Quality defects of oil well pipes mainly come from three aspects: First, the quality defects of the oil well pipe body itself, such as the mechanical properties, internal connection, and weighing of the pipe body that does not meet the requirements; second, the quality defects of oil well pipes during the processing process, such as thread parameters (taper, pitch, tooth height, Ken shape, and concentricity and close distance of the threads at both ends of the coupling) exceeding the standard, black skin buckle, broken buckle, thread deviation, screw torque exceeding the standard, leakage, thread damage (scratches, bumps), drill pipe weld quality not meeting the requirements, etc.; third, the performance of oil well pipes, including anti-collapse performance, anti-corrosion performance, perforation performance, and anti-sticking performance, fail to meet the requirements.

1. Quality defects and prevention of oil well pipe thread processing
During the thread processing of oil well pipes, the threads may have quality defects such as black skin buckle, thread deviation, broken buckle, thread scratch (bump), and thread parameters exceeding the standard.
(1) Black buckle of thread: Black buckle of thread is manifested as an "unsmooth" appearance due to too little local processing of thread, which is related to the outer diameter and wall thickness accuracy, ovality, and straightness of pipe end of steel pipe. The black buckle of the pipe body is often caused by the outer diameter of the pipe body being too small, the pipe end is not straight enough or the ovality being too large. The black buckle of coupling is generally caused by the outer diameter of the steel pipe exceeding positive tolerance the pipe wall exceeding 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 the black buckle of thread, which is caused by uneven wall thickness, bending, or too much ovality of the pipe end. Sometimes, when thread processing is wall deviation or processing control is unreasonable, the wall thickness of the threaded bottom may exceed negative tolerance, which will seriously affect the connection strength of the oil well pipe.
(3) Thread breakage: When the thread comb cutter is cutting the thread at high speed and forcefully, once the thread is broken or the thread is "lost", it will cause the thread to break. Generally speaking, thread 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, and 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 external protective ring with internal threads should be screwed on the threads of the oil well pipe body, and an internal protective ring with external threads should be screwed on the coupling threads.
API Spec 5CT standard stipulates:
① The thread processing plant should screw on internal and external thread protective rings. The design, material, and mechanical strength of the thread protection ring are required to protect the threads and pipe ends to avoid damage during normal loading and unloading, and transportation;
② During the transportation and normal storage of oil and casing, the design and material of the thread protection ring are required to isolate the threads from dirt and water. The normal storage period 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 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 floatingly clamped. The tool for processing threads uses a carbide tool, and the spindle rotation is stepless.
There are two ways to process threads: one is that the workpiece rotates and the tool does a plane feed motion; the other is that the workpiece does not move and the tool rotates and does a feed motion. These two types of machine tools have their characteristics. The former is flexible to use. It not only has a 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 a higher productivity in processing general tapered threads than the former, but special buckles require a pre-processing machine. The various parameters of the thread (mid-diameter, tooth height, taper, pitch, tooth 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 individual parameters of the thread are qualified, its close distance may not be qualified. The accuracy of the various parameters of the thread is not only related to the quality of the tube blank but also 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 and casing refers to the make-up torque generated when the coupling and the pipe body are screwed together. The purpose of controlling the torque is to ensure the connection strength between the coupling and the 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 anti-leakage of the 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 that determine the quality of the threaded connection.
(7) Leakage: To avoid oil and casing leakage due to insufficient contact pressure between the oil and casing pipe body and the coupling thread, the oil and casing with coupling are subjected to a 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, the screwing of the oil and 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 and casing screwing torque are conducive to improving the sealing performance of threads. Thread sealing grease can play a role in lubrication, filling thread gaps (sealing), and anti-corrosion during the screwing of couplings and the use of oil and casing.

2. Performance of oil well pipes
The performance of oil well pipes includes anti-sticking performance, anti-collapse performance, anti-corrosion performance, and perforation performance.
(1) Anti-sticking performance: According to standard requirements, threaded joints of oil and casing need to be made and unmade. It is stipulated that each joint must be made and unmade 6 times each. Make up to the maximum torque recommended by the manufacturer, then unmake, and check the sticking of the internal and external threads of the oil and casing. The sticking of oil and casing threads is related to factors such as thread quality, thread surface hardness, make-up speed, surface friction coefficient, and contact stress (coupling screwing torque). To improve the anti-sticking performance of oil and casing threads, the finish of the threads, the hardness and uniformity of the threads should be improved, the threading speed should be reduced and the screwing torque should be controlled. At the same time, a softer metal or non-metal film layer should be plated on the inner thread surface of the coupling to separate the pipe body of the oil and casing from the coupling to prevent the metal surface of the two threads from sticking and to avoid tearing or even tearing of the threads. Before the coupling is screwed, the thread surface needs to be coated with thread grease to prevent the threads from sticking after the coupling is screwed and to improve the sealing performance of the threads. There are many coating methods for the surface of the coupling thread: such as galvanizing and phosphating processes; for some special materials and special connection threads, copper plating is often required. Factory-related sticking factors: thread parameters (pitch, tooth height, taper, tight moment, tooth profile half angle, etc.), internal and external thread matching (surface treatment, surface finish, phosphating, galvanizing, copper plating, etc.), thread grease (function: lubrication, filling and sealing, etc., composed of metal powder and grease), thread control (threading torque, threading speed, etc.), material factors, etc. Factors related to the use and operation of oil fields include: lifting without thread protectors, skewed threading (the pipe swings in the air and is not concentric with the well thread), no threading or fewer threading, thread grease (not meeting standard requirements, sand and other debris), threading speed and threading torque, and clamping force of large tongs.
(2) Anti-collapse (crush) performance: As the drilling depth increases, the pressure on the oil and casing in the oil and gas wells increases, especially in deep wells, ultra-deep wells, or oil and gas wells in complex formations such as rock salt, salt paste, shale, and soft rock formations that need to isolate plastic flow. When the external pressure exceeds a certain limit, the oil well pipe body will produce a groove or elliptical deformation, which is called oil well pipe collapse.
(3) Corrosion resistance: Some oil and gas fields contain a large amount of corrosive media such as hydrogen sulfide, carbon dioxide, or chloride ions, which puts forward corrosion resistance requirements for oil and casing, including resistance to sulfide stress corrosion, resistance to CO2 and Cl- corrosion, etc. The corrosion resistance of oil and casing is mainly related to factors such as the chemical composition of steel and the residual stress value of steel pipe. Reducing the content of non-metallic inclusions and harmful elements in steel, increasing the corrosion resistance elements such as Cr and Ni, reducing the residual stress in steel pipe, and improving the yield strength ratio of steel pipe are all conducive to improving the corrosion resistance of oil and casing.
(4) Perforation performance: The oil production part of the oil layer casing (layered oil production in multi-oil layer oil wells) requires perforation to allow crude oil to flow into the casing from the designated oil-bearing oil sand layer. For this reason, the oil layer casing is required to have good perforation performance, especially when using gunless perforation operations, the perforation performance of the casing is required to be higher. The perforation performance of the casing is obtained through perforation tests. That is, the casing to be tested is suspended in a simulated well, and a string of shaped perforators with a certain number, a certain distance, and different directions are hung in the casing. Then perforation is carried out. After perforating, if there are no cracks around the holes of the test casing, the perforating performance is evaluated as good; if there are a small number of small cracks around each hole, but their number and length do not exceed the requirements of the technical conditions, then the perforating performance is evaluated as qualified; and if the number or length of cracks around each hole exceeds the requirements, especially if the cracks between two adjacent holes are connected, the perforating performance is evaluated as unqualified. The oil field also has clear requirements for the amount of expansion of the casing after perforation and the height of the internal and external burrs around the hole.

Thread Inspection Procedure (Based on API Spec 5B)
I. Preparation before inspection:
①. Check whether all inspection tools are within the effective identification period.
② Check all inspection tools to ensure the accuracy and authenticity of the inspection results.
II. Basic rules for inspection.
① The thread length should be measured parallel to the thread axis.
② The thread tooth profile height and the cone diameter are measured roughly perpendicular to the thread axis.
③ The thread pitch should be measured along the center cone parallel to the thread axis.
④ For the internal thread of the eccentric type, the pitch of the external thread should be measured roughly along the center diameter cone parallel to the thread axis.
⑤ The taper of the round thread should be measured along the diameter of the center diameter cone;
⑥ The taper of the external thread of the eccentric type should be measured along the small diameter cone, and the taper of the internal thread of the eccentric type should be measured along the large diameter cone on its diameter.
III. Appearance inspection:
① Within the minimum length (Lc) of the complete thread from the pipe end, and the interval between the end face of the boring hole and the plane J+1 tooth away from the center of the coupling, the thread should have no obvious tearing, inverted marks, wear marks, shoulders or any other defects that damage the continuity of the thread.
② For the occasional surface scratches, slight pits, and surface irregularities, if they do not affect the continuity of the thread surface, they shall not be considered harmful; since it is difficult to determine the extent of the surface scratches, slight pits, and surface irregularities and their impact on the continuous performance of the thread, such defects cannot be used as the basis for rejecting the pipe: as an acceptance criterion, the most critical consideration is to ensure that no thread can damage the coupling.
③ Manual finishing of the thread surface is allowed. Imperfections are allowed between the Lc length and the vanishing point of the thread, as long as their depth does not extend below the thread root cone, or is not greater than 12.5% of the specified wall thickness (measured from the pipe surface where the imperfection extends), and the allowable depth is the larger of the two. In this area, grinding and resting are allowed to eliminate defects, and the limit of the grinding depth is the same as the depth of the imperfection in this area.
④ Imperfections also include other discontinuities, such as folds, pits, knife marks, indentations, handling damage, etc., and micro-pits and stains may be encountered, but they are not necessarily harmful. Since micro-pits and stains and their impact on the continuous performance of the thread are difficult to determine, such defects cannot be used as the basis for rejecting the pipe: as an acceptance criterion, the most critical consideration is to remove any corrosion products on the thread surface without leaving leakage channels.
⑤ Grinding and filing methods are not allowed to eliminate pits.
⑥ The external chamfer (60°) of the pipe end must be guaranteed to be complete on the 360° circumference of the pipe end. The chamfer diameter should be selected so that the groove of the thread bottom disappears on the chamfer surface rather than on the pipe end face, and no knife-like edges should appear.
⑦ The root of the coupling thread should start from the inner diameter chamfer surface and extend to the center of the coupling.
⑧ Blacktop thread: For round threads, it is allowed to exist between the Lc length and the thread vanishing point, and it is not allowed within Lc. However, if the black top thread within Lc is in the shape of a point or a line, and the buckle type is complete, it can be considered that it does not affect the continuity of the thread surface. For partial trapezoidal threads, 2 black top threads are allowed within the Lc length range, but the total length of the black top thread does not exceed 1/4 of the circumference of the pipe thread and is considered qualified.
⑨Broken Thread: It is not allowed to appear within the Lc length range of the trapezoidal thread. It is allowed as long as the defect depth does not extend to the bottom cone of the thread or does not exceed 12.5% of the specified wall thickness between the Lc length and the vanishing point of the thread.
⑩Burr: Minor burrs can be polished to pass the test. Burrs on the inner and outer edges of the pipe end are unqualified.
⑪Chatter: Check with fingernails or sharp needles. If it feels slight, it is qualified. If there is an obvious sense of beating, it should be judged as unqualified.
⑫Cut: Minor cuts that do not produce leakage channels are acceptable.
⑬Tears: Cuts on the tooth side are unqualified. Minor cuts that do not affect the thread engagement, cause the coupling coating to fall off, or do not destroy the continuity of the thread are acceptable.
⑭Dent: Any slight dent within the Lc length range that does not destroy the continuity of the thread or create a leakage channel is acceptable. Dents outside the Lc length range that do not extend below the bottom cone of the thread or do not exceed 12.5% of the specified wall thickness are acceptable.
⑮Fin: Minor fins are allowed to be ground. If they are in the shape of a knife edge or a wing-shaped severe fin, they are unqualified.
⑯Tear: Fish scale scratches on the thread tooth surface are all judged as unqualified.
⑰Pitted Threads: Pitted non-floating rust corrosion within the Lc length range or peeling of the coating on the coupling, rust are all unqualified.
⑱Improper Thred Form: Abnormal tooth shape is judged as unqualified.
⑲False teeth, random buckles, and overburned thread surfaces are all judged as unqualified.