TECHNOLOGICAL SOLUTIONS FOR HIGH STRENGTHGAS PIPELINES

H.-G. Hillenbrand1, A. Liessem1, C. Kalwa1,M. Erdelen-Peppler2, C. Stallybrass2

1) EUROPIPE GmbH, Pilgerstraße 2, 45473 Mülheim an der Ruhr, Germany2) Salzgitter Mannesmann Forschung GmbH, Ehinger Strasse 200, 47259 Duisburg, Germany

ABSTRACT

Long distance pipelines are a safe and economic way to transport gas from remote gas fields tothe end users. The development of high-strength steels is a very fundamental necessity in orderto find technological solutions in view of increasing pipeline length and operating pressure.

EUROPIPE has delivered about 1,000km line pipe made of X80 and comparable grades. Wetherefore consider X80 as a standard grade in the entire supply chain from steel making to platerolling and to pipe manufacturing. Production results of the latest X80 projects are presented inthis paper. Manual combined-electrode welding and mechanized gas metal arc welding(GMAW) as field welding methods for pipeline construction are well-established. The pipematerial X80 is suitable for unrestricted use in onshore applications.

After having developed X100, several demonstration lines have been installed and have beenput into operation. As the development to X100 gives an idea of future challenges andpossibilities, updated production results of X100 pipes are presented to describe the materialproperties as well as the service behaviour.

1. Introduction

The ever increasing demand for energy world wide requires the construction of high-pressuregas transmission lines with the greatest possible transport efficiency, so that the cost of pipelineconstruction and gas transportation is minimised. This is particularly true when long distancesare to be covered. Therefore there is a trend towards using line pipe of larger diameter and/orincreasing the operation pressure of the pipeline. This, in turn, necessitates the use of higherstrength steel grades to avoid a large wall thickness that would be otherwise needed.

In addition, a reduction of the wall thickness at constant diameter and pressure can beeconomically attractive where an increase of the capacity is not required. This makes the use ofX80 pipes in long distance lines feasible.

The development of high-strength steels started more than 30 years ago, along with theintroduction of thermo-mechanical rolling practices, and will continue in the future. Figure 1shows the historical development of line pipe steels. It was mainly governed by the large-

diameter pipe manufactures [1-4], due to the fact that TM-treatment (with or without acceleratedcooling) can be applied optimally only for plate production.

In the early seventies, the hot rolling and normalising process route was replaced by thermo-mechanical rolling. The latter process enables materials up to X70 to be produced from steelsthat are micro-alloyed with Niobium and Vanadium and have a reduced Carbon content. Animproved processing method, consisting of thermo-mechanical rolling plus subsequentaccelerated cooling, emerged in the eighties. By this method, it has become possible to producehigher strength materials like X80, having a further reduced Carbon content and therebyexcellent field weldability. Additions of Molybdenum, Copper and Nickel enable the strengthlevel to be raised to that of grade X100, when the steel is processed to plate by thermo-mechanical rolling plus modified accelerated cooling.

1965 1970 1975 1980 1985 1990 1995 2000

TM + Acc.Cooling

TM + Acc.Cooling

TM -treatment

Hot rolledand normalized

0.06 CNb Ti

0.06 C 0.2 MoNb Ti

0.10 CNb V

0.20 CV

API grade

0.05 C CuNiCrMoV Nb Ti B

TM +Heavy Acc.

2005

X 120

X 100

X 80

X 70

X 60

X 521965 1970 1975 1980 1985 1990 1995 2000

TM + Acc.Cooling

TM + Acc.Cooling

TM -treatment

Hot rolledand normalized

Nb Ti

Nb Ti

Nb V

0.20 CV

API grade

0.05 C CuNiCrMoV Nb Ti B

2005

X 120

X 100

X 80

X 70

X 60

X 522008

TM +Heavy Acc.

1965 1970 1975 1980 1985 1990 1995 2000

TM + Acc.Cooling

TM + Acc.Cooling

TM -treatment

Hot rolledand normalized

0.06 CNb Ti

0.06 C 0.2 MoNb Ti

0.10 CNb V

0.20 CV

API grade

0.05 C CuNiCrMoV Nb Ti B

TM +Heavy Acc.

2005

X 120

X 100

X 80

X 70

X 60

X 521965 1970 1975 1980 1985 1990 1995 2000

TM + Acc.Cooling

TM + Acc.Cooling

TM -treatment

Hot rolledand normalized

Nb Ti

Nb Ti

Nb V

0.20 CV

API grade

0.05 C CuNiCrMoV Nb Ti B

2005

X 120

X 100

X 80

X 70

X 60

X 522008

TM +Heavy Acc.

Figure 1: Development of linepipe steels.

Microstructural features such as dislocations, grain boundaries and precipitates, govern themechanical properties of steels. In low-alloy steels, they develop in the course of transformationof the austenite during cooling, and the development depends on the cooling rate and coolingstop temperature.

Figure 2: Microstructural effects for enhancing strength and toughness properties.

Figure 2 shows, how the combination of the different types of microstructures contribute toincrease mechanical strength and toughness of steels starting from normalised X60 grade,

which was mainly used in the early seventies [5]. The steel typically contains about 0.2%Carbon, 1.55% Manganese, 0.12% Vanadium, 0.03 % Niobium and 0.02% Nitrogen.

The thermomechanically processed X70 steel mentioned in the figure was microalloyed andcontains only just 0.12% Carbon. Thermomechanical rolling results in a significant reduction ofthe ferrite grain size. Grain refinement is the only method by which both strength and toughnesscan simultaneously be improved. The loss of strength resulting from reduced Pearlite contentscan be offset by precipitation hardening and dislocation hardening. Reduction of Pearlitecontent, grain refining, dislocation hardening and precipitation hardening contributed individuallyand in combination to the development of X70 steel with improved weldability and favourableductile-brittle transition temperatures.

Further increases in strength and toughness, which led to the development of X80 steel, canonly be attained by changing the microstructure of the steel matrix from ferrite-Pearlite to ferrite-bainite. In comparison with the thermomechanically rolled X70 steel, the X80 steel has a furtherreduced Carbon content, reduced grain size and an increased dislocation density. These twosteel grades also differ in their precipitation characteristics.

Figure 3 shows typical microstructures of three modern types of linepipe steel. Themicrostructure of TM rolled X70 steels is uniform and contains ferrite grains (ASTM 10–11).Even more uniform and extremely fine microstructure (ASTM >12) is attained by acceleratedcooling that follows thermomechanical rolling, as shown for the X80 and X100 steel. Theimproved properties of these steels can be attributed to their bainitic microstructure.

Figure 3: Microstructure of high strength linepipe steels: X70 (left), X80 (centre), X100 (right).

Owing to the small grain size, the possibilities to characterise the microstructure using light-optical microscopy are limited. An alternative method is to use electron microscopy incombination with electron backscatter diffraction (EBSD). With this method it is possible tocharacterise the microtexture, measure the effective grain size and obtain information on thetype of bainite that is formed depending on the processing conditions [6]. An example of aninverse pole figure of a high strength linepipe steel is given in Figure 4. Each colour in thisimage represents a certain crystal orientation. The differences in orientation make it possible todistinguish between high angle (black) and small angle grain boundaries (grey) and to identifyorientation gradients within grains. While high-angle boundaries constitute strong obstacles fordislocation motion, the energy for dislocation motion across a low-angle boundary is far lower.Therefore this domain size influences the mechanical properties of the material significantly.

Figure 4: Inverse pole figure of high strength linepipe steel measured by EBSD.

2. Production results of grade X80

Since 1984, longitudinal welded X80 pipes used within several pipeline projects in Europe andNorth America (Table 1). In 1984, EUROPIPE produced grade X80 line pipe to be installed forthe first time in history in the Megal II pipeline. Manganese-Niobium-Titanium steel, additionallyalloyed with Copper and Nickel, was used for the production of the 13.6mm wall pipe.Subsequent optimisation of production parameters enabled the CSSR order to be executedusing a Manganese-Niobium-Titanium steel without the additions of Copper and Nickel. Thishas simultaneously led to an improvement (i.e. reduction) in the Carbon equivalent of the steelused. In 1992, 48” diameter pipe for a Ruhrgas pipeline project in Germany with 259 km lengthrequiring GRS 550 (X80) was produced [7]. More recently, 690 km of X80 pipes weremanufactured.

Year Project Main PipeGeometry

Pipeline Length

1984 Megal II 44“ x 13.6mm 3.2km

1985 CSSR 56“ x 15.4mm 1.5km

1991/92 Ruhrgas 48“ x 18.3mm

48“ x 19.4mm

259km

2001/03 CNRL 24“ x 25.4mm 12.7km

2003 Murray 20“ x 20.6mm 2.4km

2004 Stadtwerke Münster 56“ x 20.5mm 1.6km

2004/05 SnamReteGas 48“ x 16.1mm 10km

2001-

2007

National Grid (Transco) 48“ x 14.3mm

48“ x 15.9mm

48” x 20.6mm

48“ x 22.9mm

690km

Table 1: Projects executed with line pipes made of X80.

An example for alloying concepts for the base material is given in Table 2. Addition ofMolybdenum was not necessary in order to fulfil the required mechanical properties whilemaintaining a low Carbon equivalent. Use of two different alloying concepts makes it possible toreact to fluctuations in the price of alloying elements.

The mechanical properties are presented in the Figures 5 to 7. The pipe dimension was48”O.D. with a wall thickness between 14.3mm and 22.9mm. All results of the tensile tests onstrip specimens in transverse direction and impact tests in transverse direction were within thespecification for grade X80 and the properties were very similar independent of the wallthickness. The minimum yield strength was 556MPa and the mean yield strength was around590MPa. The highest standard deviation was 16MPa for the yield strength values and 16MPafor the tensile strength values. The yield-to-tensile ratio was between and 0.80 and 0.90. Theaverage value of impact energy was 297J for base material tested at 0°C.

0.430.18Cr, V, Nb, Ti<0.0015<0.0151.90.30.06

0.450.19Cu, Cr, Ni, Nb, Ti<0.0015<0.0151.90.30.06

IIWPCMOtherSPMnSiC

0.430.18Cr, V, Nb, Ti<0.0015<0.0151.90.30.06

0.450.19Cu, Cr, Ni, Nb, Ti<0.0015<0.0151.90.30.06

IIWPCMOtherSPMnSiC

Table 2: Chemical compositions in wt.% used in the production of X80

0

5

10

15

20

25

30

35

40

530 555 580 605 630 655 680 705 730 755 780

Stress [MPa]

Fre

qu

en

cy

[%]

14.3 mm (N=330)

15.9 mm (N=440)

20.6 mm (N=65)

22.9 mm (N=202)

YSYSmin=556 MPa

SMYS SMTS

TSTSmin=648 MPa

0

5

10

15

20

25

30

35

40

530 555 580 605 630 655 680 705 730 755 780

Stress [MPa]

Fre

qu

en

cy

[%]

14.3 mm (N=330)

15.9 mm (N=440)

20.6 mm (N=65)

22.9 mm (N=202)

YSYSmin=556 MPa

SMYS SMTS

TSTSmin=648 MPa

Figure 5: Distribution of the yield and tensile strength of X80 (48” OD, 14.3–22.9 mm w.t.)measured on strip specimens in transverse direction.

0

5

10

15

20

25

0 100 200 300 400 500

Av @ 0°C [J]

Fre

qu

en

cy

[%]

14.3 mm (N=330)

15.9 mm (N=440)

20.6 mm (N=65)

22.9 mm (N=202)

Figure 6: Distribution of the Charpy impact energy at 0°C of X80 (48” OD, 14.3–22.9 mm w.t.)in transverse direction.

The drop-weight-tear tests at 0°C showed that a mean shear of 98% could be maintainedindependent of the wall thickness. A minimum shear area of 85% was found at 14.3mm wallthickness.

80

82

84

86

88

90

92

94

96

98

100

10 12 14 16 18 20 22 24

Wall thickness [mm]

Sh

ear

are

a[%

]

Mean shear area

min-max range

80

82

84

86

88

90

92

94

96

98

100

10 12 14 16 18 20 22 24

Wall thickness [mm]

Sh

ear

are

a[%

]

Mean shear area

min-max range

Mean shear area

min-max range

Mean shear area

min-max range

Figure 7: Shear area of X80 measured in drop weight tear tests at 0°C in transverse direction(48” OD, 14.3–22.9 mm w.t.).

Table 3 contains the results obtained on commercially produced 36” diameter pipe with 32.0mmwall thickness in API grade X80. The Manganese-Niobium-Titanium steel used is additionallyalloyed with Molybdenum. The low Carbon equivalent ensures good field weldability. Theelongation values (A2”) are particularly high. The Charpy V-notch impact energy measured at-40°C is in excess of 200 J and the shear area of DWTT specimens tested at –20°C is greaterthan 85%. The forming and welding operations carried out on this high strength steel did notcause any problems.

0.420.19Mo, Nb, Ti<0.0015<0.0151.90.30.07

IIWPCMOtherSPMnSiC

0.420.19Mo, Nb, Ti<0.0015<0.0151.90.30.07

IIWPCMOtherSPMnSiC

4686674579Transverse

(round bar)

90852242312192224782685559Transverse

(flat bar)

S.A.

%

S.A.

%

Aver.

JJ%%MPaMPa

DWTT,-20°C

Charpy V-notch (1/1),

transverse, -40°CA2“Rt/RmRmRt0.5Specimen

orientat.

(and type)

4686674579Transverse

(round bar)

90852242312192224782685559Transverse

(flat bar)

S.A.

%

S.A.

%

Aver.

JJ%%MPaMPa

DWTT,-20°C

Charpy V-notch (1/1),

transverse, -40°CA2“Rt/RmRmRt0.5Specimen

orientat.

(and type)

Table 3: Alloying concept and mechanical properties of X80 for offshore application (36” OD,32 mm w.t.).

3. Component testing

The development of a new pipe grade facilitates the need to verify the correlation betweenlaboratory and full scale test as well as the validity of known concepts to predict componentbehaviour. This may also become necessary if other limits such as usage factors or pressureranges are exceeded. In these cases, the basic load condition of a pipe subjected to internalpressure is tested in terms of tensile tests and, finally, hydraulic burst tests to show componentbehaviour.

Figure 8: Transverse yield strength measured with different tensile test specimens (acc.[8])

As regards tensile tests, it is acknowledged that the specimen type has a major influence on theresults of tensile tests [8]. This is more apparent in transverse than in longitudinal direction asthe preparation of transverse full thickness specimens involves plastic deformation. Yieldstrength is generally more affected than tensile strength which remains more or less stable.Consequently, the Y/T ratio in a tensile test is also highly dependant on the specimen type.Because the Bauschinger effect is more pronounced with material grades above X70 (Figure8), the use of flattened rectangular specimens is not recommended if true strength anddeformation properties are required. The latest edition of ISO 3183 [9] recommends the use ofround bar specimens for grade X80 and prescribes them for grade X100. The effect of reversebending on the stress strain curve depends on the D/t ratio of the pipe, too. It is expected toincrease with decreasing D/T ratio.The integral yielding properties of the pipe can be tested by ring expansion tests. These testsare conducted up to a stress level well above the technical yield point and terminated withoutfailure of the specimens. The point of ultimate failure is always determined in tensile tests. Thiscombination of ring expansion and tensile test gives the closest measure of strength properties.As ring expansion tests are costly and time-intensive, especially for large diameter pipes, it hasbecome common practice to substitute them completely by tensile tests. Figure 9 shows a goodagreement between the yield strength determined with ring expansion and round bar tensiletests, whereas flattened strip specimens tend to underestimate the material strength. Again, thisfeature becomes more apparent in high strength steels. These findings confirm the necessity touse round bar specimens for these pipes.

Figure 9: Comparison of yield strength measured in tensile and ring expansion test (acc.[8])

The final step in the chain of verifying pipe properties is made by determining the componentproperties. Here, the hydraulic burst test plays a very important role as it characterises thecomplete pipe in terms of integral mechanical and geometrical properties that are of majorimportance to the end-user. Due to the multi-axial stress state induced by the longitudinal stressin the test vessel, the yield strength is higher than in a tensile test whereas the strain decreasescompared to the uni-axial tensile test (Figure 10). The strains at maximum load in tensile tests(uniform elongation) and in burst tests are parted by a factor of 2-3 over a large range of gradesup to X100. The ultimate strength remains unaffected by the test method. Therefore, burstpressure predictions based on tensile strength (e.g. Barlow formulae) are a robust method todesign pipelines subjected to internal pressure loads. Typically, the difference betweencalculated burst pressure on basis of Barlow and test results is less than 5%.

Figure 10: Stress-strain relationship measured in tensile and hydraulic burst test (acc. [8])

4. Relevance of HAZ requirements

High strength large diameter linepipes are manufactured in the most economic way bysubmerged arc welding (SAW) in two passes. This high performance welding process ischaracterised by a high heat input with cooling conditions that impacts on the toughnessproperties in the zone adjacent to the weld. The HAZ is characterised by a wide range ofdifferent microstructures, depending on the distance from the fusion line and the coolingconditions (Figure 11).

Figure 11: HAZ microstructural regions within two pass weld with typical Fusion Line CVNposition (acc. [10])

Although the alloying and welding techniques have been optimized to reduce the amount of anylow toughness areas, commonly referred to as local brittle zones (LBZ), they cannot becompletely avoided. The important question that has been subject of discussion within the lastyears is the significance of such LBZ to the structural integrity of welded components. In judgingthis question, not only the material properties but also the assumed flaw size and probability aswell as the applied load should be subject of investigation.Different welding processes are associated with characteristic probabilities of introducingdefects. Due to the high heat input of the submerged arc process, the number of possibledefects is small and their detectability is high if modern NDT techniques are used. The qualitystandard of EUROPIPE includes visual, automated UT and X-Ray inspection [11]. Theseinspection steps are performed before expansion. Hereafter the pipe is plastically deformed bythe expander. Although this process virtually constitutes only the last forming step, it fulfils alsoa function as quality control because the capability of every pipe to withstand stresses wellabove yield is demonstrated at the same time.When judging possible impacts of low toughness on structural integrity, engineering criticalityassessments (ECA) on basis of BS 7910 [12] are commonly conducted. The results of thesecalculations are known to be very conservative when compared to actual test results [13]. Asinput for such a calculation, an estimate of realistic loads and defects as well as knowledge oftoughness is required. In the past, the focus was clearly held on the toughness. In particular theestimation of a realistic defect size is of large importance when assessing the impact of lowtoughness. Modern NDT systems allow a safe detection especially of the more detrimentalplanar defects, a fact that should be considered in an ECA. A calculation based on BS 7910utilizing the computer program Crackwise [14] was conducted to demonstrate the correlationbetween toughness and defect size. The load was assumed to be 70% of SMYS of grade X80which is realistic for an internal pressure load in a pipeline. The critical flaw height for a surfaceflaw was determined varying the assumed toughness from CTOD = 0.01 mm to 0.1 mm. It wasshown that CTOD values as low as 0.02 mm could be accepted taking into account the NDTdetection level.

Figure 12: Critical flaw height as a function of fracture toughness in terms of CTOD

EUROPIPE commissioned SZMF to conduct a burst test on an API5L grade X100 pipe with anartificial defect to demonstrate the overall safety of a pipe containing LBZs and to compare thetest results with different prediction methods. The defect height was more than 15% of the wallthickness; the length was around 150 mm. CTOD values in the FL were below 0.05 mm.Regardless of the low toughness recorded in the heat affected zone, the pipe failed at apressure level of around 280 bar equalling a stress of u = 860 MPa.

0,0

0,4

0,8

1,2

1,6

0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0

a/t [ ]

spre

dic

tion/ s

act

ua

l

Test results available inpublic domain

X100 test result (assumedlow toughness)

X100 test result (assumedhigh toughness)

Conservative prediction

Figure 13: Ratio of calculated-to-actual failure stress based on a Kiefner model for X100pipe with actual (low) toughness and assumed (high) toughness as function of the wallthickness

Figure 13 shows the test result plotted in a graph with results available in public domain. Thefailure stress was calculated according to Kiefner [15] who developed a model for bothtoughness dependant and toughness independent failures. This model is known to give fairlyrealistic predictions. A calculation with the actual pipe properties considering the low toughnessvalues recorded in HAZ toughness and a second calculation based on the parent metaltoughness properties was conducted. The graph clearly shows that calculations based on lowtoughness totally underestimate the failure stress. On the other hand, a conservative predictionis maintained even if taking the high parent metal properties into account.

In contrast to these research results, it seems that the production testing was intensified innumber while conducting the tests in a more stringent manner with the aim to find LBZs. In thiscontext, it is important to realize that published research results on the significance of lowtoughness to the structural integrity support the thesis that due the localised nature of the brittlezones and the generally low constraint in the actual structure, the LBZs do not lead topremature fracture. These findings are not accounted for in current requirements. Solutions tosolve the problem have been discussed [16] but are not common practice up to now. An opendiscussion could help to emerge with meaningful test methods and requirements to reducecosts and to differentiate between structurally significant toughness and local effects notaffecting the overall safety.

5. Safety aspects

Incidents in the past showed that pipelines transporting compressible fluids are in risk ofsuffering propagating ductile fractures in the case of damages to the pipes. Intensive researchwork lead to a number of acknowledged semi-empirical concepts which are incorporated inlinepipe standards to serve as common design basis. Foremost, it is important to guarantee forductile fracture propagation whilst excluding brittle propagating fractures. Drop Weight Teartests (DWTT) have emerged as the most suitable laboratory test to substitute for full scale testson pipes to assess the fracture surface of fractured pipes, known as West Jefferson tests. Thetransition temperature in terms of the fracture surface evaluation of DWTT specimenscorresponds well with that of West Jefferson tests (Figure 14). In contrast to this, Charpy impacttests lead to significantly lower transition temperatures, thereby indicating a safe operating attemperatures at which a pipe will show brittle failure behaviour. It was demonstrated that ashear area fraction of 85% in DWT tests is required to ensure ductile fracture mode.

Figure 14: Transition temperature determined with full scale (West Jefferson) and laboratorytest specimens (acc. [17])

Depending on the pipe geometry and grade as well as the usage factor, intrinsic crack arrestemerges a function of the toughness of the pipe that is measured by Charpy impact tests.Series of full scale fracture propagation tests were conducted to serve as basis for the semi-

empirical approaches to predict crack arrest. The test data base includes pipe grades up to andincluding grade X80, pipe diameters up to 1420 mm, wall thickness up to 25 mm and pressureup to 80 bar. A number of tests on grade X100 pipes were conducted but crack arrest could notbe predicted safely. Therefore, it is currently recommended to use crack arrestors or to lowerthe operating pressure in X100 lines.

6. Current status of Grade X100

In order to cope with the market requirements for enhanced pipe strength, EUROPIPE put itseffort to the development of grade X100. Since 1995, EUROPIPE has developed differentapproaches to produce high strength materials. An overview of EUROPIPE activities is given inTable 4. No technological breakthroughs in TM rolling and accelerated cooling were necessary.Only optimization of the existing technology was required for the production of grade X100plate. As a result, the production window became narrower.

As shown in Figure 15, three different approaches are generally possible for selecting thechemical composition and plate rolling conditions [18].

A

B

C

Steelchemistry

Coolingparameters

C-content CEIIW

Cooling rate(AcC)

Cooling stoptemperature(AcC)

0.08%

0.06%

0.05% 0.43

0.480.49

high

highlo

w

low

A

B

C

Steelchemistry

Coolingparameters

C-content CEIIW

Cooling rate(AcC)

Cooling stoptemperature(AcC)

0.08%

0.06%

0.05% 0.43

0.480.49

high

highlo

w

low

Figure 15:Relationship between alloying concepts and strategies for accelerated cooling for theproduction of grade X100 heavy plate.

The medium Carbon content of about 0.06% ensures excellent toughness as well as fullysatisfactory field weldability, despite the relatively high Carbon equivalent of about 0.46. Thechemical composition should therefore be considered acceptable for the purpose of currentstandardisation. EUROPIPE has already produced several trials of grade X100 pipe adoptingapproach C. These trials have covered the wall thickness range between 12.7 and 25.4mm, asshown in Table 4. It was demonstrated that the same steel composition could be used and onlyslight changes in the rolling conditions would be necessary.

In the case of X100, the yield stress at 0.2% plastic deformation RP0.2 is used as the yieldstrength criterion instead of Rt0.5. At lower strength levels, both values are relatively similar, asillustrated in Figure 16. In the case of X100, however, the plastic deformation after unloading islimited after testing up to a total strain of 0.5%. For this reason, RP0.2 constitutes a better yieldstrength criterion.

Year Action1994 First request of some clients to supply pipe with grades ≥ X1001995 Lab trials (higher CE, lower acc. Cooling)

First production of pipes 36” x 19.1 mm1997 Lab trials (lower CE, higher acc. Cooling

Second production of pipe 30” x 15.1 mm1998 Pipe production for the first X100 full scale burst test, ECSC-project

56” x 19.1 mm (third production)1999 New lab trials (medium CE, medium acc. Cooling), ECSC-project

4th production of pipes for second full scale burst test 36” x 16 mm2001 Production of pipe 36” x 12.5 to 25.4 mm, Demopipe project

(5th production)2003/2004 Production of pipe 48” x 16.4 to 18.6 mm for TAP order

(6th production)2006 Production of pipe 48” x 19.8 mm for X100 trial pipeline

(7th production)2007 Production of 36” x 13.0 mm for X100 trial pipeline

(8th production)

Table 4: EUROPIPE’s development actions for Grade X100 large diameter pipes.

0,0 0,5 1,0 1,5 2,0

Strain [%]

Str

es

s[M

Pa

]

X80

X100

0,2

Rp0.2

Rt0.5

Figure 16:Schematic comparison of the stress-strain curves of X80 and X100.

7. Conclusions

During the past 25 years, EUROPIPE has carried out extensive R&D work in order to producehigh strength X80 material for large diameter pipes and thus supported our customers inreducing weight and costs for the pipeline construction. These EUROPIPE efforts have beenhonoured by several customers. Thus EUROPIPE has not only delivered the first small pipelinepart worldwide in 1985, but supplied in 1992/93 also the first big X80 pipeline. From this firstRuhrgas pipeline there exists in the meantime already 15 years of positive experience.

EUROPIPE sold the largest quantities of X80 pipes worldwide, over 500,000t or close to 1,000pipe km. Thus there is extensive X80 experience in the following areas under heaviestoperational conditions which have been confirmed repeatedly from customer’s side: steel andplate technology, welding technique for longitudinal weld seam, testing experience, corrosionresistance tests, coating, welding technique for girth weld seam incl. recommendations formanual and automatic girth welding, pipeline safety, full scale burst tests, field bending andmanufacturing of induction bends.

Attention is drawn to the issue of HAZ testing of longitudinal welds. While measures have beentaken to optimise the alloying concepts ot the steel to minimise the occurrence of localisedbrittle zones in the heat affected zone, welding equipment has been continuously modernised toallow for an optimal and stable process to facilitate good HAZ properties and, finally, best NDTmethods are applied to detect reliably any defects in the weld, testing requirements havebecome more and more stringent. It is questionable if this provides a good measure to enhancesafety. Joint efforts should be made to find appropriate methods to differentiate between HAZproperties that endanger structural safety and such that are not relevant to the component.

This work was extended to Grade X100, and especially the questions concerning pipeline safetyare under discussion.

References

[1] W. M. Hof, M. K. Graef, H.-G. Hillenbrand, B. Hoh and P. A. Peters: “New high-strengthlarge-diameter pipe steels”; Journal of Materials Engineering 9 (1987), 191 – 198

[2] M. K. Graef, H.-G. Hillenbrand and K. A. Niederhoff: “Production and girth welding of doublesubmerged-arc welded grade X80 large-diameter linepipes”; 8th EPRG PRCI Meeting, Paris,Mai 1991

[3] H.-G. Hillenbrand and P. Schwaab: “Quantitative determination of the microstructure ofHSLA steels for correlation with their mechanical properties”; Materials Science andEngineering 94 (1987), 71-78

[4] H. Engelmann, A. Engel, P. A. Peters, C. Düren and H. Müsch: “First use of large-diameterpipes of the steel GRS 550 TM (X80)”; 3R International 25 (1986), No. 4, 182 – 193

[5] P.A. Peters and H.G. Hillenbrand, “Experience in Supply of Arctic Grade Line Pipe for SovietConstruction Projects”, World Materials Congress 1988, Chicago, USA, 1988

[6] Zajac, S., Schwinn, V., Tacke, K.-H. Characterisation and Quantification of Complex BainiticMicrostructures in High and Ultra-High Strength Linepipe Steels. Proc. Int. Conf. onMicroalloying for New Steel Processes and Applications, Donostia-San Sebastian, Spain (2005)

[7] M. K. Gräf and H.-G. Hillenbrand: “High Quality Pipe – a Prerequisite for Project CostReduction”, 11th PRCI-EPRG Joint Technical Meeting, Arlington, Virginia, April 1997

[8] Knauf, G.; Hohl, G.; Knoop, F. M.: The effect of specimen type on tensile test results andits implications for linepipe testing. In: 3R International 10-11, (2001), S. 655-661

[9] ISO 3183: 2006: Steel pipe for transportation systems. 2006

[10] Niederhoff, K and Gräf, M.K., 1990, “Toughness Behaviour of the Heat-AffectedZone (HAZ) in Double Submerged-arc Welded Large Diameter Pipe”, Proc. PipelineTechnology Conference, Oostende, Belgium, Vol. B, pp. 13.1-13.9

[11] Liessem, A., Grimpe, F. and Oesterlein, L.: “State-of-the-Art Quality Controlduring the Production of SAW Linepipes”, Proc. 4th Int. Pipeline Conference, Calgary,Alberta, ASME, paper IPC2002-27141, 2002

[12] BS7910:1999, Guide on methods for assessing the acceptability of flaws inmetallic structures, British Standards Institute BSI 10-2000

[13] Fu, B., Guttormsen, S., Vu, D.Q., Chauhan, V. and Nokleebye, A. Significanceof low toughness in the seam weld HAZ of a 42-inch Diameter Grade X70 DSAW Linepipe – Experimental studies, 13th Biennial PRCI/EPRG Joint Technical Meeting, NewOrleans, 2001

[14] Crackwise 3, BS7910:1999 fracture/fatigue assessment procedures, version3.14

[15] Kiefner et al.: Failure stress levels of flaws in pressurised cylinders. In: Progress in FlawGrowth and Fracture Toughness Testing. ASTM STP 536, pp. 461-481, 1972

[16] Erdelen-Peppler, M., Liessem, A.: A critical view on the significance of HAZ testing.Proceedings of IPC 2004, International Pipeline Conference, October 4-8 2004, Calgary,Alberta, Canada

[17] Pistone, V. et al: Transition Temperature Determination for Thick Wall Line Pipes. In: 3RInternational, 2000, pp. 199-204

[18] H.G. Hillenbrand et al., “Development of Large-Diameter Pipe in Grade X100”, PipelineTechnology Conference, Brugge, Belgium 2000