Pipeline Industry Looks to New Processes for Mechanized Weld Quality
In 2000, more than 149,665km (93,000 miles) of pipeline construction were underway, planned, or actively under study worldwide. This consisted of gas, crude oil, refined products, as well as offshore pipelines. 29,413km (18,277 miles) were actually laid in 1999 with a 2000 forecast slightly higher than this.
Pipeline in Dubai desert
By far, the Shielded Metal Arc (SMAW) remains the most common welding process, using cellulosic electrodes run in the vertical down direction. This electrode type has advantages as it generates a significant amount of shielding gas in use and produces significant arc force for better root pass and puddle control at high surface speeds. In recent years, there has been a move by transmission pipeline operators to design new lines to withstand higher maximum allowable operating pressures (MAOP). To withstand these pressures, the industry has moved to using high strength steels which minimize required wall thickness as an alternative to simply using heavy wall pipe. Use of higher strength steels results in considerable savings in steel (a large pipeline project may be measured in thousands of tons), as well as transportation/handling costs and reduced costs for filler metal. These higher strength steels, such as APL 5L Grade X70 and X80, must be welded using a low hydrogen process. Levels of disfusable hydrogen must be kept to a minimum to prevent crack formation. Use of low hydrogen-type electrodes, while reducing the risk of cracking problems, have several negative aspects. They are chiefly designed for uphill welding, and require wider root gaps to achieve sufficient root penetration, resulting in slower welding speeds and reduced productivity.
Making Pipeline in Dubai desert
This factor, in addition to more severe operating environments and more stringent governing codes which must be met, has caused pipeline owners and operators to demand tighter control of weld quality and has increased the use of mechanized welding by contractors to produce welds of more consistent quality.
Many pipeline owners around the world have adapted the acceptance standards and practices of the ANSI/API Standard 1104 "Welding of Pipelines and Related Facilities". However, this standard accepts defects of numerous types when compared to other pressure pipe standards such as ASME IX, B31.3, etc., that are much more stringent concerning allowable defects. In the past, pipelines were designed to provide an adequate safety margin within the allowable defects of API 1104. With the increasing use of thinner wall higher strength pipe, the rejectable fault criteria has had to be re-evaluated, and many owners have adapted higher quality standards, such as the DNV (Det Norske Veritas) or Lloyd's standards used for offshore pipelines.
Making Pipeline in Dubai desert
Mechanized pipe welding has now been used for over 30 years in the pipeline industry. While it is extensively used for marine pipeline applications, only a small percentage of all cross country lines have been done using mechanized equipment. (The 2,973 km Alliance Pipeline, constructed in 1999-2000, was the first significant project to use mechanized welding in the U.S.) There are a number of suppliers to the mechanized pipeline welding industry. Although the systems vary considerably in configuration and operation, one common factor is that virtually all of the systems use the GMAW process in a short circuiting mode, welding downhill. The equipment presently being marketed generally consist of the following components:
- Weld Power Supply
- Weld Head, which mounts the torch and manipulates it with motions similar to those of a manual welder.
- Band or Guide Ring that clamps on the pipe, provides mounting for the Head, and is involved in weld Head propulsion.
- Filler Wire Feeder which may be mounted either on or off the rotating Head. (Most cross country applications use miniature Head-mounted feeders to minimize potential feeding problems.)
- Weld Head Programmer/Controller which may be integrated with the Power Supply or built into the weld Head. (A remote controlled Pendant is sometimes provided.)
In pipeline industry parlance, these are referred to as "bug and band systems".
While the main discussion is to be on advances in welding techniques/processes, it may be useful to describe the components of a standard cross country pipeline "spread" and a typical sequence of operation.
Pipe is generally delivered with the standard 30° factory bevel. This could be used for machine welding, but more commonly it is modified to a "narrow bevel geometry" using a large hydraulically-powered end facing machine. These clamp on the pipe ID and simultaneously round the pipe ends. Multiple cutting tools are mounted on a rotating plate which is fed at relatively high speed into the pipe ends. Quite frequently it is not possible to perfectly round the ends and the cutting tools are mounted on a spring-loaded tool holder which is allowed to "float" and follow the pipe ID surface. This ensures a more uniform land thickness.
The pipe is then preheated as required by the specification which can be done with gas torches or electrically with induction heaters.
Most cross country pipelines, whether welded manually or by machine, use a large pneumatically-powered internal alignment clamp. These clamps generate sufficient force to round the pipe ends to achieve proper alignment, and restrain the pipe ends from movement during the welding operation. On smaller diameter pipes, the clamps may be pulled through using a cable and winch. On larger pipes, the clamps are equipped with their own tractor modules and can be remotely driven from one joint to the next joint to be welded. The welder's helper will mount the guide ring, or band, on the pipe at some time prior to the line up and clamping operation.
For equipment which uses an internal welding system incorporated with the line up clamp, the root pass is done at this point automatically from the pipe ID.
External welding is typically done using separate teams, with one team doing one individual pass on the pipe (whether manual or mechanized). With mechanized welding, a tractor equipped with sideboom lowers a "welding house" over the pipe joint. The house is specifically constructed to create a seal around the pipe OD to prevent wind from affecting the shielding gas and also to provide some protection from the elements for the welders. Typically, two welding systems would be installed in the "welding house" or shed with the power supplies mounted on the tractor.
The weld Head would be clamped on the guide ring and the first weld Head rotated to the 12 o'clock position. The arc would be initiated with the first weld Head traveling from 12 to 6 o'clock clockwise. As soon as the first Head clears the top of the pipe, the second weld Head is moved into position on the opposite side and the process repeated. (The exact starting position may vary from 12 o'clock depending upon system configuration and other factors.) Starts and stops are "feathered" with a grinder by the welder's helper to ensure proper fusion at tie in of the two segments of the weld pass.
As soon as one weld pass is completed, the weld Heads are removed from the band, and the welding house lifted from the pipe and moved forward to the next joint. The next pass welding house is moved into position and the process repeated on through the final cap pass.
The standard short circuiting GMAW process cannot be used to accomplish a satisfactory root pass without using either one or two techniques:
an internal clamp fitted with segmented copper backing which serves to contain the weld puddle and limit ID bead penetration. (The copper segments are machined to the precise pipe ID and designed so that they can be extended outward to the pipe ID with only minimal gaps between segments or "shoes".)
a special ID welding system incorporated into the internal alignment clamp. This equipment is used to make the root pass from the ID with the hot pass and subsequent fill and cap passes being done from the OD using the conventional bug and band equipment.
Both of these techniques have their advantages and disadvantages. With copper backing, either a standard or special narrow gap bevel geometry can be used and very uniform ID bead penetration can be achieved. The disadvantages are possible copper contamination and subsequent corrosion of the root pass bead. The modified internal clamp is higher in cost and the copper shoes wear out and require frequent replacement. Several regulating bodies in the EEC will not allow copper backing because of possible contamination. Both the root and hot pass must be deposited before the internal clamp can be released to prevent possible cracking. This requirement delays move-up to the next joint.
The ID welder consist of a number of independent torches and wire feeders. (An internal welder for a 36" pipe would be equipped with six torches.) The first three torches begin welding at the 12 o'clock position and weld clockwise until reaching 6 o'clock. The other three torches, having moved to the 12 o'clock position, initiate welding and weld counterclockwise until reaching 6 o'clock. (Each torch welds 60° of the pipe circumference.) The internal root pass alone, however, lacks sufficient strength to allow clamp release and move up. A hot pass is put on externally which occurs immediately following the root pass welding.
The chief advantage of the ID welder is the speed to complete the root and hot pass and allow move up to occur. Essentially, the speed at which the root pass can be completed is the ultimate factor in determining the completed joints per day. Fill pass welding is not a factor as more welders can simply be used.
The ID welder has a number of significant disadvantages. The ID welds must be made "blind" without the possibility of any operator correction. This and the short circuit GMAW process results in a relatively high defect and repair rate. In addition, the equipment is expensive, especially considering a back-up spare ID welder must be maintained at the job site for immediate use should the primary unit need require service.
NEW WELD PROCESS USED FOR ROOT/HOT PASS WELDING
A new process for making the root pass from the outside of the pipe has now been field proven. The Surface Tension Transfer® (STT®) process developed by Lincoln Electric is essentially a highly-controlled short circuit process. The evolution of high-speed inverter power source technology in conjunction with microprocessor control allows the current to be precisely regulated during the entire welding cycle. It is unique in that it is neither constant current (CC) nor constant voltage (CV). The current output of the power source is guided by the instantaneous state of the arc voltage as it changes during the weld metal transfer cycle. For pipeline applications, a 1.3mm (0.052") solid wire is typically used with 100% CO2 shielding gas. The process has been used for both semiautomatic and mechanized root pass applications. Because the STT® process allows the weld current to be controlled independent of wire feed speed, the welder has the ability to control the heat input and fluidity of the weld puddle to ensure proper penetration with complete edge fusion and low spatter.
This process has been validated by successful use on numerous pipeline projects both in the semiautomatic form, as well as use with mechanized welding systems. Mechanized welding equipment using this process is now available from several manufacturers, including the Autoweld System which is manufactured for Lincoln Electric by Magnatech.
The Welding Institute, in 1996, published a report titled "Evaluation of Low Hydrogen Processes for the Pipeline Construction and High Strength Steel" (PR 164-9330). They stated that "The most successful root welding performance was obtained using the Lincoln Electric STT® Power Source and the LA90 electrode wire. The STT® Power Source provided very precise control of short circuiting metal transfer, which resulted in good handling characteristics, well-fused beads, minimal spatter, and lower fume emissions. The TWI welder involved in the trials was depositing satisfactory root beads within two hours of being introduced to the welding machine. The root welding speeds were comparable with cellulosic electrodes, while at the same time the welded ligament dimension was significantly greater." Using the STT® process, the deposit thickness for the root pass was equivalent to a typical SMAW process root and hot pass combined. It was found that the ligament dimension was adequate to allow the alignment clamp to be released and move up to take place.
Grinding is not required after completion of the STT® root pass, only a wire brushing to remove silicon.
For mechanized welding, a modified version of the internal alignment clamp is used. A Spacer clamp not only rounds the pipe ends, but precisely gaps the two pipe faces by an adjustable, predetermined dimension.
The mechanized STT® process was first used on a gas pipeline project in Argentina. The 2000 Gasoducto Centro project involved welding 36" pipe with a wall thickness of 8mm (0.315"). The API 5L X70 pipe was double-jointed to field weld lengths of 24m (80'). The total project consisted of welding 340 km, 170km of which involved root pass welding using a mechanized pipe system incorporating STT® technology. The piping contractor, Techint, split the pipeline project as a trial, with half being done using manual SMAW and the other half with mechanized STT®. The mechanized equipment achieved 100 joints per day (10 hour day). It was completed in early 2000. All welds were inspected to API 1104 with a defect rate of less than 1% and repair rate also less than 1%.
As previously discussed, the API Standard 1104 allows a significant amount of various types of weld defects before repair or cut out is required as compared to other pressure pipe codes. The Dusup Gas Distribution project (Dubai United Supply, Dubai, U.A.E.) required the contractor to meet much more stringent standards: only 50% of the allowable defects under API 1104 before repair was required. The project involved welding 48" pipe with 16mm wall (0.625"). The X70 grade pipe was welded in 12m (40') lengths. McConnell Dowell was contractor on the 16 km project which was completed in 2000. There was a 0% defect rate in the root pass and only a 1.2% repair over all the project, which included the SMAW fill passes.
The STT® process has been field proven as a viable, low-hydrogen process for root pass welding, whether semiautomatically or with mechanized pipe welding systems.
Use of FCAW for Fill Passes
Gas shielded flux cored arc welding (FCAW) has been recognized as capable of producing welds meeting the most stringent standards on high pressure pipe by the power generation, ship building, chemical and refining industries. It has been used to weld heavy wall and low temperature service pipe at Central Processing Facilities which separate out gas, water and other impurities at a producing oilfield. Much of this piping sees severe service as the gas is pressured and reinjected into the oilfield for enhanced recovery. FCAW has only recently been used for cross country pipeline applications.
Possibly the first use of mechanized FCAW for cross country pipeline was the onshore portions of the South Arne-Nybro Pipeline in Denmark for the Danish natural gas system operator, DONG A/S. The contractor was Joint Venture Aarsleff - Freytag J/V. This was a significant project to bring gas produced offshore in the North Sea into the Danish distribution network. The project in the southern part of Jutland, Denmark consisted of 187 km of 24" pipe with a wall thickness of 15.1mm (0.594"). The pipe grade was X65. What was unique about this project was that the PQR referenced both API 1104 and ASME IX. All joints were 100% ultrasonically inspected with 10% of the joints radiographically inspected as well. This significant elevation of the expected quality standards by the pipeline owner, DONG, resulted in the contractor, Per Aarsleff, deciding to evaluate the use of mechanized FCAW equipment for fill pass welding. A standard 30° bevel (60% included angle) was used. The root and hot pass were made using E6010 electrode for the root and E9010 electrode for the hot pass. For the fill and cap passes 1.2mm diameter alloy E81T1-G flux core wire was used. Shielding gas was a mix of 82% argon/18% CO2. Out of 1,800 welds, there was a 3% repair rate with only two rejects (which included the manual SMAW passes). Dong Project Manager, Ole Munch Andersen, stated that "the South Arne-Nybro project (the on-shore portion of the pipeline), was satisfactorily completed on time, on December 11, 1999. Magnatech's welding system was used for fill and cap passes. The project was completed with an acceptable fault rate and the quality of the welds were high. The welding method and welding system proved to be reliable for the job and can be recommended for other jobs of similar character."
A more severe test of the new processes were the Kalstø Landfall and Åsgaard Transport projects for Statoil, Norway. This involved welding a portion of a marine pipeline as it emerged from the water at a steep grade. A total of 300 welds had to be made for the two projects. The 1.118m (44") pipe had a wall thickness varying between 45mm - 55mm for the coastal landfall and 38mm - 48mm for the Åsgaard project. To minimize fill time, a compound V-bevel was used (30° for 15mm and a 15° bevel used for the balance). The semiautomatic STT® process was employed for the root pass using a modified ER70S-6 (Thyssen K Nova) solid wire. Filler wire was an E81T1Ni 1 flux core electrode, 1.2mm in diameter. Both processes used a gas mix of 77% argon/23%CO2 . Because of the specified low temperature impact properties and very low micro hardness required in both the deposited weld metal and heat affected zones, the pipe was continuously heated using electric induction type heaters to maintain a minimum temperature of 130°C and a maximum interpass temperature of 180°C. The weld procedure called for the heat input on the root pass to be maintained within 0.5 and 0.7 kJ/mm with the fill pass maintained between 0.6 and 1.4 kJ/mm.
The tunnel was just 3m (10') in width, resulting in an extremely confined work area. A special pipe transport vehicle was designed to work within the narrow confines of the tunnel, and locate each 12m (40') pipe. The pipeline was built partly in the tunnel approximately 100m below sea level and 800m out from the coastline. The environment in the tunnel was very hostile to electrical equipment. Salt water seeping into the tunnel had to be constantly pumped out, resulting in a welding environment of salt-saturated 100% humidity.
To maintain the heat input within allowable standards, each pass was limited to 2 - 3mm in thickness. A total of 75 passes were required to weld out the pipe from root to cap.
Once again, 100% ultrasonic inspection was required with 10% of the welds being radiographically inspected as well. The connecting Åsgaard transport portion of the pipeline (outside the tunnel) had a 0% repair rate.
Tony van der Stelt, Project Manager for JV Kaarstoe Pipeline Contractors stated that "I am very satisfied with the performance of the mechanized pipe welding system and intend to use them on future projects."
- Pipeline and Gas Industry - January 2000
- A Process Solution for Pipeline Construction, P.L. Nicholson, International Institute of Welding Commission XII, EWI.
- TWI: Evaluation of Low Hydrogen Welding Processes for Pipeline Construction in High Strength Steel - PR-164-9330.
- Advanced Weld Repair Technology Extends Plant Life, V. Viswanathan, D.W. Gandy, S.J. Findlan, Power Engineering, December 1996.
- Technology Gets to the Root of Pipe Welding, E.K. Stava, Tube & Pipe Technology March/April 2000
John Emmerson has a Bachelor of Science from Boston University, and a Master of Science from Cornell University. He has spent 24 years in the welding industry, and serves on the AWS D.10 Committee of Piping and Tubing. For the last 14 years he has been President of Magnatech, based in Connecticut, USA with a European office in The Netherlands. Magnatech specializes in the manufacture of orbital pipe and tube welding equipment.
The author would like to extend his gratitude to the following companies which contributed materials for this article: Lincoln Electric, USA, Per Aarsleff, Denmark, JV Kaarstoe Pipeline Contractors, Norway