TP45

High Strength Steel Pipes:New Developments and Applications

Dr. Ing. Christoph Kalwa EUROPIPE GmbHDr. Ing. Hans-Georg Hillenbrand EUROPIPE GmbHDr. Ing. Michael Gräf EUROPIPE GmbH

Onshore Pipeline ConferenceJune 10-11, 2002, Houston, Texas, USA

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High Strength Steel Pipes: New Developments and ApplicationsDr.-Ing. Christoph Kalwa,

Dr.-Ing. Hans-Georg Hillenbrand, Dr.-Ing. Michael Gräf

EUROPIPE GmbHFormerstr. 49

40878 RatingenGermany

Onshore Pipeline Conference, Houston,June 10th/11th 2002

Introduction

EUROPIPE’s efforts in the development of high-strength steels make a significantcontribution to pipeline project cost reduction in view of the ever-increasing pipeline lengthand operating pressure. On the one hand cost reduction subjects by use of high strengthsteels are caused by wall thickness reduction having a significant impact on quantity ofmaterial, transportation and welding costs. On the other hand high strength steels makepossible to operate pipelines with higher pressures resulting in larger quantities oftransported gas. The development focuses on the strength properties, achieving higher andhigher levels, without reducing toughness properties of the steel. Another important item ofsteel development is the ability of the steels to be processed by welding especially under fieldwelding conditions.

The presentation gives an overview over the development of high strength low alloyed steelline pipes. Some pipeline projects being performed in X 80 and the benefits of applicatingX 80 are shown. Also aspects about properties of base material and welds are presented.The development to higher grades to X 100 gives an intention of future challenges andpossibilities.

Development of High Grade Steels

Over the past 25 years, severe demands have been placed on the pipe manufacturer withrespect to the development and processing of materials to linepipe. Generally, longitudinallywelded large-diameter linepipe is used for the transportation of oil and gas, because it offersthe highest safety in pipeline operation and represents the most economic solution. From thepoint of view of pipeline economy, the pipe must favourably respond to laying in the field andpermit high operating pressures for the pipeline. These requirements imply that the pipelinesteel has to possess high strength and toughness and good weldability and that the pipeshall have optimised geometry.

The development of high strength steels is shown in figure 1. In the seventies, the hot rollingand normalising was replaced by thermomechanical rolling. The latter process enablesmaterials up to X70 to be produced from steels that are microalloyed with niobium andvanadium and have a reduced carbon content. An improved processing method, consistingof thermomechanical rolling plus subsequent accelerated cooling, emerged in the eighties.By this method, it has become possible to produce higher strength materials like X80, havinga further reduced carbon content and thereby excellent field weldability. Additions ofmolybdenum, copper and nickel enable the strength level to be raised to that of grade X100,when the steel is processed to plate by thermomechanical rolling plus modified acceleratedcooling.

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Figure 1: History in Line Pipe Steels (Large Diameter Pipe)

Figure 2 shows typical microstructures of three types of linepipe steel. Banded ferrite andpearlite and coarse ferrite grain size (ASTM 7–8) are the characteristic features ofconventionally rolled and normalised X60 steels. The microstructure of TM rolled X70 steelsis more uniform and the ferrite grains are finer (ASTM 10–11). The most uniform andextremely fine microstructure is attained by accelerated cooling that followsthermomechanical rolling, as shown for the X80 steel. The improved properties of this steelcan be attributed to its ferritic-bainitic microstructure.

Figure 2: Typical microstructures of normalised, TM-treated andaccelerated cooling steels

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X 80 Projects

In the past two decades, EUROPIPE has carried out extensive work to develop high-strengthsteels in grades X80 and X100 to assist customers in their endeavour to reduce weight andpipelaying costs.

Since 1984 longitudinal welded X 80 pipes were applied to several pipeline projects inEurope and North America (Figure 3). In 1984, EUROPIPE produced grade X80 line pipe tobe installed for the first time in history in the Megal II pipeline. A manganese-niobium-titaniumsteel, additionally alloyed with copper and nickel, was used for the production of the 13.6 mmwall pipe. Subsequent optimisation of production parameters enabled the CSSR order to beexecuted using a manganese-niobium-titanium steel without the additions of copper andnickel. This has simultaneously led to an improvement (i.e. reduction) in the carbonequivalent of the steel used.

Pipe geometryOrderYear

1984

1985

1991/92

2001/02

MEGAL II

CSSR

Ruhrgas

Transco

44“ x 13.6mm

56“ x 15.5mm

48“ x 18.3mm

48“ x 15.1mm

Pipeline length

3.2km

1.5km

250km

42km +

2001 CNRL 24“ x 25.4mm 5km

Figure 3: Projects executed with line pipes made of X 80

The first pipeline using GRS 550 (X80) in its entire length was the Ruhrgas Werne-to-Schlüchtern pipeline project in Germany with 250 km length in 1992. EUROPIPE supplied allpipe of 48” diameter with up to 19.3 mm wall thickness including induction bends. Since thestrength decreases as the wall thickness increases, it was necessary to raise the carbon andmanganese levels marginally. The concentrations of all other elements remained unchanged.

The measured tensile and the impact energy values conformed fully to the specificationrequirements in all cases. The standard deviation for the yield and tensile strength values isvery low. The impact energies measured on Charpy V-notch impact specimens at 0°C is veryhigh, resulting in an average value of about 180 J. The 85% shear transition temperaturesdetermined in the drop weight tear (DWT) tests are far below 0°C.

In 2001 and 2002 further X 80 pipes were manufactured for a project in UK. Several parts ofgas pipeline network were ordered to a total of 42km in the past. Further quantities beingabout 70km are also booked for this and next year.

A challenging project of 2001 was a hot steam system for CNRL in Canada. The longitudinalwelded line pipe was qualified for operation temperatures up to 354°C. The high temperatureproperties with respect to creep were tested, and the results showed sufficient creepresistance.

Figure 4 gives an idea of the Ruhrgas pipeline project (1). After welding, non destructiveexamination and field joint coating, the girth welded segments were lowered onto the

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prepared trench bottom. By using GRS 550 TM with a thinner wall instead of StE 480.7 TMabout 20000 t of pipe material could be saved (145000 t instead of 165000 t), which had astrong impact on material and pipe laying costs concerning transportation and welding.

Figure 4: First large scale use of GRS 550 TM (X80) Pipe LineMaterial Ruhrgas Werne-to-Schlüchtern Line

Figure 5 illustrates that the use of X 80 led to material reduction of about 20 000 t comparedwith X 70 pipes by reducing wall thickness from 20.8mm for X 70 to 18.3mm for X 80. Thisresults in reduction of pipe laying costs with respect to transportation and with a biggercontribution welding caused by reduced welding time for thinner walls.

X 80 properties

One of the latest projects using X80 was a UK pipeline for Transco in 2001. The results onEUROPIPE’s production tests, performed in the context of certification of the pipe, are shownin figures 5 and 6. All values of the tensile and impact tests conform to the requirements ofX80. The standard deviations of tensile testing are 15 MPa for yield strength and 13 MPa fortensile strength. Average values of impact testing were 227 J for base metal and 134 J forweld metal.

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Base Material - Tensile Test

YieldStrength

Elongation / %

560 600 650 700 750

35302520151050

4035302520151050

Yield/Tensile Strength / MPa

TensileStrength

0.76 0.80 0.92 28 30 32 34 36

Frequency / %

Frequency / %

25

20

15

10

5

0

Rx / Rm0.84 0.88

Frequency / %

2181 pipes

Figure 6: Strength properties of X80 Pipes(48” O.D. x 15.1 mm W.T.

Frequency / %50

45

40

35

30

25

20

15

10

5

0

Impact Energy / J

50

45

40

35

30

25

20

15

10

5

0

Frequency / %

2000Impact Energy / J

Base MaterialTest Temperature 0°C

Weld MaterialTest Temperature 0°C

100 300 2000 100 300

Figure 7: Impact properties of X80 Pipes(48” O.D. x 15.1 mm W.T.)

One of the most important processes for pipe laying is field welding. From case to casemanual or automatic welding processes are used. Welding parameter and consumables formanual X80 girth welds are shown in Figure 7. For root pass and hot pass cellulosicelectrodes were used. Filler passes and cap passes were welded with basic electrodes.

Typical impact energy values for different welding techniques and processes were comparedduring development of X80 HSLA steel. As can be seen in figure 8 the lowest toughnessvalues were achieved by welding filler passes with cellulosic electrodes. Gas-Metal-Arc-Welding techniques result in sufficient toughness values and transition temperatures as theyare at down hill welding with basic electrodes.

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Figure 8: Welding parameter for manual X80 welding

Figure 9: Typical toughness of weld metals suitable for manualgirth welding of GRS 550 TM (X80) line pipe

Beside manual welding automatic welding processes become more and more important asan economic process by reducing welding time due to narrow gaps. These can reduce thenumber of layers and passes. One example of a very efficient automatic welding process isCRC welding that was used for the Werne–to–Schlüchtern pipeline as well as for the latestTransco project. Figure 10 gives an overview over the weld structure and weldingparameters. Figure 11 shows that material properties of the girth weld met the requirementssufficiently.

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PassConsumables Current

[A]Voltage

[V]RootPassHotPassFillerPasses

CapPass

ThyssenK NovaThyssenNiMo 80

0.9

0.9

0.9

0.9

Trade namedia.

[mm]

190/220

240/260

200-230

19/21

24/26

22/25

20/22

ThyssenNiMo 80

ThyssenNiMo 80

Shielding Gas

ThyssenK Nova Ar/CO2 75/25

Ar/CO2 75/25

CO2

CO2

210/250

Weldingspeed

[cm/min]

Oscil-lation

75

127

36/45

26/41

n

n

y

y

Figure 10: Welding parameter for X80 CRC welding

CVN Toughness

300

270

240

210

180

150

120

90

60

30

0

800

700

600

500

400

300

200

100

0

Strength [MPa]

Strength Properties(Flat Specimen crossweld )

Fracture Toughness

WM-30°C

WM0°C

HAZ0°C

550

690

YS TS

Figure 11: Test results for X80 CRC welding

To show manufacturability of heavy wall X80 EUROPIPE commercially produced 36”diameter pipe with 32.0 mm wall thickness of which the mechanical properties are fixed inFigure 7. The manganese-niobium-titanium steel used here has a sufficiently high ratio oftitanium to nitrogen and is additionally alloyed with molybdenum. The low carbon equivalentensures good field weldability. The elongation values (A2”) are particularly high. The CharpyV-notch impact energy measured at -40°C is in excess of 200 J and the shear area of DWTTspecimens tested at –20°C is greater than 85%. The forming and welding operations carriedout on this high strength steel did not cause any problems.

Development of X100

To cope with market requirements for enhancing strength EUROPIPE put its effort to thedevelopment of grade X100. No technological breakthroughs, such as TM rolling andaccelerated cooling which increased the strength and toughness respectively, but onlyimprovements in the existing technology were involved in the production of grade X100 plate.As a result, the production window is quite narrow. Heat treatment of plate or pipe isobviously not advisable.

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As can be seen in figure 12, EUROPIPE took three different approaches with respect to theselection of chemical composition and cooling conditions (2).

Approach A, which involves a relatively high carbon content, has the disadvantage that thecrack arrest toughness requirements to prevent long-running cracks, may not be fulfilled.Moreover, this approach is also detrimental, e.g. to field weldability.

Approach B, which is practically the same as direct quenching with fast cooling rates down toa very low cooling-stop temperature, results in the formation of uncontrolled fractions ofmartensite in the microstructure, which have a detrimental effect on toughness properties ofbase metal and leads additionally to the softening in the heat affected zone. This effectcannot be adequately compensated for extremely low carbon contents, without adverselyaffecting productivity. Moreover, it is very difficult to produce pipe with adequate uniformity ofstrength properties. This difficulty cannot be attributed solely to the Bauschinger effectassociated with the variation in local deformation occurring during the heavy straighteningoperation needed in the case of thin section plate, which distorts heavily during directquenching.

Experience gained meanwhile indicates that Approach C is the best choice. This approachenables the desired property profile to be achieved through an optimised two-stage rollingprocess in conjunction with a reduced carbon content, a relatively high carbon equivalent andoptimised cooling conditions. The special potential of the existing rolling and cooling facilitiescontributes significantly to the success of this approach.

Approach C, which involves a low carbon content, ensures excellent toughness as well asfully satisfactory field weldability, despite the relatively high carbon equivalent of the chemicalcomposition. The chemical composition should therefore be considered acceptable for thepurpose of current standardisation.

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

highlow

low

Figure 12: Modification of steel chemistry and cooling parametersto achieve the strength level of API X100

Large diameter pipes of the three approaches were produced and EUROPIPE decided tofollow approach C regarding strength, toughness and weldability properties The mechanicalproperties determined on the pipes (Figure 13) are quite appreciable. The currentrequirements particularly for yield to tensile ratio and elongation, which are applicable to

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material grades up to X80, are too severe to be fulfilled for grade X100 on a statistical basis.This difficulty is attributable to the basic finding that as the strength increases, the yield totensile ratio increases and the elongation decreases. The X100 material produced respondsfavourably to manual and mechanised field welding, a finding which can be attributed to itsreduced carbon content (3).

For reasons of technical feasibility and cost-effective production, it is necessary in the contextof grade X100 to reassess and redefine some of the requirements for the mechanicalproperties, considering the anticipated service conditions for the pipe.

Heat pipe sizeOD X WT

C Mn Si Mo Ni Cu Nb Ti N CEIIW PCM

III

Heat

III

yield strengthRt0.5 *

tensilestrength

Rm *

yield totensile ratioRt0.5 / Rm *

ElongationA5 *

CVN(20°C)

DWTT-transition

temperature

737 MPa 800 MPa 0.92 18 % 200 J - 20 °C

56" x 19.1 mm 0.07 1.90 0.30 0.17 0.33 0.20 0.05 0.0180.005 0.46 0.20

IV

V

36" x 16.0 mm 0.06 1.90 0.35 0.28 0.25 - 0.05 0.0180.004 0.46 0.19

36"x12.7 - 25 mm 0.06 1.93 0.32 0.30 0.24 - 0.05 0.0180.005 0.46 0.19

IV 752 MPa 816 MPa 0.92 18 % 270 J ∼ - 50 °C

V 767-799 MPa 796-836 MPa ~0.94 15-18 % 200-270 J

~-60 - -10°C**

* transverse tensile tests by roundbar specimens** -60°C for WT 12.7mm -10°Cfor WT 25mm

Figure 13: Mechanical properties of X100 (Pipe body)

EUROPIPE produced some pipes with the intention for use as test pipes to determine thebehaviour of grade X100 material in full-scale burst tests (figure 14), which are conducted aspart of several ECSC-funded research projects (4).

Figure 14: Full scale burst tests on X100 line pipe

In the mean time further pipes were produced to be tested in a further ECSC Demopipeproject to demonstrate full scale usage.

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Conclusion

The predicted growth in energy consumption in the coming decades necessitates severeefforts for transporting large amounts of natural gas to the end user. Large-diameter pipelinesserve as the best and safest means of transport. This paper gives an overview of the currentrequirements of high strength steels and the associated developments. The technicalpossibilities were described. Also for the future additional substantial improvements can berealised.

Several pipeline projects in the past two decades in Europe and North America show that theuse of X 80 line pipe causes no problems with respect to mechanical properties and welding.X 100 development came to the result that mechanical properties can be achieved and crackarrest properties for certain dimensions were proven in full scale burst tests.

The enormous pressure on the price of natural gas forces the pipeline operator to explore allthe possibilities to reduce the cost of a pipeline project in the future. The pipe manufacturercan assist him in his endeavour by supplying high quality pipe. The effect of pipe quality onproject costs will be more substantial when the pipeline is constructed to the limit statedesign. For an offshore pipeline in the North Sea, the project team could reduce the projectcosts especially for transportation in several stages by optimising the pipe diameter, that ledto a cost reduction of 5%, increasing the material grade, that led to 4%, and using of limitstate design, that led to 6% reduction.

Finally, the pipe manufacturer makes his contribution to reducing operational costs of apipeline over its life by investigating the fatigue, corrosion and ageing behaviour of pipe andpipe materials. These properties have a significant bearing on the integrity of a pipeline andconsequently the operating costs. These properties are currently being extensively studied.The knowledge gained these studies can be made available to assist the pipeline operatorwhen planning a new pipeline project or when estimating the residual life of an ageingpipeline.

For reasons of technical feasibility and cost-effective production, it is necessary in the futureto reassess and redefine some of the materials requirements for pipe, considering theanticipated service conditions for the pipe. EUROPIPE likes to initiate close co-operationbetween the pipeline designer, operator, laying contractor and the pipe manufacturer in thisrespect that would be conducive to finding good solutions in modern pipeline construction.

Acknowledgements

Thanks to Thyssen Boehler Schweisstechnik and Transco for supporting us in the field ofCRC welding.

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References

1. H.G. Hillenbrand et al., “Procedures, Considerations for Welding X80 LinepipeEstablished”, Oil & Gas Journal, 37 (1995)

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

3. L. Barsanti, G. Pozzoli and H.G. Hillenbrand, “Production and Field Weldability Evaluationof X100 Line Pipe”, 13th Joint Meeting PRCI-EPRG, New Orleans, USA 2001

4. G. Demofonti et al., “Large Diameter API X100 Gas Pipelines: ‘Fracture PropagationEvaluation by Full Scale Burst Test’”, Pipeline Technology Conference, Brugge, Belgium2000