JPH0798082A - Screw coupler for oil well pipe having good fatigue characteristic - Google Patents

Abstract

PURPOSE:To exhibit good fatigue characteristic when a screw coupler for an oil well pipe is used in an oil well having strict load condition in which tensile load and bending load act. CONSTITUTION:A screw coupler for an oil well pipe is formed in such constitution that a trapezoidal pin part 11 having a male screw is connected to a trapezoidal box part having a female screw, and the screw coupler for the oil well pipe having good fatigue characteristic is formed in such constitution that a spiral stress release groove 16 whose lead is equal to the lead of a screw is provided on the screw bottom 32 of one end or both ends of the screw engaging end part of the pin part 11 or/and the box part in the range of at least 1 pitch. It is thus possible to release stress centralization on the screw engaging end part so as to improve fatigue characteristic of a coupler.

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 an oil country tubular good used for exploration and production of underground-produced natural gas and petroleum. In particular, the well bend is large and acts on a screw. It is intended to provide a threaded joint for oil country tubular goods having excellent fatigue properties, which is suitable for use under severe load conditions.

[0002]

2. Description of the Related Art Today, threaded joints are widely used as a technique for connecting oil well pipes used for exploration and production of natural gas fields and crude oil fields having a depth of several thousand meters.

FIG. 9 is an axial sectional view showing two examples of a threaded joint for oil country tubular goods. FIG. 1A shows a pin portion at one end of the pipe 10.
11 is an example of an integral system in which a box portion 21 is formed at the other end and the pipes are connected to each other. In the same figure (b), a pin 20 is formed at both ends by a coupling 20 having two box portions 21 formed. It is an example of a coupling system for connecting the pipes 10 formed with. Further, FIG. 6C shows an enlarged view of the screw engagement portion in the coupling system.

In any of the examples, the male screw 12 provided on the pin portion 11
It is connected by tightening the female screw 22 provided on the box part 21 with that of the oil well pipe in recent years.
There are few things that are composed only of screw elements consisting of 12 and female threads 22, and many things are equipped with a seal portion 13 and a shoulder portion 14 at the same time. The seal portion 13 is a metal seal to improve the airtightness against internal pressure or external pressure of the fluid inside or outside the tube 1. Further, the shoulder portion 14 includes a pin portion 11 and a box portion 21.
By abutting this portion of the above, a high contact surface pressure that causes excessive plastic deformation is not generated in the seal portion 13, and the fastening of the screw joint is ensured.

[0005] In recent years, with respect to such oil well pipe threaded joints, 1) With the development of the ocean oil field, it is often connected to a pipe body provided on an offshore hull, and therefore repeated tensile loads due to waves 2) In order to improve the efficiency of excavation work, there are new required performances such as to withstand the repeated bending load caused by intentionally bending the well. It is required to have excellent dynamic fatigue properties.

The most problematic requirement for such fatigue characteristics is that the meshing end portion 15 of the screw is used for the following reason.
It is the strength at a and 15b.

When a tensile load or a bending load is applied to the threaded joint, the load is most shared by the screw engagement end 15 a on the base side of the pin portion 11 and the screw engagement end 15 b on the inner side of the box portion 21. Therefore, a large stress concentration occurs at the screw root R portion at the meshing end portion of the screw.

In the joint having the shoulder portion 14 as illustrated in FIGS. 9A and 9B, the reaction force generated by the abutment of the shoulder at the time of fastening already causes stress on the screw meshing end portions 15a and 15b. Therefore, when a tensile load or a bending load is further applied, the stress concentration at the screw root R portion at the meshing end of the screw is promoted.

Conventionally, the screw root R at the meshing end of the screw
Several proposals have been made in order to reduce the stress concentration in the part, and there are the following, for example.

FIG. 2 shows a screw shape proposed in JP-A-4-15385 to reduce stress concentration. That is, the radius of curvature of the trapezoidal screw head R and the bottom screw root R is machined within a predetermined range so that the stress concentration of the screw root R 17 does not become excessive (stress concentration factor α _n ≦ 2.5). It is characterized by that.

FIG. 3 shows another means for preventing stress concentration.
The joint proposed by JP-A-4-157280 is shown. As is clear from the figure, in this joint, the stress relief groove 30 is provided at one end on the root side of the pin portion 11 to reduce the stress concentration at the screw root R portion at the screw meshing end portion.

However, the above-proposed threaded joints for oil country tubular goods each have the following problems.

In the threaded joint shown in FIG. 2, it is necessary to increase the radius of curvature of the screw root R portion in order to reduce the stress concentration in the screw root R portion 17. However, if this radius of curvature is made too large, the contact portion 18 of the screw load surface shown in the figure becomes relatively small, and the effective contact area between the male screw and the female screw decreases, so if a tensile load or a bending load acts. , There is a high risk that the joint will come off easily and the oil well pipe will fall into the well. Once an oil well pipe falls into a well, it is extremely difficult to reconnect the oil well pipe within the well, which causes the worst situation in which the well being developed or produced must be abandoned. It will be. In order to avoid this situation, the radius of curvature of the screw root R portion cannot be machined so large. Therefore, as a result, the stress concentration in the screw root R portion 17 cannot be reduced so much. This problem is particularly remarkable in the case of a thin-walled pipe in which the contact portion 18 of the screw load surface cannot be made large, that is, the screw height cannot be made high, and becomes a serious problem.

In the threaded joint shown in FIG. 3, since the stress relief groove 30 of the pin portion 11 is provided with reference to the surface orthogonal to the joint axis, there is a problem that the shape is not symmetrical with respect to the surface of the male screw 12. .

FIG. 4 is an enlarged perspective view showing the vicinity of the stress relief groove 30 provided in the pin portion 11 of the joint of FIG. 3 proposed by Japanese Patent Laid-Open No. 4-157280. As is clear from the figure, the surface of the spiral male screw 12 that is closest to the stress relief groove 30 is obliquely cut by the stress relief groove 30. Therefore, looking at the threads of the male screw 12 for one pitch at the screw meshing end portion 15a, the thread 15a 'in contact with the stress relief groove 30 and the thread that is approximately one pitch away from the stress relief groove 30.
There will be 15a ″.

The effect of reducing the stress in the R part of the screw root by providing the stress relief groove 30 decreases as the distance from the stress relief groove 30 increases.
The effect can be expected at the portion of the screw thread 15a 'which is in contact with the
You can't expect much from this part. Further, in order to solve this problem, a means of increasing the depth of the stress relief groove 30 may be considered, but in this case, the pin portion 11 is thinned, and the cross-sectional area of the groove bottom portion of the stress relief groove 30 is caused. Therefore, there is a risk that the stress relieving groove 30 is relatively easily broken by the action of tensile load or bending load.

[0017]

DISCLOSURE OF THE INVENTION The present invention solves the problems of the above-mentioned threaded joint for oil well pipes, and when used in an oil well under severe load conditions in which tensile load and bending load act,
An object of the present invention is to provide a threaded joint for oil country tubular goods that exhibits excellent fatigue properties.

[0018]

The gist of the present invention is basically a threaded joint for oil country tubular goods as shown in FIG. 9, in which a stress relaxation groove is provided as shown in FIG. That is, the threaded joint of the present invention, a pin portion 11 having a trapezoidal male screw 12,
A threaded joint for oil country tubular goods which is also formed by connecting a box portion 21 having a trapezoidal female thread 22, and wherein the pin portion or /
And a fatigue characteristic characterized in that a spiral stress relaxation groove 16 equal to the lead of the screw is provided in at least one pitch range on the screw bottom 32 at one end or both ends of the screw meshing end portions 15a and 15b of the box portion. It is an excellent oil well pipe threaded joint. That is, the stress relaxation grooves 16 of the present invention are provided on the same screw bottom 32, and the number thereof is one or plural in parallel.

The oil country tubular goods threaded joint is usually composed of a single-start thread, but the oil country tubular goods threaded joint of the present invention is not limited to a single-start threaded joint, and a multi-start threaded joint of two or more threads. May be In this case, assuming that the number of stress relaxation grooves provided on the same screw bottom within the range of at least 1 pitch is m and the number of screw threads is n, the total number of stress relaxation grooves provided on the screw bottom is m × n. Become.

FIG. 5 is a diagram for explaining the names of the respective parts of the male screw 12 (in the case of a double thread screw). Here, the pitch means the distance measured in the axial direction at two corresponding points of the screw threads 33 adjacent to each other, and the lead means the distance traveled in the axial direction when going around the axis line along the screw threads. Say. Therefore, regarding the relationship between the pitch and the lead, in the case of the double-thread screw shown in the same figure, the lead is twice the pitch, but in the case of the single-thread screw, the lead and the pitch are equal.

[0021]

The screw of the threaded joint of the present invention belongs to the so-called trapezoidal screw. This screw shape is adopted because it is generally superior in performance against repeated tensile loads and bending loads, as compared with a triangular screw.

FIG. 6 is a diagram for explaining a stress sharing situation on a screw load surface when a tensile load or a bending load is applied. As shown in the figure, when a tensile load or a bending load acts on the joint, a force F perpendicular to the face is applied to the load face 31 of the male screw.
However, this force F is decomposed into an axial component F · cos θ and a radial component F · sin θ of the joint. Here, the smaller the θ is, the larger the axial component F · cos θ becomes, and the larger the tensile load and the bending load acting on the joint can be shared. Further, since the radial component acts in a direction to reduce the catching of the screw, the smaller θ is, the more difficult the joint is to come off. Therefore, the smaller the value of θ, the better the performance against tensile load and bending load.

Comparing the trapezoidal screw and the triangular screw,
Assuming that the screw height and the screw bottom width are the same, since the trapezoidal screw can reduce θ, the trapezoidal screw has excellent performance against tensile load and bending load.

In the threaded joint of the present invention, at least one of the male thread of the pin section and the female thread of the box section is provided with a lead equal to the thread on the thread bottom of one end or both ends of the thread engaging end for at least one pitch. The feature is that a stress relaxation groove is provided. Here, the stress relaxation groove is provided in a range of at least one pitch from the screw meshing end portion, and is made to go around the joint axis line, because the screw thread at the most end portion of the meshing end portion having the highest stress concentration is used. Root R
This is for alleviating the stress generated in the part uniformly over the entire circumference.

In the threaded joint, not only the root R portion (for one pitch) of the outermost end of the meshing ends,
Since a relatively high stress concentration occurs in the R part of the thread root for a few pitches, it is preferable to provide a stress relaxation groove also in this part. Further, not only the single-thread screw but also the multi-thread screw having two or more threads is essential to provide the stress relaxation groove for one pitch.

The reason why the stress relaxation groove is provided at the screw bottom is that the stress relaxation effect works effectively when the stress relaxation groove is provided close to the screw root R portion (see FIG. 7, which will be described later). (See FIG. 8). On the other hand, in the sense that it is provided in the vicinity of the screw root R portion, it is conceivable to provide the stress relaxation groove on the screw load surface, but in this case, the contact portion (area) of the screw load surface is narrowed, This is not appropriate because it causes problems such as easy disconnection of the joint.

The reason why the stress relaxation groove is provided spirally with the same lead as the screw is that the relative positions of the root portion R of the screw thread and the stress relaxation groove are the same in any section in the circumferential direction of the joint. This is because By providing the stress relaxation groove in this manner, the effect of relaxing the stress concentration becomes uniform at any of the thread root R portions in the circumferential direction.

FIG. 1 is a sectional view of a pin portion 11 showing an example of a stress relaxation groove provided in the threaded joint of the present invention. In the figure, two stress relaxation grooves 16 are provided spirally on the screw bottom 32 in the vicinity of the root R portion 17 of the load surface 31 of the screw. The stress relaxation groove is provided on the basis of the above findings, and its effect will be described in more detail.

FIG. 7 shows a tensile model of a semi-infinite plate having multiple notches (cited document: Masataka Nishida, "Stress Concentration",
602 to 603, Morikita Publishing). In this model, n semi-circular notches (radius ρ) are 2p in the straight edge of a semi-infinite plate.
They are arranged at equal intervals (pitch) and a uniform tensile stress σ ₀ is applied in the direction of the straight edge. This corresponds to the case where the radius ρ of the notch is smaller than the plate width even if the plate has a finite width. Therefore, when the tension model shown in FIG. 7 corresponds to the example of the threaded joint of the present invention shown in FIG. 1, the notch at the right end of FIG. 7 corresponds to the root R portion 17 of FIG. 1, and the other of FIG. The notch will correspond to the stress relief groove 16 of FIG. According to the photoelasticity test result in the above cited document, the maximum stress in the model shown in FIG. 7 is 5 from the deepest point A of the notches at both ends.
Appears at point A ', which is outside by 7 °.

FIG. 8 is a diagram showing the stress concentration rate in the tensile model described in the cited document.

In the figure, σ _{A is} the stress at the notch bottom points A at both ends, σ _A ′ is the maximum stress occurring at the point A ′ slightly outside the point _A , and stress at the second notch bottom point B from the plate edge is σ _B ,
The stress at the center C of the bottom of the notch is σ _C, and each stress concentration rate (α _A = σ _A / σ ₀ , α _A ′ = σ _A ′ / σ ₀ , α _B = σ
The relation between _B / σ ₀ , α _C = σ _C / σ ₀ ) and 2p / ρ is
The number n of notches is shown as a parameter. Figure 8
From the above, it can be seen that the stress concentration rate is minimum when 2p / ρ is approximately 2 (when p = ρ), and decreases with increasing n. Therefore, it is preferable that the stress relaxation groove 16 provided in the threaded joint of the present invention is located so as to contact the root R portion 17 of the load surface 31 and the number thereof is more than one. Although the above description is for a tensile load, the same can be said for a bending load.

With respect to the shape of the stress relaxation groove 16, its depth is important. If the depth is too shallow, the effect of relaxing the stress concentration is small. Since the cross-sectional area is reduced, the performance against tensile load and bending load is reduced. Therefore, the screw root R
The depth is preferably 0.5 to 1.5 times the radius of curvature of the part. Within this range, the cross-sectional area at the groove bottom is sufficiently secured as compared with the stress relief groove 30 shown in FIG. 3, so the risk of breakage at the groove bottom is significantly reduced. Further, the shape of the stress relaxation groove 16 itself may be a semicircular shape shown as an example in FIG. 1 or a semielliptic shape so that a sufficient effect can be obtained. It is necessary that the shape is

In the threaded joint of the present invention, the stress relaxation groove is provided in at least one of the male screw of the pin portion and the female screw of the box portion at one end or both ends of the screw engaging end portion. As described above, the screw meshing end portions in which stress concentration on the screw root R portion poses a problem are not limited to one portion in the pair of pin portions and the box portion, but there are a plurality of portions. However, the degree of stress concentration there is different depending on the joint shape (for example, joint method, wall thickness, screw shape, etc.),
It is more realistic to determine the necessity and conditions for providing the stress relaxation groove depending on the stress concentration situation. Providing the stress relaxation groove increases the working cost of the joint. Therefore, if the stress relaxation groove is provided under more conditions than necessary, there is a possibility that the machining cost will simply increase rather than the effect of extending the life of the joint. Because.

The threaded joint of the present invention may be a coupling type joint or an integral type joint. The above-mentioned effects are the same in any method.

[0035]

EXAMPLES The performance of the threaded joint of the present invention will be described based on examples. The joint used was a coupling system having the configuration shown in FIG. 9 (b), and was a single-thread screw joint having the specifications shown in Table 1, and the screw at the screw engaging end on the base side of the pin portion was used. A stress relief groove was machined in the bottom. The processing conditions of the stress relaxation groove at this time were as follows.

1. Shape: Semicircle 2. Radial dimension: 0.3mm (of the radius of curvature of the R part of the screw root)
1.5 times) 3. Pitch and lead size: 5.08mm (same as screw pitch and lead) 4. Length and number of stress relaxation grooves: One groove was provided over the range of 1 pitch from the outermost end of the screw engaging end (hereinafter referred to as A sample) 2 pitches from the outermost end of the screw engaging end Two grooves (distance between the groove centers: 0.6 mm) were provided over the range (hereinafter, referred to as B test piece)

[0037]

[Table 1]

Each of the above-mentioned A and B specimens was produced, and for comparison with these, one joint in which the stress relaxation groove was not provided in the pin portion (hereinafter referred to as C specimen) was produced. Bending moment 5000kgf ・ m for these 3 threaded joints
A rotary bending fatigue test was conducted to evaluate each performance. The evaluation results are summarized in Table 2.

[0039]

[Table 2]

As is clear from Table 2, fatigue cracks occurred in the R part of the screw root at the end of the thread engagement end of all three threaded joints, but the fatigue life of the joint was It has been extended significantly. Even when the stress relaxation groove is provided, the B sample has a longer life than the A sample, so that the stress relaxation groove has a length of 2 pitches rather than 1 pitch. It can be seen that the fatigue characteristics of the joint are improved by providing two grooves instead of one.

Although an example of the coupling system is shown here, similar characteristics were obtained even in the integral system.

[0042]

In the threaded joint for oil country tubular goods according to the present invention, the stress concentration at the screw meshing end portions is relieved, and the fatigue characteristics are greatly improved. Therefore, if this joint is used, the efficiency of excavation work can be improved even under severe oil well conditions associated with the development of an ocean oil field, and it will be possible to greatly contribute to the oil industry in the future.

[Brief description of drawings]

FIG. 1 is an axial sectional view showing an example of a stress relaxation groove provided in a threaded joint of the present invention.

FIG. 2 is an axial cross-sectional view illustrating a thread shape of a conventional oil country tubular goods threaded joint.

FIG. 3 is an axial cross-sectional view illustrating a stress relief groove provided in a conventional oil well pipe threaded joint.

FIG. 4 is an enlarged perspective view of the vicinity of a stress relief groove provided in a conventional oil well pipe threaded joint.

FIG. 5 is a diagram illustrating names of respective parts of the male screw.

FIG. 6 is a diagram illustrating a stress sharing state on a screw load surface when a tensile load or a bending load is applied.

FIG. 7 is a diagram showing a tensile model of a semi-infinite plate having multiple notches for explaining the effect of the stress relaxation groove of the present invention.

8 is a diagram showing a stress concentration rate in the tensile model shown in FIG. 7. FIG.

FIG. 9A is an axial cross-sectional view showing the basic structure of an integral type oil well pipe threaded joint, and FIG. 9B is an axial cross-sectional view showing the basic structure of a coupling type oil well pipe screw joint. It is a figure and (c) is an axial enlarged view of the engagement part of the screw in a coupling system.

[Explanation of symbols]

10: Pipe, 11: Pin part, 12: Male screw, 13: Seal part, 14: Shoulder part 15a, 15b: Screw engaging end part, 16: Stress relaxation groove,
17: R part of screw root 18: Contact part of screw load surface, 20: Coupling, 21:
Box part 22: Female thread, 30: Stress relief groove, 31: Load surface, 32:
Screw bottom 33: Thread

Claims (1)

[Claims] 1. A threaded joint for oil country tubular goods comprising a pin portion having a trapezoidal male screw and a box portion having a trapezoidal female screw connected to each other, wherein one end of a screw meshing end portion of the pin portion or / and the box portion. Or on the screw bottoms at both ends,
A threaded joint for oil country tubular goods having excellent fatigue characteristics, characterized in that a spiral stress relaxation groove equivalent to a screw lead is provided in a range of at least 1 pitch.