Types and specifications of threaded pipes for oil pipes

casing pipe is a deep machining product with high requirements and large consumption in petroleum drilling and production engineering. Use threads to connect a single oil casing into a long pipe string that can withstand hundreds of atmospheres of thousands of meters—a tubular high-pressure vessel. In 1924, API formulated the first oil well pipe standard. The thread of the oil casing pipe joint is a V-shaped thread with 10 threads per inch and 8 threads per inch, but it was later replaced by API8 round thread and partial ladder thread, and it is still in use today. The API SPEC 5B standard stipulates that the commonly used casing threads are round thread (abbreviated as CSG) and buttress thread (abbreviated as BCSG).

With the exploration and development of oil and gas, especially the increase in deep wells, ultra-deep wells, high-pressure gas wells, directional wells, and wells containing hydrogen sulfide, higher requirements are placed on the performance of oil-casing joints. Airtightness, connection strength, and corrosion resistance are no longer suitable for the requirements. For this reason, various countries have launched the development and application of special threaded joints. The following describes API round thread, buttress thread and some special threads one by one.

●3. The crown of the arc surface is not sensitive to damage due to local scratches or dents.

Since the outer diameter of the casing is as small as 41/2 and as large as 20 inches, the shape of the threaded joint of the same type of outer diameter circular threaded casing can be divided into long and short, the wall thickness of the pipe body can be divided into thick and thin, and the steel grade of the material is divided into There are differences in height and tightening torque, which makes the other thread parameters of the casing and coupling, such as: hand-tightening distance, the number of teeth A and other basic dimensions are different. Therefore, it is necessary to test round thread casings of different specifications and The tight distance of the coupling thread should be inspected with a thread gauge of the corresponding specification, and the detection data should be processed accordingly if necessary.

In the API SPEC 5B standard, there is only one kind of inspection gauge for the casing round thread of the same outer diameter size, and they are all designed according to the size of the short round thread of the corresponding specification. That is to say, the basic size of the gauge is the same as the corresponding The basic dimensions of the short round thread are the same, which means that one gauge is multi-purpose, that is, the gauge should not only test the long round thread of the same outer diameter, but also test the short round thread of the same outer diameter.

This kind of thread is designed to improve the ability to resist axial tension or axial compression load and provide leakage resistance. It is abbreviated as BCSG in English. It has no shoulder taper pipe thread and needs to be connected by a coupling. The tooth type is partial trapezoidal and flat. Flat top.

For casing threads with specifications of 41/2-135/8, the taper on the diameter is 62.5mm/m, every 25.4mm5 thread (pitch is 5.08mm); the included angle between the guide flank and the vertical line of the thread axis is 10 °; the angle between the load side and the perpendicular to the thread axis is 3 °; the crest and the bottom are conical, parallel to the thread taper; the fillet radius (0.762mm R) of the leading side crest is larger than the load side crest The large fillet radius (0.203mm R) facilitates stabbing and make-up. When tightening, the thread is a full profile fit, and the maximum gap between the crest and the bottom of the thread is 0.051mm. Machining tolerances in the threads themselves cause stress on one thread flank on one end of the threaded member of the fitting and forces on the opposite thread flank of the mating fitting threaded member at the other end. In any case, the use of suitable thread compound or plating (or both) is another means of ensuring thread leakage resistance. Leakage resistance can only be controlled by proper assembly (interference amount) over the full thread length. The root of the thread of this joint extends along a continuous cone until it disappears on the outer surface of the pipe, and the coupling (internal thread end part) and the incomplete thread start to the point of disappearance.

The 3° load flank makes the thread slip-resistant under high tensile loads, while the 10° pilot flank makes the thread withstand high axial compressive loads. Repairing threads by hand should be done with caution and limited to a small portion of the full thread length. Careful repair of incomplete threaded portions of external threads will not affect control of leakage resistance.

The standard is not less than 16-inch partial ladder casing thread, the taper on the diameter is 83.33mm/m, every 25.4mm5 thread, the flat top and the flat bottom are parallel to the pipe axis, which is helpful for stabbing and make-up. All other dimensions and thread fillet radii are the same as for casing sizes 133/8 and up. Use of proper thread compound and plating is important to ensure leak resistance.

2) The inclination of the crest and bottom plane is the same as that of the thread, and the crest has a circular arc. The arc radius of the leading flank at the tooth crest is larger than the arc radius of the load flank at the crest, which is beneficial to the screwing of the thread.

However, the partial trapezoidal thread has low sealing performance, especially after the casing is run into the well, its anti-air sealing pressure will be further reduced under the action of axial tension and certain bending stress, and at the same time, after a primary leakage of the threaded joint, its secondary gas Sealing will be further reduced. From the schematic diagram of the casing joint structure and the meshing diagram of the partial ladder thread, it can be seen that for the partial trapezoidal thread casing joint, the sealing part mainly has two parts: one is the torque shoulder AB, and the other is the thread bearing surface S. In addition, the annular gap The thread sealant in it also has a sealing effect under certain conditions. When the partial trapezoidal thread casing joint is subjected to the combined load of internal pressure, tension and bending, the torque shoulder AB and the thread bearing surface S will superimpose the bending normal stress, and the contact pressure of the torque shoulder decreases, so the sealing pressure reduce.

Like all other oil pipes, the casing threaded connection is the weakest link. The quality of the threaded connection directly affects the structural integrity and sealing integrity of the casing string, and the thread processing accuracy is one of the important factors affecting the quality of the threaded connection. The 5B standard has more than ten control indicators for thread quality. Individual thread parameters such as taper, pitch, tooth height, tooth profile angle, etc. can be measured with the help of a thread individual parameter measuring instrument. The measurement results are very intuitive and do not require data processing. , and it is not easy to make mistakes. And the most important parameter that comprehensively reflects the individual parameters and surface processing quality - close distance, needs to be inspected with a working gauge. Due to the structural type of the gauge and the transmission value with the calibration gauge, the length of the thread, the wall thickness of the casing, the steel grade, etc., it is necessary to make necessary judgments and processing on the measurement data to obtain the required tight distance.

API stipulates that there are 8 types of casing steel grades: H-40, J-55, K-55, C-75, L-80, N-80, C-95, and P-110, of which H-40 steel grade The steel grade P-110 has the highest strength. Depending on the steel grade, the color of the bushing is also different. The commonly used steel grades are J-55 painted green, N-80 painted red, and P-110 painted white.

Φ 339.7 casing has 6 wall thicknesses in total, of which steel grades below K-55 contain four wall thicknesses of 8.38, 9.65, 10.92 and 12.19 mm, and steel grades above C-75 contain two wall thicknesses of 12.19 and 13.06 mm.

It can be seen from the above that the thread connection strength and tightness are the two main technical indicators of the oil casing. API round thread and partial ladder thread are not suitable for use in harsh conditions such as heavy oil thermal recovery, ultra-deep well, heavy corrosion, etc., because of the sealing and strength problems related to its structure and thread profile. The round thread can only withstand the tensile load equivalent to 60%~80% of the strength of the pipe body. Although the ladder thread joint has high connection strength, its sealing performance is poor under high internal pressure. These two kinds of threads are generally sealed by means of thread grease containing lead, zinc, copper, graphite and silicone oil in a suitable carrier. This form of sealing generally only works at temperatures below 60~95 ºC.

Therefore, the sealing of API standard threaded joints is mainly realized by methods such as thread grease, metal coating and thread interference engagement. API round thread root to tooth crest clearance is 0.152mm; the maximum clearance of partial ladder thread is within the entire tooth height range of the leading side, the tooth crest clearance of the casing below 193.7mm is 0.178mm, and the casing of 219.1mm and above specification is as large as 0.229mm mm. The sealing of API standard threaded joints is to fill the gap with thread grease and make the internal pressure generate a certain pressure drop at both ends of the gap within the fair meshing thread length (usually 3~5 thread teeth length), so as to realize the sealing effect. The second is to form a number of uncertain metal-to-metal contact seals (the sealing position and contact pressure are affected by the thread size, coating, and thread grease) by the interference engagement of the thread flanks, so as to achieve the sealing effect.