Welding the First ERW X80 Grade Pipeline

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WELDING THE FIRST ERW X80 GRADE PIPELINEBarbaro F J, Bowie G F and Holmes W

ABSTRACT

Pipeline materials and construction costs are the most significant components of

major transmission pipelines. In Australia these costs have been contained over

the past two decades by the utilisation of high strength thin walled pipe. API 5L

X70 grade pipe is common place and 13km of X80 grade pipe has been installed

in a looping section of the Roma - Brisbane pipeline. The aim of the Roma -

Brisbane looping project was to fully evaluate the economic benefits associated

with the use of 8.8mm thick, 406mm diameter X80 grade pipe. The evaluation

involved development of weld procedures using both the conventional cellulosic

manual metal arc (MMA) process and a mechanised gas metal arc welding

(GMAW) system to determine the influence of weld metal strength on allowable

girth weld defect tolerance.

Although currently available cellulosic consumables have been shown

to undermatch the strength of X80 pipe, the full section pipe tension test

demonstrated tolerance to both Tier 1 and Tier 2 girth weld defect allowances.

These results support recent research which has shown that the tolerable level of

weld metal strength undermatching is related to the pipe wall thickness and the

defect depth assumption

Weld metal strength matching with an appropriate level of toughness was shown

using engineering critical assessment procedures to provide increased defect

tolerance. Defect tolerance under axial yield stress loading is more accurately

determined using destructive test methods.

KEYWORDS

Pipelines, X80, GMAW, Cellulosic, Full section pipe tension test, Defect

acceptance, ECA, Destructive test.

AUTHOR DETAILS

Frank J Barbaro, Chief Development Officer and Graham F Bowie, Senior

Development Officer, BHP Steel Flat Products, Port Kembla Steelworks, New

South Wales and William Holmes, Technology Manager Pipeline & Coatings,

Agility Team Build, Fyshwick Canberra, ACT

First published in the proceedings of the Welding Technology Institute of

Australia International Conference on Pipeline Construction Technolog, 4-5

March 2002, Novotel North Beach, Wollongong, Australia.

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The significant cost of pipeline materials and construction in

conjunction with competition with other energy sources hasdriven the development of high strength linepipe for transmission

of natural gas. The cost savings associated with high strength

pipe arise from a reduction in pipe wall thickness which reduces

both required steel tonnage and also welding cost. In comparison

with X70 grade pipe, X80 grade pipe represents approximately

12% reduction in total steel weight and up to 25% less deposited

weld metal. These benefits however are balanced by any increase

in the pipe / weld consumable costs and require that field welding

productivity is not compromised.

In Australia, where thin walled small diameter pipe is

commonplace, maximum economic benefits have been obtained

by the use of high strength linepipe up to and including X70grade pipe [1]. The continued use of conventional manual metal

arc (MMA)welding using cellulosic consumables for such pipe

designs has enabled field construction rates which have been as

high as 8kms per day.

The strength of X80 however, challenges the continued use of

cellulosic welding consumables because of their limit in strength

and also high inherent hydrogen content. The main issues in

the welding of high strength linepipe are resistance to hydrogen

assisted cold cracking (HACC) and sufficient weld metal strength

to match the pipe [2, 3]. Extensive investigations have shown

that under normal field construction practices HACC can be

avoided [4, 5]. The limited strength of cellulosic consumables is

a more serious concern and has been shown to undermatch the

yield strength of X80 grade pipe [6] and even X70 grade pipe at

the upper end of the normal strength range [7, 8]. It is pertinent

to point out however, that it is not simply the weld metal yield

strength that is the governing factor but rather the level of defect

tolerance relative to the pipe design. From an economic viewpoint

adequate weld metal strength matching is required to ensuresufficient tolerance to the typical weld defects which occur during

pipeline construction in order to avoid unnecessary repairs.

There is an important difference between weld metal yield

strength matching and weld metal strength matching. The latter is

directly related to weld defect tolerance, which not only depends

on the actual yield strength of the weld metal and the pipe, but

also the specified defect limits (particularly depth) and pipe wall

thickness. Yield strength matching will provide maximum defect

tolerance but is difficult to determine [7], particularly where

different yielding phenomena can occur in different suppliers of

high strength pipe grades.

To address these issues of weld metal requirements and fieldwelding productivity, AGL Pipelines undertook to construct a

section of the Roma to Brisbane Looping line using 8.8mm

thick, 406mm diameter API 5L X80 grade pipe. This particular

looping project was selected because the pipe dimensions

closely represented a number of proposed pipelines which could

also benefit economically by the use of X80 grade pipe. The

justification for the use of X80 grade pipe involved evaluation

of different welding processes with particular emphasis on

the assessment of defect tolerance. Cellulosic MMA and two

commercial automatic GMAW processes were evaluated by full

section pipe tension (FSPT) [9] tests to determine limits in girth

weld defect tolerance. Further evaluation was undertaken using

approved fracture mechanics methods to support the FSPT tests.

This paper details the results of the investigation and some field

welding experience using automatic GMAW.

01 INTRODUCTION

02 PROCEDURE

2.1. Pipe material

The pipe used in this program was 406mm diameter, 8.8mm thick

seam welded using the electrical resistance welding (ERW) process.The chemical composition of the pipe (Table 1) is characterised by a

low carbon content and controlled additions of microalloys required

for advanced thermomechanical rolling to optimise the level of

strength and toughness as well as control of weldability.

2.2. Mechanical properties

Tensile tests were performed in both the longitudinal and transverse

direction of the pipe. Pipe body Charpy impact tests were carried

out over a range of temperatures. Girth weld Charpy impact tests

were carried out at the minimum design operating temperature

which was defined as 0C

2.2.1. Welding procedure

Pipe girth welding trials involved conventional MMA welding using

cellulosic consumables and two different commercially availableautomatic GMAW processes.

The cellulosic welding procedure, C1, employed the standard

pipe mill prepared end bevel, which consisted of a 60 included

angle with a 1.5mm root face. Root opening varied between 1 and

1.5mm.

The first GMAW process, which will be referred to as G1, utilised

the standard pipe mill bevel but was ground just prior to welding

to remove the root face. The pipe ends were aligned without a

gap and relied on the welding procedure to ensure full penetration.

Internal segmented copper shoes were employed to prevent

excessive internal weld reinforcement and/or burnthrough.

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The second GMAW process, which will be referred to as G2,

utilised a narrow gap J type preparation machined on site

just prior to welding. This welding process employed a newly

developed mode of metal transfer, the surface tension transfer

(STT) technique, to deposit the root pass which avoided the

need to use internal copper shoes. Detailed welding conditions

are presented in (Table 2.).

2.3. Defect tolerance determination.

Defect tolerance was determined using a FSPT test, which was

developed by the Cooperative Research Centre for Welded

Structures. The test basically involves loading a complete

section of pipe, containing a girth weld and the defined

defect, in uniaxial tension up to the point of fracture. Defects

were produced in the root pass on the inside surface of the

pipe using electro discharged machining to a depth of 3mm

which is the assumed maximum depth of a girth weld defect.Assessment of the complete pipe diameter eliminates the

conservatism associated with other smaller scale tests such

as the well-known wide plate test. The test rig used in this

investigation is described elsewhere [6, 7, 9].

The aim of the test is to demonstrate that gross section

yielding (GSY), and not net section yielding (NSY), occurs

before fracture. The GSY criteria, which is defined below, is

designed to ensure that the weld metal containing the defect

has sufficient strength to transfer strain to the adjacent pipe

and so ensure a reasonable level of overall elongation of the

pipe before failure. NSY occurs when the strain is concentrated

in the weld metal and fails at low levels of elongation. It is

important to state that the GSY criteria is not designed to

prevent catastrophic failure but to ensure a defined level of

defect tolerance.

The GSY criteria was originally defined by the European

Pipeline Research Group (EPRG) [10] and requires that the girth

weld, containing the maximum allowable defect, under load in

uniaxial tension achieve a:

maximum test stress >= the parent pipe yield stress,

total elongation >= 0.8%, and,

remote or parent pipe strain >= 0.5%

In the FSPT test the maximum load is determined using a

calibrated load cell. Total elongation was measured with a

linear displacement transducer attached along the length of the

welded test pipes. Remote or parent pipe strain was measuredusing strain gauges. Recorded strain levels are verified by

subtracting the weld strain, measured by a clip gauge opening

across the weld defect, ie crack mouth opening displacement,

on the inside of the pipe, from the total elongation.

The crack mouth opening displacement (CMOD) not only

provides a check on strain levels but also uniquely defines the

onset of strain transfer to the pipe body. At the point of strain

distribution to the parent plate (or yielding of the pipe) the

CMOD is interrupted as the uniaxial load increases.

03 RESULTS AND DISCUSSION

The Australian Pipeline Standard AS2885.2 has a 3 tier approach

to assessment of girth weld defects, which is designed to improve

economics in pipeline construction. An increased level of weld

imperfections is permitted provided the girth weld possesses a

minimum level of strength matching and toughness. Tier 1 is a

workmanship level which, in general, permits 25mm long surface

breaking defects and 50mm long embedded defects. Tier 2

defect limits however, are a function of pipe diameter and wall

thickness and for the 406mm diameter 8.8mm wall thickness pipe

in the present investigation, the maximum defect length is 84mm,

irrespective of its position through the wall thickness.

The following results detail the pertinent characteristics of girth

welds produced in API 5L X80 grade pipe using different welding

procedures and its ability to meet the above mentioned defect

limits.

3.1. Radiography of welds

All welds fabricated as part of this investigation were examined

by conventional radiographic techniques and complied with

the requirements of AS2885.2. It was however noted that

radiographs of welds produced by the GMAW, G1 process

contained marks which corresponded with artefacts produced

on the surface of the root pass by the segmented copper shoes.

In general this did not interfere with the inspection process.

3.2. Chemical composition and microstructure

of welds

The chemical composition of final cap weld deposits is given in

(Table 3).It will be noted that the carbon equivalent of the GMAW

consumables were markedly different. The carbon equivalent of

G1 was the highest of all assessed and is related to the addition

of Ni along with the Cr and Mo levels. The carbon equivalent of

G2 was extremely low and is reflected in mechanical properties.

The MMA weld C1 also had a high carbon equivalent which

predominantly relied upon a relatively high carbon content and

additions of Mo and V for strengthening.

The relative level of deposited weld metal strength however

depends upon welding conditions and it is evident from the

macrophotographs presented in (Figure 1), that both GMAW

welds were welded at low heat inputs as evidenced by the

narrow width of visible weld HAZ. It is apparent from (Figure 1.)

that the MMA weld was carried out at weld heat inputs greater

than both GMA welds, refer (Table 2.).

The microstructure of weld C1 primarily consisted of ferrite

and pearlite throughout the entire weld thickness. This as

mentioned above is related to the weld heat input and also the

alloy design. A high weld heat input produces a low cooling

rate which promotes the formation of coarse equilibrium

microstructures and also increases the extent of recrystallisation

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of previously deposited underlying weld runs. The level of

heat input employed in weld C1 was evident by the complete

recrystallisation of the root pass (Figure 2a.) which was up to

4mm of the girth weld thickness.

Both GMA welds were characterised by distinct columnar

structures which persisted throughout the weld thickness.

The low weld heat input employed limited the extent of

recrystallisation of underlying weld runs. As a result both welds

contained relatively fine grained acicular ferrite and martensite

microstructures outlined by columnar grain boundary ferrite

(Figure 2b and c). The difference between welds G1 and G2

can be related to the consumable alloy design. Weld G1 with

a carbon equivalent some 16 points higher than G2 contained

significantly higher levels of martensite. This was most

prominent in the root pass of weld G1 where rapid cooling

over the copper shoes employed during root pass welding

further enhanced martensite formation. This observation wassupported by the hardness results presented later.

3.3 Mechanical properties

The results of tensile tests carried out on the parent pipe in both

the transverse and longitudinal directions are presented in Table

4. Recorded yield strengths in the transverse direction were

within a tight range with the maximum less than 70 MPa above

the minimum specification.

Conventional cross weld tensile tests demonstrated that all

weld procedures satisfied traditional workmanship requirements

with a tensile strength greater than that of the specified

minimum of the pipe (Table 5.). It should be noted however, thatboth welds C1 and G2, with the weld reinforcement removed,

failed in the weld metal at a strength less than that of the pipe.

Assessment of weld metal strength matching as determined

by the notched tensile test, although acknowledged as difficult

to interpret, revealed significant differences in weld metal yield

strength(Table 6.). Clearly the MMA weld C1 undermatched the

yield strength of the pipe by approximately 17% while the high

carbon equivalent of weld G1 provided a considerable level of

overmatching, approximately 8%. It is interesting to note that

the low carbon equivalent GMAW weld G2, that indicated slight

tensile strength undermatching in the standard cross weld

tensile test, in fact demonstrated yield strength matching in the

notched tensile test.

Clearly the recorded weld metal strength is directly related to

the alloy design and the welding conditions as evidenced by

the weld metal microstructures detailed above. The results

also highlight that the level of weld metal yield strength

undermatching may go unnoticed in the standard cross weld

tensile test. It is however important to emphasise that, as

mentioned in the introduction, strength matching is not the

only characteristic of a girth weld that influences weld defect

tolerance.

3.4. Hardness

Through thickness hardness (HV5) profiles were conducted

on all weld deposits, refer (Table 7). As expected the higher

strength GMA weld, G1, recorded the highest hardness with

values in the root pass up to 50 points higher than the pipe.

Both welds C1 and G2 recorded hardness values which were

below that of the pipe which supports the cross weld tensile

tests discussed previously. The level of hardness of weld C1

could again be attributed to weld microstructure.

It is important to note that the level of hardness recorded in

the root pass of weld G1 could not be solely attributed to the

carbon equivalent or weld conditions. The root pass of this

weld was carried out with the use of internal copper shoes

which has increased the cooling rate to produce the high levels

of martensite in the microstructure.

3.5. Toughness

Girth weld toughness was evaluated using the Charpy test at

0C and results are presented in Table 8. All welds satisfied

the minimum requirement of 22J minimum individual and 30J

minimum average specified in AS2885.2 which is required to

ensure that in the event of girth weld failure, fracture would

occur by plastic collapse and not in a brittle manner.

It is however evident that weld G2 clearly possessed a superior

level of toughness compared to both welds C1 and G1. The

low carbon equivalent of weld G2 along with the controlled low

heat input welding appear to have combined to provide a fine

grained microstructure. The outcome is an optimum balanceof toughness and, as shown later, adequate strength for the

welding of X80 grade pipe. Although a similar fine grained

ferritic microstructure was produced in weld G1, the higher

carbon equivalent and weld cooling rate has increased the level

of martensite to the detriment of toughness. The toughness

of the conventional MMA weld C1 can be explained by the

relatively coarse ferritic microstructure.

CTOD fracture toughness values for both weld C1 and G1 are

consistent with the measured Charpy impact test results Table

8. Unfortunately weld G2 was not tested, but based on Charpy

toughness, a CTOD significantly exceeding that achieved with

C1 and G2 would be expected.

3.6. Full section pipe tension (FSPT) tests

Limited girth weld samples prevented a complete assessment

of all weld procedures. Four tests covering defect lengths

of 75-150mm were carried out on weld C1. Unfortunately

only two tests were carried out on weld G2 while insufficient

material prevented any tests on weld G1. The artificial defects

produced were accurately controlled around the maximum

assumed depth of 3mm and provided a thorough assessment

of tolerance as defined by Australian Standard AS2885.2.

It is apparent from Table 9. that GSY was satisfactorily

demonstrated in weld C1 with a defect length up to 100mm.

The 125mm defect, which did not meet the gross stress

requirement by just 2 MPa could also be considered

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satisfactory if the slight increase in defect depth of this test

is taken into consideration. Despite this however, the results

clearly demonstrate that the maximum defined limit of the less

conservative Tier 2, i.e. 84mm, was quite easily achieved.

The defect lengths selected for GMAW procedure G2 was

based on the level of weld metal yield strength and previous

experience and unfortunately for the two tests carried out,

neither completely satisfied the GSY criteria. Interpolation of

the data however, in the form of a plot of the maximum stress

versus defect area (Figure 3.) strongly suggests that the defect

limit to be a length approximately 170mm which is significantly

greater than AS2885.2 Tier 2 limit of 84mm.

3.7. Field welding

The production sequence consisted of the normal pipe

stringing and alignment with an internal compressed air clamp.

The welding technique employed was identical to that of weld

procedure G1 above including pipe end preparation and the

use of internal segmented copper shoes. Only one welding

station was employed however.

The welding system consisted of two welding bugs each with

twin heads, which travelled around a metal band attached to

the pipe. Welding commenced by deposition of the root and hot

pass using one welding bug down one side of the pipe from the

12 oclock position to the 6 oclock position. Before completion

of this first run the weld start position was ground in preparation

for a similar run down the other side of the pipe, which

commenced as soon as possible but generally on completion

of the first side. As the root / hot pass on the second side ofthe pipe was being deposited, preparation for the fill and cap

was underway in a similar sequence to the root / hot passes.

Unfortunately however, severe arc blow was experienced

during welding of the root / hot pass run on the second side of

the pipe. Efforts to eliminate the effect indicated that the root

cause may originate from induced magnetic effects from the

twin welding heads. As a result of these issues the majority of

the X80 section was successfully welded with MMA cellulosic

consumables in accordance with procedure C1.

3.8. Engineering critical assessment of girth

weld defect limits

The determination of critical defect dimensions in a girthweld using fracture mechanics is not only dependent on the

mechanical properties of the weld metal and pipe but also the

assumed operating stresses. For a gas transmission pipeline

the stress imposed, which could be considered normal,

includes the field hydrostatic test and the maximum allowable

operating pressure. These are defined levels of stress for

which a defect limit can be estimated. More recent attention

however, has focussed on the capacity of girth welds,

containing defects, to withstand displacement controlled

loading, i.e. axial yield stress loads, which could occur in areas

of unstable ground.

An engineering critical assessment (ECA) was carried out

on the current pipe design using The Welding Institute

software program CRACKWISE 3 which is based on British

Standard BS7910 - 1999 Guide on methods for assessing the

acceptability of flaws in metallic structures, Level 2 analysis.

(Table 10.) presents the calculated critical length of 3mm deep

defects for the above three stress conditions as afunction of

fracture toughness and enables a direct comparison with the

current experimental FSPT test results.

Evident from Table 10. is a significant difference in the ECA

calculated critical defect length and the measured FSPT test

value under yield stress loading conditions. It is also apparent

that with the ECA under this loading condition (600MPa),

fracture toughness does not effectively influence defecttolerance since predicted tolerance is very low (length


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The chemical composition of X80 pipe-steel

C P Mn Si S Ni Cr Mo Cu Al V Nb Ti Ceq IIW

X80 pipe .065 .014 1.55 .31 .002 .023 .027 .28 .019 .026 .002 .068 .018 .39

TABLE 01

Reported conditions for each welding process

Weld Process Weld Pass Consumable Amps VoltsTravel Speed

mm/minHeat Input kJ/

mm

Cellulosic C1 root E8010 120 25 400 0.45

hot E9010 180 28 390 0.77

fill E9010 165 27 210 1.27cap E9010 140 27 195 1.16

GMAW G1 **

one consumable

root 0.9mm 230 22 ) )

hot austmig 250 22 ) ~ 950 ) ~ 0.35

fill NiCrMo 240 21 ) )

cap 80/20Ar/CO2 235 20 ) ~ 400 ) ~ 0.70

GMAW G2

one consumable

root 0.9mm 205 16 380 0.51

hot Hobart / 215 23 508 0.56

fill Thyssen 215 23 508 0.56

cap ER70S-6 80/20Ar/CO2 150 20 330 0.56

** Weld G1 employed internal copper shoes to avoid blow through during root pass welding

TABLE 02

The chemical composition of weld capping deposits

C P Mn Si S Ni Cr Mo Cu Al V Nb Ti Ceq IIW

C1 .155 .009 0.70 0.13 .007 0.47 0.39 0.33 .047


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Cross weld tensile properties of girth welds

Weld Code Weld Consumable UTS (MPa) Fracture Location Weld Reinforcement

C1 E8010/ E9010 650 weld removed

709 weld/HAZ reinforced

G1 NiCrMo 727 pipe removed

724 pipe reinforced

G2 ER70S-6 692 weld removed

725 pipe reinforced

TABLE 05

Notched tensile properties of X80 pipe and deposited weld metal

X80 parent pipe Weld metal (notched tensile test)

0.5% TEYS

(MPa)TS (MPa) Y/T ratio (%) YS (MPa) TS (MPa) Y/T ratio (%) YS matching ratio

C1 E8010/ E9010 585 703 83 483 629 77 0.83

G1 NiCrMo 600 714 84 649 741 88 1.08

G2 ER70S-6 591 717 83 590 692 85 1.00

Through thickness hardness profile of each Weld Procedure,Vickers HV5

Root pass Hot Pass Fill Pass Cap Pass Parent Pipe

Cellulosic C1 185 207 199 191 228

GMAW G1 287 252 245 244 233

GMAW G2 228 226 196 203 232

Weld Metal Charpy V-notch Test Results, test temperature 0C

Weld Specimen Size (mm) Min Value (J) Average (J)AS2885.2 reqts

min Indiv min AverageCTOD (mm)

Cellulosic C1 10 X 7.5 34 43 22 30 0.156

GMAW G1 10 X 7.5 54 72 22 30 0.113

GMAW G2 10 X 7.5 108 117 22 30 not tested

TABLE 06

TABLE 07

TABLE08

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Full Section Pipe Tensile Test Results

Defect Results

Weld Length (mm) Depth (mm) Area (mm2) Overall Elong (%) Parent Strain (%) Max Stress (MPa) Yielding Mode

C1 75 2.9 177 1.02 1.5, 1.4, 0.55, 0.5 587 GSY

100 2.9 236 0.74 0.5, 0.63, 0.48, 0.38 599 GSY

125 3.1 295 0.87 1.5, 0.53, 0.80, 0.51 583 GSY/NSY

150 2.9 353 0.75 1.7, 0.83, 0.54, 0.46 575 NSY

Tier 2 Reqt 84 3.0 >0.6 0.5 585 GSY

G1 not tested

G2 200 3.0 471 0.53 0.71, 0.55, 0.41, 0.26 573 NSY250 3.0 589 0.52 0.42, 0.46, 0.60, 0.32 586 NSY

Tier 2 Reqt 84 3.0 >0.6 0.5 591 GSY

Calculated critical length of 3mm deep surface breaking defects inAPI 5L X80 grade pipeline girth welds for 3 different stress conditions

and limits experimentally determined using the FSPT testFracture toughness c,

mmYield stress loading-

600MPaHydro test loading

167MPaMAOP Loading 119Mpa

FSPT test, yield stressloading, 600MPa

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Macrographs of girth welds. a) Weld C1 b) Weld G1 c) Weld G2FIGURE 01

a)

b)

c)

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Photomicrographs showing the characteristic microstructure of each weld

a) Weld C1 b) Weld G1 c) Weld G2.

FIGURE 02

0.10 mm

0.10 mm

0.10 mm 0.10 mm

0.10 mm

0.10 mm


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Full section pipe tension test results of Weld G2. Maximum stress plotted against defect area in

conjunction with measure parent strain enables estimation of the maximum defect area to meet the

GSY criteria

FIGURE 03

Defect Area mm2

Stress(MPa)

Welding the First ERW X80 Grade Pipeline

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