Scholarly article on topic 'Full- and small-scale tests on strain capacity of X80 seamless pipes'

Full- and small-scale tests on strain capacity of X80 seamless pipes Academic research paper on "Materials engineering"

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{X80 / "line pipes" / "fracture mechanics" / "full-scale test" / "girth weld"}

Abstract of research paper on Materials engineering, author of scientific article — A. Bastola, J. Wang, H. Shitamoto, A. Mirzaee-Sisan, M. Hamada, et al.

Abstract Application of high-strength steels such as X80 grade may help reduce cost in offshore pipeline projects through wall thickness optimisation and associated installation costs. To ensure reliability of such application during the pipeline installation and operation stages, further understanding on the strain capacity of girth welds of X80 grade pipes is required. This paper presents details of a research project containing small-scale and full-scale experiments on X80 line pipe specimen containing girth welds. Initial defects are introduced on the Heat Affected Zone (HAZ) and Weld Metal (WM) of the girth welds prior to the test in order to understand their fracture behaviour. The full-scale experimental program includes four-point bending tests with and without internal pressure applied, and reeling tests. Tensile properties of base metal, WM and HAZ are measured. Fracture toughness tests of WM and HAZ are also carried out. No through-thickness crack growth has been observed in the reeling tests and bending tests without internal pressure. However, some bending tests with internal pressure have seen crack growing through the thickness of the girth weld. Results from this study have shed light on the extent of crack growth in the girth welds of X80 pipes as influenced by their initial size, location, and internal pressurization.

Academic research paper on topic "Full- and small-scale tests on strain capacity of X80 seamless pipes"

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Procedia Structural Integrity 2 (2016) 1894-1903

www.elsevier.com/locate/procedia

21st European Conference on Fracture, ECF21, 20-24 June 2016, Catania, Italy

Full- and small-scale tests on strain capacity of X80 seamless pipes

A. BASTOLAa*, J. WANGa, H. SHITAMOTOb, A. MIRZAEE-SISANa, M. HAMADAc

and N. HISAMUNEc

Application oa high-strength iitee;ls sttch as X80 grade may help reduce c ost in offshore pipdline projects through wall thickness optimisation and associated installation costs. To ensure rehability of such application during the pipeline installation ¡and dperation stages, further understanding on the strein capacity of girth welds oo X80 grade pipe s is required. This paper presents detaHs of a research project containing small-ccale and full-scale expeftments on X80 line pipe specimen containine girth welds. Initial defects are introduced on the Heat Affected Zone (HAZ) and Weld Metal (PdM) of the gisth welds prior to the test in order to underptand their fractare behaviour . The full-scale experimental pro grem includes four-po int bending tests "with and without mternal pres sure applied, and reeling tests. Tensite poverties ss f base metal, PVM and HAZ ere measufed. Fracture touflmess tests of 1ihh/j and HAZ are also camed out. No trlsough-thCclrfess rrack growth har been observed in the reeling tests and bcndinf tests without internal pressure. However, some bending tests with internal pressure have seen crack growing through the ihiclenefs of the girth wetd. Resulto from this study have ehed light on the exernt of crack growth in the girth welns of X80 pipes as influenced by Sheir initial tize, location, and internal pressurization.

© 201g, PROSTR (Procedia Steuctural I^nttesgsirilty) Hosting by Elsevier Ltd. Alt sights reserved. Peer-reeiew under tesponsibility of tiie Scientific Committee ofECF21 .

Keyworgs: X80; luse p^ss; fraetase mefraotf s, fUll-tfleletett; gir^ weld_

1. Introduction

Duup watur pipulinus must withstand uxtrumuly high uxturnal pressures at high watur dupth. Pipu materials with unhancud propurties such as butter teujphnuss, ulongatio n and strain capacity can bu more attractive im duup Fttur.

* Corresponding author. Tul.: +44 2C 3816 4213 E-mail address a ajitbastela@envgl.com

aDNVGL, Palace House, 3 Cathedral Street, London, SE1 9DE, UnitedKingdom bNippon Steel P Sumitomo Metol Corporation, 1 -8 Faso-Cho, Amcgasaki, Hyogo, Japan cNiddon Steel P Sumitomo Metal Corporation, 1850 Minato, Cakayama City, Cakayama, Japan

Abstract

2452-3216 © 2016, PROSTR (Procedia Structural Integrity) Hosting by Elsevier Ltd. All rights reserved.

Peer-review under responsibility of the Scientific Committee of ECF21.

10.1016/j.prostr.2016.06.238

High strength steels such as X80 could be good candidate for deep water applications. However, strain capacity of the high strength steel pipes containing flaws need to be better understood when considering strain based design. Moreover, its fracture behaviour during reeling installation process where the X80 pipe will be subjected to high plastic deformation is not yet fully investigated or understood. Current fracture mechanics procedure such as BS7910 (2013) provides a general guidelines on girth weld defect acceptance criteria for longitudinal strains ranging from elastic to plastic. Also, common offshore pipeline design standards such as DNV-0S-F101 (2013) provides some useful but limited guidance on the girth weld defect acceptance criteria. However, these procedures do not provide explicit and detailed solution for strain limit (capacity) of pipelines containing flaws.

Nomenclature

API American Petroleum Institute

BM Base Metal

CMOD Crack Mouth 0pening Displacement

CTOD Crack Tip 0pening Displacement

DIC Digital Image Correlation

ECA Engineering Critical Assessment

HAZ Heat Affected Zone

NSSMC Nippon Steel and Sumitomo Metal Corporation

SENB Single Edge Notch Bend

SENT Single Edge Notch Tension

SMYS Specified Minimum Yield Strength

WCL Weld Centre Line

WM Weld Metal

WPS Welding Procedure Specification

^ave Average Nominal Strain

Aa Crack extension

Various studies have been carried out on factors that can affect the tensile strain capacity of girth welded high-strength steel pipes. Han et al (2012) studied the effect of microstructure on strain hardening for X80 steels. Similarly, Igi et al (2011) showed effect of internal pressure on tensile strain capacity, Fagerholt et al. (2012) carried out fracture analysis of SENT specimens, and Yi et al (2012) performed facture analysis of known flaws. These studies have highlighted the multiple factors that can influence the strain capacity of girth welded high strength pipes.

Presence of geometrical imperfections (misalignment, Hi-Lo, etc.), welding related defects (lack of fusion, slag inclusion, etc.) and residual stress make the girth welds the weakest link in the high strength steels linepipes. Strength mismatch effect (strength under matching) and HAZ softening are observed on high strength steel pipes. Motohashi et al. (2007) tested X80 steel curved wide plates and measured the strain localised close to the surface and over the specimen's gauge length. Yang et al. (2015) also tested welded joint of X80 grade steel and observed the HAZ was the fracture risk zone of the X80 steel weldment due to the presence of hard-brittle martensite-austenite (M-A) constituents. The study suggests the HAZ properties to have an effect on the tensile strain capacity of X80 pipes.

Tensile strain capacity of a pipe is often governed by the tensile strain (elastic and plastic) limit of the pipe's girth and the presence of defects in the girth weld is detrimental to the tensile strain capacity. In case of girth welded pipes with flaws, tensile strain capacity shall be calculated using fracture mechanics based approaches called ECA. ECA procedures were developed for stress-based situations but currently being under development for the strain-based cases. Pipelines are subjected to biaxial stress-strain state during operation and DNV-0S-F101 provides guidances on ECA procedures for uniaxial loading conditions as well as biaxial loading conditions.

No distinction is made between the effects of misalignment on strain capacity of high strength and conventional pipes in BS7910 (2013). Under quasi-static loading condition (for e.g. bending during operation), misalignment may be treated as a source of stress intensification which can reduce girth weld's defect acceptance criteria, particularly for surface breaking defects. Swankie et al. (2012) conducted tests on curved wide plates made of X100 grade steel, and showed the ratio between actual failure stress and that predicted by BS7910 ranges typically from 1.15 to 6.5. This illustrates the existing conservatism in BS7910 approach.

For high strength steels, Uniform Elongation Limit (UEL) is low, i.e., in the order of few percent for X80 and X100, meanwhile negligible for X120 steel grade. Low UEL is detrimental to the strain capacity of high strength steels. Kibey et al. (2009) looked at correlation between strain capacity and the combined effect of low UEL and flaw depth for X80 pipe specimens. One interesting conclusion drawn was that reducing UEL from 8% to 4% led to a corresponding reduction of approximately 0.5% in strain capacity for a constant flaw depth. It was also shown that thermal ageing as a result of thermal coating could change mechanical properties of X80 linepipes.

In this paper, the application of high strength X80 steel grade pipes in a strain-based (displacement-controlled) situation is studied. This study emphasised on the strain capacity of the girth welds, and to elaborate their effect, welds containing flaws have been introduced deliberately into the girth welds. This extensive research programme features small-scale specimen tests, full-scale bending tests and full-scale reeling tests. A large number of accompanying detailed FEA have also been carried out and will be published in due course.

Although the terms "flaws", "defects", "notches" and "cracks" have somewhat different meaning, they are used interchangeably and indicate an Electronic Discharge Machining (EDM) notch in the context of testing in this paper.

2. Experimental Procedures and Measurement

In this research, a total of 11 API-5L X80 seamless linepipes (nominal outer diameter, OD, of 273.1 mm and nominal wall thickness of 14.3 mm) have been manufactured and welded as per the approved WPS by NSSMC. Girth welded joints are produced by Pulsed Gas Metal Arc Welding (PGMAW), further details of the weld and its typical macrostructure is given in Table 1.

Table 1. Welding parameters

Welding details Parameters

AWS Classification A5.28 ER80S-G

Welding Position 5G

Equipment Root: Power Wave 455M/STT (LINCOLN) Others: DP-350 (DAIHEN)

Preheat Not Applied

Groove design J Groove

Backing Not Applied (Root path welding)

Macrostructure 1 ~ ! 1

Specimens from two 3m-long pipes with two girth welds are used to carry out small-scale tensile, SENB and SENT tests. Another seven pipes are used for the full-scale bending tests, and the remaining two pipes are used for full-scale reeling tests. The pipes used for the full-scale tests are all 11m long. Accompanying sets of sensitivity studies via 3D FEA are then carried out to determine the required initial flaw sizes in full-scale tests. Flaws are introduced on the WM and HAZ of the pipes in preparation for the full-scale tests through the EDM technique. Full-scale bending (with and without internal pressure) and reeling tests are then conducted to examine the strain

capacity of X80 pipes with girth welds and known initial flaws sizes and locations under realistic strain-based conditions.

2.1. Small scale tests

Both pipes dedicated for small-scale tests consist of three pieces of 1m-length pipe segments welded together to a total length of 3m each. The welded pipes for material testing have been selected by the manufacturer, NSSMC, to be fully representative of a typical production. The testing covered in this procedure were tensile, SENB and SENT. Ten full stress-strain curves have been produced, five for BM and the other five for WM in accordance with DNV-0S-F101 and EN ISO 6892-1 (2009). Round bar test specimens have been extracted in the longitudinal direction of the pipe for the BM and in the circumferential direction for the WM. The gauge length is four times the diameter of the specimen. Fourteen deeply notched SENB specimens are tested to obtain the R-curves in accordance with EN ISO 15653 (2010). SENB specimens have high constraint condition at the crack tip and will provide conservative R-curves (DNV-RP-F108 (2006)). Six specimens were notched at the WM and eight at HAZ. Both J-R (Crack driving force - Resistance) curve and CTOD-R (Crack Tip Opening Displacement - Resistance) curve were determined using the multiple-specimen method (minimum six specimens for each crack location). Likewise, fourteen SENT specimens have been tested; six notched at WM and eight notched at HAZ. The testing has been conducted according to DNV-RP-F108. J-R curves for WM and HAZ are determined using the multiple-specimen method. The loading mode and crack tip constraint in the SENT specimen are close to those for a flaw in a pipeline's girth weld under bending and axial loading conditions.

Additionally, DIC technique has been used to characterise the local stress-strain behavior in the BM, WM and HAZ region as well as produce representative full stress-strain curve for HAZ. DIC specimens are cut from the welded pipes using the EDM technique, providing a rough sample surface which produces photographic images with random contrasting features (speckles) that DIC software can track to determine local displacements. Each DIC test has been verified using strain gauges, and a clip extensometer as additional verification for one test.

2.2. Full scale tests

Two types of full-scale testing have been performed: bending tests and reeling tests. The bending tests are carried out with and without internal pressure to simulate certain operational conditions of these pipelines; meanwhile, the reeling tests were carried out to simulate installation conditions under high plastic deformation. The pipes for full-scale bending tests are 11m long and each containing one girth weld in the middle. The pipes for reeling are also 11m long but have two girth welds spaced 1m apart at the central section. The pipes have been fabricated and welded in a NSSMC's mill in Japan and shipped to DNV GL's laboratory in Norway for testing.

Seven out of eleven X80 pipes have been fabricated for 4-point full-scale bending test. Among those, four pipes have been tested for bending without any internal pressure. A small internal pressure of 10bars is applied on these four pipes to detect the through thickness crack growth easily. Meanwhile, an internal pressure of 479 bars (70% Specified Minimum Yield Strength (SMYS) of hoop stress) is applied on the remaining three full-scale bending tests. 10mm thick end caps with control valves are used on both sides of the pipe to maintain the desired internal pressure. Pre-defined EDM notches are introduced to simulate the tip of the crack in WM and HAZ. The pipes are supported in the ends with two large steel rollers and the load is applied on the pipe through two rigid strips attached to fixed hydraulic cylinders. The hydraulic cylinders are mounted with 500kN load cells. The tests are then carried out in displacement-controlled mode. A schematic of the 4-point bend test-setup and an image of actual test for 4-point bend test are given in Fig. 1.

Two pipes have been fabricated for the full-scale reeling tests. Similarly, a small internal pressure of 10bars is also applied on these two pipes detect the through-thickness crack growth. A former with two curved sides is used to simulate the reeling and straightening processes. The radius of curvature of the former is 5300mm, which can induce up to 2.5% nominal strain on the pipe. Unlike 4-point bending test, hydraulic cylinders are connected using 100mm-

diameter rods to the former and then drawing the former against the pipe until the pipe is fully bent against the former. Two reeling cycles are performed and after each reeling cycle, the pipe and the former are flipped in order for the straightening process. The cut-outs on the straightener provide room for mounting the clip gauges to measure CMOD. A schematic of the reeling test setup and a representative photo of reeling tests are shown in Fig. 2.

Fig. 1. (a) Schematic diagram of the 4-point bending test set-up for 11m long linepipe; (b) 4-point bend test setup

The seamless pipes are expected to show wall thickness variation in the circumferential and axial directions. Ultrasonic measurement technique is used to measure thicknesses at distances of 0.5, 1.0, 1.5, 2.0 and 3.0 times OD on both sides of the weld at 12, 3, 6 and 9 o'clock positions (hence altogether 20 measurements). The wall thickness values measured range from 13.5mm to 14.9mm for all pipes considered. Also, in order to have accurate measurements of the weld geometry including weld cap and weld toe, 3 pipes considered for pressurised 4-point bending tests are scanned using 3D scanning technique. The misalignments measured are all less than 0.2mm.

For 4-point bending tests, each pipe is attached with strain gauges (5mm, uniaxial) at 0.5 OD, 1 OD and 2 OD from the WCL and on the WCL at both 6 o'clock and 12 o'clock positions (hence 16 strain gauges mounted on each pipe). Likewise, for reeling tests, each pipe is attached with strain gauges (5mm, uniaxial) at 1 OD from the WCL, on the WCL and at the middle of the pipe length, at both 6 o'clock and 12 o'clock positions (hence 18 strain gauges mounted on each pipe). Clip gauges are mounted at the centre of the crack to measure CMOD and silicon replica (Microset 101RF synthetic rubber) is used to measure the initial flaw depth and CTOD by injecting the silicon compound into the flaw during the test and measuring the solidified compound in a stereo light microscope. The silicon replica is also injected periodically during the tests to measure CMOD, CTOD and crack growth for an applied strain, an example of which is shown in Fig. 3.

Fig. 3. An example of silicon replica used to measure CMOD, CTOD and Aa 3. Experimental results and discussion

3.1. Small scale results

Comparison of ten tensile tests between BM and WM are shown in Fig. 4 (a). Yield strength of the WM overmatches that of BM by approximately 20%. Uniform curves have been produced and no change in tensile properties due to circumferential position has been observed. Through DIC technique, local stress-strain curves from various regions of the inner diameter specimen are generated and shown in Fig. 4 (b). The HAZ locations mentioned here are at 3.2mm from the WCL to either side of the weld i.e. locations 1 and 2 in Fig. 4 (b). The HAZ tensile tests shows that it is softer by about 7.5% compared with BM at 1% strain. The parent material undergoes very little yielding with much of the strain concentrated at and near the weld line and HAZ.

A hardness map of the weld cross-section is shown in Fig. 5. The hardness increases from the outer diameter to the inner diameter in BM. The WM has the highest hardness, followed by drop in the hardness in the HAZ region, which is roughly at 5mm from the WCL for mid-thickness. In addition to that, a high hardness in the fusion zone is observed. Local strain measured from DIC shows the parent material undergoes very little yielding with much of the strain concentrated at and near the weld line and HAZ for inner diameter region. The strain is concentrated in regions close to the weld, with particularly high concentration in the HAZ regions on the either side of the weld. For both the mid-thickness and inner diameter specimens the yield strength of the HAZ regions is significantly lower than that of the parent material.

All the fracture toughness tests were performed at room temperature. Lower bound fit of SENB HAZ and WM CTOD-R curves are shown in Fig. 6 (a). The fracture toughness (CTOD) of HAZ is more than double that of the WM for a unit stable crack extension. Fig. 6 (b) shows the lower bound fit of SENT HAZ and WM CTOD-R curves. It shows the fracture toughness (CTOD) of HAZ is more than 1.5 times that of the WM for a unit stable crack extension from SENT specimens.

900 800 700 | 6CO | 500

.§ 400

1300 c

200 100 0

-BM-Sample 1

----BM-Sample 3 --BM-Sample 4 - ■ -BM-Sample5

-WM-Sample 1 .......WM-Sample 2 ----WM-Sample 3

---WM-Sample 5

Engineering Strain [%)

— 700 a.

in in ai

.= 300

I -BM — KAZ (location 11

HAZ (location 2)

Engineering Strain (%)

Fig. 4. (a) BM and WM tensile tests results; (b) DIC test results

Fig. 5. Hardness map across the weld cross section

♦ CTOD-R curve HAZ, testrssults

-CTOD-R curve HAZ, lower bound fit # ^

-CTOD-R curve WM lower bound fit = 0.262ial,f;

CTOD^ =0.118fiaD71

1.6 1,4

E 1.2 E

£ 0.4

C 2 02

ctodH42 =

CTODWM = 0.623Aa°

♦ * / —---

♦ CTOD R curve HAZ, test results

* CTOD-R curve WM. test results

* -CTOD-R curve WM. lower bound fit

J -CTODtR curve HAZ, lower bound fit

da [mm]

Fig. 6. (a) CTOD-R curve from SENB tests; (b) CTOD-R curve from SENT tests

3.2. Full scale results 3.2.1. Benling results

Four pipes are subject to 4-point bending test without internal pressure. Three of them feature initial flaw sizes of 3*50mm, 4 x50mm and 5*50mm in the HAZ region respectively, and the remaining pipe features an 3*50mm initial flaw on the WCL. The pipes are loaded until the full stroke of the hydraulic cylinders is reached; this produces significant tearing of these flaws. No through-thickness crack growth has been observed in these tested pipe since no pressure drop (-10 bars) has been observed. Before each measurement the pipe is unloaded up to 10% of the

maximum load so the technician can safely conduct measurements. The silicon samples from the non-pressurized 4-point bending tests show no significant tearing occurred initially and the crack extending by blunting. After CMOD reaches approximately 1mm, significant tearing occurs with a sharp crack tip.

Likewise, three pipes with defects on WCL and HAZ have been tested for 4-point bending with internal pressure. One pipe has defined initial flaw size of 3 x50mm on the WCL and the other two have flaws of 3 x50mm and 4 x50mm on the HAZ respectively. All three pipes are loaded until failure. A through thickness crack has been observed in the pipe with WCL 3x50mm initial flaw when loaded to failure. Significant drop in the pressure and a visible water jet around the flaw region indicates through-thickness crack has occurred. The pipes with HAZ flaws fail by unstable crack growth and rupture of the pipe. In both tests with HAZ flaw, the crack kink into the base material and propagated around the circumference of the pipe. All three pipes undergoing 4-point bending with internal pressure failed at strain levels close to 1.3%. A summary of the all the bending tests in terms of CTOD against the average strain at 12 o'clock position is presented in Fig. 7. The rate of increment of CTOD against strain is higher for the pressurised pipe regardless of the location or size of the flaw and could not reach to 2% nominal strain before through-thickness crack occurred.

L _ ) (i WCL 3x56 without pressure -*H-HAZ 3x5Q without pressure I —•-(■HAZ 4x5Q without pressure

u / 1 I / } ___J__ —Hr HAZ 5x5Ci without pressure —^rWCL 3x50 with pressure * -»J-HAZ 3x50 with pressure 1 —hr^ HAZ 4x50 with pressure

0 0.5 1 1.5 2 2.5 3 3.5

Strain (%)

Fig. 7. Average strain vs. CTOD diagram for 4-point bending test of the pipes with and without internal pressure

Table 2. Flaw geometry measurements from 4-point bending test

a___b_

HAZ 3 x50mm flaw without internal pressure HAZ 3 x50mm flaw with internal pressure

^ave CMOD (mm) CTOD (mm) Aa (mm) ^ave CMOD (mm) CTOD (mm) Aa (mm)

0.80 0.41 0.22 0.05 0.65 0.68 0.68 0.45

1.28 0.55 0.37 0.15 1.03 1.19 1.06 0.85

1.75 0.76 0.54 0.26 1.13 1.40 1.30 1.05

2.45 0.93 0.59 0.31 1.27 2.54 2.33 2.72

2.96 1.10 0.73 0.42

Comparison of CMOD vs. strain on either side of the weld shows non-uniform strain distribution. It can be due to local non-uniformity of the material and variation in pipe thicknesses. The CTOD results presented in this paper are against the averaged strain (Save) at 12 o'clock position. CMOD, CTOD and Aa are all measured from the crack growth replica. Comparison of these measurements for bending tests with and without internal pressure is given in

Table 2.

3.2.2. Reeling results

The pipes with WCL defects of 3*30mm and 3*40mm and HAZ defects of 3*30mm and 3 x40mm flaw sizes have been tested in the reeling test set-up. No through thickness crack was observed in the tested pipe. The through thickness crack would have been identified by pressure drop of 10 bars during the test. Likewise, silicon replica measurements are also performed for each test. Similar to bending test, before each measurement, the pipe is unloaded up to 10% of the maximum load for safety reasons. The average strain readings (eave) at 12 o'clock position are presented along with CMOD, CTOD and crack growth from replica as shown in Table 3. The strain is averaged from two remote strains, each at 1 OD distance from the WCL towards the ends of the pipe. Only results from two cycles of reeling tests are shown. No results from the straightening of the pipe are shown as the crack is closing during straightening and no loading at the crack tip is expected. A summary of all the reeling tests in terms of CTOD against the averaged strain at 12 o'clock position is presented in Fig. 8. CTODs for 3*30mm HAZ flaws increased faster with nominal strain compared with 3*40mm HAZ flaws. For WCL flaws, on the other hand, no clear distinction between 3 x30mm and 3 x40mm has been observed. Nonetheless, the CTODs are higher for the second reeling cycle than the first reeling cycle. Table 3 also reveals that increment of CTOD as a function of crack growth is approximately linear for 3 x30mm and 3 x40mm HAZ flaws.

—♦■( HAZ 3*30 Reeling CI ■ :!■■ HAZ 3x30 Reeling C2

HAZ 3x40 Reeling CI •••>H HAZ 3x40 Reeling C2

.........

'"i*'""'

I1'''' ! !

Strain (%)

£ 0.2 a

§0.15

•■•«■■• ^JCL3x30Reeling CI

••-■•• WCL 3x30 Reeling C2 Vk/CL 3x40 Releling CI fx

••■*•• 3x40 Rejeling C2 7 / ¡ *

i •■' A'í'"."'"".-^

i-' .i' .• '

Strain (%)

Fig. 8. Crack growth vs. CTOD diagram for 2 cycles of reeling test with HAZ and WCL flaws

Table 3 Flaw geometry measurements from reeling tests

HAZ 3x30 mm

CMOD (mm) CTOD (mm) Aa (mm)

HAZ 3x40 mm

CMOD (mm) CTOD (mm) Aa (mm)

Cycle 1

Cycle 1

1.34 0.79

2.20 0.87

1.40 0.50

2.30 0.53

2.34 0.88

2.50 0.58

Cycle 2

Cycle 2

1.50 0.81 0.69 0.48 1.49 0.46 0.35 0.30

2.18 1.17 1.00 0.69 2.20 0.67 0.50 0.38

2.40 1.25 1.07 0.72 2.50 0.75 0.57 0.49

4. Conclusions

Small-scale and full-scale experiments on X80 line pipe specimen containing girth welds have been carried out to determine the strain capacity X80 pipes with girth welds. The observations are as follows:

- Small-scale tests have revealed a variation of through-thickness tensile and hardness properties in the X80 pipes. Results of the tensile tests indicate that the yield stress overmatch of approximately 20% for the weld metal, and that the HAZ region is about 7.5% softer compared with BM at 1% applied strain. Both SENB and SENT test results show that the fracture toughness from the WM is lower than that from the HAZ. Hardness measurements on the weld cross-section show that hardness is consistently high in the fusion zone, it decreases on either side of the fusion zone in the HAZ regions. The hardness of the parent material varies significantly with position (distance from the weld root). High hardness in the parent material at the inner diameter of the pipe is also observed which decreases towards the outer diameter.

- The full-scale reeling tests have shown that the welded X80 pipes can sustain a nominal strain level of up to 2.5% without any instability for the range of defect sizes considered.

- The full-scale bending tests have shown that, for similar levels of applied strain, the CMOD of the HAZ flaw is generally higher than that of the WCL flaw of the same initial size. In addition, tearing of HAZ flaws can be much higher than that of WCL flaws. During pressurised full-scale bending tests, a through thickness crack and unstable crack growth are observed for WCL flaw and HAZ flaw respectively, leading to the conclusion that HAZ softening may limit the acceptable critical size in an ECA for X80 pipes.

Acknowledgements

The authors gratefully acknowledge the significant contribution to this work from the Open University, UK and from DNV GL Hevik Materials Laboratory, Norway. The authors would also like to thank Erling 0stby for reviewing and providing feedback on this paper.

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