By Rick Beeson, midframe business manager, Miller Electric Mfg. Co.
Once the sugar beets and potatoes have been pulled from the rich brown earth of far northwestern Minnesota, the big iron moves in. Flatbed trucks deliver more than 3,000 segments of double-joint, 36 in. diameter pipe. Sideboom cranes with their 15,000 lb. counter-weights roll into position on the right-of-way. Excavators bite through the top soil and into the clay, digging a trench eight feet deep, stretching the 52 miles between Clearbrook, Minnesota and Kerrobert, Saskatchewan.
Then, on the American side of the border, Tommy Coker and his crew of 35 pipeline welders and their welder’s helpers create what can only be described as a human assembly line. Welding began the first week of October, and only a few tie-ins remained by Christmas.
“The pipeline industry is based on precision welds and lots of footage,” says Coker, welding foreman, Welded Construction, L.P. of Perrysburgh, Ohio. “How fast we weld directly relates to how many feet of pipe the contractor can put in the ground. On a good day, we’ll weld more than a hundred joints. And each one of those welds passes visual and x-ray inspection, meeting API 1104 Weld Procedure Specification.”
The assembly line begins by placing sections of the pipe on supports so that several hundred yards parallel the trench. Any dirty bevels will be ground clean. An internal, pneumatically operated clamp secures the sections in place after crane operators maneuver them 1/16 in. apart (one quickly gains respect for the finesse demonstrated by crane and excavator operators).
Welder’s helpers use propane torches to preheat the pipe to between 250 and 400 degrees Fahrenheit (accuracy is checked with a temp stick). On higher strength pipe like the X70 grade used for this project, good preheating and interpass temperatures help: ensure good sidewall fusion, eliminate cold lap and reduce susceptibility to hydrogen cracking. When temperatures drop (and they occasionally dipped below zero), preheat temperatures tend to fall on the higher side. Other than setting up wind shields on particularly cold and windy days (and that’s largely for the welders), no other low temperature precautions were required.
The welders use the Shielded Metal Arc (Stick) process, DC reverse polarity to join all of the pipe, which has a wall thickness of 0.345 in. The joint requires a 30 degree angle (± 5 degrees), a 1/16 in. land and five passes to weld. Specifications call for a 5/32 in. E6010 electrode on the root pass (stringer bead) and a 3/16 in. E8010 electrode for the hot, two fill and cover passes. The welder’s helper will clean the root bead with a power grinder and switch to a power or hand brush for the remaining passes.
To speed assembly, three individual welders lay down the stringer bead, then move to the next joint. Moving in behind them, two “firing line” welders make the hot pass and one of the fill passes, then move on. Following the firing line welders, two “back end” welders add the second fill and cover passes. A final crew heats the completed joint to prepare it for spray-applied epoxy coating that protects it from corrosion.
“This many men coordinating work and moving so quickly – and safely – is amazing,” Coker says. “It’s like a manufacturing assembly line, except the people move.” To keep track of which welders perform which passes, Coker assigns each of them a number. These numbers are then written on the pipe next to the joint. This aids in troubleshooting welding flaws.
Once joined, sections of pipe several hundred yards long are lowered into the trench and butted together with a 1/16 in. gap. An external clamp holds the joint in place while two welders and their helpers complete the tie-in, stringer bead through cover pass. This task requires even more skill than when the pipe is above the trench. The welders often contort their bodies, sprawled in the mud, to reach the bottom of pipe. Fit-up might not be as perfect. The tie-in crew, the x-ray crew, the “dope” crew (who apply the epoxy) and the backfill crew are waiting. And a bad weld (one that fails visual or x-ray inspection) can cost $10,000 to $15,000 of men and machine time.
“If the welder strikes an arc outside of the bevel (called an “arc burn”), we have to cut it out because it’s considered a flaw that could lead to cracking,” Coker states. “In the event of a flaw,” he continues, “we remove it with an acetylene and oxygen cutting torch and redo the weld. That’s why in the back of every welding rig you’ll see a cutting band that fits around the pipe and positions the torch so that the cut creates a 30 degree bevel.” A project this long typically has 10 to 15 welds cut out.
Traditional welding equipment performed most of the work on this project. As long as ground conditions permitted it, the pipeline welders drove their welding rigs on the right-of-way. The welding rig is typically a one-ton pick-up truck with a diesel engine driven welder filling the back. When rain or snow turns the right-of-way into a bottomless mud pit, or when a number of arcs need to be concentrated in one spot, tack rigs provide the welding power. A tack rig is a crawler-type tractor where a belt from the tractor motor drives the armatures of four welding generators mounted in the back.
Before being permitted on the right-of-way, every welder must pass a welding test where his welds are subjected to visual, mechanical and radiographic tests. Further, during this test, each welding machine is checked to ensure that its welding output is within specified bead parameters. Parameters for the X70, 0.354 in. wall thickness pipe used here are:
Bead 1: 139 – 178 amps, 22 – 28 volts
Bead 2: 148 – 185 amps, 23 – 30 volts
Bead 3: 175 – 186 amps, 23 – 29 volts
Bead 4: 148 – 192 amps, 24 – 28 volts
Bead 5: 140 – 175 amps, 23 – 28 volts
In addition to the classic CC-only output machines, a new type of engine drive proved its Stick welding performance on this tough winter job. The machine, Miller Electric’s PipePro 304, uses a 26 HP Kubota diesel to power a multiple-process, inverter-type power source based on Miller’s XMT 304 technology. With a welding output of 5 to 375 amps/10 to 35 volts and Stick, DC-TIG, MIG, pulsed MIG, flux cored and gouging capabilities, this first-of-its-kind welding generator can perform any task related to pipeline welding.
“You won’t find much worse conditions than the mud and cold of northern Minnesota,” says Coker. “This inverter engine drive worked without fail in temperatures as low as 10 degrees below zero. But what’s really important is the arc. The PipePro’s got an unbeatable downhill pipe arc. It carries more iron and creates a beautiful bead.”
Richard Ball, a pipeline welder with 38 years experience (including the Alaskan pipeline), used this inverter engine drive and reports that “The PipePro 304 is a real easy machine to weld with. It’s got a very smooth and very consistent Stick arc in every application – bead, hot pass, fill and cap.” He also notes that you can turn down the machine extremely low and the arc will not break, nor will the rod stick, as it will with other machines.
“Good low amperage and voltage capabilities let me run wider spaces on my stringer bead if I have to,” he says. “Back here, we can run real wide. We have torch cuts and bad fit-ups, and that machine’s done an excellent job.”
Advanced Stick machines like this inverter engine drive get their good low end performance from a feature called “dig.” Dig control works by sensing when the voltage drops below a certain level, roughly around 19 – 19.5 volts. At lower voltages (and only at lower voltages), the inverter boosts the amount of amperage proportionally (see graph), keeping the overall power at the end of the electrode sufficient enough to sustain the arc.
This particular inverter also features an amperage “drooping” control. Above a range of about 30 volts (see graph), the inverter automatically removes amperage from the arc, giving the puddle of a cellulosic electrode (e.g., E6010, E8010) a chance to chill and freeze.
“Experienced welders appreciate this feature for the cap pass because they can hold a longer arc,” says Coker. “It helps prevent the arc flares and excessively fluid weld puddles that can occur with a longer arc. You can’t do this with other engine drives for pipe welding.”
Coker also notes that the PipePro contains a host of other features designed for the pipeline welder. “It’s seven inches shorter so you can see out the back window, and you can put more gear in back of your truck because it weighs just 950 lb.,” he says. “It’s got a hinged top for fuel/oil fill and access to the engine. The digital volt/amp meters let the welding inspector know that the welder is running at the right specs, and it uses a lot less fuel.” The PipePro also features 12 kW of auxiliary power.
New Trends in Pipewelding
If Stick electrodes and a CC output machine could weld all pipelines, there wouldn’t be much demand for a multiple-process inverter-based engine drive. However, the pipeline industry and pipe metallurgy is changing, and this demands different welding processes and products.
On the Alliance Pipeline for example, approved procedures for making the tie-ins, mainline repairs and field fabrication work (e.g., valves, flanges), called for Stick welding the stringer bead, hot pass and first fill, then switching to the flux cored process for the remaining passes.
“Only an engine drive like the PipePro 304, with its CC/CV output, provides such multiple process capabilities,” notes Coker. “Individual pipeliners looking at the future need to consider how the industry is changing before buying a new welding generator, and they need to think about the welding skills and knowledge required.”
Information courtesy of Miller Electric