Design and application of anti-drop drill pipe with fracture self-locking function

Drill pipe will inevitably break and drop during its long-term service, which not only seriously affects the drilling efficiency and the rate of hole formation, but also brings great economic losses to coal mines. In terms of drill pipe drop caused by the random fracture during drilling, the shortcomings of fishing tap were pointed out. By analyzing the fracture characteristics of drill pipe, it is concluded that the key to solve the problem of drill pipe drop is to develop the axial connection technology after the fracture of pin joint. Herein, an active anti-drop technology with fracture self-locking function was proposed, with its technical principles and the design principles of anti-drop drill pipe introduced. On this basis, a ø73 mm anti-drop drill pipe was designed and developed. Laboratory tests showed that the anti-drop drill pipe had a torsional resistance of 20 120 N·m, a tensile strength of 1 444 kN, and a secondary tensile strength of 384 kN after the pin joint fracture. Besides, the anti-drop drill pipe was subjected to the industrial test by Shanxi Coking Coal XiShan Coal Electricity Group Co. Ltd. During the test, the accumulated drilling footage reached 43 757 m by 200 drill pipes, and 5 fracture accidents occurred at the root area of pin joint thread. Specifically, the drill pipe below the fracture point was fully taken out in 4 accidents, but the drill pipe was failed to be taken out in one accident due to its deep burial in the borehole. The field test results show that the anti-drop drill pipe has reasonable structure, long service life and reliable active anti-drop capability. Generally, the research could provide a new direction for drill pipe design, as well as drilling tool support for the safe, efficient and low-cost drilling in underground coal mine.