The defects of OCTG mainly come from three aspects: defects of the OCTG itself, defects of the OCTG generated in the manufacturing and the use performance of the OCTG. Defects of the OCTG itself such as the mechanical properties and weighing of the OCTG not meeting the requirements. Defects generated in the manufacturing mainly are thread parameters such as the taper, pitch, tooth height, tooth shape and the concentricity of the threads at both ends of the coupling not meeting the standard, blackened threads, breaking threads, uneven threads, damage to the screw thread and the welding seam of the drilling pipe not meeting the standard. The use performance of OCTG does not meet the requirement, including anti-collapse, anti-corrosion, perforation and anti-stick performance.
Defects of OCTG pipes in the manufacturing of threads and their prevention
In the process of processing threads of OCTG pipes, defects such as blackened threads, broken threads, scratches of threads and thread parameters exceeding the standard.
(1) Blackened threads
The blackened thread happens, because the partial processing amount of the thread is too small, which is related to the accuracy of the outer diameter and wall thickness, ovality and flatness of the pipe end of the steel pipe. The appearance of blackened threads on the pipe body is often caused by the small outer diameter of the pipe body, the insufficient straightness of the pipe end and too much ovality. Blackened threads that occurred for couplings are generally caused by the outer diameter of the steel pipe being too positive, the wall being too negative, or the ovality being too much.
(2) Uneven threads
The reason for the uneven thickness of the thread is similar to that of the blackened threads, which is caused by the uneven wall thickness, bending or too much ovality of the pipe end of the steel pipe. Sometimes when uneven threads happen in the threading process or the control of the processing amount is unreasonable, there may also be an over-negative difference in the wall thickness of the threaded bottom, which will seriously affect the connection strength of the OCTG pipe.
(3) Breaking threads
When the thread chaser is cutting the thread at high speeds and great force, once the thread is chipped, it will cause the thread to break. Usually, the breaking thread is mainly caused by the large-sized non-metallic inclusions in the steel, and it is also related to the quality of the thread chaser and the stability of the threading process.
(4) Damage to threads
Damage to threads of OCTG pipes is generated during production and transportation and storage of finished products. In order to prevent the exposed threads of the OCTG from being scratched, crushed and rusted, in addition to ensuring that the thread does not collide with hard objects such as transport rollers and inclined grate bars. Screw the outer protection ring with internal threads on the thread of the body, and screw the inner protection ring with external threads on the coupling thread.
API Spec 5CT standard stipulates the following aspects:
- The thread processing factory should screw on the inner and outer thread protection rings. The design, material and mechanical strength of the thread protection ring are required to protect the threads and pipe ends from damage during normal loading, unloading and transportation.
- During the transportation and normal storage of oil and casing, the design and material of the thread protection ring should be able to isolate the thread from dirt and water. The normal inventory cycle is about one year.
- The material selection of the thread protection ring should not contain any material that may cause thread corrosion or the thread protection ring to bond to the thread. It can be applied to the service temperature of -46℃ to +66℃:
- Bare steel thread protection rings shall not be used on L80 grade 9Cr and 13Cr pipe bodies.
(5) The thread parameters not meeting the standard
Thread processing is the most important process in the production of OCTG, and it is also a key process that determines the thread quality of OCTG. At present, most OCTG threads are processed by special CNC machine tools. When machining threads, the workpiece is automatically centered and clamped by floating. The tool for machining the thread uses a carbide tool, and the spindle rotation is infinitely variable. There are two kinds of threading methods: one is that the workpiece rotates, and the tool performs plane motion; the other is that the workpiece does not move, and the tool needs to rotate and perform feed motion. These two types of machine tools have their characteristics. The former is flexible, which not only has high productivity in processing general tapered threads, but also can process threads with direct connections, special connections and good air tightness; the productivity of processing general tapered threads of the latter is better than that of the former, but the processing of special threads requires pre-processing machine tools. Various parameters of the thread such as the pitch diameter, tooth height, taper, pitch, tooth angle and tight pitch will affect the connection strength and sealing performance of the thread. The tight pitch of the thread is the comprehensive value of the fluctuation of each single parameter of the thread. Even if the individual parameter of the thread is qualified, the tight pitch may be unqualified. In addition to the quality of the pipe billet, the accuracy of the parameters of the thread is also related to the method of thread processing, the type of machine tool and the stability of the processing, as well as the dimensional accuracy and wear resistance of the thread chaser. When other conditions are the same, the dimensional accuracy of the thread chaser determines the accuracy of the thread size. Generally, the dimensional tolerance of the thread chaser is only l/3 to l/4 of the product tolerance, or even higher.
(6) Torque and J value exceeding the standard
The torque of oil and casing refers to the make-up torque generated when the coupling and the pipe body are screwed together. The control torque is to ensure the connection strength of the coupling and the pipe body and the contact pressure stress on the side of the thread, and cooperate with the corresponding thread sealant to achieve anti-leakage of oil and casing. For API standard threads, the J value represents the distance from the pipe end to the center of the coupling after the coupling and the pipe body are tightened, which is one of the important parameters to determine the quality of the threaded connection.
(7) Leakages
In order to avoid leakage of oil and casing due to insufficient contact pressure between the oil and casing body and the coupling thread, the oil and casing with the coupling shall be subjected to the hydrostatic pressure test according to the standard. The leakage of the thread connecting the pipe body and the coupling is related to the type and quality of the thread, quality of oil, casing screw connection and thread sealant and other factors. As far as the thread type is concerned, the sealing performance of the round thread is better than that of the trapezoidal thread, and the special thread is better. The high-precision thread shape and reasonable oil and casing screwing torque are beneficial to improving the sealing performance of the thread. Thread sealant can lubricate, fill thread gaps (seals), and prevent corrosion when coupling and oil and casings are used.
