This is Part 5 in our series on proper pipeline installation and tile repair with Mad Dog Foam Bridges. We previously examined the stringing process, or the process where pipeline contractors lay out the individual sections of pipe for installation. Today we’ll look at welding and the final review process before the pipeline is finally lowered into the trench.
If you haven’t figured it out yet, pipeline installation is a complex process. That’s why we’re into Episode 5 of this series and the tube still hasn’t gone into the ground yet. Few steps in the process are as complicated as the next one, however. There’s a reason why licensed welders are among the best paid workers on any construction site.
Now that the pipe has been strung along the trench, welders can more formally attach them. Welding equipment is a far more sensitive tool than your common blowtorch (although it looks just as cool). There are multiple methods of welding, designed for different jobs. Pipeline welding requires Shielded Metal Arc Welding (or SHAW, for short), a process best for metal four mm or thicker. Direction of the weld is also important: Pipeline welders travel “downhill,” or from the top of the pipe down. Again, this is based on the thickness of the pipe involved. The pipes used during refinery construction are welded “uphill.”
The pipe must be warmed to high temperatures prior to the weld—250 to 400 F°. The colder the outdoor temperature (welders often need to work in subzero conditions, especially in the upper Midwest), the hotter the preheat temperature.
Individual welds can take a while, and these pipelines stretch for thousands of miles, so teams have built efficient strategies to boost performance.
This often involves three different teams coming through. First, several welders lay a “weld bead,” or an initial layer of “filling.” They then move onto the next joint, while the second team makes a “hot pass”—the nickname for a second welding layer—and also make the first filler pass. The third team adds another filler pass—this term refers to rounds where layers of metal are added—and then they provide a cover pass. The filler passes will bring the joint flush with the surface of the pipe, and the final cover pass will bring the weld slightly higher.
So yes, that’s at least six welds, barring any problems. Teams can do as many as 100 joints during a day of work.
“The pipeline industry is based on precision welds and lots of footage,” says Tommy Coker, a foreman at Welded Construction of Perrysburg, Ohio. “On a good day, we’ll weld more than a hundred joints. And each one of those welds passes visual and X-ray inspection.”
The aforementioned X-ray is a reference to how weld joints are reviewed before approved to move forward. Most flaws won’t be visible to the naked eye, so X-ray crawler machines are sent through the pipeline to examine each joint.
If everything is up to snuff, the joints are heated again in preparation for a layer of protective coating. This application will protect the pipeline from corrosion, as well as providing insulation.
And after that…satisfaction at last for our patient readers: The pipeline is ready to be lowered into the trench. Come back for our next post for that moment of glory.