WELD-DEFECTS-PART-B

8/7/2019 WELD-DEFECTS-PART-B

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Draft for review B - 1 Part B

EXTENSION OF EPRG-TIER 2 GUIDELINES

ON ALLOWABLE FLAW SIZES

Part B Experimental validation and recommendations

1 INCENTIVES

The increased use of X80 pipeline steels has created a great interest to extend the EPRG-Tier

2 guidelines for girth welds in X80 pipeline. Even so, there exist interest to apply the

guidelines outside the pipe wall thickness range of 7 mm to 25,5 mm. In particular, the

extension of the guidelines for wall thickness exceeding 1 inch would be of benefit for the

offshore industry. Extension to thinner pipe welds (< 7 mm) would be valuable for small

diameter and or high strength thin wall pipelines.

At present, the maximum allowable flaw height for EPRG-Tier 2 is 3mm. This restriction is

based on the assumption that weld flaws would typically be only one weld run high. This limit

is an unfortunate oversight, because with the development of new high productivity welding

processes bigger weld passes might be produced. More importantly, when planar flaws are

detected the critical dimension or flaw height sentencing is not easily quantified. Thus,

possible errors in flaw sizing are an important issue for consideration. Sizing errors exceeding

1 mm cannot be excluded. When taken together, one can conclude that an actual flaw of 3

mm high is not allowed. This problem is compounded by the fact that larger sizing errors are

very likely for surface-breaking flaws in combination with hi-lo. Therefore, it is desirable to

have flaw length limits as a function of flaw height, Fig. B1

Fig. B1 Schematic illustrating the effect of flaw height on allowable flaw length

EPRG Tier 2

Defect length (mm)

Defectheight(mm)

l h = 7wt.3 mm

GSY


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In simple terms, Fig. B1 gives a schematic presentation of the method used to relate the

predicted (curve) to the maximum allowable flaw size as a function of flaw height. In

numerical terms, the approach followed in this study consists of using the current ERPG Tier

2 flaw area limit of 7 %1, and on the basis of this limit to determine the corresponding

allowable length limit for flaw heights of 4mm and 5 mm.

2 MEANS

The original EPRG-Tier 2 guidelines were based on, and validated by means of the results of

187 Curved Wide Plate (CWP) tests that were carried out in the late 1980s and the early

1990s. Since that period of time, an additional 195 single notched CWP tests and

corresponding small-scale tests, namely Charpy V impact, CTOD, all-weld metal and pipe

metal tensile, have been conducted on newer types of steels and welding processes.

2.1 Laboratory Soete CWP data base

Today, Laboratory Soete database contains 382 fully documented2 results of single notched

curved wide plate (CWP) tests. The results of another group of 47 CWP test could not used in

the study since the mechanical and toughness properties of the pipe and girth weld were

neither determined nor provided3. Before the integrated database was created all original test

reports were reviewed. The large majority of the CWP specimen was extracted from welded

large diameter (> 30 inch) pipe. The girth welds were made by one of the following welding

processes: SMAW, GMAW or FCAW. It should be noted that 60 out of the 382 CWP testswere conducted on pipe material.

The CWP specimens contained either a machined or a fatigue-sharpened surface-breaking

notch. For the welded specimens, the notch was introduced from the root side of the girth

weld. The fatigue cracks were normally deeper than 3 mm. Flaw lengths varied from 12 mm

up to 350 mm. All CWP specimens were tensile loaded to failure or interrupted at maximum

load. For each CWP test, information on the failure stress and strain, the stress-strain

behaviour during loading and the deformation behaviour, visualized by the Moire technique,

at specimen failure or at maximum load is available.

1 This limit was derived from the ratio between the cross sectional area of the allowable flaw and the gross cross sectionalarea per 300 mm length of weld: 7t x 3 mm/300mm x t (see also Section 2.3)

2 In addition to the CWP tests, the pipe and girth welds were subjected to pipe metal tensile (axial direction), hardness, all-

weld metal tensile, Charpy V impact and CTOD testing. The Charpy and CTOD toughness properties were measured at the

weld metal centerline. In most cases, the Charpy and CTOD full temperature transition curves are available.

3 Furthermore, CWP specimens containing natural flaw (porosity, slag, lack of fusion, lack of penetration, etc.) were excluded

from the assessment (157 test results). Also, the tests on pipes and girth welds containing multiple flaws have not been

reviewed (120 results). These two sets of test results cannot be used in the present study since the development of specificflaw interaction criteria for ductile material behaviour is needed. Thus, the present assessment will be focussed on the test

results of 382 single notched CWP specimens.


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The composition of the single-notched CWP database used for the study of the extension of

the EPRG-Tier 2 guidelines is shown in Table B1. In this Table, the data is arbitrarily grouped

on the basis of flaw height and level of weld metal yield strength mismatch (UM, pipe metal

and OM). Plain pipe (60 results), undermatched (77 results) and overmatched girth welds

(219 results) in API 5LX52, X60, X65, X70 and X80 steels in the wall thickness range of 5.0

mm up to 30.5 mm were tested.

Test Variable (Number of specimens)

Length/wall

(l/t) ratio

FlawHeight

Range

(mm)

Level of

matching

Totalnumberof CWP

tests 7 mm > 7 mm

Y/T > 0.90

(Pipe)CVN < 40J

RemoteStrain

at failure

< 0.50 %

OM 73 38 35 6 22 7

UM 43 24 19 9 9 14 3

Pipemetal

51 9 42 8 - 3

OM 54 51 3 9 29 13

UM 25 3 22 - 6 163 - 4

Pipemetal

13 2 11 4 - 4

OM 38 16 22 1 9 7

UM 9 9 - - 5 74 - 5

Pipe

metal

4 2 2 - - 1

OM 20 15 5 - 5 3

UM 1 - 1 - - 15 - 6

Pipemetal

10 6 4 - - 0

OM 33 27 6 - 15 14

UM - - - - - - 6

Pipemetal

8 6 2 6 - 7

Table B1 - Database of single notched curved wide plate test (CWP)

Table B1 illustrates that a broad range of test variables (flaw height, wall thickness, level of

weld metal mismatch, etc,..) and a wide range of material properties (Y/T ratio,

toughness,etc,..) have been studied. Therefore, the one might conclude that the current CWP

and corresponding small-scale test database is sufficiently documented for use in a re-

assessment of the current EPRG-Tier 2 guidelines, and to verify whether the EPRG-Tier 2

guidelines can be used or extended for the assessment of:

girth weld flaws in pipe grades exceeding X70.

girth weld flaws in thin (< 7 mm) and heavy (> 25.4 mm) wall pipe.

flaw heights exceeding 3 mm.


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2.2 Method of presentation

2.2.1 Entire CWP database

The CWP test results available are presented in term of failure strain versus the relative flaw

cross sectional area in Fig. B2. It may be noted that the relationship between the failure

strain and the relative cross section area is far from unique. The observed scatter in failure

strain is due to the variation in toughness, level of weld metal yield strength mismatch and

Y/T ratio.

Fig. B2 Remote strain as a function of the relative cross-sectional area (entire database)

This preliminary assessment also illustrates that the number of free parameters must be

reduced in order to identify the effect of the individual material properties on failure strain.

0

2

4

6

8

10

0.0 0.1 0.2 0.3 0.4

lh/tW

Remotestrainatfailure(%)

Undermatching

OvermatchingPlain pipe

See detail

Labo Soete - Gent

l = flaw length

h = flaw height

t = wall thickness

W = arc length

0

0.4

0.8

1.2

1.6

2

0.00 0.04 0.08 0.12 0.16 0.20

lh/tW


Undermatching

Overmatching

Plain pipe

Strain = 0.5 %

Labo Soete - Gent


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2.2.2 Detailed presentation of CWP database

The most simple and expedient approach to analyse the data is to separate the data on the

basis of pipe grade, wall thickness and flaw height. In presenting the data, plots of the failure

strain versus the flaw size in dimensionless form were used to determine the correlations.

The following Sections present a series of plots of the relevant CWP test results in terms of

remote strain at failure (or maximum load instability) as a function of

flaw length normalized by wall thickness (l/t), and/or,

flaw area normalized by the wide plate gross cross section area (lh/Wt).

For convenience of interpretation, and to obtain more information from the experimental

database, the Figures to be presented contain a key to identify the individual data points. Theidentification of these points is limited to variables such as wall thickness range, flaw size

(length or height), level of weld metal yield strength mismatch (undermatching or

overmatching), toughness (level of Charpy V impact toughness) and pipe Y/T ratio. Moreover,

for the cases where a large amount of data is available, the Figures also contain an enlarged

view of the low strain area

Furthermore, the horizontal dashed line these Figures represents the performance criterion of

0.5 % remote strain while the intersecting horizontal and vertical solid lines (further denoted

as the EPRG-box) delineate the current EPRG -Tier 2 flaw limits. That is, a data point

located within the EPRG-box should be result of a CWP test, which failed to comply with theERPG Tier 2 material property requirements. The three requirements are related to weld

metal toughness (30J minimum/40J mean) and mechanical properties (weld metal is

matching or overmatching and pipe metal Y/T ratio in the longitudinal direction does not

exceed 0.90). Data points outside this box indicate that the measured failure strains are

greater than assumed, and are therefore conservative

3 EXPERIMENTAL VALIDATION OF ERPG-TIER 2

3.1 Current limit

The current EPRG-Tier 2 flaw acceptance limit is based on the DEN plastic collapse model,

adapted for pipe yielding (GSY), pc = YS, Eq. (A7). The maximum allowable flaw length for

GSY can be derived the following equation (or Eq. (A9)):

h

ts

R1

)R1(l

+

= (A9)

By using Eq. A9, it is assumed that the pipe yields over an arc length s (or plate width, W)

while R is the pipe metal yield to tensile ratio as measured in the longitudinal direction of


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pipe, l is the total length of the flaw, h is the flaw height and t is the wall thickness. A

schematic presentation of Eq. (A9) is shown in Fig. B1 (see curve labelled by GSY)

Curved wide plate test results have shown that conservative prediction are obtained if the arc

length, s, is set at 300 mm, i.e. approximately 10 % of the circumference of a large diameter

pipe. For a yield to tensile ratio, R, of 0.87, the limiting length, l, for a 3mm high flaw is

obtained from Eq. (A9) as 6.95t, or in practical terms, 7t. This length gives a flaw area limit

of 7 % per 300 mm length of weld. In other words, the ERPG Tier 2 maximum allowable flaw

size has been derived from and validated by CWP test results. The same CWP database

indicated that GSY can be guaranteed for flaw smaller than the maximum allowable flaw size,

7t (length) x 3 mm (height) in any 300 mm length of weld, if a Charpy toughness of 30J

minimum, 40J mean is achieved at the design temperature, the Y/T ratio of the pipe in the

longitudinal direction is less than 0.90 and the weld metal is matching or overmatching. The

Y/T ratio and weld metal yield strength requirements are specified to demonstrate that the

weld metal flow stress is greater than that of the pipe metal. Finally, recall that EPRG-Tier 2

does not require CTOD toughness testing.

3.2 Database

Among the 382 CWP test results available, 166 contained a flaw with a height of less than 3

mm and are thus directly relevant to validate the reliability of the EPRG-Tier 2 allowable flaw

size limits. Table B2 gives the details of this subset of CWP data.

Variable Number of results /

Value

%

CWP test data

Overmatching welds

Plain pipe

Undermatching welds

166

72

51

43

-

44

31

25

Thickness range (mm) 5 30.5 -

Pipe Yield/Tensile > 0.90 23 22

Average Charpy impact < 40 J 31 19

Average CTOD < 0.127 mm 19 11Failure strain < 0.5 % 24 14

Table B2 Overview of database for specimens containing a flaw less than 3 mm in height

It can be seen from Table B2 that not all test materials satisfied the toughness and material

property requirements. For example, 43 CWP specimens contained an undermatch girth weld

while the majority (73 results) contained an overmatched girth weld. On the other hand, GSY

(failure strain > 0.5 %) was not achieved in 24 instances. Thus, the database covers a

representative range of material properties.


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3.3 Assessment and discussion

The new CWP database is analysed using the original ERPG Tier 2 flaw size limit of l = 7t, Fig.

B3. Fig, B3 shows the remote failure strain against flaw length l, normalised by wall thickness,t. It will be noted that the upper plot gives the entire (new) database. The lower plot gives an

enlarged view of the lower failure strain area (strain range from 0 to 2 %)

Fig. B3 - Remote strain at failure versus normalised flaw cross sectional area (lh/Wt) of CWPtest specimens containing a single flaw smaller than 3 mm in height.

The data points located in the EPRG-box can be excluded since they do not satisfy the Tier

0

2

4

6

8

10

0 5 10 15 20 25 30

l/t

RemoteStrainatfailure(%)

All data, h < 3 mm

Remote strain = 0,5 %

EPRG Tier 2

h < 3 mm

Labo Soete - Gent

0.0

0.5

1.0

1.5

2.0

0 2 4 6 8 10

l/t

RemoteStrainatfailure(%)

Undermatching

OM - CVN < 40 J

OM - Y/T > 0.90

OM - CVN > 40 J

Remote strain = 0.5 %

h < 3 mm

Labo Soete - Gent

EPRG Tier 2 - h = 3 mm


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2 toughness (CVN > 30J/40J) and or mechanical properties (matching weld and pipe metal

Y/T < 0.90) requirements. A more rigorous examination of these results has revealed that

undermatching weld metal (open circles) is the major cause for rejection. Thus, contrary to

some opinion, the level of weld metal yield strength mismatch is a key factor in the

assessment of flawed girth welds. The remaining two CWP tests which failed at strain below

0,5 % (open triangles) contained a low toughness (Charpy < 40 J) girth weld. It is also

interesting to note that none of the 36 CWP specimens for which the pipe metal Y/T ratio

exceeded the 0.90 level, failed at strains below 0.5 %. This suggests that the Y/T ratio is not

a critical factor for shallow (flaw height < 3 mm) flaws.

The CWP specimens complying with the ERPG Tier 2 toughness and mechanical property

requirements (solid diamonds) are safely predicted. However, it is of interest to note that,

despite the fact that a significant number of CWP tests on materials which did not meet the

toughness or the mechanical property requirements (open data point located outside the

ERPG-box) failed at strains exceeding the 0.5 % level. The explanation is that ERPG Tier 2 is

based on a deterministic approach. This approach assumes that explicit input parameters

predict explicit values of the results. Moreover, the predicted flaw size limits assume minimum

values as input. On the other hand, it is known that a high degree level of weld metal yield

strength overmatch alleviates the detrimental effect of low toughness on weld performance.

4 EXTENSION OF ERPG-Tier 2 TO FLAWS IN X80 PIPELINES

4.1 Data base

The number of CWP test results in X80 steel and the breakdown by test variable is presented

in Table B3. It can be observed that this set CWP test results gives a representative database

for the wall thickness range between 12 and 17 mm. The SMAW and GMAW processes were

used to make the girth welds. However, it should be noted that all SMAW welds were

overmatching. The undermatching GMAW welds were obtained by welding pipe with an

actual yield strength exceeding the properties of X90 grade pipe (YS > 621 MPa).


Value

%

CWP test data

Overmatching welds

Plain pipe

Undermatching welds

68

19

-

-

-

28

-

-

Thickness range (mm) 12 17 mm -



Average CTOD < 0.127 mm 21 31

Failure strain < 0.5 % 22 32

Table B3 Overview of X80 CWP test database


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The remote strain at failure (or maximum load) is plotted against the flaw sectional area

normalized by the CWP gross cross sectional area in Fig. B4a (all results of tests on X80 pipe)

and B4b (detailed view of the low failure strain area). The horizontal chained line in Fig. B4a

indicates the current EPRG-Tier 2 failure criterion of 0.5% remote strain. The EPRG-box in

Fig. B4b represents the maximum allowable flaw size limits normalised by gross sectional

area.

Fig. B4a shows that for flaw areas below about 2% failure strains up to 6% can be obtained.

However, the examination of the original test reports has shown that this strain levels were

obtained for overmatching GMAW weld metals. In contrast, the undermatched welds (open

squares) generally show lower failure strains than overmatched welds (solid circles). The level

of weld metal yield strength undermatch was in the range -2 % down to -7 %.

Fig. B4a Remote strain at failure versus normalised flaw cross sectional area (lh/Wt) of CWPtest in X80 pipe (all results of tests in X80 pipe)

Fig. B4b indicates the girth welds where the average Charpy energy was less than 40J (mean

value) or the plain pipe had a yield to tensile ratio exceeding 0.90. All the results represented

by data points located in the ERPG-box but one do not comply with the ERPG Tier 2

toughness and material qualification criteria, and can therefore be excluded. As before, it

might be concluded that weld metal yield strength undermatching is the main cause for

rejection.

The single CWP result (solid circle in the ERPG-box) violating the ERPG Tier 2 guidelines was

obtained from a CWP test on a specimen containing a matching girth weld (level of weld

metal yield strength overmatch = 0 %) having a Charpy impact toughness of 33 J (min) / 40J (mean) while the pipe metal Y/T ratio was 0,899. Moreover, this specimen contained

0

2

4

6

8

0.00 0.05 0.10 0.15 0.20

lh/tW


Overmatching

Undermatching


lh/tW = 0,07 (7 %)

S e e d e t a i l


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scattered pockets of Cu-induced cracks, implying that the flaw size should be greater than

indicated. In other words, if the natural variability of the material properties is taken into

account, one might conclude that this result can be neglected. Thus by ignoring this result,

one can conclude that the current ERPG Tier 2 guidelines are applicable for flawed girth

welds in X80 pipeline provided the weld metal matching/overmatching, the pipe metal Y/T

ratio in the longitudinal direction and the weld metal toughness requirements are satisfied.

Fig. B4b Remote strain at failure versus normalised flaw cross sectional area (lh/Wt) ofCWP test in X80 pipe (detailed view)

5 EXTENSION OF EPRG- Tier 2 TO FLAW IN THIN AND HEAVY WALL PIPELINES

5.1 Thin wall pipelines (t < 7 mm)

5.1.1 Database

The number of relevant wide plate test results available and the breakdown by test parameteris summarized in Table B4. The database contains 25 wide plate test results that can be used

to validate ERPG Tier 2 for the wall thickness range of 4.9 to 6.9 mm. The database

contains no wide plate results on plain pipe. The welded specimens contained either

undermatching (12 results) or overmatching (13 results) weld metal. In all instances, test

welds were made in X80 pipeline steel.

5.1.2 Assessment and discussion

The measured Charpy values, using sub-sized Charpy V specimens, were converted using the

pro-rata rule. In all instances, the normalised Charpy toughness properties of the weld metals

0.00

0.50

1.00

1.50

2.00

0.00 0.05 0.10 0.15

lh/Wt

Remotestrain(%)

Overmatching

ERPG-box

Undermatching

OM - Mean CVN < 40 J

OM - Pipe metal Y/T > 0.90

Pipe metal results

Rdenys - labo Soete - G ent


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exceeded 40 J (The normalised impact properties reflect those of a standard Charpy impact

test bar)


Value

%

CWP test data

Overmatching welds

Plain pipe

Undermatching welds

25

12

None

13

-

48

-

52

Thickness range (mm) 4.9 6.9 -


Average Charpy impact < 40 J None -

Average CTOD < 0.127 mm Not measured -Failure strain < 0.5 % 8 32

Table B4 Overview of the CWP test database for thin wall pipe (t < 7 mm)

A related point relevant to mention here is that in nearly all cases, the height of the flaw was

equal to or greater than 3 mm. This also means that the flaw height is greater than half the

wall thickness (h/t > 0.5), Fig. B5.

Fig. B5 Effect of flaw height and flaw height-wall thickness ratio on CWP failure strain for

flawed girth welds in thin wall pipelines (t < 7 mm)

0

1

2

3

4

5

0.0 0.2 0.4 0.6 0.8

h/t



UM - t = 5.0 mm - CVN > 40 J

OM - t < 6.9 mm - CVN > 40 J

h/t = 0.5

rdenys - Labo Soete

0

1

2

3

4

5

0 1 2 3 4

h (mm)


UM - t = 5.0 mm

OM - 4.9 < t < 6.9 mm


EPRG limit - h = 3 mm

rdenys - Labo Soete


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Fig. B6 compares the EPRG-Tier 2 flaw size limit (EPRG-box) to the CWP results available.

The comparison shows that the solid squares representing the remote strain at failure of

overmatched (range from 1% to 8 %) girth welds lie on the safe side of the EPRG-box. For

undermatched girth welds, the remote strain at failure can be less than 0.5%.

Although it appears that the guidelines can be extended for thin wall pipelines, one must note

that the number of CWP is too small to draw binding conclusions. Therefore, it will be

appreciated that variations in mechanical properties and weld reinforcement can lead to quite

different conclusions. In particular, the weld reinforcement can have a very significant effect

on the strain failure at failure.

Fig. B6 Remote strain at failure versus normalised flaw cross sectional (lh/Wt) of flawed

girth welds in thin wall pipes

Therefore, if one considers using an ECA assessment based on the EPRG-Tier 2 guidelines, itseems reasonable to document the gaps in our current understanding of the failure behaviour

of thin wall pipelines. At short term, the only acceptable solution consists of conducting ad-

hoc CWP tests if one wants to justify the use of an ECA in thin wall pipelines.

5.2 Heavy wall pipelines (t 25.4 mm)

5.2.1 Database

The range of relevant CWP results is presented in Table B5. All CWP test specimens

contained overmatching SMAW or GMAW weld metal in X65 or X70 pipe. It was found in

surveying the test reports that the lowest level of weld metal yield strength overmatch was

20 %. On the other hand, the pipe metal Y/T ratio was lower than 0.90; the Y/T ratio ranged

0

1

2

3

4

5

0.00 0.05 0.10

lh/Wt


UM - t = 5.0 mm - CVN > 40 J

OM - 4.9 < t < 6.9 mm - CVN > 40 J


EPRG-box - hl/Wt = 0.07

rdenys - Labo Soete


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from 0.76 to 0.88. It is also worthy of note that the Charpy impact properties did not always

comply with the EPRG-Tier 2 requirements (mean CVN > 40 J). The 21 CWP specimens

containing low toughness weld metal were made by SMAW process.

The specimens of wall thickness equal to or exceeding 1 inch (25,4 mm) contained a single

surface breaking weld metal notch ranging from 66 to 240 mm in length and 3 mm to 12 mm

in height, Fig. B7.


Value

%

CWP test data

Overmatching welds

Plain pipe Undermatching welds

56

42

14none

-

75

25

Thickness range (mm) 25.4 30.5 -

Pipe Yield/Tensile > 0.90 None -


Average CTOD < 0.127 mm 22 39

Failure strain < 0.5 % 16 29

Table B5 Overview of the CWP test database for heavy wall pipe

Fig. B7 Remote strain versus flaw height of CWP tests on flawed girth welds in

heavy wall pipes (wall thickness 25.4 mm)

(The solid box shows the current EPRG-Tier 2 flaw size limits)

0

2

4

6

8

0 2 4 6 8 10 12

h (mm)

Remotes

trainatfailure(%)

t > 1 inch - CVN > 40 J

t > 1 inch - CVN < 40 J

t = 1 inch - CVN < 40 J

t = 1 inch - CVN > 40 J

t > 1 inch - pipe metal

EPRG - h = 3 mm

rdenys - Labo Soete


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Fig. B8 shows the effect of normalised flaw size area (lh/Wt) on remote strain at failure. For

convenience of interpretation the data have been divided into two separate plots. Distinction

has been made between the 1-inch (25.4 mm) thick pipe and the 1 inch+ (t > 25.4 mm) wall

pipes.

Fig. B8 Remote strain at failure versus normalised flaw cross sectional (lh/Wt) of flawedgirth welds in heavy wall pipe (t 25.4 mm)

0

2

4

6

8

0.00 0.10 0.20 0.30

hl/Wt

Remotestrain

atfailure(%)

t = 1 inch - CVN < 40 J

t = 1 inch - CVN > 40 J

EPRG limit - lh/w t = 0.07


rdenys - Labo Soete

0

2

4

6

8

0.00 0.10 0.20 0.30 0.40

hl/Wt


t > 1 inch - CVN > 40 J

t > 1 inch - CVN < 40 J

t > 1 inch - pipe metal

EPRG limit - lh/w t = 0.07


rdenys - Labo Soete


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5.2.2 Assessment and discussion

The first subset of CWP data (t = 25.4 mm) was obtained from SMAW girth welds in X65

pipeline steel and from GMAW girth welds in X70 pipeline steel. The toughness properties of

the SMAW girth welds were lower than 40 J (open circles). The CWP results located in the

ERPG-box can thus be rejected. The acceptable CWP results (solid circles) located above the

0.5% remote strain line were obtained from the GMAW girth welds, Fig. B8 upper plot.

The second subset of CWP data (t > 25.4 mm) was generated for GMAW girth welds in X65

and X70 pipeline steel. Only one CWP test failed at a remote strain below 0.5 %. In this

particular case, the weld metal toughness was 31 J so that this result can be rejected. It is

important to note here that the low toughness girth welds (open squares) were made

according special welding procedures. This is to say that standard GMAW welding procedures

produce girth welds with ample toughness (Charpy V impact >> 40 J).

Finally, Figs. B7 and B8 demonstrate that girth welds and materials meeting the qualification

requirements (solid data points) failed at strain well beyond 0.5 %. In other words, the

current flaw size area of 7 % can easily be extended for standard GMAW girth welds in heavy

wall pipelines.

6 EXTENSION OF EPRG- Tier 2 TO FLAW HEIGHTS EXCEEDING 3 MM

6.1 Predicted allowable flaw lengths

The DEN plastic collapse model, Eq (A7) outlined in Part A, can be applied to derive the

allowable length for flaw heights exceeding 3 mm. Using the flaw area limit of 7% (lh/Wt =

0.07), the corresponding maximum allowable flaw lengths for any other flaw heights can be

determined. For example, for 4 mm and 5 mm high flaws the calculated maximum allowable

lengths are 5.2t (h = 4 mm) and 4.2t (h = 5mm), Table B6.

Flaw height, h < 3mm 4 mm 5mm

Flaw area, %* 0.07 0.07 0.07

MaximumAllowable

Flaw length, l7.0 t 5.3 t 4.2 t

* Flaw area = lh/Wt where W = 300 mm

Fig. B6 Predicted maximum allowable flaw lengths as a function of flaw height

The predicted flaw size limits assume that the current EPRG-Tier 2 toughness and mechanical

property requirements are satisfied (see Section 3.1).


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With this background, the next sections of this report can be focussed on the assessment of

the accuracy of the proposed flaw length limits. This assessment should be possible if one

selects the test results of CWP specimen containing a flaw in the height range of 3 to 5 mm.

6.2 Database

A rigorous screening of the entire CWP database has revealed that a total of 148 CWP tests

have been conducted on specimens containing a flaw in the height range of 3 to 4 mm (97

results) and 4 to 5 mm (51 results). As shown in Table B7, this database contains a

representative set of data because the assessment can be conducted for good as well as

poor CWP results. The classification good is used if the specified material properties and

the remote failure strain meet the ERPG requirements.

Number of results / ValueVariable

3 < h 4 mm 4 < h 5 mm

CWP test data

Overmatching welds

Plain pipe

Undermatching welds

97

54

13

25

51

38

4

9

Thickness range (mm) 6.9 27.0 11.6 30.5


Mean Charpy impact < 40 J 35 14

Average CTOD < 0.127 mm 27 9Failure strain < 0.5 % 33 15

Table B7 Overview of the CWP test database for flaws exceeding 3 mm in height


The results of the two subsets of CWP test results are plotted in Figs. B9 through B11. The

horizontal dashed (red) line in these Figures indicates the failure criterion of 0.5% remote

strain. Figs. B9 and B10 give a summary plot and a detailed view of the results in the lowstrain range to show the effect of flaw length normalised by wall thickness on the remote

strain at failure. The predicted maximum flaw length limit normalised by wall thickness (l/t) is

represented by a vertical chained line. Fig. B11 shows the test results in terms of remote

strain at failure versus the flaw height to wall thickness ratio (h/t) or the relative cross

sectional area of the flaw normalised by gross cross sectional area (lh/tW).

6.3.1 Flaw height less than 4 mm

The upper plot in Fig. B10 compares the predicted allowable l/t ratio to the CWP test results

available. The comparison shows that the predicted limit (l/t = 5.3) is safe. This conclusion is


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based on the observation that the CWP database includes a wall thickness range from 6.9 to

27.2 mm, a flaw height to wall thickness (h/t) ratio range from 0.13 to 0.53, a pipe metal

Fig. B9 Overall view of remote strain at failure versus normalised flaw length (l/t) of CWPspecimens containing a single flaw ranging from 3 to 4 mm and from4 to 5 mm in height (entire database). Details are shown in Fig. B10

0

2

4

6

8

0 5 10 15 20 25 30l/t


All data, 3 < h < 4 mm


Current EPRG Tier 2

Labo Soete - Gent

3 < h < 4 mm

See detail

0

2

4

6

8

0 5 10 15 20 25 30

l/t

Rem

otestrainatfailure(%)

All data, 4 < h < 5 mm


Current EPRG Tier 2

Labo Soete - Gent

4 < h < 5 mm

See detail


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Fig. B10 Detailed view of remote strain at failure versus normalised flaw length (l/t) ofCWP specimens containing a single flaw ranging from 3 to 4 mm and

from 4 to 5 mm in height.

0.0

0.5

1.0

1.5

2.0

0 2 4 6 8 10l/t


Undermatching

OM - Y/T > 0,90

OM - CVN < 40 J

OM - CVN > 40 J

Predicted limit

4 < h < 5 mm

Labo Soete - Gent

Proposed limit : l/t = 4

0.0

0.5

1.0

1.5

2.0

0 2 4 6 8 10l/t



Plain Pipe resultsOM - CVN < 40 J

Undermatched welds

OM - Y/T > 0.90OM - CVN > 40 J

Predicted limit

Labo Soete - Gent

3 < h < 4 mm

Proposed limit : l/t = 5


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yield to tensile ratio range from 0.688 to 0.956, a toughness range from 17 J to 103 J, steel

grades from X60 to X80 with most of the data being for grades X70 to X80, and weld metal

yield strength mismatch levels from -25 % to +49 % (SMAW and GMAW welds). Therefore, it

is suggested that the maximum allowable flaw length is limited to 5t. With this choice, the

maximum flaw area is limited is to 6,7 %.

6.3.2 Flaw height less than 5 mm

The lower plot in Fig. B10 also shows that the predicted allowable l/t ratio (l/t = 4.2) provides

a conservative prediction by a reasonable margin of safety. As for the previous case, it can

also be observed the CWP database covers abroad range of parameters. The database

includes results for the wall thickness range from 11.6 to 30.5 mm, a flaw height to wall

thickness (h/t) ratio range from 0.16 to 0.43, a toughness range from 16 J to 110 J, steel

grades from X60 to X80 with most of the data being for grade X60, a pipe metal yield to

tensile ratio range from 0.651 to 0.901, and weld metal yield strength mismatch levels from -

25 % to +56 %. Therefore, it is suggested that a flaw length limit of 4t can be safely applied

(this limit also corresponds to a flaw area limit of 6.7 %).

6.3.3 General assessment

The comparison of the two sets of data, Figs. B9 and B10 (upper plot: 3 mm < h 4mm and

lower plot: 4 mm < h 5 mm), shows that flaw height has apparently little effect on failure

strain if the results of undermatched and low toughness (Charpy < 40 J) welds, and the high

Y/T ratio results are excluded. Furthermore, the solid data points (overmatched welds andCharpy impact > 40 J) located below the 0.5 % performance line could be ignored if the pipe

metal property variation (reflected in the Y/T ratio), the h/t ratio of the said flaws and the

level of overmatch are taken into account.

Fig. B11 also shows that for this database, except for one single results (Y/T = 0.895), the

remote strain at failure is greater than 0.5 % (GSY) if the height of the flaw is less than half

the wall thickness (h/t < 0.50). In particular, provided the toughness and mechanical

properties are met, the CWP results suggest that for flaw height to wall thickness ratios less

than 0.3, failure should occur by GSY (remote strain at failure 0.5 %). However, it possible

that long flaws would cause the girth welds to behave in a different way than observed in Fig.B11. Therefore, additional CWP tests could be performed to affirm the validity of this

conclusion for flaws longer than 7t. Note that the value of 7t is based on the flaw area limit of

7 % (see left hand plots in Fig. B11).

The above observations can be explained if one explores the effects of the interaction

between flaw size, pipe metal Y/T ratio and the level of weld metal yield strength mismatch

on straining capacity. The points is that the straining capacity of a flawed girth weld is not

just a function of flaw height per se, but is also related to the actual mechanical properties of

the weld and pipe metal. In other words, there exist various combinations of flaw size, Y/T

ratio and weld overmatch contributing to remote plastic deformation. For example, for a


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Fig. B11 - Detailed view of remote strain at failure versus normalised flaw height (h/t) and relative crosCWP specimens containing a single flaw ranging from 3 to 4 mm and from 4 to 5 m

0.0

0.5

1.0

1.5

2.0

0.00 0.05 0.10

lh/tW


Undermatching

OM - Y/T > 0,90

OM - CVN < 40 J

OM - CVN > 40 J

Labo Soete - Gent

Max. flaw area = 7 %

0.0

0.5

1.0

1.5

2.0

0.00 0.05 0.10

lh/tW


Labo Soete - Gent

Max. flaw area = 7 %

0.0

0.5

1.0

1.5

2.0

0 0.1 0.2 0.3 0.4 0.5 0.6

h/t


Undermatching

OM - Y/T > 0,90

OM - CVN < 40 J

OM - CVN > 40 J

Labo Soete - Gent

4 < h < 5 mm

h/t = 0.30

0.0

0.5

1.0

1.5

2.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6

h/t


Plain Pipe res ults

OM - CVN < 40 J

Undermatched w elds

OM - Y/T > 0.90

OM - CVN > 40 J

Plain pipe Y/T = 0.96

Labo Soete - Gent

3 < h < 4 mm

h/t = 0.50


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Draft for review B - Part B21

(relatively but detectable) small flaw in an overmatching girth weld joining pipe with good

strain hardening characteristics (Y/T < 0.90), there will be little strain concentration in the

weld. In that case, the applied strain will go to the pipe metal. In contrast, for large flaws, or

if the pipe metal Y/T is high and the level of overmatch low, strain concentration will be at

the flaw. That is, this situation will result in low failure strains no matter what the toughness

is.

This consideration can be illustrated by the study of the following set of test results. Table B8

gives the results of CWP and corresponding small-scale tests on girth welds in X65 pipe with

a wall thickness of 25,4 mm. The pipes were taken from the same heat. However, unnoticed

variation in the plate manufacturing parameter gave pipe of different yield strengths (these

pipes are further termed as L and H). The minimum pipe yield strength (0.5%) was 463 MPa

and the maximum was 546 MPa. The yield strength of the girth weld metal was measured

using six all-weld-metal tensile tests. The mean weld metal yield strength used for

comparison purposes was 565 MPa, giving an overmatching level of 3,5 % (pipe H) and 22,0

% (pipe L). The weld metal Charpy V-notch impact and B x 2B CTOD specimens were

tested at temperatures as indicated in Table B8. The CWP specimens were notched in the

weld metal with a fatigue-sharpened surface-breaking crack introduced from the root side.

The CWP tests were performed at -10C.

PIPE AND WELD PROPERTIES

Tensile properties Weld metal toughnessPipe

&Weld

metal

YS

(MPa)

TS

(MPa)Y/T

Yield

strengthmismatch

(%)

CVN 30C

(J)

Min/Ave/Max

CTOD -10C

(mm)

Min/Ave/Max

L

H

E9010

463

546

565

537

614

647

0.86

0.89

0.87

22,0

3,5

-

-

-

8 / 16 / 36

-

-

0,024 / 0,070 / 0,156

WIDE PLATE TEST RESULTS

FailurePipe

combinations

Defect size

Length x depth

(mm)

Pipe Metal

Failure Strain

(%) Mode Location

H - H

H - L

L - L

139,4 x 3,6

139,3 x 3,3

117,8 x 3,4

0,30

7,18

1,31

Brittle

GSY

GSY

Weld

Pipe

Weld

GSY = Gross Section Yielding (pipe metal yielding)

Table B8 - Small and large-scale test results of weld metal notched curved wide plate tests


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The CWP test results summarized in Table B8 include the flaw size measured after testing,

the remote strain at failure, the failure mode and the failure location. For this particular

situation, the CWP results demonstrate that the strain at failure depends on the level of

mismatch. The straining capacity of the overmatched welds (L-L and H-H) is significant while

the heterogeneous weld combination (H-L) failed in the pipe metal. The other key

observation is that low toughness weld metal containing a significant root flaw may fail in the

pipe metal. In other words, the results of these tests quite clearly the trade-off between

toughness and the level of weld metal mismatch.

6.3.4 Recommendation

The preceding comparisons and the clarifying discussions provide the experimental evidence

that the current ERPG Tier 2 maximum allowable flaw length can be extended for flaw heights

exceeding 3 mm. Tentatively, it is proposed to apply the limits given in Table B9. Note that

the flawed area is limited to 6.7 %. This limit on flawed area is smaller than 7 % for 3 mm

high flaws (current guideline). With this assumption the maximum allowable flaw length limits

are 5t and 3t.

Flaw height, h < 3mm 4 mm 5mm

Flaw area, %* 0.07 0.067 0.067

Maximum

AllowableFlaw length, l 7t 5t 4t

* Flaw area = lh/Wt where W = 300 mm

Table B9 Proposed maximum allowable flaw lengths as a function of flaw height

The analysis of the data also demonstrated that the maximum allowable ratio of flaw height

to wall thickness should be limited to 0.50. This restriction implies that the maximum

allowable flaw height in 10 mm thick pipe must be smaller than 5 mm.

7 SUMMARY, CONCLUSIONS AND RECOMMENDATIONS

This report outlines the proposed modifications and extensions of the EPRG-Tier 2 guidelines

on flaws in pipeline girth welds, and provides justification for these changes. In addition,

reference is made to topics which need further consideration.

7.1 Failure behaviour

The assessment of the database containing 382 results of single surface notched curved wide

plate specimens has confirmed that:


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1. Toughness-dependent failure behaviour can be excluded if the Charpy impact properties

are greater than 30 J minimum and 40 J mean. That is, this level of toughness ensures

toughness-independent failure behaviour.

2. The failure stresses and strains for toughness-independent materials are related to flaw

size, strain hardening behaviour of the pipe metal in the longitudinal direction and the

level of weld metal yield strength mismatch. The best prediction of the experimental data

is obtained by using the pipe metal flaw stress.

7.2 ERPG Tier 2 guidelines

7.2.1 Current guidelines

The current ERPG Tier 2 guidelines for determining allowable flaws in pipeline girth welds are

based on the limitation of the flawed area to 7 % in any 300 mm length of weld. That is, girth

welds containing flaws smaller than this limit fail by GSY (remote failure strain 0.5 %)

provided the weld metal is matching or overmatching and the weld metal toughness exceeds

30 J minimum and 40 J mean. Since the maximum flaw height is limited to 3.0 mm, the

limitation on flaw length assumes that flaws would typically be one weld run high. This height

limit is a serious restriction if an allowance for the likely error in flaw sizing has to be taken

into account.

The ERPG Tier 2 guidelines are considered to be simple to use while they are conservative for

elastic designs. In addition, the GSY failure criterion requires no detailed judgement andeliminates the need for detailed analyses to determine the applied stress.

7.2.2 Proposed extension

The present study has demonstrated that improvements over the existing EPRG-Tier 2

allowable flaw size procedures can be implemented. The proposed extensions are consistently

conservative provided:

The Charpy impact toughness of the weld exceeds 30 J minimum / 40 J mean, and

The pipe Y/T ratio in the longitudinal direction is smaller than 0.90, and

The actual yield strength of the girth weld metal is at least equal to that of the pipemetal.

In other words, the approach adopted is consistent with the original material qualification

requirements as well as with the results of the current study.

The plastic collapse method of analysis, originally used to derive current flaw acceptance

levels, provides a simple curve giving allowable individual flaw length and heightcombinations. Therefore, this curve can also be used to extend the guidelines for flaw heights


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greater than 3 mm. This approach was applied by limiting the flawed area to 7 %.

Comparison of this flaw area limit with the experimental CWP database has confirmed that:

1. Except for girth welds in thin wall pipe (t 7 mm), the current ERPG Tier 2 guidelines can

also be applied for X80 pipeline. However, it should be noted that (a) it is desirable to

establish the variability of both weld metal toughness and tensile properties of the weld

metal and the surrounding (pipe) material, and (b) conservative estimates of these input

parameters should be used4.

2. The current Tier 2 guidelines are not applicable to girth welds in pipelines of wall

thickness less than 7 mm. The first concern is that there is not sufficient experimental

data available so far for quantifying the effect of the interaction between flaw height and

wall thickness. The second problem is that any correction for flaw sizing error would

generate allowable flaw heights greater than half the wall thickness (see also Table B10).

Pipe

Grade

(SMYS)

Flaw height

(with h 0.5 t)*

(mm)

Allowable flaw length

per 300 mm length of weld

(mm)

3 7 t

4(1) 5 th

(mm)

5(2)

l

(mm)

4 t

X80

Wall thickness range: 7* mm t 30 mm

> X80 Not applicable

* The following restrictions apply: wall thickness, t, should be greater than 8 mm for a flaw height

equal to 4 mm (1) and greater than 10 mm for a flaw height equal to 5 mm (2)

Table B10 Extended EPRG-Tier 2 maximum allowable flaw size limits

3. The current Tier 2 guidelines can also be safely applied to girth welds in the thickness

range from 25.4 mm up to 30.5 mm. In fact, the allowable dimensions of flaws in heavy

wall pipelines are significantly larger than those of flaws in thin wall ones. This feature

indicates that the required level of accuracy in flaw sizing decreases with increasing wall

thickness.

4. The proposed extensions of the current guidelines to heights exceeding 3.0 mm give

consistent factors of safety for flaw heights in the range from 3 mm to 5 mm. Table B10

4 Unpublished CWP data (research is ongoing) on flawed girth welds in steel grades exceeding X80 show that standard plastic

collapse solutions might give non-conservative failure stress predictions. According to comparisons between experimentaland predicted data, the EPRG-Tier 2 guidelines are not suitable for very high strength pipes, such as X100, and for pipe

materials with a uniform elongation of less than about 7 %.


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summarises the maximum allowable flaw size limits as a function of pipe grade and wall

thickness.

It will be noted that the proposed flaw size limits are equally applicable to girth welds in

X80 pipe as well as for heavy wall pipe. Furthermore, the minimum wall thicknesses

specified for flaws heights exceeding 3 mm will prevent that the flaw height exceeds half

the wall thickness.

5. The applied strains in overmatched girth welds concentrate in the pipe metal. The

implication is that overmatched welds can tolerate larger flaws than matching or

undermatching ones do.

Finally, since embedded flaws are less severe than surface breaking ones, the proposed

maximum allowable flaw sizes are equally applicable to embedded flaws.

7.3 Recommendations

7.3.1 Issue

Prior to use, the proposed guidelines require the following basic input:

Pipe wall thickness

Pipe metal yield strength and tensile strength in the longitudinal direction

The Charpy V impact energy at the lowest possible pipe laying and operatingtemperature

Weld metal yield strength and tensile strength

Measured maximum height and overall length of the flaw

Provided the pre-qualified weld procedures are followed, girth welds in modern pipeline steels

possess adequate toughness and will at least be matching in yield strength. However, it must

be pointed out that it is possible to have undermatched welds in high strength pipelines. If

this possibility cannot be excluded, toughness and tensile data should be obtained in sufficient

quantity to identify possible systematic (non-random) variations.

7.3.2 Material testing requirements

The ERPG guidelines do not include specific requirements for pipe and weld metal

qualification testing. Instead, reference is made to existing industry standards. These

standards, however, do not reflect the current state-of-the-art with regard to the effect of

weld metal yield strength mismatch and Y/T on deformation and failure behaviour.

For this reason, guidance on sampling position and number of tests to be conducted is

needed to establish variations in the material properties and to eliminate possibleunconservative estimates. However, if materials and welding processes can be selected to


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avoid this circumstance, detailed testing is not required.

Unless other information is available to determine the degree of variation, detailed testing is

essential if the measured values are near to critical. The following tentative near critical

values are proposed:

Toughness Charpy V impact values are less than 33 J / 44 J (or 1,1 times the statedrequirements)

Pipe metal Y/T ratio is greater than 0.88

Level of weld metal yield strength overmatch is less than 3 %

These tentative recommendations are not meant to increase conservatism. They are only

meant to ensure that the required minimum toughness and tensile properties are satisfied.

Finally, it is recommended to provide for high strength pipe (x80 and above) stress-strain

curves up to the tensile strength of the pipe material in the longitudinal direction, from which

the uniform elongation can be deduced. This information is normally not needed for standard

pipe grades (below X80).

7.3 Flaw sizing

Assuming that the required material specifications are met, the maximum allowable flaw size

can be determined and compared to the size of the detected flaw. Thus, the use of the

current and proposed EPRG-Tier 2 allowable flaw size limits presupposes that an NDEinspection is used capable of detecting and sizing the critical flaws. The use of an automated

ultrasonic technique of flaw height measurement (AUT) is recommended. However, it has to

be demonstrated that the assumed accuracy is achievable for the girth weld and pipe

materials under consideration. The present study does not include recommendations for

adding inspection allowance values to the reported on-line NDE inspection results.

8 FUTURE WORK

Several other aspects affecting allowable flaw size have not been addressed. The issues thatneed to be documented include weld heterogeneity, plastic design and girth welds in extra

high (> X80) steel pipes.

.

8.1 Weld heterogeneity

All ECA approaches, including EPRG-Tier 2, are based on the simple premise that the

properties of a pipeline in the axial direction are homogenous. In reality, adjacent pipes in a

pipeline string have different mechanical properties. These differences affect the crack

driving force for failure. The differences between pipe and weld metal stress-strain behaviour

lead to significantly different strain distributions and strain concentrations in and around the


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weld. Also, the stress and strain situation can be affected by misalignment and out-of-

roundness. These issues are not considered in the formulation of the plastic collapse

estimation methods.

8.2 Extra high strength pipe

For extra high strength pipes, ongoing research has already shown that not only the Y/T ratio

but also the uniform elongation of the pipe metal has a dominant effect on the deformation

behaviour and failure of flawed girth welds. The quantification of the failure behaviour of

girth welds in extra high strength pipeline steels in terms of the Y/T ratio alone may,

therefore, be an oversimplification of the actual situation. In other words, for extra high

strength pipeline steels, a completely new method of assessments has to be developed. At

this time, no further guidance can be given.

8.3 Strain based design

Traditional ECA methodologies cannot be used to predict the relationship between the level of

flaw tolerance and plastic straining capacity. For non-elastic longitudinal deformations, current

strain based pipeline design codes offer the designer little assistance for longitudinal strains

exceeding the 0.5 % level. For strains in excess of 0.5 %, the development of a flaw

acceptance criterion needs information on the limit strain for failure. This strain limit cannot

be estimated form standard plastic collapse solutions as these are based on the limit stress

for failure. Thus, a methodology to assess the plastic straining capacity of flawed pipeline is

needed. That is, work has to be undertaken to determine the tolerable flaw size underlongitudinal plastic strains.

WELD-DEFECTS-PART-B

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