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Development of line pipe for deep water applications

H. G. Hillenbrand, M. K. GraefEuropipe GmbH, Ratingen, Germany

J. Groß-Weege, G. Knauf, U. MarewskiMannesmann Forschungsinstitut GmbH, Duisburg, Germany

1

Development of linepipe for deep-water applications

H.-G. Hillenbrand, M.K. GraefEuropipe GmbH, Ratingen, Germany

J. Groß-Weege, G. Knauf, U. MarewskiMannesmann Forschungsinstitut GmbH, Duisburg, Germany

ABSTRACT

The requirements for large-diameter linepipe for offshore applicationsare becoming increasingly stringent. As a result of the shifting ofoffshore oil and gas production into ever-remote offshore regions, thepipelines have to be laid in deeper waters and over longer distances.For economic reasons, the pipelines need to be operated at higherpressures, thereby requiring higher strength and toughness for the pipematerial.

The latest results on high strength steels for use as offshore pipelinestogether with a survey of typical requirements for deep-water linepipeare presented. Improvements in the manufacture of heavy wall pipe aswell as the collapse behaviour of the UOE pipe and the development ofappropriate testing criteria are highlighted. On the basis of thedescribed developments, Europipe is capable of serving the marketneeds for linepipe for deep-water applications meeting the moststringent requirements.

KEY WORDS: UOE linepipe, pipeline steels, material properties,deep water, collapse behaviour, pipe forming, compressive yieldstrength

INTRODUCTION

Analysis of the market needs indicates that the requirements forpipeline steels, especially those intended for use in offshoreapplications, are becoming more and more stringent. The gas transporthas to be safe even if the pipeline is installed at large depths and overlonger distances. For economic reasons, the pipeline needs to becapable of being installed easily and operated at very high pressures.

Europipe continuously monitors the market needs and tries to meet thestringent requirements by means of extensive research anddevelopment activities.

The development of high strength steel pipes with high toughness andoptimised geometry is the key to fulfilling the requirements for theprojects of pipelines in deeper waters and has to cover linepipe for bothsour and non-sour service. Use of higher strength steels provides theopportunity to reduce overall materials and construction costs.Moreover, certain offshore projects can only be implemented with pipehaving reduced weight and optimised strength and toughness.

The collapse behaviour is an essential design parameter, especially fordeep-water linepipe. Therefore, emphasis is placed on developing theproduction route to achieve optimum pipe geometry and low variabilityin pipe properties. The collapse behaviour of deep-water linepipe isanalysed by finite element simulations as well as by collapse tests andthe material properties are characterised by tensile and compressiontesting.

The transverse compression test on round bar specimens is a suitablelaboratory test method which can be applied to verify the collapseproperties of pipe material. A comparison is made between the collapsepressures calculated from compressive yield strength with the results ofcollapse tests on pipe sections to demonstrate that the collapsebehaviour of the UOE pipe can be characterised by means of thecompression test and that the UOE pipe has risen to the requirementsfor a deep-sea pipeline.

REQUIREMENTS FOR DEEP-WATER PIPELINES

Offshore pipelines in deep-waters are a great challenge to the designer,the pipe manufacturer and the pipelaying contractor. Table 1 contains alist of significant deep-sea projects, which are either in planning orcurrently under construction. The Oman-India pipeline was notconstructed in the end. But, it is of great value in the context of thegeneral development of deep-sea pipelines, because the requirementsfor such a deep-sea pipeline were formulated and the suitability of UOEpipe for such applications has been established for the first time.

Table 1: Major deep-sea pipeline projects

Project Pipe size(OD x WT) Material

Length ofpipeline

(km)

Max.water depth

(m)

Oman-India 28" x 41 mm X 70 1 200 3 500

Blue Stream 24" x 31.8 mm X 65(sour) 374 2 200

Libya-Sicily 32" x 30 mm X 65 560 ~ 800

Mardi Gras 28" x 38 mm X 65 712 2 000

Iran-India 29" x ~50 mm X70 / X80 1200-1500 3 500

The external pressure on the pipe from ambient water is directlyproportional to water depth (1 bar per 10 m of water depth). Thispressure and the bending stresses occurring during pipeline installationjointly constitute the major loads on the pipeline. Hence, safety againstbuckling (collapse) under external pressure is an essential designcriterion.

As shown in Figure 1, the collapse resistance of a pipe increases withincreasing wall thickness-to-diameter ratio (t/D). In view of thisrelationship, offshore pipelines, as a rule, need pipe with a smalldiameter and a large wall thickness. While the collapse resistance isnearly independent of materials yield strength at small t/D ratios, highercollapse pressures are achieved at large t/D ratios by increasing thecompressive yield strength.

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Fig. 1: Effect of t/D ratio and compressive yield strength on thecollapse pressure of pipes6

Another factor that has a profound effect on the collapse resistance isthe out-of-roundness of the pipe (Kyriakides, 1990). Therefore,increasingly tight tolerances with respect to the out-of-roundness arelaid down in pipe specifications. This situation is applicable not only topipe ends, but also to the entire pipe length.

Figure 2 shows, as an example, the buckling behaviour of a 28” OD x42 mm WT pipe with ovality. The series of curves designated “A”derive from the relationship for elastic buckling of a pipe (Schwaigerer,1993) with ovality, which is as follows:

Po = 2 E t3 (1 - ∆r/D) (1) 1 - ν2 D3

where

Po = collapse pressure t = wall thickness D = outside diameterE = elastic modulus ν = Poisson’s ratio ∆r = deviation from truly circular shape

Fig. 2: Analytical model for the prediction of the collapse pressure ofa 28” OD x 42 mm WT pipe for various levels of ovalityThe series of curves designated “B” describe the relationship betweenthe collapse pressure and pipe ovality in the case of plastic buckling.Plastic collapse occurs when the deformation is so high that thecompressive yield strength is exceeded.

When the pipe is subjected to external pressure, the curves designated“A” will be followed in accordance with the initial ovality, Uo. That

means the ovality increases as the pressure increases. If the deformationleads to a stress in excess of the compressive yield strength (point ofintersection with the curve designated “B”), the pipe will fail by plasticcollapse.

The figure shows clearly that in the case of thick-wall pipes initialovality and materials yield strength have a strong effect on achievablecollapse pressure. This finding leads to typical specifications formaterials properties for offshore linepipe, as given in Table 2.

Table 2: Typical requirements for deep-water pipelines

Steel grade X65, X70Longitudinal and transversal direction

CVN toughness,base material

> 150 J at -10°C > 45/38J at -30°C

CVN toughnessweld metal

> 100 J at -10°C > 45/38J at -30°C

DWT shear area > 75% at -10°C

Out-of-roundness < 4 mm

As a result of the increasing application of the limit state design (LSD),besides the specified minimum values for strength and toughness therequirements for statistical distribution of the properties play animportant role. LSD is used as a practical method of incorporatingreliability methods in the normal design process. This design method isa challenge also to the pipe producer because it requires that theindividual properties determined on commercial pipe should fulfilcertain statistical requirements. For example, the mean value of theyield strength has to be greater than the specified minimum yieldstrength by at least 2 x standard deviation.

PIPE PRODUCTION

The manufacture of high-strength linepipe with a small diameter andthick wall, i.e. a large t/D ratio, implies large loads on the presses andtools in all stages of pipe forming. Simultaneously, difficulties have tobe overcome in producing pipe which is as round as possible with theavailable capacities of the presses. Therefore, there is a need tooptimise the individual forming operations with a view to:

• improving the geometry of finished pipe (roundness);• achieving uniform distribution of strength properties;• avoiding overloads on presses and tools and their failures.

In the manufacture of linepipe by the UOE process the longitudinaledges of the plate are bevelled by planing for welding. The planededges are then crimped to the pipe radius over a width of 200 to400 mm in the crimping press. The actual forming of the plate into pipeoccurs in two matched consecutive operations, namely pressing into theU-shape in the U-ing press and pressing to an open-seam pipe in the O-ing press. The open-seam pipe is welded and subsequently expandedwith a mechanical expander, thereby receiving its final shape.

0

10

20

30

40

50

60

70

80

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07

Wall Thickness - to - Diameter Ratio, (t/D) [-]

Co

llap

se P

ress

ure

[M

Pa]

Grade X 80 - SMYS = 553 MPa

Grade X 70 - SMYS = 484 MPa

Grade X 60 - SMYS = 415 MPa

28"

x 41

.0 m

m

24"

x 31

.8 m

m

29"

x 50

.0 m

m

Collapse pressure calculated inaccordancewith BS 8010 for a truly circularpipe(i. e. no ovality)

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The finite element method (FEM) is an excellent tool, available today,to simulate numerically such forming operations. It has been found thatan integrated consideration of the entire production process from edgecrimping through U-ing, O-ing and welding to expanding is aprerequisite for successful optimisation of the process. Figure 3 shows,by way of example, a FE simulation of the initial state of the O-ingprocess. FE simulation is the only way by which the entire history ofpipe forming and the interactions among individual forming steps canbe covered.

Fig. 3: FE simulation of UOE pipe forming, O-ing

In the context of optimisation, the design and calibration of the toolsand optimised matching of the consecutive forming steps with eachother play an important role. Extensive FE simulations have shown thatthe crimping of the plate edges in the crimping press forms thefoundation for good roundness of the finished pipe and uniformdistribution of strength properties. This applies not only to the crimpedregion itself (peaking, etc.), but also to the entire area between the12 o’clock and 3 o’clock positions as well as between the 9 o’clock and12 o’clock positions. Optimisation is also possible in the subsequent U-ing operation, compared to the conventional forming process. In bothcases, the shape of crimping tools, the shape and size of the U-ing presspunch and the operation of the presses play an important role. Theseoptimisation measures lead to an improvement in pipe quality withrespect to roundness and strength properties, but also to favourablestress conditions in subsequent O-ing and expanding operations.

A further result of the extensive FE simulations was the development ofa new design for the O-ing press tools. Compared to the conventionaldesign, the lifetime of the tools could be increased markedly.

The results of the numerical simulations have been checked byextensive measurements in the mill. There was, in general, goodagreement between the simulation and the measurement.

PRODUCT DEVELOPMENT AND PIPE PROPERTIES

Europipe uses plate with optimised chemical composition for theproduction of linepipe with high strength and thick wall (Graef andHillenbrand, 1995). The plate is produced by thermo-mechanical (TM)rolling or thermo-mechanical rolling plus accelerated cooling. Theobjective is to achieve good toughness and weldability, and whereapplicable, suitability for sour service.

High Strength Pipe, Grade X80

The use of higher strength steels (i.e. higher material grades) results inbetter collapse resistance and enables pipe with thinner walls to bedesigned. As a consequence, pipeline construction cost can be reduced.Certain projects requiring reduced pipe weight, i.e. thinner pipe wallcan be realised. A grade X80 offshore pipe material was developed inthis context (Graef and Hillenbrand, 2000). The chemical compositionof the steel (Table 3) used for the X80 pipe (36” OD x 32.0 mm WT)corresponds to the known MnNbTi steel. This steel has a sufficientlyhigh ratio of Ti to N and is additionally alloyed with molybdenum. Thelow carbon equivalent ensures good field weldability. Circumferentialwelding trials showed that this high grade material can be welded on alay-barge without any problems.

Table 3: Production results on 36” OD x 32 mm WT API grade X80linepipe, mean chemical composition (wt.%)

C Si Mn P S Al CuX80

.07 .27 1.86 .015 .0010 .036 .02

Cr Ni Mo Nb Ti N IIW PCM

.03 .02 .15 .04 .02 .0057 .419 .186

The mechanical properties are summarised in Table 4. The elongationvalues (A2”) are particularly high. The Charpy V-notch impact energymeasured at –40°C is in excess of 200 J and the shear area of theDWTT specimens tested at –20°C is greater than 85 %. The formingand welding operations carried out at our Mülheim works on this highstrength steel did not cause any problems.

Table 4: Production results on 36” OD x 32 mm WT API grade X80linepipe, mechanical properties

Mechanical properties Mean valuestransverse

Yield strength (flat bar) Rt0.5 (MPa) 559

Tensile strength (flat bar) Rm (MPa) 685

Yield–to-tensile ratio (flat bar) Rt0.5/Rm (%) 82

Elongation (flat bar) A2” (%) 47

Yield strength (round bar) Rt0.5 (MPa) 579

Tensile strength (round bar) Rm (MPa) 674

Yield–to-tensile ratio (round bar) Rt0.5/Rm (%) 86

Elongation (round bar) A2” (%) 46

Charpy toughness at –40°C, average CVN (J) 224

Battelle drop weight shear area at –20°C SA (%) 87

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Pipe Grades for Sour Service

While it has been possible to develop material grades from X65 to X80for offshore pipelines for non-sour service, the strength levels of steelsfor pipelines intended for the transportation of H2S need to be limitedtoday to that of grade X65. The steels for sour service require a specialtreatment in the steelmaking shop. The restriction of the sulphurcontent to very low values and the addition of calcium to form non-deformable sulphides help prevent the formation of nucleation sites forHIC (hydrogen induced cracking). If the HIC test environment is a pH3solution with 1 bar H2S, the C and Mn levels of the steel have to berestricted to reduce the centreline segregation and avoid HIC.

In order to make the step from a heavy wall grade X 65 pipe on to aheavy wall grade X 70 pipe, two different approaches have beendeveloped and applied. The first approach to increase the strength ofthe steel was concerned with the distribution and type ofmicrostructural constituents and at achieving additional solid solutionhardening. The classical composition of the NbV-type steel, used forgrade X 65 pipe, was modified by adding or increasing theconcentrations of Cu, Ni, Cr and Mo in the steel analysis (Table 5).This concept results in a carbon equivalent of 0.39 according to IIW.

Also shown in Table 5 is the other approach, which is based onincreasing Nb concentration and adding Ti to the steel (Hulka, Grayand Heisterkamp, 1990). The Nb content is increased to have higheramounts of Nb in solid solution in order to increase the strengtheningby precipitation hardening. Ti was added to bind nitrogen therebypreventing the precipitation of NbCN and making Nb more effective inincreasing the strength. This approach leads to a carbon equivalent ofonly 0.32 according to IIW.

It should be noted that both the approaches (referred to as NbV andNbTi type in the table) were applied in combination with adaptedTMCP condition, such as increased cooling rate after final rolling, toproduce HIC resistant plates with 30 mm wall thickness in grade X 70.

These plates were formed by the UOE process into 30" OD pipes.Following the pipe expansion, samples were taken to determine theirmechanical properties and corrosion behavior.

Table 6 contains the mean values of the mechanical properties for thetwo different approaches. As can be seen from the data the requirementfor a shear area of 85 % minimum in the DWT test at –10°C is met byboth variants. The Charpy V-notch impact energy values measured at-30°C are as expected for a low-carbon steel, at above 450 J. In theweld seam and the heat-affected zone impact energy values in the rangeof 100 to 160 J were measured. In general, the NbV approach yieldedmore favorable strength values so that the strength requirements forgrade X 70 can be met in both transverse and longitudinal directions.

Table 5: Steel concepts for grade X 70 with a wall thickness of 30 mm

Type C Si Mn V Nb Ti others CEIIW*

NbV 0.04 0.25 1.35 0.05 0.05 - CuNiMoCr

0.39

NbTi 0.04 0.25 1.40 - 0.08 0.02 CuNi 0.32

Table 6: X70 sour service, mechanical properties

Orientation /

Position

Temp.

[°C]

NbTi NbV

Rt0.5 [MPa] Long. RT 504 519

Trans. RT 502 509

Rm [MPa] Long. RT 566 582

Trans. RT 582 595

Y/T Long. RT 0.89 0.89

Trans. RT 0.86 0.86

A5 [%] Long. RT 27 25

Trans. RT 25 24

DWTT Base metal -10 92 97

[% SA] Base metal -20 82 91

CVN [J] Base metal -30 470 480

Weld seam -30 140 160

Fusion line -30 95 135

Pipe Grades for Slightly Sour Service

Experience shows that the standard HIC test (NACE Standard, 1996) iscapable of differentiating between susceptible and resistant steels. Full-scale tests have demonstrated that a steel suffering certain amount ofcracking in the laboratory test can withstand the full-scale test, withoutcracking, in the same test environment (Provou et al.,1990).

Therefore, steels exhibiting cracking below the maximum allowablelimits are considered HIC resistant for conventional sour servicelinepipe. It becomes however increasingly difficult, if not impossible,to fulfil these requirements as the market demands ever-increasingstrength and/or special pipe geometries, e.g, high t/D ratio.

The application of the standard HIC test therefore results in discardingpipe that would perform satisfactorily in actual service. In such cases, itis therefore necessary to adopt a fit-for-purpose approach in selectingthe test conditions for the laboratory test.

Work was carried out to specify fit-for-purpose HIC test requirementsfor grade X65 pipe with 40” ID and 29.8 mm or 37.9 mm WT. Thevery lean chemical composition that was typically used for theproduction of linepipe that was intended for use in sour service at thetime could not be adopted here because of the heavy wall. Thechemical composition was optimized for the most part to fulfil therequirements for mechanical properties. Attention was paid to each andevery production step with a view to improving HIC resistance of thesteel.

Different HIC test variants were tried out to work out a procedure thatis close to the predicted service conditions and that is not too difficultto implement. In particular, the specimen type, partial pressures of H2Sin the H2S + N2 mixture and the number of specimen sides exposed tothe test solution were varied. Finally, the production HIC tests werecarried out adopting the conditions listed in Table 7.

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Table 7: HIC test conditions adopted to qualify thick-wall grade X65pipe for “slightly sour service” (fit-for-purpose testing)

Type of exposurepHp(H2S)c(H2S)

1 – sided3.00.1250 ppm

About 200,000 t of pipe was produced for this order. Table 8 shows themean chemical composition. The mean mechanical propertiesdetermined on the 37.9 mm thick wall pipe are shown in Table 9.

Table 8: API grade X65 linepipe for “slightly sour service”, chemicalcomposition

C Si Mn P SX65SlightlySour 0.07 0.20 1.60 0.008 0.0008

Nb Ti N Others IIW PCM

0.035 0.017 0.004 Cu, Ni 0.37 0.18

Compared to a conventional grade X65 material intended for sourservice, the present steel had a relatively high carbon and a highmanganese content. The higher carbon and manganese contents servedto ensure that the steel attained the required mechanical properties,particularly toughness, at the heavy wall involved. The steel was madeto the standard sour service practice, including vacuum degassing. Itwas desulphurised to a very low sulphur level of 8 ppm and Ca-treated.This practice was adopted to ensure that the steel meets therequirements for HIC resistance.

Table 9: API grade X65 linepipe for “slightly sour service”,mechanical properties

Mechanical properties Meanvalues

Standarddeviation

Yield strength Rt0.5

(MPa)TransverseLongitudinal

504514

2116

Tensile strength Rm

(MPa)TransverseLongitudinal

607596

1816

Yield–to-tensile ratio Rt0.5/Rm

(%)TransverseLongitudinal

8387

0.0210.016

Elongation A2”(%)

TransverseLongitudinal

22.122.8

1.271.67

Charpy toughness at –30°C, average

Base material CVN (J) 266 35

Weld metal CVN (J) 110 37

HAZ CVN (J) 76 37

As can be seen from the mean values and the standard deviations givenin the table, the pipe could be produced with a high statistical

confidence level, despite the heavy wall. Both the transverse and thelongitudinal tensile properties of the pipe are comfortably above thoserequired for X65. The Charpy V-notch impact energy values measuredon the base material at -30°C are in excess of 250 J.

The HIC specimens, which were hydrogen-charged to the fit-for-purpose procedure, agreed upon between the customer and Europipe,showed no indications, thereby complying with the specification.

Thus, the steel chemistry and the steelmaking practice adopted haveproven the right approach to execute this special order successfully.

Pipe Geometry

In addition to the mechanical properties, the pipe geometry, as notedpreviously, is an important parameter, because it has an effect on thecollapse strength of pipe.

Figure 4 shows the distribution of out-of-roundness values in the caseof two thick wall pipes, namely 28” OD x 41.0 mm WT grade X 70pipe and 36” OD x 28.6 mm WT grade X 65 pipe. The majority of thepipes in both sizes exhibited an out-of-roundness of 2 to 3 mm. This isa good result, considering that a truly circular pipe is technically notfeasible for different reasons.

Fig. 4: Distribution of the out-of-roundness values for two pipe lots

Figure 5 shows, by way of example, a highly magnified ovality profileof a 28” OD x 35.6 mm WT pipe. The initial ovality measured on thispipe was Uo = 0.5%. This ovality resulted from local deviations fromroundness alone and can be attributed to the expanding process in thecourse of pipe manufacture. Such profile measurements are used in thefinite element analyses to predict exactly the collapse pressures. It hasbeen found that it is not the local deviations from truly circular shape,but the overall out-of-roundness that governs the collapse strength ofthe pipe.

0

10

20

30

40

50

1 2 3 4 5 6

Ovality (mm)

Fre

qu

ency

(%

)

36" x 28.6 mm wt., X 65

28" x 41.0 mm wt., X 70

6

Fig. 5: Highly magnified ovality profile of a 28” OD x 35.6 mm WTpipe

COLLAPSE BEHAVIOUR

As noted previously, the collapse strength of thick-wall pipes dependsessentially on the strength properties (particularly the yield strength incompression) and ovality.

Also in this context, FE simulations are valuable in determiningspecifically the effects of individual parameters (e.g. ovality) on thecollapse pressure of pipe. These simulations, in principle, are capableof taking into account the entire forming process from crimpingthrough U-ing and O-ing to expanding

Fig. 6: FE simulation of collapse behaviour

The results of such a simulation of the collapse behaviour is shown inFigure 6. Of great significance in this context is the most realisticmodelling of the materials behaviour under alternating tensile andcompressive stresses so that the Bauschinger effect, which occurs as aresult of the stress reversal during pipe manufacturing, can be takeninto account.

To perform collapse tests, a pressure vessel was fabricated, whichserved as part of the test set-up. The test pipes were converted into testspecimens by closing their ends with hemi-spherical caps. After this,

the test specimens were placed in the pressure vessel. The annulusbetween the test specimen and the pressure vessel was filled with waterand pressurized to generate a hydrostatic external pressure on the innertest pipe.

Fig. 7: Photograph showing a collapse tested pipe

It was possible to monitor the ovalisation (by the use of strain gauges)and buckling of the test pipe continuously during the test. The buckleformed in the middle of the pipe can be clearly seen in Figure 7.

The results of collapse tests (Graef, Marewski, Vogt, 1996) made onlinepipe are given in Figure 8 (Langner, 1990) The figure shows alsothe collapse pressures or the maximum allowable water depthscalculated from theoretical models as a function of D/t. The modeldeveloped by Timoshenko, de Winter and Tamano holds good for trulycircular pipes and yields the highest collapse pressure.

Fig. 8: Dependence of collapse pressure on D/t ratio

7

The formula given in the DNV Offshore code (Table 10) and thatdeveloped by Shell yield nearly identical curves. They take pipe ovalityinto account and yield smaller collapse pressures. The experimentaldata lie very close to the data obtained by the Shell and DNV formulas,so that the recommended design limits would yield conservativedesigns for the tested pipes.

Table 10: Calculation of collapse pressures (DNV, 2000)

Fabrication Factor for the UOE Pipe

According to DNV OS F101 a fabrication factor has to be used,depending on the method of pipe manufacture (Table 10) forcalculating the collapse pressure. It is well known (Gresnigt et al.,2000) that the compressive yield strength of the UOE pipe is reducedby the Bauschinger effect. As the DNV code prescribes the use oftensile yield strength of the material to calculate the collapse pressure areduction of 15 % is applied to the calculated collapse pressure in thecase of the UOE pipe.

In the course of the development of the UOE pipe for the Oman-IndiaProject (Stark and Mckeehan, 1995) Europipe had demonstrated, forthe first time in the context of a large-scale application, that thecollapse properties of the pipe can be improved significantly by heatinput during pipe coating.

Full-scale tests on linepipe (Table 11) showed (Graef, Marewski, Vogt,1996) that the adverse effect of the Bauschinger effect on compressiveyield strength and hence the collapse strength of the pipe can be largelycompensated for by using a moderately elevated temperature (about220°C) during pipe coating (Müsgen and Kaiser, 1984). Accordingly,pipes so treated attained significantly higher collapse strengths becauseof their higher compressive yield strengths.

Table 11: Results of collapse tests on pipes in the coated and as-rolledcondition

Pipe

OD x WT

Mea

sure

d co

llaps

e

pres

sure

(ba

r)

Cal

cula

ted

colla

pse

pres

sure

(ba

r)

Ova

lity

Uo

(%)

Com

pres

sive

yie

ld

stre

ngth

Rc0

.2 (

MP

a

Ten

sile

yie

ld

stre

ngth

Rt0

.5 (M

Pa)

28” x 42.0 mm

(X65) 421 427 0.7 425 490

28” x 44.0 mm

(X70) 468 459 0.6 435 550

28” x 41.0 mm

(X70; coated) 484 475 1.4 525 540

The DNV code takes this into account and permits improved collapsestrength properties achieved through heat treatment, provided they aredocumented (see Table 10). It is more sensible and physically correct,to predict the collapse strength from experimentally determinedcompressive yield strength of the pipe material than from the yieldstrength determined in the tensile test. This entails however astandardised compressive test that can be carried out correctly from themeasurement point of view and results in reproducible test results.

Compression Testing

The test set-up shown in Figure 9 has proven suitable to carry out thecompressive tests. These and similar test set-ups were used in thecontext of the last deep-sea pipeline projects. A cylindrical specimenwith a length equal to 2.5 times its diameter is compressed between twomovable parallel plates. Finite element analyses carried out to backupthese tests showed that longer specimens tend to buckle. On the otherhand, when shorter specimens are used, frictional effects result ininhomogeneous distribution of the strain on the specimen outer surface,thereby making the load measurements possibly inaccurate.

Fig. 9: Set-up for a compression test on cylindrical specimens

8

It has proven useful to make the strain measurements on at least two orthree locations around the specimen circumference and to averagethem. This measure minimises any inaccuracies associated with strainmeasurements resulting, e.g. from any slight buckling of the specimen.

A comparison was made of the collapse pressures measured in full-scale tests on pipes and those predicted on the basis of small-scale testsfor pipe sections for the as-rolled condition and for the condition ofcoating simulation after the UOE pipe manufacturing process. Thepredictions were made using the specified minimum yield strength100% SMYS and 85% SMYS (which corresponds to the fabricationfactor for the UOE pipe), as well as using measured compressive yieldstrength values (σt0.5 and σp0.2). The results are given in Figure 10.

Fig. 10: Collapse pressures predicted under different conditions

Referring to Figure 10, the collapse pressure measured on a 28” OD x42 mm WT grade X65 pipe in the as-rolled condition was p = 421 bar.The use of 100% SMYS as well as the use of the measuredcompressive strength σt0.5 for the calculations would predict collapsepressures that are significantly higher than the measured collapsepressure. A good agreement is found between the measured collapsepressure and that calculated using the measured compressive strengthσp0.2 . Use of 85% SMYS, as required by DNV OS F101, only resultedin a marginal underestimation of the pipe collapse pressure for the as-rolled condition.

The same comparison made for a coated pipe (28” OD x 41 mm WTgrade X65) gives a completely different impression. The collapsepressure measured, at 484 bar, is significantly higher than that of thepipe tested without subsequent coating and higher than that predictedfrom the DNV expression, based on the 85% SMYS. Even the use of100% SMYS, i.e. without the fabrication factor, results in unjustifieddowngrading of the pipe with respect to collapse pressure.

The best agreement between the prediction and the experiment for theas-rolled as well as for the coated pipe is obtained when thecompressive yield strength at 0.2 % permanent strain (σp0.2) is used tocalculate the collapse strength from the DNV expression.

CONCLUSIONS

The pipe manufacturer is also required to make his contribution, incooperation with the pipeline designer and pipelaying contractor, inrealising the challenging offshore projects in ever-deep waters.

Collapse strength of pipe is of paramount importance in the context ofoffshore pipelines in deep waters. Pipe geometry and the materialsyield strength in compression were found to have a profound effect onthe collapse strength.

Based on our know-how, we can produce high strength linepipe fornon-sour and sour service with optimised pipe geometries and materialsproperties for deep-water pipelines. This holds good particularly topipes with a small diameter and large wall thickness. We can guaranteethe compressive yield strength values necessary in ensuring safetyagainst collapse and hence meet the requirements of the designer. Weare in a position to fulfil the most challenging requirements for linepipeintended for use in the construction of deep-water pipelines.

A still better cooperation between designers and materials experts isnecessary in preparing and identifying pipeline codes to enable low-cost and safe pipelines to be laid, especially in deep waters.

REFERENCESDNV (2000). “DNV OS-F101, Submarine Pipeline Systems”, Det Norske

Veritas.Graef, M. K. and Hillenbrand, H. G. (1995).“Production of Large Diameter Line

Pipe - State of The Art and Future Development Trends” Europipe GmbH.Graef, M.K. and Hillenbrand, H.G. (2000). “Development of larger-diameter

linepipe for offshore applications”, 3rd International Pipeline TechnologyConference, 22-24 May 2000, Brugge, Belgium.

Graef, M.K. , Marewski U. and Vogt G. (1996) ”Collapse Behaviour ofOffshore Line Pipe under external Pressure”, 9th Symposium on PipelineResearch, September 30-October 2, 1996, Houston, Texas.

Gresnigt, van Foeken, Shilin Chen (2000). “Collapse of UOE ManufacturedSteel Pipes”, Proceedings of the Tenth (2000) international Offshore andPolar Engineering Conference Seattle, USA, May 28-June 2, 2000.

Hulka, K., Gray J.M. and Heisterkamp F. (1990) . “Metallurgical concept andfull-scale testing of a high toughness, H2S resistant 0.03 % - 0.10 % Nb steel”,Niobium Tech. Report, NbTR-16/90.

Langner, C.G. (1990).”History and Review of Collapse”, Seminar on Collapseof Offshore Pipelines, Houston Texas.

Kyriakides S. (1990). “Factors affecting Pipe Collapse”, Seminar on Collapse ofOffshore Pipelines, Houston Texas.

Müsgen, B. and Kaiser, H.J. (1984).“Auswirkungen des Bauschinger-Effektesauf das Bauteilverhalten hochfester Stähle”, Thyssen Technische Berichte,issue 2/84.

NACE Standard (1996). TM0284-96Provou, Y., Bennet, C., Brown, A., Pöpperling, R., Pontremoli, M. (1990).

“Testing linepipe steels under sour gas conditions – Comparison of the resultsof full scale and laboratory tests“, Pipeline Technology Conference, 15.-18.10.1990, Oostende, Belgium

Schwaigerer, S. (1993). “Festigkeitsberechnung im Dampfkessel-, Behälter- undRohrleitungsbau“, Springer-Verlag, Berlin/ Heidelberg/ New York/Tokyo.

Stark, P.R. and Mckeehan D.S. (1995), “Hydrostatic Collapse Research inSupport of Oman India Gas Pipeline”, 27th Annual OTC Houston, Texas, 1-4May 1995.

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28" x 42.0 mm wt., X65 as-rolled

28" x 41.0 mm wt., X65 coated