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Quality defects and prevention of oil well pipe thread processing

Date:2025-01-17
Quality defects and prevention of oil well pipe thread processing: During the thread processing of oil well pipe, the thread may have quality defects such as black skin, thread deviation, broken thread, thread scratch (bump), and thread parameter exceeding the standard.

(1) Black skin of thread: The black skin of the thread is manifested as "unsmooth" due to too small local processing of thread, which is related to the outer diameter and wall thickness accuracy, ovality, and straightness of steel pipe end of steel pipe. The black skin of the steel pipe body is often caused by the outer diameter of the steel pipe body being too small, the steel pipe end being not straight enough or the ovality being too large. Black skin of coupling is generally caused by the outer diameter of the steel pipe exceeding the positive tolerance the steel pipe wall exceeding the negative tolerance or the ovality being too large.

(2) Thread deviation: Thread deviation is the uneven wall thickness of steel pipe after threading, with one side thin and the other side thick. The reason for thread deviation is similar to that for thread, which is caused by uneven wall thickness, bending, or excessive ovality at the end of the steel pipe. Sometimes, when the thread processing is biased or the processing volume is not properly controlled, the wall thickness of the threaded bottom may exceed the negative difference, which will seriously affect the connection strength of the oil well pipe.

(3) Thread breakage: When the thread comb cutter cuts the thread at high speed and forcefully, once the thread breaks or "falls off", it will cause the thread to break. Generally speaking, the breakage is mainly caused by large non-metallic inclusions in the steel and is also related to the quality of the thread comb cutter and the stability of the threading process.

(4) Thread damage: The thread damage of the oil well pipe includes bruises and abrasions. It is caused during the production, transportation, and storage of finished products. To prevent the exposed threads of the oil well pipe from being bruised, crushed, or rusted, in addition to ensuring that the threads do not collide with hard objects (such as transport rollers, inclined grate bars, etc.) during production, an outer protective ring with internal threads should be screwed on the threads of the oil well pipe body, and an inner protective ring with external threads should be screwed on the threads of the coupling.
API Spec 5CT standard stipulates:
① The thread processing plant should screw on the internal and external thread protection rings. The design, material, and mechanical strength of the thread protection rings are required to protect the threads and steel pipe ends to avoid damage during normal loading and unloading, and transportation;
② During the transportation and normal storage of oil casing, the design and material of the thread protection rings are required to isolate the threads from dirt and water. The normal storage cycle is about 1 year;
③ The material selection of the thread protection ring should not contain material components that may cause thread corrosion or cause the thread protection ring to adhere to the thread, and can be suitable for a service temperature of -46℃ to +66℃:
④ Bare steel thread protection rings shall not be used on L80 steel grade 9Cr and 13Cr steel pipe bodies.

(5) Thread parameters exceed the standard: Thread processing is the most important process in the production of oil well pipes and is also a key process that determines the thread quality of oil well pipes. At present, most oil well pipes are threaded using special CNC machine tools. When processing threads, the workpiece is automatically centered and floating clamped. The tool for processing threads uses a carbide tool, and the spindle rotation is stepless.
There are two ways of thread processing: one is that the workpiece rotates and the tool performs plane feed motion; the other is that the workpiece does not move and the tool rotates and performs feed motion. These two types of machine tools have their characteristics. The former is flexible to use. It not only has high productivity when processing general tapered threads, but can also process direct-connected and special-connected threads with good airtightness (special buckles); the latter has higher productivity in processing general tapered threads than the former, but special buckles require pre-processing machine tools.
The various parameters of the thread (mid-diameter, tooth height, taper, pitch, tooth profile angle, close distance, etc.) will affect the connection strength and sealing performance of the thread. The thread close distance is the comprehensive value of the fluctuation of each single parameter of the thread. Even if the single parameters of the thread are qualified, its close distance may not be qualified.
The accuracy of each parameter of the thread is related to the quality of the tube blank. It is also related to the thread processing method, machine tool type, and stability of the processing process, as well as the dimensional accuracy and wear resistance of the thread comb. When other conditions are the same, the dimensional accuracy of the thread comb determines the accuracy of the thread size. Generally, the dimensional tolerance of the thread comb is required to be only 1/3 to 1/4 of the product tolerance, or even higher.

(6) Torque and J values exceed the standard: The torque of the oil casing refers to the make-up torque generated when the coupling and the steel pipe body are screwed together. The purpose of controlling the torque is to ensure the connection strength between the coupling and the steel pipe body that the contact pressure stress on the thread side is large enough and to cooperate with the corresponding thread sealing grease to achieve the leakage resistance of the oil casing. For API standard threads, the J value represents the distance from the steel pipe end to the center of the coupling after the coupling and the steel pipe body are tightened, which is one of the important parameters that determine the quality of the threaded connection.

(7) Leakage: To avoid leakage of the oil casing due to insufficient contact pressure between the oil casing body and the coupling thread, the oil casing with coupling is subjected to a hydrostatic pressure test according to the standard. The leakage of the thread connecting the steel pipe body and the coupling is related to the type and quality of the thread, the screwing of the oil casing, and the quality of the thread sealing grease. In terms of thread type, the sealing performance of round threads is better than that of trapezoidal threads, and special threads are even better. High-precision thread shape and reasonable oil casing screwing torque are conducive to improving the sealing performance of the thread. Thread sealing grease can play a role in lubrication, filling thread gaps (sealing), and corrosion prevention during the screwing of couplings and the use of oil casing.
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