JP2705506B2 - 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. The threaded joint for oil country tubular goods uses an integral method to connect the pipes by forming a pin at one end of the pipe and a box at the other end, and a coupling with two boxes at both ends. There is a coupling method for connecting tubes having pin portions formed at both ends.

[0003] Since oil country tubular goods are buried in wells and used under high pressure, high load and bad environment, these threaded joints include:
(1) Withstand the tensile force in the axial direction due to the weight of the connected pipe, (2) Withstand the internal pressure due to the internal fluid,
(3) There is a demand for performance such as no corrosion due to the internal fluid, and (4) repeated use several tens of times.

In recent years, the depth of wells has tended to increase,
The required performance described above is becoming more severe.

[0005] In response to such demands, many proposals have been made on threaded joints for oil country tubular goods.

[0006] FIG. 5A shows a state in which the coupling 20 is used.
FIG. 2 is a cross-sectional view showing a state in which the pipe bodies 10 are connected to each other, and FIG. 2B 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.

[0007] Further, a threadless portion for forming a seal of the pin portion 11 is provided.
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 to adjust the fastening torque to an appropriate value so that abutting these portions does not generate a high contact surface pressure that causes excessive plastic deformation in the seal portion.

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

First, the trapezoidal screw 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 diagram showing a contact state of a thread surface in a threaded joint of the present invention. The flank angle α of the load surface and the flank angle θ of the insertion 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.

The API trapezoidal screw has a load flank angle of 3 ° and an insertion flank angle of 10 °. The contact state of the screw surface at the time of fastening is as shown in FIG.
, A non-contact at the insertion surface 2, and a contact at least one of the top surface 3 and the bottom surface 4. According to the dimensional tolerance of the API standard, a gap of 0.03 to 0.19 mm exists on the insertion surface 2.

Second, there is a screw shape called an improved trapezoidal screw (Improved Buttress) as shown in FIG. This screw has a load plane flank angle of 0 ° and an insertion plane flank angle of 45 °. Further, as shown in FIG. 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 3 have a gap. In FIG. 3B, the same parts as those in FIG. 2B are indicated by the same reference numerals.

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 good. However, when a tensile force is applied by the weight of the pipe, 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 occurs, the joint is easily detached, and there is a risk that the oil country tubular good may fall into the well. At this time, since the reconnection of the oil well pipe in the well is extremely difficult, the worst situation in which the oil well under development or production must be abandoned may be caused. 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 relatively wide gap in the insertion surface, so when a compressive force is applied, the screw portion can bear the compressive load until this gap disappears. Instead, the compressive load concentrates on the seal and torque shoulder, causing significant plastic deformation in this area.If a tensile force subsequently acts on the seal and torque shoulder, the seal and torque shoulder undergo plastic deformation. This is because the contact surface pressure at this portion decreases, and the holding torque of the joint decreases.

In the second improved trapezoidal screw shown in FIG.
The thread surface may be damaged by repeated fastening about 10 times. This damage is not as severe as burn-in, but is accompanied by a distinctive scratch. These are in contact with both the load surface and the insertion surface at the time of fastening, and the contact surface pressure on the insertion surface is considerably reduced by increasing the flank angle of the insertion surface, but the contact surface pressure is still high It is because. Therefore, if this joint is used further continuously, there is a risk that seizure may occur from this damage. In order to avoid this danger, it is necessary to re-cut the screw by machining, which causes a problem that the cost is increased.

[0018]

SUMMARY OF THE INVENTION The present invention has been made in consideration of the above-mentioned problems of the threaded joint for oil country tubular goods, and has excellent airtightness against the internal fluid of the oil country tubular goods even after a compressive load, and has a high attachment / detachment torque. With the object of providing a threaded joint that, when a torsion torque acts on the threaded joint in the direction of loosening the joint for any reason, retains the torque against this and prevents damage to the threaded surface due to repeated fastening. I have.

[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. The load surfaces 1 are in contact with each other, one of the top surface 3 and the bottom surface 4 is in contact with the other, and there is a gap on the other side.
This is a threaded joint for oil country tubular goods characterized by having a gap of 03 mm or less, having excellent airtightness and detachable torque, and enduring repeated use.

[0020]

The present inventor has conducted detailed studies on the behavior of a threaded joint when a compressive force is applied, and on the damage to the thread surface during repeated fastening of the threaded joint, and as a result, has obtained the following findings.

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 concentrates on the seal portion and the torque shoulder portion, causing significant plastic deformation in the portion. Thereafter, when a tensile load is applied by the weight of the oil country tubular good, the male screw and the female screw are shifted 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 screw insertion surface is brought into contact at the time of fastening. However, the contact surface pressure of the screw surface is increased, and the screw insertion surface is larger than when the screw insertion surface has a gap. Is likely to be damaged.

Based on the above findings, various conditions at the time of fastening in the present invention were determined as follows.

As shown in FIG. 1, the thread of the threaded joint of the present invention belongs to a so-called trapezoidal thread. 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) )
Is set higher.

The thread faces of the trapezoidal screw are load face 1, insertion face 2,
Although there are four surfaces, the top surface 3 and the bottom surface 4, in the joint having a torque shoulder, the load surface 1 of the screw is always in contact because the reaction force accompanying the contact of the torque shoulder is shared by the load surface 1 of the screw. To stabilize the screw engagement, at least one of the other three surfaces must be in contact. In the threaded joint of the present invention, either the top surface 3 or the bottom surface 4 is brought into contact, and the insertion surface 2 is brought into non-contact. (Gap is provided).

The reason why one of the top surface 3 and the bottom surface 4 is brought into contact is that such a contact state of the screw surface is preferable to prevent damage to the screw surface. That is, when either the top surface 3 or the bottom surface 4 is further contacted while the load surface 1 is in contact, the contact surface pressure generated on the top surface 3 or the bottom surface 4 is determined by bringing the insertion surface 2 into contact. Insertion surface 2
Is smaller than the contact surface pressure generated at the time. This is because the action direction of the contact surface pressure is substantially in the radial direction of the pipe on the top surface 3 or the bottom surface 4 and substantially in the axial direction of the tube on the insertion surface 2. Although the contact surface pressure does not increase so much, adjacent screws restrain each other in the tube axis direction. (The contact surface pressure on the load surface 1 is almost in the tube axis direction, This increases the contact surface pressure in this direction.

At the same time, in the threaded joint of the present invention, a gap is provided on either the top surface 3 or the bottom surface 4.
This is because lubricant such as grease used at the time of joint fastening penetrates into this gap to further prevent damage to the screw surface, and at the time of fastening, the pin portion and the seal portion of the box portion come into contact with each other. This is to allow the air trapped by the air to escape to the outside through this gap. That is,
Since this gap spirally communicates with the outside, the trapped air can escape to the outside of the tube.

One of the features of the threaded joint of the present invention is that the insertion surface 2 is non-contact.
The gap between the screw and the female screw insertion surface 2b is set to 0.03 mm or less. Here, this gap is set to be small in order to reduce the plastic deformation of the seal portion and the torque shoulder portion generated when a compressive force is applied to the joint. The contact surface pressure of the seal portion and the torque shoulder portion hardly decreases. Further, the reason why the gap limit value is set to 0.03 mm or less is to ensure a sufficient torque for attaching and detaching the threaded joint due to a compressive load.

FIG. 4 shows how, after an axial compressive force load such that a stress equivalent to 50% of the yield strength is generated in the pipe body, the ratio of the attaching / detaching torque of the threaded joint to the fastening torque depends on the clearance in the insertion surface. It is a figure showing whether it changes. The threaded joint used has the dimensions shown in the examples described later, and the fastening torque is the torque required for connection and fastening of the threaded joint.

As can be seen from the drawing, the attachment / detachment torque / fastening torque decreases monotonously with an increase in the clearance of the insertion surface, and the fastening torque is reduced by 60%.
% Of the insertion surface is required to secure
It is understood that it is necessary to set it to be not more than mm. Fastening torque 60
The reason for securing the attachment / detachment torque of more than% is that even if a torsion torque acts in the direction of loosening the joint for some reason while using the oil country tubular goods, the joint will not loosen easily and will not fall into the well. That's why.

By setting the screwing conditions at the time of fastening as described above, even if a compressive force is applied and then a tensile force due to the weight of the tube is applied, the contact surface pressure of the seal portion hardly decreases. It is possible to secure the airtight performance inside the oil country tubular goods, and to avoid the risk that the oil country tubular goods fall into the wells with almost no decrease in the coupling torque.

The joint of the present invention may be a coupling joint or an integral joint.
The above-described effects are the same in any of the methods.

[0033]

EXAMPLES The performance of the threaded joint according to the present invention will be described below based on examples. A coupling type joint having the configuration shown in FIG. 5 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 Pipe and coupling material: API standard L80 (yield strength 56.3 kgf / mm ² ) : Trapezoidal thread Thread pitch: 5.08mm Thread taper: 1/16 Thread height: 1.575mm

[0035]

[Table 1]

These threaded joints were repeatedly subjected to a fastening test and a composite load test, and the performance was evaluated. here,
In the repeated fastening test, apply lubricating grease to the thread,
The joint was repeatedly tightened and removed, and the damage on the thread surface was examined. The maximum number of repetitions was set to 20 times. In the combined load test, the joint was fastened, and an axial compressive force was applied to the pipe body to generate a stress equivalent to 50% of the yield strength. After that, a stress equivalent to 80% of the yield strength was applied to the pipe body. 80% of the tensile and yield strength in the axial direction (API
(Specified in the standard) was simultaneously applied, and the leakage of the internal pressure was investigated.

Table 2 shows the test results.

[0038]

[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 a failure occurred in any of the tests in the comparative example.

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

[0041]

[Effect of the Invention] The threaded joint for oil country tubular goods according to the present invention can maintain excellent airtightness and high attachment / detachment torque even after a compressive force is applied, and can withstand several tens of times of repeated fastening and disengagement. It can be used economically for wells.

[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 an influence of an insertion surface gap on a change in attachment / detachment torque due to a compressive load.

FIG. 5 is an axial cross-sectional view showing a basic configuration of a coupling type oil country tubular good threaded joint.

[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 oil country tubular goods that constitutes a torque shoulder part by abutting the threadless part for torque shoulder formation provided at the tip and the screwless part for torque shoulder formation provided in the box part, the load surfaces of the male and female threads are mutually A threaded joint for an oil country tubular good, characterized in that one of the top surface and the bottom surface is in contact with the other and there is a gap on the other side, and a gap of 0.03 mm or less is provided on the insertion surface of the male screw and the female screw.