays an important role in stable crack tearing resistance as with crack tip hydrogen generation.

3.2. Micro mechanisms of fracture behaviour

For the material system in the present study, the failure behaviours of the CMn pipeline steels and related weldments

are primarily governed by the process of hydrogen diffusion. Atomic hydrogen (H0) is produced as a natural byproduct of the

metal dissolution process at local cathodic sites and is readily adsorbed by the metal surface. Substantial levels of hydrogen

are adsorbed on the surface through such electrochemical processes. The kinetics of recombination can be significantly

retarded in the presence of poisons that consist of sulphur species resulting from H2S (i.e. dissolved H2S, HS and S2) [25].

Fig. 16. F versusV-curves determined for in asour environment of base metal at 24 C.

Fish eyes

AC

B

(a) (b) (c)

(d) (e) (f)

Fig. 17. Representative macro-fracture surface of (a) WM, (b) HAZ/FL, (c) BM specimens tested in a sour environment and (d) WM, (e) HAZ/FL, (f) BM

specimens tested in an air environment. The regions of (A) rising load, (B) fatigue pre-crack and (C) EDM notch are also indicated in (d). Fisheyes aremost

evident on the fracture surfaces of the weld metal specimen tested in a sour environment.

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Anode:Fe ! Fe2 2eCathode:H2S H2O ! H HS H2O

HS H2 O ! H S2 H2O

Fe H2 S H2 O ! FeS 2H0 H2 OThis leads to increased absorption of atomic hydrogen within interstitial sites in a metallic crystal as well as at grain

boundaries. The solubility of hydrogen is especially high in regions in which the lattice expands as a result of high degree

of stress triaxiality. This is particularly so for the fracture process zone near the crack tip in which hydrogen will be

Fig. 18. Representative SEM fractograph of weld metal specimens tested in (a) sour environment taken at the crack initiation zone, located at region X. (b)

Sour environment taken further from the crack initiation zone, located at region Y. (c) Air environment, taken at region Z.

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concentrated. The high local concentration of hydrogen causes the process zone to be embrittled [26].Fig. 20illustrates the

abovementioned crack-tip chemistry. The atomistic mechanisms for hydrogen embrittlement have been reviewed exten-

sively and the three major theories proposed are: hydride-induced embrittlement, hydrogen enhanced localised plasticity

(HELP) and hydrogen enhanced decohesion (HEDE)[27].

The increase in yield strength was likely associated with the diffusion of hydrogen atoms into the metal interstitial sites,

pinning down any dislocations, and hence an additional stress is required to free the dislocations causing an increase in yield

strength and ultimate tensile strength. However the decrease in elongation to fracture is most likely due to embrittlement.

Hydrogen diffusion is a time dependent process and the crack extension occurs via diffusion of hydrogen through a crit-

ical distance associated with the fracture process zone[28,29]. The presence of absorbed hydrogen in the region of material

directly ahead of a crack tip is known to have an adverse effect on the properties of common pipeline steels. It was observed

from the SEM fractographs as per Sections 3.1.1 and 3.1.2.1, that quasi-cleavage features were more pronounced at the crack

initiation area and this was consistent with the higher concentration of absorbed hydrogen ahead of the crack tip during the

Fig. 19. J R-curves of coated specimens tested in a sour environment of weld metal and HAZ/FL at 24 C.

Fig. 20. Schematic illustration of the crack tip processes involved in the hydrogen induced fracture mechanism.

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pre-charging stage. Dimples were observed intermittently further away from the initiation site as a result of the reduced

concentration of hydrogen within the metal matrix.

The fish eyes in the SENT specimens were likely to be induced by the crack tip strain and the molecular hydrogen accu-

mulated at inclusions. The atomic hydrogen that had diffused to the HAZ or weld metal had recombined at the discontinuity

sites, e.g. interstitials or small inclusion cavities, to form molecular hydrogen. The applied pressure by the hydrogen gas cre-

ated location deformations, forming a blister or fisheyes. This may have resulted in a further reduction in fracture resistance

after the hydrogen exposure[30].

3.3. Relevance of the research findings for engineering critical assessments

Fracture limit states are required to be evaluated for the girth welds in pipelines in order to avoid failure during the

installation and operation phases by determining the criticality of the possible weld flaws. Unstable fracture, plastic collapse,

tearing and fatigue crack growth assessments are assessed under both static and dynamic (fatigue) loading. The flaw accep-

tance criteria for the pipeline girth weld defects are defined based on mechanical properties and design parameters using

fracture and fatigue assessments procedures as detailed in DNV-OS-F101. It has been shown from this study that the

mechanical properties of the pipeline girth welds are sensitive to the environmentally induced embrittlement and degrade

due to effects of hydrogen ingress. In such situation, when performing the ECA, the mechanical properties i.e. tensile, fracture

resistance and fatigue properties shall be representative of the actual environment [1].

It is imperative to use an appropriate test methodology to obtain an estimate of tensile stress strain properties as well as

toughness, to generate the relevant material input parameters for the fracture mechanics-based structural integrity assess-

ments. This may not be straightforward approach as hydrogen embrittlement is itself affected by the materials microstruc-ture, the specimen geometry, the amount of bulk hydrogen and the concentration of the test solution. In this study, NACE

Solution A with pH 2.7 was selected with reference to NACE TM0177-2005 which provides guidance for sulphide stress

cracking test of metals exposed to oilfield environments containing H2S. This is a standard test solution and the pH value

is on the conservative side as a result of the built in safety margin. Engineering projects involving sour service pipelines typ-

ically lie in the range of pH 34. It is therefore recommended that test solution be formulated in accordance with the specific

fluid composition, taking into consideration the presence of inhibitors, the H2S and CO2 ppm and pH of the solution.

The data established from this study may be used to bridge with the existing material database performed by other

authors in the same field. This will serve the purpose of establishing a reference database for use in the desk-top or actual

ECAs based on numerical and analytical methods. The material database will be essential in evaluating the fracture capacity

of NACE grade CMn pipelines operating in a sour service environment.

4. Conclusions

The current research documents the effects of wet H2S containing environment on the tensile and fracture properties of

NACE grade CMn pipeline girth welds. Tensile and fracture test specimens (pre-charged with H2S) were tested in solution

using customised test rigs. The following conclusions are drawn:

The tensile test results demonstrated different degrees of reduction in ductility i.e.35% in BM and 50% in WM as com-

pared to a non-sour environment.

Similarly, the fracture resistance test results of the BM, WM and HAZ showed reduction of0.3.

However, there was noticeable increase in the yield strength i.e. 3%in BM, 13% in WM and the tensile strength i.e.5%

in BM and 13% in WM.

When using specimens coated with ceramic epoxy on all sides except the top face and the notch, higher fracture

resistance J R-curves were obtained, with reduction in the fracture resistance by about 0.4 times and this was mainly

contributed by crack tip hydrogen and less so by the bulk hydrogen. It is recommended to use coated specimens during

testing as it is more representative in the case of fluid flowing inside a pipeline

Fractographs of the tensile specimens tested in an air environment showed fracture characteristic of microvoid coales-

cence or dimple fracture. The fractogaphs of specimens tested in a sour environment showed mainly quasi-cleavage

features. The presence of such brittle features was likely due to hydrogen ingress resulting in embrittlement.

Macro-section examination of the fracture surface of the SENT specimens tested in a sour environment contained fish

eyes, which demonstrated that the material had been saturated with hydrogen and that the fracture process involved

hydrogen diffusion. The fracture surface of specimens tested in an air was relatively smooth with no fish eyes present.

The fractographs of SENT specimens tested in a sour environment exhibited predominantly quasi-cleavage features,

which was more pronounced at the crack initiation area.

The data established from this study will serve the purpose of establishing a reference database for use in the desk-top or

actual ECAs based on numerical and analytical methods.

It is recommended to formulate the test solution in accordance with the specific fluid composition, taking into consider-

ation the presence of inhibitors, the H2S and CO2 ppm and pH of the solution.


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Acknowledgement

The authors acknowledge the Economic Development Board (EDB) of Singapore and Det Norske Veritas Pte. Ltd. (DNV) for

the financial support in pursuing this research via National University of Singapore (NUS) project number R265000440.

References

[1] DNV Offshore Standard DNV-OS-F101, Submarine Pipeline Systems. Det Norske Veritas; October 2013.

[2] BS 7910: 2005, Guide on methods of assessing the acceptability of flaws in metallic structures. British Standards; July 2005.[3] DNV Recommended Practice DNV-RP-F108, Fracture control for pipeline installation methods introducing cyclic plastic strain. Det Norske Veritas;

January 2006.

[4] Yi D, Sridhar I, Xiao ZM, Kumar SB. Fracture capacity of girth welded pipelines with 3D surface crackssubjected to biaxial loading conditions. Int J Press

Vess Piping 2012;92:11526.

[5]Zhang YM, Yi D, Xiao ZM, Huang ZH, Kumar SB. Elasticplastic fracture analyses for pipeline girth welds with 3D semi-elliptical surface cracks

subjected to large plastic bending. Int J Press Vess Piping 2013;105:90102 .

[6] ANSI/NACE MR0175/ISO 15156-1/2/3. Petroleum and natural gas industries materials for use in H2S-containing environments in oil and gas

production. 2nd ed. National Association of Corrosion Engineers; 2009.

[7] EFC publications number 16. Guidelines on materials requirements for carbon and low alloy steels for H 2S containing environments in oil and gas

production. 3rd ed. European Federation of Corrosion; 2009.

[8] Holtam CM. Structural integrity assessment of CMn pipeline steels exposed to sour service. Ph.D. Thesis, Loughborough University; 2010.

[9] Maddox SJ, Woollin P, Pargeter RJ.Corrosion fatigue of welded CMn steel risersfor deepwater applications: a state of theart review. In: Proceedings of

the ASME 24th International Conference on Ocean, Offshore and Artic Engineering; 2005. Paper no. OMAE2005-67499.

[10] Holtam CM. Investigation of fatigue crack growth performance of CMn pipeline steel in a sour environment. The Welding Institute (TWI) technical

paper no. 17840.02/2009/1366.3; 2009.

[11] Holtam CM. Engineering critical assessment of steel catenaryrisers in sour deepwater oil and gas developments. The Welding Institute (TWI) technical

paper no. 17840.03/2009/1372.3; 2009.[12] Holtam CM, Baxter D, Ashcroft IA, Thomson RC. Effect of crack depth on fatigue crack growth rates for CMn pipeline steel in a sour environment. Int J

Fatigue 2010;32:28896.

[13] Holtam CM, Baxter D. Fatigue performance of sour deepwater riser welds: crack growth vs endurance. In: Proceedings of the ASME 30th international

conference on ocean, Offshore and Artic Engineering, American Society of Mechanical Engineers; 2011. Paper no. OMAE2011-49581.

[14] Fassina P, Bolzoni F, Fumagalli G, Lazzari L, Vergani L, Sciuccati A. Influence of hydrogenand lowtemperature on mechanical behaviour of two pipeline

steels. Engng Fract Mech 2012;81:4355.

[15] Fassina P, Brunella MF, Lazzari L, Re G, Vergani L, Sciuccati A. Effect of hydrogenand lowtemperature on fatigue crack growthon pipeline steels. Engng

Fract Mech 2013;103:1025.

[16] Qi YM, Luo HY, Zheng SQ, Chen CF, Lv ZG, Xiong MX. Comparison of tensile and impact behaviour of carbon steel in H 2S environments. Mater Des

2014;58:23441.

[17] Yang ZM, Tronskar JP, Xu DQ. Fracture toughness testing using SENT specimens in a sour environment. In: Proceedings of the ASME 28th international

conference on ocean, Offshore and Artic Engineering; 2009. Paper no. OMAE2009-79305.

[18] NACE TM0177-2005. Laboratory testing of metals for resistance to sulfide stress cracking and stress corrosion cracking in H2S environments. National

Association of Corrosion Engineers; 2005.

[19] ASTM A370-13. Standard test methods and definitions for mechanical testing of steel products. American Society for Testing and Materials; 2013 .

[20] ASTMG 129-00. Standard practice for slow strain rate testing to evaluate the susceptibility of metallic materials to environmentally assisted

cracking. American Society for Testing and Materials; 2000.[21] Yuan WG, Wu YY, Xu DQ, Tronskar JP. Effects of strain rate on fracture toughness in sour environment. In: Proceedings of the ASME 29th International

Conference on Ocean, Offshore and Artic Engineering; 2010. Paper no. OMAE2010-20298.

[22] ASTM E1823-13. Standard terminology relating to fatigue and fracture testing. American Society of Testing and Materials; 2013 .

[23] Zaleski TM. Investigation of laser peening effects on hydrogen charged stainless steels. Masters Thesis, San Jose State University; 2008.

[24] Wu XQ, Kim IS. Effects of strain rate and temperature on tensile behavior of hydrogen-charged SA508 Cl.3 pressure vessel steel. Mater Sci Engng A

2003;A348:30918.

[25] Kane RD, Cayard MS. Roles of H2S in the behavior of engineering alloys: a review of literature and experience. Corrosion 98 paper no. 274; 1998.

[26] Andersen TL. Fracture mechanics: fundamentals and applications. 3rd ed. Boca Raton: Taylor & Francis; 2005.

[27] Jones DA. Principles and prevention of corrosion. 2nd ed. Prentice Hall; 1996.

[28] Tognarelli MA, Ramgopal T, Shademan S. Fatigue crack growth rate and fracture toughness of API5l X65 in sweet environments. In: Proceedings of the

ASME 32nd international conference on ocean, Offshore and Artic Engineering; 2013. Paper no. OMAE2013-10216.

[29] Feng G, Ramgopal T, Muller T. Role of sour environmentson thecorrosion fatigue growthrate of X65Pipe Steel. NACE corrosion thejournal of science

and engineering Aug 2012. ISSN 0010-9312: p.730-738.

[30] API Recommended Practice 571. Damage mechanisms affecting fixed equipment in the refining industry. 2nd ed. American Petroleum Institute; 2011.

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