A Cut in the Field
Plasma Cutting Equipment for Pipeline Construction and Maintenance
By Jim Colt
In my 35 plus years of working with metal cutting applications around the world, I have to admit that the toughest field conditions for both workers and equipment have always been in the oil and gas pipeline construction industry.
While most of my tenure has been spent reviewing and advising users of flat plate cutting applications inside shops, the last 10 or so years I have seen demand from the field pulling much more metal cutting expertise into pipeline, as well as structural steel shape cutting applications. Furthermore, while much of pipeline job components are produced in a shop, this industry has demanded better field cutting technology to support the necessary cutting, weld prep beveling and repair needs for pipeline installations and long-term maintenance of these crucial oil and gas delivery systems.
Back in the fabrication facilities for large pipeline systems, it is quite common to see large Computer Numeric Controlled (CNC) pipe cutting and fabrication machines that can precisely cut weld prep bevels effectively, making field work faster and easier. The equipment used in these cutting and beveling applications has advanced dramatically in the last 15 years or so, taking full advantage of advanced design, production and control software, as well as equipment that can produce pipe sections that are ready for fit-up and welding in far less time than in the past.
Much of this automated equipment employs the latest plasma cutting technology, including high definition plasma (e.g., the Hypertherm HPRXD plasma systems), which cuts faster, cleaner and more accurately than ever before, while leaving a metallurgically ready-to-weld edge on the critical ends of the pipe.
This plasma cutting technology was first developed for the steel plate cutting industry, where it was used for weld-prep beveling steel plates for shipyard and other applications. This technology, which includes special torch and consumable designs for cutting as many as three separate bevels on an edge (often called a “K” bevel) was relatively easy to adapt for pipe and structural steel cutting and coping machines. Ultimately, this technology produces better quality pipe sections, at a faster pace and with lower production costs than earlier systems used in the industry.
With plasma cutting systems, there has always been the “other side” of the process that is different, yet uses similar technology as compared to the high technology mechanized plasma systems used in the shop for plate, pipe and structural cutting applications. Hypertherm manufactures a line of air plasma cutting systems that are primarily designed to use regular compressed air as the cutting gas.
Twenty or more years ago, a 100-amp air plasma system would weigh about 500 lbs and would efficiently cut up to about ½-in. at productive cut speeds. These systems were heavy, expensive and not as reliable as today’s plasma systems. The latest model 105-amp air plasma system, such as Hypertherm’s Powermax105, weighs less than 100 lbs and can cut 1 ½-in. steel fast and clean, while serving as a reliable portable cutting tool for field use. These machines have plug-and-play connectors to make them interface easily for mechanized use, as well as quick disconnect torches for easy switch out from hand cutting or gouging to a machine torch. This new metal cutting technology can play a role in those pipeline applications required after the pipe arrive in the field.
Pipeline fabrication and long-term pipeline maintenance must occur under tough field conditions. By nature there is some rework and fit-up that will always be required when laying pipelines. One such need that has always required a lot of manual labor is when the need arises for a custom length of pipe to be cut — a method of weld prep beveling that could challenge the accuracy of the cuts made at the shop. Typically, this type of precision cutting in the field has been done using the oxygen-acetylene process with the following steps:
- Grinding off the pipe coating using hand operated wire brushes or grinders, or by burning the coating off using a flame from a large oxygen acetylene torch.
- Once the coating is burned back adequately (usually 2 to 4 in.), a mechanized pipe cutting machine with an oxygen-acetylene torch is installed on the pipe section square to the pipe and then set up to cut at the correct length on the pipe.
- Next, the pipe cutting apparatus is moved, the torch set at the appropriate bevel angle required for welding and another cut is made.
- Subsequent grinding by hand and fit-up is often necessary once the pipe section is in place to weld to the next section.
- If this cutoff is part of a transition — when the nominal diameter pipe must pass under a highway or river bed, often the wall thickness must be heavier — then a third undercut bevel must be made on the inside diameter of the pipe as well.
- Further complication and time is required when this type of three-bevel cut must be done on an induction bend section of pipe, generally requiring some manual cutting as opposed to using the standard pipe cutting machine.
The above procedures can generally (with a four man crew) produce two cuts per shift. It is a time-consuming job that requires a high amount of manual labor. Furthermore, the hand grinding and burning of the pipe coating, as well as handling of the high-pressure oxygen and flammable acetylene gases, all create safety issues on the jobsite.
In the portable plasma cutting side of the plasma industry, demand has steered the major manufacturers to produce more powerful hand cutting plasma systems that are both portable and more reliable than ever before. Using the latest inverter based technology, today’s air plasma systems, such as Hypertherm’s Powermax 65, Powermax 85 and Powermax 105 systems, can be lifted in and out of a truck or trailer by one man, and can operate from portable power generators with ease. One of the best things is that, unlike the oxygen-acetylene cutting torches, plasma simply uses electricity and compressed air at relatively low flow rates to provide very accurate cuts at speeds four to six times faster than an oxygen-acetylene torch. The plasma system can either use a hand torch or a machine-mounted torch, and also has the ability to do plasma arc gouging (often used to repair defective welds or cracks in a pipe).
To add to the portability and safety of this plasma cutting technology for remote field use, at least one other company that provides equipment and expertise for pipeline applications, Barracuda, has developed a portable, diesel fueled combination generator and compressor that houses a Hypertherm air plasma system that is designed exclusively for this application. Barracuda also supplies some custom automated field pipe cutting equipment that can bevel and cut straight pipe sections or induction bends, all with less setup time as is required by conventional machines.
So let’s take a look at the process of custom bevel cutting for fit-up in the field, but this time using an air plasma cutting system:
- Grinding off the pipe coating using hand operated wire brushes or grinders, or by burning the coating off using a flame from a large oxygen acetylene torch. — This step is eliminated with the plasma process. Plasma will transfer a cut right through the pipe coating, and will only burn back coating about a quarter-inch for easy rework.
- Once the coating is burned back adequately (usually 2 to 4 in.), a mechanized pipe cutting machine with an oxygen-acetylene torch is installed on the pipe section square to the pipe and then set up to cut at the correct length on the pipe. — This step remains the same, except that using the air plasma torch instead of oxygen-acetylene will create about three to five times faster cutting speeds and a cleaner, dross-free cut with no flammable or high-pressure gases required.
- Next, the pipe cutting apparatus is moved, the torch set at the appropriate bevel angle required for welding and another cut is made. — This process remains the same.
- Subsequent grinding by hand and fit-up is often necessary once the pipe section is in place to weld to the next section. — This step is deleted, as typically the plasma will produce an accurate, ready-to-weld bevel.
- If this cutoff is part of a transition — when the nominal diameter pipe must pass under a highway or river bed, often the wall thickness must be heavier — then a third undercut bevel must be made on the inside diameter of the pipe as well. — This process is done with the plasma as well, at much higher speeds and more safely.
- Further complication and time is required when this type of three-bevel cut must be done on an induction bend section of pipe, generally requiring some manual cutting as opposed to using the standard pipe cutting machine. — Using the air plasma with the new technology pipe cutter from Barracuda saves a significant amount of setup time when cutting transitions and induction bends.
Typically, using an 85-amp air plasma system such as the Hypertherm Powermax85 and the Barracuda pipe cutting attachment three to four induction bend bevel cuts can be done in a day by a four-man crew. This represents a 50 to 100 percent improvement in productivity in the field, while improving safety accuracy and minimizing secondary operations at the same time.
Another capability of these air plasma systems is with plasma arc gouging. In most pipeline installations when weld faults or cracks are detected, the area of the crack is clearly marked out and a worker comes to the location with a large propane torch, burning back the pipe coating. Then an engine powered welder of at least 600 amps is brought to the location and the crack is carbon arc gouged (carefully to not fully penetrate the pipe wall), then ground out (to remove impurities) with hand grinders before being inspected carefully and then rewelded. Similar operations are used on pipelines that have been inspected after periods of use when cracks are detected.
By using the air plasma system, the first step of burning back the coating is eliminated, as the plasma will attach its arc right through the coating. The plasma gouging arc, operating at between 65 and 105 amps is fast, yet the depth is easily controllable by the operator, minimizing the chance of burn through of the pipe wall. After gouging, generally the gouged area is ready for rewelding without secondary grinding.
The plasma process speeds up this process and can eliminate eye injuries from grinding, as well as eliminating the need for flammable gases for burning back the coating. The process provides a significant improvement in productivity and is easier on the worker than oxygen-acetylene, as it reduces ultraviolet glare when the amperage of the two processes are compared.
The use of plasma cutting systems in the shop and in the field to build and install modern, high-technology pipelines will continue to expand. As the oil and gas pipeline industry continues to grow in North America, it is important for contractors to take notice of better ways to improve safety, productivity and quality, and if you can do that by also lowering production costs, it will be a winning situation for all involved.
Jim Colt is application technologies manager for Hypertherm Inc., based in Hanover, N.H., with offices in the Netherlands and Germany.
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