Threaded fittings for steel pipes

Abstract

To provide a threaded joint for steel pipes with high tightening torque performance. The joint comprises a pin provided at the tip of the steel pipe and having a male threaded portion (5a) made of tapered threads, and a box fastened to the pin and having a female threaded portion (5b) made of tapered threads, wherein the male threaded portion (5a) and the female threaded portion (5b) have a wedge-shaped thread cross-section, the thread base surfaces (6a, 6b) and threaded surfaces (7a, 7b) are parallel to the pipe axis, the insertion surface pitch is smaller than the load surface pitch, and the male threaded portion (5a) has an insertion surface ( The screw has a first connecting portion (11a1) that smoothly connects 8a) and the thread surface (7a), and a second connecting portion (11a2) that smoothly connects the load surface (9a) and the thread surface (7a). The first connecting portion (11a1) and the second connecting portion (11a2) are formed by two arcs, with the radius of curvature of the arc on the thread surface side being greater than the radius of curvature of the arc on the thread base side, and the first connecting portion (11a1) is taller than the second connecting portion (11a2).

Description

Technical Field

[0001] The present invention relates to a steel pipe screw joint in which the thread shape is a wedge shape.

Background Art

[0002] Screw joints are widely used for connecting steel pipes used in oil production industry equipment such as oil well pipes (Oil Country Tubular Goods (OCTG)). Conventionally, standard screw joints defined by API (American Petroleum Institute) standards have typically been used for connecting steel pipes used in oil and gas exploration and production.

[0003] In recent years, oil and natural gas wells have become deeper, and horizontal wells and inclined wells, etc., rather than vertical wells, have increased. As a result, the drilling and production environment has become more severe, and the development of wells in harsh environments such as the ocean and polar regions has increased. Along with these environmental changes, the required performance of screw joints, such as compressive resistance performance, bending resistance performance, external pressure seal performance (external pressure resistance performance), and tightening torque performance, has become diversified. In response to such required performance, a screw joint called a premium joint, which is a high-performance screw joint that passes the screw test standard APIRP 5C5, has been developed. On the other hand, a screw joint called a semi-premium joint has been developed, which does not require the seal performance required by the above screw test standard but has performance superior to that of API screw joints.

[0004] The basic design of a semi-premium joint is to provide tapered threads at the pipe ends of the pipes to be connected to each other. These are composed of a pin, which is a male part provided on one pipe, and a box provided on the other pipe. These elements of the pin and the box are designed to face each other when the joint (meaning a screw joint, the same hereinafter) is tightened. A seal part (metal touch seal part) or a shoulder part may be provided as necessary.

[0005] In semi-premium joints, tapered threads are necessary for securely fastening the joint. Furthermore, in threaded joints with a shoulder section, the shoulder section acts as a tightening stopper, as contact during the tightening process causes a sudden increase in torque, confirming that the joint is sufficiently tightened. Additionally, if axial compressive force is applied after tightening is complete, the shoulder section also bears the load of this compressive force.

[0006] Incidentally, in order to reduce the amount of drilling required during well development, there is a demand to reduce the outer diameter of the joint. Eliminating the shoulder portion is effective in reducing the outer diameter of threaded joints, but the threaded portion must take on the role of replacing it. In order to achieve the above effect, the threaded connection of the tapered thread requires that the male and female threaded portions make contact not only in the normal radial direction, but also on both the load surface and the insertion surface. This type of tapered thread is called an axial interference tapered thread. A typical shape of an axial interference thread is the wedge thread shape. Joints with axial interference tapered threads have a larger total area that receives axial force, and therefore have high compression resistance and high tightening torque performance.

[0007] Patent Document 1 describes a substantially rectangular wedge threaded joint in which the threads and thread root are substantially parallel to the pipe axis direction and the threaded side is substantially perpendicular to the pipe axis direction, characterized in that a clearance is secured in the radial direction of the threaded portion when the screw tightening is completed. A lubricant is applied to the threaded joint to prevent seizing during tightening. The lubricant needs to be applied thinly and uniformly to the entire male and female threaded joint, including the threaded portion and, if present, the seal portion and shoulder portion. If too little lubricant is applied, the effect of preventing seizing during screw tightening described above will be insufficient, scratches will occur on the threaded portion, and the subsequent airtightness will be adversely affected.

[0008] The dope, which acts as a lubricant, is held in a gap (called dope relief) secured between the male and female threads during the tightening process. When tightening a screw joint with a dope relief gap that is too small, there is a concern that the dope pressure will increase as the dope has nowhere to escape. When the apparent torque is high due to the effect of dope pressure, the torque will reach the specified acceptable range even if the amount of tightening of the screw joint is insufficient, making it impossible to determine the proper tightening state from the measured torque. As a result, with such tightening, the airtightness after tightening of the screw joint cannot be fully achieved. To address this problem, Patent Document 1 proposes a design that can secure a sufficient dope relief gap.

[0009] Premium joints typically use torque management to determine when tightening is complete, and similar torque management is sometimes applied to semi-premium joints. The graph drawn during tightening is called a torque turn chart, with the horizontal axis representing turns (amount of tightening rotation) and the vertical axis representing tightening torque. In axial interference screws (wedge screws), the torque increases gradually as the screw is tightened until the sides of the screw threads come into contact (corresponding to the point indicated as the axial interference position in Figure 8). After that, further tightening causes the torque to increase rapidly, and tightening is complete when the torque reaches the specified acceptable range (corresponding to the point indicated as the tightening completion position in Figure 8). The torque change when tightening is continued is shown by a dotted line in Figure 8. If overtightened, the linearity is lost and the curve changes. This is because the screw undergoes plastic deformation, and the torque at the point where linearity is lost is called the yield torque. The acceptable range for tightening torque must be sufficiently higher than the torque at the axial interference point to ensure that the torque at the completion of tightening does not reach the yield torque, while simultaneously confirming that the screw is securely tightened.

[0010] Patent Document 2 specifies the threads at the pipe end, which are generally considered to have a high risk of failure, in a wedge-shaped thread joint for steel pipes. A wedge thread shape generally has a spiral shape in which the thread width changes continuously. In the male thread, the thread width is wide at the distal part from the pipe end and gradually narrows towards the pipe end, with the thread width being smallest at the pipe end. The female thread is configured to mesh with the male thread, and the thread groove width is wide at the pipe end and gradually narrows towards the inner part, with the thread groove width being smallest at the innermost part. In a wedge thread shape, both sides of the thread interfere in the direction of the pipe axis, so when tensile and compressive forces in the direction of the pipe axis are applied to the pipe, tensile and compressive forces in the direction of the pipe axis are applied to the sides of the thread. Since the thread with the smallest thread width of the male thread is more likely to fail, Patent Document 2 specifies that the minimum thread width should be designed to withstand forces in the direction of the pipe axis. This enables premium joints employing wedge threads to exhibit both high torque performance and high sealing performance. [Prior art documents] [Patent Documents]

[0011] [Patent Document 1] U.S. Patent No. 11371293 [Patent Document 2] Patent No. 7237084 [Overview of the Initiative] [Problems that the invention aims to solve]

[0012] Incidentally, Patent Document 1 attempts to achieve the expected high torque performance and high sealing performance of a premium joint employing wedge threads by securing a dope relief gap to suppress the increase in dope pressure. However, if a parallel gap is secured between the threads and the thread root, which are substantially parallel to the pipe axis direction, the contact area on the load surface side of the thread, where the thread height is low, becomes shorter, which is thought to impair the high torque performance. This problem of impaired torque performance is also present in semi-premium joints.

[0013] Furthermore, Patent Document 2 attempts to achieve high torque performance and high sealing performance, which are expected performance characteristics of premium joints employing wedge threads, by ensuring the minimum rigidity of the thread to suppress failure at the minimum thread width of the male thread. However, Patent Document 2 does not take dope relief gap into consideration, which leads to the problem of increased dope pressure. Moreover, paragraph 0006 of Patent Document 2 suggests that the contact area of ​​the thread is closely related to torque performance, and it is thought that simply reducing the thread height to secure a dope relief gap will impair high torque performance. This problem of impairing torque performance is also present in semi-premium joints.

[0014] Therefore, the present invention has been made in view of the above-mentioned problems, and aims to provide a threaded joint for steel pipes that has high tightening torque performance. [Means for solving the problem]

[0015] (1) According to one aspect of the present invention, a threaded joint for steel pipes is provided, comprising: a tubular pin provided at the tip of a steel pipe; and a tubular box into which the pin is screwed and fastened with the pin, wherein the pin has a male threaded portion consisting of a tapered thread formed on the outer circumference of the pin; the box has a female threaded portion consisting of a tapered thread formed on the inner circumference of the box; the male threaded portion and the female threaded portion have a wedge-shaped thread cross-section, the threaded surface and the threaded bottom surface are parallel to the pipe axis, the insertion surface pitch is smaller than the load surface pitch; the male threaded portion has a first connecting portion that smoothly connects the insertion surface and the threaded surface, and a second connecting portion that smoothly connects the load surface and the threaded surface; the first connecting portion and the second connecting portion are formed by two arcs, the radius of curvature of the arc on the threaded surface side is larger than the radius of curvature of the arc on the threaded bottom surface side, and the height of the first connecting portion is higher than the height of the second connecting portion.

[0016] (2) In the threaded joint for steel pipes described in (1) above, the first and second connecting parts have a ratio of the radius of curvature of the arc on the threaded surface side to the radius of curvature of the arc on the threaded bottom side of 5 or more.

[0017] (3) In the threaded joint for steel pipes described in (1) or (2) above, the first and second connecting parts have a radius of curvature of the arc on the threaded surface side of 0.05 inches or more.

[0018] (4) In any one of the steel pipe threaded joints described in (1) to (3) above, the ratio of the height of the insertion surface side to the height of the straight section on the load surface side of the male threaded section is 0.95 or more and 1.15 or less.

[0019] (5) In any one of the steel pipe threaded joints described in (1) to (4) above, the female threaded portion has a first connecting portion that smoothly connects the insertion surface and the threaded surface, a second connecting portion that smoothly connects the load surface and the threaded bottom surface, a third connecting portion that smoothly connects the insertion surface and the threaded bottom surface, and a fourth connecting portion that smoothly connects the load surface and the threaded bottom surface, wherein the first and second connecting portions of the female threaded portion are formed by two arcs, with the radius of curvature of the arc on the threaded surface side being greater than the radius of curvature of the arc on the threaded bottom surface side, and the third and fourth connecting portions of the female threaded portion are formed by two arcs, with the radius of curvature of the arc on the threaded bottom surface side being greater than the radius of curvature of the arc on the threaded surface side. [Effects of the Invention]

[0020] According to one aspect of the present invention, a threaded joint for steel pipes having high tightening torque performance is provided. [Brief explanation of the drawing]

[0021] [Figure 1] This is a cross-sectional view of a pipe axis showing a wedge screw coupling of the coupling type in one embodiment of the present invention. [Figure 2] This is an enlarged cross-sectional view of section A in Figure 1. [Figure 3] This is an enlarged cross-sectional view showing the male threaded portion of a pin in one embodiment of the present invention. [Figure 4]This is an enlarged cross-sectional view showing the female thread portion of a box in one embodiment of the present invention. [Figure 5] This is an enlarged cross-sectional view showing the tapered thread row of a wedge screw fitting in the prior art. [Figure 6] This is an enlarged cross-sectional view showing the male threaded portion of a pin in the prior art. [Figure 7] This is an enlarged cross-sectional view showing the female thread portion of a box in the prior art. [Figure 8] This graph shows an example of a torque turn chart when a bolt is tightened normally and then overtightened. [Figure 9] This graph shows an example of a torque turn chart when there is disturbance in the chart before axial interference. [Figure 10] This graph shows an example of a torque turn chart when there is distortion in the chart after axial interference. [Modes for carrying out the invention]

[0022] The following detailed description will illustrate embodiments of the present invention with reference to the drawings. In the drawings, identical or similar parts are denoted by the same or similar reference numerals, and redundant descriptions are omitted. Each drawing is schematic and may differ from reality. Furthermore, the embodiments shown below are illustrative of apparatus and methods for realizing the technical idea of ​​the present invention, and the technical idea of ​​the present invention is not limited to the materials, structure, arrangement, etc., of the components described below. The technical idea of ​​the present invention can be modified in various ways within the technical scope defined by the claims described in the patent claims.

[0023] The present invention will now be described with reference to the drawings. As shown in Figures 1 and 2, the threaded joint 1 for steel pipes comprises a pin 2 and a box 3. The threaded joint 1 for steel pipes is, for example, a threaded joint for oil well steel pipes. The pin 2 is a tubular portion provided at the tip of the steel pipe. The box 3 is a tubular member into which the pin 2 is screwed and connected. In this embodiment, the threaded joint 1 for steel pipes is a threaded joint for oil well steel pipes, in which the box 3 and the pin 2 are connected by a screw connection. Although Figure 1 shows a state in which the pin 2 of the steel pipe is connected only to one side (right side) of the box 3, the pins of other steel pipes can also be connected to the opposite side (left side) of the box 3, thereby connecting the steel pipes to each other via the box 3.

[0024] Pin 2 has a male threaded portion 5a consisting of a tapered thread formed on its outer circumference. Box 3 has a female threaded portion 5b consisting of a tapered thread formed on its inner circumference. The male threaded portion 5a and the female threaded portion 5b are collectively referred to as the tapered thread row 5. The tapered thread row 5 is an axial interference thread, and the cross-sectional shape of the thread passing through the pipe axis is wedge-shaped (wedge thread shape). Furthermore, the thread base surfaces 6a, 6b and thread surfaces 7a, 7b of the male threaded portion 5a and the female threaded portion 5b are parallel to the pipe axis. Therefore, radial contact is achieved by contact between the thread base surface 6b of the female threaded portion 5b and the thread surface 7a of the male threaded portion 5a, or by contact between the thread base surface 6a of the male threaded portion 5a and the thread surface 7b of the female threaded portion 5b. Furthermore, the tapered thread row 5 has a spiral shape in which the thread width changes continuously. Focusing on the thread width of the male thread, the thread width is wider distal to the pipe end and gradually narrows toward the pipe end. In addition, the tapered thread row 5 has a stepped thread shape in which the radial positions of adjacent thread bases 6a, 6b and adjacent thread surfaces 7a, 7b are different in the male thread section 5a and the female thread section 5b, respectively.

[0025] Furthermore, in the male thread portion 5a and the female thread portion 5b, the side surfaces of the respective threads, that is, the surfaces extending from the thread surfaces 7a, 7b to the thread base surfaces 6a, 6b, are called the insertion surfaces 8a, 8b or the loading surfaces 9a, 9b. Note that in the same thread, the side surfaces of the male thread on the pipe end side (left side in Figure 2) are the insertion surfaces 8a, 8b, and the side surfaces on the opposite side of the male thread from the pipe end (right side in Figure 2) are the loading surfaces 9a, 9b. As described above, the thread width of the male thread gradually narrows towards the pipe end side, so the insertion surface pitch is smaller than the loading surface pitch. The loading surface pitch and the insertion surface pitch are the axial spacing between the loading surfaces 9a, 9b and the insertion surfaces 8a, 8b in the male thread portion 5a and the female thread portion 5b.

[0026] As shown in Figure 3, which shows a cross-section including the pipe axis, the male threaded portion 5a is composed of an insertion surface 8a, a load surface 9a, and a straight section 10a. The straight section 10a that forms the insertion surface 8a is called the first straight section 10a1, and the straight section that forms the load surface 9a is called the second straight section 10a2. Furthermore, the straight section 10a and the thread surface 7a, and the straight section 10a and the thread base surface 6a are connected via curved connecting sections 11a. The connecting section 11a between the first straight section 10a1 (insertion surface 8a) and the thread surface 7a is called the first connecting section 11a1, and the connecting section 11a between the thread surface 7a and the second straight section 10a2 (load surface 9a) is called the second connecting section 11a2. Furthermore, the connection portion 11a between the screw base 6a and the first straight section 10a1 (insertion surface 8a) is called the third connection portion 11a3, and the connection portion 11a between the second straight section 10a2 (load surface 9a) and the screw base 6a is called the fourth connection portion 11a4. In addition, the length from the screw base 6a on the insertion surface side to the screw thread surface 7a in the radial direction of the screw (vertical direction in Figure 3) is called the screw height H on the screw insertion surface side. as Furthermore, the length from the screw base surface 6a on the load surface side to the thread surface 7a of the adjacent thread on the smaller diameter side (the thread on the left in Figure 3) in the height direction is defined as the screw height H on the load surface side. al Furthermore, the length of the first straight section 10a1 in the height direction is the height h on the screw insertion surface side. as The length of the second straight section 10a2 is defined as the height h on the screw load surface side. al That's what they say.

[0027] In the male screw portion 5a, the height h on the thread load surface side al and the height h on the thread insertion surface side with respect to as is preferably 0.95 or more and 1.15 or less. By doing so, the male screw portion 5a can properly receive tension due to the axial load on the load surface 9a and compression on the insertion surface 8a.

[0028] As shown in FIG. 4 in the cross-section including the pipe axis, the female screw portion 5b is composed of an insertion surface 8b, a load surface 9b, and a straight portion 10b. The straight portion 10b forming the insertion surface 8b is referred to as the first straight portion 10b1, and the straight portion forming the load surface 9b is referred to as the second straight portion 10b2. Further, the straight portion 10b and the thread crest surface 7b, and the straight portion 10b and the thread bottom surface 6b are respectively connected via a curved connection portion 11b in the cross-section view of the pipe axis. The connection portion 11b between the first straight portion 10b1 (insertion surface 8b) and the thread crest surface 7b is referred to as the first connection portion 11b1, and the connection portion 11b between the thread crest surface 7b and the second straight portion 10b2 (load surface 9b) is referred to as the second connection portion 11b2. Also, the connection portion 11b between the thread bottom surface 6b and the first straight portion 10b1 (insertion surface 8b) is referred to as the third connection portion 11b3, and the connection portion 11b between the second straight portion 10b2 (load surface 9b) and the thread bottom surface 6b is referred to as the fourth connection portion 11b4. Further, the length from the thread crest surface 7b on the insertion surface side to the thread bottom surface 6b in the radial direction of the thread (the vertical direction in FIG. 4) is referred to as the thread height H on the thread insertion surface side bs Furthermore, the length from the thread crest surface 7b on the load surface side to the thread bottom surface 6b of the adjacent thread groove with a smaller diameter (the left thread groove in FIG. 4) in the height direction is referred to as the thread height H on the thread load surface side bl Furthermore, the length of the first straight portion 10b1 in the height direction is referred to as the height h on the thread insertion surface side bs and the length of the second straight portion 10b2 is referred to as the height h on the thread load surface side bl as such.

[0029] In the male threaded portion 5a and the female threaded portion 5b, the screw base surfaces 6a, 6b, the threaded surfaces 7a, 7b, the insertion surfaces 8a, 8b, and the load surfaces 9a, 9b are smoothly connected to the connecting portions 11a, 11b. In this embodiment, a smooth connection means that the tangents at the connection points are common tangents. That is, at the connection points between the screw base surfaces 6a, 6b and the threaded surfaces 7a, 7b and the straight portions 10a, 10b (insertion surfaces 8a, 8b and load surfaces 9a, 9b), the tangents are common tangents.

[0030] As shown in Figures 2 to 4, when the pin 2 and the box 3 are fastened together, the length in the height direction at which the first straight portion 10a1 of the male threaded portion 5a and the second straight portion 10b2 of the female threaded portion 5b come into contact is called the contact height h1 of the insertion surface. Also, when the pin 2 and the box 3 are fastened together, the length in the height direction at which the second straight portion 10a2 of the male threaded portion 5a and the first straight portion 10b1 of the female threaded portion 5b come into contact is called the contact height h2 of the load surface.

[0031] Furthermore, the straight sections 10a and 10b are inclined by a predetermined angle with respect to the height direction, which is perpendicular to the pipe axis direction. Here, the angle (the acute angle) between the first straight sections 10a1 and 10b1 on the screw insertion surface side and the height direction is defined as the flank angle α on the insertion surface side, and its sign is positive when it is pointing in the direction of the threads of pin 2. Note that the sign of flank angle α is negative. Also, the angle between the second straight sections 10a2 and 10b2 on the screw load surface side and the height direction is defined as the flank angle β on the load surface side, and its sign is positive when it is pointing in the direction of the threads of pin 2. Note that the sign of flank angle β is negative. The flank angles α and β need to be negative values ​​to achieve a strong connection and must be -1 degree or less. However, if the absolute values ​​of the flank angles α and β are made too large, the thread surface 7a becomes too wide, making it difficult to engage with the screw groove (stubbling performance deteriorates). For this reason, the flank angles α and β need to be -10 degrees or more.

[0032] Furthermore, as shown in Figure 3, the first connecting portion 11a1 and the second connecting portion 11a2 of the male threaded portion 5a are each formed by two arcs with different radii of curvature. The two arcs are smoothly connected. Here, for the two arcs of the first connecting portion 11a1, the radius of curvature of the arc on the thread bottom surface side is R asc1 Let R be the radius of curvature of the arc on the thread side. asc2 Let's assume the radius of curvature R on the thread side. asc2 The radius of curvature R on the bottom surface of the screw asc1 Larger than (R asc2 >R asc1 ). For the two arcs of the second connection part 11a2, the radius of curvature of the arc on the screw bottom side is R alc1 Let R be the radius of curvature of the arc on the thread side. alc2 Let's assume the radius of curvature R on the thread side. alc2 The radius of curvature R on the bottom surface of the screw alc1 Larger than (R alc2 >R alc1 ). Also, the connection height H is the length (height) of the first connection part 11a1 in the height direction. asc The connection height H is the length (height) of the second connection part 11a2 in the vertical direction. alc Larger than (H asc >H alc The central angles of the two arcs are set appropriately to satisfy the above conditions.

[0033] In this embodiment, the first connection part 11a1 and the second connection part 11a2 are made into two arcs, and the heights of the first connection part 11a1 and the second connection part 11a2 are optimized. This not only improves the ability to withstand both compressive and tensile axial loads, but also ensures a screw contact area, thus achieving high torque performance. Furthermore, the contact height h2 on the load surface side becomes larger, and the contact height h1 on the insertion surface side becomes smaller by eliminating excess height, so that a dope relief gap d can be secured on the insertion surface side as shown in Figure 1. Screw joints are generally tightened by applying dope. During the screw tightening process, the gap between the box 3 and the pin 2 decreases, and when tightening is complete, the dope applied to the dope relief gap d accumulates. The definition of the dope relief gap d is the maximum value within the distance from the thread base surface of the female screw and points on both ends of the connection parts to the thread surface of the male screw (the length of the shortest line segment connecting to the thread surface). If the dope relief gap d is small, the dope pressure increases when a large amount of dope is applied, as the excess dope cannot be held in the gap. This creates a risk that the performance of the screw joint may be impaired due to instability in the torque during tightening. However, according to this embodiment, since the dope relief gap d can be secured, this risk can be reduced.

[0034] Furthermore, by making the connecting parts 11a1 and 11a2 into two arcs, the connecting parts 11a1 and 11a2 are less likely to catch on the female thread portion 5b when engaged, thereby improving stubbling performance. In order to effectively achieve this effect, the height H of the threaded connecting part asc and H alc The radius of curvature R of the smaller arc of each connecting part 11a asc1 ,R alc1 When calculating the magnification for each, it is desirable to set it to be 1.1 times or more and 2.0 times or less.

[0035] Furthermore, the first connecting portion 11a1 and the second connecting portion 11a2 have a radius of curvature R of the arc on the screw bottom surface side. asc1 ,R alc1 Radius of curvature R of the arc on the thread side asc2 ,R alc2It is preferable that the ratio of each is 5 or greater (5 × R asc1 ≤R asc2 ,5×R alc1 ≤R alc2 ). This is useful for constructing a design that can secure contact height h1, contact height h2, and dope relief gap d. In this case, from the viewpoint of the above effect, the radius of curvature R on the thread surface side asc2 ,R alc2 It is more preferable that the length be 0.05 inches or more.

[0036] Furthermore, it is preferable that the third connection portion 11a3 and the fourth connection portion 11a4 of the male threaded portion 5a are each formed by a single arc in a cross-sectional view of the pipe axis. When the third connection portion 11a3 and the fourth connection portion 11a4 on the threaded bottom side are subjected to compressive or tensile loads due to axial loads, stress concentration occurs in the region close to the insertion surface 8a or the load surface 9a. On the other hand, when subjected to bending and unbending, stress concentration occurs in the region close to the threaded bottom surface 6a of the third connection portion 11a3 and the fourth connection portion 11a4 on the threaded bottom side. However, by making the third connection portion 11a3 and the fourth connection portion 11a4 a single arc, both of the above-mentioned stress concentrations can be suppressed. In addition, in order to suppress stress concentration, the radius of curvature R of the third connection portion 11a3 is sr and the radius of curvature R of the fourth connection part 11a4 lr It is preferable that the length be 0.010 inches or more.

[0037] As shown in Figure 4, it is preferable that the first connection portion 11b1, the second connection portion 11b2, the third connection portion 11b3, and the fourth connection portion 11b4 of the female thread portion 5b are each formed by two arcs with different radii of curvature in a cross-sectional view of the pipe axis. In this case, the radius of curvature R of the arc on the thread surface 7b side of the first connection portion 11b1 and the second connection portion 11b2 is bsc2 ,R blc2 However, the radius of curvature R of the arc on the straight section 10b side bsc1 ,R blc1 It is larger than that. Also, in the third connection part 11b3 and the fourth connection part 11b4, the radius of curvature R of the arc on the screw bottom surface 6b side. bsr2 ,R blr2 However, the radius of curvature R of the arc on the straight section 10b sidebsr1 ,R blr1 It is larger than that. By making the first connection part 11b1 and the second connection part 11b2, which are the connection parts 11b on the thread surface side of the female thread portion 5b, the stubbling performance can be improved compared to the case where the connection part of the female thread portion 5b is formed by a single arc, similar to the male thread portion 5a. In addition, by making the third connection part 11b3 and the fourth connection part 11b4, which are the connection parts 11b on the thread bottom surface side, each consist of two arcs, stress concentration when loads such as axial loads are applied can be mitigated. This prevents failure at the critical cross-section of the box 3 due to stress concentration. As described above, by making the connection parts 11b on the thread surface side and the thread bottom surface side consist of two arcs, the contact heights h1 and h2 can be increased, and the dope relief gap d can also be secured.

[0038] In the steel pipe threaded joint 1, upon completion of tightening, a strong connection in the pipe axial direction is achieved by the contact between the insertion surfaces 8a and 8b and the load surfaces 9a and 9b, as shown in Figure 2. In the wedge threaded joint, the thread width changes continuously, and the thread width of the male threaded portion 5a decreases toward the pipe end, and correspondingly the thread groove width of the female threaded portion 5b decreases, thereby achieving the connection shown in Figure 2.

[0039] Here, Figures 5 to 7 show the tapered thread rows of a wedge thread joint in the prior art. In Figures 5 to 7, components similar to those in Figures 2 to 4 are indicated with the same reference numeral followed by an apostrophe ('). As shown in Figures 5 to 7, the axial cross-sectional shape of the thread is a nearly rectangular thread shape, where the thread surfaces 7'a, 7'b and thread base surfaces 6'a, 6'b are parallel to the pipe axis, and the insertion surfaces 8'a, 8'b and load surfaces 9'a, 9'b are nearly perpendicular to the pipe axis. As shown in Figure 5, in the conventional design, the thread shape is nearly rectangular, resulting in a small dope relief gap d'. Therefore, when a large amount of dope is applied, the dope pressure becomes very high. In contrast, with the steel pipe thread joint 1 according to this embodiment, as described above, a sufficient dope relief gap d can be secured, thereby improving the stability of the torque during tightening and preventing performance degradation of the thread joint due to insufficient thread engagement.

[0040] Furthermore, in the conventional tapered thread arrangement shown in Figures 5 to 7, the connecting portions 11'a and 11'b are all formed by a single arc. However, in the case of a wedge thread joint with a nearly rectangular axial cross-sectional shape, there is a problem that the corners tend to catch, resulting in poor stubbling performance. In contrast, according to this embodiment, by making the first connecting portion 11a1 and the second connecting portion 11a2 of the male thread into two arcs, stubbling performance can be improved compared to the conventional design. By making the first connecting portion 11b1 and the second connecting portion 11b2 of the female thread into two arcs, stubbling performance can be further improved.

[0041] Furthermore, a wedge screw is a tapered screw in which the thread surface and the thread base are parallel to the pipe axis. In other words, conventionally, as shown in Figure 5, the radial positions of adjacent thread bases 6'a, 6'b and adjacent thread surfaces 7'a, 7'b are different, resulting in a stepped screw. In the conventional wedge screw shown in Figure 5, the contact height h'1 of the insertion surfaces 8'a, 8'b is clearly larger than the contact height h'2 of the load surfaces 9'a, 9'b. Therefore, as a screw design that receives axial loads at the thread portion, the magnitude of the tensile load that the load surfaces 9'a, 9'b can withstand becomes a bottleneck, making it difficult to withstand large axial loads. In contrast, in this embodiment, by making the connecting portion 11a into two arcs and optimizing the height of the connecting portion 11a, it is possible to withstand both compressive and tensile axial loads.

[0042] Figure 8 shows the tightening chart (torque turn chart) when the steel pipe threaded joint 1 according to this embodiment is tightened. Figures 9 and 10 show the tightening chart when a steel pipe threaded joint having the tapered thread rows shown in Figures 5 to 7 is used. In conventional thread designs with dope pressure problems, there was a problem of chart distortion occurring before axial interference as shown in Figure 9 and after axial interference as shown in Figure 10.

[0043] In contrast, the threaded joint 1 for steel pipes according to this embodiment provides a normal tightening chart without irregularities, as shown in Figure 8. If the chart is irregular, there is a risk that axial interference may not occur as intended, or that the tightening completion position may not be as intended. Therefore, if the tightening is not performed properly, the threaded joint will not be able to perform to its full potential. However, this embodiment solves these problems.

[0044] <Variation> Although the present invention has been described above with reference to specific embodiments, this description is not intended to limit the invention. By referring to the description of the present invention, those skilled in the art will also see other embodiments of the invention, including various modifications, in addition to the disclosed embodiments. Accordingly, the embodiments of the invention described in the claims should be understood to include embodiments that include these modifications described herein, either individually or in combination.

[0045] For example, in the above embodiment, the connecting portion 11b of the female threaded portion 5b consists of two arcs, but the present invention is not limited to such examples. The connecting portion 11b may consist of one arc in a cross-sectional view of the pipe axis. [Examples]

[0046] The inventors will now describe an embodiment they have performed. In this embodiment, a pipe body with an outer diameter of 9.625 inches (244.48 mm), a wall thickness of 0.545 inches (13.84 mm), and steel grade API5CT L80, as well as a coupling corresponding to this pipe body, were prepared. The length of the threaded joint was set to 5 inches. A pin threaded joint was machined onto the pipe body to form pin 2, and a box threaded joint was machined onto the coupling to form box 3. The insertion surface pitch, which is the axial spacing between insertion surfaces 8a and 8b, was set to 0.300 inches, and the load surface pitch, which is the axial spacing between load surfaces 9a and 9b, was set to 0.315 inches. The flank angle α on the insertion surface side was set to -5 degrees, the flank angle β on the load surface side was set to -5 degrees, and the thread taper relative to the load surface side was set to 1 / 16. The difference in height of the stepped threads is calculated by multiplying the thread pitch by the thread taper and dividing by 2, which in this embodiment is 0.0098 inches. The thread height H on the thread load surface side of box 3. bl The thread height H on the thread load side of pin 2 is set to 0.0550 inches. al The thread height H on the insertion side was set to 0.0500 inches. as ,H bs The screw height H on the load-bearing surface side al ,H bl This is obtained by adding 0.0098 inches, which is the difference in height between the stepped screws, to each of these values.

[0047] Table 1 shows various designs (radius of curvature) of the arc shape of the connecting parts 11a and 11b of the screw root surfaces 6a and 6b and the thread surfaces 7a and 7b. In Table 1, the conditions for connecting parts 11a with two radii of curvature are shown, which are formed by two arcs. In the examples, the conditions in which the first connecting part 11a1 and the second connecting part 11a2 are two arcs, and the other connecting parts 11a and 11b are one arc are designated as Examples 1 and 2 of the present invention. Furthermore, the conditions in which the first connecting part 11a1, the second connecting part 11a2, and the first to fourth connecting parts 11b1 to 11b4 are two arcs, and the other connecting part 11a is one arc are designated as Example 3 of the present invention. In addition, as a comparison with the prior art, the conditions in which all connecting parts 11a and 11b are one arc are designated as Comparative Examples 1 to 3.

[0048] [Table 1]

[0049] Table 2 also shows the connection heights at connection points 11a and 11b for various designs. Based on the dimensions shown in Tables 1 and 2, the screw contact heights h1 and h2 and the dope relief gap d for various designs are calculated as shown in Table 3. In step screws, the contact heights h1 and h2 are generally lower on the load surface than on the insertion surface. In this embodiment, where only the connection point is changed, the tightening limit torque (yield torque) is determined by the screw contact area calculated from the contact height h2 on the load surface.

[0050] [Table 2]

[0051] [Table 3]

[0052] Table 4 shows the results of yield torque tests conducted under each condition as a result of the examples. For Examples 1-3 of the present invention, it was confirmed that the tightening charts were all normal. On the other hand, in the screw designs of Comparative Examples 1-3 using the prior art, undulation was observed in the tightening charts. It can be seen that the yield torque has a strong correlation with the contact height h2 of the load surface shown in Table 3. Because Examples 1-3 of the present invention had normal tightening charts, the yield torque could be accurately read. However, in Comparative Examples 1-3 using the prior art, there were cases where the curvature of the chart indicating the yield could not be accurately read due to the presence of undulation. This is thought to be because the dope relief gap was insufficient, resulting in excessive dope pressure and preventing normal tightening.

[0053] [Table 4]

[0054] Table 5 shows the results of the examples, in which the axial strength was evaluated by repeatedly applying tension and compression 10 times after tightening to 40,000 ft-lbs for each condition. The tension and compression loads were set to 95% (1182 kps) of the tensile load of the pipe body. It was confirmed that the tightening charts for Examples 1 to 3 of the present invention were all normal. On the other hand, wavering was observed in the tightening charts of the screw designs in Comparative Examples 1 to 3 of the prior art. Examples 1 to 3 of the present invention passed the tension and compression tests. On the other hand, Comparative Examples 1 to 3 of the prior art failed because the screws came loose during tension. This is thought to be the result of excessive dope pressure preventing normal tightening, causing the screws to loosen due to repeated tension and compression loads.

[0055] [Table 5] [Explanation of Symbols]

[0056] 1. Threaded fittings for steel pipes 2 pins 3 boxes 5 Tapered thread rows 5a, 5a' Male threaded section 5b, 5b' Female thread section 6a, 6b, 6a', 6b' Screw bottom surface 7a, 7b, 7a', 7b' thread surface 8a, 8b, 8a', 8b' Insertion surface 9a,9b,9a',9b' Load surface 10a,10b,10a',10b' Straight section 10a1,10b1,10a'1,10b'1 1st straight section 10a2,10b2,10a'2,10b'2 2nd straight section 11a, 11b, 11a', 11b' connection section 11a1,11b1,11a'1,11b'1 First connection section 11a2, 11b2, 11a'2, 11b'2 Second connection section 11a3,11b3,11a'3,11b'3 Third connection section 11a4, 11b4, 11a’4, 11b’4 Fourth connection part α, β Frank angle h1, h2, h’1, h’2 Contact height h al , h as , h bl , h bs Height H al , H as , H bl , H bs , H’ al , H’ as , H’ bl , H’ bs Screw height H asc , H alc , H bsc , H bsr , H blc , H blr Connection height R asc1 , R asc2 , R asr , R alc1 , R alc2 , R alr , R bsc1 , R bsc2 , R bsr1 , R bsr2 , R blc1 , R blc2 , R blr1 , R blr2 , R’ asc , R’ asr , R’ alc , R’ alr , R’ bsc , R’ bsr , R’ blc , R’ blr Radius of curvature

Claims

1. It comprises a tubular pin provided at the tip of a steel pipe, and a tubular box into which the pin is screwed and fastened, The pin has a male threaded portion consisting of a tapered thread formed on the outer circumference of the pin, The box has an internal threaded portion consisting of a tapered thread formed on the inner circumference of the box, The male and female threaded portions have a wedge-shaped cross-section, the thread surface and thread base are parallel to the pipe axis, and the insertion surface pitch is smaller than the load surface pitch. The male threaded portion has a first connecting portion that smoothly connects the insertion surface and the threaded surface, and a second connecting portion that smoothly connects the load surface and the threaded surface. The first and second connecting portions are formed by two arcs, and the radius of curvature of the arc on the thread side is greater than the radius of curvature of the arc on the thread base side. A threaded joint for steel pipes, wherein the height of the first connection portion is higher than the height of the second connection portion.

2. The threaded joint for steel pipes according to claim 1, wherein the first and second connecting portions have a ratio of 5 or more of the radius of curvature of the arc on the threaded surface side to the radius of curvature of the arc on the threaded bottom surface side.

3. The threaded joint for steel pipes according to claim 1 or 2, wherein the first and second connecting portions have a radius of curvature of the arc on the threaded surface side of 0.05 inches or more.

4. The threaded joint for steel pipes according to claim 1 or 2, wherein the ratio of the height of the straight portion on the insertion surface side to the straight portion on the load surface side is 0.95 or more and 1.15 or less for the male threaded portion.

5. The female thread portion has a first connecting portion that smoothly connects the insertion surface and the threaded surface, a second connecting portion that smoothly connects the load surface and the threaded base surface, a third connecting portion that smoothly connects the insertion surface and the threaded base surface, and a fourth connecting portion that smoothly connects the load surface and the threaded base surface. The first and second connecting portions of the female thread are formed by two arcs, with the radius of curvature of the arc on the thread side being greater than the radius of curvature of the arc on the thread base side. The third and fourth connecting portions of the female threaded portion are formed by two arcs, wherein the radius of curvature of the arc on the thread base side is greater than the radius of curvature of the arc on the threaded surface side, according to claim 1 or 2.

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