What are the causes of drill pipe breakage? How to avoid it?

　A terrible failure mode for drill pipes is breakage. If a drill pipe breaks during guided, directional drilling, or pipe laying, it will seriously affect the project, not only causing economic losses but also affecting the schedule, losing credibility, and resulting in unimaginable consequences. Therefore, as a construction company, it is essential to learn how to correctly select and use drill pipes; while as a drill pipe manufacturer, it is important to understand the special requirements of drill pipes for trenchless engineering and produce high-strength, high-elasticity, and high-reliability drill pipes.

　　Causes of drill pipe failure (mainly breakage): In addition to problems with the drill pipe itself, improper operation during construction is the main reason for drill pipe failure.

　　1. Analysis of main failure modes and causes of drill pipes

　　Drill pipes used in trenchless guided and directional drilling are often subjected to bending forces in addition to large tensile and torsional forces. When the drill pipe passes through a curved section, it is forced to bend, with compressive stress acting on the inner side of the arc and tensile stress acting on the outer side. When the drill pipe rotates in the curved section, the rod body is subjected to alternating tensile and compressive stresses, and the smaller the curvature radius of the borehole, the greater the alternating stress. Studies have shown that when this alternating stress reaches a certain value, it is easy to cause fatigue cracks in the drill pipe. The fatigue cracks initially generated in the drill pipe are very small and difficult to detect with the naked eye, but the fatigue cracks develop rapidly, eventually resulting in sudden brittle fracture. Tests have shown that fatigue fracture of drill pipes due to alternating stress is the main cause of failure in trenchless drill pipes. Many drill pipe breakage accidents during the construction of guide holes have confirmed this conclusion.

　　Currently, many trenchless construction teams in China are professional companies with rich experience and high quality, but more are temporarily assembled teams without any construction experience or technology. They often haven't even heard of the curvature radius of a drill pipe (or borehole), and letting them direct trenchless construction is the main human factor causing drill pipe breakage.

　　2. Measures to avoid abnormal failure of drill pipes

　　Based on the analysis of the causes of drill pipe failure, in addition to normal wear and tear, abnormal failure of drill pipes can be divided into two aspects: worker operation reasons and drill pipe quality reasons. Therefore, we can avoid abnormal failure of drill pipes from two aspects: improving drill pipe quality and standardizing operations.

3. Improving drill pipe quality

　　(1) Drill pipe material selection: To adapt to the stress analysis of trenchless guided and directional drill pipes, the drill pipe body should have high tensile strength, good bending resistance, and good impact toughness. The rod body material should be selected from medium-carbon alloy seamless steel pipes. The alloy elements should contain Cr, Mo, etc., to improve the material's tensile strength and impact toughness, and Mn, Si, etc., to improve the material's elasticity (i.e., bending resistance). Sometimes, trace amounts of B, V, etc., are also added to improve the material's hardenability. Commonly used rod body materials include: 36Mn2V, 35CrMo, 42MnMo7, 35CrMnSi, 45MnMoB.

　　The wall thickness uniformity and rolling defects of seamless steel pipes used for the rod body are also important factors affecting the quality of drill pipes. The wall thickness uniformity of steel pipes rolled by small steel mills is seriously problematic. When the drill pipe is subjected to large torsional forces, it is easy to crack longitudinally at thinner sections. Some steel pipes have defects such as heavy skin and pores, and the drill pipe is prone to breakage or leakage at these points.

　　The drill pipe joint is subjected to the most complex forces, and the joint material requires high comprehensive mechanical properties. Drill pipe joints are mostly made of 35CrMo or 42CrMo bar materials, but forged joint materials can greatly improve their comprehensive mechanical properties.

　　(2) Processing technology selection: Currently, domestic trenchless drill pipes are mainly divided into integral forged drill pipes (referred to as integral drill pipes), upsetting + friction welded drill pipes (referred to as upsetting welded drill pipes), and purely friction welded drill pipes (referred to as friction welded drill pipes) according to their processing technology.

　　(3) Treatment of the upsetting thickened section: Whether it is an integral drill pipe or an upsetting welded drill pipe, the quality of the transition section between the thickened and unthickened parts is the main factor affecting the upsetting forging quality. The transition section should have sufficient length, uniform and smooth wall thickness transition, and no wrinkles. After upsetting forging, the rod body should preferably undergo overall quenching and tempering treatment to remove internal stress in the transition section and improve its comprehensive mechanical properties.

　　(4) Joint structural type

　　a. Stress dispersion groove: Experimental studies have shown that fatigue fracture due to cyclic bending stress is the main failure mode of trenchless guided and directional drill pipes, and this failure mainly occurs at the root of the drill pipe male joint or the root of the male joint thread. Designing a stress dispersion groove at the root of the drill pipe male joint can effectively reduce stress concentration at this point, thereby improving its bending resistance. In addition, increasing the thread diameter, using a tapered thread design, and increasing the radius of the thread root can also significantly improve the bending resistance at the root of the drill pipe male joint.

　　b. "Double top" structure: The so-called "double top" structure means that when the male and female couplings of the drill pipe are tightened, the female coupling end is in contact with the male coupling shoulder, and the inner shoulder of the female coupling is also in contact with the end of the male coupling. The "double top" structure drill pipe is difficult to process, but it has many advantages compared to the "single top" drill pipe:

　　(1) Good sealing effect: Drill fluid leakage from the inside to the outside must pass through two sealing surfaces, resulting in high leakage resistance.

　　(2) Can withstand greater torque: The "double top" drill pipe provides two anti-torsion shoulders, and its total shoulder contact area is much larger than that of ordinary drill pipes, so it can withstand much greater torque than ordinary drill pipes.

　　(3) Improve the service life of the drill pipe: Ordinary drill pipes often fail because the joint wears out, the female coupling end becomes thin, and the male coupling cone forms a bell mouth. The "double top" drill pipe can still be used even if the joint is severely worn because the inner shoulder is still effective.

　　(4) Surface treatment of threads: Nitriding treatment of the thread surface can effectively prevent sticking and improve the wear resistance of the threads, thereby increasing their service life. The nitriding layer thickness is generally 0.2~0.3mm. If the thickness is too large, brittle cracks are easily generated, and if the thickness is too small, the anti-sticking effect is poor. Integral drill pipes cannot be nitrided, and instead, a much thinner phosphating treatment is used.

　　(5) Thread machining accuracy: The joint threads should be machined on a CNC lathe using a forming thread cutting tool to ensure the interchangeability of the drill pipes. The joint thread pitch should be controlled within a reasonable range to ensure good thread engagement.

　　4. Improvements in Construction Technology

　　(1) Standardized Operation: During construction, drill pipes must be used strictly in accordance with the allowable tensile force, torque, and radius of curvature for various drill pipes. For construction companies, only by using drill pipes according to the specifications can the abnormal failure of drill pipes be effectively avoided.

　　(2) Role of Transition Rods: In the process of reaming and pulling pipes, the radius of curvature of the drill pipe connected to the reaming bit is likely to be much smaller than the design radius of curvature of the borehole, i.e., r<, causing this drill pipe to be subjected to large cyclic stress under rotation and tensile force, resulting in fatigue damage, and then cracks and breakage. This breakage often occurs between 0.3~0.8m from the drill pipe coupling. Connecting a transition rod between the ordinary drill pipe and the reaming bit can allow the transition of the small curvature between the reaming bit and the ordinary drill pipe to be borne by the transition rod, thus protecting the ordinary drill pipe. This function of the transition rod is often overlooked by users, but the reason for many seemingly inexplicable drill pipe breakage accidents lies in this.

　　(3) Preventing Drill Pipe Instability: Drill pipes are slender rods in mechanics and tend to become unstable under pressure. Therefore, during the construction of the guiding hole, the drill pipes in the free state should be constrained to prevent the drill pipes from becoming unstable and bending during the inclined drilling process. Drill pipe instability often occurs in the drill pipe section between the drill rig chuck and the entry point. This section should be shortened as much as possible, or appropriate constraints should be taken. To prevent drill pipe instability, the length of the free section of the drill pipe should generally be less than 20 times the diameter of the drill pipe.

　　(4) Use of Threading Oil: High-quality threading oil can effectively prevent drill pipe sticking, reduce uncoupling torque, and reduce wear on the coupling surface.

　　(5) Use of Flushing Fluid: When drilling in sand layers and gravel layers, high-quality mud should be used as the flushing fluid. The mud forms a mud cake on the outer wall of the drill pipe, acting as a lubricant, reducing the drilling torque, reducing drill pipe wear, and playing a very important role in protecting the drill pipe.

　　(6) Inspection of Drill Pipes: After using the drill pipes for a period of time, the wear, bending, and surface scratches of the drill pipes should be checked. Drill pipes with excessive wear, obvious bending, and deep surface scratches should be eliminated in time. In particular, the wear of the upset transition section should be carefully checked, as this part is often the easiest to wear thin and break.