growth of through-wall fatigue cracks in brace members
TRANSCRIPT
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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
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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
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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]
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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
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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.
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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.
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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.
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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.
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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.
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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
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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
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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.
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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.
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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.
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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
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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
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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).
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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.
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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.
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2 0
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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 .
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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
External -x- 23 170 -x- -x-
Internal 5 93+23 185 500 193W09-01
External -x- 35 178 -x- -x-
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
External -x- 54 142 -x- -x-
Internal 23 118 180 408 210W04-01
External -x- 36 170 -x- -x-
-x- Data not available
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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
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APPENDIX A
Material Certificates
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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.
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APPENDIX C
Weld Inspection Certificates
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APPENDIX D
Fatigue Test Certificates
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APPENDIX E
Engineering Critical Analysis
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