Threaded joint for steel pipes

Abstract

A threaded coupling is configured of a pin (10) and a box (20). The pin (10) is provided, in order from the pipe main body of the pin (10) toward the distal end, with a male threaded part (11) and a lip part (12) including a seal surface (13), and the box (20) is provided with a female threaded part (21) corresponding to the male threaded part (11) of the pin (10), and a concave part (22) including a seal surface (23) corresponding to the lip part (12). The lip part (12) is provided, in order from the male threaded part (11) toward the distal end of the pin (10), with a neck part (14) and a sealing head part (15) including the seal surface (13), and the maximum outside diameter (D1) of the area of the seal surface (13) in the sealing head part (15) is greater than the outside diameter (D2) in the position of the neck part (14) that borders the male threaded part (11). This threaded coupling can reliably exhibit high sealing performance while maintaining high torque-resistant performance due to a tapering thread having a dovetail shape.

Description

This invention relates to a threaded joint used for connecting steel pipes. In oil wells, natural gas wells, etc. (hereinafter collectively referred to as "oil wells"), steel pipes called oil country tubular goods (OCTG), such as casings and tubing, are used to extract underground resources. These steel pipes are connected in sequence, and threaded joints are used for these connections. Threaded joints for steel pipes are broadly classified into two types: coupling type and integral type. In the coupling type, one of the pair of pipes to be connected is a steel pipe, and the other pipe is a coupling. In this case, male threads are formed on the outer circumference of both ends of the steel pipe, and female threads are formed on the inner circumference of both ends of the coupling. The steel pipe and the coupling are then connected. In the integral type, both of the pair of pipes to be connected are steel pipes, and no separate coupling is used. In this case, a male thread is formed on the outer circumference of one end of the steel pipe, and a female thread is formed on the inner circumference of the other end. The steel pipe is then connected to the other steel pipe. Generally, a pipe end joint with a male threaded section is called a pin because it includes an element that is inserted into the female threaded section. On the other hand, a pipe end joint with a female threaded section is called a box because it includes an element that receives the male threaded section. Since both pins and boxes are ends of pipe material, they are both tubular in shape. In threaded fittings for steel pipes, the male threaded portion of the pin is screwed into the female threaded portion of the box, causing the male and female threaded portions of the tapered thread to fit together tightly. Basically, the sealing performance of the threaded fitting is ensured by this thread seal resulting from the tight fit between the male and female threaded portions. In recent years, drilling techniques such as horizontal and inclined drilling have become widespread in oil wells, resulting in situations where threaded joints are subjected to high torque. Therefore, there is a growing demand for threaded joints with high torque resistance. Furthermore, the oil well environments in which these drilling techniques are applied are harsh environments of high temperature and pressure. To cope with such harsh environments, threaded joints are required to have not only high torque resistance but also high sealing performance. Generally, coupling-type threaded joints are more widely used than integral-type joints because they offer superior sealing performance. As a screw joint having high torque resistance, there is one that employs a double-tail type tapered screw, also called a wedge screw. In a wedge screw, the thread width of the male thread portion becomes narrower and narrower in the direction in which the right-handed screw advances along the helical winding line of the screw, and the thread groove width of the corresponding female thread portion also becomes narrower and narrower in the direction in which the right-handed screw advances along the helical winding line of the screw. Further, both the load surface and the insertion surface have a negative flank angle, and when the fastening is completed, the load surfaces and the insertion surfaces contact each other, so that the entire screw portion fits firmly together. Furthermore, in the state where the fastening is completed, the thread crest surface and the thread valley bottom surface of the screw portion contact and adhere to each other. By these, the screw joint employing a wedge screw can exhibit high torque resistance while ensuring sealing performance. However, in the case of a screw seal using a wedge screw, for each of the male thread portion and the female thread portion, manufacturing tolerances such as the radius of curvature must be strictly set and adhered to each other, otherwise the formation of the seal will be insufficient. Also, since the screw seal is formed only in the region of substantially complete threads, if the length of the region of incomplete threads is designed to be large, there is a risk that the expected sealing performance cannot be obtained. In particular, when the internal pressure or external pressure is excessively loaded on the screw joint, there is a risk that high-pressure fluid will penetrate into the gap of the screw seal and leakage will occur. As described above, there is a limit to the sealing performance that can be ensured only by a screw seal. For this reason, conventionally, by installing an inner seal and an outer seal separately from the screw seal, sealing performance against internal pressure and external pressure has been ensured. The inner seal is formed by the contact between the seal surface provided at the tip of the pin in front of the male thread portion of the pin and the seal surface provided in the box corresponding thereto. The outer seal is formed by the contact between the seal surface provided behind the male thread portion of the pin and the seal surface provided in the box corresponding thereto. For example, U.S. Patent Application Publication No. 2010 / 0181763 (Patent Document 1) and International Publication WO2004 / 106797 (Patent Document 2) describe threaded joints that have an internal seal and an external seal in addition to the thread seal. In the threaded joint described in Patent Document 2, a gap is provided between the thread crest and the thread root when the fastening is completed. This gap contains excess lubricant applied during fastening (hereinafter also referred to as "dope"), and contributes to preventing abnormal pressure increases due to the dope. While conventional threaded joints described in Patent Documents 1 and 2 certainly offer improved sealing performance against pressure, threaded joints with an external seal reduce the cross-sectional area of ​​the pin's critical cross-section (the cross-section at the rear end of the male thread, i.e., the cross-section closest to the pipe body), thus lowering tensile strength. For this reason, the installation of an external seal is undesirable from the standpoint of the strength of the threaded joint. In threaded joints with internal seals, the sealing surface area at the pin tip may not have sufficient wall thickness. Therefore, when excessive external pressure is applied, the sealing surface area may undergo plastic deformation. This could result in a loss of sealing performance. U.S. Patent Application Publication No. 2010 / 0181763, International Publication WO2004 / 106797 In threaded joints, fluid leakage when excessive pressure (internal or external) is applied becomes a serious problem. In particular, for threaded joints using wedge-type threads, ensuring reliable sealing performance against pressure is crucial. The object of the present invention is to provide a threaded joint for steel pipes having the following characteristics: to reliably exhibit high sealing performance while maintaining high torque resistance performance due to wedge-type threads, i.e., dovetail-shaped tapered threads. A threaded joint for steel pipes according to one embodiment of the present invention comprises a tubular pin and a tubular box, wherein the pin is screwed into the box to fasten the pin and the box together. The pin comprises, in order from the pipe body toward the tip, a dovetail-shaped tapered male thread portion and a lip portion including a sealing surface. The box comprises a dovetail-shaped tapered female thread portion corresponding to the male thread portion of the pin and a recess including a sealing surface corresponding to the lip portion. The lip portion comprises, in order from the male thread portion toward the tip of the pin, a neck portion and a sealing head portion including the sealing surface. The maximum outer diameter of the sealing surface region in the sealing head portion is greater than the outer diameter at the boundary position with the male thread portion in the neck portion. The above-described threaded joint can be configured such that the inner diameter of the sealing head portion is smaller than the inner diameter of the pipe body. In this configuration, the inner circumferential surface of the lip portion includes a tapered surface that widens towards the tip of the pin, and a cylindrical surface connected to this tapered surface, with the boundary between the tapered surface and the cylindrical surface as the reference point, and it is preferable that the length of the cylindrical surface in the pipe axial direction is 3 mm or more. Alternatively, the above-described threaded joint can be configured such that the inner diameter of the sealing head portion is larger than the inner diameter of the pipe body. The above-mentioned screw joint preferably has the following configuration: When fastening is complete, a gap is provided between the thread crest surface of the male thread portion and the thread root surface of the female thread portion, and between the thread root surface of the male thread portion and the thread crest surface of the female thread portion. The above-mentioned threaded joint preferably has the following configuration: The wall thickness at the maximum outer diameter position of the sealing surface area in the sealing head portion is 55% to 80% of the wall thickness of the pipe body, and the minimum wall thickness of the neck portion is 45% to 70% of the wall thickness of the pipe body. The above-mentioned threaded joint preferably has the following configuration: When fastening is complete, the gap between the tip surface of the lip portion and the end of the recess of the box on the pipe body side is 0.1 mm or more and 3.0 mm or less. The above-mentioned threaded joint preferably has the following configuration: The taper angles of the male threaded portion and the female threaded portion are 1° to 5° with respect to the pipe axis. The above-described threaded joint preferably has the following configuration: The outer circumferential surface of the neck portion is a cylindrical surface, and the outer circumferential surface of the sealing head portion includes a tapered surface that is connected to the outer circumferential surface of the neck portion and widens at an angle of 5° to 20° with respect to the pipe axis, a cylindrical surface connected to this tapered surface, and a sealing surface which is composed of a curvatured surface connected to the cylindrical surface and a tapered surface that narrows towards the tip of the pin. The above-described threaded joint preferably has the following configuration: The sealing surface of the lip portion includes a tapered surface, and the taper angle of this tapered surface is 3° to 10° with respect to the pipe axis. The above-mentioned screw joint preferably has the following configuration: The screw height of the male screw portion and the female screw portion are 1.0 mm or more and 3.0 mm or less. The threaded joint for steel pipes of the present invention has the following remarkable effect: It can reliably exhibit high sealing performance while maintaining high torque resistance performance due to the wedge-type thread, i.e., a dovetail-shaped tapered thread. Figure 1 is a longitudinal cross-sectional view showing a threaded joint for steel pipes according to the first embodiment. Figure 2 is a longitudinal cross-sectional view showing the threaded portion of the threaded joint for steel pipes according to the first embodiment. Figure 3 is a longitudinal cross-sectional view showing an example of the tip of a pin in the threaded joint for steel pipes according to the first embodiment. Figure 4 is a longitudinal cross-sectional view showing an example of the tip of a pin in the threaded joint for steel pipes according to the second embodiment. Figure 5 is a diagram showing the evaluation results of torque resistance performance in Example 1. Figure 6 is a diagram showing the evaluation results of sealing performance in Example 1. Figure 7 is a schematic diagram showing the FEM analysis model in Example 2. Figure 8 is a diagram showing the evaluation results of sealing performance in Example 2. Figure 9 is a schematic diagram showing the FEM analysis model in Example 3. Figure 10 is a diagram showing the evaluation results of sealing performance in Example 3. The inventors had discovered that conventional screw joints, which employ wedge-type screw seals but lack internal and external seals, have particular problems with sealing performance against external pressure. This is based on numerical simulation analysis using the elastoplastic finite element method conducted by the inventors and the evaluation of the results. Therefore, the inventors first considered improving sealing performance against external pressure by installing an external seal, and further, an internal seal, as described in the screw joints in Patent Documents 1 and 2. However, as mentioned above, the installation of an external seal reduces the tensile strength of the threaded joint. Therefore, the inventors considered whether it would be possible to actively utilize an internal seal to maintain sealing performance against internal pressure while also ensuring sealing performance against external pressure, and investigated possible configurations. The pin has a lip portion that extends from the male threaded portion in the direction of the pipe axis. A sealing surface that forms an internal seal is provided on the outer surface of this lip portion. External pressure penetrates the threaded portion from outside the threaded joint and reaches the vicinity of the sealing surface of the pin's lip portion. The external pressure that reaches the pin's lip portion promotes a reduction in the diameter of the lip portion and acts to loosen the contact of the sealing surface. The inventors of this invention believe that increasing the rigidity of the lip portion of the pin and suppressing the diameter reduction deformation of the lip portion would be effective in preventing this phenomenon, and therefore focused on the shape of the lip portion. To increase the rigidity of the lip, one can simply enlarge the outer diameter of the lip and increase its wall thickness. Indeed, increasing the wall thickness of the lip improves the sealing performance against external pressure. However, in the case of a pin, if the outer diameter of the lip is drastically enlarged and its wall thickness is drastically increased, the starting position of the threading on the male screw shifts towards the outer circumference of the pin as the outer diameter of the lip increases. Therefore, it becomes necessary to either lower the screw height or shorten the length of the screw from the original design. As a result, the high torque resistance performance of the wedge-type screw may be lost. Furthermore, as the outer diameter of the lip increases, maintaining the wall thickness in the sealing surface area of ​​the box necessitates an increase in the outer diameter of the box (e.g., coupling). This significantly narrows the clearance between the internal and external threaded joints in the multi-layered well structure. Therefore, increasing the outer diameter of the box is undesirable. If the outer diameter of the box is maintained while the outer diameter of the pin's lip is enlarged to increase the wall thickness of the lip, the wall thickness of the sealing surface area in the box will inevitably decrease. In this case, the sealing performance against internal pressure may actually decrease. Based on the above, the inventors concluded that optimizing the shape and thickness of the pin's lip is effective in ensuring high sealing performance while maintaining high torque resistance with a wedge-type screw. The threaded joint of the present invention was completed based on the results of repeated studies following the above ideas. The embodiments of the threaded joint for steel pipes of the present invention are described below. [First Embodiment] Figure 1 is a longitudinal cross-sectional view showing a threaded joint for steel pipes according to the first embodiment. Figure 2 is a longitudinal cross-sectional view showing the threaded portion of the threaded joint for steel pipes. Figure 3 is a longitudinal cross-sectional view showing an example of the tip of a pin in the threaded joint for steel pipes. As shown in Figures 1 to 3, the threaded joint of the first embodiment is a coupling type threaded joint employing a wedge-type screw (dovetail tapered screw), and is composed of a pin 10 and a box 20. The pin 10 comprises a male threaded portion 11 and a lip portion 12, arranged in order from the pipe body to the tip. The lip portion 12 extends in the direction of the pipe axis CL, connected to the male threaded portion 11. The lip portion 12 is divided into a neck portion 14 and a sealing head portion 15, arranged in order from the tip of the pin 10. The sealing head portion 15 is provided with a sealing surface 13. On the other hand, the box 20 is provided with a female threaded portion 21 and a recessed portion 22, in order from the tip of the box 20 toward the pipe body. The female threaded portion 21 is provided corresponding to the male threaded portion 11 of the pin 10. The recessed portion 22 is provided corresponding to the lip portion 12 of the pin 10. The recessed portion 22 is provided with a sealing surface 23 corresponding to the sealing surface 13 of the pin 10. The neck portion 14 of the pin 10 connects the male threaded portion 11 and the sealing head portion 15. The outer circumferential surface 14a of the neck portion 14 is a cylindrical surface (hereinafter also referred to as the "neck outer circumferential cylindrical surface") centered on the pipe axis CL. In the threaded joint shown in Figure 3, the inner circumferential surface 14b of the neck portion 14 includes a cylindrical surface (hereinafter also referred to as the "neck inner circumferential cylindrical surface") 14ba on the male threaded portion 11 side that is the same as the inner diameter of the pipe body, and a tapered surface (hereinafter also referred to as the "neck inner circumferential tapered surface") 14bb on the sealing head portion 15 side, where the diameter decreases towards the tip of the pin 10. However, there may be cases where the neck inner circumferential cylindrical surface 14ba is absent. The outer circumferential surface 15a of the sealing head portion 15 includes a tapered surface (hereinafter also referred to as the "inner tapered surface of the head outer circumference") 15aa that is continuous with the outer circumferential surface 14a of the neck portion 14 and whose diameter increases towards the tip of the pin 10. A cylindrical surface (hereinafter also referred to as the "cylindrical surface of the head outer circumference") 15ab is continuous with this inner tapered surface of the head outer circumference 15aa, centered on the pipe axis CL. A sealing surface 13 is continuous with this cylindrical surface of the head outer circumference 15ab. The sealing surface 13 is composed of, for example, a curvatured surface (hereinafter also referred to as the "curvatured surface of the seal") 13a and a tapered surface (hereinafter also referred to as the "tapered surface of the seal") 13b. The curvatured surface of the seal 13a is a surface corresponding to the circumferential surface of a body of revolution obtained by rotating a curve such as a circular arc around the pipe axis CL. The tapered surface of the seal 13b is a tapered surface whose diameter decreases towards the tip of the pin 10. In the screw joint shown in Figure 3, the inner circumferential surface 15b of the sealing head portion 15 includes a tapered surface (hereinafter also referred to as the "inner tapered surface of the head") 15ba that is connected to the inner tapered surface 14bb of the neck at the same taper angle. A cylindrical surface (hereinafter also referred to as the "inner cylindrical surface of the head") 15bb centered on the pipe axis CL is connected to this inner tapered surface of the head 15ba. A tapered surface (hereinafter also referred to as the "tip tapered surface of the head") 15bc whose diameter slightly increases towards the tip of the pin 10 is connected to this inner cylindrical surface of the head 15bb. However, there are cases where the inner tapered surface of the head 15ba is absent. In this case, the inner cylindrical surface of the head 15bb extends into the area of ​​the neck portion 14 and connects to the inner tapered surface 14bb of the neck. The maximum outer diameter D1 of the sealing surface 13 region in the sealing head portion 15 is greater than the outer diameter D2 at the boundary position with the male thread portion 11 in the neck portion 14. The boundary position with the male thread portion 11 in the neck portion 14 corresponds to the starting position of threading into the male thread portion 11. With this configuration, the thickness of the sealing head portion 15 having the sealing surface 13 can be increased in the lip portion 12 of the pin 10 without shifting the starting position of threading into the male thread portion 11 towards the outer circumference of the pin 10. In particular, in the threaded joint of the first embodiment, the inner diameter d1 of the sealing head portion 15 is smaller than the inner diameter d0 of the pipe body. The inner diameter d1 of the sealing head portion 15 is smallest in the region of the inner circumferential cylindrical surface 15bb of the head. This can be easily achieved by pre-swaging the tip of the pin 10. The inner circumferential tip tapered surface 15bc and the inner circumferential cylindrical surface 15bb of the head are formed by cutting after swaging. The inner circumferential tapered surface 14bb of the neck may be the surface as it is after swaging or may be machined, depending on the specifications. Since the inner diameter d1 of the sealing head portion 15 is smaller than the inner diameter d0 of the pipe body, it is possible to make the wall thickness of the sealing head portion 15 sufficiently thick without significantly increasing the outer diameter of the sealing head portion 15. The male threaded portion 11 of the pin 10 and the female threaded portion 21 of the box 20 are dovetail-shaped tapered threads (wedge-type threads) that interlock with each other. The load surface 11c of the male threaded portion 11 and the load surface 21c of the female threaded portion 21, and the insertion surface 11d of the male threaded portion 11 and the insertion surface 21d of the female threaded portion 21, all have a negative flank angle θ of about 1° to 10° with respect to the pipe axis CL. The male threaded portion 11 and the female threaded portion 21 can be screwed into each other, and when fastening is complete, the load surfaces 11c, 21c and the insertion surfaces 11d, 21d come into contact with each other, and the entire threaded portion is firmly fitted together. The sealing surface 13 of the pin 10 and the sealing surface 23 of the box 20 come into contact with each other as the pin 10 is screwed in, and when fastening is complete, they fit together tightly in an interference fit state, forming an internal seal by surface contact. As described above, the screw joint of the first embodiment employs a wedge-type screw, making it possible to maintain high torque resistance. Furthermore, since the thickness of the sealing head portion 15 having the sealing surface 13 can be increased, the rigidity of the sealing surface 13 region is increased. As a result, even if external pressure reaches the vicinity of the sealing surface 13 of the lip portion 12, the internal seal provided by the sealing surface 13 exhibits high sealing performance. This is because the diameter reduction deformation of the sealing surface 13 region is suppressed. Moreover, as an essential function, the internal seal also exhibits high sealing performance against internal pressure. Furthermore, since the starting position for threading the male thread portion 11 does not need to be shifted towards the outer circumference of the pin 10, the length of the threaded portion can be secured while maintaining a high thread height. This ensures sufficient high torque resistance performance with the wedge-type screw. The following provides further details regarding preferred embodiments of the threaded joint according to the first embodiment. In the threaded joint of the first embodiment, when fastening is completed, a gap 31 is provided between the thread crest surface 11a of the male threaded portion 11 and the thread root surface 21b of the female threaded portion 21. This gap 31 prevents abnormal pressure rise due to doping. On the other hand, the thread root surface 11b of the male threaded portion 11 and the thread crest surface 21a of the female threaded portion 21 are in contact with each other. However, gaps may be provided both between the thread crest surface 11a of the male threaded portion 11 and the thread root surface 21b of the female threaded portion 21, and between the thread root surface 11b of the male threaded portion 11 and the thread crest surface 21a of the female threaded portion 21. A gap may be provided only between the thread root surface 11b of the male threaded portion 11 and the thread crest surface 21a of the female threaded portion 21. The preferred range for the gap is 0.05 to 0.5 mm. Within this range, the engagement height between the male thread portion 11 and the female thread portion 21 is ensured, allowing for high torque performance. The taper angle of the male threaded portion 11 and the female threaded portion 21 is preferably 1° to 5° with respect to the pipe axis CL. If the taper angle of the threaded portion is too large, the length of the threaded portion becomes too short, reducing the torque resistance performance. On the other hand, if the taper angle of the threaded portion is too small, the length of the threaded portion becomes too long, increasing manufacturing costs. A more preferable lower limit for the taper angle is 1.5°, and an even more preferable lower limit is 2°. A more preferable upper limit for the taper angle is 4°. The thread height of the male thread portion 11 and the female thread portion 21 is preferably 1.0 mm or more and 3.0 mm or less. If the thread height is too high, manufacturing costs will increase, the wall thickness of the box 20 will decrease, and the sealing performance against internal pressure will deteriorate. On the other hand, if the thread height is too low, the torque resistance performance will deteriorate. A more preferable lower limit for the thread height is 1.2 mm, and an even more preferable lower limit is 1.5 mm. In the sealing head portion 15 that constitutes the lip portion 12 of the pin 10, it is preferable that the wall thickness at the maximum outer diameter position of the sealing surface 13 region is 55% to 80% of the wall thickness of the pipe body. If the wall thickness of the sealing surface 13 region is too thin, the rigidity will decrease, and sealing performance against external pressure cannot be ensured. On the other hand, if the wall thickness of the sealing surface 13 region is too thick, the rigidity will increase. Conversely, if the thickness of the sealing surface 13 region is increased by increasing the outer diameter of the sealing head portion 15, the diameter of the sealing surface 23 of the box 20 will increase, and the resulting decrease in the wall thickness of the box 20 may reduce the sealing performance against internal pressure. Furthermore, if the thickness of the sealing surface 13 region is increased by reducing the inner diameter of the sealing head portion 15, it is necessary to keep the inner diameter of the sealing head portion 15, especially the inner diameter of the inner circumferential cylindrical surface 15bb of the head, within the inner diameter specified by the API standard. In contrast, the minimum wall thickness of the neck portion 14 that constitutes the lip portion 12 of the pin 10 is preferably 45% to 70% of the wall thickness of the pipe body. If the minimum wall thickness of the neck portion 14 is too thin, the rigidity of the neck portion 14 will decrease, making it impossible to ensure sealing performance against external pressure. On the other hand, if the minimum wall thickness of the neck portion 14 is too thick, the threading start position will be shifted towards the outer circumference of the pin 10, resulting in a situation where the thread height is reduced or the length of the threaded portion is shortened, and the torque resistance performance of the wedge-type screw is not achieved. Of the outer peripheral surface 15a of the sealing head portion 15, the tapered surface 15aa on the inner outer peripheral surface of the head that is connected to the outer peripheral surface 14a of the neck portion 14 is preferably 5° to 20° with respect to the pipe axis CL. If the tapered angle of the inner outer peripheral tapered surface 15aa of the head is large, the rigidity of the sealing surface 13 region increases with the increase in wall thickness of the sealing head portion 15, but the wall thickness of the box 20 decreases, and the sealing performance against internal pressure deteriorates. On the other hand, if the tapered angle of the inner outer peripheral tapered surface 15aa of the head is too small, it is not possible to secure the wall thickness of the sealing surface 13 region, and the sealing performance against external pressure deteriorates. Of the inner circumferential surfaces 14b and 15b of the lip portion 12, the length L1 in the direction of the pipe axis CL of the head inner circumferential cylindrical surface 15bb (including cases where it extends to the area of ​​the neck portion 14) is preferably 3 mm or more. This length L1 is the length based on the boundary between the tapered surface 15bc at the tip of the head inner circumferential surface and the head inner circumferential cylindrical surface 15bb. The length L2 of the tapered surface 15bc at the tip of the head inner circumferential surface in the direction of the pipe axis CL is approximately 8 to 12 mm. If the length L1 of the head inner circumferential cylindrical surface 15bb is too short, the rigidity of the sealing head portion 15 will not increase sufficiently, and thus sealing performance against external pressure cannot be ensured. A more preferable lower limit for the length L1 is 4 mm or more. On the other hand, the longer the length L1, the better. However, the length L1 depends on the degree of swaging. For this reason, a preferable upper limit for the length L1 is 15 mm or less. Regarding the seal tapered surface 13b that constitutes the seal surface 13 of the lip portion 12, it is preferable that its taper angle is 3° to 10° with respect to the pipe axis CL. If the taper angle of the seal tapered surface 13b is large, the rigidity decreases as the wall thickness decreases at the tip of the seal surface 13 region, resulting in a decrease in sealing performance against external pressure. On the other hand, if the taper angle of the seal tapered surface 13b is too small, when a tensile load is applied to the threaded joint, the seal surfaces 13 and 23 may slip against each other, potentially causing them to lose contact. When the fastening is complete, a gap 30 is provided between the tip surface of the lip portion 12 of the pin 10 and the pipe body side end of the recess 22 of the box 20 (see Figure 1). Preferably, the gap 30 is 0.1 mm or more and 3.0 mm or less. If there is no gap 30, the tip surface of the pin 10 will inadvertently come into contact with the recess 22 of the box 20 during the fastening process. As a result, the fit between the male thread portion 11 and the female thread portion 21 will be insufficient, and torque resistance performance cannot be obtained. On the other hand, if the gap 30 is too large, turbulence of the fluid flowing inside the screw joint will occur near the gap 30, which may induce erosion. [Second Embodiment] Figure 4 is a longitudinal cross-sectional view showing an example of the tip of a pin in the threaded joint for steel pipes of the second embodiment. The threaded joint of the second embodiment shown in Figure 4 is a modification of the threaded joint of the first embodiment shown in Figures 1 to 3, and explanations that overlap with the first embodiment will be omitted as appropriate. In the threaded joint of the second embodiment, the inner circumferential tapered surface 14bb of the neck portion 14 constituting the lip portion 12 is a tapered surface whose diameter increases towards the tip of the pin 10. As a result, the inner diameter d1 of the sealing head portion 15 is larger than the inner diameter d0 of the pipe body. However, in the second embodiment as in the first embodiment, the maximum outer diameter D1 of the sealing surface 13 region of the sealing head portion 15 is larger than the outer diameter D2 at the boundary position with the male thread portion 11 of the neck portion 14. Therefore, the threaded joint of the second embodiment also produces the same effects as in the first embodiment. However, in the second embodiment, increasing the wall thickness of the sealing surface 13 region leads to an increase in the outer diameter of the sealing surface 13 region. As a result, the diameter of the sealing surface 23 of the box 20 increases, and the resulting decrease in the wall thickness of the box 20 may reduce the sealing performance against internal pressure. In this respect, the first embodiment is superior. Furthermore, the present invention is not limited to the embodiments described above, and various modifications are possible without departing from the spirit of the invention. For example, the screw coupling of the above embodiment can be applied not only to coupling type but also to integral type couplings. To confirm the effects of the present invention, numerical simulation analysis was performed using the elastoplastic finite element method. [Example 1] <Test Conditions> The FEM analysis model was based on a coupling-type threaded joint for oil well pipes employing a wedge-type thread. As an example of the present invention, a model based on the threaded joint of the first embodiment shown in Figures 1 to 3 was prepared. As a comparative example, a model of a conventional threaded joint was prepared in which the maximum outer diameter of the sealing surface area in the sealing head portion is smaller than the outer diameter at the threading start position of the neck portion. The main dimensions of each model are shown in Table 1 below. The common conditions are as follows: • Threaded fitting (steel pipe) size: 7-5 / 8 [inch] x 55.3 [lb. / ft.] • Steel pipe (pin) and coupling (box) grade: API standard T95 (carbon steel with a yield strength of 95 [ksi] (655 [MPa])) <Evaluation Method> In the FEM analysis, a load history simulating the ISO 13679 Series A test was applied to each model in a completed fastening state. The value of the point at which the torque curve begins to yield (yield torque) was adopted, and the torque resistance performance was evaluated by comparing these values. Furthermore, the sealing performance was evaluated by comparing the minimum value of the sealing contact force of the sealing surface (product of the average sealing contact pressure and sealing contact width of the sealing surface) in the internal pressure cycle (first and second quadrants) and external pressure cycle (third and fourth quadrants) of the history (a higher value indicates better sealing performance of the sealing surface). <Test Results> Figure 5 shows the evaluation results of torque resistance performance in Example 1. Figure 6 shows the evaluation results of sealing performance in Example 1. As shown in Figure 5, both the present invention example and the comparative example were able to exhibit high torque resistance performance. As shown in Figure 6, contact of the sealing surface was lost in the comparative example. In contrast, the sealing performance of the present invention example was significantly improved. [Example 2] <Test Conditions> The FEM analysis model was based on a coupling-type oil well pipe threaded joint employing a wedge-type thread, and various changes were made to the shape of the lip portion based on the threaded joints of the first and second embodiments shown in Figures 1 to 4. Specifically, as shown in Figure 7, the inner diameter of the sealing head portion was changed, thereby changing the ratio of the wall thickness at the maximum outer diameter position of the sealing surface area of ​​the sealing head portion to the wall thickness of the pipe body (hereinafter also referred to as "wall thickness ratio of the sealing portion to the pipe body") to five levels: 50%, 56%, 61%, 74%, and 83%. The main dimensions of each model are shown in Table 2 below. The common conditions are as follows: • Threaded fitting (steel pipe) size: 7 [inch] x 35.0 [lb. / ft.] • Steel pipe (pin) and coupling (box) grade: API standard L80 (carbon steel with a yield strength of 80 [ksi] (552 [MPa])) Tests No. 1 to 3 are based on the threaded joint of the second embodiment shown in Figure 4, in which the inner diameter of the sealing head is larger than the inner diameter of the pipe body. Tests No. 4 and 5 are based on the threaded joint of the first embodiment shown in Figure 3, in which the inner diameter of the sealing head is smaller than the inner diameter of the pipe body. <Evaluation Method> Similar to Example 1, a load history simulating the ISO 13679 Series A test was applied to each model in the completed fastening state, and the sealing performance was evaluated by comparing the minimum values ​​of the sealing contact force on the sealing surface. <Test Results> Figure 8 shows the evaluation results of the sealing performance in Example 2. From the results shown in Figure 8, it was found that the sealing performance improved with increasing the ratio of the wall thickness of the sealing portion to the pipe body, that is, with increasing the wall thickness of the lip portion. In particular, regarding the degree of increase in the wall thickness of the lip portion, it was found that sufficient sealing performance can be ensured if the ratio of the wall thickness of the sealing portion to the pipe body is 55% or more. [Example 3] <Test Conditions> The FEM analysis model was based on a coupling-type oil well pipe threaded joint employing a wedge-type thread, and various changes were made to the shape of the lip portion based on the threaded joint of the first embodiment shown in Figures 1 to 3. Specifically, as shown in Figure 9, the length of the inner cylindrical surface of the head was changed in four stages: 2 mm, 3 mm, 4 mm, and 5 mm. The main dimensions of each model are shown in Table 3 below. The common conditions are as follows: • Threaded fitting (steel pipe) size: 3-1 / 2 inches x 9.2 lb. / ft. • Steel pipe (pin) and coupling (box) grade: API standard L80 (carbon steel with a yield strength of 80 kSi (552 MPa)) <Evaluation Method> Similar to Examples 1 and 2 described above, a load history simulating the ISO 13679 Series A test was applied to each model in the completed fastening state, and the sealing performance was evaluated by comparing the minimum values ​​of the sealing contact force on the sealing surface. <Test Results> Figure 10 shows the evaluation results of the sealing performance in Example 3. From the results shown in Figure 10, it was found that the sealing performance improved with increasing length of the inner cylindrical surface of the head. In particular, it was found that sufficient sealing performance could be ensured when the length of the inner cylindrical surface of the head was 3 mm or more. The threaded joint of the present invention can be effectively used to connect steel pipes used when excavating, producing, or transporting underground resources. Underground resources include fossil fuels such as crude oil, natural gas, shale gas, and methane hydrate, as well as gaseous or liquid underground resources such as groundwater and hot springs. 10: Pin, 11: Male thread portion, 11a: Thread crest, 11b: Thread root bottom, 11c: Load surface, 11d: Insertion surface, 12: Lip portion, 13: Sealing surface, 14: Neck portion, 15: Sealing head portion, 20: Box, 21: Female thread portion, 21a: Thread crest, 21b: Thread root bottom, 21c: Load surface, 21d: Insertion surface, 22: Recess, 23: Sealing surface, 30, 31: Gap, D1: Maximum outer diameter of the sealing surface area in the sealing head portion, D2: Outer diameter at the boundary position with the male thread portion in the neck portion, d1: Inner diameter of the sealing head portion, d0: Inner diameter of the pipe body, CL: Pipe axis, L1: Length in the pipe axis direction of the inner cylindrical surface of the sealing head, L2: Length in the pipe axis direction of the inner tapered tip surface of the sealing head

Claims

1. A threaded joint for steel pipes comprising a tubular pin and a tubular box, wherein the pin is screwed into the box to fasten the pin and the box, wherein the pin comprises, in order from the pipe body of the pin toward the tip, a dovetail-shaped tapered male thread portion and a lip portion including a sealing surface, the box comprises a dovetail-shaped tapered female thread portion corresponding to the male thread portion of the pin and a recess including a sealing surface corresponding to the lip portion, the lip portion comprises, in order from the male thread portion toward the tip of the pin, a neck portion and a sealing head portion including the sealing surface, and the maximum outer diameter of the sealing surface area in the sealing head portion is greater than the outer diameter at the boundary position with the male thread portion in the neck portion.

2. A threaded joint for steel pipes according to claim 1, wherein the inner diameter of the sealing head portion is smaller than the inner diameter of the pipe body.

3. A threaded joint for steel pipes according to claim 2, wherein the inner circumferential surface of the lip portion includes a tapered surface that widens towards the tip side of the pin, and a cylindrical surface connected to the tapered surface, and the length of the cylindrical surface in the pipe axis direction is 3 mm or more, with the boundary between the tapered surface and the cylindrical surface as the reference.

4. A threaded joint for steel pipes according to claim 1, wherein the inner diameter of the sealing head portion is larger than the inner diameter of the pipe body.

5. A threaded joint for steel pipes according to any one of claims 1 to 4, wherein, when fastening is completed, a gap is provided between the thread crest surface of the male thread portion and the thread root surface of the female thread portion, and between the thread root surface of the male thread portion and the thread crest surface of the female thread portion.

6. A threaded joint for steel pipes according to any one of claims 1 to 5, wherein the wall thickness at the maximum outer diameter position of the sealing surface area in the sealing head portion is 55% or more and 80% or less of the wall thickness of the pipe body, and the minimum wall thickness of the neck portion is 45% or more and 70% or less of the wall thickness of the pipe body.

7. A threaded joint for steel pipes according to any one of claims 1 to 6, wherein, when fastening is completed, the gap between the tip surface of the lip portion and the recess of the box is 0.1 mm or more and 3.0 mm or less.

8. A threaded joint for steel pipes according to any one of claims 1 to 7, wherein the taper angles of the male threaded portion and the female threaded portion are 1° or more and 5° or less with respect to the pipe axis.

9. A threaded joint for steel pipes according to any one of claims 1 to 8, wherein the outer circumferential surface of the neck portion is a cylindrical surface, and the outer circumferential surface of the sealing head portion includes a tapered surface that is connected to the outer circumferential surface of the neck portion and widens by 5° to 20° with respect to the pipe axis, a cylindrical surface connected to this tapered surface, and a sealing surface which is composed of a curvatured surface connected to the cylindrical surface and a tapered surface that narrows towards the tip of the pin.

10. A threaded joint for steel pipes according to any one of claims 1 to 9, wherein the sealing surface of the lip portion includes a tapered surface, and the taper angle of the tapered surface is 3° or more and 10° or less with respect to the pipe axis.

11. A threaded joint for steel pipes according to any one of claims 1 to 10, wherein the thread height of the male threaded portion and the female threaded portion are 1.0 mm or more and 3.0 mm or less.

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