JPH08320091A - Oil well tubing thread coupling - Google Patents

Abstract

PURPOSE: To decrease stress to be generated on a ring charging groove in a thread coupling as little as possible by providing a first groove into which a ring is to be inserted on a coupling, and providing a second groove just above a coupling shoulder part. CONSTITUTION: A screw coupling is composed of a coupling 1, pins 2, the pin extends to a thread part 3 and is provided with a shoulder part 4 and a seal part 5 extending to the shoulder part, and a ring 6 is so arranged as to be pinched between the tips of facing pins. A first groove, namely, a ring inserting groove 7 is provided on a coupling, and a second groove, namely, a stress alleviating groove 8 is provided just above the shoulder part. The structures of the shoulder part 4 and the seal part 5 are the suitable structures adversely not affecting the seal part since excess torque and compressed load are absorbed on the shoulder part under the excess torque or compressed load, and compressibility can be controlled by controlling tolerance between the shoulders and the tolerance in the axial direction of the pin seal part.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oil country tubular good used for crude oil mining, and the like, in which a pipe having a plastic coating on its inner surface can be screwed without damaging the coating, and the joint base metal is also used at the fastening portion. The present invention also relates to a threaded joint that can prevent damage due to an inner surface fluid.

[0002]

2. Description of the Related Art In the drilling of oil wells, the development of so-called water injection wells, in which water is injected through tubing into oil wells in order to improve crude oil recovery efficiency, has become popular. When ordinary water or seawater is directly injected into a low alloy oil country tubular good, the steel tube is significantly corroded by the effect of oxygen contained, which causes the steel tube to collapse in a short period of time. Although the injected water may be degassed, it may not always be sufficient, and the most reliable method is to use a corrosion resistant material such as 13Cr steel, but this causes a cost increase of more than three times.

[0003] Here, what has begun to be studied in recent years is an inner surface coating tubing in which ordinary steel is coated with a resin to protect the base material from a corrosive fluid on the inner surface. In such an inner surface coating tubing, a fastening portion by a screw joint also needs a corrosion resistance function, and various studies have been conducted in recent years.

For example, a ring is generally arranged at the tip of a pin as shown in Japanese Utility Model Laid-Open No. SHO 142-142475, but in the case of an inner surface coating joint, coating is performed against repeated tightening and loosening. In order to prevent damage, the material of the ring is preferably a plastic resin such as Teflon ring. However, when a plastic resin is used for the ring in such a structure, the role as a torque shoulder cannot be expected. In order to solve this, there is, for example, Japanese Utility Model Laid-Open No. 4-49290 in which a torque shoulder is provided on the outside, but it is necessary to perform hot working called upset on the pipe end portion, which is expensive.

As an example of the development of a joint which has both of these problems, that is, has a torque stopper function and is inexpensive,
JP-A-62-98088, JP-A-4-63884
There are inventions described in Japanese Patent Publication No. 5,236,230. The points not mentioned in these inventions are the ease of mounting the ring and the stability of the ring after mounting, but generally, the deeper the ring insertion groove is, the more stable the ring is. In particular, when the ring is sandwiched so that the tube ends abut against each other as seen in US Pat. No. 5,236,230, it is necessary to take the groove deep from the viewpoint of ring stability when the actual use is kept in mind. It is a very important design factor.

However, as shown in FIG. 1, the deeper the groove, the thinner the cross-sectional area of the coupling,
In particular, under axial force, stress exceeding the yield point is concentrated on the groove bottom. Under these circumstances, the corrosive fluid on the inner surface will easily attack the stress concentrating area if the ring sealing function is insufficient or if the ring is not attached for some reason. Also causes fatal SSC (sulfide stress corrosion cracking). As a means for preventing these, there has been a device belonging to the design know-how such as increasing the corner R (radius) of the groove portion, but none of the inventions aimed at positively reducing the stress of the ring groove.

[0007]

DISCLOSURE OF THE INVENTION In the present state of the art, the present invention provides a ring joint in a threaded joint including a coupling having a ring at a pin tip and a groove for inserting the ring. An object of the present invention is to provide an oil country tubular goods threaded joint that reduces the stress generated in the groove as much as possible.

[0008]

DISCLOSURE OF THE INVENTION The present invention relates to a pipe threaded joint in which a coupling having an internal thread on an inner surface and a pin having an external thread on an outer surface are screwed to each other. Has a non-threaded portion formed with a ring, and a ring is interposed between the tips of the pipes or between the pipe tip and the coupling shoulder, and a first groove for inserting the ring is formed on the coupling. And, further, characterized in that a second groove is engraved immediately above the coupling shoulder portion, and the second groove depth is shallower than the valley depth of the coupling first screw, or further The inner diameter of the first groove is smaller than the inner diameter of the second groove, whereby the stress generated in the first groove can be minimized.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below based on the embodiments shown in the drawings. FIG. 2 shows a threaded joint according to the present invention, and FIG. 2B is an enlarged view of a main part (circle portion) of FIG. In the figure, the screw joint consists of coupling 1 and pin 2,
The pin follows the screw part 3, then the shoulder part 4 and the following,
The seal 6 is provided, and the ring 6 is arranged so as to be sandwiched between the tips of the opposing pins 2. Here, a first group = ring-loading groove 7 is engraved on the coupling, and a second groove = stress relaxation groove 8 is engraved immediately above the shoulder. 9 means a coating that protects the base material.

Here, the structure of the shoulder portion 4 and the seal portion 5 is optimal in that it does not adversely affect the seal portion because excessive torque or compression load is absorbed by the shoulder portion under excessive load torque or under compressive load. The compression ratio of the ring can be accurately controlled by appropriately controlling the tolerance between the shoulders on both sides of the coupling and the tolerance in the axial direction of the pin seal portion.

A mechanism by which the groove 8 can relieve the stress of the ring charging groove 7 will be described. FIG.
(A) schematically shows the flow of axial stress when there is no stress relaxation groove. The stress transmitted by the screw portion diffuses at the same time as the coupling cross-sectional area increases in the coupling portion directly above the seal portion, hits the ring charging groove, and causes stress concentration at the ring bottom portion. If this stress exceeds the limit value for the SSC of the material, there is a risk of sulfide stress corrosion cracking starting from the groove bottom.

On the other hand, as shown in FIG. 3B, when the stress relaxation groove 8 is present, the flow of stress is less likely to reach the groove 7, and as a result, the stress concentration in the groove 7 is relaxed.

FIG. 4 shows a quantitative representation of such effects using the finite element method. It can be seen that the stress of the ring charging groove is significantly reduced by providing the stress relaxation groove 8. Stress is concentrated at the bottom of the stress-releasing groove, but such groove surface is not directly exposed to the corrosive fluid, and there is no problem in practical use.

Next, the reason for making the depth of the stress relaxation groove 8 and the depth of the ring charging groove 7 shallower than the root depth of the first screw of the coupling 1 is that the coupling strength of the coupling is the original cup having no groove. This is to maintain the same strength as the ring strength. This design maintains the joint strength without reducing the critical cross section of the coupling.

FIG. 2 shows an example in which the inner surface of the pin and the tip of the threaded joint according to the present invention are coated. Normal,
Such joints are often used in combination with inner surface coated pipes. As an effect, neither the pipe base material nor the joint portion directly comes into contact with the fluid on the inner surface, and when the fluid is corrosive, the corrosion resistance of the pipe body or the joint portion can be improved. With such a structure, since the coupling portion does not directly contact the internal fluid, there is no need to coat the coupling portion, which is more preferable in terms of cost, and is similar to the invention described in Japanese Utility Model Laid-Open No. 4-63884. However, this is different from the present invention in that no stress relaxation measures for the ring charging groove are taken. In addition, FIG.
(B) is an example in which rings 6 and 6 are interposed between the tube tip and the coupling, but is included in the present invention in that it has a stress relaxation groove.

Regarding the shape of the invention ring,
As mentioned in Japanese Patent No. 142475, such a shape is effective for preventing the ring from popping out when the internal pressure drops, or the ring from protruding due to axial compression. If it is extremely soft, it does not have a torque control function, and as described above, it is necessary to provide a stress relief measure for the torque stopper and the ring mounting groove. FIG. 6 shows a technique for embodying it. This is also included in the present invention.

[0017]

As described above, according to the present invention, by providing the stress relaxation groove immediately above the shoulder portion adjacent to the seal portion, the effect of keeping the axial stress flow away from the ring charging groove occurs. The maximum stress generated in the groove can be reduced.

[Brief description of drawings]

FIG. 1 is an explanatory view of a general screw joint structure.

FIG. 2A is an end surface of a threaded joint portion of the present invention, and FIG.

FIG. 3 is a schematic view showing the flow of stress in the axial direction of the joint portion,
(A) shows the case where there is no stress relaxation groove, and (b) shows the case where it does.

4A and 4B are diagrams quantitatively showing stress concentration in a ring charging groove by using a finite element method. FIG. 4A is a schematic view of a joint structure, and FIG. 4B is a ring charging groove diameter and a maximum equivalent. The figure which shows the relationship with stress.

5A and 5B are other examples of the threaded joint of the present invention, in which FIG. 5A is a cross section thereof, and FIG.

FIG. 6 shows a main part of a screw joint showing another structure of the present invention.

[Explanation of symbols]

 1 Coupling 2 Pin 3 Screw part 4 Shoulder part 5 Seal part 6 Ring 7 Ring insertion groove 8 Stress relaxation groove 9 Coating

Claims (3)

[Claims] 1. A threaded joint for pipes, in which a coupling having an internal thread on an inner surface and a pin having an external thread on an outer surface are screwed together, wherein a threadless portion is formed in the order of a shoulder portion and a seal portion. A ring is interposed between the tips of the tubes or between the tube tip and the coupling shoulder, and a first groove for inserting the ring is formed in the coupling, and the coupling is formed. An oil country tubular goods threaded joint characterized in that a second groove is engraved just above the shoulder portion. 2. The oil well pipe threaded joint according to claim 1, wherein, in the threaded joint, the first and second groove depths are shallower than the valley depth of the coupling first screw. 3. The screw joint as described above, wherein the inner surface of the pin is
Also, a threaded joint characterized in that the tip end surface of the pin is coated with resin or FRP.