INOX 2010 NÚCLEO INOX

Orbital Welding of Duplex Stainless Steel Tubing and Pipe for Critical Offshore Applications

Barbara K. Henon, Ph.D. and Eng. Angel Brond
Arc Machines, Inc.

ABSTRACT
Extremely harsh offshore and subsea environments have mandated the development of improved grades of duplex stainless steels over the last decade or two. These newer materials have the ability to retain a good balance of austenite and ferrite and thereby toughness and corrosion resistance in the welded condition. However, in order to retain the desirable properties of these technically complex materials, special care and attention is required for the joining of tubing and pipe.

Orbital GTA welding with preprogrammed weld schedules provides maximum parametric control and repeatability of process and is preferred to manual welding of duplex stainless steel wherever practical. The fine control of heat input makes it possible to meet the highest quality specifications for phase balance, corrosion resistance and mechanical strength. A major advantage of orbital welding over manual welding is that once a procedure has been established for a particular duplex alloy, consistent welds which meet all of the requirements of the qualified test coupons can be achieved with a high degree of repeatability throughout a project.

The use of orbital GTA welding for subsea and offshore applications includes autogenous welding of small diameter 0.250 inch tubing, as well as tubing installations on subsea “Christmas Tree” well heads. Orbital GTA welding with filler metal has also been done successfully on small diameter super duplex 2507 (UNS 32750) tubing for subsea umbilical coils as well as for large diameter headers.

A recent example of orbital welding of larger diameter duplex stainless steel pipe was done for the 800,000 barrel P-52 Platform. Built in Angra-Rio and owned by Petrobras; this is the largest offshore platform in the world. The installer welded 22 inch Schedule 80 2205 (UNS 31803) duplex stainless steel pipe using two orbital welding power supplies and two full-function weld heads mounted on a single guide ring. The welds were accomplished with low heat input and the radiographic results were excellent. The index of repair over the six months of production welding was so outstanding that the fabricator received a special award from the Brasfels shipyard for production quality. This project will be described in depth.

This presentation will describe orbital welding procedures and fabrication techniques as well as welding equipment used for autogenous and filler metal applications. The importance of tube and pipe end preparation, joint design, purge gas composition and purging techniques will be discussed. Data comparing corrosion test results to ASTM A923 of welds on one inch diameter 2205 duplex tubing welded with and without the addition of filler material will be presented.

INTRODUCTION
As the development of engineered duplex stainless steel alloys for the offshore, subsea, and deepwater industries has advanced, so has the need for advanced welding technology for these materials. Orbital welding, which is defined as “automatic or machine welding of tubes or pipe in-place with the electrode rotating (or orbiting) around the work,” has been shown to offer many advantages compared to manual for welding the new generation of duplex stainless steels.

During the past 10-15 years a number of major companies have been successful using orbital GTA welding technology for welding duplex and super duplex stainless steels on a wide variety of offshore and subsea projects. The following examples show the range of applications and range of orbital welding equipment used for welding of duplex stainless steels:

In the early 1990’s, orbital welding technology was used by Cameron Forged Products Division of Cooper Ltd. (Great Britain) to fabricate a super duplex (UNS S32760) test manifold for Philips Petroleum11. A full-function track-mounted Model 15 weld head was used to join the larger diameter (8 inches/203.2 mm at the joint) heavy-wall (32 mm/1.259 inches), specially forged tees to form the manifold used for a high-pressure application on an offshore platform in the North Sea. (Figure 1.)


Figure 1. The Embla Test Header built by Cameron from forged super duplex tees using orbital GTA welding11. The Weld Procedure Specification (WPS) required preparation and examination of metallographs for sigma phase and ferrite counts. (Photos from Stainless Steel Europe, December, 1992)

Sandvik Chomutov Precision Tubes in the Czech Republic was one of the first companies to develop the combination of orbital welding technology with an advanced duplex grade, SAF 2507 super duplex (UNS 32507), for the fabrication of umbilical tubing used in today’s remote deepwater fields10. Arc Machines, Inc. Model 227 Power Supplies and Model 95 weld heads were used for this orbital wirefeed application on several small-bore tube sizes thus providing the ultra-high strength and corrosion resistance needed for wellhead control systems in enhanced oil and gas recovery systems. (Figure 2.)


Figure 2. Left: Sandvik orbital welding set-up with orbital welding power supply with cooling unit and orbital weld head. Center: orbitally welded lengths of super duplex tubing being wound onto large coils for umbilical tubing. 100% radiography of the orbital welds was performed. (Photos from Welding Design and Fabrication, 2001)10
Right: close up of Model 95 wire feed weld head on a similar duplex umbilical tube project. (Photo from Kvaerner)

In the late 1990s FMC Kongsberg (UK) performed autogenous welding of small-bore thin-wall duplex stainless steel tubing on numerous subsea “Christmas Tree” wellheads using a Model 227 power supply and a weld head designed for autogenous orbital welding.12 FMC developed and qualified welding procedures and required performance qualification of the welding operators. Welding personnel were trained to maintain consistent operating procedures (SOPs) throughout the project. (Figure 3.) FMC has performed similar welding projects in Houston, Texas, and is presently orbitally welding Christmas tree assemblies in Malaysia.


Figure 3. Left: Christmas tree tubing installed with autogenous orbital welding by FMC in Scotland. Right: An AMI Model 9AF-750 orbital weld head for autogenous welding on tubing and a Model 227 power supply on the tool box. This power supply can feed wire and operate either tube or pipe weld heads. (Photos from FMC and Arc Machines, Inc.)12

Acute Technological Services (ATS), based in Houston, Texas USA, made an early decision to specialize in welding engineering, consulting and fabrication of duplex stainless steels. ATS has developed highly specialized manual and automatic welding procedures that consistently maintain all relevant welding parameters within a range that yields optimum weldment properties and minimizes deleterious effects.8

These same companies and others continue to use orbital GTA welding today for their critical duplex stainless steel projects all over the world.

A recent example of orbital pipe welding of duplex stainless steel was the orbital welding of up to 22 inch schedule 80 2507 (UNS 32750) pipe at the Brasfelds Shipyard in Brazil in 2010. The installers used two AMI full-function track-mounted Model 15 orbital pipe weld heads for the 22 inch pipe and they also used a Model 79 open frame orbital weld head for welding smaller diameter pipe in the pipeshop.

MATERIAL AND METHODS
Material. Duplex stainless steels consisting of iron alloyed with chromium, nickel, molybdenum and nitrogen are engineered and manufactured to produce a material with a balanced phase microstructure of approximately 50% face centered cubic (fcc) austenite (γ) phase and 50% body-centered cubic ferrite(α) phase in the annealed condition. The balanced phase structure combines the favorable properties of austenitic and ferritic stainless steels when welded properly. During welding, solidification is primarily as ferrite which then transforms partially to austenite as the material cools. However, if the cooling occurs too quickly, the desired transformation to austenite fails to reach base metal levels. If cooling is too slow, deleterious phases such as sigma (σ) chi (χ) and others will form in the temperature range of 475-955°C (887-1750°) and the material may become embrittled. Precise control of welding parameters is required to consistently produce the right amount of heat input into the weld. This must be determined as part of the WPS.

While a balanced microstructure of 50% ferrite to 50% austenite is considered ideal, a range of 35% to 60% ferrite after welding is generally considered sufficient to maintain the favorable properties of the base metal.14 A full solution anneal after welding, which is impractical in the field, would be required to restore the base metal phase balance.

The alloying elements nickel and nitrogen promote the formation of austenite during welding while chromium and molybdenum, which enhance corrosion resistance, promote the formation of ferrite during welding. Duplex grades reflect the degree of alloying, with super duplex more highly alloyed. Super duplexes are those with a pitting resistance equivalent greater than 40 according to the formula: PREN = Cr + 3.3Mo + 16N where Cr is the percentage of chromium, 3.3 times the percentage of molybdenum and 16 times the percentage of nitrogen in the alloy. Tungsten (W) is sometimes used in the calculation of this formula.

Weld procedure qualification of duplex materials usually requires corrosion testing of the weldments. Ferrite counts, either point counts from sections which are destructive tests, or ferritscope measurements which can be done on tubing in the field or between passes on pipe welds are commonly made. The ferritscope does not work on very small diameter tubing as the curve of the surface does not permit the positioning of the instrument. Duplex stainless steel has a smaller HAZ and thus inspectors are more likely to miss pockets of high ferrite which is detrimental. For weldments to be placed in low temperature service, low temperature Charpy impact testing may be required as a test for ductility.

Equipment. For orbital GTA welding there are basically two types of equipment: equipment for autogenous welding used to weld tube, and pipe welding equipment with the capability of adding filler wire to the weld. In general, orbital tube welding on thin-wall material is an autogenous process on a square butt end preparation completed in a single pass, while pipe welding on thicker-wall material is normally done with a modified “J” preparation and the addition of wire requiring multiple passes. However, single pass square butt welding with wire feed is commonly done on small bore tube and pipe for subsea umbilical coils.

Orbital tube welding equipment. Orbital tube welding is a nearly automatic process in which the operator installs the tubing or components to be welded into an enclosed weld head, selects the appropriate welding program or schedule from the power supply memory, sets the ID purge flow and initiates the weld sequence. The power supply controls weld parameters which include Prepurge Time, a timed Rotation (travel) Delay before the arc strikes, Primary and Background Amperage, Pulse Times, Rotation Speed of the electrode (RPM), Level Time or Position for any number of levels, (usually four) followed by a timed Downslope and a timed shield gas Postpurge. There is no operator intervention during the weld sequence in which the electrode and, thus the arc, complete a single pass weld with full penetration. Each weld head can accommodate a range of tube sizes with the smallest head welding a maximum OD of 0.0250 inch and the largest head a maximum of 6.625 inches. Water cooling of weld heads is strongly recommended, especially for wall thicknesses greater than 0.049 inch or for high production welding.

Orbital tube weld heads are enclosed to form a chamber of inert gas during the weld that provides optimal protection of the weld surface from discoloration. Purging of the ID is not typically controlled by the power supply but usually must be set up and operated by the welding operator or technician. The Force Institute in Denmark has investigated the correlation between weld discoloration and loss of corrosion resistance in duplex and austenitic stainless steel. Duplex stainless steel has been shown to be even more susceptible to loss of corrosion resistance from heat tint oxidation than austenitic stainless steel so protection from oxidation is essential6. The end preparation for tube welding is typically a butt weld with the ends precision squared by machine.

Techniques for orbital tube welding: While millions of autogenous orbital welds have been performed on 316L for semiconductor process gas lines and biopharmaceutical process piping, autogenous orbital welds on duplex and super duplex materials require special attention. For autogenous orbital welding, Acute Technological Services (ATS) explored the use of shield gas containing 10% helium, 88% argon and 2% nitrogen to provide lower ferrite counts and better penetration7. ATS guaranteed ferrite levels of 50-60% on autogenous welds done with this gas mixture. ATS also began the use of I.D. pressure balancing to gain better control of the weld bead profile.

Insert rings overalloyed with nickel to promote the formation of austenite can be used to control ferrite levels when welding with equipment designed for autogenous welding7. Electrode travel in STEP mode in which the electrode remains stationary during the high current pulse and moves during the Background Pulse can be used for autogenous welds on heavier wall tubing and small bore pipe to provide good penetration and to control the weld profile.

Orbital Pipe welding equipment. Orbital pipe welding power supplies generally provide higher welding current than tube welding power supplies and have the ability to feed wire into the weld. In addition to controlling the parameters listed above for tube welding, they may also have arc length control (AVC) and controls for torch oscillation for weaving the torch back and forth across the weld seam. For both tube and pipe systems, the weld program or schedule is stored in the memory of the microprocessor controlled power supply, but with pipe welding power supplies, weld parameters such as AVC, welding current and torch centering may be adjusted or modified by the welding operator during welding.

There are several types of pipe weld heads. The most basic is an open-frame head (AMI Model 95) that clamps onto the pipe. The Model 95 weld head is able to execute basic weld parameters, can feed wire, but has no electronic AVC or oscillation function. Arc gap is controlled by a follower. The Model 79 is a similar open-frame head which also has AVC and oscillation functions in addition to wire feed. The power supplies used with these heads must also provide control for these functions. Also available are large full-function track-mounted heads such as the AMI Model 15 for welding pipe from 4 inches nominal diameter and larger. These heads move around the weld joint on a track while the pipe remains in place. Other heads for limited clearance are used for boiler tubes and heads and torches exist for a large variety of applications.

Orbital Pipe Welding at Brasfels Shipyard
In Brazil, installers at the Brasfels Shipyard used two AMI full-function track-mounted Model 15 orbital weld heads for welding up to 22 inch schedule 80 2507 (UNS 32750) duplex stainless steel pipe. They also used a Model 79 open frame head for welding smaller diameters up to 4 inch pipe in the pipeshop. At Brasfels, there were two different joint configurations, pipe to pipe using a modified J end preparation and pipe to fitting which was a standard V groove with a 37 ½ ° bevel. The preferred end preparation for pipe welding up to about 0.500 inch wall thickness is the modified-J preparation with a land of about 0.065 to 0.095 inches. The land extension may be adjusted depending on the material. It should be wide enough to prevent the weld pool from climbing the side walls of the joint. Adjustments of the end prep may be part of the WPS which can vary with the material and the application. Precision machining is necessary to assure repeatability of weld end preparation. The fittings at Brasfels were received with a 37 ½ ° end prep which could not be machined or modified.

The PQR reflects the distribution of weld sizes and joint configurations for fully orbital welds or manual root passes combined with orbital welding. Pipe to elbow or tee fittings welds were done with a manual root followed by orbital filler passes, while pipe-to-pipe welds with a J end preparation (Figure 5.) were completely orbitally welded. In the pipeshop the pipe was segregated by ODs larger and smaller than 4 inches. The larger pipe was welded with two Model 15 weld heads mounted on the same track, while the Model 79 weld heads were used for the smaller pipe diameters.


Figure 4. Left: Detail from Brasfels’ PQR showing separation of pipe by sizes over 4 inches and 4 inches and less. It also shows manual root passes for pipe to fitting welds and complete orbital welds for pipe to pipe. Right: A modified “J” end preparation is recommended for orbital welding of pipe.

Arc Machines employee, Eng. Angel Brond, worked closely with Brasfels to develop their PQR according to the requirements for limiting the heat input in duplex stainless steel welds listed in Offshore Standard DNV-OS-F101, Submarine Pipeline Systems, October 2007, App. C4. This standard requires the control of heat input to avoid detrimental weld cooling rates. In order to achieve optimum control of heat input they recommend faster welding times and higher welding currents. Stringer beads are recommended to ensure a constant heat input and any weaving of the weld bead was limited to a maximum of 3 times the filler wire or electrode diameter. In this case the wire diameter was 0.035 inches supplied by Sandvik. The heat input for girth welds was limited to a range of 0.5 – 1.8 kJ/mm, with the lower end of the range specified for thinner wall pipe.

Additionally heat input minimum and maximum in both root and fill passes were recommended on the PQR. The heat input range on their PQR was listed separately on each pass for welds up to 4 inches and welds over 4 inches OD. The Sandvik Steel brochure Stainles Steel Products for Oil and Gas Production S-133 EWG August 199716 also recommends the use of stringer beads with a maximum of 1 kJ/mm (25 kJ/inch) with weld current maximized to 100-120 Amps for TIG/GTA welding. Brasfels used a travel speed of 4-5 inches per minute (ipm) on fill passes and 2.5 ipm on the cover pass which was slower due to oscillation. This procedure is different from standard practices recommended by AMI for orbital welding of stainless steel alloys in which wider oscillation is used.

Appendix C of DNV-OS-F101 required that welding personnel be qualified to ISO 14732 and EN 1418. Qualification of welding operators was done by Brasfels with AMI consulting on site. As specified by this standard, the qualification tests were done on the actual equipment and premises that were used during production welding.

An additional standard DNV-RP-F112 Recommended Practice - Design of Duplex Stainless Steel Subsea Equipment Exposed to Cathodic Protection October 20085 was used as a Recommended Practice to asses the quality and repeatability of the ferrite and austenite composition of the base metal before welding and after welding.

NORSOK M 601 Standard, Edition 5, April 200815 is also referenced with respect to 4.4.4 Heat Input and 5.1 Welding Requirements. The maximum variation allowed in heat input is +/- 15%. Table A.3 lists acceptance criteria for welds including the amount of weld bead reinforcement or internal protrusion. A normative annex was added in this edition with color photos for acceptable oxidations/coloration on the I.D of pipes. Color ranges from no color (good) to heavy black, brown and blue discoloration which is unacceptable. Loss of corrosion resistance has been shown to be proportional to the amount of color.

NORSOK M 601 requires the use of filler metal in root passes of Type 25Cr duplex and Ni-alloys for seawater service. Filler metal on 4 inch 2 lbs spools was supplied by Bohler.

A separate welding procedure (WPS) was required for repair welds. Norsok Standard M-601 allows only one attempt at repair in the same area. Re-welding must include complete removal of the original weld and HAZ.

RESULTS
Comparing autogenous orbital welding with filler wire welds on 1 inch OD duplex stainless steel tube for bioprocess applications:
In 2008, Arc Machines, Inc., in a Company funded weld research project, welded 1 inch OD (25.4 mm) 0.061 inch (1.55mm) wall S32205 tubing in order to compare autogenous welds with welds with filler to determine acceptability for bioprocess piping applications according to the ASME Bioprocessing Equipment (BPE) Standard9. The shielding gas was argon with 2% nitrogen.

Orbital welds on 1 inch OD tubing with and without the addition of filler wire were tested to ASTM A923 Parts A and C. As expected the ferrite numbers were lower for the wire feed welds and no sigma phase was seen on any of the welds on micrographs according to Part A. ASTM A923 Method C1 is a corrosion test with a corrosion rate maximum allowable of 10 mdd. Both autogenous and filler welds passed the requirement, but the filler metal weld had a lower corrosion rate of 5.81 Mdd compared to 7.49 Mdd for the autogenous welds. The ASME Bioprocessing Equipment (BPE) Standard now requires the use of appropriate filler wire or insert rings when welding 2205 for biopharmaceutical applications. The BPE Committee members agreed that although the autogenous welds passed the required tests, welds with overalloyed filler wire or insert rings had superior corrosion resistance. Since insert rings of the same chemical composition as the recommended filler wire are not generally available, BPE specified insert rings of UNS N06022, a nickel-based alloy, for welding S32205 duplex stainless steel with equipment designed for autogenous orbital welding.

Brasfels welding of duplex pipe
The Brasfels Shipyard was very successful in orbital welding of all of their sizes of duplex stainless steel pipe. Their welding operation lasted for 1-1/2 years welding two shifts per day. The results of radiographic examination was excellent with a very low reject rate. They received an award from Petrobras for their excellent welding.


Figure 5. Left: Orbital welding of 2507 (UNS 32750) 20 inch duplex pipe with the AMI Model 15 and 4 inch pipe with the Model 79 weld heads in the prefabrication environment in the pipeshop at the Brasfels Shipyard in Brazil.
Right: Some orbital welds on pipe and fittings for the Petrobras P-52 Platform. Photos from Arc Machines, Inc.

DISCUSSION
In deepwater installations, defined as over 1,000 feet or more beneath the surface, materials are expected to perform under higher pressures (15,000 psi) and to withstand higher temperatures (300° Fahrenheit) than ever before3. To satisfy the requirements for deepwater applications, the use of stainless steels, especially duplex and super duplex have become nearly indispensible. Since proper welding of these materials is essential for maintaining their corrosion resistance and mechanical properties it is absolutely critical that good welding procedures be developed and carried out in a repeatable manner throughout a project.

Each application of duplex and super duplex material is unique and weld procedures must be developed accordingly. The steel tube umbilical must be manufactured to meet the requirements of the specific subsea application which varies with environmental conditions. Material selection and welding procedures must be determined independently for each individual project. For example, in the early 1990s FMC initially had problems welding Christmas tree tubing. They worked on their procedures and educated their welding personnel until they could achieve consistently good welds. They ended up with an acceptable overall reject rate with some Christmas trees having zero rejects and they are continuing with this work today.

It is generally agreed that duplex stainless steels benefit from the addition of filler metal overalloyed in nickel and nitrogen. However, for some small bore tubing, acceptable ferrite counts and corrosion resistance have been achieved with autogenous welding. This has been accomplished by the use of shield gas containing helium, argon and 2% nitrogen. Autogenous manual welding of duplex stainless steel is not recommended. Arc Machines’ results with the 1 inch OD biopharmaceutical tubing indicated that acceptable results were possible with autogenous orbital welding, but that definite advantages were seen with the use of the proper filler wire.

Orbital welding of duplex stainless steel for subsea umbilical tubing is continuing. Several manufacturers of coiled tubing have told AMI, that this would not be possible without the use of orbital welding. Umbilical tubing is typically welded with a Model 95 or a Model 79 weld head feeding wire. AMI has recently been active in assisting with the development of PQR/WPS for umbilical tubing in Brazil.

Petrobras in Brazil is a major player in this industry. The P-52 platform, recently commissioned, is largest in the world. Brasfelds Shipyard was given an award by Petrobras for their excellence. Weld criteria for duplex stainless steels must include ferrite counts or evaluation of the mechanical properties of the weldment since a good looking weld alone does not assure a good weld. A weld can have a good appearance while still have unacceptable ferrite numbers or deleterious phases that result in embrittlement. In order to accomplish repeatable welds with all of the favorable properties of the qualification welds, there must be proper and repeatable end preparation, joint configuration, purging techniques, and operator training. The orbital welding of duplex stainless steel on this installation had such a high degree of productivity that Brasfels elected to weld their carbon steel pipe with the orbital equipment.

Installers must be aware of the unique properties of the material and to understand the need for strict adherence to the qualified weld procedures (WPS). Precise, consistent control of heat input is essential. In addition, purging with inert gas is even more critical for duplex than for austenitic stainless steels for the retention of corrosion resistance. The recommendations for cleaning, end-preparation, welding environment, purging for manual welding of duplex stainless steels apply to orbitally welded duplex stainless steel as well and the reverse is also true.

The PQR for the Petrobras duplex stainless steel pipe made specific requirements for welding of duplex stainless steel, as outlined in the Sandvik and DNV Standards that would be very difficult to achieve by manual welding. The welding had to be done with a low heat input with a maximum of 0.5-2.5 kJ/mm (10-65 kJ/inch). The requirement for stringer beads and limitations on the amount of torch oscillation were instrumental in minimizing the heat input into these sensitive materials. In order to comply with these requirements, a higher travel speed than usual was recommended. Compliance with these requirements virtually demanded the use of mechanized or orbital welding equipment.

Orbital welding with a proven WPS is the most reliable method for welding of duplex alloys. Unfortunately, installing contractors using manual welders working in the pipeshop or field environments are not always able to consistently meet the same quality requirements as those of the qualification welds that were done under ideal conditions. There have been several occasions when manually welded duplex and super duplex weldments have failed in service. It should be apparent that working in the deepwater environment drives up cost of repairing welds and weld failures may be catastrophic. However, there is very little in the welding literature to document the causes and effects of weld failures.

In an article in Stainless Steel World by Van Wijngaarden and Chater on CalEnergy’s use of corrosion resistant alloys they stated “With failures we reach a topic that is sometimes overlooked: welding17.” The authors interviewed Project Manager for CalEnergy, George Furmanski who indicated that in the United States there are not many companies that can provide proper welding specialists or welding equipment. Mr. Furanski is quoted as saying “The majority of the stress corrosion cracking failures that we have seen occurred in the weld or the heat-affected zone. We therefore now tend to favor companies that use fully automatic welding machines. Our biggest technical challenge right now is to find people who can weld 2507 duplex.”

The use of duplex stainless steels for deepwater applications is increasing worldwide and this trend likely to continue as most new oil and gas discoveries are in deepwater fields. Michael Hayes of Acute Technological Services, Inc. estimates that presently about 60% of umbilical tubing for subsea applications is joined by orbital GTA welding, while only about 20-25% of heavier wall duplex and super duplex pipe are joined with this technology. Orbital welding is becoming more and more indispensible for welding these critical materials.

CONCLUSIONS

1. Orbital GTA welding is uniquely suited for the joining of advanced duplex stainless steel materials due to the precise control of welding parameters.

2. A wide range of orbital GTA welding equipment is available for welding of small diameter, thin-wall tubing to large diameter heavy wall pipe and all sizes in between.

3. Whether the alloy is duplex or superduplex and, whether the size is small bore or large diameter, these materials and applications have in common the requirement for welding procedures that are optimized for the specific material. Precise control of heat input, shield gas composition and filler metal chemistry are critical for achieving weldments with balanced phase microstructure and minimal deleterious phases so as to retain the favorable mechanical properties and corrosion resistance that were designed into the alloy.

4. Installers of duplex stainless steel pipe must understand the properties of these complex materials and welding operators and technicians must be aware of all of the requirements of the WPS in order to carry out the weld procedures consistently from joint-to-joint with a high degree of repeatability throughout a project.

5. Companies that began orbital GTA welding of duplex stainless steels for offshore applications in the early 1990’s are still using this technology and have expanded their operations worldwide.

6. The recent installation of pipe sizes up to 22 inch schedule 80 2205 duplex pipe with orbital GTA pipe welding equipment at the Brasfels Shipyard was highly successful. The fabricator received an award from the Shipyard for completing the welding with a very low index of repair. This installer was clearly aware and focused on all of the details of the WPS.

7. Orbitally welded duplex and superduplex stainless steels will be critical in expanding the exploration and recovery of gas and oil from deepwater fields.

ACKNOWLEDGEMENTS
David Just, Arc Machines, Inc. 10500 Orbital Way, Pacoima, California 91331 USA www.arcmachines.com
Michael D. Hayes, Acute Technological Services, Inc. 11925 Brittmoore Park Drive, Houston, Texas, 77041 USA www.acutetechserv.com

BIBLIOGRAPHIC REFERENCES

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2. AWS D10.18M/D10.18:2008 Guide for welding ferritic/austenitic duplex stainless steel piping and tubing. American Welding Society, 550 N.W. LeJeune Road, Miami, FL 33126 United States

3. CHATER, J. Mount Everest in the sea: how duplex stainless steels are transforming the offshore
industry. Stainless Steel World, June, 2007

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5. DNV-RP-F112 Recommended Practice - Design of Duplex Stainless Steel Subsea Equipment Exposed to Cathodic Protection October 2008

6. HANSEN, J.V. Influence of residual oxygen on the welding result. FORCE Institute, March, 1997

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8. HAYES, M.D. and B.K. HENON. Automatic orbital GTA welding of duplex stainless steels for critical piping applications. Stainless Steel World, June, 1993

9. HENON, B.K. Considerations for orbital welding of corrosion resistant materials to the ASME Bioprocessing Equipment (BPE) Standard. Presented at the Stainless Steel America 2008 Conference, Houston, Texas, 2008

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11. HENON, B.K. Orbital welding of super duplex header. Stainless Steel Europe, December, 1992

12. HENON, B.K. Orbital welding of Christmas tree assemblies for the Terra Nova Project. Development of SOPs for autogenous orbital welding of 2205 duplex and 316 stainless steel tubing www.arcmachines.com, Applications: Tube and Pipe Welding, 2001

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14. MESSER, B., V. OPREA, A. WRIGHT. Duplex stainless steel welding: best practices* Stainless Steel World, December, 2007

15. NORSOK STANDARD M-601 Welding and inspection of piping. Edition 5, April, 2008

16. SANDVIK STEEL. Stainless Steel Products for Oil and Gas Production. S-133 EWG August 1997

17. VAN WIJNGAARDEN, M. and J. CHATER. CalEnergy goes for duplex. Stainless Steel World, October, 2006