1996_Knauf, Hopkins - EPRG Weld Defect Guidelines - 3R

The EPRG guidelines on the assessment of defects in transmission pipeline girth welds Dr. rer. nat. Gerhard Knauf and Dr. Phil Hopkins

published in 3R international 10-11/1996 Vulkan-Verlag Vulkan-V erlag GmbH, Essen Contact: N. Hülsdau (Tel. 0201/82002-33, 0201/82002-33, E-mail: [email protected])

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3R international á 35 (1996) á Heft 10/11 á Oktober/November

The EPRG guidelines on the assessment of defects in transmission pipeline girth welds EPRG-Richtlinie zur Beurteilung von Fehlern in Rundschwei§nŠhten bei Fernleitungsrohren

Summary The EPRG guidelines on the assessment of defects in transmission pipeline girth welds are based on literature reviews, an extensive laboratory test programme, published experimental data and accepted Þtness for purpose methodologies. The guidelines are structured in three tiers and specify defect acceptance levels in Tier 1 (good workmanship) and defect limits in Tiers 2 and 3 (Þtness-for-purpose). The application of current welding standards can lead to quite different defect limits, but the EPRG guidelines provide uniform acceptance levels and defect limits, with a comprehensive technical justiÞcation.

Zusammenfassung Die EPRG-Richtlinie zur Beurteilung von Fehlern in Rundschwei§nŠhten bei Fernleitungsrohren wurde auf Basis von Literaturrecherchen, einem umfangreichen Versuchsprogramm, veršffentlichten Versuchsergebnissen, und Berechnungen erstellt. Die Richtlinie ist in drei Stufen (ãTierÒ 1 bis 3) unterteilt und speziÞziert in ãTier 1Ò nach dem Stand der Technik erlaubte Schwei§nahtfehler und in ãTier 2 und 3Ò aufgrund von Berechnungen maximal zulŠssige Fehlergrš§en. Die Anwendung der zur Zeit gŸltigen Normen zur Beurteilung von Schwei§nŠhten kann zu deutlich unterschiedlichen zulŠssigen Fehlergrenzen fŸhren. Die EPRG Richtlinie benennt einheitliche Fehlergrenzen auf einer umfangreichen technischen Grundlage.

Dr. rer. nat. Gerhard Knauf * 20. November 1951 in Duisburg Mitarbeiter der Abteilung Mechanik, OberßŠchentechnik, Werkstoff- und BauteilprŸfung der Mannesmannršhren-Werke AG, Mannesmann-Forschungsinstitut, Duisburg; Tel. (02 03) 9993160.

Dr. Phil Hopkins * 26. September 1951 in HuddersÞeld/England Business Manager of British Gas, Gas Research Centre, Loughborough/England; Tel. (0044/15 09) 28 3227. Ð Member of European Pipeline Research Group, American Gas Association, British Standards Committee and the Institute of Gas Engineers.

pose methods [5]. Additionally, the standards require the repair of non-planar defects such as slag or porosity, which are generally accepted as innocuous [6], and consider many planar defects unacceptable despite them having high failure stresses even under the most severe loadings at low temperatures [7]. The European Pipeline Research Group (EPRG) considered these dis-

Table 1: Summary of the technical basis of the EPRG guidelines Tafel 1:

Zusammenfassung der technischen Grundlagen der EPRG-Richtlinie

Introduction Pipeline girth welds have a good operating record, and are not a major cause of pipeline failure [1]. The welds are fabricated to stringent standards, e.g. API 1104, BSI 4515, CSA Z182 [2 to 4]. The defect acceptance levels in these standards vary signiÞcantly even when they are based on Þtness-for-purThe authors would like to thank the members of the EPRG sub-committee Weld Defects for their assistance, and Professor R. Denys of University of Gent for assisting in the production of the current EPRG guidelines.

crepancies, and the absence of universally accepted defect acceptance levels, and decided to produce independent guidelines. It commissioned literature reviews [5, 6] and conducted an extensive laboratory programme of small scale and full scale tests [7]. The philosophy adopted for setting pipeline girth weld defect limits is novel. It requires a girth weld to meet a mini-

Tier 1

Tier 2

Tier 3

Shape/proÞle defects BSI 4515 [3]

BSI 4515 [3]

BSI 5415 [3]

Undercut

BSI 4515 [3] API 1104 [2]

Wide plates [9, 10]

Full scale tests [7, 12 to 16]

Planar defects

BSI 4515 [3] API 1104 [2]

Wide plates [9, 10]

Full scale tests [7, 12 to 16]

Non-planar defects

BSI 4515 [3]

BSI 4515 [3] BSI PD 6493 [11]

BSI PD 6493 [11] Full scale tests [7, 12 to 16]

Interaction criteria

BSI 4515 [3]

BSI 4515 [3] BSI PD 6493 [11]


Accumulation criteria BSI 4515 [3]

BSI 4515 [3] BSI PD 6493 [11]



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Table 2: Requirements for defect acceptance levels (Tier 1), and defect limits (Tiers 2 and 3) Tafel 2:

Anforderungen fŸr zulŠssige Schwei§nahtfehler (Tier 1) und zulŠssige Fehlergrš§en (Tiers 2 und 3) Type of requirement

Geometry

Wall thickness (t)

Tier 1(A) 7 £ t £ 25.4 mm wall thickness outside this range by agreement

7 £ t £ 25.4 mm

Defect height Additional remarks Toughness

Strength

3 mm (single weld run)

£

Surface breaking non planar defects should be treated as planar defects Only girth welds between pipes of equal thickness

Charpy and CTOD values for the weld at minimum design temperature

CVN - Average CVN - Minimum

40 J 30 J

Cross weld tensile tests with weld reinforcement removed

Acceptable if the specimen breaks in the base material or when it breaks in the weld metal with a tensile strength ³ the speciÞed minimum tensile strength (SMTS)

³ ³

Sub-size specimen have toughness requirements reduced pro rata with their dimensions CTOD - Average ³ 0.15 mm CTOD - Minimum ³ 0.10 mm

Yield strength (YS) Yield strength matching

Loading

(A) (B)

SMYS (pipe) £ 483 MPa Tests can be carried out by special agreement to ensure that YS(weld) ³ YS(pipe)

YS(weld) ³ YS(pipe) Measurement of the yield strength, location, type, and number of specimen by agreement

Yield to ultimate tensile strength ratio (Y/T)

Y/T (pipe) £ 0.90

Additional remark

Only girth welds between pipes of equal grade

Applied strain/stress Additional remark

NDT

Tier 3(B)

Tier 2

strain £ 0.5 %

Y/T (pipe) £ 0.85

stress £ YS(pipe)

Onerous fatigue duty, or servere environmental effects are not included

Non destructive testing

100 % non destructive testing of pipeline girth welds

Alternatively existing company standards, CEN standards, API 1104, or BSI 4515 within their known and their deÞned limitations may be used The user can specify other Engineering Critical Assessment (ECA) methods based on documented Þtness-for-purpose calculation, or appropriate tests

mum toughness requirement [8, 9], then defect limits based on limit load/net section collapse calculations can be used. This document speciÞes both defect ÔacceptanceÕ levels (Tier 1) and defect ÔlimitsÕ (Tiers 2, 3). Acceptable defects (Tier 1) are those considered by welding workmanship standards not to require repair. Unacceptable defects will require some remedial action, to maintain good workmanship, but may not affect the Þtness-for-purpose of the weld. The defect limits (Tiers 2, 3) detailed below, are unlikely to be acceptable to most workmanship standards, but do not affect the Þtness-for-purpose of the girth weld and hence do not require repair. Defects outside these limits will require repair because they will affect the Þtness-for-purpose of the weld. The technical background to these guidelines and their full justiÞcation is given in references [5 to 10]. They make full use of accepted international codes [2, 3] and new data as listed in table 1. It should be noted that the guidelines make extensive use of published experimental data. Consequently, parts of the guidelines are restricted by empirical limits. For example, most of the data were from pipes of wall thickness between 7 and 25 mm, and most of the data were on welds whose yield strengths were higher than for the par-

ent materials. Additionally, only limited data on high yield to tensile strength ratio linepipe steels are available. It is likely that restrictions in these guidelines caused by these empirical limits will be relaxed as further data becomes available.

EPRG guidelines: good workmanship level (Tier 1) The defect acceptance levels used in Tier 1 can be from recognised, appropriate and acceptable transmission pipeline girth welding codes. Existing company standards, a CEN standard, API 1104, or BSI 4515 would be appropriate, providing they are used within their known and their deÞned limitations. The Tier 1 acceptance limits detailed below can be applied to pipeline girth welds if geometrical, toughness and strength requirements are fulÞlled.

their dimensions. The number of toughness specimens, their orientation and location will be speciÞed by the Responsible Engineer, and take due account of the welding parameters, pipeline, its operation and the weld defects expected. Cross weld API tensile tests shall be carried out, with the weld reinforcement removed. The tests are acceptable if the specimens fail in the pipe material or if the specimens break in the weld metal with a tensile strength greater than the minimum speciÞed tensile strength of the pipe material. Additionally, and by agreement, further tests can be carried out to ensure that the yield strength of the weld is equal to, or greater than, the yield strength of the pipe material. Pipelines subjected to onerous fatigue duty, or severe environmental loading or effects may require higher levels of acceptance which are not included in these guidelines. Tier 1: Defect acceptance levels

Tier 1: Requirements

Tier 1 requirements are given in table 2. The required minimum Charpy V energy of the weld at the minimum design temperature (40 J average, 30 J single value) is valid for full size specimens. Sub-size specimens have toughness requirements reduced pro rata with

The defect acceptance levels for Tier 1 are mainly based on acceptance levels given in API 1104 [2] and BSI 4515 [3]. The acceptance levels are speciÞed for individual and accumulated defects in terms of maximum defect depth and length. In the case of non-planar defects the percentage of these defects in

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Table 3: Defect acceptance levels (Tier 1), and defect limits (Tiers 2 and 3); proÞle, concavity and undercut Tafel 3:

ZulŠssige Schwei§nahtfehler (Tier 1) und zulŠssige Fehlergrš§e (Tiers 2 und 3); ProÞl, WurzelrŸckfall und Einbrandkerben

Type of defect

Tier 1 Acceptance criteria

External ProÞle

Excess weld metal should be uniform and not more than 3 mm in height. It should merge smoothly with the parent metal and not extend beyond the original joint preparation by more than 3 mm on each side. No area should have the weld face lower than the adjacent pipe surface.

Internal ProÞle

The root bead or any concavity should merge smoothly into the adjacent surface but at no point should the weld be thinner than the pipe thickness.

Root Concavity Undercut (cap)

25 % weld circumference

Depth

1.5 mm or 0.1 t (lesser)

Length 50 mm

7xt

Length 25 mm

7 x t in any 300 mm Figure 1

7 x t in any 300 mm

Depth 1 mm or 0.1 t (lesser)

the projected weld area on the radiograph is limited. Acceptance criteria are summarized in tables 3 to 5 for weld geometry, planar and non-planar defects, accumulation, as well as interaction of these defects. Table 3 summarizes acceptance levels for proÞle, concavity and undercut defects. In table 4 the acceptance levels for planar defects and non-planar defects are listed. Care should be taken in using the limits for non planar defects. The Engineer should ensure that the presence of any non-planar defect does not mask the presence of a more severe defect. Interaction and accumulation criteria are given in table 5. Root concavity is not included in accumulation calculations, unless it causes the weld thickness to be less than that of the pipe.

Figure 1

7xt

Total 25 mm in 300 mm or 8 % circumference (lesser) Undercut (cap, root)

Tier 3 Limit criteria

Length

Total 50 mm in 300 mm or 15 % circumference (lesser) Undercut (root)


EPRG guidelines: Fitness-forpurpose levels (Tiers 2 and 3) Tiers 2 and 3 defect limits are beyond good workmanship limits. Although such defects are unacceptable according to Tier 1 they do not affect the Þtness-for-purpose of the girth weld, if they are within Tier 2 or Tier 3 limits. The Tier 2 and Tier 3 limits given below can be applied to pipeline girth welds providing the Tier 1 requirements and additional requirements are satisÞed. Tier 2 and Tier 3: Requirements

Tier 2 and Tier 3 are listed in table 2. In addition, and as distinct from Tier 1, there are restrictions in wall thickness (7 £ t £ 25.4 mm), yield to ultimate tensile ratio of the pipe material (Y/T £ 0.90 for Tier 2 and Y/T £ 0.85 for Tier 3) and

Assumed to be < 3 mm

pipe grade (speciÞed minimum yield strength SMYS £ 483 MPa). The defect limits are based on the assumption that each defect is conÞned to a single weld run of height not greater than 3 mm. If there is suspicion of a defect being of depth greater than this weld run, it shall be repaired or assessed using other appropriate Engineering Critical Assessment methods. Coplanar defects in the through thickness direction should be assessed for interaction using PD 6493 [11]. Furthermore the defect limits are based on the assumption, and require, that the yield strength of the weld metal should be equal to, or greater than, the yield strength of the pipe material. The measurement of this yield strength, and the location, type and number of specimens, shall be by agreement. Tier 2 can be applied to girth welds providing the applied axial strain is not greater than 0,5 %. Tier 3 can be applied to axial stresses not greater than the pipe yield strength. Girth welds shall be between pipes of equal grade, thickness and be subjected to 100 % non-destructive testing. For Tier 3 (additional to the Charpy toughness requirement in Tiers 1 and 2) a minimum mean CTOD toughness of 0.15 mm and a minimum single CTOD toughness of 0.10 mm, at the minimum design temperature is required. The number of toughness specimens, their orientation and location, will be speciÞed by the Responsible Engineer, and take due account of the welding parameters, pipeline, its operation and the weld defects expected. Tier 2 and Tier 3: Defect limits

Fig. 1:

Girth weld defect limits for Tier 3

Bild 1:

Fehlergrenzen in Rundschwei§nŠhten fŸr Tier 3

Defect limits for Tiers 2 and 3 are results of Þtness-for-purpose calculations


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Table 4: Defect acceptance levels (Tier 1), and defect limits (Tiers 2 and 3); planar and non-planar defects Tafel 4:

ZulŠssige Schwei§nahtfehler (Tier 1) und zulŠssige Fehlergrš§en (Tiers 2 und 3); ebene und nichtebene Fehler

Type of defect


Inadequate root penetration


Length 25 mm

Incomplete (lack of) fusion root and/or cap Incomplete fusion, cold lap

7xt

Total 25 mm in 300 mm or 8 % circumference (lesser) Length 50 mm

Lack of sidewall fusion Lack of inter-run fusion

Tier 3 Limit criteria Figure 1

7 x t in any 300 mm 7xt

Total 50 mm in 300 mm or 15 % circumference (lesser)

Figure 1

7 x t in any 300 mm

Cracks

Not allowed

Crater Cracks

4 mm

Burn through

Individual

4 mm

Total t < 10 mm

Porosity

2 per 300 mm

Individual

3 mm or 0.25 t (lesser)

Figure 1

Total Not exceed a total area when projected radially through the weld of 2 % projected weld area in radiograph consisting of the length of the weld affected by the porosity, with a minimum length of 150 mm, multiplied by the maximum width of the weld. Hollow bead,

Length

50 mm

Slag

Total

50 mm in 300 mm, or 15 % circumference (lesser)

Inclusions

Total

12 mm in 300 mm, and 4 per 300 mm

Width t ³ 10 mm

Porosity,

Individual

Hollow bead

Total

Slag, Inclusions

Total

3 mm or 0.5 t (lesser) as for t < 10 mm

6 mm or 0.25 t (lesser)(A) 5 % projected area on radiograph

as for t < 10 mm

40 % circumference(A)

Width (A)

Figure 1

3 mm

If the pipe yield strength is above 450 MPa, limits for t < 10 mm should be used

Table 5: Defect acceptance levels (Tier 1), and defect limits (Tiers 2 and 3); accumulation and interacting criteria Tafel 5:

ZulŠssige Schwei§nahtfehler (Tier 1) und zulŠssige Fehlergrš§en (Tiers 2 und 3); Kriterien zur Aufsummierung und Wechselwirkung

Type of defect



Defect accumulation 100 mm in 300 mm, or 15 % circumference (lesser), t < 10 mm excepting porosity and root concavity Defect interaction t < 10 mm

Inherent in defect accumulation criteria

Defect accumulation as for t < 10 mm t ³ 10 mm

7 x t or 100 mm in any 300 mm (larger), excepting porosity and root concavity. Slag is exempt from accumulation, providing the yield strength of the pipe YS(pipe) £ 450 MPa. Accumulation of planar and non planar defects £ 40 % circumference.

Tier 3 Limit criteria Figure 1 Limits are given in Þgure 1. If a planar, slag or porosity defect is separated from a planar defect by a distance smaller than the length of the shorter of the two defects, then recategorise as a single planar defect of length equal to the two individual lengths and separation. Figure 1

Root concativity is not included in accumulation calculations, unless it causes the weld thickness to be less than that of the pipe. Defect interaction t ³ 10 mm

Inherent in defect accumulation criteria

If a planar, slag or porosity defect is separated from a planar defect by a distance smaller than the length of the shorter of the two defects, then recategorise as a single planar defect of length equal to the two individual lengths plus separation. Recategorised planar defect should have the same identity as the planar defect

Figure 1

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b) All Total Defect Length

a) Planar Defect Length 1000

2500

800

2000 Tier 1 Tier 2

, h t 600 g n e L m t m c e f e 400 D

Tier 3

200

m m , h t g 1500 n e L t c e f e D l 1000 a t o T

Tier 1 Tier 2 Tier 3

500

0 18" x 7 mm

36" x 25 mm

56" x 20 mm

0 18" x 7 mm

36" x 25 mm

Pipe Dimensions

Fig. 2:

Examples for the application of Tiers 1 to 3

Bild 2:

Beispiele fŸr die Anwendung der Tiers 1 bis 3

based on small scale-, wide plate- and full scale tests, on BSI 4515 [3], BSI PD 6493 [11], and on a plastic collapse approach [17]. Defect limits in Tier 2 are expressed as a function of wall thickness (planar defects) or projected area on radiographs (non-planar defects). Planar defects, including cap and root undercut, are allowed up to a length of 7 x wall thickness in 300 mm circumference. The assumed maximum depth is 3 mm. Non-planar defect limits and slag limits for t ³ 10 can be used if the yield strength of the pipe material is £ 450 MPa. If the yield strength is above this limit, the t < 10 mm limits for nonplanar defects should be used for all wall thicknesses. Defect limits for Tier 3 are given in Þgure 1. In this Þgure the limits for defect lengths are given as percentage of circumference of individual planar, interacting planar and total limit for all planar defects as a function of the wall thickness. There is a general limit for all types of planar defects in a weld of 25 % circumference. Defect limits in Tier 3 can alternatively be set for pipeline girth welds by other recognized Þtness-for-purpose methods.

Application of the EPRG guidelines Examples of the application of Tiers 1 to 3 are given in Þgure 2 for three pipe dimensions. Figure 2a shows the allowable defect sizes for an (individual) undercut in the root. For all pipe dimensions the allowed defect lengths increase from Tier 1 to Tier 2 and Tier 3. This behaviour is pronounced when applying Tier 3 to larger diameters and wall thicknesses. But even in the case of 7 mm wall thickness pipe the defect length is doubled when changing from Tier 1

56" x 20 mm

Pipe Dimensions

(the good workmanship standard) to Tier 2 (the more conservative Þtness-for-purpose level). Similar results can also be seen for the total Ôall defectÕ length in Figure 2b, showing the beneÞt in applying Tiers 2 and 3. Figure 2 also gives an impression of the safety margins using good workmanship standards.

[7]

[8]

Conclusions The European Pipeline Research Group (EPRG) has produced defect limits for pipeline girth welds. These guidelines are based on Þtness-for-purpose calculations and extensive full scale and wide plate testing. The guidelines are aimed at providing pipeline operators with uniform acceptance levels and defect limits. EPRG is committed to these guidelines, and intend to update them, as further data becomes available.

[9]

[10] [11]

[12]

References [1] Eiber, R. J.; Jones, D. J.: An analysis of reportable incidents for natural gas transmission and gathering lines, June 1984 through 1990. AGA NG-18, Report No. 200, August 1992 [2] Anon.: Welding of pipelines and related facilities. API Standard 1104, 17th Edition, September 1988 [3] Anon.: Process of welding of steel pipelines on land and offshore. BSI 4515: 1984, British Standards Institution, 1984 [4] Anon.: Gas pipeline systems. Canadian Standards Association, CAN/CSAZ184-M86, 1986 [5] Roodbergen, A. H.; Denys, R.: Limitations of Þtness-for-purpose assessment of pipeline girth welds. Int. Conf. on Pipeline Technology, AIM, Rome, 1987 [6] Mc Donald, K.; Hopkins, P.: The signiÞcance of embedded non-planar defects in transmission pipeline girth welds: A literature review. Pipes and Pipelines

[13]

[14]

[15]

[16]

[17]

International, March-April (Part 1), MayJune (Part 2), 1995 Hopkins, P.; Pistone, V.; Clyne, A. J.: A study of the behaviour of defects in pipeline girth welds: The work of the European Pipeline Research Group. Conf. on Pipeline Reliability, Calgary, Canada, June 1992 Hopkins, P.: The application of Þtnessfor-purpose methods to defects detected in offshore transmission pipelines. Conf. on Welding and Weld Performance in the Process Industry, IBC, London, April 1992 Hopkins, P.; Denys, R.: The European Pipeline Research Group's guidelines on acceptable girth weld defects in transmission pipelines. Eighth AGA Linepipe Symposium, Houston, September 1993 Denys, R.: To be published Anon.: Guidance on methods for assessing the acceptability of ßaws in fusion welded structures. BSI Standards, Published Document, PD 6493: 1991 Glover, A. G. et al.: Engineering critical assessment of pipeline girth welds. Conf. on Fitness-for-purpose of Welded Structures, Welding Institute, London, November 1981 Coote, R. I. et al.: Alternative girth weld acceptance standards in the Canadian Pipeline Code. Conf. on Welding and Performance of Pipelines, Welding Institute, London, paper 21, November 1986 Wilkowski, G. M.: Evaluation of tensile failure of girth weld repair grooves in pipes subjected to offshore laying stresses. Energy Sources Technology Conference, New Orleans, February 1980 Anderson, T. L.: Plastic collapse analysis of girth weld repair grooves in pipes subjected to offshore laying stresses. Int J Press Vess & Piping, 31, 2, 1988, pp. 105 Ð 130 Miller, A. G.: Review of limit loads of structures containing defects. Int J Press Vess & Piping, 32, nos. 1 Ð 4, 1988 Kastner, W. et al.: Critical crack size in ductile piping. Int J Press Vess & Piping, 9, 1981, pp. 197 Ð 219

1996_Knauf, Hopkins - EPRG Weld Defect Guidelines - 3R

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