OA19994A - Threaded joint for oil well pipe. - Google Patents

Abstract

High sealing performance is provided in a threaded connection used for pipes for oil wells with large wall thickness and with a connection efficiency smaller than 1. A threaded connection (10) connects pipes for oil wells (l). The threaded connection (10) includes two pins (11) and a tubular coupling. Each of the pins (11) is provided on an end of the pipe for oil wells (l) and is in continuity with the pipe body (12), which has a wall thickness not smaller than 12 mm. The coupling (2) includes two boxes (21) and a recess portion (22). The box (21) is provided on both ends of the coupling (2). The recess portion (22) has a length not smaller than 10 mm. The threaded connection (10) has a connection efficiency smaller than 1. When the wall thickness of the pipe body (12) is expressed as tpipe, the wall thickness and length of the recess portion (22) are expressed as t1 and 2xL1, respectively, and the length of each of the boxes (21) is expressed as L2, and if the following expressions (l) and (2) define T and L, then, T and L satisfy the following expression (3):

Description

The présent disclosure relates to a threaded connection, and more particularly, to a threaded connection for connecting pipes for an oil well by means of a tubular coupling.

BACKGROUND ART

In oil wells or natural-gas wells (hereinafter collectively referred to as oil wells or the like), for example, steel pipes called pipes for oil wells are used. The pipes for oil wells are connected with each other by means of threaded connections and are placed in oil wells.

Such threaded connections are generally categorized as integraLtype connections and coupling-type connections. An integraLtype connection directly connects the pipes for oil wells. More specifically, a female thread is provided on the inner periphery of an end of the pipe for oil wells and a male thread is provided on the outer periphery of an end of the pipe for oil wells, and the male thread of one pipe for oil wells is screwed into the female thread of another pipe for oil wells such that the pipes for oil wells are connected. A coupling-type connection connects the pipes for oil wells using a coupling. More specifically, female threads are provided on the inner périphéries of both ends of a coupling and a male thread is provided on the outer periphery of an end of the pipe for oil wells, and the male thread is screwed into the associated female thread of the coupling such that the pipes for oil wells are connected.

The end portion of the pipe for oil wells on which a male thread is provided includes an element to be inserted into a female thread, and thus is usually referred to as pin. The end portion of the pipe for oil wells or coupling on which a female thread is provided includes an element for receiving a male thread, and thus is referred to as box.

In recent years, oil wells hâve become deeper and deeper. Usually, the pipes for oil wells with large wall thickness and thus having high pressure résistance are used in deep wells. A threaded connection for connecting the pipes for oil wells with large wall thicknesses are not only required to hâve high strength and high sealing performance, but are also subject to tight restrictions on outer-diameter dimensions to allow multiple the pipe for oil wells to be placed inside each other.

To reduce the outer-diameter dimensions of a connection, the wall thickness of at least one of the pin and box may be reduced. Threaded connections with a reduced wall thickness or reduced wall thicknesses of the pin and/or box include, for example, slim-type connections. A slim-type threaded connection has an outer diameter substantially equal to the outer diameter of the body of the pipe for oil wells. A slim-type threaded connection has a connection efficiency smaller than 1. The connection efficiency of a connection is the ratio of the tensile strength of the connection to the tensile strength of the body of the pipe for oil wells, and is defined as the area of a transverse section of the portion of the connection that has the smallest area of résistance to a tensile load when the connection has been made up (usually referred to as critical section) divided by the area of a transverse section of the body of the pipe for oil wells.

The maximum value of internai pressure during a seal test for a threaded connection dépends on the outer diameter and wall thickness of the body of the pipe for oil wells. Accordingly, a relatively high internai pressure is repeatedly applied to a slim-type threaded connection during a seal test compared with other threaded connections. As such, the box may be deformed to increase its diameter such that the contact force ofthe seal formed by the pin and box sealing surfaces contacting each other decreases, potentially leading to a leak.

The sealing performance of a threaded connection is typically improved by devising various shapes for the thread or seal. For example, Patent Document 1 (JP H02(1990)-80886 A) describes a technique to remove the interférence margin as measured in a radial direction of a portion of the pin with perfect threads that is closest to the tip of the pin, thereby preventing the decrease in the interférence margin in the seal caused by the mating of the threads. Patent Document 2 (Japanese Patent No. 3726302) describes a technique to increase sealing performance by constructing a non-threaded portion of the pin that forms a seal in such a way that the portion has a two-section taper structure, and making the tapering of the portion adjacent to the tip of the pin smaller than the tapering of the other portion.

The following prior art documents are incorporated herein by reference.

Patent Document 1: JP H02(1990)-80886 A

Patent Document 2- Japanese Patent No. 3726302

SUMMARY OF THE DISCLOSURE

When a slim-type threaded connection with a connection efficiency smaller than 1 undergoes a seal test or is being used, an internai pressure is repeatedly applied to the connection such that the box can easily be deformed to increase its diameter, potentially leading to a leak. The deformation of the box with increased diameter caused by repeated application of the internai pressure is difficult to reduce by devising new shapes for the thread or seal. Thus, in the case of a slim-type threaded connection, it is difficult to provide high sealing performance by devising varions shapes for the thread or seal, as is conventionally done.

An object of the présent disclosure is to provide high sealing performance in a threaded connection used for pipes for oil wells with large wall thickness and with a connection efficiency smaller than 1.

A threaded connection according to the présent disclosure is a threaded connection for connecting pipes for an oil well. The threaded connection includes two pins and a tubular coupling. Each of the pins is provided on an end of a corresponding one of the pins and is in continuity with a body of the corresponding pipe having a wall thickness not smaller than 12 mm. The coupling includes two boxes and a recess portion. The boxes are provided on both ends of the coupling. Each of the boxes are capable of being made up on the associated pin. The recess portion is located between the two boxes. Each of the pins includes a nose, a pin shoulder surface, a male thread, and a pin sealing surface. The nose forms a tip portion of the pin. An outer peripheral surface of the nose is not in contact with the box made up on the pin. The pin shoulder surface is formed at a tip surface of the nose. The male thread is provided on an outer peripheral surface of the pin and located doser to the body of the corresponding pipe than the nose. The male thread is a tapered thread. The pin sealing surface is provided on the outer peripheral surface of the pin and located between the nose and the male thread. Each of the boxes includes a female thread, a box shoulder surface, and a box sealing surface. The female thread is provided on an inner peripheral surface of the coupling to correspond to the male thread. The female thread is a tapered thread. The box shoulder surface is located doser to a middle of the coupling as determined along a pipe-axis direction than the female thread. The box shoulder surface is in contact with the pin shoulder surface when the connection has been made up to form a shoulder. The box sealing surface is provided on the inner peripheral surface of the coupling and is located between the female thread and the box shoulder surface. The box sealing surface is in contact with the pin sealing surface when the connection has been made up to form a seal. The recess portion has a length not smaller than 10 mm as measured in the pipe-axis direction. The connection has a connection efficiency smaller than 1. The connection efficiency is calculated by A1/A2, where Al is the area of a transverse section of the coupling as measured at the ends, adjacent to the seal, of the male thread and the female thread engaging each other when the connection has been made up (transverse section of the portion of the connection that has the smallest area of résistance to a tensile load when the connection has been made up), and A2 is the area of a transverse section of the body.

When the wall thickness of the body is expressed as t_pi_pe, the wall thickness of the recess portion and the length of the recess portion as measured in the pipe-axis direction are expressed as ti and 2xZy, respectively, and the length of each of the boxes as measured in the pipe-axis direction is expressed as L2, and when the following expression (1) defines Tand the following expression (2) defines L, T and L satisfy the following expression (3).

r = -L (D pipe ^l = (2)

A ⁺ ^2

T L¹'¹⁵ > 1.4 (3)

The présent disclosure provides high sealing performance in a threaded connection used for pipes for oil wells with large wall thickness and with a connection efficiency smaller than 1.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1] FIG. 1 is a schematic longitudinal cross-sectional view of a threaded connection used for considering how to construct a coupling.

[FIG. 2] FIG. 2 is a longitudinal cross-sectional view of a model of a two-portion cylinder that approximates the coupling of the threaded connection shown in FIG. 1.

[FIG. 3] FIG. 3 is a graph showing a relationship between a parameter a relating to the amount of seal interférence and a parameter L relating to the length of the recess portion of the coupling.

[FIG. 4] FIG. 4 is a graph showing a relationship between the parameter a relating to the amount of seal interférence and a parameter T relating to the wall thickness ofthe recess portion of the coupling.

[FIG. 5] FIG. 5 is a schematic cross-sectional view of a threaded connection according to an embodiment.

[FIG. 6] FIG. 6 is a schematic longitudinal cross-sectional view of a threaded connection according to a variation of the embodiment.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

As discussed above, in a slim-type threaded connection, it is difficult to prevent the deformation of the box with increased diameter caused by the internai pressure even when one devises a new shape for the thread or seal. In coupling-type threaded connection, the stiffness of the coupling may be increased to prevent the box from being deformed with increased diameter. That is, the outer diameter and wall thickness of the coupling may be increased to prevent the box from being deformed with increased diameter. However, since a slim-type threaded connection has tight restrictions on the outer-diameter dimensions, there is a limit to the increase in the outer diameter of the coupling.

Typically, the portion of the coupling located between the boxes (hereinafter referred to as recess portion) has a larger wall thickness than the portions with the box seals, and thus has a higher stiffness. As such, the recess portion does not increase its diameter significantly when an internai pressure is applied to the threaded connection, and is thus thought to hâve the function to reduce the diameter of the box sealing surface.

The présent inventors focused on the recess portion of the coupling and attempted to improve sealing performance by devising a new construction of the recess portion. The présent inventors considered how to construct the recess portion to provide high sealing performance without increasing the outer diameter of the coupling.

FIG. 1 is a schematic longitudinal cross-sectional view of a coupling-type threaded connection. In the coupling of this threaded connection, 1/2 of the distance of the box shoulder surfaces as measured in the pipe-axis direction is denoted by Li, the length of the box as measured in the pipe-axis direction by L2, and the distance between the box-shoulder surface and the middle of the contact of the box sealing surface as measured in the pipe-axis direction by Ls. Although the length of the recess portion as measured in the pipe-axis direction is twice Li, Li will be referred to as the length of the recess portion, for ease of explanation.

A model of a two-portion cylinder that approximates the coupling shown in FIG. 1 was created and the necessary and sufficient stiffness for this model was investigated using elastic shell theory. FIG. 2 is a longitudinal cross-sectional view of the model of a two-portion cylinder used for this investigation. In the model of a two-portion cylinder shown in FIG. 2, the cylindrical portion SI with a larger wall thickness corresponds to the recess portion, while the cylindrical portion S2 with a smaller wall thickness corresponds to the box.

In the case of a uniform cylinder, the amount of deflection wof the cylindrical shell caused by an internai pressure P, as measured at a position where the effects of an end portion are negligible, is given by expression (4), provided below, based on material mechanics. The amount of deflection w is 1/2 of the amount of increase in the diameter of the cylindrical shell caused by the internai pressure P. In expression (4), Æ/is the Young's modulus, vis the Poisson's ratio, ris the médian diameter of the cylinder, and iis the wall thickness of the cylinder.

Pr² vP ^ = -- + -(1 + v) (4) ht t l·¹

The right side of expression (4) is the particular solution to this problem. Supposing that the amount of deflection in a direction away from the axis of the cylinder is positive, the fundamental équation of the cylindrical shell on which the internai pressure P acts is given by the following expression, (5):

+ e^^x(Cx cos βχ + C2 sin βχ) + e“^^x(C3 cos βχ + C₄ sin βχ)

If the amounts of deflection in the cylinders Si and S2 are denoted by wi and W2, wi and ws are given by the following expressions, (6) and (7). λ in the expressions (6) and (7) is the inner diameter of the recess portion of the coupling, that is, the inner diameter of the cylindrical portion Si.

[0026]

(6) + e^'^x{C_n cos^x+ C₁₂ sinβ_λχ)+β cosβ\Χ + C₁₄ sinβ_χχ) _P(r_i+t_x+t₂/2Ÿ

Wn — -------------7¾

(7) + (C₂₁ cos β₂χ + C₂₂ sin β₂χ) + e (C₂₃ cos β₂χ + C₂₄ sin β₂χ)

If the pipe-axis direction of the coupling représente the χ-axis and the middle of the coupling as determined along the pipe-axis direction représente the origin of the χ-axis, then, the angle of deflection θι and the ehearing force Qi at the right end (x=0) of the cylindrical portion Si in FIG. 2 ie zéro. Thue, the following expressione, (8) and (9), are true^ ^,.0)=-01^- =0 (8)

OX x=0

21G=_O) = - A -t =0 (9)

OX x—0

Since the cylindrical portione SI and S2 are in continuity, the angle of deflection θι of the cylindrical portion SI and the angle of deflection Θ2 of the cylindrical portion S2 are equal when x^Li. Thus, the amounts of deflection wi and W2 in the cylindrical portions Si and S2 are equal, the bending moments Mi and M2 are equal, and the shearing forces Qi and Q2 are equal. That is, the following expressions (10) to (13), are true.

Ν-Ο =( ^=2,) = ^=2,)01) ^1(+=2,)=^2(+.2,)(12)

61(+.2,)=62(+=2,)(13)

Further, since the left end of the cylindrical portion S2 in FIG. 2 (x-Li+Ls) is a free end, the following expressions, (14) and (15), are true.

3+(+=2,+2,)=°d

82(+=2,+2,)=°(15)

By solving expressions (8) to (15) simultaneously, the unknown quantifies Cn to Ci4 and C21 to C24 are determined. Thus, the amounts of deflection wi and W2 of the cylindrical portions SI and S2, respectively, are determined from expressions (6) and (7). If x=Lu+L_s is substituted into expression (7), the amount of deflection of the seal may be given by the following expression, (16). L_s is the distance between the border between the cylindrical portions SI and S2 and the position corresponding to the middle of the contact of the box sealing surface as measured in the χ-direction.

-^+6+^/2)² ,v₂P ^W2 ------~U + V₂ ) 4^2 4 + ^(02^08^(^+4) + 022 sin4(4 +4)) + X-^x (C₂₃ cos 4 (4 + 4 ) + O₂₄ sin 4 (4 + ^LsS) (16)

The présent inventors used parameters T, L and a, defined by the following expression, (17), to détermine the effects of the wall thickness of the recess portion and the length thereof as measured in the pipe-axis direction on the sealing performance.

= -^-, ₍₁₇₎ t pipe A ⁺ ^2

T is a parameter obtained by normalizing the wall thickness ti of the recess portion with the wall thickness t_pi_pe of the body of the pipe. L is a parameter obtained by normalizing the length Li of the recess portion with the length of a half of the coupling, L1+L2. a is a parameter obtained by normalizing the real amount of sealing interférence found when the coupling has increased its diameter, with the design amount of sealing interférence during make-up, 6. The amount of deflection w^used in expression (17) is the amount of deflection W2 of the seal given by expression (16).

FIG. 3 shows a relationship between Land a where Lis constant and the length Li of the recess portion varies. FIG. 4 shows a relationship between Land a where Lis constant and the wall thickness ti of the recess portion varies. The relationships shown in FIGS. 3 and 4 may be given by the following expressions, (18) and (19), respectively. f(T)ïs an arbitrary function that does not contain L, g(L) is an arbitrary function that does not contain T, and m is a constant.

a(T,L) π f(T)· L'¹·(18) a(T,L)xT-g(L)(19)

[0040] The following expression, (20), may be derived from expressions (18) and (19): [0041] a(T,L)xT-L^l/m(20)

As discussed above, a is a parameter obtained by normalizing the real amount of seal interférence found when the diameter of the coupling has increased with the amount of seal interférence during make-up. As such, the greater the value of a, the higher the contact surface pressure at the seal during application of the internai pressure becomes, improving sealing performance. Sufficient stiffness of the coupling is provided and, as a resuit, high sealing performance is provided if T-U^/m is larger than a threshold ath. The présent inventors did extended analysis and investigation and reached the conditions m=15 and ath=1A, and made the threaded connection according to the embodiment

The threaded connection according to the embodiment is a threaded connection for connecting pipes for an oil wells. The threaded connection includes two pins and a tubular coupling. Each of the pins is provided on an end of a corresponding one of the pipes and is in continuity with a body of the corresponding one of the pipes having a wall thickness not smaller than 12 mm. The coupling includes two boxes and a recess portion. The boxes are provided on both ends of the coupling. Each of the boxes are capable of being made up on the associated pin. The recess portion is located between the two boxes. Each of the pins includes a nose, a pin shoulder surface, a male thread, and a pin sealing surface. The nose forms a tip portion of the pin. An outer peripheral surface of the nose is not in contact with the box made up on the pin as made up. The pin shoulder surface is formed at a tip surface of the nose. The male thread is provided on an outer peripheral surface of the pin and located doser to the body of the corresponding pipe than the nose. The male thread is a tapered thread. The pin sealing surface is provided on the outer peripheral surface ofthe pin and located between the nose and the male thread. Each of the boxes includes a female thread, a box shoulder surface, and a box sealing surface. The female thread is provided on an inner peripheral surface of the coupling to correspond to the male thread. The female thread is a tapered thread. The box shoulder surface is located doser to a middle ofthe coupling as determined along a pipe-axis direction than the female thread. The box shoulder surface is in contact with the pin shoulder surface when the connection has been made up to form a shoulder. The box sealing surface is provided on the inner peripheral surface of the coupling and is located between the female thread and the box shoulder surface. The box sealing surface is in contact with the pin sealing surface when the connection has been made up to form a seal. The recess portion has a length not smaller than 10 mm as measured in the pipe-axis direction. The connection has a connection efficiency smaller than 1. The connection efficiency is calculated by A1/A2, where Al is the area of a transverse section of the coupling as measured at the ends, adjacent to the seal, of the male thread and the female thread engaging each other when the connection has been made up, and A2 is the area of a transverse section of the body. When the wall thickness of the body is expressed as t_pi_pe, the wall thickness of the recess portion and the length of the recess portion as measured in the pipe-axis direction are expressed as ti and Zx-Li, respectively, and the length of each of the boxes as measured in the pipe-axis direction is expressed as Lz, and when the following expression (1) de fines T and the following expression (2) defines L, T and A satisfy the following expression (3).

γ = ^!_ (i) t pipe ^{L =} -rm~ <²»

A + ^2

7·Λ^1/15>1.4 (3)

[0045] In the threaded connection disclosed above, the recess portion has a wall thickness and a length as measured in the pipe-axis direction that satisfy expression (3). This construction will increase the stiffness of the coupling. This will prevent the box from being deformed with increased diameter during application of the internai pressure, thereby preventing the contact force of the box sealing surface on the pin sealing surface from decreasing. The outer diameter of the coupling need not be changed to satisfy expression (3). Thus, the above-disclosed threaded connection will provide high sealing performance without increasing the outer diameter of the coupling.

The embodiment will now be described with reference to the drawings. The same and corresponding éléments in the drawings are labeled with the same characters, and the same description will not be repeated. For ease of explanation, the drawings may show éléments in a simplified or schematic manner, or may not show some éléments.

[Overall Construction]

FIG. 5 is a schematic longitudinal cross-sectional view of a threaded connection 10 according to the embodiment. As discussed further below in detail, the threaded connection 10 is a so-called slim-type threaded connection. The threaded connection 10 connecte pipes for oil wells 1 by means of a tubular coupling 2. Each of the pipes for oil wells 1 is a steel pipe with a relatively large wall thickness. In each of the pipes for oil wells 1, the pipe body 12 has a wall thickness equal to or larger than 12 mm.

As shown in FIG. 5, the threaded connection 10 includes two pins 11 and a tubular coupling 2. Each of the pins 11 is provided on an end of the pipe for oil wells 1, and is in continuity with the pipe body 12. The coupling 2 includes two boxes 21 and a recess portion 22. The boxes 21 are provided on both ends of the coupling 2. Each of the boxes 21 is made up on the associated pin 11. For ease of explanation, as determined along the pipe-axis direction of the threaded connection 10, the direction toward the tip of the pin 11 may be referred to as inward direction, and the direction toward the pipe body 12 as outward direction.

Each of the pins 11 includes a nose 111, a male thread 112, a pin sealing surface 113 and a pin shoulder surface 114.

The nose 111 is located doser to the tip of the pin 11 than the male thread 112 and pin seal surface 113 are. The nose 111 constitutes a tip portion of the pin 11. When the pin 11 and box 21 hâve been made up, the outer peripheral surface of the nose 111 is not contact with the box 21. That is, when the connection has been made up, the outer diameter of the nose 111 is smaller than the inner diameter of the portion of the box 21 that faces the nose 111. For example, the outer peripheral surface of the nose 111 may be formed by a concave surface recessed toward the inner periphery of the pin 11 relative to the pin sealing surface 113.

The male thread 112 is provided on the outer peripheral surface of the pin 11. On the pin 11, the male thread 112 is located doser to the pipe body 12 than the nose 111 is. The male thread 112 is constituted by a tapered thread. A pin sealing surface 113 is provided on the outer periphery of the pin 11 and located between the nose 111 and male thread 112.

The pin sealing surface 113 is generally a tapered surface that decreases its diameter as it goes from the male thread 112 toward the nose 111. The pin sealing surface 113 is formed by, for example, the periphery of a solid of révolution obtained by rotating an arc about the pipe axis CL or the periphery of a truncated cône whose axis is represented by the pipe axis CL, or a combination of two or more such périphéries.

The pin shoulder surface 114 is a toroidal surface formed by a tip surface of the nose 111. According to the présent embodiment, the pin shoulder surface 114 is sloped such that its outer periphery is located doser to the tip of the pin 11 than its inner periphery. That is, in a cross-sectional view of the pin 11 taken along a plane containing the pipe axis CL, the pin shoulder surface 114 is shaped such that its outer periphery is inclined toward the direction of advance during screw-in of the pin 11. However, the pin shoulder surface 114 is not limited to such a shape. The pin shoulder surface 114 may be substantially perpendicular to the pipe axis CL.

Each of the boxes 21 includes a female thread 212, a box sealing surface 213 and a box shoulder surface 214.

The female thread 212 corresponds to the male thread 112 of the pin 11 and provided on the inner peripheral surface of the box 21. The female thread 212 is constituted by a thread that mates the thread constituting the male thread 112. The female thread 212 is constituted by a tapered thread.

The box sealing surface 213 corresponds to the pin sealing surface 113 and is provided on the inner periphery of the box 21. The box sealing surface 213 is in contact with the pin sealing surface 113 when the pin 11 and box 21 hâve been made up.

The pin sealing surface 113 and box sealing surface 213 hâve an amount of interférence. That is, when the connection is not made up, the diameter ofthe pin sealing surface 113 is slightly larger than the diameter of the box sealing surface 213. As such, as the pin 11 is screwed into the box 21, the pin and box sealing surfaces 113 and 213 contact each other and, when the connection has been made up, fittingly adhéré to each other to achieve an interférence fit. Thus, the pin and box sealing surfaces 113 and 213 form a seal using metafto-metal contact.

The box shoulder surface 214 corresponds to the pin shoulder surface 114 and is provided on the inner end of the box 21 as determined along the pipe-axis direction. The box shoulder surface 214 is in contact with the pin shoulder surface 114 when the connection has been made up.

As the pin 11 is screwed into the box 21, the pin and box shoulder surfaces 114 and 214 contact each other and are pressed against each other. The pin and box shoulder surfaces 114 and 214, in press contact with each other, constitute a shoulder.

The area of contact between the pin and box shoulder surfaces 114 and 214, i.e. area of the shoulder, is preferably not smaller than 30 % of the area of a transverse section of the pipe body 12. Transverse section means a cross section taken along a plane perpendicular to the pipe axis CL. If the area of the shoulder is not smaller than 30 % of a transverse section of the pipe body 12, this reduces the plastic deformation of the pin shoulder surface 114 and the adjacent pin seal surface 113 occurring when an excessive compression load is applied to the threaded connection 10. This will stabilize the seal in terms of contact, thereby preventing the contact surface pressure of the seal from decreasing.

Although not limiting, the area of the shoulder is preferably not larger than 60 % of a transverse section ofthe pipe body 12 to prevent the minimum cross section ofthe coupling from decreasing.

As discussed above, the threaded connection 10 is a so'called slim'type threaded connection. Thus, the threaded connection 10 has a connection efficiency smaller than 1. The connection efficiency is calculated by A1/A2, where Al is the area of a transverse section of the portion of the coupling 2 that has the smallest area of résistance to a tensile load when the connection has been made up (i.e. critical section), and A2 is the area of a transverse section of the pipe body.

The male and female threads 112 and 212 hâve ends of their engaged length, El and E2, when the connection has been made up. The area Al of the critical section of the coupling 2 is the area of the transverse section of the coupling 2 as measured at the end El of the engaged length, which is located adjacent to the seal.

The portion of the couphng 2 located between the two boxes 21 forms the recess portion 22. The inner diameter of the recess portion 22 is substantially constant. The outer diameter of the coupling 2 is also substantially constant. Thus, the recess portion 22 has a wall thickness that is substantially uniform. The wall thickness of the recess portion 22 is larger than the wall thickness of the box 21. Thus, the recess portion 22 has a higher stiffness than the box 21.

The inner diameter of the recess portion 22 is preferably smaller than the inner diameter of the pipe body 12. This increases the wall thickness of the recess portion 22 and increases its stiffness. However, if there is a bump, between the pin 11 and recess portion 22, on the inner peripheral surface of the threaded connection 10 when the connection has been made up, a turbulent flow is generated in internai fluid as it passes through the threaded connection 10. This may not only decrease the productivity of the well, but also damage the threaded connection 10. Thus, it is préférable that the inner diameter of the recess portion 22 is substantially equal to the inner diameter of the tip of the pin 11.

To make the inner diameter of the tip of the pin 11 equal to the inner diameter of the recess portion 22, for example, drawing may be performed to make the inner diameter of the tip of the pin 11 smaller than the inner diameter of the pipe body 12. This increases the wall thickness of the portion of the pin 11 that is located doser to the tip than the male thread 112 is (i.e. lip). This increases the stiffness of the lip of the pin 11. This increases the contact force of the seal, thereby further improving the sealing performance of the threaded connection 10.

The length of the recess portion 22 is not smaller than 10 mm. The length ofthe portion of the coupling 2 that is located between the inner end of one box shoulder surface 214 and the inner end of the other box shoulder surface 214, as measured in the pipe-axis direction, will be referred to as length of the recess portion 22. If the recess portion 22 has a length of at least 10 mm, this prevents the deformation of the shoulder occurring when the one box 21 and the associated pin 11 are made up from affecting the make-up ofthe other box 21 and associated pin 11, thereby preventing improper make-up. This will prevent the sealing performance of the threaded connection 10 from decreasing.

The longer the recess portion 22, the higher the stiffness of the coupling 2 becomes, improving the sealing performance of the threaded connection 10. However, if the recess portion 22 has a certain length, saturation is reached in terms of the effect of improving sealing performance. Thus, the length of the recess portion 22 may be, for example, 100 mm at most.

The wall thickness and length of the recess portion 22 is such that the coupling 2 has a necessary stiffness. More specifically, the parameters Tand Lrelating to the wall thickness and length of the recess portion 22 are defined by the following expressions (1) and (2), and T and L are set to satisfy the following expression (3):

T = -^~ (1) t pipe ^{L =} ~rm- m

T-L^1/15>1A (3)

While the wall thickness of the recess portion 22 is substantially constant, ti in expression (1) is defined as the wall thickness of the recess portion 22 as measured at the middle thereof as determined along the pipe-axis direction. That is, ti is the wall thickness of the coupling 2 as measured at the intermediate position between the two box shoulder surfaces 214. t_pi_pe is the wall thickness of the pipe body 12.

Li in expression (2) is the distance between the innermost point of a box shoulder surface 214 and the middle of the coupling 2 as measured in the pipe-axis direction. That is, the length of the recess portion 22 as measured in the pipe-axis direction may be given by 2xLi. 2xLi is equal to the minimum distance between the opposite box shoulder surfaces 214.

Trio expression (2) is the length of each box 21 as measured in the pipe-axis direction. L2 is the distance between an end of the coupling 2 and the innermost point of the associated box shoulder surface 214.

To calculate T and L based on expressions (1) and (2), the variables may be in any units; however, ti and t_pi_pe must be in one and the same unit, and Li and L2 must be in one and the same unit.

[Effects]

According to the présent embodiment, the wall thickness of the recess portion 22 of the coupling 2 and the length thereof as measured in the pipe-axis direction are such that expression (3) is satisfied. The outer diameter of the coupling 2 need not be adjusted to satisfy expression (3). This will improve the stiffness of the coupling 2 without increasing the outer diameter of the coupling 2. Thus, even in a slim-type threaded connection, which has tight restrictions on the outer-diameter dimensions, the deformation with increased diameter of each box 21 during application of the internai pressure can be prevented, thereby maintaining the contact force of the seal. This will provide high sealing performance.

Generally, if the wall thickness of the pipe body is 15 mm or larger, the load applied to the connection is increased such that the box can more easily be deformed. On the other hand, in the threaded connection 10 according to the présent embodiment, the wall thickness and length ofthe recess portion 22 are adjusted to improve the stiffness ofthe coupling 2, thereby preventing deformation of the box 21. This suggests that the threaded connection 10 will also be very effective if the wall thickness of the pipe body is 15 mm or larger.

A slim-type threaded connection typically has a connection efficiency smaller than 1 and has a relative low stiffness. Particularly, if the connection efficiency is not higher than 0.8, the stiffness of the threaded connection is insufficient such that the sealing performance can easily decrease. On the other hand, in the threaded connection 10 according to the présent embodiment, the wall thickness and length of the recess portion 22 are adjusted to improve the stiffness of the coupling 2. This suggests that the threaded connection 10 will also be very effective if the connection efficiency is not higher than 0.8.

[Variations]

Although an embodiment has been described, the présent disclosure is not limited to the above-described embodiment, and various modifications are possible without departing from the spirit of the disclosure. For example, although the threaded connection 10 according to the présent embodiment includes a single seal, it may include two or more seals.

According to the above-described embodiment, the inner diameter ofthe tip ofthe pin 11 and the inner diameter of the recess portion 22 of the coupling 2 are smaller than the inner diameter of the pipe body 12. However, as in the threaded connection 10a shown in

FIG. 6, the inner diameter of the tip of the pin lia and the inner diameter of the recess portion 22a of the coupling 2a may be not smaller than the inner diameter of the pipe body 12. In such implémentations, too, it is préférable that the inner diameter of the tip of the pin lia is substantially equal to the inner diameter of the recess portion 22a when the connection has been made up.

EXAMPLES

To verify the effects of the threaded connection according to the présent disclosure, physical seal tests and a numerical analysis using the elastic-plastic finite element method were conducted.

[Physical Seal Testing]

Two specimens of the coupling-type threaded connection having the basic construction shown in FIG. 5 were prepared and physical seal tests were conducted. The conditions common to these specimens are described below.

Used pipe: 8-5/857.4# steel pipe (with an outer diameter of 219.1 mm and a wall thickness of 16.3 mm)

-Steel material: carbon steel Q125 according to the API standards (with a nominal yield stress of 125 ksi (=862 MPa))

-Thread: every thread had a thread pitch of 5.08 mm, a load-flank angle of-3°, a stab-flank angle of 10° and a stab-flank clearance of 0.15 mm

For the seal tests, the surface treatment for the pin was as eut and the surface treatment for the box was manganèse phosphate-based treatment, and API dope was applied to the entire surfaces of the connection. The seal tests were conducted in accordance with ISO 13679:2011, and the maximum tensile load applied to the connection was 75 % of the tensile-yield load of the pipe body.

As shown in Table 1, the two specimens hâve different values of T-D^ calculated by expression (3).

[0084] [Table 1]

Specimen	Connection efficiency	T (wall thickness of recess base / wall thickness of pipe body)	L (length of recess base / length of coupling)	T-L,/'5	Seal test resuit
comp. ex. 1	0.75	1.60	0.08	1.36	leak
inv. ex. 1	0.82	1.69	0.08	1.43	passed

As shown in Table 1, in the specimen of comparative example 1, a leak occurred during the seal test. In the specimen of comparative example 1, T-IM⁵ was smaller than 1.4 and did not satisfy expression (3), which is presumably the reason why the stiffness of the recess portion of the coupling was insufficient.

In contrast, the specimen of inventive example 1 passed the seal test as no leak occurred during the seal test. In the specimen of inventive example 1, T-IM⁵ was larger than 1.4 and satisfied expression (3), which is presumably the reason why the stiffness of the recess portion of the coupling was sufficiently high.

(Numerical Analysis Using Elastic-Plastic Finite Element Method)

A numerical analysis using the elastic-plastic finite element method was conducted for a plurality of specimens, shown in Table 2. After the make-up of the threads for each specimen was analyzed, a load simulating the ISO 13679 CAL4 Sériés A test was applied, and the sealing performance against each of the internai and external pressures was evaluated. Sealing performance was evaluated based on the minimum value of the average contact surface pressure of the seal in the load history (i.e. minimum seal average contact surface pressure). Ahigher value of the minimum seal average contact surface pressure means a better sealing performance of the seal. The elastic-plastic finite element analysis used models in which the material was an isotropically hardened elastic-plastic material and the elastic modulus was 210 GPa and the yield strength was 125 ksi (=862 MPa) with a proof stress of 0.2 %.

[Table 2]

Specimen	Connection efficiency	T (wall thickness of recess base / wall thickness of pipe body)	Length of recess base [mm]	Length of coupling [mm]	L (length of recess base / length of coupling)	T-L1'15	Minimum seal average contact surface pressure [MPa]	Sealing performance
comp. ex. 2~1	0.75	1.60	25	320	0.08	1,35	466.0	poor
inv. ex. 2-1	0.75	1.60	50	345	0.14	1.41	515.5	good
înv. ex. 2_2	0.75	1.60	75	370	0.20	1.44	536.7	good
comp. ex. 2-2	0.79	1.65	25	320	0.08	1.39	480.8	poor
inv. ex. 2_3	0.79	1.65	50	345	0.14	1.45	517.4	good
inv. ex. 2-4	0.79	1.65	75	370	0.20	1.48	525.2	good
inv. ex. 2-5	0.82	1.69	25	320	0.08	1.43	500.2	good
inv. ex. 2-6	0.82	1.69	50	345	0,14	1.49	511.8	good
inv. ex. 2-7	0.82	1.69	75	370	0.20	1.52	531.6	good

Each of the specimens illustrated in Table 2 is a coupling-type threaded connection having the basic construction shown in FIG. 5, but they hâve different coupling dimensions. The elastic-plastic finite element analysis was conducted while the length of the recess portion and the outer diameter of the coupling were varied. The used pipe, steel material, and thread conditions were common to ail the specimens and were the same as those of the above-discussed physical seal tests.

The specimen of inventive example 2-5 had a coupling dimension equal to that of the specimen of inventive example 1, which passed the physical seal test. As the minimum 10 seal average contact surface pressure of the specimen of inventive example 2-5 was not smaller than 500 MPa, those specimens with minimum seal average contact surface pressure values not lower than 500 MPa were determined to hâve good sealing performance, and those with values below 500 MPa were determined to hâve poor sealing performance.

In each of the specimens of comparative examples 2'1 and 2'2, the minimum seal 15 average contact surface pressure was smaller than 500 MPa, meaning poor sealing performance. In each of the specimen of comparative examples 2-1 and 2-2, T-LM⁵ was smaller than 1.4, suggesting that the stiffness of the recess portion ofthe coupling was insufficient.

In contrast, in each of the specimens of inventive examples 2-1 to 2-7, the minimum seal average contact surface pressure was not lower than 500 Pa, meaning good sealing performance. In each of the specimens of inventive examples 2-1 to 2'7, T-D^ was larger than 1.4, suggesting that the stiffness of the recess portion of the coupling was sufficient

The above-discussed physical seal tests and elastic-plastic finite element analysis demonstrate that good sealing performance can be achieved if 7\L^1/15>1.4.

Claims (1)

1. A threaded connection for connecting pipes for an oil well, comprising:

two pins each provided on an end of a corresponding one of the pipes and in continuity with a body of the corresponding pipe having a wall thickness not smaller than 12 mm,’ and a tubular coupling including two boxes and a recess portion, the boxes being provided on both ends of the coupling, each of the boxes being capable of being made up on the associated pin, the recess portion being located between the two boxes, each of the pins including:

a nose forming a tip portion of the pin, an outer peripheral surface of the nose being not in contact with the box made up on the pin;

a pin shoulder surface being formed at a tip surface of the nose,’ a male thread provided on an outer peripheral surface of the pin and located doser to the body of the corresponding pipe than the nose, the male thread being a tapered thread; and a pin sealing surface provided on the outer peripheral surface of the pin and located between the nose and the male thread, each of the boxes including:

a female thread provided on an inner peripheral surface of the coupling to correspond to the male thread, the female thread being a tapered thread;

a box shoulder surface located doser to a middle of the coupling as determined along a pipe-axis direction than the female thread, the box shoulder surface being in contact with the pin shoulder surface when the connection has been made up to form a shoulder; and a box sealing surface provided on the inner peripheral surface of the coupling and located between the female thread and the box shoulder surface, the box sealing surface being in contact with the pin sealing surface when the connection has been made up to form a seal, wherein the recess portion has a length not smaller than 10 mm as measured in the pipe-axis direction,

5 the connection has a connection efficiency smaller than 1, the connection efficiency being calculated by A1/A2, where Al is the area of a transverse section ofthe coupling as measured at the ends, adjacent to the seal, of the male thread and the female thread engaging each other when the connection has been made up, and A2 is the area of a transverse section of the body, and,

1° when the wall thickness of the body is expressed as t_pi_pe, the wall thickness of the recess portion and the length of the recess portion as measured in the pipe-axis direction are expressed as ti and 2*Li, respectively, and the length of each of the boxes as measured in the pipe-axis direction is expressed as L2, and when the following expression (1) defines Tand the following expression (2) defines L, Tand Lsatisfy the following expression (3):

15 T = (1) t pipe £1 + T₂

Γ·Λ^1/15>1.4 (3)