There is no real problem with threading a pipe. Most DIY types can do it in their workshop with hand tools, but when the pipe is 40′ long and ordered by the ton, you are in oil country. Pumping crude oil out of the ground requires drill pipe, casing, tool joints, tees, crosses, flanges, and couplings. All those items need to be machined. And that takes specialized workholding.
Oil country tubulars are big, heavy and hard to handle. Underground, they are subjected to extreme pressure, torque, internal expansion and contraction, and side loads. The trick is to thread the pipe within American Petroleum Industry (API) tolerances for a continuous drill string that can gradually curve over half a mile without breaking the seal and creating leaks. But there are many variables to be considered. Oil companies purchase their pipe from different mills and use different thread types for each unique subsurface condition. Threading companies are contracted to thread each pipe end. Many of these have their own proprietary thread profile geometry that must be machined to exacting tolerances.
Currently, seamless oil country tubular goods (OCTG) drill pipe, casing and tubing diameters are formed by forcing a steel billet over high-toe-angle piercer rollers with extreme force. This technology dynamically sizes the pipe to the desired diameter. All products are quality controlled to meet stringent API standards. The pipe is then shipped to threading companies all over the world.
How It Was Done
When you think of OCTG, you probably visualize the pipe being straight as an arrow. In reality, it can droop on the ends (end hook) or sag in the middle. Any material run-out exceeding the combined pipe wall thickness and thread form geometry is a problem. If the run-out is not corrected to rotate concentric within the machine spindle centerline, the thread geometry will not fully form due to the lack of material (these are known as black threads). Aligning pipe to the machine centerline is essential and used to be done manually.
A pipe rack load handling system would present 40′ long sections of pipe to a lathe, passing it through the rear of the lathe spindle to manually actuated front and rear chucks. Once clamped at both ends, the operator would manually align the pipe to the lathe spindle centerline by turning the master jaw pinion with a massive wrench. The process required plenty of muscle and could take up to 20 minutes.
Things changed considerably for most pipe diameters (14.8″ or less at first, now up to 22″) with the release of the Big Bore BB-N ES front-end pneumatic power chuck. The BB-N ES was the first chuck developed for end machining of long pipe with a full spindle bore. It was made possible by two principles invented by SMW Autoblok—air supply via distributor ring and SMW-profile seal rings. Built-in non-return valves maintain the air pressure during machining and the clamping pressure level is constantly checked by a safety control system. Pipe is loaded on a table, goes through a lathe and is threaded and clamped with less effort.
This was a great improvement over the manual method, but aligning the center could still take five minutes or more. Plus, a shim was needed between one jaw and the pipe to push it into alignment with the spindle centerline of the lathe.
Auto-centering become a reality with the introduction of the BB-FZA in the 1990s. For the first time, repeatable, accurate auto-centering of OCTG pipe was possible without human intervention (and the risk of error). Shims were no longer needed, which alone was a great safety feature.
Even though things were better, SMW Autoblok Corp. still wasn’t satisfied and the engineering teams set out to reinvent the centering process. In 2012, a new generation of chuck, the BB-FZA2G, was introduced. It uses three integrated centering jaws that move forward angularly and axially to center the pipe exactly at the area to be threaded. The three compensating jaws then grip the pipe in the eccentric position and the centering jaws retract axially into the chuck body. The whole alignment process takes 11 seconds and is the standard for high-efficiency machining of OCTG.