TP48

Production and Service Behaviour ofHigh Strength Large Diameter Pipe

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

International Conference on Application and Evaluation ofHigh Grade Linepipes in Hostile EnvironmentsNovember 8-9, 2002, Yokohama, Japan

1

Production and Service Behaviour ofHigh Strength Large Diameter Pipe

Dr.-Ing. Hans-Georg Hillenbrand, Dr.-Ing. Christoph Kalwa

EUROPIPE GmbHFormerstr. 49

40878 RatingenGermany

Abstract

The development of high strength large diameter pipe makes a significantcontribution to pipeline project cost reduction. X80 grades are already available foronshore and offshore use. Higher grades like X100 have been developed, but incombination with increased usage factor and operating pressure very high toughnessis necessary for crack arrest.

This paper describes the concepts to produce X100 pipes and discusses advantagesand limits of this high grade.

Keywords

X80, X100, project cost, pipe production, chemical composition, service behaviour

Introduction

In view of the ever-increasing pipeline length and operating pressure, thedevelopment of high-strength steels makes a significant contribution to pipelineproject cost reduction.

Project cost reduction is the result of the sum of many benefits when using high-strength steels /1/, even when the price per ton increases with higher grades:Ø lower amount of steel requiredØ lower transportation costsØ lower laying costs

As one example the materials savings of the first Ruhrgas X 80 project are shown infigure 1.

The use of X 80 led to material reduction of about 20 000 t compared with X 70 pipesby reducing wall thickness from 20.8mm for X 70 to 18.3mm for X 80. This resultsalso in reduction of pipe laying costs with respect to transportation and with a biggercontribution to welding caused by reduced welding time for thinner walls.

2

Therefore gas companies try to look to further cost reduction. Preliminary evaluationhighlighted, that X100 steel could give investment costs savings of about 7% withrespect to X80 grades. Other studies claim cost savings of up to 30% when X70 andX100 is compared /2/.

The production of high strength large-diameter pipes will be presented and in somecases the service behaviour will be discussed.

Historical Background

Over the past 25 years, severe demands have been placed on the pipe manufacturerwith respect to the development and processing of materials to line pipe. Generally,longitudinally welded large-diameter line pipe is used for the transportation of oil andgas, because it offers the highest safety in pipeline operation and represents themost economic solution. From the point of view of pipeline economy, the pipe mustfavourably respond to laying in the field and permit high operating pressures for thepipeline. These requirements imply that the pipeline steel has to possess highstrength and toughness and that the pipe shall have optimised geometry.

The development of high strength steels is shown in figure 2. In the seventies, thehot rolling and normalising was replaced by thermomechanical rolling. The latterprocess enables materials up to X70 to be produced from steels that aremicroalloyed with niobium and vanadium and have a reduced carbon content. Animproved processing method, consisting of thermomechanical rolling plussubsequent accelerated cooling, emerged in the eighties. By this method, it hasbecome possible to produce higher strength materials like X80, having a furtherreduced carbon content and thereby excellent field weldability. Additions ofmolybdenum, copper and nickel enable the strength level to be raised to that ofgrade X100, when the steel is processed to plate by thermomechanical rolling plusmodified accelerated cooling /3/.

Figure 3 shows typical microstructures of three types of line pipe steel. Banded ferriteand pearlite and coarse ferrite grain size (ASTM 7–8) are the characteristic featuresof conventionally rolled and normalised X60 steels. The microstructure of TM rolledX70 steels is more uniform and the ferrite grains are finer (ASTM 10–11). The mostuniform and extremely fine microstructure is attained by accelerated cooling thatfollows thermomechanical rolling, as shown for the X80 steel. The ferritic-bainiticmicrostructure attributes to the improved properties of this steel.

High Strength Line Pipes

Over the past two decades, extensive work has been carried out to develop high-strength steels in grades X80 and X100 to assist customers in their endeavour toreduce weight and pipe laying costs /3//4/.

3

X 80 for onshore use

The developments in alloy design since 1984 can be seen in the results on pipeproduced commercially for the Megal II, CSSR and Ruhrgas projects (Figure 4) /5/.

A manganese-niobium-titanium steel, additionally alloyed with copper and nickel, wasused in the production of the 13.6 mm wall pipe for the Megal II order in 1984.Subsequent optimisation of production parameters enabled the CSSR order to beexecuted using a manganese-niobium-titanium steel without the additions of copperand nickel. This has simultaneously led to an improvement (i.e. reduction) in thecarbon equivalent of the steel used.

In 1992, 48” diameter pipe for a Ruhrgas pipeline project in Germany with 250 kmlength requiring GRS 550 (X80) was produced. Also pipes of 48” diameter with19.3 mm wall thickness were included. Since the strength decreases as the wallthickness increases, it was necessary to raise the carbon and manganese levelsmarginally. The concentrations of all other elements remained unchanged.

The table shows also the mean values of the mechanical properties measured on thepipes produced for the three orders. The measured tensile and the impact energyvalues conformed fully to the specification requirements in all cases. The standarddeviation for the yield and tensile strength values is very low. The impact energiesmeasured on Charpy V-notch impact specimens at -20°C is very high, resulting in anaverage value of about 180 J. The 85% shear transition temperatures determined inthe drop weight tear (DWT) tests are far below –20°C.

Figure 5 shows the pipe laying operation of the Ruhrgas pipeline project. Afterwelding, non destructive examination and coating, the girth welded segments werelowered onto the prepared trench bottom. By using GRS 550 TM with a thinner wallinstead of StE 480.7 TM about 20000 t of pipe material could be saved, which had astrong impact on pipe laying costs concerning transportation and welding. For rootpass and hot pass cellulosic electrodes were used. Filler passes and cap passeswere welded with basic electrodes.

One of the latest projects using X80 (L555MB) was a UK pipeline for BG Transcodelivered in 2001 and 2002. The results of the tensile and impact tests, performed inthe context of certification of the pipe, are shown in the figures 6 and 7. All valuesconform to the requirements of X80. The standard deviations are 15 MPa for yieldstrength, 13 MPa for tensile strength, 0.02 for yield to tensile ratio and 1.8% forelongation. Standard deviations of impact testing was 58 J fo r base metal and 29 Jfor weld metal.

Beside manual welding automatic welding processes become more and moreimportant as an economic process by reducing welding time due to narrow gaps.These can reduce the number of layers and passes. One example of a very efficientautomatic welding process is CRC welding that was used for the Werne–to–Schlüchtern pipeline as well as for the latest Transco project. Figure 8 gives anoverview of the typical weld structure and typical welding parameters. The materialproperties of the girth weld met the requirements sufficiently.

4

X80 for offshore use

X 80 material was mainly developed for onshore use. But there were already severaltrials to test the use of X80 offshore. Figure 9 contains the results obtained oncommercially produced 36” diameter pipe with 32.0 mm wall thickness in API gradeX80. The manganese-niobium-titanium steel used here has a sufficiently high ratio oftitanium to nitrogen and is additionally alloyed with molybdenum. The low carbonequivalent ensures good field weldability. The elongation values (A2”) are particularlyhigh. The Charpy V-notch impact energy measured at - 40°C is in excess of 200 Jand the shear area of DWTT specimens tested at –20°C is greater than 85%. Theforming and welding operations carried out on this high strength steel did not causeany problems.

The effect of boron on high strength line pipe steels =X80

It is of paramount importance to the plate producer, and ultimately the customer, thatthe required properties are attained with the minimum of alloying additions, in order tocontrol the pipe production costs, and thereby increase the viability of the use of highstrength steel pipelines for long distance/high pressure transportation of gas. Thematerials used are obtained by means of a suitable combination of chemicalcomposition and thermomechanical treatment parameters in order to have a correctbalance between strength, toughness and weldability. Besides niobium, titanium andvanadium the micro-alloying element boron should also be effective. Therefore, aseries of laboratory plate rolling trials were performed based on the well knownchemical composition for grade X80 material starting up with a low CEIIW of about0.38 %. Besides the cooling rate (ca. 15 and 25 °C/s) all rolling and coolingconditions were kept constant.

Figure 10 illustrates the influence of boron on the yield strength for increasing carbonequivalents in comparison to boron-free heats of the same chemical composition. Inthe case of the boron alloyed steels the carbon equivalent is mainly increased byincreasing the carbon content itself (up to 0.06 %). As can be seen from the figuregrade X100 plate with 20 mm wall thickness was achieved with a CEIIW of about 0.40to 0.41%, which is very low. The increase of the yield strength by adding boron isabout 70 to 100 MPa. In all cases the base material was characterised mainly by abainitic microstructure. The Charpy-V-notch energy at –40 °C was in excess of 200 J.Only boron-microalloyed heats alloyed with 0.06 % C dropped down to Charpyvalues between 100 and 170 J at –40 °C.

Figure 11 shows the influence of boron and the effect of the cooling rate on thestrength properties. Above 15 °C/s the influence is relatively small. It is evident fromthis figure that the addition of boron led to an increase of the steel grade from X80 toX90 in the wall thickness range up to 32 mm. The higher carbon content of 0.06 %led to grade X120 but with inadequate toughness.

There is a strong influence of the micro-alloying elements on the toughness of thecoarse-grain HAZ of the longitudinal seam weld of large-diameter pipes. Incombination with boron the permitted max. carbon content has to be restricted toabout 0.04 %. Therefore, boron may be used in combination with low carboncontents and low carbon equivalents. But this result has be experienced by pipeproduction.

5

Development to X100

To cope with market requirements for enhancing strength Europipe put its effort tothe development of grade X100. No technological breakthroughs, such as TM rollingand accelerated cooling which increased the strength and toughness respectively,but only improvements in the existing technology were involved in the production ofgrade X100 plate. As a result, the production window is quite narrow. Heat treatmentof plate or pipe is obviously not advisable.

Since 1995, Europipe developed different approaches with respect to the selection ofchemical composition /1/. As can be seen in figure 12, three different approaches aregenerally possible with respect to the selection of chemical composition and coolingconditions. More details on chemistry and rolling conditions will be given by DillingerHuette during this conference /6/.

Approach A, which involves relatively high carbon equivalents of 0.49, has thedisadvantage that the crack arrest toughness properties are not as high andtherefore requirements to prevent long-running cracks, may not be fulfilled. Moreover,this approach is also detrimental, e.g. to field weldability. Results of that approach areas follows:

Approach B, with a carbon equivalent of only 0.43, which was used in combinationwith fast cooling rates in the plate mill down to a very low cooling-stop temperatures,resulting in the formation of high fractions of martensite in the microstructure, whichhave a detrimental effect on toughness properties of base metal. In addition, asoftening in the heat affected zone was observed. This effect cannot be adequatelycompensated using extremely low carbon contents, without adversely affecting theproductivity.

Heat pipe sizeOD X WT

C Mn Si Mo Ni Cu Nb Ti N C EIIW PCM

I 30" x 19.1 mm 0.08 1.95 0.26 0.26 0.23 0.22 0.05 0.018 0.003 0.49 0.22

Approach A

Heat

I

CVN(20°C)

DWTT-transition

temperature

739 MPa 792 MPa 0.93 18.4% 235 - 15 °C

* transverse tensile tests by round bar specimens

yield strengthR t0.5 *

tensile strengthRm *

yield to tensile ratioR t0.5 / Rm *

ElongationA 5 *

6

Experience gained meanwhile indicates that Approach C is the best choice. Thisapproach enables the desired property profile to be achieved through an optimisedtwo-stage rolling process in conjunction with a medium carbon content, a mediumcarbon equivalent and optimised cooling conditions. The special potential of theexisting rolling and cooling facilities contributes significantly to the success of thisapproach.

Approach C, which involves a medium carbon content, ensures excellent toughnessas well as fully satisfactory field weldability, despite the carbon equivalent, which is inthe range of 0.46. The chemical composition should therefore be consideredacceptable for the purpose of current standardisation.

Europipe already produced hundreds of tons of grade X100 according Approach C.The latest trials were covering the wall thickness range between 12.7 and 25.4 mm. Itwas demonstrated that the same steel composition could be used and only slightchanges in the rolling conditions were necessary.

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 to tensile ratioR t0.5 / 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 0.018 0.005 0.46 0.20

IV 36" x 16.0 mm 0.06 1.90 0.35 0.28 0.25 - 0.05 0.018 0.004 0.46 0.19

IV 752 MPa 816 MPa 0.92 18 % 270 J ∼ - 50 °C200-

270 J* transverse tensile tests by round bar specimens**-60°C for WT 12.7mm -10°C for WT 25mm

Approach C

Heatpipe sizeOD X WT

C M n S i Mo N i C u Nb Ti N C EI IW PC M

II

Approach B

Heat

II

CVN(20°C)

DWTT-transition

temperature

755 MPa 820 MPa 0.92 17.1 % 240 - 25 °C

30" x 15.9 mm 0.07 1.89 0.28 0.15 0.16 - 0.05 0.015 0.004 0.43 0.19

* transverse tensile tests by round bar specimens

yield strengthRt0.5 *

tensile strengthR m *

yield to tensile ratioRt0 .5 / Rm *

ElongationA5 *

7

As can be seen in figure 13, the production results show same strength properties forall wall thickness tested. Tensile tests were performed using round bar specimens.Yield/Tensile ratios are still relatively high and elongation values lower than known forgrade X70. Charpy toughness was measured in excess of 200 J in all cases, but itseems to be impossible to guarantee values in excess of 300 J at low temperaturesfor a big project. In figure 14 the DWT shear areas at –20°C are shown for thedifferent wall thickness. As a typical result the ductile area is higher for thin wallX100. Due to the relatively high carbon equivalent and the high strength level, thetoughness of the longitudinal weld seam and the HAZ is limited. The X100 materialproduced responds favorably to manual and mechanized field welding, a findingwhich can be attributed to its reduced carbon content.

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

The pipes produced were used in different tests to simulate the service behaviour.Figure 15 shows the influence of an ageing treatment on the Charpy transition curve.There is only a slight decrease in toughness properties after 30min 250°C treatment.

There are papers on this conference that describe the field welding behaviour andthe fracture behaviour. Also field bending tests are already performed. Figure 16shows a photo of full scale burst tests, which were conducted by CSM as part of anECSC-funded research project /7/. So far our experience has shown that it isimpossible to realise a pipeline project in Arctic regions in grade X100 without the useof crack arrestors. We also try to serve the industry with different types of crackarrestors, but on the other hand the use of grade X90 without crack arrestors couldbe the most economic solution.

Conclusion

The predicted growth in energy consumption in the coming decades necessitatessevere efforts for transporting large amounts of natural gas to the end users. Large-diameter pipelines serve as the best and safest means of transport. This paper givesan overview of the current requirements of high strength steels and the associateddevelopments. The technical possibilities were described. Also for the futureadditional improvements can be realised.

The enormous pressure on the price of natural gas forces the pipeline operator toexplore all the possibilities to reduce the cost of a pipeline project in the future. Thepipe manufacturer can assist him in his endeavour by supplying high strength andhigh quality pipe.

X80 line pipe material is already proven. Higher grades are under development.Usually not only the steel grade, but also the usage factor and the operating pressureis increasing steadily. From the manufacturers point of view there are some boarderlines, that limits the use of higher grades. For grade X80 the min. thickness should be12 mm and for grade X100 16 mm, respectively. In any cases thickness/diameterratio of high strength steels should be in excess of 1%.

8

References

/1/ M.K. Graef, H.G. Hillenbrand, “High Quality Pipe – a Prerequisite for Project CostReduction”, 11th PRCI-EPRG Joint Technical Meeting, Arlington, Virginia, April 1997

/2/ L. Barsanti et. al, “Possible use of new materials for high pressure line pipeconstruction: An opening on X100 grade steel”, International Pipeline Conference,Calgary, Alberta, September 2002

/3/ H.G. Hillenbrand et al., “Development of Large-Diameter Pipe in Grade X100”,Pipeline Technology Conference, Brugge, Belgium, May 2000

/4/ A.W. Gaertner, M.K. Graef, and H.G. Hillenbrand, “A Producer’s View of Large-Diameter Line Pipe in the Next Decade”, International Conference in PipelineReliability, Calgary, Canada, 1992

/5/ M.K. Graef, H.G. Hillenbrand, “Production of large-diameter line pipe – state of theart and future development trends”, 4. Int. Conference on Corrosion Prevention of theEuropean Gas Grid System, Amsterdam, The Netherlands, May 1995

/6/ V. Schwinn, P. Fluess, J. Bauer,. "Production and progress work of plates forpipes with strength level of X80 and above" , International Conference on theApplication and Evaluation of high-grade line pipes in hostile Environments,Yokohama, Japan, November 2002

/7/ G. Demofonti et al., “In field fracture behaviour of X100 gas pipeline by full scaletests”, International Conference on the Application and Evaluation of high-grade linepipes in hostile Environments, Yokohama, Japan, November 2002

9

Figures and Tables

Figure 1: Material savings of Ruhrgas X80 Project

Figure 2: Development of high-strength steels

10

X 60 NormalizedASTM 7/8

X 70 TMTASTM 10/11

X 80 Acc. cooledASTM 12/13

50µm

Figure 3: Effect of accelerated cooling on the microstructure of TM.-processed steels

Figure 4: Chemical composition and mechanical properties ofGRS550 pipe

11

Figure 5: Pipe laying operation of Ruhrgas GRS 550 project

Figure 6: Tensile properties of Transco X80 pipe (48” x 15.1 mm)

12

Figure 7: Toughness properties of Transco X80 pipe(48”OD x 15.1 mm)

Figure 8: CRC welding parameters for X80 pipe

13

Figure 9: Mechanical properties of heavy wall X80 line pipe(36” OD x 32.0mm)

500

550

600

650

700

750

800

850

900

0.37 0.39 0.41 0.43 0.45 0.47

Boron-alloyed(20 ppm)

SMYS (X100)

SMYS (X90)

SMYS (X80)

Yie

ld S

tren

gth

Rp2

.0 p

late

[MP

a]

Carbon Equivalent CEIIW [%]

C

Si

Mn

Mo

Nb

Ti

N

%

0.03-0.06

0.36

1.95

0.20

0.040

0.02

40ppm

Figure 10: Influence of boron on yield strength of high strength line pipematerial

14

SMYS (X80)

500

550

600

650

700

750

800

850

900

SMYS (X100)

SMYS (X90)

Yie

ld S

tren

gth

Rp2

.0 p

late

[M

Pa]

Cooling rate:15 K/secThickness:20 mm

Cooling rate:15 K/secThickness:32 mm

Cooling rate:25 K/secThickness:20 mm

B-freeCE = 0.38C = 0.03%

20 ppm BCE = 0.38C = 0.03%

20 ppm BCE = 0.42C = 0.06%

Figure 11: Influence of boron and cooling conditions on yield strength ofhigh strength line pipe material

Figure 12: Different approaches to reach strength level of X100 byvariing steel chemistry and cooling parameters

15

Figure 13: Influence of wall thickness on tensile properties of X100 pipe

Figure 14: Influence of wall thickness on DWT test results at –20°C

16

Figure 15: Charpy test results on 36” x 16mm X100 pipe in theas delivered and aged condition

Figure 16: Full scale burst tests on X100 line pipeA) 56” x 19.1 mmB) 36” x 16.0 mm