Technical field
[0001]The present disclosure relates to a threaded joint, and more particularly to a threaded joint for connecting oil well pipes to each other via the coupling of the tubular.
BACKGROUND
[0002]Conventionally, oil wells and natural gas wells and the like (hereinafter, collectively referred to as oil well), the steel pipe called oil well pipe is used. OCTG are installed in oil wells are connected to each other by a screw joint.
[0003]Format of the threaded joint is roughly classified into a integral type coupling type. The integral-type, oil well pipes to each other are directly connected. More specifically, the female screw portion provided on an inner periphery of an end portion of one oil well pipe, by male screw portion provided on an outer periphery of an end portion of another oil well pipe is screwed, OCTG each other are connected that. The coupling type, oil well pipes to each other are connected via a coupling. Specifically, each of the female screw portion provided on the inner periphery of both end portions of the coupling, by male screw portion provided on an outer periphery of an end portion of the oil well pipe is screwed, oil well pipes to each other are connected.
[0004]Generally, the end of the oil well pipe male thread portion is formed is referred to as pin from the containing elements to be inserted into the female screw portion. End of the oil well pipe or coupling female screw portion is formed is referred to as boxes because it contains elements for receiving the external thread portion.
[0005]In recent years, it is progressing deep wells of the oil well. In deep wells, usually OCTG thick with high pressure resistance is used. The threaded joint for connecting oil well pipes to each other the thick, not only high strength and sealing performance is required, constraints tight outer diameter in order to place the oil well pipe to multiplex is imposed.
[0006]To reduce the outer diameter of the joint portion may be thinner at least one of the pin and box. Pins and / or box as thinned screw joint, for example, may include those called slim type. Slim-type threaded joint has an outer diameter approximately the same as the outer diameter of the oil well pipe body. Joint efficiency slim type threaded joint is less than 1. Joint efficiency is the ratio of the tensile strength of the joint portion to the tensile strength of the oil well pipe body, the cross section (typically of the joint portion where the area to withstand tension in the engaged state load smallest, referred to as "dangerous section". It is defined by the area of ​​the cross-sectional area / OCTG body).
[0007]Maximum pressure value in the seal test of the threaded joint is determined by the outer diameter and wall thickness of the oil well pipe body. Therefore, when the slim type threaded joint, in the sealing test, so that the relatively high internal pressure is repeatedly load compared to other threaded joint. Thus, boxes and diameter variations, decreased contact force of the seal portion is formed by a contact between the pin seal surface and the box sealing surface, there is a possibility that leakage is induced.
[0008]In order to improve the sealing performance of the threaded joint, to devise the shape of the screw portion or the seal portion is generally used. For example, Patent Document 1 (JP-A-2-80886), among the complete thread portion, by eliminating the interference margin in the radial direction in some of the pin tip side, due to the mating of the threaded seal techniques to prevent the deterioration of the interference margin parts is described. Patent Document 2 (Japanese Patent No. 3726302), a seal forming unthreaded portion provided on the pin a two-stage tapered structure, the distal end side of the taper of the pin is made smaller than the other taper, seal It describes a technique for enhancing the performance.
[0009]Herein, the following prior art documents are incorporated by reference.
Patent Document 1: JP-A-2-80886 JP
Patent Document 2: Japanese Patent No. 3726302
SUMMARY OF THE DISCLOSURE
[0010]For small slim type threaded joint than the joint efficiency 1, by the internal pressure is repeatedly loaded in a sealed test or in use, the box is easily expanded deformation, leakage as a result may occur. Cyclic loading expanded deformation of the box by the internal pressure, it is difficult to suppress by devising the shape of the screw portion or the seal portion. Therefore, when the slim type threaded joint, even devised as in the conventional shape of the screw portion or the seal portion, it is difficult to ensure high sealing performance.
[0011]The present disclosure is used in the thick oil well pipes, in smaller screw joint than the joint efficiency is 1, and its object is to ensure high sealing performance.
[0012]A threaded joint according to the present disclosure is a threaded joint for connecting oil well pipes to each other. Threaded joint is provided with two pin portions and a coupling tubular. Each pin is provided at an end of the oil well pipe, it is formed continuously with the tube body having a wall thickness of more than 12 mm. Coupling includes two box portion, and a recessed portion. Box portion is provided at both ends of the coupling. Each box section is fastenable to the corresponding pin. Recess is located between two box portion. Each pin includes a nose portion, and Pinshoruda surface, and the male screw portion, and a pin seal surface. Nose constitutes the tip of the pin portion. The outer peripheral surface of the nose portion does not contact the box portion that is fastened to the pin portion. Pinshoruda surface is formed on the distal end surface of the nose portion. External thread portion, than the nose portion in the outer peripheral surface of the pin portion is formed in the tube body. Male screw portion is composed of a taper screw. Pin seal surface is formed between the nose portion and the male screw portion on the outer peripheral surface of the pin portion. Each box unit includes a female screw portion, and the box shoulder surface, the box sealing surface. Internal thread portion is formed corresponding to the external thread portion at the inner circumferential surface of the coupling. Internal thread portion is composed of a taper screw. Box shoulder surface is formed on the center side of the tube axis direction of the coupling than the female threaded portion. Box shoulder surface constitutes a shoulder portion in contact with the Pinshoruda surface in engagement state. Box sealing surface is formed between the female screw portion and the box shoulder surfaces in the inner circumferential surface of the coupling. Box sealing surface, constituting the sealing portion in contact with the pin seal surface in the engaged state. Recessed portion has a length of 10mm or more in the tube axis direction. Joint efficiency is less than 1. Joint efficiency, the area of ​​the cross section of the coupling at the end of the sealing portion of the external thread portion and the female threaded portion meshed with each other in the engagement state (the cross-section of the joint portion area to withstand tensile loads in the engaged state is the smallest) A1 , the area of ​​the cross section of the pipe body as A2, is calculated by the A1 / A2. The wall thickness of the tube body t PIPE , thickness and tube axis direction of the length of each t of the recess 1 and 2 × L 1 , each of the tube axis direction of the box length L 2 as the T defined by the following equation (1) , when the L defined by the following equation (2), T, L satisfies the following equation (3).
[0013]
[Number 1]

[0014]
　According to the present disclosure, is used in the thick oil well pipe joint efficiency in smaller screw joint than 1, it is possible to ensure the high sealing performance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015]
[1] Figure 1 is a longitudinal sectional view showing a schematic configuration of a threaded joint used in consideration of the configuration of the coupling.
FIG. 2 is a longitudinal sectional view of a model of a two-stage cylinder in which is approximated to the coupling of the threaded joint shown in Figure 1.
FIG. 3 is a graph showing the relationship between the parameter L of the length of the recessed portion of the parameter a and the coupling about seals interference amount.
[4] FIG. 4 is a graph showing the relationship between the parameter T relating to the thickness of the recessed portion of the parameter a and the coupling about seals interference amount.
FIG. 5 is a longitudinal sectional view showing a schematic configuration of a threaded joint according to the embodiment.
FIG. 6 is a longitudinal sectional view showing a schematic configuration of a threaded joint according to a modification of the embodiment.
DESCRIPTION OF THE INVENTION
[0016]
　As described above, in the case of the slim type threaded joint, even when devising the shape of the screw portion or the seal portion, it is difficult to suppress the diameter expansion deformation of the box due to internal pressure. If the coupling type of threaded joint, by increasing the rigidity of the coupling, it is conceivable to suppress the diameter expansion deformation of the box. That is, in order to suppress the diameter expansion deformation of the box, may be increased outer diameter and wall thickness of the coupling. However, in the slim-type threaded joint, stringent by the outside diameter of the constraints, there is a limit to the increase in the outer diameter of the coupling.
[0017]
　In general, the coupling portion located between the boxes (hereinafter, referred to as recessed portions), since the wall thickness as compared to portions box sealing surface is provided is large, it has a high rigidity. Therefore, when the loaded internal pressure is the screw joint, the recessed portion is considered to have a function of diameter not much expanded, the box sealing surface.
[0018]
　The present inventors have focused on the recessed portion of the coupling, thought to improve the sealing performance by devising the configuration of the recessed portion. The present inventors have without increasing the outside diameter of the coupling was examined the configuration of the recess in order to obtain high sealing performance.
[0019]
　Figure 1 is a longitudinal sectional view showing a schematic configuration of a coupling type of threaded joint. In the coupling of the threaded joint, a 1/2 of the tube axis direction of length between the box shoulder surface L 1 , a tube axial direction length of the box L 2 , from the box shoulder surface to contact the center of the box sealing surface a tube axis direction of length and Ls. The length of the tube axis direction of the recess portion is L 1 is a 2-fold, for convenience of explanation, L 1 is called the length of the recess.
[0020]
　To prepare a two-stage cylindrical model which approximates the coupling shown in Figure 1, for the model was examined necessary and sufficient rigidity of an elastic shell theory. Figure 2 is a longitudinal sectional view of a two-stage cylinder model used for the study. In a model of two-stage cylinder as shown in FIG. 2, the cylindrical portion S1 thickness is large corresponds to the recessed portion, the cylindrical portion S2 thickness is small corresponds to the box.
[0021]
　For uniform cylindrical in position negligible impact end, the deflection amount w of the cylindrical shell due to internal pressure P is the strength of materials is given by the following equation (4). Amount of deflection w is 1/2 of the diameter of the cylindrical shell due to internal pressure P. In the formula (4), E is Young's modulus, [nu is the Poisson's ratio, r is the central diameter of the cylindrical, t is the wall thickness of the cylinder.
[0022]
[Number 2]

[0023]
　Right side of the equation (4) is a particular solution in this problem. When the deflection amount in the direction axis away sincere cylinder is positive, basic equations of cylinder shell acting internal pressure P is expressed by the following equation (5).
[0024]
[Number 3]

[0025]
　Here, the amount of deflection of the cylindrical portion S1, S2 w respectively 1 , w 2 When, w 1 , w 2 is represented by each of the following formula (6) and (7). R in the formula (6) and (7) i is the inner diameter of the recess of the coupling, that is, the inner diameter of the cylindrical portion S1.
[0026]
[Formula 4]

[0027]
　Take x-axis in the tube axis direction of the coupling, when the tube axis direction center of the coupling to the origin of the x-axis, the right end in FIG. 2 of the cylindrical portion S1 (x = 0) in the deflection angle theta 1 and shear forces Q 1 There is zero. Therefore, the following equation holds (8) and (9).
[0028]
[Formula 5]

[0029]
　Furthermore, since the continuous cylindrical portion S1 and the cylindrical portion S2, x = L 1 deflection angle θ of the cylindrical portion S1 at 1 deflection angle θ and of the cylindrical portion S2 2 are equal. Therefore, the amount of deflection of the cylindrical portion S1, S2 w 1 , w 2 , the bending moment M 1 , M 2 , shear force Q 1 , Q 2 becomes equal to each other. That is, the following equation (10) to (13) hold.
[0030]
[Number 6]

[0031]
　Further, the left end in FIG. 2 of the cylindrical portion S2 (x = L 1 + L 2 ) Since a free end, the following equation holds (14) and (15).
[0032]
[Number 7]

[0033]
　By solving simultaneous equations above formula (8) to (15), the unknowns C 11 ~ C 14 , C 21 -C 24 are determined. Accordingly, from the formula (6) and (7), each of the deflection amount w of the cylindrical portion S1, S2 1 , w 2 is obtained. Amount of deflection in the sealing portion, in formula (7) x = L 1 + L S if put and can be expressed by the following equation (16). However, L s is the boundary between the cylindrical portion S1 and the cylindrical portion S2, the x-direction of the distance to a position corresponding to the contact center of the box sealing surface.
[0034]
[Number 8]

[0035]
　The present inventors have used a parameter T, L, a, defined by the following equation (17), the length of the wall thickness and tube axis direction of the recess has examined the effect on sealing performance.
[0036]
[Number 9]

[0037]
　T is the thickness t of the recessed portion 1 a wall thickness t of the pipe main body pipe is a parameter normalized by. L is the length L of the recess portion 1 a half of the coupling length L 1 + L 2 is a parameter normalized by. a is the seal interference amount of design [delta] at the time of fastening, substantially seal interference amount [delta]-2w when the coupling is enlarged 2 (L s is a parameter of) Deflection of w used in Equation (17) 2 is deflection amount w of the sealing portion of the formula (16) 2 and.
[0038]
　As constant T, the length L of the recess portion 1 shown in FIG. 3 the relation between L and a when changing the. A constant L, and the thickness t of the recessed portion 1 shown in FIG. 4 the relationship between T and a when changing the. 3 and the relationship shown in FIG. 4, respectively, can be expressed by the following equation (18) and (19). However, any function f (T) is free of L, g (L) is any function that does not contain T, m is a constant.
[0039]
[Formula 10]

[0040]
　From equation (18) and (19) can be derived the following equation (20).
[0041]
[Number 11]

[0042]
　As described above, a is a parameter normalized by the seal interference amount at the time of fastening a substantial seal interference amount at the enlarged diameter of the coupling. Thus, as the value of a is large, the contact surface pressure of the seal portion becomes large when the load of the internal pressure, the sealing performance is improved. Securing a sufficient rigidity of the coupling, in order to obtain a high sealing performance as a result, T · L 1 / m threshold a is th be larger than. The present inventors, as a result of extensive analysis and study, m = 15, a th reaches the condition that = 1.4, to complete the threaded joint according to the embodiment.
[0043]
　A threaded joint according to the embodiment is a threaded joint for connecting oil well pipes to each other. Threaded joint is provided with two pin portions and a coupling tubular. Each pin is provided at an end of the oil well pipe, it is formed continuously with the tube body having a wall thickness of more than 12 mm. Coupling includes two box portion, and a recessed portion. Box portion is provided at both ends of the coupling. Each box section is fastenable to the corresponding pin. Recess is located between two box portion. Each pin includes a nose portion, and Pinshoruda surface, and the male screw portion, and a pin seal surface. Nose constitutes the tip of the pin portion. The outer peripheral surface of the nose portion does not contact the box portion that is fastened to the pin portion. Pinshoruda surface is formed on the distal end surface of the nose portion. External thread portion, than the nose portion in the outer peripheral surface of the pin portion is formed in the tube body. Male screw portion is composed of a taper screw. Pin seal surface is formed between the nose portion and the male screw portion on the outer peripheral surface of the pin portion. Each box unit includes a female screw portion, and the box shoulder surface, the box sealing surface. Internal thread portion is formed corresponding to the external thread portion at the inner circumferential surface of the coupling. Internal thread portion is composed of a taper screw. Box shoulder surface is formed on the center side of the tube axis direction of the coupling than the female threaded portion. Box shoulder surface constitutes a shoulder portion in contact with the Pinshoruda surface in engagement state. Box sealing surface is formed between the female screw portion and the box shoulder surfaces in the inner circumferential surface of the coupling. Box sealing surface, constituting the sealing portion in contact with the pin seal surface in the engaged state. Recessed portion has a length of 10mm or more in the tube axis direction. Joint efficiency is less than 1. Joint efficiency, the area of the cross section of the coupling A1, the area of the cross section of the pipe body as A2 in the male screw portion and the end of the sealing portion side of the female threaded portion meshed with each other in the fastening state, is calculated by the A1 / A2 . The wall thickness of the tube body T Pipe, The wall thickness and tube axis direction of the length of each t of the recess 1 and 2 × L 1 , each of the tube axis direction of the box portion length L 2 as the T defined by the following equation (1) , when the L defined by the following equation (2), T, L satisfies the following equation (3).
[0044]
[Number 12]

[0045]
　In the threaded joint, the recess has a length of thick and axial direction of the tube so as to satisfy the equation (3). According to this configuration, it is possible to increase the rigidity of the coupling. Therefore, when the load of the internal pressure, becomes box portion is hardly expanded deformed, the contact force of the box sealing surface with respect to pin seal surface can be suppressed. To satisfy equation (3), there is no need to change the outer diameter of the coupling. Therefore, according to the screw joint, without increasing the outside diameter of the coupling, it is possible to ensure the high sealing performance.
[0046]
　It will be described below with reference to the accompanying drawings embodiments. Given the same reference and the corresponding configuration in the figure does not repeat the same description. For convenience of explanation, in each figure, or are simplified or schematized configuration, in some cases or are omitted a part of the configuration.
[0047]
　[Overall Configuration]
　FIG. 5 is a longitudinal sectional view showing a schematic configuration of a threaded joint 10 according to the embodiment. Although this will be described in detail later, the screw joint 10 is a so-called slim-type threaded joint. Threaded joint 10 connects the oil well pipe 1 to each other via a coupling second tubular. Each OCTG 1 is a steel tube of relatively thick. In each OCTG 1, the pipe body 12 has a wall thickness of more than 12 mm.
[0048]
　As shown in FIG. 5, the screw joint 10 is provided with two pin portions 11, and a coupling 2 of the tubular. Each pin portion 11 is provided at an end of the oil well pipe 1, it is continuously formed in the pipe main body 12. Coupling 2 includes two box portion 21, and a recessed portion 22. Box portion 21 is formed at both ends of the coupling 2. Each box section 21 is fastened to the corresponding pin portion 11. For convenience of explanation, in the tube axis direction of the threaded joint 10, the distal end side of the pin portion 11 inwardly, may be referred to the pipe main body 12 side and the outside.
[0049]
　Each pin portion 11 includes a nose portion 111, a male screw portion 112, a pin seal surface 113, and a Pinshoruda surface 114.
[0050]
　The nose portion 111 is disposed on the distal end side of the pin portion 11 than the male screw portion 112 and a pin seal surface 113. The nose portion 111 constitute the tip of the pin portion 11. In the fastening state between the pin portion 11 and the box portion 21, the outer peripheral surface of the nose portion 111 does not contact the box portion 21. That is, in the fastening state, the outer diameter of the nose portion 111 is smaller than the inner diameter of the portion facing the nose portion 111 of the box portion 21. For example, the outer peripheral surface of the nose portion 111 is constituted by a concave surface which is recessed on the inner peripheral side of the pin portion 11 than pin seal surface 113.
[0051]
　Male screw portion 112 is formed on the outer circumferential surface of the pin portion 11. Male screw portion 112, the pin portion 11 is disposed on the pipe body 12 than the nose portion 111. Male screw portion 112 is composed of a taper screw. In the outer peripheral surface of the pin portion 11, pin seal surface 113 is disposed between the nose portion 111 and the male screw portion 112.
[0052]
　Pin seal surface 113 is a generally tapered surface whose diameter decreases toward the male screw portion 112 side to the nose portion 111 side. Pin seal surface 113, for example, the peripheral surface of the rotated rotator and around the arc tube axis CL, comprising a combination of frustoconical peripheral surface centered on the tube axis CL 1 or 2 or more.
[0053]
　Pinshoruda surface 114 is an annular surface formed on the distal end surface of the nose portion 111. In the present embodiment, Pinshoruda surface 114 is inclined so as to be disposed on the distal end side of the pin portion 11 than the inner peripheral side outer circumferential side. In other words, in cross-sectional view of the pin portion 11 taken along a plane including the tube axis CL, Pinshoruda surface 114 has a shape outer peripheral side is inclined in the screwing direction of travel of the pin portion 11. However, the shape of Pinshoruda surface 114 is not limited thereto. Pinshoruda surface 114 may be a plane substantially perpendicular to the tube axis CL.
[0054]
　Each box section 21 is provided with a female screw portion 212, a box sealing surface 213, and a box shoulder surface 214.
[0055]
　Internal thread portion 212, corresponding to the male screw portion 112 of the pin portion 11, is formed on the inner peripheral surface of the box portion 21. Internal thread portion 212 is constituted by a screw that meshes with screw constituting the male screw portion 112. Internal thread portion 212 is composed of a tapered thread.
[0056]
　Box sealing surface 213, corresponding to the pin seal surface 113 is formed on the inner peripheral surface of the box portion 21. Box sealing surface 213 is in engagement state between the pin portion 11 and the box portion 21, contacts the pin seal surface 113.
[0057]
　Pin seal surface 113 and the box sealing surface 213 has an interference amount. That is, in the disengaged state, the diameter of the pin seal surface 113 is slightly greater than the diameter of the box sealing surface 213. Thus, pin seal surface 113 and the box sealing surface 213, with the threaded pin portion 11 relative to box portion 21 in contact with each other, and because of the state interference fit in close contact with the fitting in the engagement state. Thus, pin seal surface 113 and the box sealing surface 213, the metal - constituting the sealing portion by the metal contact.
[0058]
　Box shoulder surface 214, corresponding to Pinshoruda surface 114 is formed in a tube axis direction of the inner end of the box portion 21. Box shoulder surface 214 contacts the Pinshoruda surface 114 in engaged state.
[0059]
　Pinshoruda surfaces 114 and box shoulder surface 214, the threaded pin portion 11 relative to box portion 21 is pressed in contact with each other. Pinshoruda surfaces 114 and box shoulder surface 214 constitutes the shoulder portion by pressure contact with each other.
[0060]
　The area of ​​the contact area, i.e. the shoulder portions of the box shoulder surface 214 of the Pinshoruda surface 114 is preferably at least 30% of the area of ​​the cross section of the tube body 12. And the transverse plane refers to cut cross-section in a plane perpendicular to the tube axis CL. If the area of ​​the shoulder portion is 30% or more of the cross-sectional area of ​​the pipe main body 12, when an excessive compressive load is applied to the threaded joint 10, the plastic deformation of Pinshoruda surface 114 and pin seal surface 113 connecting to this is suppressed . Therefore, the contact state of the seal portion can be stabilized, it is possible to suppress the reduction of the contact surface pressure of the seal portion.
[0061]
　In particular but not limited to, the area of ​​the shoulder, from the viewpoint of suppressing a reduction in the minimum cross-sectional area of ​​the coupling 2, is preferably not more than 60% of the cross-sectional area of ​​the tube body 12.
[0062]
　As described above, the screw joint 10 is a so-called slim-type threaded joint. Therefore, a threaded joint 10 has a joint efficiency of less than 1. Joint efficiency, the area of ​​the cross section of the coupling 2 to the area to withstand a tensile load in the engaged state becomes smallest (dangerous section) A1, the area of ​​the cross section of the pipe body as A2, be calculated by A1 / A2 it can.
[0063]
　Area A1 of dangerous section of the coupling 2, out of the meshing end E1, E2 of the male thread portion 112 and the female screw portion 212 in the engagement state, the area of ​​the cross section of the coupling 2 in the engagement end E1 of the sealing portion.
[0064]
　Between the two box portion 21 of the coupling 2, the recessed portion 22 is formed. The inner diameter of the recess 22 is substantially constant. The outer diameter of the coupling 2 is also substantially constant. Therefore, the recessed portion 22 has a thickness substantially uniform wall. The thickness of the recessed portion 22 is greater than the thickness of the box portion 21. Therefore, the recessed portion 22 has a higher rigidity as compared to the box portion 21.
[0065]
　The inner diameter of the recessed portion 22 is preferably smaller than the inner diameter of the tube body 12. In this case, the thickness of the recessed portion 22 is increased, thereby improving its rigidity. However, the inner peripheral surface of the threaded joint 10 of the fastening state, the step between the pin portion 11 and the recess 22 occurs, the threaded joint 10 internal fluid turbulence generated when passing through. This not only lowers productivity in oil well, a threaded joint 10 may be damaged. Therefore, the inner diameter of the recess 22 is preferably substantially the tip of the inner diameter of the pin portion 11 to the same.
[0066]
　The tip inner diameter of the pin portion 11 to match the inner diameter of the recess 22, for example by drawing or the like, the tip inner diameter of the pin portion 11 can be made smaller than the inner diameter of the tube body 12. In this case, the tip-side portion than the male thread portion 112 of the pin portion 11 (the lip portion) is thickened. Therefore, the rigidity of the lip portion of the pin portion 11 is increased. Therefore, the contact force of the seal portion is increased, the sealing performance of the threaded joint 10 can be further improved.
[0067]
　The length of the recess portion 22 is 10mm or more. In the coupling 2, the length of the tube axis direction from the inner end of one of the box shoulder surface 214 to the inner end of the other box shoulder surface 214, that the length of the recessed portion 22. If the recess portion 22 has a length of at least 10 mm, deformation of the shoulder portion when the one of the box portion 21 is fastened to the pin portion 11, affects the engagement of the other box portion 21 and pin portion 11 it is possible to prevent the failure signed occur Te. Therefore, it is possible to suppress the reduction of the sealing performance of the threaded joint 10.
[0068]
　Recess 22 increases the rigidity of the coupling 2 longer, thereby improving sealing performance of the threaded joint 10. However, when the recessed portion 22 becomes long to some extent, the effect of improving the sealing performance is saturated. Therefore, the length of the recess 22, for example, can be defined as 100mm at maximum.
[0069]
　Thickness and length of the recessed portion 22, the coupling 2 is set so as to obtain the necessary rigidity. Specifically, the parameter T relating to thickness and length of the recess 22, the L defined by the following equation (1) and (2), sets T, the L so as to satisfy the following equation (3) .
[0070]
[Formula 13]

[0071]
　Although the thickness of the recessed portion 22 is substantially constant, t in equation (1) 1 is the thickness in the tube axis direction center of the recessed portion 22. That, t 1 is the coupling 2, a thickness of the coupling 2 as measured at the intermediate position of the two boxes shoulder surface 214. T Pipe is a wall thickness of the tube body 12.
[0072]
　L in formula (2) 1 , in the tube axis direction is a length from the inner end of the box shoulder surface 214 to the center of the coupling 2. That is, the length in the tube axis direction of the recess 22, 2 × L 1 can be expressed by. 2 × L 1 is equal to the shortest distance between the box shoulder surface 214 that faces.
[0073]
　L in formula (2) 2 is the length of the tube axis direction of the box portion 21. L 2 from one end of the coupling 2, the distance in the tube axis direction to the inner end of the box shoulder surface 214 closer to the one end.
[0074]
　When calculating T, the L by the equation (1) and (2), the unit of each variable is arbitrary, t 1 and t PIPE , L 1 and L 2 , it is necessary to unify the units, respectively.
[0075]
　[Effect]
　In the present embodiment, the recessed portion 22 of the coupling 2, the length of the wall thickness and tube axis direction is adjusted so as to satisfy the equation (3) above. There is no need to adjust the outer diameter of the coupling 2 to satisfy the equation (3). In this way, without increasing the outside diameter of the coupling 2, it can improve the rigidity of the coupling 2. Therefore, also restrictions outer diameter a tight slim type threaded joint, it is possible to suppress the enlarged deformation of the box portion 21 at the time of loading of the internal pressure, to maintain the contact force of the seal portion it can. Therefore, it is possible to secure a high sealing performance.
[0076]
　Generally, when the thickness of the pipe body is more than 15 mm, increases the load applied to the joint portion, the deformation of the box is more likely to occur. In contrast, a threaded joint 10 according to this embodiment, since thereby improving the rigidity of the coupling 2 by adjusting the thickness and length of the recessed portion 22, it is possible to suppress the deformation of the box portion 21 . Therefore, a threaded joint 10, the wall thickness of the pipe body is considered to be very effective in the case of more than 15 mm.
[0077]
　Slim-type threaded joint, typically, it has a joint efficiency of less than 1, has a relatively low rigidity. If joint efficiency is 0.8 or less, the rigidity of the joint portion becomes insufficient, especially, reduction in sealing performance is likely to occur. On the other hand, a threaded joint 10 according to this embodiment is to improve the rigidity of the coupling 2 by adjusting the thickness and length of the recessed portion 22. Therefore, a threaded joint 10 is considered to be very effective when joint efficiency is 0.8 or less.
[0078]
　[Modification]
　Although embodiments have been described, the present disclosure is not intended to be limited to the above embodiments but can be variously modified without departing from the spirit. For example, in the threaded joint 10 according to the above embodiment, the seal portion is provided in one place, the sealing portion to two or more portions may be provided.
[0079]
　In the above embodiment, the inner diameter of the tip of the inner diameter and the recessed portion 22 of the coupling 2 of the pin portion 11 is smaller than the inner diameter of the tube body 12. However, as the threaded joint 10a shown in FIG. 6, the inner diameter of the recess 22a of the tip of the inner diameter and the coupling 2a of the pin portion 11a may be of internal diameter than the tube body 12. Again, in the engaged state, the distal end of the inner diameter of the pin portion 11a is preferably an inner diameter that is substantially equal to the recessed portion 22a.
Example
[0080]
　To confirm the effects of a threaded joint according to the present disclosure were carried out numerical analysis by entity Seal Test and Elastic-plastic finite element method.
[0081]
　[Entities seal Test
　specimens of the coupling type of threaded joint two prepared having the basic configuration shown in FIG. 5 were carried entities seal test. The common conditions for both specimens are shown below.
- Using Target: 8-5 / 8 "57.4 # steel (outer diameter 219.1Mm, wall thickness 16.3 mm)
, steel: carbon steel Q125 of API standard (nominal yield stress 125ksi (= 862MPa)) · Screw: commonly for all screws, the thread pitch 5.08 mm, flank angle -3 ° of the load surface, insert flank angle 10 °, the insertion surface gap 0.15mm
[0082]
　The sealing test, while cutting the surface finish of the pin portion, a manganese phosphate treatment of the surface finish of the box portion, was applied on the entire surface API dope joint part. Seal test, ISO13679: 2011 was carried out according to the maximum tensile load applied to the joint portion was 75% of the tensile yield load of the pipe main body.
[0083]
　As shown in Table 1, two specimens, T · L determined by the above equation (3) 1/15 different from each other.
[0084]
[Table 1]

[0085]
　As shown in Table 1, specimen of Comparative Example 1, leakage occurs in the seal test. The specimen of Comparative Example 1, T · L 1/15 less than 1.4, because they did not satisfy equation (3) described above, the rigidity of the recessed portion of the coupling is considered to have insufficient.
[0086]
　In contrast, specimens of Example 1, the leakage is not generated in the sealing test, it passed the seal test. Specimens of Example 1, T · L 1/15 is larger than 1.4, since it satisfies the formula (3) described above, the rigidity of the recessed portion of the coupling is considered sufficiently high.
[0087]
　(Numerical Analysis by elastic-plastic finite element method)
　for a plurality of specimens in Table 2, were carried out numerical analysis by elastic-plastic finite element method. After analysis of the tightening of the screw with respect to each specimen, a load simulating the ISO13679 CAL4 Series A test to evaluate the sealing performance against an inner external pressure. Sealing performance was evaluated by the minimum value of the average contact surface pressure of the seal portion in a load history (minimum sealing average contact surface pressure). The higher the value of the minimum sealing average contact surface pressure, which means that sealing performance of the seal portion is good. The elasto-plastic finite element analysis, the elasto-plastic body of isotropic hardening the material, the elastic modulus 210 GPa, a yield strength at 0.2% yield strength was used a model of so that 125ksi (= 862MPa).
[0088]
[Table 2]

[0089]
　A plurality of specimen shown in Table 2, although both are coupling type of threaded joint having the basic configuration shown in FIG. 5, the dimensions of the coupling are different each. The elasto-plastic finite element analysis, was verified by changing the outer diameter of the length and the coupling of the recess. Use subject steel screws conditions are common to all specimens, the same as the entity sealing test described above.
[0090]
　Specimen of Example 2-5, the same size of the specimen and the coupling of Example 1 that have passed the real seal test. Since the minimum sealing average contact surface pressure of the specimen of this Example 2-5 is not less than 500MPa, for each specimen, the minimum sealing average contact surface pressure of not less than 500MPa sealing performance excellent, sealed of less than 500MPa It was evaluated with the performance FuKaoru.
[0091]
　The specimen of Comparative Example 2-1 and 2-2, the minimum sealing average contact surface pressure becomes less than 500 MPa, the sealing performance was FuKaoru. The specimen of Comparative Example 2-1 and 2-2, both T · L 1/15 is less than 1.4, the rigidity of the recessed portion of the coupling can be considered to have been insufficient.
[0092]
　On the other hand, in the specimens of Examples 2-1 to 2-7, both the minimum sealing average contact surface pressure becomes higher 500 Pa, sealing performance was good. The specimen of Examples ~ 2-7 2-1, T · L 1/15 greater than 1.4 is considered that it was possible to sufficiently secure the rigidity of the recessed portion of the coupling.
[0093]
　The above entities seal test and elasto-plastic finite element analysis, T · L 1/15 if> 1.4, it was possible to confirm that good sealing performance is obtained.

The scope of the claims
[Requested item 1]A threaded joint for connecting oil well pipes to each other,
and each is provided at an end of the oil well pipe, the two pin portions formed continuously with the tube body having a wall thickness of at least 12 mm,
at both ends provided part, a pin portion and fastening two boxes units each corresponding, and coupling tubular and a recess located between said two box part
comprises a,
each of said pin portion may
constitute the distal end portion of the pin portion, a nose portion outer peripheral surface does not contact the box portion which is fastened to the pin portion,
and Pinshoruda surface formed on the front end surface of the nose portion,
the outer periphery of the pin portion than the nose section in a plane formed in the tube body, and a male screw portion formed in tapered thread,
a pin seal surface formed between the outer circumferential surface of the pin portion and the nose portion and said externally threaded portion,
wherein the
said box Each part contains
the formed corresponding to the external thread portion at the inner circumferential surface of the coupling, the female thread portion formed in tapered thread,
formed on the center side of the tube axis direction of the coupling than the female threaded portion is a box shoulder surface forming a shoulder portion in contact with the Pinshoruda surface in engagement state,
is formed between the female thread portion in the inner peripheral surface of the coupling and the box shoulder surface, it said in engaged pin seal and a box seal surface forming a seal in contact with the surface,
Wherein the
said recess has a length of 10mm or more in the tube axis direction,
the area of the cross-section of the coupling in the seal portion side end of the external thread portion and the female threaded portion meshed with each other in the engaged state the A1, the area of the cross section of the pipe body as A2, less than 1 joint efficiency calculated by the A1 / A2,
the wall thickness of t of the tube body pIPE , thickness and tube axis direction of the recessed portion length, respectively t 1 and 2 × L 1 , the length of each tube axis direction of the box portion L 2 as to define T by the following equation (1), L the following equation (2) in when you defined, T, L satisfies the following equation (3), a threaded joint.
[Number 1]