JPH0535317B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a threaded joint having a barrel portion, such as an oil country tubular joint, and particularly relates to a threaded joint that can improve tensile fatigue strength. It is something.

[Prior art] As shown in Fig. 8, a tool joint for oil country tubular goods is generally a screw-in type joint that is tightened with a pin and a box in contact with each other in a barreled part made thickened by upset processing, etc. used. The figure is a cross-sectional view of the main parts of a threaded joint for oil country tubular goods.
1 is pin (male thread side), 2 is box (female thread side)
It is. When examining the tensile strength of such a threaded joint, conventionally, the A-A cross section and the B-B cross section of the screw clearance parts 1a and 2a shown in the figure were
Generally, strength calculations are performed by focusing on the area of the cross section.

[Problems to be Solved by the Invention] When an external force is applied to this joint, the cross-sectional areas of the A-A cross section and the C-C cross section shown in FIG.
The stress value applied to the A-A cross section of the pin 1 varies widely depending on the shape of the joint formed by the length l of the tip end 2b of the box 2.

Figure 10 shows the A-A cross section of pin 1 and box 2 when an external force P due to tension is applied in a state where F ₀ force is generated due to initial tightening (point A).
It is a characteristic diagram which shows the change of the stress of the CC cross section of.
In the figure, the vertical axis F represents the force acting on the A-A cross section of pin 1 and the C-C cross section of box 2, and the horizontal axis Δl represents the amount of deformation of pin 1 and box 2 between l shown in FIG. . 3 is the deformation characteristic of the A-A cross section of pin 1,
4 is the deformation characteristic of the cross section of box 2 taken along the line C--C, P is the applied external force, and a and b are the areas where pin 1 and box 2 receive the external force due to tension.

9 and 10, when the box 2 receives an external force P due to tension, the compression force of the box 2, which was initially compressed toward the pin 1 due to the fastening, is reduced by the external force P. On the other hand, a tensile load is applied to the pin 1 in both axial directions due to the fastening at the A-A cross section of the relief portion 1a, and when it receives an external force P due to tension, the tensile load acting on the A-A cross section is further increased. increase

Of the external force P, the force applied to pin 1 is P×
It is given by the value of {b/(a+b)}. Therefore, the action of an external force such as a repeated load will cause the A-
The stress value changes and acts on the A section (weak point in terms of strength). This has not been considered in the design of conventional barreled joints, and has become a major problem for threaded joints that are subject to repeated stress.

In view of the above points, the present invention aims to provide a threaded joint that can improve the fatigue strength of the joint by keeping the stress amplitude applied by tension to at least the pin side within the allowable stress amplitude value. purpose.

[Means for solving the problem] The screw-type joint according to the first invention of the present invention has the following features:
In a case where a barreled pin having a male thread with a relief part at the base is screwed into a barreled box having a female thread until the end surfaces of the barreled parts come into contact with each other, the rigidity ratio of the pin and the box in the barreled part is determined. , by adjusting the cross-sectional area of the relief part of the pin, the cross-sectional area of the box covering the relief part, and the axial length of the barreled part of the box, the stress amplitude applied by tension can be reduced. This is within the allowable stress amplitude value.

Further, in the screw-in joint according to the second aspect of the present invention, a barreled pin having a male thread with a relief portion at the root is screwed into a barreled box having a female thread until the end surfaces of the barreled portions of the two are in contact with each other. In the case where the rigidity ratio between the pin and the box in the barreled portion is determined by the axial length of the pin relief portion surrounded by the end of the box, and the end surface of the barreled portion of the pin continuous from the relief portion, By adjusting the length of the slit, which is formed by recessing in the axial direction, the stress amplitude applied due to tension is kept within the allowable stress amplitude value.

[Function] In the present invention, by maintaining an appropriate rigidity ratio between the pin and the box, the stress generated when subjected to repeated external force due to tension is reduced at least on the pin side, and thereby the above-mentioned The stress amplitude due to external force is reliably kept within the allowable stress amplitude value on the pin side, improving the fatigue strength of the joint.

[Example] The present invention will be explained below with reference to illustrated embodiments.

First, when considering the cross-sectional area as the basis, let A and C be the respective cross-sectional areas of the screw relief part 1a of the pin 1 and the tip part 2b of the box 2 shown in FIG. When the width is △σ and the allowable stress amplitude is △σ _a , pin 1 as static strength
After determining the cross-sectional area A, we add A/(A+C)=
By combining the conditions Δσ _a /σ, the cross-sectional area C of box 2 can be determined.

According to this, in the vicinity of the barreled part, Fig. 1B
As shown in the figure, the thickness of the tip of box 2 will be increased. l in the figure is the tip 2b of the screw in box 2.
L indicates the length of the portion with an increased outer diameter, which is the length of L plus 2 to 3 threads.

In addition, since the outer diameter of the tool is often limited for various reasons, in order to adjust the rigidity ratio of the joint without increasing the outer diameter, it is necessary to
By adjusting the length of b, a slit 6 can be provided as shown in FIG. In the figure, the slit length of pin 1 is L ₁ , and the tip of box 2 is
If the length of b is l and the outer diameter is D, first, the cross-sectional area A against external force is determined from the static strength, and the cross-sectional area C is also determined from the constraint of the outer diameter D, and the value of C/A is accordingly determined. is determined. Therefore, the stress amplitude for stress σ due to external force is Δσ={A/(A+C)}+σ.

If the allowable stress amplitude is △σ _a , generally △σ>△
Since σ _a , in this case, the slit 6 shown in Figure 2
The length L ₁ of (△σ−△σ _a )/△σ _a = l/(L ₁ +
If L is set to ₁ or more, which is determined by the condition l),
△σ′ is attenuated and becomes △σ′, and furthermore, △σ′≦△σ _a .

How the difference in shape between pin 1 and box 2 of the screw-in joint designed in this way results in a difference in rigidity will be determined using a characteristic curve of changes in rigidity.

Figures 3A and 3B show changes in the stress and stress amplitude applied to the relief part 1a of pin 1 due to the difference in shape between pin 1 and box 2, and show the rigidity ratio (F
=E・A・△l/L (represented by E・A/L). In the figure, F is the force used on the cross section A of the pin 1, Δl is the amount of change in the pin 1 and the box 2 between the length l of the tip 2b of the box 2, 3' is the rigidity of the pin 1, and 4 ' is a characteristic curve showing the rigidity of box 2, and Δσ and Δσ' are the cyclic stress amplitudes. In Fig. 3A, by increasing the stiffness curve 4' of box 2 as shown by the arrow (increasing E・C/L of F=E・C・△l/L), for the same external force P, This shows that the external force amplitude △σ acting on the pin decreases to △σ'.

Similarly, in FIG. 3B, by lowering the stiffness curve 3' of pin 1 as shown by the arrow, the pressure amplitude △σ acting on the pin for the same external force P is reduced to △
It shows that it decreases to σ′.

Figure 4 is an example of a diagram showing the fatigue strength in seawater of a tool joint (material: chrome molybdenum steel) used as a joint for oil country tubular goods in highly corrosive environments such as seawater. The vertical axis Δσ represents the stress amplitude, and the horizontal axis N represents the number of repetitions until fatigue failure occurs. 5 is a curve showing fatigue strength.

Next, a specific example of the first invention of the present invention will be explained by citing numerical values, but before that, for comparison with this specific example, an example of a conventional general tool joint is shown and explained in Fig. 11. do. In the figure, 1 is a pin,
2 is box, d ₁ = 270mmφ, d ₂ = 285mmφ, d ₃ =
320mmφ, d ₄ = 200mmφ, l = 25mm, pin 1
and the load applied to the joint of box 2, respectively.
If F _pio and F _BOX are used, the calculation formula is as follows.

A cross-sectional area S ₁ = π・(27-3.5)×3.5 ≒258.4cm ² C cross-sectional area S ₂ = π(32-1.75)×1.75 ≒166.3cm ² F _pio = E・A/l・△l _p = 2.1×10 ⁶ ×258.4/2.5 ×△l _p =2.17×10 ⁸・△l _p F _BOX =E・C/l・△l _p =2.1×10 ⁶ ×166.3/2.5 ×△l _p =1.40×10 ⁸・△l _pThe characteristic diagram based on the above calculation formula is as shown in Fig. 12. In the figure, A and C are A cross section, respectively.
It shows the stiffness ratio of the C section, and the other symbols are the same as in the above description.

Now, if an external force of P = 1000 tons is applied, the load on the pin is F _pio = 1000 x {258.4/(258.4+166.3)}
= 600.84ton, and the resulting stress σ =
600.84×10 ³ /258.4=2325Kgf/cm ² =23.25Kgf/
mm ² . This is the average acting load on a conventional joint of this size, and the number of repetitions until failure in this case is N =
It becomes 2×10 ⁵ .

Next, a case will be described in which the present invention is incorporated into a tool joint having the same diameter as described above and the thickness of the tip end 2b of the box 2 is increased as shown in FIG.

In the figure, the dimensions of pin 1 and box 2 are d ₁ = 270mmφ, d ₂ = 280mmφ, d ₃ = 320mmφ, d ₄
=200mmφ, _d5 =360mmφ, and L=50mm. As a result, the cross-sectional area of C cross-section S = π (36-3.75) × 3.75≒
380.0, and the load on pin 1 for external force P = 1000 tons F _pio = 1000 x {258.4/(380.0 + 258.4)} =
The resulting stress is 404.8ton, σpin=
404.8/258.4=15.67Kgf/mm ² . Comparing this with Figure 4, the number of repetitions leading to breakage N = 8 x
10 ⁵ .

In addition, according to an experiment using a small-diameter screw-in joint, when using a conventional joint shape, P = 100 tons.
However, it became clear that by increasing the rigidity according to Example ¹ , σ could be reduced to 3.9 Kgf/mm ² .

Next, a specific example of the second aspect of the present invention will be described. In this example, a slit 6 is provided at the base of the pin 1 of the tool joint, as shown in FIG. In this case, the dimensions of the slit 6 are width D ₁ =
5 mm, and the length L ₁ = 50 mm. Other dimensions are the same as in FIG. 11. In this case, the rigidity ratio of pin 1 and box 2 is (cross section A) E・A/l+L ₁ = 2.1×258.4×10 ⁶ /2.5+
5.0 (C section) E・C/l=2.1×166.3×10 ⁶ /2.5, so it becomes 34.5:66.5.

Therefore, pin 1 for external force P = 1000 tons
Load F _pio = 1000×{34.5/(66.5+34.5)}=
341.6ton, generated stress σ _pio = 341.6×10 ³ /258.4=
It becomes 13.22Kgf/ ^mm2 . The number of repetitions leading to breakage in this case is N=9.2×10 ⁵ when compared with FIG. 4.

In addition, when the above two embodiments according to the present invention are combined, the stiffness ratio of the A section and the C section is 258.4/
7.5:380/2.5=34.45:152.

Therefore, the load on the pin for P=1000ton
F _pio = 1000 x {34.45/(34.45+152)} = 184.77ton,
Generated stress σ _pio = 184.77×10 ³ /258.4 = 7.15Kgf/mm ²
Therefore, the number of repetitions leading to destruction is N=2×10 ⁶ when compared with FIG. 4.

In this way, the slit 6 is effective in adjusting the rigidity ratio, and since it is parallel to the direction in which the external load is applied, no cracks will occur. Furthermore, as shown in FIG. 7, if a filler 7 made of adhesive rubber or the like is attached to the slit 6, intercalary corrosion can be prevented.

[Effects of the Invention] As described above, according to the present invention, by maintaining an appropriate rigidity ratio between the pin and the box, stress generated when subjected to repeated external force due to tension is reduced at least on the pin side. Since the stress amplitude is reduced, the stress amplitude due to repeated external forces is reliably kept within the allowable stress amplitude value on the pin side, which has the effect of improving the fatigue strength of the joint. This made it possible to increase the life before breakage by 4 to 4.6 times.
When this is applied to the actual ocean, the lifespan of conventional oil country tubular goods (OCTG) of 1 or 2 years will be increased to 4.8 to 5.5 years, and the effect of implementation will be extremely large.

[Brief explanation of the drawing]

Figures 1A and B are configuration diagrams of an embodiment of the first invention of the present invention, Figure 2 is a configuration diagram of an embodiment of the second invention of the invention, and Figures 3A and B are rigidity diagrams of the joint portion. Figure 4 is a diagram of the changes in added stress and stress amplitude depending on the ratio, Figure 4 is a diagram of repeated stress and the number of times it takes to break, Figure 5
The figure is an explanatory diagram of a specific example of the first aspect of the present invention in which the wall thickness of the box is increased, FIG. 6 is an explanatory diagram of a specific example of the second aspect of the present invention in which a slit is provided, and FIG. Explanatory diagram of another embodiment of the second invention, No. 8
Figure 9 is a structural diagram of a conventional joint, Figure 9 is a partial detail view of a conventional joint structure, Figure 10 is a characteristic diagram showing changes in stress applied to a conventional joint, and Figure 11 is an example of a conventional joint. FIG. 12 is a characteristic diagram of a conventional embodiment. 1: Pin, 1a: Recess, 2: Box, 3:
Pin deformation characteristic curve, 4: Box deformation characteristic curve, 5: Fatigue strength curve, 6: Slit.

Claims (1)

[Scope of Claims] 1. A threaded joint in which a barreled pin having a male thread with a relief portion at the base is screwed into a barreled box having a female thread until the end surfaces of the barreled parts abut each other, comprising: By adjusting the rigidity ratio between the pin and the box in the part by adjusting the cross-sectional area of the pin relief part, the cross-sectional area of the box covering the relief part, and the axial length of the barreled part of the box, A screw-in joint characterized by keeping the stress amplitude added due to tension within the allowable stress amplitude value. 2. In a screw-in joint in which a barreled pin having a male thread with a relief portion at the base is screwed into a barreled box having a female thread until the end surfaces of the barreled portions touch each other, the connection between the pin and the box in the barreled portion The rigidity ratio is determined by the axial length of the pin relief portion surrounded by the end of the box and the length of a slit formed by concave in the axial direction from the end surface of the barreled portion of the pin, continuing from the relief portion. A screw-in joint characterized in that the stress amplitude added due to tension is kept within an allowable stress amplitude value by adjusting the stress amplitude.