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What Is The Reason For The Failure Of The Drill Joint Thread?

Dec 20 , 2019

After years of production practice, the main failure modes of drill pipe joint threads are as follows. Oil and gas drill pipe manufacturers take you to know.

1) Hook-and-loop buckle: Occurs when there is a high axial pressure, the external thread is forced into the internal thread, causing the internal thread to swell or stick and cause the connection to fail; the tightening torque is too high or the downhole is too high When torque is applied, it will fail due to expansion or sticking.

2) Screw shear failure: Screw shear failure often occurs at the end of the complete buckle. The larger the thread taper and the shorter the thread, the more likely it is that the shear failure will occur.

3) Fracture: Because the stress is greater at the end of the complete buckle, the fracture often occurs at the end of the thread.

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4) Slip: When the taper of the thread is large, the number of tightening turns has not reached the appropriate number of turns and the torque has reached the recommended value. At this time, it is subject to the axial tensile force and it is easy to trip; when the filling of the thread gap is unreasonable There is also this possibility.

5) Inverse buckle: A small tightening torque may cause the thread to be unable to bear the applied axial load and downhole torque, which will cause the inverse buckle to fail, may cause some drill strings to fall into the well, and may even cause the borehole to be scrapped.

6) Barbed buckle and seal failure: The torsional vibration of the drill string causes the drill string to rotate faster and slower. When the drill string suddenly accelerates and rotates, the torque may suddenly increase due to the difference between the drill string and the good wall, the external thread, and the internal thread. The interaction causes high heat at the joint so that the thread grease flows out from the thread gap, which may cause seal failure. At the same time, high-pressure fluid flows out of the pipe along the thread gap, causing barbed buckles; axial tension will also reduce the sealing ability; Tight torque is too small, too low thread interference also leads to lower sealing capacity; lateral vibration of the drill string makes the drill string subjected to alternating bending stress will also cause seal failure; machining errors, unreasonable tolerance fit is also an important reason.

7) Application of threaded joints for high fatigue strength drilling tools. With the increase of deep well drilling year by year, drilling tool fracture accidents are also increasing. Ninety percent of them are fatigue fractures of threaded joints in bottom drilling tools, and 80% of them are fatigue fractures of externally threaded joints. The main reason is that due to the uneven distribution of the external load and preload on the threads of the outer thread joint of the drill bit, the threads in the later engagement area (LET) of the thread bear most of the load, so the drill's male threaded joints are prone to fatigue fractures here. Therefore, reducing the thread load in the LET area has become an important way to prolong the fatigue life of the drill's external thread joint.

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