JP2001124253A - Screw joint for steel pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a screw joint for a steel pipe which causes no fluid leakage even when not only the compression but also the tension or the bending is loaded during the execution. SOLUTION: In the screw joint for the steel pipe comprising a pin which is reduced in outside diameter toward the tip and has a lip part having a male screw on an outer circumferential surface and a seal surface on the tip, and a box having a female screw screwed in the male screw of the pin on the inner circumference and a shoulder part in contact with the lip part to the distal side of the inner circumference, load surfaces formed by the male screw and the female screw are brought into contact with each other, and a gap of 0.035-0.170 mm is formed in an insertion surface.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a threaded joint for steel pipes, and more particularly to a technique for preventing fluid leakage even when an external pressure, a tensile force, a compressive force, a bending stress or the like is applied.

[0002]

2. Description of the Related Art Today, threaded joints are widely used as a technique for connecting steel pipes (for example, oil well pipes) used for exploration and production of natural gas fields, crude oil fields and the like having a depth of several thousand meters. . This threaded joint is used for oil country tubular goods (hereinafter sometimes simply referred to as pipes) under high pressure, high load and bad environment.
There is a demand for performance such as the ability to withstand the tensile force in the axial direction due to the weight of the connected pipe, the ability to withstand external pressure, and the ability to be used repeatedly several tens of times. In addition, this screw joint is provided with a pin portion having a male thread at one end of the pipe and a box portion having a female thread at the other end, and an integral type in which the pipes are connected to each other through them. As shown, there is a coupling method in which a coupling 2 having two box portions 1 at both ends is used, and an oil country tubular good 4 having pin portions 3 at both ends is connected to each other.

[0003] The inner circumference of the box portion 1 is tapered, and the inner diameter is reduced as it progresses in the axial direction from the tip to the back, and a female screw 5 as shown in FIG. As shown in FIG. 5, an annular shoulder 6 (referred to as a shoulder) protruding toward the axis is formed. On the other hand,
A male screw 7 to be screwed to the female screw 5 is engraved on the pin portion 3 screwed to the box portion 1, and a screwless portion (lip portion 8) is provided at the tip of the male screw 7. Is provided). The outer peripheral surface of the lip portion 8 is also tapered, and its protruding end is overhanging, as apparent from FIG. Therefore, when the lip portion 8 comes into contact with the inner peripheral portion of the box portion 1, a seal surface (referred to as a metal surface-to-metal surface seal) is formed by abutting therewith.

On the other hand, as shown in FIG. 4, the surfaces of the male screw 7 and the female screw 5 that come into contact with each other by the engagement of the male screw 7 and the female screw 5 at the time of fastening, as shown in FIG. In other words, the inclination of the surface 9 is referred to as a load flank angle α (counterclockwise is negative) with reference to a line perpendicular to the tube axis.
The surface on the side where the gap 13 is formed between the male screw 7 and the female screw 5 is referred to as the respective insertion surface 10, and the inclination of the surface 10 is the stubbing flank angle θ (positive in the counterclockwise direction).
That. Further, as shown in FIG. 4, the contact state of the screw surface at the time of fastening is such that the load surface 9 makes contact, the insertion surface 10 makes no contact, and at least one of the top surface 11 and the bottom surface 12 makes contact. The seal can be ensured even in contact with. In other words, such a threaded joint maintains high confidentiality due to the contact between the screws of the pin portion 3 and the box portion 1 and the contact of the abutting seal surface.

[0005] However, recently, when the oil country tubular goods 4 are constructed, the frequency of applying a compressive force in the axial direction of the oil country pipes 4 and the threaded joints has increased. In this case, after the compression, a tensile force or a bending force is further applied to the screw by its own weight of the oil country tubular good 4. For this reason, even with the above-mentioned threaded joint having excellent sealing properties, a problem often occurs in that the internal fluid (natural gas, crude oil) leaks to the outside. In addition, if a torque acts in a direction in which the joining is loosened in a state where this phenomenon occurs, the joint may come off, and the oil country tubular good 4 may fall into the well. In such a case, reconnection of the joints is extremely difficult, so that the well digging during development or production is abandoned.

[0006]

SUMMARY OF THE INVENTION In view of the above circumstances, the present invention provides a threaded joint for a steel pipe which does not leak even if it is subjected not only to compression but also to tension or bending during construction of an oil country tubular good. It is an object.

[0007]

The inventor of the present invention has studied the causes of fluid leakage in order to achieve the above object, and stated that "when a strong compressive force is applied, the seal surface of the threaded joint is deformed,
Next, even if a tensile force is applied, it does not return to its original state and remains as a gap. " In other words, when a large compressive force is applied to the oil country tubular goods (pin side), the forces repel each other at the sealing surface where the tip of the pin and the inner periphery of the box abut, and the movement of the material itself occurs. Both the sealing surfaces on the box side are plastically deformed. Thereafter, when a tensile force is applied, the plastically deformed portion is held as it is, so that the gap 13
Is formed. Then, the inventor continued to study diligently for measures against fluid leakage, and found that it could be solved by changing the engraved position of the screw thread, and embodied it in the present invention.

That is, according to the present invention, the outer diameter is reduced toward the tip, and a pin having a male screw on the outer peripheral surface and a lip portion forming a sealing surface at the tip is screwed into the male screw of the pin on the inner circumference. Female screw, a box having a shoulder portion in contact with the lip portion at the inner periphery,
A load joint formed by the male screw and the female screw is in contact with each other, and the insertion surface forms a gap of 0.035 to 0.170 mm.

The present invention is also a threaded joint for steel pipes, wherein the thread of the male screw and the female screw is trapezoidal.

According to the present invention, even if a tensile force is applied after the compression, the plastic deformation of the screw and the sealing surface is alleviated, so that the sealing property of the threaded joint is well maintained.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with the background of the invention.

First, the inventor examined the conventional structure of the threaded joint in detail in consideration of the cause of the fluid leakage. Then, as shown in FIG. 4, attention was paid to the gap 13 between the insertion surface 9 formed by the male screw 7 of the pin 3 and the female screw 5 of the box 1. The reason for focusing on this is that if there is a gap 13 in the oil well pipe 4 or the threaded joint, the screw or the sealing surface may be deformed when a compressive force or a tensile force is applied to the oil country tubular good 4 or the screw joint. The result of the investigation showed that many screw joints having similar structures had a gap 13 of 0.200 to 0.350 mm on the insertion surface 9. Among them, there is an example of 0.03 mm or less disclosed in Japanese Patent No. 2705506, but this dimension is not considered because it cannot be implemented with the current thread cutting technology. Also, even if it can be manufactured, if it is so narrow, the thread "Murashi" at the time of construction of the oil well pipe 4
(Referred to as galling), which is considered to be difficult to use.

Therefore, the inventor of the present invention has studied diligently whether the size of the gap 13 does not cause plastic deformation of the sealing surface during compression, that is, whether the material itself can be deformed within the elastic limit. Based on the current materials and dimensions (outer diameter, wall thickness, and length) of the oil country tubular goods and threaded joints and the magnitude of the applied compressive force, material dynamics are considered.
It has been found that 035 to 0.170 mm is preferable. Then, many material tests were conducted with various kinds of materials, and this gap 1
It was confirmed that three ranges were sufficient, and the present invention was completed. The threaded joint having the gap 13 of less than 0.035 mm is preferable from the viewpoint of avoiding plastic deformation, but cannot be actually manufactured. Therefore, the threaded joint is excluded from the scope of the present invention. It was inevitable, so we decided to exclude it.

The shape of the thread is not particularly limited in the present invention. That is, as shown in FIG. 1, some screws have a negative inclination (referred to as a load flank angle) of the load surface 9, but the present invention takes only the gap 13 of the insertion surface 10 into consideration. API (American)
It is also effective for trapezoidal screws specified in the Petroleum Institute standard.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS On the outer periphery of a pin 3 provided at the tip of an oil country tubular good and on the inner periphery of a box 1 provided at both ends of a coupling 2, each screw having the shape shown in FIG. 1 and each sealing surface shown in FIG. Was provided, and the pin 3 and the coupling 2 were connected to investigate the fluid leakage. Table 1 shows the sizes of the oil country tubular goods 4 and the couplings 2 used at that time. In addition, an investigation was conducted using the same oil well pipe 4 and coupling 2 provided with a conventional threaded joint (the gap 13 on the insertion surface was as large as 0.274 mm). Both studies used high-pressure water and gas instead of crude oil and natural gas as the fluid,
The evaluation of the degree of leakage was performed according to the A1 class (highest grade) according to API standards. In addition, the load flank angle α of the load surface 9 and the insertion surface 10 of each joint and the stubbing angle
The flank angle θ is made to combine various values.

Table 2 shows the results of the survey. As is clear from Table 2, in the threaded joint according to the present invention, the gap 13 in the screw insertion surface 10 prevents plastic deformation, and in any case, no gap is generated in the sealing surface during construction. As a result, fluid (gas and liquid) leakage did not occur. On the other hand, in the case of the conventional threaded joint, damage and fluid leakage also occurred. In addition, the damage rate or the fluid leakage rate in Table 2 is shown by the occurrence rate of each connection of 50 oil country tubular goods in each example.

[0017]

[Table 1]

[0018]

[Table 2]

In the above embodiment, the threaded joint is of the coupling type, but it goes without saying that the present invention can be applied to the integral type.

[0020]

As described above, according to the present invention, even if a tensile force or a bending force acts after a large compressive force at the time of connecting an oil country tubular good, damage and liquid leakage as in the prior art do not occur.

[Brief description of the drawings]

FIG. 1 is a view showing an example of a screw portion according to the present invention in which a gap between insertion surfaces is set to 0.03 to 0.170 mm.

FIG. 2 is a view showing a seal part connected to a screw part of FIG. 1;

FIG. 3 is a view showing a conventional coupling type screw joint.

FIG. 4 is an enlarged view of a thread portion of the threaded joint shown in FIG. 3;

FIG. 5 is an enlarged view of a seal portion of the threaded joint shown in FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Box part (box) 2 Coupling 3 Pin part (pin) 4 Oil well pipe (pipe) 5 Female screw 6 Shoulder (shoulder) 7 Male screw 8 Lip part 9 Load surface 10 Insertion surface 11 Top surface 12 Bottom surface 13 Clearance

Claims (2)

[Claims] A pin having a male thread on an outer peripheral surface and a lip portion forming a sealing surface at a distal end, and a female screw screwed on an inner periphery with a male screw of the pin,
In a threaded joint for steel pipes provided with a box having a shoulder portion in contact with the lip portion at the inner periphery, a load surface formed by the male screw and the female screw is in contact with each other, and an insertion surface is 0.035-0. A threaded joint for steel pipes, wherein a 170 mm gap is formed. 2. The threaded joint for a steel pipe according to claim 1, wherein the threads of the male screw and the female screw are trapezoidal.