Automation Allows Production of Long-Length Tubing
Automation Allows Production of Long-Length Tubing
Automated orbital welding offers the critical step to join many single lengths of seamless tubing together before winding into coils for a variety of subsea applications.
Until recently, mills produced either single lengths of straight tubing or wound single lengths into small coils. Orbital welding made it practical to produce tubing in long lengths. Its repeatability makes it possible to consistently achieve a high level of weld quality while maintaining the mechanical properties and corrosion resistance of the tubing. These advantages make long-coil technology feasible.
Long lengths of tubing can reduce installation costs. Fewer fittings reduce the risk of leaks and installation time. Long lengths are easier to install, particularly for vertical applications. In addition, the long lengths allow the end-user to cut the tubing to the required length virtually eliminating the scrap that occurs with shorter tube lengths. Finally, the larger coil size makes shipping more efficient.
AB Sandvik Steel, based in Sweden, bought a plant in Chomutov, The Czech Republic, in 1994 to produce coiled duplex tubing for subsea applications. The facility has manufactured tubing since 1870.
The long-coil project began at Sandvik Chomutov Precision Tubes in June of 1996. Equipment purchases included three Arc Machines, Inc. Model 227 microprocessor-controlled orbital welding power supplies, with cooling units to circulate water through the cables to cool the weld heads, and three Model 95-1500 weld heads. The equipment welds tubing of 30 mm and below.
The plant ships completed coils to Norway to use in hydraulic umbilical tubing for subsea use. The umbilical tubing provides the final communication link between the platform and control valves. Other tubing, used as the center tube, carries methanol for methanol injection. Final destinations for these products are the oil and gas fields of the North Sea and the Gulf of Mexico.
Material and weld procedure
At Sandvik Chomutov single lengths of duplex stainless steel tubing are orbitally-welded together and wound onto huge coils for use by the offshore oil and gas industry.
Sandvik Chomutov orbital weld takes 7 to 10 minutes including preparation and purging, plus 4 minutes to X-ray. The X-ray images of each weld verify full penetration, no porosity, and no undercutting.
Model 95-1500 orbital weld head from Arc Machines, Inc. is suspended on a cable for production welding on the long coiled tubing.
The material selected for the long coil project was SAF 2507 super duplex stainless steel (UNS 32750). Duplex stainless steels have a balanced phase structure of approximately 50 percent austenite and 50 percent ferrite and combine the stress corrosion resistance of ferritic materials with the ductility of austenitics. They have high mechanical strength suitable for high-pressure applications and excellent corrosion resistance to chlorides and other chemicals that makes them highly suitable for service in the marine environment. SAF 2507 is a super duplex that means that it has a pitting resistance equivalent (PRE) of greater than 40.
The 15.7 mm o.d. seamless tubing has an 1.5 mm wall thickness. The firm plans to add an orbital welding system for 11.5 mm o.d. and then one for 9.5 mm o.d. tubing.
Alternate materials considered for this application were plastic tubing and coated carbon steel. The carbon steel would have had a 20 year projected lifetime but less corrosion resistance than the duplex. Plastic tubing is not suitable for such high pressure applications and could not pass hydrotests at 1,000 bar.
Obital Welding Advantages
The chemical composition of duplex determines to a large extent the phase balance of the welds. Rapid cooling can make the phase balance shift to a too high ferrite content. Using a filler material overalloyed in nickel adjusts for the shift.
The use of orbital welding makes it possible to develop weld procedures, evaluate the phase composition of sample welds, and exactly repeat the same weld parameters that produced the favorable results. This is not the case with manual welding during which the heat input varies from weld to weld with unpredictable effects on the phase balance and a greater likelihood of sigma formation.
Sandvik recommends the use of filler wire for welding SAF 2507 material. Chumotov uses Sandvik wire from Sweden that has a chemical composition of 25 percent chromium, 10 percent nickel, 4 percent molybdenum and a carbon content of below 0.02 percent. The low carbon content minimizes the formation of carbides. The wire was overalloyed in nickel compared to the tubing that had a nickel content of 7 percent to promote the formation of weld metal with a balanced phase structure. The tubing also has a carbon content of less than 0.02 percent and contained 25 percent chromium, 4 percent molybdenum and 0.3 percent nitrogen with the balance being iron. Specifications call for low impurity levels. The tubing and filler materials met the requirements for certification to ANSI/ASME B31.3 code for pressure piping.
Tests show that using nitrogen as an alloying element restores the austenite-ferrite phase balance more rapidly during cooling from welding temperatures. This effect of nitrogen results in a significant improvement in corrosion resistance, especially to pitting.
The Model 95-1500 comes with an external wire feeder. Its microprocessor program adjusts the wire feed rate in synchronization with the primary and background current pulses to produce a smooth, even weld.
A good and consistent end-preparation is essential for achieving repeatability of the welding process. Operators prepare square butt joints with a mechanical tube squaring machine. They remove burrs to maintain a consistent wall thickness and to assure a good fitup without a gap between the ends when installed in the orbital weld head.
The production line feeds straight lengths of tubing into the orbital welding station.
The orbital weld head hangs upside down from a cable and engages the two tubing ends to make the weld. The weld head has a scissor grip, which adjusts to different tube o.d. sizes, to hold the tube faces together. After purging the joint with nitrogen, the equipment supplies a 20 He-77.8 Ar-2.2 N shielding gas to the torch. An arc strikes between the tungsten electrode and the weld joint, and the electrode rotates around the joint to complete the weld. Travel speed is typically 125 mm/min.
The firm's goal was 1,000 welds per week from the two orbital welding machines. Each weld takes 7 to 10 minutes including preparation and purging, plus 4 minutes to X-ray. The X-ray images of each weld verify full penetration, no porosity, and no undercutting.
A local independent testing laboratory qualified the first orbitally-welded test welds in December 1996.
The stringent requirements for this application included non-destructive testing, such as radiography, and destructive tests including bend and tensile tests. Bend and tensile tests demonstrate that the weld meets the minimal tensile strength specified for the material and that the weld is ductile and bends without cracking. The firm met ASTM material specifications, Norwegian specifications, and ASME B31.3 Process Piping Specifications.
Accelerated corrosion testing of weldments helps to predict how the welded material will resist corrosion in a particular type of service environment. The lab determinated pitting corrosion resistance according to ASTM G-48A. Technicians placed the welded sample in a ferric chloride solution at a temperature below the specified critical pitting temperature (CPT) for the material for a set period of time, typically 24 hours. Then they check for weight loss and visible signs of corrosion. If no corrosion is detected, they return the sample to the solution and increase the temperature by 5 C and repeat the process until observing measurable corrosion. The temperature at which this occurs is the critical pitting temperature.
The lab did Vickers hardness testing of the weld and heat-affected zone on the i.d. and o.d. of the tubing. Each size of tubing required a separate Weld Procedure Specification (WPS) and Procedure Qualification Reports (PQRs) for ASME and to qualify to the Norwegian standards, NORSOK. Sandvik also qualified its procedures to ISO 9001.
Digital information about each weld feeds to an on-line computer supplied by the European Office of Arc Machines, Inc.
This quality control information includes the welding operator number, heat and lot number of the two tubes being welded, weld number, and results of the x-ray inspection.
Acknowledgments: The author acknowledges the technical assistance of the following people from Sandvik Chumotov Precision Tubes: Anders Sjoden, quality control manager; Tomas Jelinowicz, production manager; and Per Eklund, welding engineer. Photography by Milan Surik of foto-porst, Audio- Video- Electronik, Chomutov, Czech Republic.
Barbara K. Henon, Ph.D., is Manager of Technical Publications for Arc Machines, Inc.,10500 Orbital Way, Pacoima, California 91331. Phone 818-896-9556, fax 818-890-3724.
Reprinted from Welding Design and Fabrication, May 2001