JPH0769028B2 - Oil well fitting - Google Patents

Description

TECHNICAL FIELD The present invention relates to a threaded joint for firmly connecting thick-walled, high-strength, large-diameter oil well steel pipes.

[Prior Art] The technique of connecting an oil country tubular good with a screw is well known, and the thread shape is a round thread (Round Thread) defined by API (American Petroleum Institute) as shown in Fig. 3. , Trapezoidal screw (Buttress Thread) and so on. The Maruyama screw has a good sealing property because the loading surface and the insertion surface are sealed, and the trapezoidal screw has a good sealing property by the screw itself, but is used when it is desired to secure good joint strength.

Further, in the case of a thin-walled joint in which the thread height cannot be sufficiently obtained, a hook-shaped screw (Hook Thread) as shown in FIG. 4 is used. Even in joints for line pipe piping, screws are sometimes used, and although there are slight differences in screw pitch, taper, etc., the same thread shape as that for oil well pipes is used.

[Problems to be Solved by the Invention] In the case of a round mountain screw, the radius of curvature of the screw is γ ₁ = 0.356 to 0.508.
Since it is as large as mm, the stress concentration is small, while the load surface forms 30 ° with the steel pipe axis, and when the axial load is applied, the screws are dislocated, so sufficient strength cannot be secured.
In the case of trapezoidal screw, the loading surface is as small as 3 ° to the steel pipe axis,
No deviation occurs with respect to the axial tensile load,
The corner radius γ ₂ is as small as 0.15 to 0.25 mm, and seizure of screws may occur due to an increase in local surface pressure. Particularly, in the case of a hook-shaped screw, the load axis becomes negative, so that the screws are likely to bite each other, and if the radius of curvature γ ₃ is small,
Easy to burn. From the viewpoint of screw machining, if the radius of curvature of the machining tool is too small, tool wear will occur and the tool life will be shortened. Therefore, it is preferable that the radius of curvature of the tool is large. However, if the radius of curvature is made too large, the fitting surface of the screw cannot be sufficiently obtained, the screw is likely to be dislocated, and the forces in the axial direction cannot be sufficiently transmitted to each other. There is also a method of increasing the thread height in order to secure the fitting surface of the screw, but this also causes an increase in stress concentration, leading to a decrease in joint strength.

Therefore, the present invention takes the radius of curvature of the screw head corner as large as possible, reduces the stress concentration due to the screw fitting, prevents seizure of the screw, and is an optimum screw head for ensuring the life of the processing tool. This is a calculation of the range of the corner radius of curvature.

[Means for Solving the Problem] In the oil country tubular goods joint of the present invention, the female screw and the male screw have a certain taper and are fitted to each other to form a trapezoidal shape in which the screw thread head is flat, It is a joint member that has a mountain height of 2.0 to 3.0 mm and is screwed with each other, and it is possible to machine the trapezoidal screw head and the corner of the bottom with a radius of curvature of 0.25 to 0.55 mm. It has a feature. In particular, the second feature is that the radius of curvature in the above is a processed structure of a key-shaped screw head.

The details of the present invention will be described below.

In FIG. 1, first, when the minimum allowable radius of curvature γ _n (min) is estimated, the stress concentration coefficient α _n is expressed by the equation (1) considering the case where the thread height is 2 mm.

The above formula (1) is a general formula, but using this formula, α _n ≦
If 2.5 is secured, γ _n ≧ 0.25 mm when the screw thread height α _x 2 mm and the screw thread pitch P6.35 mm. That is, 0.25
When the thickness is less than mm, the stress concentration becomes 2.5 or more, which may cause fatigue cracks.

However, if γ _n is too large, the contact portion 1 cannot be sufficiently removed in FIG. 2 and if the contact surface is displaced, the screw is easily displaced, and sufficient strength cannot be secured.

Therefore, in order to find the maximum allowable radius of curvature γ _n (max), when considering the length of l on the load surface as shown in FIG. 2, 1 μN cos θ ≧ νσ _L (2P) N is the vertical surface added to the load surface. The pressure is N2.5σ _y . σ
_L is the stress when pulled in the axial direction, and is considered as σ _L σ _y . Ν is Poisson's ratio ν = 0.3, μ is friction coefficient μ0.8
Then, the contact surface length 1 is calculated for a hook-shaped screw with θ = −3 °, and l ≧ 0.9 mm.

Further, in FIG. 2, from the relationship of 2γ _n + l = d, γ
<0.55 mm.

From the above results, in this case, 0.25 ≦ γ _n ≦ 0.55 can be proposed as the optimum range. By the way, API trapezoidal screw (Buttress Thread
In the case of d), the stress concentration factor is α _n 2.46 as shown in FIG.

Below is an example of applying a hook-shaped screw (Hook Thread) to a thick oil well pipe.

[Example] In Figs. 3 and 4, the diameter is 193.7 mm, the wall thickness is 26 mm, and the diameter is 273.05 mm, the wall thickness.
For two 26 mm thick ultra-thin oil well pipes, the thread height d = 2.0 mm, the screw pitch 2p = 6.35 mm, and the key screw with θ = -3 °,
An example of processing with γ _n = 0.5 ± 0.05 mm is shown. Since this pipe is a thick-walled pipe, the taper of the screw was made sharp, and the number of processed screws was reduced as much as possible. In addition, the thread height is 2.0 mm because it can be made high enough,
I thought that I could secure l. However, if the number of threads is reduced, the load on one screw increases, so the thread shearing condition Nln ≧ tσ _y is satisfied. Here, N = 2.5σ _y , t is the pipe wall thickness, and σ _y is the yield stress. Therefore, it was designed with n values such that n> 10 for the complete screw thread number n.

As an effect, the outer diameter is 273 mm, the wall thickness is 26.6 mm, and L-80 is shown in Table 1.
The results of tensile rupture test of the material under external pressure load are shown. By transmitting the screw strength sufficiently, it became possible to break the mother pipe of the pipe. At the same time, a marked increase in the strength of the biaxial joint under external pressure was observed.

Further, regarding the cutting life, as shown in Table 2, as long as the conventional type curvature is used, the cutting life of the key screw is low, whereas the cutting life is remarkable if the key screw is machined with the curvature of the present invention. You can see that it will improve. Also, as can be seen from Table 3, the present invention is superior to the conventional one in terms of galling resistance.

[Effects of the Invention] According to the joint of the present invention described above, since high joint strength can be obtained, it is possible to provide the optimum joint as a threaded joint for thick-walled, high-strength, large-diameter oil country tubular goods, The contribution to the industry is extremely large.

[Brief description of drawings]

1 and 2 are explanatory views of a screw shape for explaining a threaded joint of the present invention, FIG. 3 shows a conventional screw shape, (a) is an API screw, (b) is a trapezoidal screw , (C) are explanatory views of a screw joint, and FIG. 4 is a shape diagram of a conventional hook-shaped screw.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Eizo Takeuchi 1-1-1 Edamitsu, Yawatahigashi-ku, Kitakyushu, Fukuoka Prefecture, Nippon Steel Corporation Yawata Works (56) References JP-A-61-45188 (JP) , A)

Claims (2)

[Claims] 1. A female screw and a male screw are fitted to each other with a certain taper, and the screw thread head has a flat trapezoidal shape, and the screw thread height is 2.0 to 3.0 mm. An oil country tubular goods joint, which is a joint member to be joined, wherein the trapezoidal screw head and bottom corners are machined to have a radius of curvature of 0.25 to 0.55 mm. 2. A female screw and a male screw are fitted to each other with a certain taper, and the screw thread head has a flat trapezoidal shape, and the screw thread height is 2.0 to 3.0 mm. An oil country tubular goods joint, which is a joint member to be fitted, characterized in that the radius of curvature of the corners of the hook-shaped screw head and the bottom portion is machined to a size of 0.25 to 0.55 mm.