GRADE 550 LINE PIPE PASSES TESTS FOR CANADIAN PROJECT
Keith E. W. Coulson, Russell J. Young
NOVA Corp. of Alberta
Calgary
Hiroaki Tsukamoto
NKK
Fukuyama, Japan
Tests by NOVA Corp. of Alberta, Calgary, on Grade 550 pipe have led to the world's first specified use of the high-strength material in a large-diameter, high-pressure natural-gas pipeline.
Use of the pipe allows significant material-cost savings by permitting thinner wall requirements. This savings is potentially enhanced by the fact that this higher strength pipe requires no significant changes to conventional methods of pipeline construction.
The NOVA project also employed a mechanized, pulsed, gas metal arc welding system for the first time on this grade of pipe.
STRUCTURAL LIMITS
Improvements in the efficiency and cost effectiveness of gas-transmission pipelines would be possible by increased gas flow. Such an increase can be accomplished either by use of larger-diameter pipelines or by increases in the systems' operating pressures.
In either case, the limits that could be achieved are restricted by practical, economic, and technological capabilities.
There has been a substantial increase in the strength properties of pipe steels over the last few decades, with the employment of Grade 483 pipe (API X70) being no longer uncommon.
For pipelines with high operating pressures, however, pipeline materials currently used would require excessive wall thickness and, along with related installation equipment, would be extremely expensive.
Conversely, larger pipeline diameters of up to 200 in. (nominal; 5,000 mm) may be possible but the practicability and economics of manufacturing and constructing such a line makes these extremely large-diameter lines unrealistic.
At present, gas-pipeline systems have generally been limited to diameters of 56 in. (1,422 mm) with specified minimum yield strengths (SMYS) of 483 MPa (70 ksi). With the current advances in steel and pipe-making technology, higher strength pipe-up to Grade 550 (API X80)-with good weldability characteristics and low-temperature toughness performance has become viable.123
Because the primary material cost for a pipeline system is directly related to the diameter and thickness of the pipe, the use of higher strength pipe would allow for design with thinner wall or smaller diameter pipe.
This in turn translates to a direct cost savings. For a given diameter and design operating pressure, a Grade 550 pipeline would allow for a reduction of the pipe thickness of approximately 12.5% when compared to a Grade 483 pipeline.
TEST PROGRAM
Before actual use of Grade 550 pipe, an extensive test program was undertaken by NOVA to ensure that the required material was, in fact, readily available and that conventional pipeline methods were applicable to this type of pipe.
Test pipes in sizes 24-44 in. with various wall thicknesses were supplied by seven different pipe mills from throughout the world. Mechanical testing of the pipe body and seam weld, as well as chemical analysis of the pipe body, was conducted at NOVA's Airdrie, Alta., laboratories.
The specified mechanical properties and the results obtained are shown in Table 1.
While the results from this test program demonstrated that Grade 550 pipe is available, it also indicated that only three of the pipe mills provided samples that successfully met all the required mechanical properties for Grade 550 pipe.
The weldability of the test pipes supplied was determined with a modified Welding Institute of Canada (WIC) restraint test .4 This test was used to determine the critical preheat temperatures required to prevent a cellulosic root bead (E41010-AWS 6010) from cracking with a degree of restraint in excess of that normally encountered during pipeline construction.
The acceptance criterion for this test was set at a maximum of 3% cracking with a critical preheat temperature of 75 C. or cooler. In all cases, the required critical preheat temperatures were found to comply with the specified requirements, and NOVA's standard preheat temperature of 100 C. was specified for all cellulosic welding.
In addition to verifying that Grade 550 material could be manufactured and supplied by the various pipe mills, NOVA also used the test pipes for the development of suitable field-welding procedures.
To reduce the potential problems with regards to field installation and welder acceptance of a new material, conventional pipeline welding techniques as well as some nonstandard pipeline welding processes were investigated.
In all cases, the weld procedure was evaluated according to the requirements of CSA Z184(5), and the testing included radiographic inspection of the completed weld joint as well as mechanical testing of the weld.
A combination cellulosic/low hydrogen process was initially tested. The root and hot passes utilized E48010 (AWS 7010) electrodes with a downhill progression. Fill and cap passes were completed with E69018 (AWS 10018) low-hydrogen electrodes with an uphill progression.
Although this procedure would not be typical for pipeline construction because of its low operating speed, it was considered suitable for the Grade 550 material. The procedure qualification trials and tests demonstrated the poor productivity, and the test welds also failed to meet the CSA Z184 guided bend-test requirements.
Because of these problems, the procedure was rejected and no further development of this process was considered.
To achieve improved productivity and to meet the desire to utilize conventional techniques, the qualification of a manual shielded-metalarc welding (SMAW) process with cellulosic electrodes and a downhill weld progression was investigated.
Root and hot passes were completed with E55010 (AWS 8010) electrodes, and fill and cap passes were made with modified E62010 (AWS 9010) electrodes. Again, welds were qualified in accordance with the CSA Z184 requirements.
While this cellulosic weld procedure provided acceptable results, the strength levels obtained were marginal. The possibility of failure because of mismatched weld-to-pipe strengths 6 required that other welding procedures and processes be considered to optimize the mechanical properties and the quality of the field welds produced.
Based on tests conducted by others, 1 3 a combination cellulosic and low-hydrogen procedure with a downhill progression was tested. This procedure combined the higher productivity of these processes with improved mechanical properties.
The root and hot pass electrodes (E48010-AWS 7010) provided reduced susceptibility to hydrogen-assisted cracking with superior operating characteristics resulting in freedom from internal undercutting.
The fill and cap pass electrodes (E62018-AWS 9018) provided the required productivity and mechanical properties. This procedure proved to be the best combination for manual SMAW and was eventually used for tie-in welds.
For large-diameter gas pipelines, the mechanized gas-metal-arc welding (GMAW) process has been used extensively because it provides high productivity with superior mechanical properties. The main limitation is that the equipment costs typically limit its application to larger diameter (24 in. and larger) and longer (typically 25 km or more) pipeline projects.
It is expected that Grade 550 pipe would be used where larger diameter and longer pipelines are required, and a GMAW procedure was therefore developed for this material.
Previously, GMAW procedures employed a short-circuiting process. NOVA, however, in conjunction with CRC-Evans, has developed a pulsed system for hot-pass welding only. The pulsed GMAW was developed to reduce the potential for lack of cross penetration and lack of penetration-type defects which are common to the process. At the same time, the pulsed arc is also successful in eliminating spatter.
Because of the susceptibility of the pulsed GMAW process to the effects of residual magnetism, it is unsuitable for the internal root pass.
CRC-Evans and NOVA have recently conducted development work in the fullpulsed GMAW process (that is, pulsed arc for hot, fill, and cap passes). The advantages of the pulsed arc on the hot pass are also evident on the other passes. At the same time, good mechanical properties and productivity are maintained. This process was therefore selected for use on the Grade 550 material.
The procedure was tested at CRC-Evans in Houston, and the consistency trials proved that defect-free welds could be produced with production rates equivalent to those achieved with a conventional GMAW process.
The test welds also exhibited yield strengths compatible with Grade 550 pipe and with excellent crack-tip-opening displacement (CTOD) toughness properties.
Another process that was considered suitable and practical was the use of flux-cored consumables. While these consumables could potentially provide the required quality, mechanical properties, and productivity, the unavailability of suitable flux-cored wires for Grade 550 pipe precluded any testing.
EMPRESS EAST CROSSOVER
Based on the results of the successful test program, appropriate specification requirements were established and competitive bids for the supply of Grade 550 pipe were obtained. The successful bidder was the Japanese mill, NKK-Fukuyama with Marubeni Canada Ltd., Calgary, acting as the agent.
The project selected for the Grade 550 pipe was the Empress East crossover in south eastern Alberta. It comprised approximately 2.5 km of 42-in. pipe with 10.6 mm and 16.9 mm W.T.
The design operating pressure for the project is 8,700 kPa (approximately 1,263 psi) and the resulting design factors are 0.8 and 0.5, for the thinner and thicker wall pipe, respectively.
While this project is relatively small, the terrain, design, and operating requirements allowed for the complete evaluation of this material for pipeline construction.
Pipe bending in the field was necessary, as was the use of various weld-joint designs because of the differences in the wall thicknesses and strengths of the matching pipeline components.
Fusion-bonded epoxy (FBE) coating was plant-applied at Shaw Pipe Protection's Regina, Sask., facilities in accordance with NOVA and CSA Z245.20(7) specifications (Fig. 1).
MANUFACTURE
In order to achieve the desired strength, low-temperature toughness, and weldability, the steel chemistry and plate rolling processes are critical. The steel chemistry contributes to the strength and toughness of the pipe and is the determining factor so far as the weldability is concerned.
- Chemical composition. Table 2 lists the chemistries of the Grade 550 pipe used on this project. In consideration of the rolling practices that were to be utilized, the thinner wall pipe was produced with a slightly richer chemistry than the heavy-wall pipe.
To get superior toughness and ductility, both in the pipe body and the seam weldment, such impurity elements in the steel as nitrogen, oxygen, sulfur, and phosphorous must be reduced.
New refining processes such as mechanical stirring method and R-H degassing were therefore applied to reduce these elements.
Furthermore, it is also important to reduce the possibility of center-line segregation to improve field weldability. For this purpose, "soft reduction" during continuous casting (minor slab-thickness reduction during steel solidification) was also applied.
With these treatments, superior clean steel containing < 100 ppm phosphorous, < 30 ppm sulfur, and approximately 40 ppm nitrogen were obtained.
Microalloying elements such as molybdenum (Mo), niobium (Nb), vanadium (V), and low carbon contents, combined with the selected plate-rolling practices, provide improved strength and low-temperature toughness. Table 2 shows the range of chemistries obtained for the two different thicknesses of pipe produced.
- Plate rolling. For the 10.6-mm W.T. pipe, a process known as "intensified controlled rolling" (ICR) was used. This process differs from a "normal" controlled rolling practice in that the reduction ratio under the recrystallization temperature is higher and the finish rolling temperature is lower.
This rolling practice produces a fine-grained microstructure and a high dislocation density for high strength and improved toughness.8
The 16.9-mm W.T. heavy-wall pipe was rolled with "online accelerated cooling" (OLAC).
The OLAC process features plate cooling after finish rolling within a specified temperature range and with a specific, controlled cooling rate. This process results in transformation hardening and grain refinement of the steel which in turn provides high strength and toughness.9 10
The primary reason for utilizing the two different plate-rolling methods was the potential difficulty in controlling the flatness of the thinner wall plate if OLAC was used.
TENSILE STRENGTH, TOUGHNESS
Fig. 2 shows the relationship between carbon equivalent (Ceq), as determined with the IIW formula, and the tensile strength of plates manufactured by the CR, IRC, and OLAC processes.
The tensile strength of the Nb-bearing plate manufactured by the CR process increases with the addition of Mo and V combined with the IRC or the OLAC processes.
The rectangular area shown in the middle portion of the figure indicates the target strength and optimum Ceq range for Grade 550 steel plates. This range was determined based on the strength, toughness, and weldability requirements of the pipe .4
The maximum target strength and minimum Ceq value were determined based on heat-affected zone (HAZ) softening of the seam weld. Production stability was also considered in establishing the minimum target strength.
The Ceq value has been restricted to less than 0.45 in consideration of HAZ toughness of the seam weld and HAZ hardness of the girth weld (Fig. 3)."
Table 3 shows the actual mechanical test results of the pipe. In all cases, the mechanical properties met the specified requirements of the pipe for this project.
STANDARD PRACTICES
Project construction commenced mid-September of last year with 0. J. Pipelines Inc., Edmonton, the contractor (Fig. 4).
Although this project represented the first specified use of Grade 550 pipe, all attempts were made to ensure that standard pipeline construction practices were followed and adhered to.
This included pipe handling, field bending, lowering-in, hydrostatic testing, and back filling. In all cases, no problems were encountered.
Only in welding and inspection were any special procedures followed.
Main line welding of both pipe thicknesses was performed with the mechanized, pulsed GMAW procedures listed in Table 4 (Fig. 5).
Because of the short length of the project, welder training time was limited and, for the main line welds, was combined with actual production. Weld productivity could not, therefore, be accurately determined because of this combination of training and production welding.
At the same time, the limited welder training resulted in initial relatively high repair rates: approximately 50% of the welds required some repair.
The defects encountered were primarily lack of fusion; the total length of the repairs in comparison to the total weld length was well below that typically encountered on pipeline projects. Repairs in accordance with CSA Z184 were, therefore, readily performed.
With increased experience with the full pulsed process, repair rates were found to decrease drastically. At the same time, an analysis of the welding equipment used showed that some additional development work could lead to improved performance.
The repairs required by CSA Z184 at the end of the project were found to be well within acceptable levels. (Approximately 14% of welds made on the last day required repair with repair lengths approximately 3-4% of the total length of welds made.)
Main line weld inspection was accomplished with X-ray radiography complemented by automatic ultrasonic inspection. While the specified defect acceptance criteria (CSA Z184) are based on radiographic inspection, injurious defects detected by the ultrasonic inspection were also repaired resulting in weld quality superior to that required by CSA. The ultrasonic inspection employed for this project also provided immediate feedback with regard to weld quality and allowed for the adjustment of the procedures and process during production welding.
The types of defects encountered were also more easily detected, categorized, and sized by the ultrasonic inspection. This capability, combined with CTOD testing of the weld, would have allowed for the assessment of any defects on a fitness-for-purpose basis as permitted by GSA Z184, Appendix K.
For this project, no fitness-for-purpose analysis was conducted because all injurious defects were repaired.
Welder training and qualifications for the manual SMAW procedure (Table 5) were conducted at NOVA's Airdrie service center. This process was used for tie-in and repair welds which were completed with only one rejectable repair as a result of porosity at a start location.
For tie-ins, the use of the vertical down, low-hydrogen electrodes proved to be comparable in terms of productivity as the usual cellulosic electrodes. Repair welding is usually performed with low hydrogen, vertical-up electrodes; the repair procedure used on this project proved considerably faster.
ACKNOWLEDGMENTS
The authors acknowledge the contributions of NKK and NOVA personnel involved in this project. In particular, the authors would like to thank A. Wong (NOVA, project engineering), A. Loyer (NOVA, design engineering), and Marubeni Canada, Calgary office.
REFERENCES
- Engelmann, H., et al., "First Use of Large-Diameter Pipes of the Steel GRS 550 TM (X80) in a HighPressure Gas Pipeline," Sonderdruck aus "3R International," 1986.
- Tamehiro, H., et al., "Properties of High-Toughness X80 Line Pipe Steels," International Symposium on Accelerated Cooling of Rolled Steel, 1987.
- Akao. K., et al., "Development of API X80 Grade Line Pipe with Excellent Toughness and Superior Weldability," Offshore and Arctic Pipelines. 1987.
- NOVA, "Specification for the Determination of Line Pipe Weldability Using the Modified WIC Test, for Girth Welds," Revision 2, 1989.
- Canadian Standards Association, "CAN/CSA-Z184-M86 Gas Pipeline Systems," 1986, Rexdale Blvd., Toronto.
- Denys, R., "Fracture Behavior of Large-Diameter Pipeline Girth
Welds: Effect of Weld Metal Strength," Draft Final Report to the American Gas Association on Project PR-202-922, 1990
- Canadian Standards Association. "CAN/CSA-Z245.20-M86 External Fusion Bond Epoxy Coated Steel Pipe," 1986.
- Kozasu, I., et al., "Hot Rolling as High-Temperature Thermo-Mechanical Process," Microalloying 75 Conference Proceedings, 1975, pp. 100-114.
- Tsukada, K., et al., "Application of On-line Accelerated Cooling (OLAC) to Steel Plate," I & SM, 1982, pp. 21-28.
- Abe, T., et al., "Role of Interrupted Accelerated Cooling and Microalloying of Weldable HSLA Steels," Metallurgy and Application of HSLA Steels Conference Proceedings, Beijing, 1985, pp. 103-111.
- Endo, S., et al., "Recent Design Concept for High Strength Line Pipe Steel Manufactured by Accelerated Cooling Process," 9th OMAE Conference Proceedings, Vol. V, Houston 1990, pp. 1-8.
Copyright 1991 Oil & Gas Journal. All Rights Reserved.