 | |||||||||
| |||||||||
 |
|
 |
Mechanization and inverter technology lend speed to the laying of a major gas pipeline
by: Rick Beesen, midframe business manager, Miller Electric Mfg. Co.
Spread four has bugs in the cornfield — but not to worry. The “bug” referred to here is a mechanized welding bug (Fig. 1), specifically a single welding head, computer-aided gas metal arc (GMAW) system from CRC-Evans Automatic Welding powered by a Miller Electric XMT® 304 inverter-based welding machine. The bug operator and owner of 32 inverters is Welded Construction, L. P., of Perrysburgh, Ohio. Spread four is a 147-mile stretch of the Alliance Pipeline system (Alliance) cutting through the cornfields of northeast Iowa.
While used successfully in Canada and Europe for onshore and offshore pipeline construction for 30 years, this is the first large-scale use of mechanized welding in the United States on a cross-country pipeline. On longer, larger-diameter and thicker-wall pipe projects — the Alliance mainline has 1844 miles of pipe, most of it 36-in. diameter with a 0.622-in. wall thickness — mechanized GMAW offers better productivity than manual shielded metal arc welding (SMAW). In addition, high-strength steels, such as the API 5L Grade X70 pipe used on the Alliance, benefit from the low-hydrogen content of certain solid and tubular wire electrodes.
Metal Composition Changing Pipe Welding
“The pipeline industry is going to higher strength steels,” said Dean Phillips, manager of welding engineering and applications for Hobart Brothers Co. “It lets them use a thinner wall pipe, which can reduce transportation costs and the amount of filler metal used to weld it. Higher strength also permits increasing operating pressure, which increases the pipe’s carrying capacity (the Alliance’s maximum operating pressure is 1740 lb/in.2; initial capacity is 1.325 billion cubic ft/day of natural gas). However, these higher strength steels require a welding process with very low levels of diffusable hydrogen in the weld metal to prevent transverse cracking or heat-affected zone cracking.
“Welding X80 grade pipe with covered electrodes, which future projects may call for, may demand shifting to a low-hydrogen electrode,” said Phillips. “Unfortunately, low-hydrogen shielded metal arc electrodes are typically designed for uphill welding and exhibit reduced productivity. This is unacceptable for pipeline projects.”
For the mechanized welds, Welded Construction uses an 0.035-in.-diameter AWS ER70S-6 electrode for all passes. This wire is a titanium-modified, carbon-manganese-silicon consumable manufactured by Thyssen according to CRC-Evans specifications. On tie-ins, field fabrication (such as welding flanges) and mainline repair, Welded Construction uses a gas-shielded tubular electrode, a 0.045-in.-diameter AWS E18T1-Ni1 manufactured by Hobart Brothers Co. for the second fill and remaining passes (the root pass is made with an E6010 and the hot and fill pass with an E8010-G). Sheehan Pipeline Construction and Murphy Bros. chose this flux cored wire for repair work, as well as for tie-in welds after making the root bead and hot pass with AWS ER70S-6.
Mechanized Success
Six of the seven contractors responsible for welding on the U.S. mainline use mechanized welding (the seventh contractor is working in an urban area too congested for this type of work). While each company qualifies its own equipment and technologies, the project requires that every mainline welding process involve low-hydrogen GMAW and FCAW electrodes.
After welding since mid-June, “evidence indicates that all six contractors’ equipment is performing well from a reliability and weld quality standpoint,” said Brian Laing, vice president, CRC-Evans (four of the contractors selected CRC-Evans to supply equipment and technology). “This is an important victory for mechanized welding. If one spread had to go back to covered electrode welding, it might have had a ripple effect. Fortunately, that’s not the case. Even better, contractors are seeing productivity increases.”
Laing said most contractors on this project average 100 joints per day (100 sections of 80-ft double-joint pipe, or about 1.5 miles per day) with a single-head bug using short circuiting GMAW.
A welding crew using a covered electrode would be hard pressed to average 90 joints per day on this 36-in., 0.622-wall pipe. To get the type of productivity mechanized welding offers, a SMAW crew would have put so much equipment on the right-of-way it would get unwieldy, especially if a piece of equipment failed.
Max Helms, welding supervisor, Welded Construction, said, “It’s very important to note that while mechanized welding requires fewer welding operators, it takes more support personnel. A lot of people call what we’re doing automatic welding, but it’s really not. It requires a lot of operators and other union people, so it’s a good thing for the industry. Given the trend to use higher strength steels, our ability to join pipe with mechanized equipment means there will be a lot more lines laid with us.”
Internal Root
As much as spread four resembles the right-of-way of the past — Caterpillar tractors (“tack rigs”) and side-boom cranes still crawl next to the pipe (Fig. 2) — it also heralds the future of U.S. pipeline construction. Max Helms runs through the Welded Construction’s joining process to demonstrate the differences.
The start is with an automatic end-facing machine to change the 30-deg-factory bevel on the pipe to a modified “J” bevel. This bevel, unique to the process, requires less filler metal, so it reduces welding time.
Next, the pipe is preheated to 200°-350°F (93°-177°C) (the specification calls for a temperature of 200°-400°F [93°-204°C]).
An internal line-up clamp traveling inside the pipe aligns the two pipe sections and pneumatically clamps them in position.
After operators check fitup, six weld heads on the internal clamp weld an internal root pass — Fig. 3.
Laing noted an internal bead can accommodate much broader fitup tolerances than an external bead, so it leads to better quality root beads. It also provides more productivity than an external bead with a copper backup.
The first three heads on the internal machine fire in the 12 o’clock position and weld clockwise until reaching 6 o’clock. The other three heads, now in the 12 o’clock position, fire and weld counterclockwise until reaching 6 o’clock (i.e., each head welds one-third of one-half of the pipe). Clamping to completion of the root bead takes less than two minutes.
The internal welding machine is powered by three inverter welding power sources mounted on a tack rig. If one welding head or inverter misfires, an operator uses the fourth inverter to repair the point of misfire with SMAW.
Next comes the hot pass. The first rig, like the nine that follow, carries two inverters and a side crane that hoists a welding shack. The shack prevents wind from affecting the shielding gas and provides a platform for two welding operators and two welder’s helpers.
The helper clamps the mechanized bug to a band affixed to the pipe and, if needed, clips the welding wire to proper length. The first welding operator then hits a contactor button, which pre-flows the gas. The welder then hits an arc start button and the bug begins traveling down from the 12 o’clock position (Fig.4), powered by a constant speed, 24-V DC motor. Since the band may not be perfectly parallel to the joint, the operator must carefully watch the bead, using a horizontal adjustment knob to re-center the welding head in the bevel (if needed).
After the first bug clears the top of the pipe, the operators on the opposite side of the pipe repeat the process, with the bead being completed near the 6 o’clock position.
Shacks 2 and 3 each put in the first fill pass; shacks 4 and 5 each put in the second fill; shacks 6 and 7 weld the third fill; and shacks 8 and 9 weld the cap. Shack 10 is a spare.
Each of these shacks welds every other joint. Two stations increase production. The hot pass bug moves quickly (44.5 to 50.5 in./min.) compared to the fill cap. These latter passes require slower travel speeds because they deposit more filler metal and because the torch oscillates to fill the joint and let the weld pool cool slightly.
During these passes, the welding operator can also control the torch’s oscillation width and torch-to-work distance (arc length), making adjustments as necessary to maintain a good bead.
Verifying Weld Integrity
To ensure quality, all welds are ultrasonically tested (also a major U.S. pipeline technology first). Ultrasonic testing (UT) provides the ability to look at a weld in intricate details by sending a sound wave into the pipe and having a computer map the echo. Where X-rays may take up to 10 min per joint, UT takes about 90 sec to inspect and interpret a weld.
“X-ray testing only tells the length of a defect, not the depth. With UT, we can tell a defect’s depth, height and length to within 0.0040 in.” said an employee of Shaw Pipeline Services, Ltd., who conducts tests on the Alliance Pipeline. “This can help lower repair rates. Engineers who have done some mathematics and filler metal analysis have determined that we can accept longer defects if they’re smaller in height. UT’s precision also lets us tell the repair crew exactly which pass contains the defects, and they just love that. Further, UT detects very fine defects that X-rays can’t, notably cold laps (incomplete fusion),” he continued.
Light Weight Wins
Until about 1995, the standard power source on a rig was a 500-lb DC transformer rectifier. Today, all but two of the mechanized welding systems on the Alliance Pipeline carry an inverter-based machine.
The inverters were chosen because of their compact design and light weight. If a machine needs maintenance, four bolts loosen it and two people can lift it off the rig.
People in the pipeline business recognize that equipment doesn’t last forever.“From a field maintenance perspective,” said Laing, “you have to operate on the premise that all equipment someday needs maintenance. The fundamental reason for using an inverter is that it’s much easier to swap out than a 500-lb transformer rectifier. Those heavy welders were reliable, but when one broke, you needed a crane to replace it. That’s one more vehicle a constricted right-of-way doesn’t need.”
Laing also noted that the reliability of the inverters has “exceeded initial expectations. There was initial concern whether they would be able to withstandthe vibrations of the tractor. After four years of experience with them (on equipment welding pipelines in Canada), their performance has certainly been acceptable from a reliability point of view.”
CC/CV Machines Required
“Using a combination of SMAW and FCAW for repair and fabrication also represents another ‘first’ for the Alliance Pipeline,” noted Dave Bickel, account manager, Miller Electric. “Like mechanized welding, it increases productivity. It also requires replacing traditional DC generators and their CC output with new technology, such as engine-driven inverters with a CC/CV output.”
Bickel explained the nickel-alloy FCAW electrodes used for pipeline welding work best with a CV power source because they’re voltage sensitive.
“A CC engine drive doesn’t provide real voltage control,” he stated, “because you’re trying to pick a voltage off the volt/amp curve. Conversely, an engine drive with CV capabilities paired with a wire feeder that provides constant speed performance gives you full control over setting voltage and amperage values. Importantly, the CV capabilities on such machines do not compromise CC output and stick welding characteristics. Operators still get code-quality results with E6010 and E8010 electrodes. I’d strongly recommend that anyone making FCAW and/or SMAW pipe welds use this type of system.”
On this project, Sheehan Pipeline Construction, Murphy Bros. And Welded Construction qualified procedures with the PipePro‘ 304. This machine uses a 26-hp Kubota diesel to power a CC/CV inverter-type power source.
“We purchased five engine-driven inverters because of their versatility,” said Helms.
Ferrell Feagin, a welder with Welded Construction, said, “This is my first time wire welding, and the puddle seems to have a lot of the same characteristics as a 7018 rod, except you’re welding downhill and pulling a trigger instead of changing rods.”
Though he did not have prior experience, “I passed my wire welding test after a couple of practice coupons. After welding 25 joints here in the fab shop, I haven’t had any repairs,” said Feagin. He advises any welder who has an opportunity to run flux cored wire to “get in and learn how to do it. It’s not a bad way to go.”
Dale Kennedy, another welder with Sheehan, said engine-driven inverters have very good “digging” power with covered electrodes.
“When you push on the rod, you’ve still got plenty of heat,” he said. “This helps on the bead and hot passes, as well as on repairs when pushing through existing welds. It also carries a very clean puddle on the third fill. Usually I switch over to flux cored wire after the third fill, and it’s got a good arc there, too.”
Induction Heating
Another first on this project came when Sheehan elected to replace traditional propane heaters with an inverter-based induction heating power system (IHPS) from Miller Electric. This system uses a rapidly alternating (10 to 50 kHz) electromagnetic field to induce eddy currents into the copper wire coils contained within a “blanket” wrapped around the pipe.
“An IHPS provides uniform heat to the top, sides, and especially bottom of the pipe,” said Bickel. “This technology allows the joint to be preheated to a precise temperature. If a contractor encountersespecially thick sections — such as those common near pumping stations, roads or river crossings and blow-down stacks — induction heating works substantially better than propane. It ensures that heat penetrates deeply and is delivered completely throughout the pipe wall.”
Look to the Future
The success of inverters on the Alliance pipeline will likely guarantee their use on the next major U.S. cross-country pipeline.
Those next pipelines may also see an increased use of pulsed GMAW. The one Alliance pipeline contractor using pulsed GMAW and a dual-head, dual-wire bug now welds 120 to 150 joints per day.
Reprinted with permission from Welding Journal Magazine, November 1999 by The Reprint Dept., 800-259-0470