Tool joint ( drilling rig ) – Readyzone

Tool joint

Tool - joint

Tool joint

Tool joints are subject to abrasion on their exterior surface since they invariably rub against the rock wall of the hole. Welding beads of hard surfacing material around the outside of the tool joints often doubles their useful life. The threads and shoulders of the tool joints represent surfaces which slide over each other with extreme pressures as the joints are made up. If foreign particles are present or too much friction develops, parts of the metal will tear away and roll up very much like a snowpall, which results in “galling” of the thread or shoulder surface and eventual failure of the joint. It is therefore very important that the threads and shoulders be maintained clean and covered with a suitable thread lubricant.

The drill string in a deep well has the relative dimensions of a length of thread: the long, slender shape gives the drill string certain inherent weaknesses. A new piece of drill pipe could fail from the application of excessive tension or torque, although such failure is rare. The yield strength of the various types and sizes of drill pipe is listed in tables giving the properties of tubular goods, and drill pipe will be permanently stretched and thereby weakened if the yield strength is exceeded in a direct pull on the pipe. Most drill pipe failures are caused through the process of corrosion-fatigue. Such failures usually start on the inside of the pipe. Minute cracks in the surface open and close as the pipe works in rotation and tension or compression, and corrosive fluid is pumped in and out by such working so that the crack is enlarged by both mechanical failure and corrosion until eventually the piece of pipe is discarded or fails. The inside surface of some drill pipe is plastic coated to retard such action. In some instances, sodium dichromate has been added to the drilling mud to inhibit corrosion, particularly where the mud is salty. Where a small hole has extended through the pipe during drilling operations, the hole has been rapidly enlarged by mud erosion to the extent that the pipe twisted off. An alert driller will be able to detect a washout almost immediately by a change in pump pressure, thereby eliminating a fishing job or damage to casing. Where such holes have occurred inside of the cased portion of the well hole, the casing has been damaged by erosion of the mud. Any wobbling action in the rotating drill string is conducive to fatigue failures. Such wobble may be caused at the top of the string by a bent Kelly. Formerly, wobble was common in the bottom portion of the drill pipe when part, or all, of the weight on the bit was supplied by the drill pipe. Shock such as may occur from carrying too much weight on a drag type bit is also conducive to pipe fatigue and failures.  

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Reference : Mc Cray & Cole, Oil Well drilling Technology, New India Publication.

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