Hyatt Hotel, Canberra, Australia th April 2007 CHALLENGES IN LINEPIPE MANUFACTURING FOR HIGH STRENGTH PIPELINES

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1 16 th Biennial Pipeline Resarch Joint Technical Meeting of The Australian Pipeline Industry Association European Pipeline Resarch Group Pipeline Research Council International, Inc. Hyatt Hotel, Canberra, Australia th April 2007 CHALLENGES IN LINEPIPE MANUFACTURING FOR HIGH STRENGTH PIPELINES Authors: A. Liessem (Presenter) EUROPIPE GmbH, Works Muelheim, Germany Wiesenstrasse Muelheim - Germany Andreas.Liessem@europipe.com H.-G. Hillenbrand EUROPIPE GmbH, Head Office Muelheim, Germany Pilgerstrasse Muelheim - Germany Hans-Georg.Hillenbrand@europipe.com ABSTRACT The development of high strength linepipe steels such as X100 started more than 10 years ago as a consequence of the general realization that the supply of large gas volumes over long distances from remote areas to the energy markets was becoming an economical necessity in order to replace oil and gas as the primary resources of energy. When the first steps of this development were undertaken, the grade X100 was widely considered as an advanced X80 with additional 20,000psi yield strength. Following extensive worldwide research leading to better and broader understanding of the material behaviour itself and the corresponding design requirements, the development steadily progressed until today when several demonstration pipelines mark a further crucial milestone before the first commercial projects. This paper provides an overview of the challenges of linepipe manufacturing for high strength pipelines. As the necessary microstructure for X100 consists of a very fine, fully bainitic microstructure it is essential to understand the process of bainite formation and the resulting mechanical properties. Especially the work hardening of the plate and the pipe needs to be understood, since it determines the final stress-strain behaviour of the pipe after coating. High uniformity of the mechanical properties requires tighter production control with smaller tolerances for the production parameters. Compared to X70 the stress-strain curve differs considerably, which challenges especially the cold forming of UOE pipes as the elastic springback is increased. The forming and expansion process needs to be modified accordingly. The second major challenge during pipemaking is the high performance submerged arc welding process. Overmatching strength properties, high weld seam toughness, limited HAZ softening are the essential keywords in this area. New welding consumables need to be developed to fulfil the existing requirements. The assessment of the significance of low HAZ toughness is required as low Charpy-V values adjacent to the fusion line are obtained. The paper will summarise the present status of these technical matters, identifying those issues which are solved and those where questions still need to be answered. Finally a closer look on the alloying cost development relevant for X100 will be taken, in comparison with that for conventional linepipe grades. Andreas Liessem Canberra JTM page 1 of

2 INTRODUCTION The International Energy Outlook 2006 (IEO2006) projects strong growth for worldwide energy demand over the 27-year projection period from 2003 to Despite the known increase of world oil prices world economic growth continues to increase at an average annual rate of 3.8 percent over the projection period, driving the robust increase in world energy use. Total world consumption of marketed energy expands from 421 quadrillion British thermal units (Btu) in 2003 to 563 quadrillion Btu in 2015 and then to 722 quadrillion Btu in 2030, or a 71-percent increase over the 2003 to 2030 period as shown in Figure 1 [1]. World marketed Energy Consumption Projections History Quadrillion Btu History Year Fig.1: World Marketed Energy Consumption This continuous growth of the energy demand will lead to an increase in total world natural gas consumption from 95 trillion cubioc feet in 2003 to 134 trillion cubic feet in 2015 and 182 trillion cubic feet in 2030 [1]. The application of high strength steels is considered as an economical necessity in order to supply large volumes of gas at pressures above 10 MPa over long distances in a competitive manner. However the current commercial use of high strength steels is still limited to X80. Even though the first use of X80 was more than 20 years ago, world wide not more than 0km of high strength pipelines are in operation. The number of gas companies using X80 linepipe is limited, but growing during the last years. Meanwhile X80 pipelines are operated in Canada, Germany, the United Kingdom and Italy. The construction of the Cheyenne Plains project in 2004 recently opened the door to the US-market. For the next 5 years more than km of high strength pipelines are projected as shown in Table 1. Region Length [km] Grade North America X80/X100 Russia X80 China X80 Europe 500 X80 Table 1: High strength linepipe projects world wide (forecast ) So far grade X100 or X120 is used only for trial loops or demonstration lines like the TAP project [2]. In total around 10km of X100 and 2km of X120 are estimated to be welded and to be under pressure. Andreas Liessem Canberra JTM page 2 of

3 STANDARDS AND SPECIFICATIONS Grade X80 has for many years been included in the relevant standards like API 5l, ISO 3183, CSA Z245.1 and DNV OS-F101 and is therefore not further discussed in this paper. Grades above X80 are considered in CSA Z245.1 and since March 2007 in the second edition of ISO3183 [3]. These documents define the basic requirements for the mechanical properties of high strength linepipes in terms of tensile, Charpy-V, DWTT and bend tests. Furthermore limits for the chemical composition are given in order to guarantee excellent weldability. Depending on the project specific design additional requirements are necessary and need to be covered by supplementary specifications. When the development of X100 was started by Shell, BP and BG in 1995 a guideline was given in a preliminary specification defining the basic requirements (Table 1). Dimensions Tensile Properties 30 OD x 19,1mm WT Yield strength: min. 690 MPa Elongation: min. 20% (50mm gage length) Y/T: max. 0,90 for flattenend specimens max. 0,93 for non-flattened round bar Design temperature 0 C Ch-V notch energy Min. 36/ av C DWTT Min. 75% 0 C Chemical composition At discretion of supplier Table 2: Specified requirements for X100 in 1995 After completion of the first trials it was clear that X100 linepipe can be manufactured by UOE pipe mills. Further aspects now related to pipeline safety were added to the initial requirements in the second phase of X100. The focus of several research projects was directed towards identifying the necessary minimum impact toughness to guarantee fracture arrest. Several full scale burst tests were performed in order to validate the existing models for predicting crack arrest. The results revealed that the predictions were not reliably on the conservative side and even Charpy-V toughness above 270J were not sufficient to stop a long running crack. Therefore the application of crack arrestors built in the pipeline was considered as inevitable for designs utilising high pressures and high usage factors. The requirements for the mechanical properties of the TAP project are summarised in table 3. Properties Unit Specified Values Rt0.5 MPa Rm MPa Y/T - max El (A5) % min. 11 CVN C J min. 180 / av. 240 CVN C J min. 50 / av. 75 C % SA min. 75 / av. 85 Table 3: Mechanical property requirements of L690MC in transverse direction for TAP project As major high strength linepipe projects like the Alaska Highway Pipeline or the Mackenzie Project will cross arctic regions, where frost heave or ground movement will cause lateral forces to the linepipe, additional requirements in longitudinal direction are introduced. In such strain based design cases the deformation behaviour and the shape of the stress strain curve is of crucial importance. A minimum uniform elongation and maximum Y/T ratio in longitudinal direction have to be guaranteed, taking into account the ageing effect due to coating heat treatment. In order to take the existing anisotropy into Andreas Liessem Canberra JTM page 3 of

4 account and to facilitate girth weld overmatching, the minimum yield strength requirement is lowered in longitudinal direction compared to transverse direction. The uniform elongation is measurued at the maximum load. Following the definitions of the second edition of ISO3183 [3] the 0,2% offset yield strength has to be reported instead of the yield point at 0,5% total elongation. It should be noted that the definition of Rt0.5 as acceptance criteria for X100 as in CSA Z [5] adds around 15 MPa to the yield strength requirement for Rp0.2 as shown in Figure 2. The definition of Rt0.5 therefore discriminates against the high strength material as the work hardening potential is less exploited compared to other steels with lower yield strength level Stress [MPa] ,0 0,5 1,0 1,5 2,0 2,5 Strain [%] Figure 2: Difference between Rt0.5 and Rp0.2 yield point for different strength level Table 4 presents typical requirements for tensile properties of X100 under consideration for strainbased design [4]. Tensile Properties Transverse Longitudinal Rt0.5 [MPa] Info Info Rp0.2 [MPa] Rm [MPa] Y/T Max. 0,95 Max. 0,95 A (50mm) [%] Min. 13 Min. 19 Agt [%] Min. 5 Min. 5 Specimen type Round bar Strip Table 4: Current tensile test requirements for X100-strain based design As artificial crack arrestors are included in the design the requirements of Charpy-V toughness are set below the values initially considered necessary for crack arrest. However, sufficiently high toughness to prevent brittle fracture has to be achieved. ISO 3183 defines the minimum values against brittle fracture depending on pipe grade and outside diameter. For X100 with 48 outside diameter 68J minimum average is required. Company specific requirements actually request higher values in the range of J. In the Drop Weight Tear Test (DWTT) the specimen needs to exhibit a minimum shear area of 75% at the minimum design temperature. During the last years considerable progress has been made in convergence of standards for high strength linepipe steels. After years of intensive discussions between gas companies, manufacturers and certifying bodies the second edition of ISO3183 now covers grades up to and including X120. It Andreas Liessem Canberra JTM page 4 of

5 provides a broad and reasonable basis for the linepipe requirements in the case of design against internal pressure. Additional requirements for strain-based design are currently under evaluation. Existing safety factors for design should be defined in a way that reasonably balances pipeline reliability and manufacturing capability and avoids unnecessary conservatism. MANUFACTURING CHALLENGES Based on the discussions above, the main challenges for the manufacture of X100 linepipe can be summarised as follows: - Homogenous microstructure and properties, achieved by tightly controlled production parameters - Excellent weldability, achieved by controlling chemical composition and limiting carbon equivalent - Sufficient strength properties in narrow ranges, low Y/T and mimimum uniform elongation - High toughness in the base material to resist brittle fracture - Heat affected zone/weld seam properties providing high toughness, sufficient overmatching and limited softening - Excellent pipe geometry (ovality, straightness) in order to facilitate girth welding on site The combination of these partly competing and multifaceted requirements is the real challenge in the development of grades above X80, and requires in-depth expertise in order to develop advanced manufacturing technologies for plate making, pipe forming and longitudinal seam welding. Plate Manufacture The main challenge for steel and plate making is to consistently produce high strength plate with a composition as lean as possible in order to obtain excellent weldability and high toughness while still meeting the required strength properties in narrow ranges. It should be emphasized that some of these requirements are competing against each other. Measures which provide lower Y/T can harm the toughness level or the weldability. This combination of high requirements significantly narrows the usuable production windows. In Figure 3 typical engineering stress-strain curves are shown for different strength level. With increasing strength level uniform elongation is reduced whereas the Y/T ratio is raised Stress [MPa] Strain [%] Fig. 3: Typical engineering stress-strain curves for different strength levels A further example for this property competition is given in Figure 4. It describes how the finish cooling temperature (Tfc) influences tensile strength and Charpy-V notch toughness. Maintaining the minimum required tensile strength limits the achievable toughness level and vice versa [5]. The target values for the mechanical properties on plate need carefully be defined considering the cold deformation during pipe forming and expanding. Andreas Liessem Canberra JTM page 5 of

6 850 to meet TS min CVN 400 TS in MPa TS min for X100 TS CVN transv. at -30 C in J 725 Tfc 150 Fig. 4: Influence of finish cooling temperature on tensile strength and Charpy-V notch toughness The current development in the plate mills covers the wall thickness range from 12-25mm. Clean steel treatment provides the basis to achieve high toughness properties. Appropriate casting technique results in reduced segregation and keeps the low inclusion content achieved after secondary metallurgy process. The target values for the mechanical properties on plate need carefully be defined considering the cold deformation during pipe forming and expanding. The plates are thermomechanically rolled involving strong accelerated cooling (TMCP+ACC). With incresing cooling rate special attention has to be paid that cooling is well controlled in order to provide homogenous properties over the whole width and along the complete plate length from head to tail. Cold levelling or even hot levelling might be necessary in order to prevent or properly rectify the occurrence of plate waviness, which could affect pipe forming and eventually pipe geometry. The alloying concepts can be described as a microalloyed low carbon-manganese steel with well chosen combinations of Molybdenum, Nickel, Copper and Chromium. The carbon content should be limited to maximum 0,07% in order to achieve high toughness and to maintain good weldability. The carbon equivalent according IIW is typically below 0,50, the Pcm-formula does not reach values above 0,25. Fig. 5: Typical bainitic microstructure of X100 base material produced by TMCP Andreas Liessem Canberra JTM page 6 of

7 To produce the target ultra high strength and toughness, a very fine grain bainitic microstructure is the key. To obtain the desired X100 properties it is very important to understand precisely the morphology of the target bainitic microstructure and its formation. In addition the type and distribution of second phases need to be determined. Bainite can be considered as the most complicated microstructure in steel and difficult to characterise quantitatively [6]. Light-optical microscopy offers only limited possibilities for the characterisation of bainitic microstructures because the size of the microstructural features is close to the limit of resolution (~1 µm). A typical example of the microstructure of X100 base material that results from the thermo-mechanical controlled process with accelerated cooling is shown in the light-optical micrograph Figure 5. The microstructure consists predominantly of granular bainite with minor volume fractions of ferrite and martensite. Electron microscopy is a far more powerful tool to characterise the microstructure of bainitic steels. Using electron backscatter diffraction (EBSD), the crystal orientation can be measured locally by scanning the electron beam across a predefined region of interest. This makes it possible to quantify variations of the orientation with respect to neighbouring grains and even to quantify orientation gradients within grains. An example of such an EBSD mapping of X100 steel is shown in Figure 6. Based on such mappings, the microtexture can be analysed in terms of the distribution of misorientations of neighbouring grains and subgrains. The variation of fractions of displacively transformed or relaxed bainitic ferrite depends on TMCP parameters. The analysis of the misorientation distributions makes it possible to classify the bainite modification. Fig. 6: Inverse pole figure obtained from an EBSD mapping of a longitudinal section of an X100 sample Forming From the differences of the stress-strain behaviour shown in figures 2 and 3 it is obvious that pipe forming of a high strength steel like X100 considerably differs from that of a lower grade due to the greater elastic range. A big challenge in the manufacturing process of X100 pipes is therefore the spring-back that occurs during all forming steps. From plate-bending theory it is well-known, that the spring-back increases with increasing yield strength and increasing ratio of bending radius to plate thickness. This implies that in particular, thin-walled pipes (in terms of D/t-ratio) in combination with high yield strength show the highest spring-back. The spring-back in principle affects every forming step during UOE process, i.e. crimping, U-ing, O-ing and expansion. Already the crimping as the first forming step needs to be modified in order to minimize peaking which could have a detrimental effect on the later expanding process. Furthemore Andreas Liessem Canberra JTM page 7 of

8 spring-back after U-ing process is a particular problem because it has to be ensured that the formed plate can be inserted into the O-ing press. In Figure 7 the differences between plate shapes after U-ing for material X65 and X100 are shown. The same forming conditions have been used for both material grades. It becomes obvious, that the opening of the X100 plate is so high that it cannot be inserted into the O-ing press. Extensive finite element simulations have been performed to analyze the whole forming process for production of X100 pipes. In particular, a new design of the U-ing punch has been developed in order to cope with the high spring-back and to ensure that the plate can be inserted into the O-ing press. X100 X65 Fig. 7: Pipe shape after U-ing of X65 compared to X100 After welding the final shape is given to the pipe by mechanical expanding. The elastic spring back needs again to be considered and basically requires a higher expansion. The expansion rate needs to be defined exactly in order to achieve best geometrical tolerances. Most important for the line-up before girth welding is the out-of-roundness. Besides the pipe geometry the cold expanding affects the mechanical properties of the pipe material. Especially the stress-strain curve is affected as the mechanical expanding consumes a part of the deformation capacity of the pipe body and the work hardening reduces the uniform elongation and increases the Y/T ratio. As discussed before, competing properties require again a balanced selection of the process parameters and need to be considered in the definition of requirements. Longitudinal Seam Welding The longitudinal weld of large diameter linepipe is typically performed by submerged arc welding with one multi-wire pass from inside and a subsequent inter-penetrating pass from outside. The process is characterised by high heat input high dilution between parent and weld material. Due to its high heat input this process provides high productivity, which allows economical production. Therefore the following features need to be addressed for the longitudinal seam welding of high strength steels: - weld seam strength and hardness (overmatching) - Strength softening in the HAZ - Low HAZ toughness at the fusion line boundary The weld seam should overmatch the minimum yield strength of the parent material in order to minimize local strain concentration in the weld seam. The mechanism of grain refinement by thermomechanical processing can unfortunately not be used in order to improve strength and toughness. The low cooling rate caused by the high heat input, together with a dilution rate of approximately 65% therefore necessitate higher alloying of the weld seam compared to the parent Andreas Liessem Canberra JTM page 8 of

9 material. Welding wires providing a minimum yield strength of 690 MPa and simultaneously high toughness even at low temperatures can be regarded as a considerable challenge. Commercially such solid wires are not yet available and need to be developed. Alloying elements like Molybdenum, Nickel and Chromium play a dominant role in such developments. However elements that raise the strength level have a detrimental effect on toughness at that high heat input. It is therefore crucial to define the matching ratio very carefully. This is dependent upon whether the existing weld seam height is included in the definition of the required minimum strength for the weld metal. A limited metallurgical undermatching of 5% is compensated by the real weld seam height, as the expansion process proves for UOE pipes. Figure 8 shows the macrograph for the weld seam of X100 with 19,8mm wall thickness. The all weld metal yield point at 0,2% plastic offset is depicted in figure 9 for different welding consumables. It can be seen that typically the yield strength of the inside weld meets the minimum yield strength of 690 MPa and is higher compared to the outside weld layer because of the annealing effect caused by outside welding. In the heat affected zone a certain softening can be detected by hardness measurements. This softening is limited to a narrow band and the hardness is approximately 15% below the parent material in this area. The possibilities for avoiding this localised softening are limited as major changes in the metallurgical design or the welding process would have other undesired effects. Fig. 8: Macrograph of X100 submerged arc welded longitudinal seam 900 All weld metal Rp0.2 [MPa] Inside Outside SMYS - X100 Var. 1 Var. 2 Var. 3 Var. 4 Var. 5 Welding Variant Fig. 9: All weld metal yield point (Rp0,2) for different welding variants Andreas Liessem Canberra JTM page 9 of

10 In Figure 10 typical Charpy-V notch transisition curves are presented for X100 at different notch locations. The weld seam properties correspond to the variant 5 in figure 9, and reveal a slight undermatching of the outside weld seam. It can be seen that the weld seam toughness remains on a high level even at low temperatures such as -60 C. Whereas the detrimental effect of high heat input can be offset by proper alloying in the weld seam, the heat affected zone shows localised low toughness. The usual metallurgical measures to optimise HAZ toughness can only be applied in a limited manner as the base material properties have to be maintained. One possibility to optimise the HAZ properties is to reduce heat input by modifying the weld groove and the welding parameters. Reducing the weld cross section by narrowing the weld gap and cold wire addition is currently under investigation for submerged arc welding. However these measures soon reach their limits when the productivity starts to be affected. It is therefore important to investigate the significance of low HAZ toughness for the structural integrity of high strength linepipe. 250 Base Material Weld Material 50%HAZ FL+2 CVN Energy [J] Temperature [ C] Fig. 10: Charpy-V notch transition curves of X100 for different notch locations COST CONSIDERATIONS Beside construction, operating and project management the linepipe material constitutes a major part in the cost model of an onshore pipeline. The linepipe part lies in the range of 30-35% of the total project cost. Considerable savings are expected as high strength steels permits a gas pipeline for a given gas capacity to be built with either a smaller diameter or with reduced wall thickness compared to conventional steel grades. Beside the potential cost savings resulting from the reduced steel tonnage, further economical benefits are possible due to lower transportation costs to the construction side and lower welding costs (reduced number of passes, faster welding, fewer welding stations). Budgetary calculations performed 5 years ago considered a steel price difference of 20-30% between X70 and X100. However due to the extended economical growth of China and other emerging countries over the last years the prices for raw material like iron ore and alloying elements may be drastically increased. Figure 11 shows the price development of the alloyings important for the design of high strength steels. To assist comparison the values were set to 100 for uary Especially Molybdenum and Vanadium have reached historical highs. A recent press release predicts that the high level for Molybdenum will become the status quo as the growth of capital goods projects coupled with a booming oil industry increases the Molybdenum demand over the next three years [8]. A similar situation can be found for Nickel where demand exceeds the supply. Here the price per tonne has topped the $ mark and climbs further on. Andreas Liessem Canberra JTM page 10 of

11 Price ( = 100) Fe Nb Fe V Ni Fe Mo Cu Jul Jul Jul Jul Jul Jul Year Fig. 11: Price development of alloyings used for high strength steels ( = 100) It is evident that this development will eventually influence the material costs for high strength steels to a greater extent. A verification of the assumed savings due to the reduced steel tonnage is at least advisable under consideration of the technical requirements. However the linepipe costs are only one part of the total pipeline cost structure beside installation, transportation and operation. The answer as to which grade is the optimum for a given project needs to be determined by the gas companies and designers taking into account all potential savings connected with the use of high strength linepipe. CONCLUSIONS The International Energy Outlook 2006 (IEO2006) projects strong growth for worldwide energy demand over the decades. The use of high strength steels is considered as the best economical option to transport large gas volumes under high pressure from remote areas to the market. The technical development of large diameter linepipe has progressed over the last years and the basic technical requirements are meanwhile standardised in the second edition of ISO Pipeline to be laid under conditions that necessitates strain-based design will place additional requirements in longitudinal direction for the steel and pipe producer. The main challenges for the linepipe manufacturer are: - Homogenous microstructure and properties, achieved by tightly controlled production parameters - Excellent weldability, achieved by controlling chemical composition and limiting carbon equivalent - Sufficient strength properties in narrow ranges, low Y/T and minimum uniform elongation - High toughness in the base material to resist brittle fracture - Heat affected zone/weld seam properties providing high toughness, sufficient overmatching and limited softening - Excellent pipe geometry (ovality, straightness) in order to facilitate girth welding on site An additional and maybe the real challenge arises from the fact that the required properties are partly competing against one another, and measures that improve the one can harm another. It is therefore essential that gas companies, designers and manufacturers work together to balance these requirements and find the best option for both pipeline reliability and manufacture capability. The significant increase of alloying cost especially affects the high strength steels as they contain notable amounts of cost intensive alloying elements, particularly Molybdenum and Nickel. Compared to 2001 the alloying costs are currently three times higher and a decrease is not seen in the midterm period. However the linepipe costs are only one part of the total pipeline cost structure beside installation, transportation and operation. The answer as to which grade is the optimum for a given Andreas Liessem Canberra JTM page 11 of

12 project needs to be determined by the gas companies and designers taking into account all potential savings connected with the use of high strength linepipe. REFERENCES [1] International Energy Outlook 2006 Report DOE/EIA-0484 (2006) [2] Marchesani, F., Donati, E., Spinelli, C. M., Mannucci, G., Demofonti, G., Cabrini, M., Pastore, T.: The TAP Project, Proc. Super-High Strength Steels, 2-4 November 2005, Rome, Italy, Associazione Italiana di Metallurgia [3] Hammond, J.: Development of Standards and Specifications for High Strength Line Pipe, X100 Forum Pula, 6/7 April 2006 [4] X100 Pipeline Data Sheet, British Petroleum Company plc [5] V. Schwinn, P. Flüß, K.-H. Tacke, S. Zajac: Bainitic steel plates for X100 and X120 4th International Conference on Pipeline Technology May 9-13, 2004, Oostende, Belgium [6] Zajac, S., V. Schwinn, et al. (2005). Characterisation and Quantification of Complex Bainitic Microstructures in High and Ultra-High Strength Linepipe Steels. Proceedings of the International Conference on Microalloying for New Steel Processes and Applications, Donostia-San Sebastian 2005, Spain, Trans Tech Publications [7] Liessem, A., Hillenbrand, H.-G., Schwinn, V., Grimpe, F.:Manufacturing of X100 Pipes for the TAP Project, Paper IPC , Proceedings of 6th International Pipeline Conference, September 25-29, 2006, Calgary, Alberta, Canada [8] Meghann McDonnel, American Metal Market dated 8. March 2007 Andreas Liessem Canberra JTM page 12 of