drop weight tear testing of seamless linepipe eroktjerkt ekrjtker jkerjtert ert
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8/9/2019 Drop Weight Tear Testing of Seamless Linepipe eroktjerkt ekrjtker jkerjtert ert
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Drop Weight Tear Testing of Seamless Linepipe
Andre Hasenhütl, Marion Erdelen-Peppler Salzgitter Mannesmann Forschung GmbH
Duisburg, Germany
Tanja Schmidt, Dorothee NiklaschVallourec & Mannesmann Deutschland
Düsseldorf, Germany
ABSTRACT
Resistance against propagating fractures is one of the main
requirements for gas transmission pipelines. Ductile fracture resistanceand materials toughness is commonly assessed by Charpy impact and
Drop Weight Tear testing (DWT). To investigate propagation
characteristics of long running ductile fractures, fracture surface ofbroken DWT specimens are analysed in terms of ductile and brittle
portions. DWT testing is specified in pipeline standards forqualification of pipes with OD>18”. Since seamless pipes for line pipe
applications are commonly used in dimensions below 18”, hardly anyinformation about fracture propagation is available. Beside the lack of
historical data, occurrence of problems during testing due to pipe
dimensions is assumed.
KEY WORDS: battelle; drop weight tear test; DWT; BDWT;
seamless pipes; QT, seamless linepipe, West-Jefferson
INTRODUCTION
Due to considerations in the oil and gas industry to require DWT
testing of pipes with OD down to 12”, testing of small diameter
seamless pipe would be required.
In this paper the applicability of DWT testing to seamless quenched
and tempered (QT) pipes with diameters down to 12.8” is presented.Test results from seamless pipes, produced by VALLOUREC &
MANNESMANN TUBES, with yield strength level 65 ksi (450 MPa)
with outer diameters down to 12.8” will be shown. In order to limit the
number of influencing factors, steel source, steel type andmanufacturing process was the same for all tested pipes. Geometrical
aspects like the influence of pipe wall thickness, thickness reduction
and manufacturing route are scrutinised. Limitations for DWT testingon small diameter and resulting high curvature of seamless pipes are
highlighted and discussed. Recommendations for DWT testing
performance and evaluation on small diameter quenched and tempered
pipes are given.
Fracture propagation characteristics and materials toughness in gas
transmission pipelines are major concerns for the safe operation at highinternal pressure. Typical small scale laboratory toughness test methods
are Charpy impact testing and Battelle Drop Weight Tear testing(BDWT). Charpy impact testing is performed to quantify if the
toughness properties or rather the Charpy energy meet the requirements
in product specifications. Additionally to Charpy energy, the fracture
surface of broken specimens can be analysed in terms of brittle andductile surface.
BDWT testing is performed to investigate if crack propagation occursin ductile or brittle manner. Ductile to brittle transition curves are
established by testing at various temperatures in order to establish
ductile to brittle transition temperature (DBTT), which is specified as
the temperature, where the portion in ductile fracture is 85% (T85%).Additionally information regarding total energy and crack propagation
energy can be obtained by instrumented DWT testing.Beside small scale lab testing, West–Jefferson test on full length of pipe
is used in order to investigate resistance against propagation of longrunning ductile fracture. The pipe is exposed to different test
temperatures and fracture is initiated by explosive charge. After the
test, the fracture surface is examined concerning the amount of sheararea fraction. The test environment and test conditions are comparable
to conditions in operating pipelines. Thus, the real pipe behavior
concerning fracture propagation can be estimated using West-Jeffersontest method.Fracture propagation characteristic (toughness) strongly depends on
operating temperature. Toughness decreases with decreasing
temperature. In pipeline applications, it is strongly recommended that
toughness is in the upper shelf (ductile fracture appearance) at
operating conditions. West-Jefferson tests need to be carried out at
temperatures comparable to or below minimum design temperature ofpipe lines. Execution of West-Jefferson testing is very complex in
terms of time, cost and test setup. Furthermore, tests with explosive
charge can just be done on a limited number of test sides around theworld. Hence this test method is not appropriate as a standard qualitytest in running pipe production. Therefore, other reliable, faster and
viable small scale test methods for evaluating environmental conditionsfor long running ductile fractures are required.As an alternative test method BDWT testing is considered. In this test a
specimen with a mechanically inserted notch is cracked by an impact of
a drive-hammer. Subsequent testing, fracture surface of specimens areevaluated in terms of the amount in shear area fraction. The tests can be
carried out at different temperatures to investigate the ductile to brittle
230
Proceedings of the Twenty-fir st (2011) In ternational Of fshore and Polar Engineeri ng Conference
Maui, Hawaii, USA, June 19-24, 2011
Copyri ght © 2011 by the International Society of Of fshore and Polar Engineers (I SOPE)
ISBN 978-1-880653-96-8 (Set); ISSN 1098-6189 (Set); www.isope.org
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transition behavior. The resulting ductile to brittle transition
temperature and curve must be compared to results of West-Jeffersontests to quantify the correlation between laboratory small scale test
(BDWT) and West-Jefferson tests. If there is a good correlation, results
from small scale tests can be used to estimate fracture propagation
behavior or rather the ductile to brittle transition during full pipetesting. For seamless pipes, the correlation between BDWT and West-
Jefferson tests has not been investigated up to now. Especially forquenched and tempered (QT) seamless pipes, the statistical databaseconcerning results from BDWT and West-Jefferson tests is very
limited.
The common international standards for pipeline steels in the petroleumand natural gas industries are
• ISO 3183 (2nd ed. 2007)
• API5L (4th ed. 2007/2008)
• DNV-OS-F101 (2007)
According ISO 3183 and API5L, BDWT tests shall be performed onwelded pipes (PSL2) with diameters equal to or exceeding 20” (508
mm) only (see Table 1). DNV-OS-F101 requires BDWT testing to be
performed on welded pipes only, but limits the testing to diametersexceeding 500 mm and wall thickness (WT) exceeding 8 mm. In all
three standards, BDWT testing on seamless pipes is not foreseen.
Table 1: Requirements for BDWT testing depending on pipe geometry(linepipe standards)
OD WT
ISO 3183 / API5L ≥ 508 mm
DNV-OS-F101 > 500 mm > 8 mm
The test method itself and its execution are described in the teststandards EN 10274: Metallic materials – Drop Weight Tear test and
API RP 5L3: Recommended practice for conducting Drop-Weight Tear
tests on line pipe. Contrary to linepipe specifications like API 5L,
BDWT test standard EN 10274 allows BDWT testing on pipes with
outer diameters down to 300 mm.The requirements regarding pipe geometry for performing BDWT tests
are listed in Table 2.
Table 2: Requirements for BDWT testing depending on pipe geometry(test specifications)
OD WT
EN 10274 > 300 mm > 6 mm
API RP 5L3 as prescribed in API5L
The purpose of this paper is to disclose testing issues, which were
observed during Charpy impact, BDWT and West-Jefferson tests.
Numerous test results are shown to fortify the problematic in toughnesstesting on seamless QT linepipe steels. Additionally, test limitations
due to pipe geometry (outer diameter, wall thickness) and hence
applicability of BDWT testing on seamless pipes is investigated.
TOUGHNESS TESTING OF SEAMLESS QT PIPES
Seamless pipes in strength level higher than 52 ksi (360 MPa) areusually manufactured by hot rolling method and subsequent quenching
and tempering of pipe. Depending on the wall thickness and the pipe
outer diameter the cooling rate during quenching is different over the
wall and as the consequence the phase fractions vary depending on wall
thickness and location over the wall.
In general, for lower wall thickness up to 25 – 30 mm, themicrostructure can be considered as consistent over full wall. With
increasing wall thickness, the temperature gradient between outside
surface and midwall increases during quench of pipe. Hence
microstructure differences in wall accompanied by variations inmechanical properties can occur. Therefore, location of specimens in
pipe wall have a decisive influence on test results of Charpy impact andBDWT tests, if specimen volume does not occupy representative
amount of pipe wall thickness (e.g. BDWT test specimens with reducedthickness). However, minimum requirements can be guaranteed at any
point of the wall thickness.Seamless pipes for linepipe applications are usually ordered in heavy
wall thicknesses (WT >19 mm). If the pipe wall thickness exceeds 19
mm, testing may be performed on full wall thickness specimens or on
specimens with a thickness reduced to 19 mm. Thickness reduction
offers the possibility to perform BDWT tests using test equipment
which has insufficient energy for full wall thickness testing. Thicknessreduction leads to a shift in ductile to brittle transition to lowertemperatures and therefore, if testing is performed on specimens with
reduced thickness, test temperature must be decreased as described in
the test standards. As an example test temperature reduction is shown inTable 3 (API RP 5L3). According both test standards, wall thickness
reduction shall be performed by machining one or both surfaces of the
original wall.
Table 3: Test temperature reduction according API RP 5L3
API RP 5L3
specified pipethickness
test temperaturereduction °F (°C)
3/4“ to 7/8“ 10 (6)
7/8” to 1 1/8” 20 (11)
1 1/8” trough 1 9/16” 30 (17)
The influence of microstructure and phase composition on fracturepropagation characteristics of seamless QT pipes has not been
investigated up to now. Furthermore, the temperature reduction was
determined based on BDWT test results from welded pipes only and
the validity of the absolute values for temperature reduction was not
established for seamless pipes, yet.
Another challenge in BDWT testing of seamless QT pipes is related tospecimen flattening. As described in test standard API RP 5L3,
specimens shall be cold flattened unless the diameter to thickness ratio
(D/t) is less than 40. If D/t is less than 40, the middle part of the
specimen may be left unflattened on 1” to 2” length, as it is shown in
Fig. 1. If buckling occurs, testing is invalid and replacement tests shallbe conducted. In EN 10274 both flattening methods may be used
independent of D/t ratio.
Fig. 1: BDWT test specimen with unflattened middle part (source: EN10274)
Plastic deformation and resulting cold hardening during flattening may
lead to decreased shear area fractions and thus to conservative or even
incorrect test results, depending on amount of plastic pre-deformation.
If there are differences between flattened and non flattened specimens,results from non flattened specimens shall govern according to API RP
5L3 and EN10274. In this paper flattening procedures are denoted as:
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• Flattened: plastic deformation close to crack propagation path
• Non flattened: the middle part was left unflattened (Fig. 1)
Usually, seamless QT pipes have OD/WT ratios below 40. If cold
flattening method is applied on BDWT specimens, the plastic
deformation depends on outer diameter and wall thickness. Colddeformation is expected to result in change of shear area fraction due to
material embrittlement; hence to determine “real” pipe properties, the
middle part (Lc) of the tested specimens was left unflattened as shown
in Fig. 1.
Depending on OD/WT ratios, plastic deformation during flattening can
theoretically be above 25% as it is shown in Fig. 2. Cold deformation
during flattening is increasing with increasing WT and decreasing OD.
Therefore, specimens extracted from pipes with small OD/WT ratios
incur higher plastic deformation than pipes with high OD/WT ratios.
0
5
10
15
20
25
30
0 5 10 15 20 25 30 35 40 45 50 55
wall thickness [mm]
p
l a s t i c d e f o r m a t i o n [ % ]
OD219.1 mm OD323.9 mm OD406.4 mm OD508.0 mm
Fig. 2: Plastic deformation during flattening for different OD and WT
Beside BDWT testing all linepipe standards require Charpy impact
testing for all types of pipes regardless of manufacturing method. In
general, the position of Charpy impact specimens in the initial pipe wallis not explicitly specified in the pipeline standards. For heavy wall pipe,depending on specimen position the variation in microstructure lead to
different results for Charpy impact specimens even in one pipe.
Consequently, some standards and customer specifications requireadditional alternative test locations, i.e. closely underneath the outsidesurface of pipe in order to accommodate the possible property
variations of thicker wall pipes. DNV-OS-F101, for example, requires
one set of Charpy V-notch specimens subtracted 2mm above inner pipesurface to be tested during manufacture procedure qualification test ofseamless pipes with a wall thickness above 25mm.
EXPERIMENTAL ACTIVITIES
Investigations concerning toughness were performed within the last
years R&D testing programs. BDWT, Charpy impact and West-
Jefferson tests were used to investigate toughness and crack
propagation characteristics. During these tests unexpected incidents
during testing were observed.
BDWT testing
BDWT tests were performed on one of the most powerful drop weight
tear tester in Europe with a maximum in drop energy of 105 kJ and a
max. drop height of 3.8 m.
BDWT test specimens were extracted in transversal direction and apressed notch was inserted. Broken specimens were evaluated in terms
of ductile and brittle fracture portions. At each test temperature, a set of
at least two specimens was tested. To get additional informationregarding total and propagation energy, the striker was instrumented.
Using the force-time record, the energy portions for crack initiation and
crack propagation can be evaluated.
A characteristic force-time record from a BDWT test in the upper shelfis shown in Fig. 3.
Fig. 3: BDWT test force-time record in the upper shelf
Assuming that crack initiation takes place at the maximum force Fmax
,
crack initiation and crack propagation energy can be calculated from
force-time record. The total energy is the sum of both energy portions.
During BDWT testing of seamless QT pipes, inverse fracture was
observed in the ductile to brittle transition regime.
Inverse fracture is characterised by ductile crack initiation and earlystages crack propagation and subsequent change to brittle fracture after
some distance. This phenomenon is known from welded pipes (Halsen
and Heier, 2004) where it is found on pipes with high toughness. It is
attributed to embrittlement of the ligament material by plasticdeformation during impact of the drive hammer while the crack is not
initiated yet. According to standards, tests of specimens exhibitinginverse fracture are invalid. On all BDWT test specimens of seamless
QT pipe of strength level 65 ksi in the dimension 16” x 0.819”,
showing portions of brittle fracture (except 100% brittle fracture),
inverse fracture was observed. Inverse fracture example is shown in
Fig. 4.
0
50
100
150
200
250
300
350
400
450
500
1,272 1,274 1,276 1,278 1,28 1,282 1,284 1,286 1,288 1,29
time [s]
f o r c e [ k N ]
crackinitiation
energy crackpropagation
energy
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Fig. 4: Inverse fracture appearance
In this figure, the plastic deformation at the impact side of the hammer
and lateral expansion are visible.
Increasing test temperature to determine upper shelf temperatures, a
second issue for invalid test results was pointed out. In the upper shelfand at test temperatures in the transition regime where 100% ductile
fracture occurs, BDWT test specimens show enormous plasticdeformation.
In several cases unbroken specimens were observed as it is shown inFig. 5.
Fig. 5: Unbroken BDWT test specimens
According to standards, results of specimens exhibiting huge plastic
deformation and unbroken specimens are invalid. During BDWT
testing, only in the lower shelf, where 100% brittle fracture occurs,
valid test results were observed. An example for fracture surface in theupper and lower shelf is shown in Fig. 6.
Fig. 6: Fracture surface in the upper shelf: left and lower shelf: right
The observation of huge plastic deformation and fracture absence show
that specimens are plastically deformed prior to crack initiation.
Due to these observations, the deflection and crack initiation behaviorwas determined using a high speed camera, which was mounted to the
drop weight tear tester. A series of snapshots is given in Fig. 7.
6
4
21
3
5
Pressed notch
Ductile crack initiation
and propagation
Brittle crack
propagation
Notch side
Ductilecrack
propagation
Brittlecrack
propagation
Huge plasticdeformation
crack
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Fig. 7: High speed camera observation BDWT
As it can be seen in Fig. 7 huge deflection occurs prior to crack
initiation. The crack is visible first time in snapshot 6 and is thenpropagating very slowly. Huge specimen deflection was observed to be
typical in the upper shelf and in the transition regime. Additionally to
fracture surfaces and deformed specimens the camera observationshows that crack initiation in seamless QT pipes is not unproblematic.
According API RP 5L3 two notch types may be used as a crack starter,
the pressed notch and the Chevron notch. The pressed notch is thepreferred one for low toughness linepipe steels. For higher toughness
linepipe steels, the Chevron notch is the preferred notch type as it
should facilitate brittle crack initiation.
To investigate the influence of the notch type on crack initiation, one
test series with pressed (PN) and Chevron notch (CN) was performedon a pipe of strength level 65 ksi in the dimension 14” x 0.626”. Acomparison of shear area fractions is shown in Fig. 8.
0
10
20
30
40
50
60
70
80
90
100
-90 -80 -70 -60 -50 -40 -30 -20 -10 0
temperature [°C]
s h e a r a r e a [ % ]
Pressed notch
Chevron notch
Mean value pressed notch
Mean value Chevron notch
Fig. 8: Comparison shear area BDWT pressed and Chevron notch
The test results show a similar transition behavior for the pressed and
the Chevron notch. Both notch types show a sharp drop in shear area
fraction between -25°C and -30°C. A comparison of specific total and
propagation energies are shown in Fig. 9. Using Chevron instead of
pressed notch, leads to a small decrease in total energy, whereas thepropagation energy is nearly the same for both notch types. The
difference in energy values is not as significant as to be out of the
statistical scatter.
0
200
400
600
800
1000
1200
1400
1600
-90 -85 -80 -75 -70 -65 -60 -55 -50 -45 -40 -35 -30 -25 -20 -15 -10 -5 0
temperature [°C]
s p e c i f i c e n e r g y [ J / c m ² ]
PN total
PN propagation
CN total
CN propagation
Fig. 9: Comparison specific energies BDWT pressed and Chevron
notch
Furthermore, it was observed that inverse fracture also occurred on
specimens with Chevron notch. Thus, the inverse fracture problematic
was not avoided using the Chevron notch instead of the pressed notch.
The ductile to brittle transition behavior was examined by BDWT
testing of 95 specimens extracted from pipe in the dimension 12.8” x
0.5”. Quantity of tests was chosen in order to increase statistical database. DWT testing was performed at six test temperatures: 0°C (10
specimens); -10°C (10 specimens); -20°C (20 specimens); -25°C (20
specimens); -30°C (18 specimens); -35°C (17 specimens). Although werecognise that testing was invalid, fracture surfaces were evaluated
concerning shear area fractions as described in the test standards. The
resulting transition curve is shown in Fig. 10.
Fig. 10: BDWT transition behavior
As described before, specimens in the upper shelf (theoretically 100%
shear area) exhibit huge plastic deformation. Therefore, test results in
the upper shelf are invalid. In the ductile to brittle transition region,
where the amount in shear area fraction is less than 100% and morethan 0%, inverse fracture was observed. Thus, all test results areinvalid. The scatter in shear area fractions is very high, e.g. at a test
temperature of T=-35°C, shear area fractions were observed between
18% and 100%.
87
0
10
20
30
40
50
60
70
80
90
100
-40 -35 -30 -25 -20 -15 -10 -5 0
temperature [°C]
s h e a r a r e a f r a c t i o n [ % ]
shear area fraction
average shear area
85% SA 85% SA
≈-27°C
Massive plastic deformation
Plastic deformation
+
Inverse fracture
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DISCUSSION
During BDWT testing of seamless QT pipes with outer diameters down
to 12.8”, invalid test results were observed in the upper shelf (ductile
fracture) and in the ductile to brittle transition regime. Invalidity of test
results was found to be due to main issues like inverse fractureappearance, plastic deformation during impact and unbroken
specimens. These issues can be summarized as the main problem ofcrack initiation.In the upper shelf, BDWT test specimens from pipes with different
outer diameters exhibit plastic deformation (buckling). According to
test standards those specimens are invalid. In some case, highlydeformed unbroken specimens were observed. High speed camera
observations revealed huge deflection prior to crack initiation.
In the ductile to brittle transition regime, inverse fracture appearance
was observed on all tested seamless QT pipes of strength level 65 ksi.
Inverse fracture is characterised by ductile crack initiation and thechange to brittle fracture after some distance. The combination ofplastic deformation caused by impact and by huge specimen deflection
lead to embrittlement of the rest ligament due to strain hardening
effects. Thus, all test results in the ductile to brittle transition regime areinvalid, too.
Due to huge deflection, which was observed during high speed camera
observations, plastic deformation in the ligament increased more and
more until the combination of embrittlement and stress condition leadto brittle fracture of the remaining ligament.
As it is described in API RP5L3 specification, the Chevron notch is thepreferred one for higher toughness linepipe steels. The Chevron notch
leads to a decrease in crack initiation energy and therefore to easier
crack initiation.
The fracture behavior of specimens with Chevron and pressed notch
type was investigated. Shear area fractions of specimens with Chevron
and with pressed notch show similar ductile to brittle transition
behavior. Furthermore, it was observed that specific total energy fromspecimens with Chevron notch is lower compared to specimens with
pressed notch, but the propagation energies were nearly the same for
both notch types. The difference in energy values are not as significantas to be out of the statistical scatter.
Looking on the fracture surfaces, it was observed that specimens with
Chevron notch show inverse fracture, too. Thus, the inverse fractureproblematic cannot be solved using Chevron instead of pressed notch.
The scatter in shear area fraction was observed to be huge in the
transition regime. At even one test temperature, specimens can exhibit
shear area fractions between 18% and 100%.
Charpy impact testing on pipes of strength level 65 ksi and wall
thickness above 20 mm was performed on transversal specimens.
Testing was performed in a temperature range between -100°C and
0°C. The transition behavior in Charpy impact test was determined
using a database of more than 3700 single values. In the upper shelf,nearly all specimens exhibit incomplete fracture. Upper shelf energies
up to 463 J were measured. Down to a test temperature of -80°C
Charpy impact energy was observed to exceed 300 J.
West-Jefferson tests were performed on pipes of strength level 65 ksi.
At ambient test temperature, it was not possible to initiate a longrunning fracture. In both pipe directions, the crack propagated only 80mm. A second West-Jefferson test was performed at a temperature of -
10°C. In this test, the crack was propagating a short distance in pipe
axis direction and was then changing in circumferential direction.
CONCLUSION
The main problems concerning toughness testing of seamless QT pipes
with outer diameters down to 12.8”, can be summarized in the maintopic “crack initiation”.
During testing a lot of invalid test results were produced due todifferent causes. These can be subjected to following main issues:
•
BDWT testingo Inverse fracture
o Plastic deformation during impact
o Unbroken specimens
• Charpy impact testing
o Incomplete fracture
• West-Jefferson testing o Initiation of running ductile fracture
In BDWT test method, crack initiation problems lead to huge deflection
resulting in enormous plastic deformation during impact and thus to
ligament embrittlement. In some cases crack initiation was completelyabsent leading to unbroken specimens.
In West-Jefferson testing, crack initiation problems manifest by the
disability to initiate running ductile fractures.
FURTHER WORK
As a result of observed issue with crack initiation in BDWT and West-
Jefferson tests of seamless QT pipes and based on the fact that the
interpretation of fracture behavior is unclear, more detailedinvestigations will be conducted. The database of BDWT tests with
heavy wall using specimens with full and reduced thickness will be
further increased. The temperature reduction which is necessary for
specimens with a reduced thickness was established on welded pipematerial. The applicability of these temperature reductions shall be
proven for seamless QT pipe. The fracture behavior will be investigated
in more detail using high speed camera observations. West-Jeffersontests will be performed to increase knowledge concerning fracture
behavior of representative pipe segments and to determine a correlation
between BDWT and West-Jefferson tests.
REFERENCES
API 5L (2nd edition 2008). Specification for Line Pipe.
API RP 5L3 (3rd edition 1996). Recommended Practice for Conducting
Drop-Weight Tear Tests on Line Pipe.
DIN EN 10274 (1999). Fallgewichtsversuch.
DNV-OS-F101 (2007). Submarine Pipeline Systems.
Halsen, Kjell Olav and Heier, Espen (2004). “Drop Weight TearTesting of High Toughness Pipeline Material” Proceedings of IPC
2004, IPC04-0609.
ISO 3183 (2nd edition 2007). Petroleum and natural gas industries –
Steel pipe for pipeline transportation systems.
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