Tool Joint - Drill Pipe Welding Techniques
Authors: Smith Thomas B. 
Published: December 2007

The safe and economical joining of tool joints to drip pipe has challenged the ingenuity of designers for many years. This paper will take a look at the methods in current use. i.e., "Flashweld" precess, inertia welding, and friction welding. Comparisons will be made as to how these methods differ and how they can be used to achieve the same end. This study will show the attention given to the small but critical area of the weld so the drill string user can safely assume the attachment will be trouble free and not a limiting factor in his drilling operation.

With the advent of rotary drilling. drill string became a necessity and a problem. It was necessary if the drilling; was to go deeper than one joint, and it became a problem because severe service made failure an all too frequent occurrence. The oilfield has always been good at getting by with whatever is available until something better can be made. Because of this practice, the first drill string was thought to have been lap welded line pipe and couplings, the two being joined with sharp "V" threads. (Fig. 1) This drill string worked bat there were problems. Repeated makeup and breakout of the sharp "V" threads resulted in galling and handling damage and if these did not put the drill string out of service, fatigue failure at the last engaged pipe thread did.

The first step toward a solution to this thread problem was the Whittier tool joint. (Fig. 2) The coarse tapered threads, similar to cable tool connections of that day, meant the makeup and breakout were faster. These threads also were less susceptible to handling damage and to galling. The tool joints were more durable but they were still connected to the pipe with sharp "V" threads so emphasis was added to failures resulting from pipe fatigue at the last engaged thread.

Some of the next steps in drill string improvement were to upset the pipe and to use tool joints of heat treated steel (Fig. 3). Both of these changes helped but the sharp "V" threads were still a problem so the next improvements were the use of rounded "V" threads and controlled make-up of the tool joint to the pipe. These changes were introduced in the late 1920's.

Still fatigue failure in the pipe at the last engaged thread continued to limit drill string life severely (Fig. 4). A conventional weld was applied to give support outside the last engaged thread (Fig. 5). This helped, but the additional expense caused it to be superseded by the counterbore weld tool joint in about 1937 (Fig. 6).

Other solutions for better drill string were tried. One of these, about 1937, was the Pittsburgh "Double-Grip" Tool Joint (Fig. 7). An interference fit on a tapered band outside the threads provided mechanical support outside the troublesome last engaged thread.

Soon behind the Pittsburgh "Double-Grip" Tool Joint came a barrage of shrunk-on tool joints. National Supply's "Double Seal Shrink Thread", Reed's "Super Shrink Grip", American's "Straight Grip", and Hughes' "Sealgrip" Tool Joints (Fig. 8). began to take large portions of the market away from the counterbore weld. The shrunk-on tool joints were better because they were truly field replaceable and because the support outside of the last engaged thread greatly increased the fatigue life at that point. But all the screwed-on and shrunk-on assemblies were expensive. They required pipe upsets which were long and heavy. The pipe ends and the mating portions of the tools joint required close tolerance machining and the loss of length, when the pipe ends were screwed into the tool joint members, was considerable

These shortcomings pointed the way for the next drill string improvement -" Flashweld" tool joints were introduced in 1938 and the first welds were as shown in Fig. 9A with the welds in the heavy portion of the tool joint. In nine, these gave way to the weld in neck assembly (Fig. 9B), which used a still fighter and less expensive upset. From their introduction. "Flashweld" tool joints began to push shrunk-on designs out of the market; they were in the enviable position of being not only less expensive but better:Tool joints butt-welded to plain end (unthreaded) upset pipe made expensive machining of pipe ends and tool joint sockets unnecessary.The loss of length of the shrunk-on joint (5 or 6 inches per end) was reduced to a length loss due to weld burnoff per end) was reduced to a length loss due to weld burnoff of some 3/8 to 3/4 inches per end.The long heavy upset was soon replaced with a shorter smaller upset (The "Flashweld" tool joint was not field replaceable but this became less important over the ensuing years.)

There were other efforts to make serviceable drill string. One area of endeavour was the integral tool joint. It was machined from an extra long, extra heavy upset on each end of the pipe. Both Jones and Laughlin and Hydril had entries in the integral field.