After years of production practice, Kaiqiu Drilling Tools Manufacturer ( https://www.kqdrill.com/ ) believes that the main failure modes of drill pipe joint threads are as follows.
1) Velcro and buckle: Occurs when there is a high axial pressure, the external thread is forced into the internal thread, causing the internal thread to expand or buckle and cause the connection to fail; the tightening torque is too high or the downhole is too high When torque is applied, it will also fail due to bulging or hooking.
2) Thread shear failure: Thread shear failure often occurs at the last complete buckle. The larger the thread taper and the shorter the thread, the more likely to have shear failure.
3) Fracture: Because the stress is greatest at the last complete buckle, fracture often occurs at the last complete buckle.
4) Sliding buckle: When the thread taper 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 easy to trip due to axial tension; when the thread gap filling is unreasonable There is also this possibility.
5) Undercut: A small tightening torque may cause the thread to be unable to bear the applied axial load and downhole torque, resulting in undercut failure, which may cause part of the drill string to fall into the well, and even cause the borehole to be scrapped.
6) Barb and seal failure: the torsional vibration of the drill string causes the drill string to rotate at a fast and sometimes slow speed. When the drill string suddenly accelerates and rotates, the torque may suddenly increase due to the gap between the drill string and the well wall, external thread and internal thread The interaction causes high heat to be generated at the joint, so that the thread grease flows out from the thread gap, which may cause sealing failure. At the same time, high-pressure fluid flows out of the pipe along the thread gap, causing stabs; axial tension will also reduce the sealing ability; Too small tightening torque and too low thread interference also lead to lower sealing ability; the transverse vibration of the drill string causes the drill string to withstand alternating bending stresses, which will also cause sealing failure; machining errors and unreasonable tolerances are also a problem Important reason.
7) The application of threaded joints for high-fatigue-strength drilling tools has increased year by year as deep well drilling has increased year by year, and drilling tool fracture accidents have also increased. Among them, 90% are fatigue fractures of bottom hole threaded joints, and 80% of these are fatigue fractures of external threaded joints. The main reason is that the external load and preload are not uniformly distributed on the thread teeth of the external thread joints of the drill bit. The thread teeth in the last engagement area (LET) of the thread bear most of the load, of which the first thread bears the load. It accounts for about 30% of the total load, so the external threaded joint of the drilling tool is prone to fatigue fracture here. Therefore, reducing the thread load in the LET area has become an important way to extend the fatigue life of the external threaded joint of the drill. Accordingly, the Petroleum Pipe Research Institute has developed and studied a high-fatigue strength drilling tool external thread joint—LETR external thread joint. The load borne by the first thread in the last engagement area (LET) of this joint thread is reduced by about 25%, and the load is distributed evenly on the threads of each circle than in ordinary joints. Its structural feature is to reduce the thread profile height in the final engagement area (LET) of the thread, while improving the fatigue strength, while maintaining the same torsion strength, top-off performance and sealing performance as API drill tool threaded joints. The joint threads are interchangeable. The machining principle is to cut a part of the thread profile height in the final engagement area (LET) of the API external thread.