The most dreaded form of drill pipe failure is the breaking of drill pipe. The fracture of drill pipe in the process of guiding and directional drilling and laying pipe will cause serious impact on the project, not only the economic loss, but also affect the construction period, the loss of credibility, the consequences unimaginable. Therefore, as a construction enterprise, we must learn how to choose and use drill pipe correctly; As a drill pipe manufacturer, we should know the special requirements of trenchless engineering for drill pipe, so as to produce drill pipe with high strength, high elasticity and high reliability.
Causes of drill pipe failure (mainly fracture) : In addition to the quality of drill pipe itself, improper operation during construction is the main cause of drill pipe failure.
1. Analysis of main failure forms and causes of drill pipe
The api drill pipe used in trenchless guiding and directional drilling is often subjecting to the action of bending force as well as large pulling force and torsion force. As the drill pipe passes through the curve section, it is forcing to bend, with compressive stresses on the inside of the arc and tensile stresses on the outside of the arc.
As the drill pipe rotates in the curve section, the rod is subjecting to alternating tension and pressure stress, and the smaller the radius of curvature of the hole, the greater the alternating stress. The research shows that when the alternating stress reaches a certain value, it is very easy to cause fatigue cracks in drill pipe. The initial fatigue crack in drill pipe is very small and difficult to be seen by the naking eye, but the fatigue crack develops very fast and finally appears as a sudden brittle fracture. The test results show that fatigue fracture of drill pipe under alternating stress is the main reason for failure of drill pipe without excavation. This conclusion is supporting by the fact that many drill pipe fracture accidents occurring during the construction of guide holes.
At present, there are many professional companies with rich experience and high quality in the trenchless construction team in our country, but more temporary teams without any construction experience and technology, they are often not even heard of the radius of curvature of the drill pipe (or drilling hole), leading to the trenchless construction is the main man-made reason that causes the fracture of the drill pipe.
2. Measures to avoid abnormal failure of drill pipe
According to the analysis of the failure causes of drill pipe, in addition to the failure caused by normal wear and tear, the abnormal failure causes of drill pipe can be dividing into two aspects: worker operation reasons and drill pipe quality reasons. Therefore, we can avoid abnormal failure of drill pipe by improving the quality of drill pipe and standardizing operation.
3. Improve the quality of drill pipe
(1) Selection of drill pipe materials:
In order to adapt to the stress analysis of trenchless oriented and directional drill pipe, the drill pipe body should have high tensile strength, good bending performance and good impact toughness. The rod body material should be selecting as seamless steel pipe with medium carbon alloy structure. The alloy elements should contain Cr, Mo and other elements to improve the tensile strength and impact toughness of the material, and Mn, Si and other elements to improve the elasticity of the material (i.e., flexural performance). Sometimes also contains trace B, V and other elements to improve the hardenability of materials. Commonly used rod body materials are: 36Mn2V, 35CrMo, 42MnMo7, 35CrMnSi, 45MnMoB.
The wall thickness uniformity and rolling defects of seamless steel pipe used in the rod body are also important factors affecting the quality of drill pipe. Small steel mill rolled steel pipe wall thickness uniform situation is serious, when the drill pipe greater torque action, easy in the thin wall thickness of the longitudinal crack. Some steel pipes have defects such as heavy skin and pores, and the drill pipe is easy to break or leak from here.
Drill pipe joint force is the most complex, joint material high comprehensive mechanical properties. Most of the drill pipe joints are made of 35CrMo or 42CrMo bar material, but the forging joint material can greatly improve its comprehensive mechanical properties.
(2) Selection of processing technology:
At present, trenchless drill pipe in China mainly includes integral forging drill pipe (referred to as integral drill pipe), upsetting + friction welding drill pipe (referred to as upsetting welding drill pipe) and simple friction welding drill pipe (referred to as friction welding drill pipe).
(3) Processing of upsetting and thickening section:
Whether the whole drill pipe or upsetting welding drill pipe, the processing quality of the transition section between the upset and unupset part is the main factor affecting the quality of upsetting. The transition section shall be of sufficient length, and the transition of wall thickness shall be uniform, smooth and wrinkle-free. In order to remove the stress in the ITD and improve its mechanical properties, the rod body should be treating by upsetting.
(4) joint structure type
a. Stress dispersion trough: Test studies show that fatigue fracture of trenchless guided and directional drill pipe under cyclic bending stress is the main failure mode of drill pipe, and this failure mainly occurs at the root of male joint or thread of drill pipe. The stress concentration can be reducing effectively by designing stress dispersing groove at the root of drill pipe male joint, thus improving its bending ability. In addition, the bending resistance of drill pipe joint root can be improved by increasing the diameter of thread, the design of taper thread and the radius of thread bottom.
b. “Double top” structure: the so-called “double top” structure, that is, when the drill pipe male and female buckle is tightening, the end of the female buckle is tightening with the shoulder of the male buckle, and the inner shoulder of the female buckle is tightening with the end of the male buckle. “Double top” structure drill pipe is difficult to process, but it has many advantages compared with “single top” drill pipe:
l Good sealing effect drilling fluid leakage from inside to outside through two sealing surface, leakage resistance.
The “double top” drill pipe provides two torsion-resistant shoulders with a greater total contact area than normal drill pipe, resulting in greater torque.
l Improve the service life of drill pipe Common drill pipe is often because of joint wear, the end of the female buckle becomes thinner, and the male buckle cone into a bell, thus making the whole drill pipe failure. The “double top” drill pipe continues to function effectively on the inner shoulder after the joint has been severely worn.
(5) Surface treatment of the wire buckle:
Nitriding treatment of the surface of the wire buckle can effectively prevent sticking, but also improve the wear resistance of the wire buckle, so as to improve its service life. The thickness of nitriding layer is generally 0.2-0.3mm. If the thickness is too large, it is easy to produce crushing cracks. If the thickness is too small, the anti-bonding effect is poor. The whole drill pipe cannot be nitrizing and is instead treating with a much smaller layer.
(6) Machining accuracy of thread:
The joint thread should be processing with forming thread turning tool on CNC lathe to ensure the interchangeability of drill pipe. The joint thread tightness should be controlling within a reasonable range to ensure good mesh of the wire buckle.
4. Improvement of construction technology
(1) Standard operation:
API drill pipe shall be uingd in strict accordance with the allowable tension, torque and curvature radius of various drill pipes during construction. For construction enterprises, only according to the standard use of drill pipe, in order to effectively avoid the abnormal failure of drill pipe.
(2) The role of the transition rod:
During reaming and pipe drawing, the radius of curvature of the drill rod connected to the reaming bit is likely to be much less than the designed radius of curvature of the drill hole, that is, r< < r< span=””> , so that the drill pipe in rotation and tension under the action of greater cyclic stress, so that its fatigue damage, and then crack and fracture. This kind of fracture occurs in the distance between 0.3~0.8m from the drill pipe male joint. By connecting the transition rod between the common drill pipe and the reaming bit, the small curvature transition between the reaming bit and the common drill pipe can be borne by the transition rod, thus protecting the common drill pipe. This function of the transition rod is often overlooking by users, but it is the cause of many seemingly unexplaining pipe breaks.
(3) Prevent drill pipe instability:
Drill pipe is a slender rod in mechanics, which tends to become unstable under pressure. Therefore, the drill pipe in the free state should be restraining in the construction stage of the guide hole to prevent the drill pipe from buckling and failure in the process of oblique jacking. The instability of drill pipe usually occurs in the drill pipe section between the rig holder and the entry point, so the length of this section should be shortening as far as possible or appropriate restraint measures should be taken. In order to prevent drill pipe instability, the length of the free section of drill pipe should be less than 20 times the diameter of drill pipe.
(4) The use of thread oil:
Good quality thread oil can effectively prevent drill pipe sticking, reduce shackle torque, reduce buckle surface wear.
(5) The use of washing fluid:
When drilling in sand layer and gravel layer, the application of high quality mud as washing fluid. Mud forms mud skin on the outer wall of drill pipe, plays a lubricating role, reduces drilling torque, reduces drill pipe wear, and plays a very important role in protecting drill pipe.
(6) Inspection of drill pipe:
After the drill pipe has been using for a period of time, the wear, bending and surface scratches of the drill pipe should be checking. Excessive wear, obvious bending and deep surface scratches should be eliminating in time. In particular, the wear condition of the upsetting transition section should be carefully examining, which is often the most prone to be worn thin and broken.