Threaded joint for steel pipe

Abstract

A threaded joint is provided with a pin (10) and a box (20). The male buckle (10) and the female buckle (20) which are mutually fastened comprise a threaded part (101), an inner sealing part (102) and an outer sealing part (103). The ratio Lth of the amount of interference deltath of the threaded part (101) with respect to tan (thetath) when the angle of inclination of the threaded part (101) is thetath, the ratio Lint of the amount of interference deltaint of the inner seal part (102) with respect to tan (thetaint) when the angle of inclination of the inner seal part (102) is thetaint, and the ratio Lext of the amount of interference deltaext of the outer seal part (103) with respect to tan (thetaext) when the angle of inclination of the outer seal part (103) is thetaext satisfy the following condition (1) or condition (2). Lth > Lext and Lint is not equal to Lth (1), Lth > Lint and Lext is not equal to Lth (2).

Description

Technical Field

[0001] The present invention relates to a threaded joint for a steel pipe. Background Art

[0002] Oil wells, natural gas wells, and other wells (hereinafter collectively referred to as "oil wells") use steel pipes called oil country tubular goods (OCTG) to extract underground resources. These pipes are connected sequentially, using threaded joints.

[0003] Threaded joints for steel pipes are generally classified into coupling type and integral type. In the case of a coupling type threaded joint, one of a pair of pipes to be connected is a steel pipe, and the other is a coupling. In this case, external threads are formed on the outer periphery of both ends of the steel pipe, and internal threads are formed on the inner periphery of both ends of the coupling. The external threads of the steel pipe are screwed into the internal threads of the coupling, whereby the two are fastened and connected. In the case of an integral type threaded joint, both of the pair of pipes to be connected are steel pipes. In this case, an external thread is formed on the outer periphery of one end of each steel pipe, and an internal thread is formed on the inner periphery of the other end. The external thread of one steel pipe is screwed into the internal thread of the other steel pipe, whereby the two are fastened and connected.

[0004] The end of a pipe with an external thread includes a feature that inserts into the internal thread, and is therefore called a pin. On the other hand, the end of a pipe with an internal thread includes a feature that receives the external thread, and is therefore called a box. These pins and boxes are the ends of pipes and are therefore both tubular.

[0005] Threaded joints for steel pipes utilize tapered threads. Therefore, the male thread of the pin is a tapered male thread. The female thread of the box is a tapered female thread. These two threads mesh together to form the threaded portion. An example of a tapered thread is a trapezoidal thread, exemplified by the API standard's trapezoidal thread. In a trapezoidal thread, the male and female threads each have a thread top surface, a thread bottom surface, a load-bearing flank surface, and a thrust-flank surface.

[0006] Typically, the pin and box each have a shoulder surface. When the pin and box are fastened, the pin is screwed into the box so that the shoulder surface of the pin contacts the shoulder surface of the box. The moment when the shoulder surfaces contact each other is also called shouldering. After the shouldering, the pin is continued to rotate slightly to complete the fastening of the pin and box. As a result, the load-bearing tooth side surfaces of the pin and box strongly contact each other, generating a tightening axial force. In the state where the tightening is completed (hereinafter also referred to as the "tightening state"), the bottom surface of the thread of the external thread part interferes with and contacts the top surface of the thread of the internal thread part. On the other hand, a gap is formed between the top surface of the thread of the external thread part and the bottom surface of the thread of the internal thread part.

[0007] In order to improve the sealing performance of the threaded joint, a sealing surface is formed on the outer periphery of the pin and a sealing surface is formed on the inner periphery of the box. In the tightened state, the sealing surface of the pin interferes with and contacts the sealing surface of the box. The sealing surfaces of the pin and box interfere with each other and contact each other, forming a sealing portion based on metal contact. For example, the sealing portion is arranged at a position closer to the top end of the pin than the threaded portion. Since this sealing portion is close to the inside of the threaded joint, it is also called an inner sealing portion. The inner sealing portion contributes to the sealing performance of the fluid inside the threaded joint. In addition, the sealing portion is sometimes also arranged at a position closer to the side of the pin opposite to the top end than the threaded portion. Since this sealing portion is close to the outside of the threaded joint, it is also called an outer sealing portion. The outer sealing portion contributes to the sealing performance of the fluid outside the threaded joint.

[0008] Here, in the case where the threaded joint has both an inner sealing part and an outer sealing part, the following problems may arise. When the pin and the box are tightened, a lubricant is applied to the pin and / or the box in advance. The lubricant is a compound grease (Japanese: コンパウンドグリス), also known as paint (Japanese: ドープ). In the case where the amount of paint applied is excessive, the paint is not fully discharged during the tightening process, and the remaining paint is sealed between the inner sealing part and the outer sealing part. As the tightening proceeds further, the gap between the outer thread part and the inner thread part between the inner sealing part and the outer sealing part narrows. As a result, the pressure of the paint sealed between the inner sealing part and the outer sealing part increases. The pressure of this paint is also called paint pressure.

[0009] The coating pressure at this point can reach hundreds of MPa. In this situation, the high coating pressure causes elastic deformation of the pin and / or box, reducing the amount of interference between the inner and / or outer seals, potentially degrading sealing performance. In particular, in slim or flush threaded joints, where the difference between the outer diameter of the box and the pin body is small, the risk of reduced sealing performance is particularly high due to the thin wall thickness of the inner and outer seals.

[0010] Various countermeasures have been proposed to address the increase in paint pressure. For example, Japanese Patent Application Publication No. 2000-314489 (Patent Document 1) discloses a threaded joint for steel pipes in which grooves are formed along the pipe axis in the threads of one or both of the external and internal threads. In the threaded joint of Patent Document 1, the paint enclosed between the threaded portion and the sealing portion is discharged through the grooves from the end of the threaded portion in the pipe axis direction where the sealing portion is not provided. This suppresses the increase in paint pressure.

[0011] However, in the threaded joint of Patent Document 1, when both an inner seal and an outer seal are provided, the groove-based paint drainage is ineffective. This is because the seal exists at the destination of the paint discharge through the groove in the threaded portion, trapping the paint within. Therefore, even if the technology of Patent Document 1 is applied to a threaded joint with both an inner seal and an outer seal, the increase in paint pressure cannot be suppressed. Furthermore, in the threaded joint of Patent Document 1, the groove edge appears at the thread ridge. This edge can cause wear during tightening.

[0012] Japanese Patent Application Publication No. 2012-510009 (Patent Document 2) discloses a threaded joint for steel pipes that includes both an inner and outer sealing portion. The shoulder surfaces of the pin and box are located within the threaded portion. In the threaded joint of Patent Document 2, the load flank surfaces of the external and internal threaded portions are angled negatively relative to the pipe axis, and the shoulder surfaces of the pin and box are angled negatively relative to the pipe axis. This prevents the dissipation of torque caused by coating pressure.

[0013] However, in the threaded joint of Patent Document 2, once the paint is sealed between the inner seal portion and the outer seal portion during tightening, it is impossible to eliminate the increase in the paint pressure.

[0014] Prior art literature

[0015] Patent Literature

[0016] Patent Document 1: Japanese Patent Application Laid-Open No. 2000-314489

[0017] Patent Document 2: Japanese Patent Application No. 2012-510009 Summary of the Invention

[0018] Problems to be solved by the invention

[0019] An object of the present invention is to provide a threaded joint for steel pipes that can suppress an increase in coating pressure even when both an inner seal portion and an outer seal portion are provided.

[0020] Solutions for solving problems

[0021] The threaded joint for steel pipes according to the technical solution of the present invention comprises a tubular pin and a tubular box fastened to the pin. The pin has a tapered external thread, a pin inner sealing surface, and a pin outer sealing surface. The pin inner sealing surface is located closer to the pin's tip than the tapered external thread. The pin outer sealing surface is located on the opposite side of the pin's tip than the tapered external thread. The box has a tapered internal thread, a box inner sealing surface, and a box outer sealing surface. The tapered internal thread corresponds to the tapered external thread. The box inner sealing surface corresponds to the pin inner sealing surface. The box outer sealing surface corresponds to the pin outer sealing surface. The fastened pin and box comprise a threaded portion, an inner sealing portion, and an outer sealing portion. The threaded portion comprises a tapered external thread and a tapered internal thread that mesh with each other. The inner sealing portion comprises a pin inner sealing surface and a box inner sealing surface that contact each other. The outer seal portion is composed of a pin outer seal surface and a box outer seal surface that contact each other. In a threaded joint for steel pipes, the ratio Lth of the interference amount δth of the threaded portion to tan(θth) when the threaded portion has an inclination angle of θth, the ratio Lint of the interference amount δint of the inner seal portion to tan(θint) when the inner seal portion has an inclination angle of θint, and the ratio Lext of the interference amount δext of the outer seal portion to tan(θext) when the outer seal portion has an inclination angle of θext satisfy the following conditions (1) or (2).

[0022] Lth>Lext and Lint≠Lth (1)

[0023] Lth>Lint and Lext≠Lth (2)

[0024] Effects of the Invention

[0025] According to the threaded joint for steel pipes of the present invention, even when both the inner seal portion and the outer seal portion are provided, it is possible to suppress an increase in the coating pressure. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a cross-sectional view showing an example of the threaded joint for steel pipes according to the first embodiment.

[0027] Figure 2 It is an enlarged representation Figure 1 sectional view of an example of a threaded portion shown.

[0028] Figure 3 It is an enlarged representation Figure 1 A cross-sectional view of the inner seal is shown.

[0029] Figure 4 It is an enlarged representation Figure 1 A cross-sectional view of the outer seal is shown.

[0030] Figure 5This is a timing chart for explaining the tightening process of the threaded joint for steel pipes according to the first embodiment.

[0031] Figure 6 It is a timing chart for explaining the tightening process of the threaded joint for steel pipes according to the second embodiment.

[0032] Figure 7 This is a timing chart for explaining the tightening process of the threaded joint for steel pipes according to the third embodiment.

[0033] Figure 8 It is a timing chart for explaining the tightening process of the threaded joint for steel pipes according to the fourth embodiment.

[0034] Figure 9 It is a timing chart for explaining the tightening process of the threaded joint for steel pipes according to the fifth embodiment.

[0035] Figure 10 It is a cross-sectional view showing an example of a threaded joint for steel pipes according to a sixth embodiment. DETAILED DESCRIPTION

[0036] Hereinafter, embodiments of the present invention will be described. Furthermore, in the following description, examples are given to illustrate embodiments of the present invention, but the present invention is not limited to the examples described below. In the following description, specific numerical values ​​or specific materials may be exemplified, but the present invention is not limited to these examples.

[0037] To solve the above-mentioned problems, the present inventors conducted a detailed study on the conditions during tightening of a threaded joint provided with both an inner seal portion and an outer seal portion.

[0038] In the tightened state, a predetermined amount of interference is introduced into the threaded portion. During tightening, as the pin is screwed into the box, the interfering surfaces of the male and female threads (e.g., the bottom surface of the male thread and the top surface of the female thread) come into contact. This contact between the threads begins to introduce interference. When the pin is tightened, the threaded interference reaches the predetermined amount.

[0039] Similarly, in the tightened state, a predetermined amount of interference is introduced into the inner seal. During tightening, as the pin is screwed into the box, the inner sealing surfaces of the pin and box come into contact. This contact between the inner seals begins to introduce a predetermined amount of interference. Once the pin is screwed into the tightened state, the predetermined amount of interference is achieved.

[0040] Similarly, in the tightened state, a predetermined amount of interference is introduced into the outer seal. During tightening, as the pin is screwed into the box, the outer sealing surfaces of the pin and box come into contact. This contact of the outer seals begins to introduce a predetermined amount of interference. Once the pin is tightened, the predetermined amount of interference is achieved.

[0041] During tightening, when the threads come into contact, interference begins to be introduced, but paint can still flow. This is because there is a gap between at least the non-interfering surfaces of the external and internal threads (for example, the top surface of the external thread and the bottom surface of the internal thread), and this gap is continuous across the entire area of ​​the threads even in the tightened state. On the other hand, when contact occurs in the inner seal, the flow of paint is blocked as interference begins to be introduced. Similarly, when contact occurs in the outer seal, the flow of paint is blocked as interference begins to be introduced.

[0042] Therefore, the following can be used. If the inner and outer seals come into contact before the threads come into contact, the paint will already be trapped between them when the threads come into contact, causing the paint pressure to rise. This is because the paint trapped between the inner and outer seals cannot be expelled. Then, tightening is performed to introduce interference between the threads, inner and outer seals. This further increases the paint pressure.

[0043] In short, in this case, paint is enclosed between the inner and outer seals in the early stages of the tightening process. As a result, a large amount of paint is enclosed between the inner and outer seals, causing the paint pressure to rise excessively.

[0044] Therefore, in order to suppress the increase in the paint pressure, the amount of paint sealed between the inner seal and the outer seal during tightening can be reduced. Therefore, it is only necessary to promote the discharge of excess paint until the late stage of the tightening process.

[0045] Based on the above-mentioned research results, the present inventors focused on the contact timing of the threaded portion, the inner seal portion, and the outer seal portion during tightening.

[0046] When the threads come into contact last, that is, when the inner seal and outer seal come into contact before the threads come into contact, a large amount of paint is sealed between the inner seal and outer seal as described above.

[0047] In contrast, if the outer seal contacts the final contact, the paint is allowed to flow through the outer seal until the final contact. Therefore, for example, if the inner seal, threaded portion, and outer seal contact in this order, the following phenomenon occurs (condition (A)). The paint is discharged sequentially through the threaded portion and outer seal until the final contact of the outer seal occurs.

[0048] For example, if the threads, inner seal, and outer seal come into contact in this order, the following phenomenon occurs (condition (B)): Paint is discharged through the inner and outer seals, respectively, until the inner seal comes into contact. Furthermore, paint is discharged through the threads and outer seals in sequence, until the outer seal finally comes into contact.

[0049] For example, if contact between the inner and outer seals occurs simultaneously after the threads have made contact, the following phenomenon occurs (condition (C)): the paint is expelled through the inner and outer seals separately until the final contact between the inner and outer seals occurs simultaneously.

[0050] Furthermore, if the inner seal contacts the final contact, the paint is allowed to flow through the inner seal until the final contact. Therefore, for example, if the threads, outer seal, and inner seal contact each other in this order, the following phenomenon occurs (condition (D)). The paint is discharged through the inner and outer seals, respectively, until the outer seal contacts the final contact. Furthermore, the paint is discharged through the threads and inner seals, in that order, until the final contact of the inner seal occurs.

[0051] For example, when the outer seal, threaded portion, and inner seal come into contact in this order, the following phenomenon occurs (condition (E)): the paint is discharged through the threaded portion and inner seal in sequence until the inner seal finally comes into contact.

[0052] Under any of conditions (A) to (E), the amount of paint sealed between the inner seal and the outer seal is relatively small. This is because, after the threaded portion contacts, the paint is discharged until the inner seal and / or the outer seal contact. In other words, this is because the discharge of the remaining paint is promoted until the late stages of the tightening process. Therefore, tightening is then performed to introduce the interference amount of the threaded portion, the inner seal, and the outer seal. As a result, the paint pressure rises for the first time. Therefore, the increase in the paint pressure can be suppressed.

[0053] To satisfy conditions (A) to (E), the threaded portion need not be the last of the threaded portion, inner seal portion, and outer seal portion to contact. Specifically, threaded portion contact only needs to occur before outer seal portion contact (condition (i)). Alternatively, threaded portion contact only needs to occur before inner seal portion contact (condition (ii)).

[0054] In other viewpoints, as long as the contact of the inner seal and / or the outer seal among the threaded portion, the inner seal and the outer seal occurs last. That is, under condition (i), as long as the contact of the inner seal and the contact of the threaded portion do not occur at the same time. In other words, under condition (i), as long as the contact moment of the inner seal is different from the contact moment of the threaded portion. In addition, under condition (ii), as long as the contact of the outer seal and the contact of the threaded portion do not occur at the same time. In other words, under condition (ii), as long as the contact moment of the outer seal is different from the contact moment of the threaded portion. Preferably, under any of conditions (i) and (ii), the contact of the inner seal and the contact of the outer seal do not occur at the same time. In other words, under any of conditions (i) and (ii), as long as the contact moment of the inner seal is different from the contact moment of the outer seal.

[0055] Here, in the threaded portion, the distance that the pin translates relative to the box in the pipe axial direction from the moment of contact until the tightened state is reached is equivalent to the ratio (Lth) of the threaded portion's interference (δth) to tan(θth) when the threaded portion has an inclination angle (θth). This distance is also referred to herein as the threaded portion translation distance (Lth).

[0056] Similarly, in the inner seal, the distance the pin moves in translation relative to the box in the pipe axial direction from the moment of contact until the tightened state is reached is equivalent to the ratio Lint of the inner seal interference amount δint to tan(θint) when the inner seal has an inclination angle of θint. This distance is also referred to herein as the inner seal translation distance Lint.

[0057] Similarly, in the outer seal, the distance the pin moves relative to the box in the pipe axial direction from the moment of contact until the tightened state is reached is equivalent to the ratio Lext of the outer seal interference amount δext to tan(θext) when the outer seal has an inclination angle of θext. This distance is also referred to herein as the outer seal translation distance Lext.

[0058] The endpoints of the threaded portion translation distance Lth, the inner seal portion translation distance Lint, and the outer seal portion translation distance Lext are each at the point in time when the tightening state is achieved. In other words, these endpoints are common. Meanwhile, the starting points of the threaded portion translation distance Lth, the inner seal portion translation distance Lint, and the outer seal portion translation distance Lext are each at the point in time when contact occurs.

[0059] Therefore, the thread translation distance Lth, the inner seal translation distance Lint, and the outer seal translation distance Lext each serve as an indicator of the contact moments of the thread, inner seal, and outer seal, respectively. Specifically, the greater the thread translation distance Lth, the earlier the thread contact moment. Similarly, the greater the inner seal translation distance Lint, the earlier the inner seal contact moment. Similarly, the greater the outer seal translation distance Lext, the earlier the outer seal contact moment.

[0060] For example, if the threaded portion translation distance Lth is greater than the inner seal portion translation distance Lint, contact of the threaded portion occurs earlier than contact of the inner seal portion. If the threaded portion translation distance Lth is greater than the outer seal portion translation distance Lext, contact of the threaded portion occurs earlier than contact of the outer seal portion. If the inner seal portion translation distance Lint is greater than the outer seal portion translation distance Lext, contact of the inner seal portion occurs earlier than contact of the outer seal portion. If the outer seal portion translation distance Lext is greater than the inner seal portion translation distance Lint, contact of the outer seal portion occurs earlier than contact of the inner seal portion. If the inner seal portion translation distance Lint is the same as the outer seal portion translation distance Lext, contact of the inner seal portion and contact of the outer seal portion occur simultaneously.

[0061] From the above, it can be seen that the conditions that can suppress the increase in coating pressure are as follows.

[0062] Under the above condition (i) (thread contact occurs before outer seal contact), inner seal contact and thread contact do not occur simultaneously. This condition can be expressed by the following condition (1) using the above Lth, Lint, and Lext.

[0063] Lth>Lext and Lint≠Lth (1)

[0064] Under the above condition (ii) (thread contact occurs before inner seal contact), outer seal contact and thread contact do not occur simultaneously. This condition can be expressed by the following condition (2) using the above Lth, Lint, and Lext.

[0065] Lth>Lint and Lext≠Lth (2)

[0066] Condition (1) includes the above-mentioned condition (A) (the inner seal portion, the threaded portion, and the outer seal portion come into contact in this order). This condition (A) can be expressed by the following condition (a) using the above-mentioned Lth, Lint, and Lext.

[0067] Lint>Lth>Lext (a)

[0068] Condition (1) includes the above-mentioned condition (B) (the threaded portion, the inner seal portion, and the outer seal portion come into contact in this order). This condition (B) can be expressed by the following condition (b) using the above-mentioned Lth, Lint, and Lext.

[0069] Lth>Lint>Lext (b)

[0070] Condition (1) includes the above-mentioned condition (C) (after the threaded portion contacts, the inner seal portion contacts and the outer seal portion contacts simultaneously). This condition (C) can be expressed by the following condition (c) using the above-mentioned Lth, Lint, and Lext.

[0071] Lth>Lint=Lext (c)

[0072] Condition (1) includes the above-mentioned condition (D) (the threaded portion, the outer seal portion, and the inner seal portion come into contact in this order). This condition (D) can be expressed by the following condition (d) using the above-mentioned Lth, Lint, and Lext.

[0073] Lth>Lext>Lint (d)

[0074] Condition (2) includes the above-mentioned condition (E) (the outer seal portion, the threaded portion, and the inner seal portion come into contact in this order). This condition (E) can be expressed by the following condition (e) using the above-mentioned Lth, Lint, and Lext.

[0075] Lext>Lth>Lint (e)

[0076] Furthermore, condition (2) also includes the above-mentioned conditions (B), (C), and (D). Therefore, condition (2) also includes the above-mentioned conditions (b), (c), and (d).

[0077] The present invention has been completed based on the above findings.

[0078] A threaded joint for steel pipes according to an embodiment of the present invention comprises a tubular pin and a tubular box fastened to the pin. The pin has a tapered external thread, a pin inner sealing surface, and a pin outer sealing surface. The pin inner sealing surface is located closer to the pin's tip than the tapered external thread. The pin outer sealing surface is located on the opposite side of the pin's tip than the tapered external thread. The box has a tapered internal thread, a box inner sealing surface, and a box outer sealing surface. The tapered internal thread corresponds to the tapered external thread. The box inner sealing surface corresponds to the pin inner sealing surface. The box outer sealing surface corresponds to the pin outer sealing surface. The fastened pin and box include a threaded portion, an inner sealing portion, and an outer sealing portion. The threaded portion comprises a tapered external thread and a tapered internal thread that mesh with each other. The inner sealing portion comprises the pin inner sealing surface and the box inner sealing surface that contact each other. The outer sealing portion comprises the pin outer sealing surface and the box outer sealing surface that contact each other. The ratio Lth of the interference amount δth of the threaded portion to tan(θth) when the inclination angle of the threaded portion is θth, the ratio Lint of the interference amount δint of the inner seal portion to tan(θint) when the inclination angle of the inner seal portion is θint, and the ratio Lext of the interference amount δext of the outer seal portion to tan(θext) when the inclination angle of the outer seal portion is θext satisfy the following condition (1) or condition (2).

[0079] Lth>Lext and Lint≠Lth (1)

[0080] Lth>Lint and Lext≠Lth (2)

[0081] In the case of condition (1), contact of the threaded portion occurs before contact of the outer sealing portion occurs, and contact of the inner sealing portion and contact of the threaded portion do not occur simultaneously. In the case of condition (2), contact of the threaded portion occurs before contact of the inner sealing portion, and contact of the outer sealing portion and contact of the threaded portion do not occur simultaneously. In any of the cases of condition (1) and condition (2), contact of the threaded portion among the threaded portion, the inner sealing portion, and the outer sealing portion does not occur last. Therefore, the discharge of the remaining paint is promoted until the late stage of the tightening process. Therefore, the threaded joint for steel pipes of this embodiment can suppress the increase in paint pressure even when both the inner sealing portion and the outer sealing portion are provided.

[0082] In a typical example, under condition (1), Lint is larger than Lth. In this case, condition (1) is rewritten as the following condition (a).

[0083] Lint>Lth>Lext (a)

[0084] Under condition (a), contact occurs in the order of the inner seal, threaded portion, and outer seal. This also promotes the discharge of excess paint until the late stages of the tightening process. Furthermore, in this case, the paint is discharged sequentially through the threaded portion and outer seal until the outer seal finally contacts. In other words, the paint is continuously discharged to the exterior of the threaded joint. In other words, the paint does not discharge into the interior of the threaded joint. Consequently, contamination of the interior of the threaded joint by the paint can be prevented.

[0085] In another typical example, under condition (1), Lint is smaller than Lth. In this case, condition (1) is rewritten as any one of the following conditions (b), (c), and (d).

[0086] Lth>Lint>Lext (b)

[0087] Lth>Lint=Lext (c)

[0088] Lth>Lext>Lint (d)

[0089] Under condition (b), contact occurs in the order of the threads, inner seal, and outer seal. This also promotes the removal of excess paint until the late stages of the tightening process. Furthermore, in this case, paint is removed through the inner and outer seals separately until the inner seal contacts. Furthermore, paint is removed sequentially through the threads and outer seal until the outer seal finally contacts.

[0090] Under condition (c), after the threads come into contact, the inner and outer seals come into contact simultaneously. This also facilitates the removal of excess paint until the final stages of the tightening process. Furthermore, in this case, paint is removed separately through the inner and outer seals until they finally come into contact.

[0091] Under condition (d), contact occurs in the order of the threads, outer seal, and inner seal. This also promotes the removal of excess paint until the late stages of the tightening process. Furthermore, in this case, paint is removed through the inner and outer seals separately until the outer seal contacts. Furthermore, paint is removed sequentially through the threads and inner seal until the inner seal finally contacts.

[0092] In another typical example, under condition (2), Lext is greater than Lth. In this case, condition (2) is rewritten as the following condition (e).

[0093] Lext>Lth>Lint (e)

[0094] In the case of condition (2), if Lext is smaller than Lth, condition (2) is rewritten as any one of the above-mentioned conditions (b), (c), and (d).

[0095] Under condition (e), contact occurs in the order of the outer seal, threaded portion, and inner seal. This also facilitates the discharge of excess paint until the late stages of the tightening process. Furthermore, in this case, the paint is discharged sequentially through the threaded portion and inner seal until the final inner seal contacts. In other words, the paint continuously discharges into the interior of the threaded joint. In other words, the paint does not discharge to the exterior of the threaded joint. Consequently, contamination of the exterior of the threaded joint by the paint can be prevented.

[0096] The threaded joint for steel pipes may also have the following structure. The pin has a second tapered external threaded portion. The second tapered external threaded portion is positioned closer to the pin's outer sealing surface than to the pin's tip. The box has a second tapered internal threaded portion. The second tapered internal threaded portion corresponds to the second tapered external threaded portion. The pin and box, which are fastened to each other, include a second threaded portion. The second threaded portion is composed of a second tapered external threaded portion and a second tapered internal threaded portion that mesh with each other.

[0097] Hereinafter, a specific example of the threaded joint for steel pipes according to the present embodiment will be described with reference to the drawings.

[0098] [First embodiment]

[0099] Figure 1 It is a cross-sectional view showing an example of the threaded joint for steel pipes according to the first embodiment. Figure 2 It is an enlarged representation Figure 1 1 is a cross-sectional view of an example of the threaded portion 101 shown. Figure 3 It is an enlarged representation Figure 1 A cross-sectional view of the inner seal 102 is shown. Figure 4 It is an enlarged representation Figure 1 A cross-sectional view of the outer seal portion 103 is shown. Figures 1 to 4 In the figure, the hollow arrow indicates the screwing direction of the male buckle 10 relative to the female buckle 20. Figures 1 to 4 A longitudinal section including the pipe axis CL of the threaded joint is shown in FIG. Figures 2 to 4 In the figure, the tightening state, i.e. the state when the tightening is completed, is shown. Figures 2 to 4 In FIG, the shape of the interfering portion indicates the designed shape before the interference.

[0100] Figure 1 The threaded connection shown is a coupling type threaded connection. Figures 1 to 4 , the threaded joint is composed of a pin buckle 10 and a box buckle 20. The pin buckle 10 and the box buckle 20 are tubular.

[0101] The pin 10 includes an external thread portion 11, a pin inner sealing surface 12, a pin outer sealing surface 13, and a pin shoulder surface 14. The external thread portion 11, the pin inner sealing surface 12, and the pin outer sealing surface 13 are arranged on the outer circumference of the pin 10. The external thread portion 11 is a tapered external thread portion. The pin shoulder surface 14 is arranged at the top of the pin 10. The pin inner sealing surface 12 is located closer to the top of the pin 10 than the external thread portion 11. In other words, the pin inner sealing surface 12 is located between the external thread portion 11 and the pin shoulder surface 14. The pin outer sealing surface 13 is located closer to the side of the pin 10 opposite the top than the external thread portion 11. In short, the pin inner sealing surface 12, the external thread portion 11, and the pin outer sealing surface 13 are arranged in sequence from the top of the pin 10 toward its pipe body. In this embodiment, the pin inner sealing surface 12 is located near the top of the pin 10.

[0102] The box 20 includes an internal thread portion 21, a box inner sealing surface 22, a box outer sealing surface 23, and a box shoulder surface 24. The internal thread portion 21, the box inner sealing surface 22, and the box outer sealing surface 23 are arranged on the inner circumference of the box 20. The internal thread portion 21 is a tapered internal thread portion. It is positioned corresponding to the external thread portion 11 and can mesh with the external thread portion 11. The box shoulder surface 24 is positioned corresponding to the pin shoulder surface 14. The box shoulder surface 24 can contact the pin shoulder surface 14. The box inner sealing surface 22 is positioned corresponding to the pin inner sealing surface 12. The box inner sealing surface 22 can contact the pin inner sealing surface 12. The box outer sealing surface 23 is positioned corresponding to the pin outer sealing surface 13. The box outer sealing surface 23 can contact the pin outer sealing surface 13. In short, the box inner sealing surface 22, the internal thread portion 21 and the box outer sealing surface 23 are arranged in this order from the tube body toward the tip of the box 20. In this embodiment, the box outer sealing surface 23 is arranged near the tip of the box 20.

[0103] The external threaded portion 11 and the internal threaded portion 21 mesh with each other to form the threaded portion 101. The internal sealing surface 12 of the pin and the internal sealing surface 22 of the box contact each other to form the internal sealing portion 102. The external sealing surface 13 of the pin and the external sealing surface 23 of the box contact each other to form the external sealing portion 103. In summary, the internal sealing portion 102, the threaded portion 101, and the external sealing portion 103 are arranged in sequence from the top of the pin 10 toward its tube body. Since the internal sealing portion 102 is closer to the interior of the threaded joint than the external sealing portion 103, it contributes to the sealing performance against the fluid inside the threaded joint. In addition, since the external sealing portion 103 is closer to the outside of the threaded joint than the internal sealing portion 102, it contributes to the sealing performance against the fluid outside the threaded joint.

[0104] In the tightened state, the pin shoulder surface 14 and the box shoulder surface 24 are in strong contact with each other. In a longitudinal cross-section of the threaded joint, the pin shoulder surface 14 is formed by a straight line. Similarly, the box shoulder surface 24 is formed by a straight line. The pin shoulder surface 14 slopes in the direction in which the pin 10 is screwed into the box 20. The box shoulder surface 24 is inclined in a manner corresponding to the pin shoulder surface 14. However, the pin shoulder surface 14 does not need to slope, and it can also slope in the direction opposite to the direction in which the pin 10 is screwed into the box 20.

[0105] The external thread portion 11 of the pin 10 includes a top surface 11a, a bottom surface 11b, a pin load flank surface 11c, and a pin insertion flank surface 11d. Meanwhile, the internal thread portion 21 of the box 20 includes a top surface 21a, a bottom surface 21b, a box load flank surface 21c, and a box insertion flank surface 21d. Figure 2 The threaded portion 101 shown adopts a trapezoidal thread represented by, for example, a partial trapezoidal thread of the API standard.

[0106] Reference Figure 2 In the tightened state, a gap is formed between the male thread top surface 11a and the female thread bottom surface 21b. In a longitudinal cross-section of the threaded joint, the male thread top surface 11a lies on a straight line across the entire area of ​​the male thread portion 11 in the direction of the pipe axis CL. In other words, the male thread top surface 11a has a tapered shape. Similarly, the female thread bottom surface 21b also lies on the same straight line and has a tapered shape.

[0107] In the tightened state, the male thread bottom surface 11b interferes with and contacts the female thread top surface 21a. In a longitudinal cross-section of the threaded joint, the male thread bottom surface 11b lies on a straight line across the entire area of ​​the male thread portion 11 in the direction of the pipe axis CL. In other words, the male thread bottom surface 11b has a tapered shape. Similarly, the female thread top surface 21a also lies on a straight line and has a tapered shape.

[0108] In the tightened state, the pin load flank 11c and the box load flank 21c are in strong contact with each other. In a longitudinal cross-section of the threaded joint, the pin load flank 11c is formed by a straight line. Similarly, the box load flank 21c is formed by a straight line. The pin load flank 11c tilts in a direction opposite to the direction of threading of the pin 10 into the box 20. In other viewpoints, the pin load flank 11c is inclined in a hook-like shape. The box load flank 21c is inclined in a manner corresponding to the pin load flank 11c. In this case, the flank angle between the pin load flank 11c and the box load flank 21c is negative.

[0109] In the tightened state, a gap is formed between the pin insertion flank 11d and the box insertion flank 21d. In a longitudinal cross-section of the threaded joint, the pin insertion flank 11d is formed by a straight line. Similarly, the box insertion thread 21d is formed by a straight line. The pin insertion thread 11d slopes in a direction opposite to the direction of threading of the pin 10 into the box 20. The box insertion flank 21d is inclined in a manner corresponding to the pin insertion flank 11d. In this case, the flank angles of the pin insertion flank 11d and the box insertion flank 21d are positive.

[0110] The inclination angle θth of the threaded portion 101 can be calculated as follows. In a longitudinal cross-section of the threaded joint, a straight line Lt is used to connect the intersection point CP1 of the extended line of the male thread top surface 11a of any pin thread and the extended line of its pin insertion flank surface 11d, and the intersection point CP2 of the extended line of the male thread top surface 11a of the adjacent thread and the extended line of its pin insertion flank surface 11d. The angle formed by this straight line Lt and the pipe axis CL is the inclination angle θth of the threaded portion 101.

[0111] The amount of interference δth of the threaded portion 101 can be calculated by the following formula (3).

[0112] δth=(Dp-Db) / 2 (3)

[0113] The meanings of the symbols in formula (3) are as follows:

[0114] Dp: The diameter of the male thread bottom surface 11b before interference at any position in the pipe axis CL direction

[0115] Db: The designed diameter of the internal thread top surface 21a before interference at this position.

[0116] In addition, Figure 2 The figure shows a case where the external thread top surface 11a is aligned with the entire area of ​​the external thread portion 11 in the direction of the pipe axis CL and has a tapered shape. However, there are also cases where the external thread top surface 11a has a stepped shape along the pipe axis CL of the external thread portion 11. In this case, the external thread top surface 11a is parallel to the pipe axis CL. In this case, the inclination angle θth and the interference amount δth of the thread portion 101 can also be calculated in the same way as above.

[0117] When the external thread top surface 11a is stepped, the external thread bottom surface 11b is also stepped. Similarly, the internal thread top surface 21a and the internal thread bottom surface 21b are also stepped.

[0118] Reference Figure 3In the tightened state, the pin inner sealing surface 12 and the box inner sealing surface 22 interfere with and contact each other. In the longitudinal cross-section of the threaded joint, the pin inner sealing surface 12 is formed by a convex curve. Similarly, the box inner sealing surface 22 is formed by a convex curve. The overlapping portion of these curves is the interference area between the pin inner sealing surface 12 and the box inner sealing surface 22. The curve is a circular arc. However, the curve can also be a curve other than a circular arc (for example, an elliptical arc) or a curve combined with straight lines.

[0119] The inclination angle θint of the inner seal portion 102 can be calculated as follows. In a longitudinal cross-section of the threaded joint, the curved lines of the pin inner sealing surface 12 and the box inner sealing surface 22 intersect at two points, Pi1 and Pi2. A straight line Li connects these two intersection points, Pi1 and Pi2. The angle formed by this straight line Li and the pipe axis CL is the inclination angle θint of the inner seal portion 102.

[0120] The interference amount δint of the inner seal portion 102 can be calculated by the following formula (4).

[0121] δint=(Dip-Dib) / 2 (4)

[0122] The meanings of the symbols in formula (4) are as follows:

[0123] Dip: The diameter of the designed inner sealing surface 12 of the male buckle before interference at the midpoint Pi3 of the straight line Li

[0124] Dib: The diameter of the designed box inner sealing surface 22 before interference at the midpoint Pi3 of the straight line Li.

[0125] Alternatively, in a longitudinal cross-section of the threaded joint, one of the pin inner sealing surface 12 and the box inner sealing surface 22 may be formed by a straight line. In this case, the straight line intersects the other curved line at two points, thus creating a straight line Li. This allows calculation of the inclination angle θint and the interference amount δint of the inner seal portion 102.

[0126] Reference Figure 4 In the tightened state, the pin outer sealing surface 13 and the box outer sealing surface 23 interfere with and contact each other. In the longitudinal cross-section of the threaded joint, the pin outer sealing surface 13 is formed by a straight line. The box outer sealing surface 23 is formed by a convex curve. The overlapping portion of the straight line and the curve is the interference area between the pin outer sealing surface 13 and the box outer sealing surface 23. The curve is a circular arc. However, the curve can also be a curve other than a circular arc (for example, an elliptical arc) or a curve combined with a straight line.

[0127] The inclination angle θext of the outer seal portion 103 can be calculated as follows. In a longitudinal cross-section of the threaded joint, the straight line of the pin outer sealing surface 13 and the curved line of the box outer sealing surface 23 intersect at two points, Pe1 and Pe2. A straight line Le connects these two intersection points, Pe1 and Pe2. The angle formed by this straight line Le and the pipe axis CL is the inclination angle θext of the outer seal portion 103.

[0128] The interference amount δext of the outer seal portion 103 can be calculated by the following formula (5).

[0129] δext=(Dep-Deb) / 2 (5)

[0130] The meanings of the symbols in formula (5) are as follows:

[0131] Dep: The diameter of the pin outer sealing surface 13 before interference as designed at the midpoint Pe3 of the straight line Le. Deb: The diameter of the box outer sealing surface 23 before interference as designed at the midpoint Pe3 of the straight line Le.

[0132] Furthermore, in a longitudinal cross-section of the threaded joint, the pin outer sealing surface 13 may be formed by a convex curve, while the box outer sealing surface 23 may be formed by a straight line. Alternatively, both the pin outer sealing surface 13 and the box outer sealing surface 23 may be formed by convex curves. In this case, the two curves intersect at two points, thus describing a straight line Le. This allows calculation of the inclination angle θext and the interference amount δext of the outer seal portion 103.

[0133] In this embodiment, the ratio Lth of the threaded portion interference amount δth to the tan value (θth) when the threaded portion 101 is tilted at an angle of θth, the ratio Lint of the inner seal portion interference amount δint to the tan value (θint) when the inner seal portion is tilted at an angle of θint, and the ratio Lext of the outer seal portion interference amount δext to the tan value (θext) when the outer seal portion is tilted at an angle of θext satisfy the above-mentioned condition (a) (Lint>Lth>Lext). In other words, the tilt angle θth, the interference amount δth, the tilt angle θint, the interference amount δint, the tilt angle θext, and the interference amount δext are designed to satisfy the above-mentioned condition (a).

[0134] The ratio Lth is the threaded portion translation distance Lth. The ratio Lint is the inner seal portion translation distance Lint. The ratio Lext is the outer seal portion translation distance Lext.

[0135] Figure 5 This is a timing chart for explaining the tightening process of the threaded joint for steel pipes according to the first embodiment. Figure 5The contact moment and completion moment (the moment when the tightening state is reached) of the threaded portion, the inner seal portion and the outer seal portion are shown in FIG. Figure 5 Threaded portion translation distance Lth, inner sealing portion translation distance Lint, and outer sealing portion translation distance Lext are shown in FIG.

[0136] When fastening the pin 10 and the box 20, by screwing the pin 10 into the box 20, the pin inner sealing surface 12 and the box inner sealing surface 22 first come into contact. At this time, the interference amount of the inner seal 102 begins to be introduced. Next, the outer thread bottom surface 11b comes into contact with the inner thread top surface 21a. At this time, the interference amount of the threaded portion 101 begins to be introduced. Finally, the pin outer sealing surface 13 comes into contact with the box outer sealing surface 23. At this time, the interference amount of the outer seal 103 begins to be introduced. By further screwing the pin 10, the interference amount of the threaded portion 101, the inner seal 102, and the outer seal 103 continue to be introduced.

[0137] Then, the pin shoulder surface 14 and the box shoulder surface 24 come into contact and abut. After abutting, the pin 10 is further rotated slightly, completing the fastening of the pin 10 and the box 20. This results in the respective interference amounts δth, δint, and δext among the threaded portion 101, the inner seal portion 102, and the outer seal portion 103.

[0138] During this tightening process, since the threaded portion translation distance Lth, the inner seal portion translation distance Lint, and the outer seal portion translation distance Lext satisfy condition (a), contact occurs in the order of the inner seal portion 102, the threaded portion 101, and the outer seal portion 103. That is, of the threaded portion 101, the inner seal portion 102, and the outer seal portion 103, the threaded portion 101 does not come into contact last. Consequently, the removal of excess paint is facilitated until the late stages of the tightening process. Consequently, the increase in paint pressure can be suppressed.

[0139] Furthermore, in this case, the paint is discharged sequentially through the threaded portion 101 and the outer seal portion 103 until the outer seal portion 103 finally contacts. In other words, the paint is continuously discharged to the exterior of the threaded joint. In other words, the paint does not discharge into the interior of the threaded joint. Therefore, contamination of the interior of the threaded joint by the paint can be prevented.

[0140] [Second embodiment]

[0141] Figure 6 This is a timing chart for explaining the tightening process of the threaded joint for steel pipes according to the second embodiment. Figure 6 Shown in Figure 5The second embodiment of the threaded joint is a modification of the first embodiment. Hereinafter, descriptions of the structure of the threaded joint that overlaps with the first embodiment will be omitted. This also applies to the following embodiments.

[0142] In this embodiment, the threaded portion translation distance Lth, the inner seal portion translation distance Lint, and the outer seal portion translation distance Lext satisfy the aforementioned condition (b) (Lth>Lint>Lext). In other words, the tilt angle θth, the interference amount δth, the tilt angle θint, the interference amount δint, the tilt angle θext, and the interference amount δext are designed to satisfy the aforementioned condition (b).

[0143] When tightening the pin 10 to the box 20, as the pin 10 is screwed into the box 20, the bottom surface 11b of the external thread first contacts the top surface 21a of the internal thread. At this point, the interference of the threaded portion 101 begins. Next, the pin's inner sealing surface 12 contacts the box's inner sealing surface 22. At this point, the interference of the inner seal 102 begins. Finally, the pin's outer sealing surface 13 contacts the box's outer sealing surface 23. At this point, the interference of the outer seal 103 begins. Further screwing of the pin 10 continues, further introducing the interference of the threaded portion 101, the inner seal 102, and the outer seal 103. Then, after the pin 10 abuts against the shoulder, the pin 10 is rotated slightly to complete the tightening of the pin 10 and the box 20. As a result, the interference of the threaded portion 101, the inner seal 102, and the outer seal 103 each reach the predetermined interference amounts δth, δint, and δext.

[0144] During this tightening process, since the threaded portion translation distance Lth, the inner seal portion translation distance Lint, and the outer seal portion translation distance Lext satisfy condition (b), contact occurs in the order of the threaded portion 101, the inner seal portion 102, and the outer seal portion 103. That is, the threaded portion 101 does not come into contact last among the threaded portion 101, the inner seal portion 102, and the outer seal portion 103. Consequently, the removal of excess paint is facilitated until the late stages of the tightening process. Consequently, the increase in paint pressure can be suppressed.

[0145] In this case, the paint is discharged through the inner seal 102 and the outer seal 103 until the inner seal 102 contacts. The paint is then discharged through the threaded portion 101 and the outer seal 103 in sequence until the outer seal 103 contacts.

[0146] [Third embodiment]

[0147] Figure 7 This is a timing chart for explaining the tightening process of the threaded joint for steel pipes according to the third embodiment.

[0148] In this embodiment, the threaded portion translation distance Lth, the inner seal portion translation distance Lint, and the outer seal portion translation distance Lext satisfy the aforementioned condition (c) (Lth>Lint=Lext). In other words, the tilt angle θth, the interference amount δth, the tilt angle θint, the interference amount δint, the tilt angle θext, and the interference amount δext are designed to satisfy the aforementioned condition (c).

[0149] When tightening the pin 10 to the box 20, the pin 10 is screwed into the box 20, first causing the male thread bottom surface 11b to contact the female thread top surface 21a. At this point, the interference of the threaded portion 101 begins. Next, the pin's inner sealing surface 12 contacts the box's inner sealing surface 22. At this point, the interference of the inner seal 102 begins. Simultaneously, the pin's outer sealing surface 13 contacts the box's outer sealing surface 23. At this point, the interference of the outer seal 103 begins. Further screwing of the pin 10 continues, further introducing the interference of the threaded portion 101, the inner seal 102, and the outer seal 103. Then, after the pin 10 abuts, the pin 10 is rotated slightly to complete the tightening of the pin 10 and the box 20. As a result, the interference of the threaded portion 101, the inner seal 102, and the outer seal 103 each reaches the predetermined interference amounts δth, δint, and δext.

[0150] During this tightening process, the threaded portion translation distance Lth, the inner seal portion translation distance Lint, and the outer seal portion translation distance Lext satisfy condition (c). Therefore, after the threaded portion 101 contacts, the inner seal 102 and the outer seal 103 simultaneously contact each other. In other words, the threaded portion 101 does not contact the last of the three components, the inner seal 102, and the outer seal 103. Consequently, the removal of excess paint is facilitated until the late stages of the tightening process. Consequently, the increase in paint pressure can be suppressed.

[0151] Furthermore, in this case, the coating material is discharged through the inner seal 102 and the outer seal 103 respectively until the final contact between the inner seal 102 and the outer seal 103 occurs.

[0152] [Fourth embodiment]

[0153] Figure 8 It is a timing chart for explaining the tightening process of the threaded joint for steel pipes according to the fourth embodiment.

[0154] In this embodiment, the threaded portion translation distance Lth, the inner seal portion translation distance Lint, and the outer seal portion translation distance Lext satisfy the aforementioned condition (d): Lth>Lext>Lint. In other words, the tilt angle θth, the interference amount δth, the tilt angle θint, the interference amount δint, the tilt angle θext, and the interference amount δext are designed to satisfy the aforementioned condition (d).

[0155] When tightening the pin 10 to the box 20, as the pin 10 is screwed into the box 20, the bottom surface 11b of the external thread first contacts the top surface 21a of the internal thread. At this point, the interference of the threaded portion 101 begins. Next, the pin's outer sealing surface 13 contacts the box's outer sealing surface 23. At this point, the interference of the outer seal 103 begins. Finally, the pin's inner sealing surface 12 contacts the box's inner sealing surface 22. At this point, the interference of the inner seal 102 begins. Further screwing of the pin 10 continues, further introducing the interference of the threaded portion 101, the inner seal 102, and the outer seal 103. Then, after the pin 10 abuts against the shoulder, the pin 10 is rotated slightly to complete the tightening of the pin 10 and the box 20. As a result, the interference of the threaded portion 101, the inner seal 102, and the outer seal 103 each reach the predetermined interference amounts δth, δint, and δext.

[0156] During this tightening process, since the threaded portion translation distance Lth, the inner seal portion translation distance Lint, and the outer seal portion translation distance Lext satisfy condition (d), contact occurs in the order of the threaded portion 101, the outer seal portion 103, and the inner seal portion 102. That is, the threaded portion 101 does not come into contact last among the threaded portion 101, the inner seal portion 102, and the outer seal portion 103. Consequently, the removal of excess paint is facilitated until the late stages of the tightening process. Consequently, the increase in paint pressure can be suppressed.

[0157] In this case, the paint is discharged through the inner seal 102 and the outer seal 103 until the outer seal 103 comes into contact. Furthermore, the paint is discharged through the threaded portion 101 and the inner seal 102 in sequence until the inner seal 102 comes into contact.

[0158] [Fifth embodiment]

[0159] Figure 9 It is a timing chart for explaining the tightening process of the threaded joint for steel pipes according to the fifth embodiment.

[0160] In this embodiment, the threaded portion translation distance Lth, the inner seal portion translation distance Lint, and the outer seal portion translation distance Lext satisfy the aforementioned condition (e) (Lext>Lth>Lint). In other words, the tilt angle θth, the interference amount δth, the tilt angle θint, the interference amount δint, the tilt angle θext, and the interference amount δext are designed to satisfy the aforementioned condition (e).

[0161] When tightening the pin 10 and the box 20, as the pin 10 is screwed into the box 20, the pin outer sealing surface 13 first contacts the box outer sealing surface 23. At this point, the interference of the outer seal 103 begins to be introduced. Next, the male thread bottom surface 11b contacts the female thread top surface 21a. At this point, the interference of the threaded portion 101 begins to be introduced. Finally, the pin inner sealing surface 12 contacts the box inner sealing surface 22. At this point, the interference of the inner seal 102 begins to be introduced. By further screwing the pin 10, the interference of the threaded portion 101, the inner seal 102, and the outer seal 103 continues to be introduced. Then, after the pin 10 abuts, the pin 10 is rotated slightly to complete the tightening of the pin 10 and the box 20. As a result, the interference of the threaded portion 101, the inner seal 102, and the outer seal 103 each reaches the predetermined interference amounts δth, δint, and δext.

[0162] During this tightening process, since the threaded portion translation distance Lth, the inner seal portion translation distance Lint, and the outer seal portion translation distance Lext satisfy condition (e), contact occurs in the order of the outer seal portion 103, the threaded portion 101, and the inner seal portion 102. That is, of the threaded portion 101, the inner seal portion 102, and the outer seal portion 103, the threaded portion 101 does not come into contact last. Consequently, the removal of excess paint is facilitated until the late stages of the tightening process. Consequently, the increase in paint pressure can be suppressed.

[0163] Furthermore, in this case, the paint is discharged sequentially through the threaded portion 101 and the inner seal portion 102 until the inner seal portion 102 finally contacts the threaded portion 102. In other words, the paint is continuously discharged into the interior of the threaded joint. In other words, the paint is not discharged to the exterior of the threaded joint. Therefore, contamination of the exterior of the threaded joint by the paint can be prevented.

[0164] [Sixth embodiment]

[0165] Figure 10 1 is a cross-sectional view showing an example of a threaded joint for steel pipes according to a sixth embodiment. The threaded joint according to the sixth embodiment differs from the threaded joint according to the first embodiment in that it includes a second threaded portion 101A independent of the threaded portion 101 .

[0166] Specifically, Figure 10The threaded joint of the present embodiment shown is similar to the first embodiment, and includes an inner sealing portion 102, a threaded portion 101, and an outer sealing portion 103 in sequence from the top of the pin 10 toward its pipe body. The threaded joint of the present embodiment also includes a second threaded portion 101A that is independent of the threaded portion 101. That is, the threaded joint of the present embodiment includes two threaded portions. The threaded portion 101 is independent of the second threaded portion 101A and can therefore be referred to as the first threaded portion 101. The second threaded portion 101A is arranged at a position on the side of the pin 10 opposite to the top end relative to the outer sealing portion 103. In short, the inner sealing portion 102, the threaded portion 101, the outer sealing portion 103, and the second threaded portion 101A are arranged in sequence from the top end of the pin 10 toward its pipe body.

[0167] The outer seal portion 103 is closer to the outside of the threaded joint than the inner seal portion 102. Therefore, the outer seal portion 103 contributes to the sealing performance against the fluid outside the threaded joint. Because the outer seal portion 103 exists between the first threaded portion 101 and the second threaded portion 101A, it can be called an intermediate seal portion.

[0168] The second threaded portion 101A has the same structure as the threaded portion 101 (first threaded portion 101) of the first embodiment. Specifically, the second threaded portion 101A is composed of a second external threaded portion 11A and a second internal threaded portion 21A. The pin 10 has a second external threaded portion 11A, and the box 20 has a second internal threaded portion 21A. The second external threaded portion 11A is a tapered external threaded portion, which is provided on the outer periphery of the pin 10. The second internal threaded portion 21A is a tapered internal threaded portion, which is provided on the inner periphery of the box 20. The second internal threaded portion 21A is arranged at a position corresponding to the second external threaded portion 11A. The second external threaded portion 11A and the second internal threaded portion 21A are engaged with each other to form the second threaded portion 101A.

[0169] The taper of the second threaded portion 101A is, for example, the same size as the taper of the first threaded portion 101. However, the taper of the second threaded portion 101A may be larger or smaller than the taper of the first threaded portion 101. A taper refers to a shape with an inclination angle on both sides relative to the pipe axis CL. Therefore, in the first threaded portion 101, the value converted into an angle of half the taper corresponds to the inclination angle θth.

[0170] Even with a threaded joint having such a structure, the same effects as those of the first embodiment can be obtained. The second threaded portion 101A in the sixth embodiment can also be applied to the threaded joints of the second to fifth embodiments.

[0171] The present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention. For example, the threaded joint may be a coupling type or an integral type.

[0172] Alternatively, the threaded portion can adopt a dovetail thread. In this case, the flank angles of the pin and box inserts are negative. In this case, when tightened, the pin and box inserts are in strong contact with each other. In other words, the external and internal threads engage firmly in a wedge-like manner. The moment when the external and internal threads engage in a strong wedge-like manner during tightening is called locking. This locking is equivalent to abutment.

[0173] Furthermore, in the tightened state, a gap may be formed between the male thread bottom surface 11 b and the female thread top surface 21 a , and the male thread top surface 11 a and the female thread bottom surface 21 b may interfere with and contact each other.

[0174] Industrial applicability

[0175] The threaded joint of the present invention can be effectively used for connecting steel pipes used as oil country tubular goods.

[0176] Description of Reference Numerals

[0177] 101. Threaded portion; 102. Inner sealing portion; 103. Outer sealing portion; 101A. Second threaded portion; 10. Pin; 11. External threaded portion; 11a. External thread top surface; 11b. External thread bottom surface; 11c. Pin load tooth side surface; 11d. Pin insertion tooth side surface; 12. Pin inner sealing surface; 13. Pin outer sealing surface; 14. Pin shoulder surface; 11A. Second external threaded portion; 20. Box; 21. Internal threaded portion; 21a. Internal thread top surface; 21b. Internal thread bottom surface; 21c. Box load tooth side surface; 21d. Box insertion tooth side surface; 22. Box inner sealing surface; 23. Box outer sealing surface; 24. Box shoulder surface; 21A. Second internal threaded portion; CL. Pipe axis.

Claims

1. A threaded joint for a steel pipe, wherein: This threaded joint for steel pipes has: a tubular pin having a tapered external thread portion, a pin inner sealing surface disposed closer to a tip end of the pin than the tapered external thread portion, and a pin outer sealing surface disposed on a side of the pin opposite to the tip end than the tapered external thread portion; and A tubular female buckle fastened to the male buckle has a tapered internal thread portion corresponding to the tapered external thread portion, a female buckle inner sealing surface corresponding to the male buckle inner sealing surface, and a female buckle outer sealing surface corresponding to the male buckle outer sealing surface. The mutually fastened pin and box include: a threaded portion consisting of the mutually meshing tapered external thread portion and the tapered internal thread portion, an inner sealing portion consisting of the mutually contacting inner sealing surface of the pin and the inner sealing surface of the box, and an outer sealing portion consisting of the mutually contacting outer sealing surface of the pin and the outer sealing surface of the box. The ratio Lth of the interference amount δth of the thread portion to tan(θth) when the inclination angle of the thread portion is θth, the ratio Lint of the interference amount δint of the inner seal portion to tan(θint) when the inclination angle of the inner seal portion is θint, and the ratio Lext of the interference amount δext of the outer seal portion to tan(θext) when the inclination angle of the outer seal portion is θext satisfy the following condition (1) or condition (2), Lth>Lext and Lint≠Lth (1) Lth>Lint and Lext≠Lth (2).

2. The threaded joint for steel pipes according to claim 1, wherein: In the case of the condition (1), Lint is greater than Lth.

3. The threaded joint for steel pipes according to claim 1, wherein: In the case of the condition (2), Lext is smaller than Lth.

4. The threaded joint for steel pipes according to any one of claims 1 to 3, wherein: The pin has a second tapered external thread portion disposed on a side of the pin opposite to the tip thereof, relative to the pin outer sealing surface. The female buckle has a second tapered internal thread portion corresponding to the second tapered external thread portion, The pin and box fastened to each other include a second threaded portion consisting of the second tapered external threaded portion and the second tapered internal threaded portion meshing with each other.

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