Growth of Through-wall Fatigue Cracks in Brace Members

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HSEHealth & Safety

Executive

Growth of through-wall fatiguecracks in brace members

Prepared by TWI Ltd for the

Health and Safety Executive 2004

RESEARCH REPORT 224



HSEHealth & Safety

Executive

Growth of through-wall fatiguecracks in brace members

Dr Marcos Pereira

TWI Ltd

Granta Park

Great Abington

Cambridge

CB1 6AL

The total fatigue life of a brace in an offshore jacket structure is conventionally considered in four parts.

N1 is the number of cycles to initiate the first discernible surface cracking as noted by any available

method. N2 is the number of cycles to detect surface cracking by visual examination without the use of

crack enhancement or optical aids. N3 is the number of cycles until the first through wall cracking and

N4 is the total number of cycles to the end of test or final separation of the member. The majority of

fatigue tests conducted on tubular connections or on girth welds in brace members obtained only N3

results, it being common practice to stop testing when a through wall crack was present. In the HSE

Guidance the S-N curves for tubular connections and girth welds in braces are therefore based on N3data. (In fact it should be noted here that there were very few test results for single sided girth welds

available at the time of drafting the HSE guidance; the choice of Class F2 for these joints was therefore

based largely on judgement rather than data).

In UK waters, flooded member detection (FMD) by ultrasonic inspection with a remotely operated

vehicle is used to check whether through cracks are present; however, in practice, fatigue cracks are

likely to continue to grow around the brace circumference after breaking through-wall. A review by

Sharp (Ref.1) concluded that detailed knowledge of the crack shape development after breakthrough

together with a value for the ratio N4 /N3 are required. From the structural safety viewpoint, there is

clearly a need to quantify the rate of fatigue crack growth after development of a through wall crack, but

prior to the point at which final separation becomes a possibility. The present study was designed to

examine these factors for circumferential welds in tubular members, and hence allow the efficacy of the

FMD strategy to be assessed.

This report and the work it describes were funded by the Health and Safety Executive (HSE). Itscontents, including any opinions and/or conclusions expressed, are those of the authors alone and do

not necessarily reflect HSE policy.

HSE BOOKS



ii

© Crown copyright 2004

First published 2004

ISBN 0 7176 2867 1

All rights reserved. No part of this publication may bereproduced, stored in a retrieval system, or transmitted inany form or by any means (electronic, mechanical,photocopying, recording or otherwise) without the priorwritten permission of the copyright owner.

Applications for reproduction should be made in writing to:Licensing Division, Her Majesty's Stationery Office,St Clements House, 2-16 Colegate, Norwich NR3 1BQor by e-mail to [email protected]



CONTENTS

EXECUTIVE SUMMARY v

Background v

Objectivesv

Work Carried Out v

Conclusions v

Recommendations vi

1. INTRODUCTION 1

2. OBJECTIVES 3

3. PROJECT OVERVIEW 5

4. SCOPE OF WORK 7

4.1. TASK 1 – SPECIMEN MANUFACTURE 7

4.2. TASK 2 – TESTING FIXTURE DESIGN AND COMMISSIONING 7

4.3. TASK 3 – FATIGUE TESTING 8

4.4. TASK 4 – ANALYTICAL EVALUATION AND VALIDATION OF ANALYTICAL MODEL 9

5. RESULTS AND DISCUSSION 11

6. CONCLUSIONS 15

7. RECOMMENDATIONS 17

8. REFERENCES 19

FIGURES 1 – 2

TABLES 1 – 4

Appendix A – Material Certificates

Appendix B – Welding Procedure

Appendix C – Weld Inspection Certificates

Appendix D – Fatigue Test Certificates

Appendix E – Engineering Critical Analysis

iii



iv



v

EXECUTIVE SUMMARY

BACKGROUND

The total fatigue life of a brace in an offshore jacket structure is conventionally considered in

four parts. N1 is the number of cycles to initiate the first discernible surface cracking as noted

by any available method. N2 is the number of cycles to detect surface cracking by visual

examination without the use of crack enhancement or optical aids. N3 is the number of cycles

until the first through wall cracking and N4 is the total number of cycles to the end of test or

final separation of the member. The majority of fatigue tests conducted on tubular

connections or on girth welds in brace members obtained only N3 results, it being common

practice to stop testing when a through wall crack was present. In the HSE Guidance the S-N

curves for tubular connections and girth welds in braces are therefore based on N3 data.

In UK waters, flooded member detection (FMD) by ultrasonic inspection with a remotely

operated vehicle is used to check whether through cracks are present; however, in practice,

fatigue cracks are likely to continue to grow around the brace circumference after breaking

through-wall. From the structural safety viewpoint, there is clearly a need to quantify the rateof fatigue crack growth after development of a through wall crack, but prior to the point at

which final separation becomes a possibility. In this way the remaining safe life of the

structure after through-wall cracking can be assessed, and hence the efficacy of the FMD

strategy examined.

OBJECTIVES

To carry out fatigue tests on tubular girth welds in order to determine their remaining fatigue

life (N3-N4) after development of through wall cracks.

WORK CARRIED OUT

A series of circumferential butt welds in steel pipe of 324mm outside diameter and 12.7mmwall thickness was fatigue tested under four point bending. The project was divided in four

major tasks as described below:

Task 1 – Specimen manufacture

Task 2 – Testing fixture design, commissioning and pre-test

Task 3 – Fatigue testing

Task 4 – Analytical evaluation and validation of analytical model

All tasks have now been completed; this report is the final one in the series.

CONCLUSIONS

A series of fatigue tests on circumferential butt welds in steel pipe of 324mm outside diameter

and 12.7mm wall thickness was conducted in which crack development prior to final failure

was examined in detail. The results lead to the following conclusions.

x A value of 1.1 can be reasonably assumed for the ratio N4/N3 for the girth welds

investigated here.

x It is evident that the remaining fatigue life could be only slightly greater than the

endurance. This has important implications on the use of flooded member detection

(FMD) and the selection of an appropriate inspection interval needs to be taken into

careful consideration in the development of a structural integrity management strategy.



vi

x Conservative estimations of fatigue endurance of tubular girth welds can be achieved

using the current formulations of BS 7910 and the fatigue crack growth mean line for

R~0.1 (Ref.3).

RECOMMENDATIONS

x It is recommended that further investigation be undertaken to incorporate in the fatigue

crack growth formulation of BS 7910, methods to estimate the growth of the first

observed through crack (based on N3) until a fully developed through crack (based on

N*) is achieved. For this purpose, further fatigue tests will be required in order to measure

the fatigue crack length and height during testing. Finite element analysis may also

provide K solutions in order to estimate the crack growth ratios that should be applied in

the N3 to N* interval.

x It is also recommended that further tests be carried out in order to study the fatigue crack

shape development, which is essential to validate fatigue crack growth formulations

developed within the N3 to N* interval.

x Behaviour is likely to be influenced by the wall thickness to diameter ratio. Further tests

for tubes of different dimensions are therefore recommended in order to allow broader

application of these findings.

x The results provide a limited statistical dataset and need to be considered further in

conjunction with the wider data in the literature.

Note: N* is the total number of cycles from the start of testing required for a surface crack to

develop and grow to a nominally straight fronted through wall crack, i.e. when the external

and internal crack lengths are roughly equal.



1

1. INTRODUCTION

The total fatigue life of a brace in an offshore jacket structure is conventionally considered in

four parts. N1 is the number of cycles to initiate the first discernible surface cracking as noted by

any available method. N2 is the number of cycles to detect surface cracking by visual

examination without the use of crack enhancement or optical aids. N3 is the number of cycles

until the first through wall cracking and N4 is the total number of cycles to the end of test or final separation of the member. The majority of fatigue tests conducted on tubular connections

or on girth welds in brace members obtained only N3 results, it being common practice to stop

testing when a through wall crack was present. In the HSE Guidance the S-N curves for tubular

connections and girth welds in braces are therefore based on N 3 data. (In fact it should be noted

here that there were very few test results for single sided girth welds available at the time of

drafting the HSE guidance; the choice of Class F2 for these joints was therefore based largely

on judgement rather than data).

In UK waters, flooded member detection (FMD) by ultrasonic inspection with a remotely

operated vehicle is used to check whether through cracks are present; however, in practice,

fatigue cracks are likely to continue to grow around the brace circumference after breaking

through-wall. A review by Sharp (Ref.1) concluded that detailed knowledge of the crack shape

development after breakthrough together with a value for the ratio N4/N3 are required. From the

structural safety viewpoint, there is clearly a need to quantify the rate of fatigue crack growth

after development of a through wall crack, but prior to the point at which final separation

becomes a possibility. The present study was designed to examine these factors for

circumferential welds in tubular members, and hence allow the efficacy of the FMD strategy to

be assessed.



2



3

2. OBJECTIVES

To carry out fatigue tests on tubular girth welds in order to determine their remaining fatigue

life (N3-N4) after development of through wall cracks.



4



5

3. PROJECT OVERVIEW

The project was divided in four major tasks as described below:

Task 1 – Specimen manufacture

Task 2 – Testing fixture design, commissioning and pre-test

Task 3 – Fatigue testingTask 4 – Analytical evaluation and validation of analytical model

All tasks have now been completed; this report is the final one in the series.



6



7

4. SCOPE OF WORK

4.1. TASK 1 – SPECIMEN MANUFACTURE

In order to be as consistent as possible with earlier studies on tubular connections, particularly

those conducted under the United Kingdom Offshore Steels Research Project (UKOSRP, Ref.2)

the pipe grade selected was a BSI 7191 GR 355EN (EN 10210-1) with specified yield strengthof 350MPa and fracture toughness measured in terms of Charpy energy of 27J at –20oC. The

main pipe dimensions were 12in (324mm) outside diameter (OD) and 12.7mm wall thickness.

The pipe material specification is given in Appendix A.

TWI developed a welding procedure for the girth welds using a single sided SMAW procedure

similar to that used for North Sea offshore structures. The welding procedure details are given in

Appendix B.

Eight specimens were prepared from 16 sections with the final specimen dimensions as shown

in Fig.1. The sections were marked prior to flame cutting and welding in order to allow for

consistent preparation of samples.

Figure 1

Schematic illustration of fatigue testing arrangement (not to scale, dimensions in mm)

All specimens were inspected visually and by magnetic particle inspection (MPI) and X-ray

non-destructive methods. The standard inspection criterion applied for the girth welds was

BS EN 288-3 with reference level B of BS EN 25817. All welds passed the criteria adopted.

The inspection certificates are given in Appendix C.

Following an accidental overload of one of the original specimens (W01-01) an additional

specimen (W09-01) was prepared using the same welding procedure and inspection methods

described above.

4.2. TASK 2 – TESTING FIXTURE DESIGN AND COMMISSIONING

The fatigue tests were conducted under bending load. Four point loading was selected since this

gives a relatively uniform stress field in the test section between the points of load application.

A specially designed four point bending rig was manufactured as shown in Fig.2. The rig was

commissioned and tested with specimen W07-01 (the first specimen to be tested in Task 3).

Originally it was planned to pressurise a rubber sleeve around the girth weld with low pressure

air, and to monitor the pressure as a means of detecting through-thickness cracking. Specimen

W07-01 was tested in this way; however, the method proved to be unsatisfactory. It was decided

1400

Weld

Applied

loads350



8

that further tests would be monitored internally using a digital camera to view the internal weld

root bead at the position of maximum tensile stress, and by visual inspection of the exterior.

This method proved to be successful and was used for all the remaining specimens.

Figure 2

Fatigue testing arrangement

4.3. TASK 3 – FATIGUE TESTING

Fatigue testing was conducted under ambient laboratory conditions using a servo-hydraulic

testing system of 1000kN capacity operating in load control. The testing frequency was in the

range 1.0 to 1.5 Hz. Four electrical resistance strain gauges of 6mm gauge length (type FLA-6-

11) were bonded to the pipe outer surface to measure the axial strain range, and hence allow the

outer fibre axial stress range local to the joint to be estimated. Gauges 1 to 3 were placed at 6

o’clock position. The centre of the gauges 1 and 2 being 5mm from the weld toe, one gauge

either side of the weld respectively. Gauge 3 was placed 85mm from the weld toe on the gauge

2 side. Gauge 4 was placed 5mm from the weld toe at 12 o’clock position. Applied stress ranges

were selected to achieve lives in the range A to B cycles approximately. All tests were

conducted with a tensile mean stress in the outer fibre at the location of failure, i.e. with a

positive load ratio, R. The nominal stress ranges and mean stress values applied are shown inTable 1.

The tests were carried out in two stages. In the first stage, loading was applied until a through

wall crack developed in the girth weld. Once the first crack was detected the number of cycles

(N2) was recorded and the fatigue crack growth was monitored and measured constantly until a

through wall crack was detected. The through wall cracks were detected by visually inspecting

the external pipe wall at convenient intervals (number of cycles) that varied accordingly with

the applied load. The size of the internal and external cracks were measured and monitored

systematically in order to record the number of cycles for the first through wall crack (N3) and

to determine the ratio of N2/N3 cycles. In the second stage, additional monitoring of the external



9

crack size was carried out in order to determine the number of cycles (N*)1

at which the crack

had developed to a nominally straight fronted through wall crack, i.e. when the external and

internal crack lengths were roughly equal. This stage was deemed to have been reached once the

external crack length had grown to 90% of the internal length, at which stage monitoring was

discontinued and the test was allowed to continue until unstable tearing occurred.

4.4. TASK 4 – ANALYTICAL EVALUATION AND VALIDATION OF ANALYTICALMODEL

In order to estimate the total number of cycles to failure for comparison with the experimental

data generated in Task 3, assessments were made using a fracture mechanics approach based on

BS 7910: amendment No.1 (Ref.3). A stress intensity factor solution derived for joints in flat

plate (Clause M.3.2, M.5.1.3 and P.4.3.2 of BS 7910) was used. Stress intensity magnification

factor (Mk ) of flaws at weld toe was assumed according to Clause M.5.1.3 of BS 7910 (Mk 3D

solutions). The assessments were undertaken in two stages. Stage 1 estimated the total number

of cycles to grow an initial surface crack using the experimentally measured first crack size at

N2 cycles obtained in Task 3 to the first through wall crack. Stage 2 estimated the total number

of cycles to failure (i.e. when the final flaw length reached the limit of validation of BS 7910

flat plate formulation) using the re-characterized through wall crack size from the estimated firstthrough wall crack size from stage 1. The initial crack sizes used in the assessments are

summarized in Tables 3 and 4.

To carry out the fatigue crack growth assessments the software CRACKWISE 3, version 3.13,

was used. Initially the BS 7910 recommended Paris Law constants for a stress ratio R>0.5 were

used, however the results obtained were very conservative in terms of the estimated total

number of cycles to failure. Paris law constants for the fatigue crack growth assumed in the

assessments were then based on the mean line for a stress ratio R~0.1 (Ref. 4). Two sets of

assessments were undertaken, i.e. two-stage fatigue crack growth and simplified fatigue crack

growth curves as described in Ref.3 and 4. The fatigue thresholds assumed in the assessments

are those from BS 7910: Amendment No.1.

1Note: N* is the total number of cycles from the start of testing required for a surface crack to

develop and grow to a nominally straight fronted through wall crack, i.e. when the external and

internal crack lengths are roughly equal.



10



11

5. RESULTS AND DISCUSSION

Nine specimens were fatigue tested and a summary of the results is shown in Table 1 and Fig.3.

The fatigue test certificates are given in Appendix D.

10

100

1000

1.E+05 1.E+06 1.E+07

Endurance N, cycles

S t r e s s r a n g e ,

M P a

Class B

ClassC

Class D

Class E

N2

N3

N4

Figure 3

Fatigue test results including N2, N3 and N4 measured cycles

As indicated above, Specimen W07-01, the first specimen tested, used air pressure to detect the

through wall crack. The system did not work as planned and the specimen fractured while it was

loaded overnight without through-thickness cracking being detected. From post mortem

examination it was found that the internal crack shape did not allow the air to pass through the

opened crack at a sufficient rate. Hence it was decided that in further tests visual inspection

aided by a video camera would be used to detect the internal and external cracks, as described

earlier. Therefore, the W07-01 initial crack size at N2 cycles and first through wall crack size at

N3 cycles reported in Table 1 were estimated based on the strain gauge measurement and the

last number of cycles monitored before a large crack was detected respectively. While these

estimates were made in an attempt to retrieve some value from the test, clearly they are not validtest results and should be treated with caution.

From Table 1 it should be noted that tests W01-01 and W03-01, both tested at 100MPa stress

range, were not continued to failure. W01-01 specimen achieved five million cycles without any

visible internal or external cracks, at which point the test was terminated. W03-01 survived 2.8

million cycles without cracking, at which point a test machine malfunction gave rise to an

overload, damaging the specimen beyond any possible repair. Due to these two different

circumstances there are no N3 or N4 data available for the tests at 100MPa stress range.



12

All remaining specimens were tested successfully to failure, i.e. until unstable tearing occurred.

For those specimens the initial internal crack sizes at N2 cycles and external crack sizes at N3

cycles, and the parameter N*

as defined above, were successfully measured. These values are

summarized in Table 1. Figure 4 shows the plots for all specimens that developed through wall

cracks (N3) and the measured fatigue endurance N4 (in this figure the N2 data plots are omitted).

Note that specimens W01-01 and W03-01 are plotted as run-out values together with the N4

values from other specimens.

10

100

1000

1.E+05 1.E+06 1.E+07

Endurance N, cycles

S t r e s s r a n g e ,

M P a

Class B

ClassC

Class D

Class E

N3

N4

Figure 4

Fatigue test results showing N3 and N4 measured cycles. Also plotted are the run out values of

specimens W01-01 and W03-01 (N2) as N4 indicated with an arrow

Examination of the results in Table 1 suggests that an N4/N3 ratio of between 1.0 and 1.2 could

be applied to the girth welds tested here.

The results of the fatigue crack growth assessments undertaken using the two stage and

simplified fatigue crack growth curves are summarized in Tables 3 and 4. Figure 5 shows plots

of the estimated and measured fatigue endurance ratio (N4est/N4) against the nominal stress rangeapplied during the fatigue testing for the simplified and two stage fatigue crack growth

assessments. The detailed assessment printouts are given in Appendix E.

The fatigue assessments were undertaken on specimens for which values of N2 were

successfully measured. Tables 3 and 4 show that the best estimation of the measured N4 cycles

is achieved using the simplified fatigue crack growth curve. The results also show that, for the

majority of the cases studied, the formulation in BS 7910 yielded conservative estimates of

actual fatigue life.



13

0.00

0.20

0.40

0.60

0.80

1.00

1.20

100 120 140 160 180 200 220 240 260

Stress range, MPa

N 4 e s t / N 4

Simplified

Two stage

limit of failure

Figure 5

Stress range plotted against the estimated/measured fatigue endurance ratio (N4est/N4)

Specimen W02-01 gave low fatigue endurances by comparison with the other specimens, as can

be seen in Table 1 and Figure 3. In addition, the fracture mechanics assessment overpredicted

the endurances for this test, see Tables 3 and 4. Results of the NDT conducted after specimen

manufacture did not provide any explanation of this low result; similarly examination of the

fatigue fracture surfaces (shown in Fig.6) found no fabrication flaw which would account for

early fatigue development. It would therefore appear that this is a valid test result towards thelower side of the expected scatter, most likely as a result of local variability of the root bead

profile.

D002506_01

Figure 6

Fracture surface of W02-01. Stress range 250MPa; Mean Stress 125MPa



14

A plot of the estimated and measured number of cycles at the first through wall crack and end of

test (N4est/N3est and N4/N3) is shown in Fig.7. The plotted data lie below the diagonal showing

that the assessments underestimated this ratio. The figure also shows that N4est/N3est converges to

a value of 1.04 whilst the experimental data (Table 1) indicates a N4/N3 converging to a value of

1.1. The difference between the measured and assessed values can be attributed to the

conservative estimation of fatigue crack growth according to BS 7910, since it did not take into

account the endurance for the crack growth from the first observation of a through wall crack to

a fully developed straight fronted through wall crack. Evidence of this is given in Tables 1, 3

and 4.

The ratio of the cycles to the first observed through crack/fully developed through crack (N*/N3)

varied from 1.02 to 1.15. Using the estimated N3est and the measured N*

the estimated ratio

(N*/N3est) varies from 1.14 to 1.65 for the simplified fatigue crack growth curve (W02-01

excluded). This reflects the general conservatism in the predicted N3 values, N3est, compared to

those observed in the test. A monitoring strategy could therefore be based on the predicted N3est

values.

1.00

1.05

1.10

1.15

1.20

1.00 1.05 1.10 1.15 1.20

N4 /N3

N 4 e s t / N 3 e s t

Figure 7

Plot of estimated (N3est/N4est) and measured (N3/N4) first through crack/fatigue endurance ratios



15

6. CONCLUSIONS

A series of fatigue tests on circumferential butt welds in steel pipe of 324mm outside diameter

and 12.7mm wall thickness was conducted in which crack development prior to final failure was

examined in detail. The results lead to the following conclusions.

x A value of 1.1 can be reasonably assumed for the ratio N4/N3 for the girth weldsinvestigated here.

x It is evident that the remaining fatigue life could be only slightly greater than the endurance.

This has important implications on the use of flooded member detection (FMD) and the

selection of an appropriate inspection interval needs to be taken into careful consideration in

the development of a structural integrity management strategy.

x Conservative estimations of fatigue endurance of tubular girth welds can be achieved using

the current formulations of BS 7910 and the fatigue crack growth mean line for R~0.1

(Ref.4).



16



17

7. RECOMMENDATIONS

x It is recommended that further investigation be undertaken to incorporate in the fatigue

crack growth formulation of BS 7910, methods to estimate the growth of the first observed

through crack (based on N3) until a fully developed through crack (based on N*) is achieved.

For this purpose, further fatigue tests will be required in order to measure the fatigue crack

length and height during testing. Finite element analysis may also provide K solutions inorder to estimate the crack growth ratios that should be applied in the N3 to N

*interval.

x It is also recommended that further tests be carried out in order to study the fatigue crack

shape development, which is essential to validate fatigue crack growth formulations

developed within the N3 to N*

interval.

x Behaviour is likely to be influenced by the wall thickness to diameter ratio. Further tests for

tubes of different dimensions are therefore recommended in order to allow broader

application of these findings.

x

The results provide a limited statistical dataset and need to be considered further inconjunction with the wider data in the literature.



18



19

8. REFERENCES

1 – Sharp J V, Stacey A, Wignall C M ‘Structural Integrity Management of Offshore

Installations Based on Inspection for Through-Thickness Cracking’, OMAE 1998

2 – Lapwood D G ‘Pedigree of Steel used in the UKOSRP-I Programme’, SDR(1984), TWI

Report No. 3460/12/77, August 1977.

3 – BS 7910:1999 (incorporating Amendment No.1): ‘Guide on methods for assessing the

acceptability of flaws in metallic structures’

4 – King R N, Stacey A and Sharp J V: 'A Review of Fatigue Crack Growth Rates for Offshore

Steels in Air and in Seawater Environments', OMAE 1996, Vol.III, pp.341-348.



2 0



2 1

T a b l e 1

F a t i g u

e t e s t i n g r e s u l t s

S p

e c i m e n

N o m i n a l S t r e s s

R a n g e

( M P a )

N o m i n a l M e a n

S t r e s s ( M P a )

N 2

( c y c l e s )

N 3 ( c y c l e s )

N 2 / N 3

N

*

N * / N 3

N 4 ( c y c l e s )

N 4 / N 3

W

0 1 - 0 1 +

1 0 0

5 0

2 , 8

8 3 , 0

8 0

- x -

- x -

-

x -

- x -

- x -

- x -

W 0 3 - 0 1 * *

1 0 0

5 0

5 , 0

9 2 , 7

9 3

- x -

- x -

-

x -

- x -

- x -

- x -

W

0 6 - 0 1

1 5 0

7 5

1 , 0

8 2 , 9

6 9

1 , 6

1 4 , 8

0 1

0 . 6

7 1

1 , 7 0

8 , 3

9 4

1 . 0

6

1 , 7

2 8 , 4 7

5

1 . 1

W

0 8 - 0 1

1 5 0

7 5

3 4 4 , 1

6 2

4 9 8 , 6

0 0

0 . 6

9 0

5 7 2

, 9 0 0

1 . 1

5

5 8 4 , 7 9 1

1 . 2

W

0 9 - 0 1

1 5 0

7 5

3 3 5 , 3

2 3

1 , 3

4 0 , 2

9 1

0 . 2

5 0

1 , 4 1

7 , 2

2 4

1 . 0

6

1 , 4

4 2 , 5 7

4

1 . 1

W

0 5 - 0 1

2 0 0

1 0 0

3 1 4 , 2

0 7

4 7 0 , 8

7 2

0 . 6

6 7

4 9 5

, 9 1 5

1 . 0

5

5 1 7 , 9 7 3

1 . 1

W 0 7 - 0 1 + +

2 0 0

1 0 0

6 2 1 , 9

3 5

6 3 6 , 6

1 3

n

0 . 9

7 6

6 4 6

, 5 5 6

1 . 0

2

6 6 8 , 1 2 2

1 . 0

W

0 2 - 0 1

2 5 0

1 2 5

1 2 0 , 0

0 0

1 3 7 , 8

4 0

0 . 8

7 1

1 4 6

, 0 0 0

1 . 0

6

1 4 9 , 4 8 9

1 . 1

W

0 4 - 0 1

2 5 0

1 2 5

2 9 0 , 9

0 0

3 8 6 , 0

3 6

0 . 7

5 4

4 0 0

, 8 1 8

1 . 0

4

4 0 3 , 7 7 7

1 . 0

- x - D a t a n o t a v a i l a b l e

+

O v e r l o a d e d .

N o c r a c k s w e r e i d e n t i f i e

d i n t h e s p e c i m e n .

N u m b e r o f c y c

l e s i n d i c a t e e n d o f t e s t .

* * T e s t s t o p p e d a t t h e n u m b e r o f c y c l e s i n d i c a t e d .

N o c r a c k s w e r e i d e n t i f i e d i n t h e s p e c i m e n .

+ + C r a

c k i n i t i a t i o n n o t c l e a r l y i d e n t i f i e d

o n v i s u a l i n s p e c t i o n .

N 2 v a l u e s e s t i m a t e d f r o m s t r a i n g a u g e m e a s u r e m e n t

n

T h e

n u m b e r o f c y c l e w a s e s t i m a t e d f r o m t h e p r e v i o u s c r a c k s i z e m o n i t o r i n g b e f o r e t h e t h r o u g h c r a c k w a

s d e t e c t e d .



22

Table 2

Internal and external measured crack length and correspondent fatigue cycles (all measures in

millimetres).

Specimen Position Crack length

at N2

Crack length

at N3

Crack length

at N

*Crack length

at N4

Weld/Parent

Interface

*

Internal -x- -x- -x- -x- -x-W01-01

External -x- -x- -x- -x- -x-

Internal -x- -x- -x- -x- -x-W03-01

External -x- -x- -x- -x- -x-

Internal 40 67 174 462 221W06-01

External -x- 44 177 -x- -x-

Internal 30 72 150 492 292W08-01


Internal 5 93+23 185 500 193W09-01


Internal 32 84 133 484 150W05-01

External -x- 36 117 -x- -x-Internal -x- -x- -x- 473 227

W07-01External -x- -x- -x- -x- -x-

Internal 37 85 152 400 235W02-01


Internal 23 118 180 408 210W04-01


-x- Data not available



23

Table 3

Result of the fatigue crack growth from BS 7910, two stage fatigue crack growth curve.

Specimen N2 N2 crack

size (mm) N3,est N2/N3,est N

* N

*/N3,est N4est N4est/N3,est N4,est/N4

W01-01 2,883,080 N/A N/A N/A N/A N/A N/A N/A N/AW03-01 5,092,793 N/A N/A N/A N/A N/A N/A N/A N/A

W06-01 1,082,969 40 1,277,681 0.848 1,708,394 1.34 1,304,785 1.02 0.75

W08-01 344,162 30 409,162 0.841 572,900 1.40 427,162 1.04 0.73

W09-01 335,323 5 749,323 0.448 1,417,224 1.89 781,323 1.04 0.54

W05-01 314,207 32 396,207 0.793 495,915 1.25 408,207 1.03 0.79

W07-01 621,935 N/A N/A N/A N/A N/A N/A N/A N/A

W02-01 120,000 37 154,000 0.779 146,000 0.95 160,000 1.04 1.07

W04-01 290,900 23 342,900 0.848 400,818 1.17 350,900 1.02 0.87

N/A – not applicable

Table 4

Result of the fatigue crack growth from BS 7910, simplified fatigue crack growth curve.

Specimen N2

N2 crack

size (mm) N3,est N2/N3,est N*

N*/N3,est N4est N4est/N3,est N4,est/N4

W01-01 2,883,080 N/A N/A N/A N/A N/A N/A N/A N/A

W03-01 5,092,793 N/A N/A N/A N/A N/A N/A N/A N/A

W06-01 1,082,969 40 1,323,965 0.818 1,708,394 1.29 1,350,948 1.02 0.78

W08-01 344,162 30 420,162 0.819 572,900 1.36 438,162 1.04 0.75

W09-01 335,323 5 857,323 0.391 1,417,224 1.65 889,323 1.04 0.62

W05-01 314,207 32 410,207 0.766 495,915 1.21 422,207 1.03 0.82

W07-01 621,935 N/A N/A N/A N/A N/A N/A N/A N/A

W02-01 120,000 37 160,000 0.750 146,000 0.91 166,000 1.04 1.11

W04-01 290,900 23 350,900 0.829 400,818 1.14 357,900 1.02 0.89

N/A – not applicable



APPENDIX A

Material Certificates

















Pipe Specification API 5L

Specification API 5L NPS 1/8 -- 26Scope Covers WELDED and SEAMLESS pipe suitable for use in conveying gas, water, and oil in both the oil and natural gas industries.

Kinds of Steel Permitted Open-hearth Basic-oxygen

For Pipe MaterialElectric-furnace

Hot-Dipped May be ordered galvanized.

Galvanizing

Permissible Variations Grade A, B, A25 X42 through X80

n Wall Thickness NPS 2 1/2 and smaller -- Seamless and welded, % +20 -- 12.5 +15 -- 12.5

NPS 3 -- Seamless and welded, % +18 -- 12.5 +15 -- 12.5

NPS 4 through 18 -- Seamless and welded, % +15 -- 12.5 +15 -- 12.5

NPS 20 and larger -- Welded, % +17.5 -- 10.0 +19.5 -- 8.0

NPS 20 and larger -- Seamless, % +15.0 -- 12.5 +17.5 -- 10.0

Chemical C max % Mn max % P max % S max %

Requirements Seamless or ERW

Grade A 0.25 0.95 0.05 0.06

Grade B 0.30 1.20 0.05 0.06

Continuous-weld - - 0.08 0.06

Tensile Lists minimum yield and tensile strength for all grads as well as a maximum tensile strength for X80.

Requirements Maximum yield-to-tensile ratios outlined for cold-expanded pipe--may be waived when a fracture toughness requirement is specified.

Hydrostatic Lists hydrostatic inspection test pressures Test Pressures are held for not less than:

Testing for all sizes and grades covered by the Seamless (all sizes) -- 5 seconds

specification. Welded (NPS 18 and smaller) -- 5 seconds

(NPS 20 and larger) -- 10 seconds

Permissible Variations For each length of Standard Weight, Regular Weight, For Special Plain End -- Not more than plus 10% minus 5%.

n Weights per Foot Extra Strong, and Double Extra Strong -- Not more than

plus 10% minus 3.5%. For Carload Lots -- Not more than minus 1.75%.

Permissible Variations Outside Diameter Sizes Over Under

n Outside Diameter at any point shall not vary from standard specified more than: -------------------------------------------------------------------

NPS 1 1/2 and smaller 1/64" 1/32"

NPS 2 through 4 1% 1% (Buttweld Only)

NPS 2 through 18 .75% .75%

NPS 20 through 26

Non-expanded 1% 1%

Mechanical Tests Tensile Test Bending Test (Cold) -- 2" and smaller Buttweld.

Specified Seamless and Buttwelded -- All Sizes -- Longitudinal Specimens Degree of Bend Diameter of Mandrel

Electric Weld -- NPS 6 and smaller -- Longitudinal For all API Uses 90 12 x OD of pipe

NPS 8 and Larger -- Transverse

Number of On One Length

Tests Required NPS From Each Lot of Flattening

Tensile 5 and smaller 40 or less Non-Expanded Electric-Weld for single lengths crop ends from each

6 through 12 200 or less length. For multiple lengths, crop ends from each length, plus 2

14 and larger 100 or less intermediate rings.

2 and smaller (Buttweld) 25 tons or less

Bending 1 1/2 and smaller (Buttweld) 50 tons or less

Lengths Minimum

Shortest Shortest Average

Length Length in 95% LengthThreaded & In Entire of Entire of Entire

Coupled Pipe Shipment Shipment Shipment

Single Random 16' 0" 18' 0" --

Double Random 22' 0" -- 35' 0"

Required Markings Paint Stenciled or Die Stamped (by agreement).

on Each Length Manufacturer's name or mark. Spec 5L, size, weight per foot, grade, process of manufacture, type of steel, length (NPS 4

On Tags attached to and larger only). Test pressure when higher than labulated(NPS 2 and larger only).

each Bundle in case Heat treat symbols, as applicable -- HN, HS, HA or HQ.

of Bundled Pipe)

General Supplementary Requirements available when specified. SR5--Charpy impact Testing--Welded Pipe 20" & larger--Grade X52 or higher.

nformation SR3 -- Color Identification. SR6 -- Drop Weight Tear Testing--Welded Pipe 20" & larger--Grade X52 or higher.

SR4 -- Nondestructive Inspection of Seamless Pipe. SR8 -- Fracture Tough ness Testing of Line Pipe.



APPENDIX C

Weld Inspection Certificates

























































APPENDIX D

Fatigue Test Certificates





































































APPENDIX E

Engineering Critical Analysis



































































































Printed and published by the Health and Safety ExecutiveC30 1/98

Printed and published by the Health and Safety Executive

C0.06 08/04



Documents

Growth of Through-wall Fatigue Cracks in Brace Members