Threaded fittings for steel pipes

Abstract

This threaded joint (100) comprises a pin (10) and a box (20). The pin (10) and the box (20) each include a shoulder surface (11, 21) and a sealing surface (12, 22). The threaded joint (100) satisfies formulae (1), (2) and (3) when: the thickness of the pin (10) and the thickness of the box (20) at a sealing point (SP) is Rpin and Rbox, respectively; the axial length from the sealing point (SP) to the tip of the pin (10) is a pin lip length L; and the angle that the shoulder surface (11) of the pin (10) makes with a plane (S1) perpendicular to the axial direction of the threaded joint (100) is a shoulder angle θ.

Description

[Technical Field]

[0001] This disclosure relates to a threaded joint for connecting steel pipes. [Background technology]

[0002] In oil wells, natural gas wells, and other similar facilities, numerous steel pipes called oil well pipes are used to extract underground resources. These steel pipes are connected to each other by threaded joints.

[0003] Threaded joints for steel pipes are broadly classified into integral type and coupling type. In the integral type, each steel pipe has a male threaded portion at one end and a female threaded portion at the other end. The steel pipes are connected by screwing the male threaded portion of one steel pipe into the female threaded portion of the other steel pipe. In the coupling type, steel pipes having male threaded portions at both ends are connected by a coupling. The coupling is a separate pipe material from the steel pipes to be connected and has female threaded portions at both ends. The male threaded portions of the steel pipes are screwed into each of the female threaded portions of the coupling to fasten them. Generally, the end of a steel pipe with a male threaded portion is called a pin, and the end of a steel pipe or coupling with a female threaded portion is called a box.

[0004] In the development of oil wells and other drilling facilities, the well wall is formed by steel pipes arranged in multiple stages to prevent the well from collapsing. The steel pipes that form the well wall are called casings. The threaded joints that connect these steel pipes require excellent sealing performance against both external pressure and internal pressure.

[0005] Conventionally, threaded joints are known that have sealing surfaces on both the pin and the box to ensure sealing performance. In these threaded joints, the diameter of the pin's sealing surface is slightly larger than the diameter of the box's sealing surface. Due to the difference in diameter between the pin's sealing surface and the box's sealing surface, when the pin and box are fastened together, the sealing surfaces come into contact with each other. In the fastened state, the elastic recovery force of each sealing surface causes them to contact each other at high pressure, resulting in sealing performance.

[0006] For example, in the threaded joint disclosed in Patent Document 1, the pin has a pin shoulder surface and a pin seal surface at its tip. The box is provided with a box shoulder surface and a box seal surface corresponding to the pin shoulder surface and the pin seal surface. In Patent Document 1, the ratio of the thickness of the box to the thickness of the pin (seal thickness ratio) at the midpoint of the tapered surface included in the box seal surface (seal point) is 1.7 or more. Also, the axial length from the seal point to the tip of the pin (pin lip length) is 13 mm or more. Furthermore, the angle (shoulder angle) that the pin shoulder surface and the box shoulder surface make with a plane perpendicular to the central axis of the threaded joint is 2° to 13°. Patent Document 1 states that with such a configuration, the sealing performance against internal and external pressure can be improved. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] International Publication No. 2019 / 093311 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] In recent years, in deep-sea wells, where development has been increasing, so-called slim-type threaded joints are used to connect steel pipes in order to enable multi-stage casing designs. A slim-type threaded joint is a threaded joint in which the outer diameter of the box is 105% or less of the outer diameter of the steel pipe body, resulting in a small protrusion of the joint portion relative to the steel pipe body.

[0009] Slim type screw joints are often integral type screw joints. However, since integral type screw joints are long, for example, when performing surface treatment to prevent sticking to a box, large-scale equipment is required. Also, in the case of integral type, usually, the end of the steel pipe that becomes the pin is subjected to pipe shrinking, and the end of the steel pipe that becomes the box is subjected to pipe expanding, so the manufacturing process of the screw joint increases and the manufacturing cost increases. Furthermore, at the end of the steel pipe plastically deformed by pipe expanding, the dislocation density becomes high, so there is a possibility of hydrogen embrittlement fracture in a sour environment containing hydrogen sulfide.

[0010] To solve such problems, a coupling type slim type screw joint is being considered. In the case of the coupling type, since a pipe material different from the steel pipe to be connected has a box, pipe shrinking and pipe expanding are unnecessary. Therefore, the manufacturing cost of the screw joint can be reduced, and the risk of hydrogen embrittlement fracture can be reduced.

[0011] On the other hand, in the coupling type slim type screw joint, due to dimensional constraints, the wall thickness of the pin becomes very small. In other words, in the coupling type slim type screw joint, the ratio of the wall thickness of the box to the wall thickness of the pin is large. In this case, since the rigidity of the tip of the pin is not sufficiently ensured, an increase in the contact pressure due to the elastic recovery force of the pin seal surface cannot be expected.

[0012] To ensure the rigidity of the tip of the pin, it is also conceivable to increase the pin lip length. However, if the pin lip length is increased, buckling due to the compressive load applied to the screw joint is likely to occur at the tip of the pin. When the tip of the pin buckles, the contact pressure of the pin seal surface provided at the tip of the pin against the box seal surface decreases. In this case, in particular, the sealing performance against internal pressure deteriorates.

[0013] In Patent Document 1, it is described that based on the results of numerical analysis by the finite element method, if the pin lip length is 13 mm or more, the sealing performance is improved regardless of the shoulder angle. However, the numerical analysis was carried out with the seal wall thickness ratio fixed at 2.0. In Patent Document 1, the appropriate pin lip length in the case where the seal wall thickness ratio is large has not been verified. As described above, when the pin lip length is increased in a state where the seal wall thickness ratio is large, buckling is likely to occur at the tip of the pin due to the compression load.

[0014] In Patent Document 1, it is described that based on the results of another numerical analysis by the finite element method, when the seal wall thickness ratio is 1.7 or more, the sealing performance against internal pressure saturates. However, in the numerical analysis, only the sealing performance up to a seal wall thickness ratio of 3.0 has been verified, and the sealing performance when the seal wall thickness ratio exceeds 3.0 has not been verified. Further, the numerical analysis was carried out with the pin lip length fixed at 13 mm and the shoulder angle fixed at 5°. Therefore, the appropriate pin lip length and shoulder angle when the seal wall thickness ratio exceeds 3.0 are unknown.

Means for Solving the Problem

[0015] An object of the present disclosure is to provide a steel pipe threaded joint capable of ensuring excellent sealing performance even when the ratio of the wall thickness of the box to the wall thickness of the pin at the seal point is large.

[0016] The threaded joint according to this disclosure is a threaded joint for connecting steel pipes together. The threaded joint comprises a tubular pin and a tubular box. The pin is provided continuously with the steel pipe body. The box has an outer diameter of 105% or less of the outer diameter of the steel pipe body. The pin is inserted into the box. The box is fastened with the pin. The pin includes a first pin shoulder surface, a male threaded portion, and a first pin seal surface. The first pin shoulder surface is annular. The first pin shoulder surface is provided on the pipe end face of the pin. The first pin shoulder surface is inclined with respect to a plane perpendicular to the axial direction of the threaded joint such that its outer peripheral edge is located closer to the tip of the pin than its inner peripheral edge. The male threaded portion is provided on the outer peripheral surface of the pin. The first pin seal surface is provided on the outer peripheral surface of the pin between the first pin shoulder surface and the male threaded portion. The box includes a first box shoulder surface, a female threaded portion, and a first box seal surface. The first box shoulder surface is annular. The first box shoulder surface is located deep inside the box, corresponding to the first pin shoulder surface. The first box shoulder surface contacts the first pin shoulder surface when the pin and box are fastened together. The female thread portion is located on the inner circumferential surface of the box, corresponding to the male thread portion. The female thread portion engages with the male thread portion when fastened together. The first box seal surface is located on the inner circumferential surface of the box, corresponding to the first pin seal surface. The first box seal surface has a smaller diameter than the first pin seal surface, so that it makes interference contact with the first pin seal surface around its entire circumference when fastened together. The thickness of the pin at the seal point, which is the position where the difference between the diameter of the first pin seal surface and the diameter of the first box seal surface is maximum, is set to R pin , the thickness of the box R box Assuming that the axial length from the seal point to the tip of the pin is the pin lip length L, and the angle that the first pin shoulder surface makes with a plane perpendicular to the axial direction is the shoulder angle θ, the threaded joint satisfies the following equations (1), (2), and (3).

number

[0017] According to the threaded joint for steel pipes described herein, excellent sealing performance can be ensured even when the ratio of the wall thickness of the box to the wall thickness of the pin at the sealing point is large. [Brief explanation of the drawing]

[0018] [Figure 1] Figure 1 is a longitudinal cross-sectional view of a threaded joint for steel pipes according to the first embodiment. [Figure 2] Figure 2 is a magnified view of a portion of the threaded joint shown in Figure 1. [Figure 3] Figure 3 is a longitudinal cross-sectional view of a threaded joint for steel pipes according to the second embodiment. [Figure 4] Figure 4 is a graph showing the relationship between the ratio of pin lip length L to wall thickness ratio Rbox / Rpin (L / (Rbox / Rpin)), the shoulder angle θ, and the sealing performance for the specimens used in the analysis. [Modes for carrying out the invention]

[0019] The threaded joint according to this embodiment is a threaded joint for connecting steel pipes together. The threaded joint comprises a tubular pin and a tubular box. The pin is provided continuously with the steel pipe body. The box has an outer diameter of 105% or less of the outer diameter of the steel pipe body. The pin is inserted into the box. The box is fastened with the pin. The pin includes a first pin shoulder surface, a male threaded portion, and a first pin seal surface. The first pin shoulder surface is annular. The first pin shoulder surface is provided on the pipe end face of the pin. The first pin shoulder surface is inclined with respect to a plane perpendicular to the axial direction of the threaded joint such that its outer peripheral edge is located closer to the tip of the pin than its inner peripheral edge. The male threaded portion is provided on the outer peripheral surface of the pin. The first pin seal surface is provided on the outer peripheral surface of the pin between the first pin shoulder surface and the male threaded portion. The box includes a first box shoulder surface, a female threaded portion, and a first box seal surface. The first box shoulder surface is annular. The first box shoulder surface is located deep inside the box, corresponding to the first pin shoulder surface. The first box shoulder surface contacts the first pin shoulder surface when the pin and box are fastened together. The female thread portion is located on the inner circumferential surface of the box, corresponding to the male thread portion. The female thread portion engages with the male thread portion when fastened together. The first box seal surface is located on the inner circumferential surface of the box, corresponding to the first pin seal surface. The first box seal surface has a smaller diameter than the first pin seal surface, so that it makes interference contact with the first pin seal surface around its entire circumference when fastened together. The thickness of the pin at the seal point, which is the position where the difference between the diameter of the first pin seal surface and the diameter of the first box seal surface is maximum, is set to R pin , the thickness of the box R box Assuming that the axial length from the seal point to the tip of the pin is the pin lip length L, and the angle that the first pin shoulder surface makes with a plane perpendicular to the axial direction is the shoulder angle θ, the threaded joint satisfies the following equations (1), (2), and (3) (first configuration).

number

[0020] In the first configuration of the threaded joint for steel pipes, at the seal point, which is the position where the difference between the diameter of the first pin seal surface and the diameter of the first box seal surface is maximum, the ratio of the box wall thickness to the pin wall thickness is large, between 3.65 and 4.25. According to the inventors' studies, within this range of wall thickness ratio, the contact pressure of the first pin seal surface to the first box seal surface can be maintained at a high level by satisfying the above equations (2) and (3). Therefore, even when the ratio of the box wall thickness to the pin wall thickness at the seal point is large, excellent sealing performance can be ensured. The first configuration makes it possible to improve the sealing performance of the threaded joint, especially with respect to internal pressure.

[0021] The shoulder angle θ may be 30° or less when the pin lip length L is 8.5 mm or more and less than 10.5 mm, 15° or less when the pin lip length L is 10.5 mm or more and less than 13.5 mm, 10° or less when the pin lip length L is 13.5 mm or more and less than 16.5 mm, and 5° or less when the pin lip length L is 16.5 mm or more (second configuration).

[0022] If the shoulder angle θ of the first pin shoulder surface is too large, the over-torque performance of the threaded joint will usually decrease. Over-torque is the torque at which plastic deformation occurs at the tip of the pin when the pin and box are further tightened after the first pin shoulder surface has contacted the first box shoulder surface. Over-torque performance refers to the ability to withstand further tightening without plastic deformation after the shoulder surfaces have come into contact with each other.

[0023] In the second configuration, the upper limit of the shoulder angle θ of the first pin shoulder surface is determined according to the lip length L of the pin. This ensures excellent over-torque performance in threaded joints for steel pipes.

[0024] In the first or second configuration of the threaded joint for steel pipes, the pin may further include a second pin shoulder surface and a second pin seal surface. In this case, the box further includes a second box shoulder surface and a second box seal surface. The second pin shoulder surface is provided at the end of the pin on the steel pipe body side. The second pin seal surface is provided on the outer circumferential surface of the pin between the second pin shoulder surface and the male thread portion. The second box shoulder surface is provided on the pipe end surface of the box corresponding to the second pin shoulder surface. The second box shoulder surface contacts the second pin shoulder surface in the fastened state. The second box seal surface is provided on the inner circumferential surface of the box corresponding to the second pin seal surface. The second box seal surface contacts the second pin seal surface in the fastened state (third configuration).

[0025] According to the third configuration, the pin includes a first pin shoulder surface and a first pin seal surface on the pipe end side, as well as a second pin shoulder surface and a second pin seal surface on the steel pipe body side. When the pin is fastened to the box, the second pin shoulder surface and the second pin seal surface contact the second box shoulder surface and the second box seal surface, respectively. This improves the sealing performance against external pressure.

[0026] In a threaded joint for steel pipes relating to any of the first to third configurations, it is preferable that the pin is provided on each of the steel pipes to be connected, and the box is provided on the coupling, which is a different pipe material from the steel pipe (fourth configuration).

[0027] The fourth configuration of the threaded joint for steel pipes is a coupling-type threaded joint. That is, a box is provided on the coupling, which is a separate pipe material from the steel pipe to be connected, and only a pin is provided on the steel pipe to be connected. Therefore, there is no need to perform pipe expansion processing on the steel pipe to allow the pin to be inserted. Thus, the manufacturing cost of the threaded joint can be reduced and the occurrence of hydrogen embrittlement fracture during use can be prevented.

[0028] Embodiments of this disclosure will be described below with reference to the drawings. In each drawing, the same or equivalent components are denoted by the same reference numerals, and the same description will not be repeated.

[0029] [First Embodiment] (Threaded joint configuration) Figure 1 is a longitudinal cross-sectional view of a threaded joint 100 for steel pipes according to the first embodiment. The longitudinal cross-section of the threaded joint 100 is the cross-section obtained when the threaded joint 100 is cut in a plane containing the pipe axis X. Hereinafter, the direction in which the pipe axis X extends will be referred to as the axial direction, and the radial direction of the threaded joint 100 will simply be referred to as the radial direction.

[0030] The threaded joint 100 is used to connect two steel pipes 30 together. The threaded joint 100 includes a tubular pin 10 and a tubular box 20. The pin 10 is provided at the end of the steel pipe 30. The box 20 is provided on a coupling 40, which is a different pipe material from the steel pipes 30 to be connected. In other words, the threaded joint 100 is a coupling-type threaded joint.

[0031] The pin 10 is inserted into the box 20 and fastened to the box 20. The pin 10 is provided continuously with the body 31 of the steel pipe 30. The pin 10 includes a pin shoulder surface 11, a pin seal surface 12, and a male threaded portion 13. The pin shoulder surface 11, the pin seal surface 12, and the male threaded portion 13 are arranged in this order from the tip side of the pin 10 toward the steel pipe body 31 side.

[0032] The pin shoulder surface 11 is provided on the end face of the pin 10. The pin shoulder surface 11 is an annular surface with the pipe axis X as its axis.

[0033] The pin seal surface 12 is provided on the outer circumferential surface of the pin 10 between the pin shoulder surface 11 and the male threaded portion 13. The pin seal surface 12 is a cylindrical surface with the pipe axis X as its axis. The pin seal surface 12 may be, for example, a curve that is convex toward the box 20 side in a longitudinal cross-sectional view of the threaded joint 100, or it may be a straight line that is inclined with respect to the pipe axis X so as it approaches the pipe axis X toward the tip of the pin 10. The pin seal surface 12 may be composed of a combination of two or more such curves and / or straight lines.

[0034] The male threaded portion 13 is provided on the outer circumferential surface of the pin 10. The male threaded portion 13 is positioned on the steel pipe body 31 side relative to the pin seal surface 12. The male threaded portion 13 extends from the vicinity of the pin seal surface 12 to the vicinity of the steel pipe body 31. The male threaded portion 13 is composed of tapered threads. Examples of the types of threads that make up the male threaded portion 13 include API standard buttress threads, trapezoidal threads formed based on buttress threads, or dovetail-shaped threads.

[0035] The box 20 has an outer diameter ODb. The outer diameter ODb of the box 20 is 105% or less of the outer diameter OD of the steel pipe body 31. The box 20 includes a box shoulder surface 21, a box seal surface 22, and a female threaded portion 23. The box shoulder surface 21, the box seal surface 22, and the female threaded portion 23 are provided on the box 20 corresponding to the pin shoulder surface 11, the pin seal surface 12, and the male threaded portion 13, respectively.

[0036] The box shoulder surface 21 is located at the back of the box 20, corresponding to the pin shoulder surface 11. Like the pin shoulder surface 11, the box shoulder surface 21 is an annular surface with the pipe axis X as its axis.

[0037] The box seal surface 22 is provided on the inner circumferential surface of the box 20 between the box shoulder surface 21 and the female thread portion 23. The box seal surface 22 is a cylindrical surface with the pipe axis X as its axis. In a longitudinal cross-sectional view of the threaded joint 100, the box seal surface 22 may be a curved shape that is convex toward the pin 10 side, or it may be a straight line that is inclined with respect to the pipe axis X so as it approaches the inner part of the box 20. The box seal surface 22 may be composed of a combination of two or more such curves and / or straight lines.

[0038] The female threaded portion 23 is provided on the inner circumferential surface of the box 20. The female threaded portion 23 is positioned on the pipe end side of the box 20 relative to the box seal surface 22. The female threaded portion 23 extends from the vicinity of the box seal surface 22 to the vicinity of the pipe end of the box 20. The female threaded portion 23 is composed of a tapered thread that engages with the male threaded portion 13 of the pin 10.

[0039] The configuration of the threaded joint 100 according to this embodiment will be described in more detail below with reference to Figure 2. Figure 2 is a partially enlarged view of the threaded joint 100 shown in Figure 1.

[0040] Referring to Figure 2, the male threaded portion 13 includes a thread crest surface 131, a thread groove bottom surface 132, an insertion flank surface 133, and a load flank surface 134. The female threaded portion 23 includes a thread crest surface 231, a thread groove bottom surface 232, an insertion flank surface 233, and a load flank surface 234.

[0041] In the male threaded portion 13, the thread crest surface 131 is connected to the thread groove bottom surface 132 by the insertion flank surface 133 and the load flank surface 134. The insertion flank surface 133 is located in front of the thread crest surface 131 in the direction in which the pin 10 advances relative to the box 20 during fastening. The load flank surface 134 is located behind the thread crest surface 131 in the direction in which the pin 10 advances.

[0042] In the female thread portion 23, the thread crest surface 231 and the thread groove bottom surface 232 are connected by the insertion flank surface 233 and the load flank surface 234. The insertion flank surface 233 is located in front of the thread groove bottom surface 232 in the direction of travel of the pin 10 during fastening. The load flank surface 234 is located behind the thread groove bottom surface 232 in the direction of travel of the pin 10.

[0043] When the pin 10 and the box 20 are fastened together, the bottom surface 132 of the thread groove of the male thread portion 13 and the top surface 231 of the thread of the female thread portion 23 are in contact, and the load flank surfaces 134, 234 of the male thread portion 13 and the female thread portion 23 are in contact with each other. On the other hand, in the fastened state, a gap is formed between the top surface 131 of the thread of the male thread portion 13 and the bottom surface 232 of the thread groove of the female thread portion 23. Also, in the fastened state, a gap is formed between the insertion flank surface 133 of the male thread portion 13 and the insertion flank surface 233 of the female thread portion 23. The size of the gap between the insertion flank surfaces 133, 233, that is, the axial distance between the insertion flank surface 133 and the insertion flank surface 233, is, for example, 0.025 mm or more and 0.230 mm or less.

[0044] In the fastened state, the pin seal surface 12 and the box seal surface 22 form a seal through metal-to-metal contact. Specifically, when the pin 10 and the box 20 are not fastened together, the box seal surface 22 has a smaller diameter than the pin seal surface 12. The difference between the diameter of the pin seal surface 12 and the diameter of the box seal surface 22 is called the radial interference amount. Due to this interference amount, in the fastened state, the box seal surface 22 interferes with the pin seal surface 12, causing the pin seal surface 12 to shrink in diameter and the box seal surface 22 to expand in diameter. Due to the elastic recovery force of the shrinking pin seal surface 12 and the expanding box seal surface 22, the box seal surface 22 makes interference contact with the pin seal surface 12 around its entire circumference, generating a high contact pressure between the pin seal surface 12 and the box seal surface 22.

[0045] The pin seal surface 12 and the box seal surface 22 interfere most strongly at the seal point SP. That is, the seal point SP is the position where the difference (interference amount) between the diameter of the pin seal surface 12 and the diameter of the box seal surface 22 is the largest. When the region where the interference amount between the pin seal surface 12 and the box seal surface 22 is the largest extends in the axial direction, the axial center of the region is defined as the seal point SP. At the seal point SP, the pin 10 and the box 20 each have a wall thickness R pin ,R box .

[0046] A portion of the pin 10 on the tip side of the seal point SP is defined as the pin lip 14, and the axial length from the seal point SP to the tip of the pin 10 is defined as the pin lip length L. In the fastened state, a portion of the outer peripheral surface of the pin lip 14 that is located on the tip side of the pin 10 with respect to the pin seal surface 12 faces the inner peripheral surface of the box 20 with a gap therebetween throughout its entirety.

[0047] A pin shoulder surface 11 is provided at the tip of the pin lip 14. The pin shoulder surface 11 is inclined with respect to the plane S1 such that its outer peripheral edge is located on the tip side of the pin 10 with respect to its inner peripheral edge. The plane S1 is a virtual plane perpendicular to the axial direction of the screw joint 100. The angle formed by the pin shoulder surface 11 with respect to the plane S1 is defined as the shoulder angle θ.

[0048] The wall thickness R of the pin 10 pin , the wall thickness R of the box 20 box , the pin lip length L, and the shoulder angle θ are set to satisfy the following equations (1), (2), and (3).

Equation

[0049] The pin lip length L and the shoulder angle θ of the pin shoulder surface 11 are related to the wall thickness ratio R box / R pinThe shoulder angle θ can be adjusted as appropriate, insofar as it satisfies equations (2) and (3) above in relation to the pin lip length L. For example, if the pin lip length L is 8.5 mm or more and less than 10.5 mm, the shoulder angle θ may be 30° or less. For example, if the pin lip length L is 10.5 mm or more and less than 13.5 mm, the shoulder angle θ may be 15° or less. For example, if the pin lip length L is 13.5 mm or more and less than 16.5 mm, the shoulder angle θ may be 10° or less. For example, if the pin lip length L is 16.5 mm or more, the shoulder angle θ can be 5° or less.

[0050] The box shoulder surface 21, like the pin shoulder surface 11, is inclined with respect to the plane S1. The angle (shoulder angle) that the box shoulder surface 21 makes with respect to the plane S1 is substantially equal to the shoulder angle θ of the pin shoulder surface 11. Due to manufacturing tolerances, there may be a difference between the shoulder angle of the box shoulder surface 21 and the shoulder angle θ of the pin shoulder surface 11, but this difference is at most about ±1°. In the fastened state, the box shoulder surface 21 contacts the pin shoulder surface 11 and together with the pin shoulder surface 11 forms the shoulder portion.

[0051] (effect) The threaded joint 100 for steel pipes according to this embodiment is a slim-type threaded joint in which the outer diameter ODb of the box 20 is 105% or less of the outer diameter OD of the steel pipe body 31. In this threaded joint 100, the wall thickness ratio R of the pin 10 and the box 20 at the seal point SP box / R pin Because the ratio is large, between 3.65 and 4.25, the rigidity of the pin lip 14 becomes relatively small. However, in the threaded joint 100, the wall thickness ratio R of the pin 10 and the box 20 box / R pin The pin lip length L and the shoulder angle θ of the pin shoulder surface 11 are set to satisfy equations (2) and (3) above. This results in the thickness ratio R of the pin 10 and the box 20 at the seal point SP. box / R pinEven if the pressure is between 3.65 and 4.25, the contact pressure of the pin seal surface 12 against the box seal surface 22 can be maintained at a high level. Therefore, the threaded joint 100 can ensure excellent sealing performance, especially against internal pressure.

[0052] In the threaded joint according to this embodiment, the shoulder angle θ can be adjusted according to the pin lip length L as described above. By adjusting the shoulder angle θ in this way, the threaded joint 100 can exhibit excellent over-torque performance.

[0053] The threaded joint 100 according to this embodiment is a coupling-type threaded joint. That is, a box 20 is provided on the coupling 40, which is separate from the steel pipe 30 to be connected, and only a pin 10 is provided on the steel pipe 30. Therefore, unlike integral-type threaded joints, there is no need to perform pipe shrinking or expanding on the end of the steel pipe 30 in order to insert the pin 10 into the box 20. Thus, the manufacturing cost of the threaded joint 100 can be reduced, and the occurrence of hydrogen embrittlement fracture during use can be prevented.

[0054] [Second Embodiment] Figure 3 is a longitudinal cross-sectional view of the threaded joint 101 for steel pipes according to the second embodiment. The threaded joint 101 according to this embodiment differs from the threaded joint 100 according to the first embodiment in that it has a shoulder portion and a sealing portion on the steel pipe body 31 side.

[0055] Referring to Figure 3, in the threaded joint 101 according to this embodiment, the pin 10 includes a pin shoulder surface 11, a pin seal surface 12, and a male threaded portion 13, as well as a pin shoulder surface 15 and a pin seal surface 16.

[0056] The pin shoulder surface 15 is provided at the end of the pin 10 on the side of the steel pipe body 31. The pin shoulder surface 15 is an annular surface with the pipe axis X as its axis. The pin shoulder surface 15 may be inclined with respect to a virtual plane S2 perpendicular to the pipe axis X, or it may be parallel to the plane S2. In the example shown in Figure 3, the pin shoulder surface 15 is inclined with respect to the plane S2 such that its inner periphery is located closer to the steel pipe body 31 than its outer periphery.

[0057] The pin seal surface 16 is provided on the outer circumferential surface of the pin 10 between the pin shoulder surface 15 and the male threaded portion 13. The pin seal surface 16 is a cylindrical surface with the pipe axis X as its axis. In a longitudinal cross-sectional view of the threaded joint 100, the pin seal surface 16 may be a curved shape that is convex toward the box 20 side, or it may be a straight line that is inclined with respect to the pipe axis X so as it moves away from the pipe axis X toward the steel pipe body 31 side. The pin seal surface 16 may be composed of a combination of two or more such curves and / or straight lines.

[0058] The box 20 includes a box shoulder surface 21, a box sealing surface 22, and a female threaded portion 23, as well as a box shoulder surface 25 and a box sealing surface 26.

[0059] The box shoulder surface 25 is provided on the pipe end face of the box 20, corresponding to the pin shoulder surface 15. The box shoulder surface 25 is an annular surface with the pipe axis X as its axis. In this embodiment, the box shoulder surface 25 is inclined with respect to the plane S2, similar to the pin shoulder surface 15. When the pin shoulder surface 15 is parallel to the plane S2, the box shoulder surface 25 is also substantially parallel to the plane S2. In the fastened state, the box shoulder surface 25 contacts the pin shoulder surface 15 and together with the pin shoulder surface 15 forms the shoulder portion.

[0060] The box seal surface 26 is provided on the inner circumferential surface of the box 20, corresponding to the pin seal surface 16. The box seal surface 26 is positioned between the box shoulder surface 25 and the female thread portion 23. The box seal surface 26 is a cylindrical surface with the pipe axis X as its axis. In a longitudinal cross-sectional view of the threaded joint 100, the box seal surface 26 may be a curved shape that is convex toward the pin 10 side, or it may be a straight line that is inclined with respect to the pipe axis X so as it moves away from the pipe axis X toward the pipe end side of the box 20. The box seal surface 26 may be composed of a combination of two or more such curves and / or straight lines. The box seal surface 26 has a radial interference amount with the pin seal surface 16. Therefore, in the fastened state, the box seal surface 26 interferes with the pin seal surface 16 over its entire circumference, and together with the pin seal surface 16, forms a seal portion by metal-to-metal contact.

[0061] In the threaded joint 101 according to this embodiment, shoulder surfaces 11, 21 and sealing surfaces 12, 22 form a shoulder portion and a sealing portion, respectively, on the tip side of the pin 10, while shoulder surfaces 15, 25 and sealing surfaces 16, 26 form a shoulder portion and a sealing portion, respectively, on the steel pipe body 31 side. The shoulder portion and sealing portion on the tip side of the pin 10 mainly improve sealing performance against internal pressure, while the shoulder portion and sealing portion on the steel pipe body 31 side mainly improve sealing performance against external pressure. Therefore, the threaded joint 101 according to this embodiment can ensure excellent sealing performance against both internal and external pressure.

[0062] While embodiments relating to this disclosure have been described above, this disclosure is not limited to the embodiments described above, and various modifications are possible as long as they do not deviate from its spirit.

[0063] For example, in the above embodiment, when fastened, the bottom surface 132 of the thread groove of the male thread portion 13 and the top surface 231 of the thread of the female thread portion 23 are in contact, and a gap is formed between the top surface 131 of the thread of the male thread portion 13 and the bottom surface 232 of the thread groove of the female thread portion 23. However, when fastened, a gap may be provided between the bottom surface 132 of the thread groove of the male thread portion 13 and the top surface 231 of the thread of the female thread portion 23, and the top surface 131 of the thread of the male thread portion 13 and the bottom surface 232 of the thread groove of the female thread portion 23 may be brought into contact. Alternatively, the bottom surface 132 of the thread groove of the male thread portion 13 and the top surface 231 of the thread of the female thread portion 23 may be brought into contact, as well as the top surface 131 of the thread of the male thread portion 13 and the bottom surface 232 of the thread groove of the female thread portion 23.

[0064] In the above embodiment, the thread crest surface 131 and the thread groove bottom surface 132 of the male threaded portion 13 are parallel to the taper line of the male threaded portion 13 in a longitudinal cross-sectional view of the screw joint. Similarly, the thread crest surface 231 and the thread groove bottom surface 232 of the female threaded portion 23 are parallel to the taper line of the female threaded portion 23 in a longitudinal cross-sectional view of the screw joint. However, the thread crest surfaces 131, 231 and the thread groove bottom surfaces 132, 232 may be parallel to the pipe axis X in a longitudinal cross-sectional view of the screw joint.

[0065] The threaded joints 100 and 101 according to the above embodiment are coupling-type threaded joints. However, the threaded joints 100 and 101 can also be changed to integral-type threaded joints. [Examples]

[0066] The present disclosure will be further described below with reference to examples. However, the present disclosure is not limited to the following examples.

[0067] The inventors have determined the thickness ratio R of the pin 10 and box 20 in the seal point SP. box / R pinTo investigate a configuration that can ensure excellent sealing performance even when the thickness ratio is large, elastoplastic finite element (FEM) analysis was performed on numerous test specimens. Each test specimen was modeled after the threaded joint 100 shown in Figure 1, with the material being an isotropically hardened elastoplastic body, with an elastic modulus of 210 GPa and a 0.2% proof stress, resulting in a yield strength of 125 ksi (861.8 MPa). Each test specimen had a wall thickness ratio R box / R pin At least one of the pin lip length L and the shoulder angle θ of the pin shoulder surface 11 differs from other specimens. The thickness ratio R of each specimen box / R pin The range was set to 3.65 or higher and 4.25 or lower.

[0068] The common analysis conditions are shown below. • Dimensions of steel pipes Outer diameter (OD): 244.5mm, wall thickness: 13.84mm • Grade of steel pipe Steel pipes specified as Q125 in the API standard (low-alloy steel with a nominal yield strength of 125 ksi (861.8 MPa)) • Screw dimensions Thread pitch: 5.08 mm, Flank angle of load flank face: -5°, Flank angle of insertion flank face: 15°, Gap between insertion flank faces: 0.03 mm, Thread taper: 1 / 8 • Coupling dimensions Outer diameter (ODb): 249.16 mm (Outer diameter OD of steel pipe body × 101.9%) • Joint efficiency: 74.6%

[0069] After analyzing the screw tightening for each specimen, repeated combined loads simulating the Series A test of ISO 13679:2011 were applied to evaluate the sealing performance of each specimen against internal pressure. Over-torque performance was also evaluated for each specimen.

[0070] (Sealing performance) Figure 4 is a graph summarizing the sealing performance of the specimens used in this analysis. In the graph in Figure 4, the horizontal axis represents the wall thickness ratio R. box / R pinRatio of pin lip length L to (R): L / (R box / R pin The vertical axis of the graph represents the shoulder angle θ. Sealing performance was evaluated by the minimum value of the contact force [N / mm] per unit length in the circumferential direction of the seal portion during the internal pressure load cycle (minimum contact force). A larger minimum contact force indicates better sealing performance. In this analysis, 115% of the minimum contact force obtained for existing slim-type threaded joints (integral type) of approximately the same size as each test specimen, i.e., those with the same steel pipe dimensions and grade, joint efficiency, and thread pitch as each test specimen, was used as the baseline. Test specimens with a minimum contact force above the baseline were judged to have good sealing performance, and test specimens with a minimum contact force below the baseline were judged to have insufficient sealing performance. In the existing slim-type threaded joints, the flank angle of the loaded flank surface is -10°, the flank angle of the insertion flank surface is 25°, the gap between the insertion flank surfaces is 0.50 mm, and the thread taper is 1 / 16. Figure 4 shows the approximate curve C, which represents the upper and lower limits of the shoulder angle θ for test specimens judged to have good sealing performance. max ,C min It is displaying.

[0071] From Figure 4, the minimum contact force of the seal portion is greater than or equal to the standard when the wall thickness ratio R box / R pin The pin lip length L and shoulder angle θ are the two approximation curves C. max ,C min When it is within the range between, that is, when the following two equations are satisfied: Wall thickness ratio R box / R pin When the ratio is between 3.65 and 4.25, the threaded joint exhibits excellent sealing performance if both of the following equations are satisfied.

number

[0072] (Over-torque performance) Over-torque performance was evaluated using the Maximum Torque Value (MTV) [ft.lb], which is one of the indicators of yield torque. A higher MTV indicates better over-torque performance. The wall thickness ratio R of pin 10 and box 20. box / R pin Table 1 shows the MTV of the test specimens with a value of 3.70.

[0073] [Table 1]

[0074] Of the specimens subjected to analysis, the specimen with the minimum contact force equal to or greater than the standard value and closest to the standard value had a wall thickness ratio R box / R pin The test specimen had a torque coefficient (MTV) of 3.70, a pin lip length L of 8.5 mm, and a shoulder angle θ of 30°. Therefore, regarding over-torque performance, the MTV of this test specimen was used as the standard, and if the MTV of each test specimen was equal to or greater than the standard, it was determined that over-torque performance was ensured for that test specimen.

[0075] As shown in Table 1, the MTV value tends to decrease as the shoulder angle θ increases. When the pin lip length L is 8.5 mm or more and less than 10.5 mm, an MTV above the standard can be obtained and over-torque performance can be ensured if the shoulder angle θ is 30° or less.

[0076] When the pin lip length L was between 10.5 mm and less than 13.5 mm, an MTV (Maximum Torque Value) above the standard was achieved when the shoulder angle θ was 15° or less. Furthermore, when the pin lip length L was between 13.5 mm and less than 16.5 mm, an MTV above the standard was achieved when the shoulder angle θ was 10° or less. In addition, when the pin lip length L was 16.5 mm or more, an MTV above the standard was achieved when the shoulder angle θ was 5° or less. Therefore, the preferred shoulder angle θ for ensuring over-torque performance is 15° or less when the pin lip length L is between 10.5 mm and less than 13.5 mm, 10° or less when the pin lip length L is between 13.5 mm and less than 16.5 mm, and 5° or less when the pin lip length L is 16.5 mm or more. [Explanation of Symbols]

[0077] 100,101: Threaded fittings for steel pipes 10: Pin 11,15: Pin shoulder side 12,16: Pin seal surface 13: Male threaded section 20: Box 21,25: Box shoulder side 22,26: Box sealing surface 23: Female thread section 30: Steel pipe 31: Steel pipe body 40: Coupling SP: Seal Point L: Pin lip length θ: Shoulder angle

Claims

1. A threaded joint for connecting steel pipes, A tubular pin is provided continuously on the steel pipe body, A tubular box having an outer diameter of 105% or less of the outer diameter of the steel pipe body, into which the pin is inserted and fastened with the pin, Equipped with, The aforementioned pin is, An annular first pin shoulder surface is provided on the tube end face of the pin and is inclined with respect to a plane perpendicular to the axial direction of the threaded joint such that its outer peripheral edge is located closer to the tip of the pin than its inner peripheral edge, A male threaded portion provided on the outer surface of the aforementioned pin, Between the first pin shoulder surface and the male thread portion, the first pin seal surface is provided on the outer circumferential surface of the pin, Includes, The aforementioned box is An annular first box shoulder surface is provided in the innermost part of the box corresponding to the first pin shoulder surface, and contacts the first pin shoulder surface when the pin and the box are fastened together, A female thread portion is provided on the inner circumferential surface of the box corresponding to the male thread portion, and which engages with the male thread portion in the fastened state, A first box seal surface is provided on the inner circumferential surface of the box corresponding to the first pin seal surface, and having a diameter smaller than the diameter of the first pin seal surface, thereby interfering with and in contact with the first pin seal surface over its entire circumference in the fastened state, Includes, The thickness of the pin at the sealing point, which is the position where the difference between the diameter of the first pin sealing surface and the diameter of the first box sealing surface is maximum, is R ｐｉｎ The thickness of the box is R ｂｏｘ A threaded joint that satisfies the following equations (1), (2), and (3), where the length in the axial direction from the seal point to the tip of the pin is the pin lip length L, and the angle that the first pin shoulder surface makes with a plane perpendicular to the axial direction is the shoulder angle θ. [Math 1]

2. A threaded joint according to claim 1, A threaded joint in which the shoulder angle θ is 30° or less when the pin lip length L is 8.5 mm or more and less than 10.5 mm, 15° or less when the pin lip length L is 10.5 mm or more and less than 13.5 mm, 10° or less when the pin lip length L is 13.5 mm or more and less than 16.5 mm, and 5° or less when the pin lip length L is 16.5 mm or more.

3. A threaded joint according to claim 1, The aforementioned pin further, A second pin shoulder surface is provided at the end of the pin on the steel pipe body side, Between the second pin shoulder surface and the male thread portion, the second pin seal surface is provided on the outer circumferential surface of the pin, Includes, The aforementioned box further, A second box shoulder surface is provided on the pipe end face of the box corresponding to the second pin shoulder surface, and contacts the second pin shoulder surface in the fastened state, A second box seal surface is provided on the inner circumferential surface of the box corresponding to the second pin seal surface, and contacts the second pin seal surface in the fastened state, Threaded fittings, including those mentioned above.

4. A threaded joint according to any one of claims 1 to 3, The aforementioned pins are provided on each of the steel pipes that are connected, The box is a threaded joint provided on a coupling, which is a different pipe material from the steel pipe.

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