Home News & Events Mechanized Pipe Welding 101 Article in October World Pipelines Magazine

Mechanized Pipe Welding 101 Article in October World Pipelines Magazine

Oct. 30, 2013

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Mechanized Pipe Welding 101, an introduction to producing pipe girth welds with a machine.


The pipeline industry is strong and growing today due in part to increased demand for oil and gas along with improved sources provided by advances in drilling and hydraulic fracturing.  However, as has been recently discussed in many forums, the supply of qualified, skilled welders is diminishing.  This, along with more critical specifications for line pipe including increases in material strengths which require a more tightly controlled welding process, present a strong case for the pipeline contractors to explore the mechanization of field welding, of pipelines.


Welding automation in general has been explored for years.  Resistance spot welds in the automotive industry have been made using automated systems since the 1960’s.  Many other consumer products have been manufactured using welds made via mechanized and automatic systems since the 1980’s.  Automatic welding is commonplace in many industries.  The welding of pipelines however, has not necessarily followed this same trend.  Automated welding of pipelines has been performed successfully since the early 1970’s, both onshore and offshore.  Due primarily to the high costs associated with the laying of offshore pipelines, the automation of these welds were more financially attractive and thus were the first to be successful.  The last 20 years have seen many successful applications of automated welding on many large diameter, onshore pipelines.  


If you are a pipeline contractor facing a reduced skilled work force, more stringent welding specifications due to the newer, higher strength pipe material and compressed time schedules, you are probably facing the possibility of being required to change the way you make pipe welds.  The following is a quick review of the basics of automated pipe welding equipment.


First we must define a few words often used when referring to this topic: automatic, mechanized and orbital.


Mechanized welding is a weld performed such that the welding gun is held by a machine which controls travel speed and motion but the physical positioning of the weld being made is controlled by the welder.  The machine may control some portion of the welding process such as the electrical weld parameters and weaving motion, but the welder adjusts the position of the welding gun via some means such as a remote steering control or by modifying the position of the device holding the gun.  


Automatic welding is a weld performed with little or no human intervention.  The machine controls all portions of the welding process along with the position of the weld with respect to the weld joint.  The welding operator puts the machine in place and starts the process.  Once started, the machine handles everything else until it is stopped by the operator.  In some cases, the machine even stops itself.


Orbital welding is most often performed using the GTAW or TIG process.  In orbital welding the process is performed continuously around the pipe, starting and stopping very near the same location on the pipe.  The welding of pipelines however is done in what is referred to a “double up” or “double down” progression.  The machine is started at the top or bottom of the pipe and is directed to travel around one side of the pipe and is stopped at the opposite side.  It is then repositioned at the original starting point and directed to travel around the other side of the pipe.  As the weld is not made in a continuous, non-stop fashion it is not considered a true orbital weld.


So, what makes up a mechanized pipe welding system?  Generally, a system is composed of a track, a tractor, welding equipment and some sort of control that connects them all together along with an enclosure that physically houses the system.  


Track – The track is often referred to as a track, a band, a ring or even a rail.  The track provides a uniform surface upon which the tractor can travel.  Traveling directly on the surface of the pipe is not typical as any scarring of the pipe surface or the coating is not acceptable, therefore, the tractor travels upon a track.  These tracks are usually made up of aluminum or a steel or stainless steel.  In the case of the aluminum, it is often an extrusion with a cross section which provides a feature for a locating surface for the tractor to ride upon.  The steel and stainless steel track is typically a thin strip or strips.  The extrusion or the strips are typically assembled with some sort of stand-off that provides a consistent position relative to the surface of the pipe.  Sometimes these stand-offs are spring loaded to allow for variation in the pipe surface.  The track also provides a resistive surface to allow the driving motion of the tractor.  This drive function can be by squeezing the edges of the track, driving against a knurled surface on the face of the track, driving against a rubber or plastic belt located in or on the track or by a gear rack located as part of the track or mounted to it.  In nearly all cases, it is necessary to use one size track for a specific size pipe.  It is sometimes possible to allow some adaptation by adjusting the size of the stand-offs supporting the track but that will obviously only allow a track of a larger diameter to fit upon a smaller diameter pipe.

   
Tractor – The tractor is very often referred to as a bug and thus the industry title of “bug and band systems” is often used.  The tractor drives upon the track and provides the motion necessary for performing the weld.  The tractor may offer 1, 2, 3 or more axes of motion.  One axis would be for travel only and would provide controlled speed as the tractor proceeds around the pipe.  A two axis system would include an axis of motion perpendicular to the direction of travel, in addition to the travel axis, to provide the ability to oscillate the welding gun during the welding process.  A third axis would add the vertical position of the welding gun.  


In many cases, these axes of motion are provided with some sort of feedback control loop that monitors the actual motion as compared to the motion that is requested and expected.  This is an important characteristic to consider when comparing these products.  Better control of the system motion will result in better weld uniformity and productivity. Using feedback control allows the travel speed to be held constant at the required speed as the tractor travels up around the pipe.  The weaver or oscillation axis is designed so as to create a uniform, consistent oscillation motion of the gun throughout the weld.  The weaver axis may be of a linear or pendulum style.  This axis very often includes the ability to cause the motion to stop for a period of time at the ends of the oscillation travel known as dwells.  This allows the weld puddle to expand and create a more uniform fill at the toes, edges of the weld bead.  The vertical travel axis gives the operator the ability to raise and lower the welding gun in order to keep the electrode extension consistent during the welding process and also to elevate the gun to allow access to the nozzle and consumable parts of the gun.  In some cases, a feedback function that monitors the welding process is added to the control of the vertical axis.  This function ensures that the welding parameters, particularly current, are held within tight control by adjusting the vertical position of the gun relative to the pipe surface.  Additional axes may provide the ability to change gun angle during welding or in some cases provide the ability for two welding guns to be mounted to the same tractor in order to increase productivity.

 
As explained in the discussion of the track, there are various methods employed to connect the tractor to the track and impart the travel motion around the pipe.  This is innate to the tractor/track design.  The design of this mounting and travel are very important in the application of the equipment.  It is critical that this mounting process be easy and quick to perform by the welder.  In most cases mounting of the tractor to the track takes only seconds and differences between styles is a matter of personal preference.  There is however advantages and disadvantages to the different drive methods.  Systems that squeeze the edges of the track for motion can slip if some external force pulls on the tractor but these systems are very quick to mount on the track.  Systems with a rack and pinion drive system render very tight control over motion that will not slip during travel but they do require a few more seconds to mount to the track.  Other drive system would lie between these two styles each having unique positive and negative traits.


Welding Equipment – The welding equipment is typically made up of the welding power source and the wire feeder.  Together, these items provide the controlled electrical power to perform the weld.  


The welding power source used can be of many types and manufacturers, each with their own characteristics.  The size and weight of the power source with respect to the output available is important to consider.  Older, large models provide basic functions and they take up considerable space and payload capacity.  They are robust, tried and proven products.  New styles of welding power sources with digital control and inverter power conversion provide high output with very precise control for nearly any required welding process in relatively small, lightweight packages.  Some of these new options are equally as tough and field sturdy as the older, larger machines and provide advanced welding process functions only available in the shop just a few years ago.  


Wire feeders for pipe welding mechanization come in all sizes and capacities.  In some cases these feeders are manufactured and supplied as part of the welding tractor.  In other cases they are standard units direct from the welding power source supplier.  Depending upon the specific application the feeder could be mounted directly on the tractor, mounted rigidly to a support structure or a unique push-pull system which shares the function between a stationary feeder and one mounted on the tractor.  The weight of the tractor would obviously increase with the feeder attached, however the feed distance is very short which reduces wire feed concerns.  

Welding power source functionality along with wire feeder placement must be considered during the specification process.  The welding process will most often dictate the style and type of equipment necessary to accomplish the weld.  If the specification requires a gas-shielded, flux cored welding process then a lower cost welding power source and feeder may be adequate.  If the requirement is for a very high wire feed speed, high travel speed, pulsed gas metal arc process or for an open root pass using one of the modified short circuit processes then much more sophisticated welding equipment would be required.  


Control System – Integrating all of these various parts is the control system.  How the various machines work together and how the operator interfaces with the system can be the most important facet when implementing mechanized welding.  

Simple systems using standard CV welding power sources with off-the-shelf wire feeders can have very simple control systems.  All welding process controls can be set directly on the power source and wire feeder just as when performing the weld manually.  The tractor system may only turn the welding contactor and wire feeder on and off via a simple switch.  All motion control would be input directly at the tractor via simple control pots and switches for travel and oscillation.  These simple controls may be mounted directly to the tractor or to a hand-held pendant. Systems of this sort would provide an economical initial solution for a contractor presented with a relatively short line or for tie-ins.  Systems of this type like the Bug-O Systems Universal Bug-O-Matic used with any of the Inverter type CV power sources and a suitcase style wire feeder present a relatively economical solution.  This would be a basic approach to mechanized welding.


A more complicated system might entail a separate control box with some sort of input device like a touch screen or keyboard.  This system would provide more extensive control of the welding process and of the motion of the tractor.  The operator would likely control the operation via a hand-held pendant.  A system like this could be equipped with a basic inverter, CV power source and a small, tractor mounted wire feeder such as the Bug-O Systems, Piper Bug.


Going a step further, the control system could provide digital control of a welding power source such as the Lincoln Power Wave S350.  With this more advanced power source the welding process options are many.  A power source of this type coupled with a tractor and control as in the Bug-o System Piper Plus would allow welding with the pulsed gas metal arc process for specialty metals or with a the modified short circuit process such as STT for open root welding.  Systems like the ones described above can be readily applied to standard weld joint designs.  


At the top end of the spectrum are the fully automatic systems that integrate not only the welding controls but also use special sensing systems to automatically adjust the position of the welding gun to the welding joint as the weld is made.  These sensors may monitor the welding parameters and make adjustments with through-the-arc seam tracking or they may use an external sensor such as a laser tracking device.  These advanced systems are capable of observing the weld joint and modifying not only the position of the gun relative to the joint but also modifying the welding parameters themselves to adapt to changes in the joint.  In addition to automatic joint tracking, many systems like this provide the ability to use 2 guns during the same weld pass, greatly improving productivity.  One limitation of this type of system is that they may require considerable preparation of the weld joint including unique J bevels to reduce the volume of the joint to be filled.  Application of these advanced systems are typically handled by crews who are very familiar with not only welding equipment itself but also the other various pieces needed to make the application of these high production systems successful.


Enclosure – In order to apply these mechanized systems it is almost always necessary to provide an enclosure.  The welding processes that allow mechanized welding to be successful all require an external gas shield for the welding arc.  Unlike familiar shielded metal arc or stick welding, these gas shielded processes require an environment of calm air so that the shielding gas is not blown away.  A cabin style enclosure which is placed over the pipe at the location of the joint provides this environment.  Further, all of the necessary portions of the mechanized system can be mounted to the inside of the enclosure to provide an organized, tidy workspace for the welder as well as protect them from the elements.  


 Mechanized welding can appear at first glance to be a complicated affair that has just too much involved for the average pipeline contractor to learn.  This is not at all the case.  Given the basic information provided here, a contractor should be able to make some initial decisions that could very quickly show results.  In fact, with a relatively small initial investment and a short period of training and education, mechanized welding of pipe joints can greatly improve your bottom line.