JP2705505B2 - Threaded fittings for oil country tubular goods - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a threaded joint of an oil country tubular good used for exploration and production of natural gas and crude oil produced from underground, particularly when the depth of a well is deep or in a corrosive environment. The present invention relates to a joint suitable for use in a case where is severe.

[0002]

2. Description of the Related Art Today, threaded joints are widely used as a technique for connecting oil country tubular goods used for exploration and production of natural gas fields, crude oil fields, and the like having a depth of several thousand meters. Because oil country tubular goods are used under high pressure, high loads and in adverse environments, oil country tubular goods threaded joints must (1) be able to withstand the axial tensile force due to the weight of the connected pipe, and (2) (3) No corrosion by internal fluid, (4)
Performance such as being able to be used several tens of times is required. In recent years, the depth of wells has tended to increase,
The required performance described above is becoming more severe.

[0003] In response to such demands, many proposals have been made regarding threaded joints for oil country tubular goods. In general, threaded joints for oil country tubular goods use an integral method in which a pin is provided at one end of the pipe and a box is provided at the other end to connect the pipes, and a coupling with two box parts provided at both ends. There is a coupling method for connecting a pipe provided with a pin portion.

FIG. 7A is a sectional view showing a state in which pipe bodies are connected to each other using a coupling 20, and FIG. 7B is an enlarged view of a main part thereof. That is, a taper-shaped sealless screwless portion 13 at the tip of a pin portion 11 having a male screw 12 provided at an end of the pipe body 10 and a box portion 21 having a female screw 22 provided inside the coupling 20. The seal portion is formed by contacting the tapered seal-free screwless portion 23 located inside. This is because a metal seal is formed in this portion to improve the airtight performance against an internal pressure load caused by the fluid inside the oil country tubular good.

[0005] Also, a threadless portion for forming a seal of the pin portion 11
13 No threaded part for forming torque shoulder at the tip of 14
A torque shoulder portion is formed by abutting the torqueless screwless portion 24 located further inside the sealless screwless portion 23 of the box portion 21.
This is because the fastening torque of the threaded joint is adjusted to an appropriate value so that abutting the two portions does not cause a high contact surface pressure that causes excessive plastic deformation to occur in the seal portion.

[0006] On the other hand, many proposals have been made regarding the shapes of screws used for these joints.

First, a trapezoidal screw (Buttress Thru) shown in FIG.
read) is defined in the API (American Petroleum Institute) standard (STD 5B). FIG. 1A shows the shape of the trapezoidal screw, and FIG. 1B shows the contact state when the male screw and female screw of the screw are fastened.

Due to the engagement at the time of fastening, the screw load surface 1 is formed by the male screw load surface 1a and the female screw load surface 1b in the figure, the screw insert surface 2 is formed by the male screw insert surface 2a and the female screw insert surface 2b, and the male screw top is formed. The screw top surface 3 with the surface 3a and the female screw bottom surface 3b
And the screw bottom surface 4 is formed by the male screw bottom surface 4a and the female screw top surface 4b.

FIG. 1 is a view showing a contact state of a thread surface in a threaded joint according to the present invention. The flank angle α and the flank angle θ of a loading surface of a trapezoidal screw which affect the performance of the threaded joint are defined. I have. The flank angle is represented by a positive or negative value based on a line XY, X'-Y 'perpendicular to a tube axis (not shown). The flank angle α of the load surface is a negative angle when the flank angle advances in the counterclockwise direction, and the flank angle θ is a positive angle in the counterclockwise direction.

[0010] The API trapezoidal screw has a load flank angle of 3 ° and an insertion flank angle of 10 °, and the contact state of the screw surface at the time of fastening is as shown in FIG.
, Non-contact (there is a gap) at the insertion surface 2, and contact at least one of the top surface 3 and the bottom surface 4.

Second, the improved trapezoidal screw shown in FIG.
A screw shape called (Improved Buttress) is used. This screw has a 0 ° load surface flank angle, 45 °
3B, the contact state of the screw surface at the time of fastening is such that the load surface 1 and the insertion surface 2 make contact and the top surface 3 and the bottom surface 4 have a gap as shown in FIG. It has become. In FIG. 3B, the same parts as those in FIG. 2B are denoted by the same reference numerals.

Further, as a modification of the improved trapezoidal screw, -3
A trapezoidal screw having a loading surface flank angle of 30 ° and an insertion surface flank angle of 30 ° has been proposed. The contact state of each surface of this screw is the same as that of the improved trapezoidal screw shown in FIG. 3 (b). is there.

Each of the threaded joints for oil country tubular goods using the above-mentioned thread shapes has the following problems.

Although the frequency is not high, a compressive force may be applied in the axial direction of the oil country tubular goods. After that, when a tensile force due to the weight of the pipe is applied, the API trapezoidal screw shown in FIG. Also, the contact surface pressure is reduced at the torque shoulder portion, or a gap is formed in an extreme case. This phenomenon lowers the hermeticity of the oil well pipe against the internal pressure load due to the fluid inside the oil country tubular good, causing a problem that the internal fluid easily leaks, and the fluid penetrates into the above-mentioned gap to cause significant gap corrosion.

In addition, if a twisting torque acts in a direction in which the joint is loosened for some reason in a state where this phenomenon is occurring, the joint is easily detached, and there is a risk that the oil country tubular good falls into the well. At this time, it is extremely difficult to reconnect the oil well pipe in the well, which may cause a worst case in which the oil well during drilling development or production must be abandoned. These problems become more serious as wells become deeper.
This is a more serious problem because the pulling force due to the weight of the tube increases. These phenomena occur because, at the time of contact at the time of fastening, there is a gap in the insertion surface, so if a compressive force is applied, the screw portion cannot bear the compressive load until this gap disappears, and the seal The compressive load is concentrated on the part and the torque shoulder part, and remarkable plastic deformation occurs in this part, and when a tensile force is applied thereafter, the seal part and the torque shoulder part undergo plastic deformation. This is because the contact surface pressure of the joint decreases and the holding torque of the joint decreases.

In the second improved trapezoidal screw shown in FIG.
When an excessive tensile force is applied, there occurs a phenomenon (jump-out phenomenon) in which the male screw detaches from the female screw before the pipe body is tensilely broken. This jump-out phenomenon occurs when the male screw and the female screw slide relatively in the radial direction on the load surface of the screw, and the tensile performance of the joint is lower than the tensile performance of the pipe body. For this reason, if the design is made in consideration of only the tensile performance of the pipe main body in application to the well, the oil well pipe falls into the well due to a jump-out phenomenon, causing the same problem as described above. In order to avoid this, if the joint is designed in consideration of the tensile performance (the limit at which the jump-out phenomenon occurs), there is still room for the performance of the pipe body, and the material efficiency is reduced accordingly, resulting in an increase in material cost. .

Further, in the modification of the improved trapezoidal screw,
At the time of fastening, the load surface and the insertion surface are in contact with each other, but since the contact surface pressure of the insertion surface increases due to the small flank angle of the insertion surface, seizure tends to occur on the thread surface at the time of joint fastening,
It cannot withstand dozens of repeated uses. For this reason, it is necessary to re-cut the screw by machining to reuse it,
This leads to an increase in cost.

[0018]

SUMMARY OF THE INVENTION In view of the above-mentioned drawbacks of the threaded joint for oil country tubular goods, the present invention provides a threaded joint which has not only superior tensile performance than a pipe main body but also excellent airtightness even after a compressive load. The purpose is to provide.

[0019]

The gist of the present invention is as follows. That is, a seal portion is formed by a sealless screwless portion provided on a pin portion having a trapezoidal male screw and a sealless screwless portion provided on a box portion also having a trapezoidal female screw, and provided at the tip of the pin portion. As shown in FIG. 1, in a threaded joint for an oil country tubular good in which a torque shoulder forming screwless portion and a torque shoulder forming screwless portion provided in a box portion are abutted to each other to form a torque shoulder portion, as shown in FIG. α is -20 ° or more and less than 0 °, the flank angle θ of the insertion surface is more than 30 ° and 60 ° or less, and the load surface 1 and the insertion surface 2 of the male screw and the female screw contact each other at the time of fastening, and the top surface This is a threaded joint for oil country tubular goods having a gap between the base 3 and the bottom surface 4 and having excellent tensile strength and airtightness.

[0020]

The present inventor has conducted detailed studies on the behavior of a threaded joint when a tensile force and a compressive force are applied.
The following findings were obtained.

1) At the time of completion of the screw connection, if a compressive force is applied when there is a gap in the screw insertion surface, the male screw and the female screw are displaced in the axial direction with each other, and the compressive load is applied to the screw portion until the gap disappears. Can not do it. For this reason, most of the compressive load is concentrated on the seal portion and the torque shoulder portion, and significant plastic deformation occurs in this portion. Thereafter, when a tensile load is applied by the own weight of the pipe main body, the male screw and the female screw shift in the opposite direction to the above, and a gap is formed again in the screw insertion surface. At this time, due to plastic deformation in the seal portion and the torque shoulder portion, the contact surface pressure is reduced, or a gap is generated in an extreme case.

2) In order to prevent this, it is preferable that the insertion surface of the screw is brought into contact at the time of fastening. At this time, if the insertion surface flank angle is small, the contact surface pressure of the screw surface becomes high, and seizure easily occurs. . Therefore, the flank angle of the insertion surface needs to have a certain angle or more.

3) The flank angle of the load surface of the screw affects the tensile performance of the joint. To prevent the jump-out phenomenon from occurring even when an excessive tensile force is applied, the flank angle of the load surface must be negative. Must be a value.

Based on the above findings, in the present invention, a predetermined range and various conditions of the flank angle of the thread surface are determined as follows.

First, the flank angle of the load surface of the screw was set to -20 ° or more and less than 0 ° in order to secure the tensile performance of the joint.

FIG. 4 shows the sharing of the axial load acting on the joint on the screw load surface 1. Due to the tensile force, a force P perpendicular to the surface is generated on the load surface 1, and this force P is decomposed into an axial component P · cos α and a radial component P · sin α of the joint. The radial component P · sin α becomes downward when α is a negative value, and acts to press the female screw toward the male screw, thereby suppressing the jump-out phenomenon. At this time,
Since the axial component P · cos α also acts at the same time, a thread falling phenomenon occurs due to elastic deformation, and the above α becomes a value larger than the flank angle of the load surface by the thread falling. For this reason,
The flank angle of the load surface must be a negative value in consideration of the thread fall, that is, less than 0 °.

On the other hand, if the load surface flank angle is less than 0 ° and the negative value is increased, the radial component P · sin α increases, so that the jump-out suppression effect increases.
If the angle exceeds 20 °, the stress concentration generated at the root of the screw becomes excessively large, and the root of the screw is destroyed at an early stage, which causes a problem that the tensile performance of the joint is reduced. For this reason, the flank angle of the load surface is -20 ° or more and less than 0 °.

In order to secure both the jump-out suppression and the stress concentration suppression, the flank angle of the load surface is set to −10 °.
It is preferable to set the above to -3 ° or less.

Next, the flank angle of the screw insertion surface is set to a range of more than 30 ° and 60 ° or less. This is to prevent seizure of the screw at the time of fastening the joint and to ensure the compression performance of the joint. That is, in the present invention, even during compression,
Preload the insertion surface to protect the shoulder
In order to ensure that they are fastened under high surface pressure,
ing. In other words, after the insertion surface starts contacting,
The fastening is completed in a state where it is not. The process after the conclusion of the conclusion
If you follow in order, the load surface will come in contact with the seal surface
The insertion surface comes into contact, and during the tightening process, the surface pressure of the insertion surface
Rises, the shoulders touch, and the shoulders
The fastening is completed with the constant surface pressure obtained.

FIG. 5 is a diagram showing how the ratio between the contact surface pressure of the screw surface (insertion surface) generated by the fastening of the joint and the yield strength of the material changes with the change in the flank angle of the insertion surface.

When both the load surface and the insertion surface of the screw are brought into contact as in the joint of the present invention, the insertion surface contact surface pressure / yield strength of the screw increases as the insertion surface flank angle decreases.
If the flank angle is 30 ° or less, the contact surface pressure exceeds the yield strength of the material, and there is a high risk of seizing of the screw.

FIG. 6 shows the sharing of the axial compression force acting on the joint on the screw insertion surface.
A force F perpendicular to the screw insertion surface 2 is generated. This force F
Is the axial component F · cos θ of the joint and the radial component F · sin
θ, and the compressive force applied to the joint is shared by the former axial component F · cos θ on the screw insertion surface. For this reason,
As the flank angle increases, the compressive force shared by the screw insertion surface 2 decreases, and the compressive force acts on the seal portion and the torque shoulder portion accordingly. This results in the plastic deformation of the seal portion and the torque shoulder portion similar to the above-described drawbacks of the conventional threaded joint. Therefore, from the viewpoint of preventing this, the flank angle is set to 60 ° or less.

It is more preferable to set the flank angle of the insertion surface in the range of 40 ° to 50 ° in order to secure both the seizure prevention and the compression performance.

Further, the reason why both the load surface 1 and the insertion surface 2 of the male screw and the female screw are brought into contact with each other when the joint is fastened is to ensure the compression performance of the joint. That is, even if a compressive force acts on the joint, the male screw and the female screw do not shift from each other in the axial direction, and the compressive force is shared by the screw portions. Thereby, the effect of the insertion surface flank angle defined as described above can be sufficiently exerted.

The joint of the present invention is characterized in that a gap is provided between the top surface 3 and the bottom surface 4 of the screw. This is because lubricant such as grease used at the time of fastening the joint is penetrated into this gap to prevent seizure of the screw, and since this gap spirally communicates with the outside world, the seal between the pin and the box is sealed. This is because the trapped air can escape to the outside through the gap due to the contact of the parts.

The screw of the present invention belongs to a so-called trapezoidal screw. The reason for adopting this shape is that the ratio of thread height to thread pitch can be reduced compared to a triangular thread, and the joint efficiency (the value obtained by dividing the dangerous cross-sectional area of the joint by the cross-sectional area of the pipe body) ) Can be set higher.
The coupling of the present invention may be a coupling type coupling or an integral type coupling. The above-described effects are the same in any of the methods.

[0037]

EXAMPLES The performance of the threaded joint according to the present invention will be described below based on examples. A joint having a basic configuration of the coupling system shown in FIG. 7 and having variously changed screw specifications as shown in Table 1 was prototyped. At this time, the specifications of the other threaded joints were as follows.

Outer diameter of pipe body: 177.8 mm Wall thickness of pipe body: 10.36 mm Outer diameter of coupling: 194.5 mm Material of pipe and coupling: 22Cr stainless steel (yield strength: 56.3 kgf / mm ² ) Screw shape: Trapezoidal thread Thread pitch: 5.08mm Thread taper: 1/16 Thread height: 1.575mm Seal taper: 1/10

[0039]

[Table 1]

These threaded joints were repeatedly subjected to a fastening test, a simple tensile test, and a composite load test, and the performance was evaluated. Here, in the repeated fastening test, lubricating grease was applied to the thread portion, the joint was fastened and detached, and this was repeated until seizure occurred in the thread portion. The maximum number of repetitions was set to 10. In the simple tensile test, after the joint was fastened, a tensile force was applied in the axial direction, and the fracture site and the fracture load were recorded. In addition, the combined load test, after fastening the joint,
After applying a compressive force in the axial direction that generates a stress equivalent to 50% of the yield strength on the pipe body, 80% of the yield strength is applied to the pipe body.
And an internal pressure (defined in the API standard) of 80% of the yield strength was simultaneously applied so as to generate a stress equivalent to the above, and leakage of the internal pressure was investigated.

Table 2 shows the test results.

[0042]

[Table 2]

From Table 2, it can be seen that the joint of the present invention gave good results in any of the tests, although the comparative example failed in any of the tests.

Although the embodiment in the case of the coupling system is shown here, the same effect can be obtained in the case of the integral system.

[0045]

[Effect of the Invention] The threaded joint for oil country tubular goods according to the present invention has excellent tensile performance more than that of the pipe main body and also has excellent airtightness even after compressive load, so that it is sufficiently used for wells that will become even deeper in the future. Can be done.

[Brief description of the drawings]

FIG. 1 is a diagram showing a thread shape of a threaded joint of the present invention and a thread surface contact state at the time of fastening.

FIG. 2A is an axial sectional view showing the shape of a trapezoidal screw defined by an API standard, and FIG. 2B is an axial sectional view showing a screw surface contact state at the time of fastening.

3A is an axial sectional view showing a shape of an improved trapezoidal screw, and FIG. 3B is an axial sectional view showing a screw surface contact state at the time of fastening.

FIG. 4 is a diagram showing the state of sharing of the tensile force in the axial direction on the screw load surface.

FIG. 5 is a diagram showing the influence of the flank angle on the screw insertion surface contact surface pressure.

FIG. 6 is a diagram showing the state of sharing of the axial compression force on the screw insertion surface.

FIG. 7 is an axial sectional view showing a basic configuration of a coupling type threaded joint for oil country tubular good.

[Explanation of symbols]

1: Screw load surface 2: Screw insertion surface 3: Screw top surface 4:
Screw bottom 10: Pipe body 11: Pin section 12: Male thread 20:
Coupling 21: Box 22: Female thread 13, 23: No thread for forming seal 14, 24: No thread for forming torque shoulder α: Load surface flank angle θ: Insertion surface flank angle

Claims (1)

(57) [Claims] A seal portion is formed by a sealless screwless portion provided on a pin portion having a trapezoidal male screw and a sealless screwless portion provided on a box portion also having a trapezoidal female screw. In a threaded joint for an oil country tubular good in which a torque shoulder forming part is formed by abutting a torque shoulder forming screwless part provided at a tip end and a torque shoulder forming screwless part provided in a box part, a flank angle of a flank of a screw is −20 °. Less than 0 °, the flank angle of the insertion surface is more than 30 ° and less than 60 °, and the load surface and insertion surface of the male screw and female screw are in contact with each other at the time of fastening, and a gap is formed between the top surface and the bottom surface. A threaded joint for oil country tubular goods.