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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



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:

231



• 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

232



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

233



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

234



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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drop weight tear testing of seamless linepipe eroktjerkt ekrjtker jkerjtert ert

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