Threaded fittings for steel pipes

Abstract

Provided is a slim threaded joint that is for steel pipes and that is capable of exhibiting high torque resistance performance and high sealing performance. A threaded joint 1 for steel pipes comprises: a pin 10 having a male thread 11 formed of a wedged thread having a two-step thread structure; and a box 20 having a female thread 21 including a wedged thread having a two-step thread structure. The pin 10 and the box 20 respectively have, at a middle portion of the two-step thread, inner seal portions 13, 23 and outer seal portions 14, 24, and respectively have shoulder surfaces 12, 22 provided between the inner and outer seal portions. In a fastened state, the male thread 11 is tightly attached to and fitted with the female thread 21 to generate a locking effect. Meanwhile, in the fastened state, there is a minute gap between the shoulder surfaces 12, 22. The size of the gap is determined such that the shoulder surfaces 12, 22 make contact with each other by single round or several rounds of a yield compression load Lc on the thread joint 1.

Description

[Technical Field]

[0001] The present disclosure relates to a threaded joint for steel pipes used for connecting steel pipes. [Background Art]

[0002] For example, in exploration or production of oil wells, natural gas wells, etc. (hereinafter collectively referred to as "oil wells"), development of unconventional resources such as oil sands and shale gas, carbon dioxide capture and storage (CCS), geothermal power generation, hot springs, etc., steel pipes called oil country tubular goods are used. Threaded joints are used to connect steel pipes to each other.

[0003] Types of this kind of threaded joint for steel pipes are broadly classified into coupling type and integral type.

[0004] In the case of the coupling type, steel pipes are connected to each other via a tubular coupling. Typically, internal threads are provided on the inner periphery of both ends of the coupling, and external threads are provided on the outer periphery of both ends of the steel pipes. Then, one end of one steel pipe is screwed into one end of the coupling, and one end of another steel pipe is screwed into the other end of the coupling, thereby connecting the steel pipes to each other. That is, in the coupling type, among a pair of directly connected pipe members, one pipe member is a steel pipe and the other pipe member is a coupling.

[0005] In the case of the integral type, steel pipes are directly connected to each other without using a separate coupling. Specifically, an internal thread is provided on the inner periphery of one end of a steel pipe, and an external thread is provided on the outer periphery of the other end. One end of a first steel pipe provided with the internal thread is screwed with the other end of a second steel pipe provided with the external thread, thereby connecting the steel pipes to each other.

[0006] Generally, the joint portion at the end of a steel pipe with a male threaded section is called a "pin" because it includes an element that is inserted into the female threaded section formed in the steel pipe or coupling. The end of a steel pipe or coupling with a female threaded section is called a "box" because it includes an element that receives the male threaded section of the steel pipe. Since these pins and boxes are the ends of pipe materials, they are both tubular in shape.

[0007] In recent years, the depth and ultra-deep-sea exploration of oil wells have been steadily increasing. In such environments, multiple layers of oil well tubular construction are used to efficiently develop oil wells. Slim threaded joints, in which the outer diameter of the box is approximately 100-105% of the outer diameter of the pin, are used to connect these multiple layers of oil well tubular construction. This is because a clearance is required between the inner and outer layers of the multiple layers, and the outer diameter of the box of the inner oil well tubular construction is limited by the inner diameter of the pin of the outer oil well tubular construction. Even under these constraints, threaded joints are required to have excellent sealing performance against both internal pressure and external pressure. Furthermore, in deep wells, sealing performance under compressive loads is particularly important.

[0008] Furthermore, with the increasing adoption of well development technologies such as DwC (Drilling with Casing) and horizontal drilling in recent years, the demand for high-torque fittings has surged. As a result, there is a need for threaded fittings that can achieve both high torque resistance and excellent sealing performance within a limited pipe wall thickness.

[0009] In the threaded joint disclosed in Patent Document 1 below, the threaded portion is divided into two along the pipe axis, and a torque shoulder is provided at the boundary between the two threaded portions, with a metal seal portion provided adjacent to this shoulder. This threaded joint aims to improve torque resistance and sealing performance by combining gapless fitting and tight contact through screwing wedge-shaped threads together with pressing contact between the shoulders.

[0010] However, in the case of the screw joint described in Patent Document 1, in order to achieve a tight, gap-free fit when fastened, the design dimensional tolerances of both the wedge thread and the shoulder must be extremely small. This not only increases manufacturing costs, but can also lead to the processed dimensions falling outside the tolerances. If tolerances are exceeded, the locking effect of the wedge thread may not be fully realized, and the expected sealing performance and torque resistance may not be achieved.

[0011] In the threaded joint disclosed in Patent Document 2 below, in order to solve the above-mentioned problems in the threaded joint of Patent Document 1, the shoulder surfaces are in contact with each other when fastened, but a gap is formed between the insertion surfaces of the male and female threads, which are made of wedge-shaped threads. [Prior art documents] [Patent Documents]

[0012] [Patent Document 1] U.S. Patent No. 7690696 [Patent Document 2] International Publication No. 2017 / 145192 [Overview of the project]

[0013] However, as with the screw joint in Patent Document 2, if a gap is formed between the insertion surfaces of the wedge-shaped screws when fastened, the torque resistance performance due to the locking effect between the wedge-shaped screws will not be achieved, and sufficient torque resistance performance may not be obtained by contact between the shoulder surfaces provided in the middle of the two-stage screw.

[0014] The purpose of this disclosure is to provide a slim-type threaded fitting for steel pipes that can exhibit high torque resistance and high sealing performance.

[0015] The threaded joint for steel pipes of this disclosure 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 includes a male thread including an internal thread portion and an external thread portion spaced apart in the axial direction, a shoulder surface provided between the internal thread portion and the external thread portion of the male thread, an internal pressure sealing portion (hereinafter also referred to as the "internal sealing portion") provided on the outer circumferential surface of the pin on the tip side of the steel pipe side of the shoulder surface, and an external pressure sealing portion (hereinafter also referred to as the "external sealing portion") provided on the outer circumferential surface of the pin on the pipe body side of the steel pipe side of the shoulder surface. The box comprises a female screw including an internal thread portion into which the internal thread portion of the male screw fits when fastened and an external thread portion into which the external thread portion of the male screw fits; a shoulder surface provided between the internal and external thread portions of the female screw and facing the shoulder surface of the pin in the axial direction; an internal seal portion provided corresponding to the internal seal portion of the pin and in contact with the internal seal portion of the pin when fastened; and an external seal portion provided corresponding to the external seal portion of the pin and in contact with the external seal portion of the pin when fastened. The internal and external thread portions of the male screw and the internal and external thread portions of the female screw are all made of wedge-shaped threads, and in the fastened state, at least a portion of the male screw has a thread profile such that its insertion surface and load surface both contact the insertion surface and load surface of the female screw.

[0016] Preferably, when fastened, the shoulder surfaces of the pin and the box are provided such that a gap is formed between them when no axial compressive load is applied. The size of the gap is determined such that the shoulder surfaces of the pin and the box come into contact with each other when a load Lc, represented by the following formula (1), is applied. Lc = Lp × JE ... (1) Here, Lp is the yield compressive load of the pipe body of the steel pipe under no internal or external pressure load, and JE is the joint efficiency.

[0017] According to this disclosure, it is possible to provide a slim-type threaded fitting for steel pipes that can achieve both high torque resistance and excellent sealing performance under limited pipe wall thickness. [BRIEF DESCRIPTION OF THE DRAWINGS]

[0018] [Figure 1] Figure 1 is a longitudinal sectional view showing a fastened state of a threaded joint for steel pipes according to one embodiment of the present invention. [Figure 2] Figure 2 is an enlarged sectional view of the threaded fitting structure of the threaded joint shown in Figure 1. [Figure 3] Figure 3 is an enlarged sectional view of the intermediate portion of the two-stage thread structure of the threaded joint shown in Figure 1. [Figure 4] Figure 4 is a diagram showing the path of load conditions in the evaluation of sealing performance by FEM analysis. [MODE FOR CARRYING OUT THE INVENTION]

[0019] The threaded joint for steel pipes according to the present embodiment includes a tubular pin provided at the distal end portion of a steel pipe, and a tubular box into which the pin is screwed and fastened to the pin. The box may be provided at a pipe end of another steel pipe connected to the steel pipe including the pin, or may be provided at a pipe end of a tubular coupling for connecting two steel pipes. When a box is provided at a pipe end of another steel pipe, after diameter expansion processing is performed on the pipe end of the other steel pipe, an internal thread, a shoulder surface, an inner seal portion and an outer seal portion can be formed on the inner circumference thereof by an appropriate processing method such as turning.

[0020] The pin includes an external thread having a two-stage thread structure including an inner thread portion and an outer thread portion spaced apart in the axial direction, a shoulder surface provided between the inner thread portion and the outer thread portion of the external thread, an inner seal portion provided on the outer peripheral surface of the pin on the distal end side of the steel pipe relative to the shoulder surface, and an outer seal portion provided on the outer peripheral surface of the pin on the pipe body side of the steel pipe relative to the shoulder surface.

[0021] Said box comprises an internal thread with a two-stage thread structure including an internally threaded portion and an externally threaded portion spaced apart in an axial direction, a shoulder surface provided between the internally threaded portion and the externally threaded portion of said internal thread and axially opposing a shoulder surface of the pin, an inner seal portion provided corresponding to an inner seal portion of the pin and contacting the inner seal portion of the pin in a fastened state, and an outer seal portion provided corresponding to an outer seal portion of the pin and contacting the outer seal portion of the pin in a fastened state. In the fastened state, the internally threaded portion of the external thread fits into the internally threaded portion of the internal thread, and the externally threaded portion of the external thread fits into the externally threaded portion of the internal thread.

[0022] The internally threaded portion and the externally threaded portion of said external thread, and the internally threaded portion and the externally threaded portion of said internal thread are each formed of wedge-shaped threads, and more preferably each formed of tapered threads whose diameter gradually decreases toward a distal end side of the pin. The wedge-shaped thread has a thread profile in which a thread width gradually narrows toward the distal end along the spiral of the thread, and a thread groove width gradually increases. In general, the cross-sectional shape of the thread ridge and thread groove of a wedge-shaped thread is dovetail-shaped, but the threaded joint of the present invention may also be a wedge-shaped thread having other conventionally known thread ridge cross-sectional shapes, such as a thread ridge cross-sectional shape similar to that of trapezoidal threads or API standard buttress threads. Furthermore, one of the internally threaded portion and the externally threaded portion may be formed of a dovetail-shaped thread ridge, and the other may be formed of a thread ridge having another cross-sectional shape.

[0023] It should be noted that both the load face and the insertion face of the dovetail-shaped thread ridge each have a negative flank angle. For example, the flank angle of the load face and the insertion face may be -10° to -1°. Furthermore, for the internally threaded portion and the externally threaded portion of the external thread, and the internally threaded portion and the externally threaded portion of the internal thread, the cross-sectional shape of the thread crest and the cross-sectional shape of the thread root may be a line segment parallel to the axis of the steel pipe, or may be a line segment inclined along the overall taper angle of each threaded portion.

[0024] In the screw joint of the present invention, the shoulder surfaces of the pin and box do not need to function as torque shoulders, but are provided primarily to bear the compressive load when an axial compressive load is applied. Torque resistance is achieved by the locking effect caused by the wedge-shaped male thread fitting into the female thread, where not only the load surfaces but also the insertion surfaces come into contact in the fastened state. However, it is not necessary for all parts of the insertion surfaces of the male and female threads to come into contact in the fastened state. As long as the required torque performance can be obtained, the insertion surface and load surface of at least a part of the fitting portion between the male and female threads can be configured to come into contact with the insertion surface and load surface of the female thread. For example, the screw profiles of the internal and external threads of the male and female threads can be designed such that, in the fastened state, the load surfaces and insertion surfaces come into contact over almost the entire length of the internal thread portion, while the load surfaces come into contact over almost the entire length of the external thread portion, but the insertion surfaces do not come into contact over almost the entire length of the external thread portion. Furthermore, the thread profiles of the internal and external threads of the male and female threads can be designed such that, in the fastened state, the load surfaces and insertion surfaces of the external threads are in contact with each other over almost their entire length, while in the internal threads, the load surfaces are in contact with each other over almost their entire length, but the insertion surfaces are not in contact with each other over almost their entire length.

[0025] Furthermore, there may or may not be a gap between the thread crest of the male thread and the thread root of the female thread, and between the thread root of the male thread and the thread crest of the female thread. If a lubricating compound grease called dope is applied to at least one of the male and female threads, the design may be such that a gap is created between the thread crest of the male thread and the thread root of the female thread when fastened, taking into consideration the discharge of the dope.

[0026] The shoulder surfaces of the pin and the box are positioned such that, in a fastened state where both the load surface and insertion surface of the male thread are in contact with the load surface and insertion surface of the female thread, a gap is formed between the shoulder surfaces of the pin and the box when there is no axial compressive load and no internal or external pressure load. Each shoulder surface may be ring-shaped when viewed from the axial direction. Each shoulder surface may be perpendicular to the pipe axis in a longitudinal cross-sectional view, or it may be inclined. Preferably, the shoulder surface of the pin and the shoulder surface of the box are parallel. Furthermore, each shoulder surface is preferably a flat surface without irregularities, but grooves or protrusions may be provided. In this case, the size of the gap described below may be the size of the smallest gap.

[0027] The size of the gap is determined such that the shoulder surfaces of the pin and the box come into contact with each other after one or more loads of the load Lc represented by the following formula (1) are applied. More preferably, when there is no internal or external pressure, the load Lc is applied so that the area near the fitting portion of the male and female threads, where the insertion surfaces are in contact, undergoes elastic deformation and comes into contact with each other.

[0028] Lc = Lp × JE ... (1)

[0029] Here, Lp is the yield compressive load of the steel pipe body under no internal or external pressure load, and JE is the joint efficiency. Lp may be the yield compressive load of the steel pipe body under no internal or external pressure load, calculated according to the von Mises yield condition. Such a yield compressive load of the pipe body under the von Mises yield condition is uniquely determined by the outer diameter, wall thickness, and material of the pipe body. For example, Lp can be the yield compressive load of the pipe body under no internal or external pressure load in the yield curve (also called the yield ellipse) for 100% VME (von Mises equivalent stress), which is the yield condition for the pipe body as defined in ISO 13679.

[0030] Furthermore, this disclosure also includes cases where the shoulder surfaces do not contact each other when the yield compressive load Lc of the threaded joint is first applied, but contact occurs when the yield compressive load Lc of the threaded joint is applied for the second time or later in a load test involving repeated compression-tensile loads. In the case of oil well pipes, it is sufficient for the shoulder surfaces to contact each other at any load step in a combined load cycle of internal and external pressure and tensile compressive load in accordance with ISO 13679, where there is no internal or external pressure and the yield compressive load of the threaded joint is applied.

[0031] More preferably, when the load Lc has been applied a predetermined number of times and damage has accumulated in the male and female threads, the size of the gap can be determined such that the shoulder surfaces come into contact with an axial compressive load smaller than the load Lc.

[0032] According to the present invention, as the axial compressive load acting on a threaded joint consisting of a fastened pin and a box increases, the shoulder surfaces of the pin and the box come into contact with each other before the yield compressive load of the threaded joint is reached. Before the shoulder surfaces come into contact, the compressive load is distributed and borne by the mating portion of the male and female threads, where both the load surfaces and insertion surfaces are in contact. After the shoulder surfaces come into contact, not only the male and female threads but also the shoulder surfaces of the pin and the box bear the compressive load. Therefore, the overall axial displacement of the box relative to the pin after the shoulder surfaces come into contact is suppressed, and damage to each threaded portion consisting of wedge-shaped threads can be reduced. Subsequently, even if the load condition changes to internal pressure + tensile load, the locking effect due to the tight mating of the wedge-shaped threads allows each sealing portion to come into contact with each other at the correct relative position, preventing a decrease in sealing performance under repeated combined loads. Furthermore, the contact between the shoulder surfaces during compressive load application suppresses deformation of the male and female threads, improving their resistance to repeated loads acting on these threaded portions and making them less prone to breakage.

[0033] Furthermore, in screw joints where the male and female threads are wedge-shaped, a gap exists between the shoulder surfaces when the pin and the box are fastened, which stabilizes the tightening torque required to determine when fastening is complete. Therefore, it becomes easier to ensure the quality of the tightening work for steel pipe screw joints.

[0034] The joint efficiency JE may be the ratio of the smaller of the two cross-sectional areas of the pin hazard section PCCS and the box hazard section BCCS to the area of ​​the cross-sectional surface of the steel pipe body. The pin hazard section PCCS is the cross-section of the pin at the root of the thread in the external thread of the male screw at the pipe body side of the engaging end of the pin between the external thread of the male screw and the external thread of the female screw in the fastened state. The box hazard section BCCS is the cross-section of the box at the root of the thread in the internal thread of the female screw at the tip side of the pin at the engaging end of the pin between the internal thread of the male screw and the internal thread of the female screw in the fastened state. Since the tensile and compressive strength of the threaded joint is governed by the smaller of the two cross-sectional areas of the pin hazard section PCCS and the box hazard section BCCS, it is customary to use the ratio of the smaller of the two cross-sectional areas of the pin hazard section PCCS and the box hazard section BCCS to the cross-sectional surface of the pipe body as the joint efficiency JE and to calculate the maximum allowable load of the threaded joint.

[0035] In the case of a two-stage threaded joint, there are also intermediate critical sections (PICCS) for the pin and BICCS for the box at the middle of the joint (see Figure 1). The PICCS is the cross-section of the pin at the root of the thread in the internal thread of the male screw at the end of the engagement between the internal thread of the male screw and the internal thread of the female screw in the fastened state, on the pipe body side of the pin. The BICCS is the cross-section of the box at the root of the thread in the external thread of the female screw at the end of the engagement between the external thread of the male screw and the external thread of the female screw in the fastened state, on the tip side of the pin. The tensile load is distributed between the PICCS and BICCS, but in order to ensure strength at these intermediate critical section locations, it is preferable that the sum of the cross-sectional areas of the PICCS and BICCS is greater than the cross-sectional area of ​​at least one of the PCCS and BCCS.

[0036] The size of the gap formed between the shoulder surfaces in the fastened state should be less than the amount by which the gap between the shoulder surfaces narrows when the yield compressive load Lc of the threaded joint, represented by formula (1), is applied, and more preferably less than half of that amount. In one example, if the male and female threads can be deformed as a whole so that the gap between the shoulder surfaces narrows by 0.2 mm before and after the application of the yield compressive load Lc of the threaded joint, the size of the gap may be less than 0.2 mm, and more preferably less than 0.1 mm. The size of the gap can be adjusted in proportion to the outer diameter of the steel pipe body. In one example, the size of the gap may be less than 0.082% of the outer diameter of the steel pipe body, and more preferably less than 0.041%.

[0037] When the internal seal portion of the pin is provided between the shoulder surface and the internal thread portion of the pin, it is preferable that the axial distance between the shoulder surface and the internal seal portion of the pin is greater than twice the pipe wall thickness of the pin in the cross-section including the internal seal portion. For example, when the size of the steel pipe body is 200 to 360 mm in diameter, the axial distance between the shoulder surface and the internal seal portion of the pin may be 20 mm or more and 45 mm or less, more preferably 25 mm or more and 35 mm or less. When a shoulder surface is provided between the male and female threads of a two-stage thread structure, and an internal seal portion is provided between this shoulder surface and the internal thread portion, the pipe wall thickness of the pin in the cross-section including the internal seal portion can be made larger compared to when the internal seal portion is provided at the pipe end of the pin, ensuring the rigidity of the pin near the internal seal portion and thereby improving the sealing performance. On the other hand, when the shoulder surfaces come into contact with each other under a relatively large compressive load and a compressive load is applied to the shoulder surface of the pin, strain due to the compressive load occurs near the shoulder surface. If the inner seal is too close to the shoulder surface, this may negatively affect the sealing performance. Therefore, by making the axial distance between the shoulder surface of the pin and the inner seal greater than twice the pipe wall thickness near the inner seal, a structure is created in which damage due to plastic deformation near the shoulder surface under compressive load is less likely to spread to the inner seal, thereby maintaining excellent sealing performance against internal pressure. Furthermore, by ensuring a certain axial distance between the shoulder surface and the inner seal, sufficient space for threading tools can be secured, resulting in good machinability. In addition, a gap exists between the shoulder surfaces when fastened, and depending on the fastening torque, strain does not substantially occur near the shoulder surface. Therefore, even under relatively large compressive loads, the strain near the shoulder surface is reduced, and sealing performance can be maintained.

[0038] When the outer seal portion of the box is provided between the shoulder surface and the external thread portion of the box, it is preferable that the axial distance between the shoulder surface and the outer seal portion of the box is greater than twice the pipe wall thickness of the box in the cross-section including the outer seal portion. For example, when the size of the steel pipe body is 200 to 360 mm in diameter, the axial distance between the shoulder surface and the outer seal portion of the box may be 20 mm or more and 45 mm or less, more preferably 25 mm or more and 35 mm or less. When a shoulder surface is provided between the male and female threads of a two-stage thread structure, and an outer seal portion is provided between this shoulder surface and the external thread portion, the pipe wall thickness of the box in the cross-section including the outer seal portion can be made larger compared to when the outer seal portion is provided at the pipe end of the box, thereby ensuring the rigidity of the box near the outer seal portion and improving the sealing performance. On the other hand, when the shoulder surfaces come into contact with each other under a relatively large compressive load and a compressive load is applied to the shoulder surface of the box, strain due to the compressive load occurs near the shoulder surface. If the outer seal is too close to the shoulder surface, it may negatively affect the sealing performance. Therefore, by making the axial distance between the shoulder surface of the box and the outer seal greater than twice the pipe wall thickness near the outer seal, damage due to plastic deformation near the shoulder surface under compressive load is less likely to spread to the outer seal, thereby maintaining excellent sealing performance against external pressure. Furthermore, by ensuring a certain axial distance between the shoulder surface and the outer seal, sufficient space for threading tools can be secured, resulting in good machinability. In addition, a gap exists between the shoulder surfaces when fastened, and depending on the fastening torque, virtually no strain occurs near the shoulder surface. Therefore, even under relatively large compressive loads, the strain near the shoulder surface is reduced, and sealing performance can be maintained.

[0039] Furthermore, if both the inner seal portion and the outer seal portion are provided between the inner thread portion and the outer thread portion, it is preferable that the sum of the axial distance between the shoulder surface of the pin and the inner seal portion and the axial distance between the shoulder surface of the box and the outer seal portion is less than three times the sum of the pipe wall thickness of the pin in the cross-section including the inner seal portion and the pipe wall thickness of the box in the cross-section including the outer seal portion. This prevents the overall length of the pin and box in the pipe axial direction from becoming too long, which would increase manufacturing costs. In one example, if the size of the steel pipe body is 200 to 360 mm in diameter, the sum of the axial distance between the shoulder surface of the pin and the inner seal portion and the axial distance between the shoulder surface of the box and the outer seal portion can be less than 80 mm.

[0040] The threaded joint for steel pipes according to this disclosure can be suitably implemented as a slim-type threaded joint in which the maximum outer diameter of the box is 100% or more and 105% or less of the outer diameter of the pipe body of the steel pipe. More preferably, the threaded joint for steel pipes according to this disclosure can be suitably implemented as an integral-type slim-type threaded joint.

[0041] The threaded joint for steel pipes according to this embodiment will be described below with reference to the drawings. In the drawings, the same and corresponding components are denoted by the same reference numerals, and the same description will not be repeated.

[0042] Referring to Figure 1, the steel pipe threaded joint 1 according to this embodiment is an integral-type slim threaded joint comprising a tubular pin 10 and a tubular box 20. The pin 10 is formed at the end of one steel pipe 2 that is connected to the other. The box 20 is formed at the end of the other steel pipe 2' that is connected to the other. The box 20 is fastened with the pin 10 by inserting the pin 10 into its interior. In this specification, the portion of the steel pipe 2 other than the end is sometimes referred to as the "pipe body". Figure 1 shows the axis CL of the steel pipes 2 and 2', the outer diameter OD of the pipe body of the steel pipe 2, i.e., the diameter of the outer surface, and the inner diameter ID of the pipe body, i.e., the diameter of the inner surface.

[0043] The pin 10 has a male thread with a two-stage thread structure including an internal thread portion 11a and an external thread portion 11b provided on its outer circumferential surface, a shoulder surface 12 provided on the outer circumferential surface of the pin 10 between the internal thread portion 11a and the external thread portion 11b, an internal seal portion 13 provided on the outer circumferential surface of the pin 10 between the shoulder surface 12 and the internal thread portion 11a, and an external seal portion 14 provided on the outer circumferential surface of the pin 10 between the shoulder surface 12 and the external thread portion 11b. The internal thread portion 11a and the external thread portion 11b are spaced apart in the direction of the pipe axis.

[0044] The box 20 has an open end for receiving the pin 10, and this open end is enlarged so that the outer diameter of the box 20 is enlarged by a few percent compared to the outer diameter OD of the steel pipe body 2. The box 20 has a two-stage threaded female thread including an internal threaded portion 21a and an external threaded portion 21b provided on its inner circumferential surface, a shoulder surface 22 provided on the inner circumferential surface of the box 20 between the internal threaded portion 21a and the external threaded portion 21b, an internal seal portion 23 provided on the inner circumferential surface of the box 20 between the shoulder surface 22 and the internal threaded portion 21a, and an external seal portion 24 provided on the inner circumferential surface of the box 10 between the shoulder surface 22 and the external threaded portion 21b. The internal threaded portion 21a and the external threaded portion 21b are spaced apart in the direction of the pipe axis.

[0045] A predetermined amount of interference is set between the inner sealing portions 13 and 23 of the pin 10 and the box 20, and a metal seal is formed when the inner sealing portions 13 and 23 come into contact with each other without any gaps around their entire circumference in the fastened state. Similarly, a predetermined amount of interference is set between the outer sealing portions 14 and 24, and a metal seal is formed when the outer sealing portions 14 and 24 come into contact with each other without any gaps around their entire circumference in the fastened state. The shape of each sealing portion 13, 23, 14, and 24 may be as appropriate. In one example, it can be made of a cylindrical sealing surface with a vertical arc cross-section, in another example the cross-sectional shape of each sealing portion can be made linear, and in yet another example one side of the pin and box can have a linear cross-section and the other side can have an arc cross-section.

[0046] The internal thread portion 11a and external thread portion 11b of the male screw, and the internal thread portion 21a and external thread portion 21b of the female screw, are all composed of tapered threads having a predetermined taper angle that gradually decreases in diameter towards the tip side of the pin 10 in the axial direction, and are also composed of wedge-shaped threads in which the thread width gradually narrows and the thread groove width gradually widens towards the tip side of the pin 10 or box 20 in the axial direction. Wedge-shaped threads are usually formed with dovetail-shaped threads and thread grooves, and the illustrated example also shows a wedge-shaped thread with a dovetail-shaped thread.

[0047] In this embodiment of the threaded joint, as shown in Figure 2, the load-bearing surface and insertion surface of the male thread 11 and female thread 21 each have a negative flank angle. The flank angles of the load-bearing surface and insertion surface may be the same, or different flank angles may be set. Furthermore, by configuring the joint so that a small gap is formed between the thread crest surface of the male thread 11 and the thread root surface of the female thread 21 when fastened, gorging during fastening is prevented and dope discharge is improved.

[0048] In this embodiment, when the pin 10 is inserted into the box 20 and screwed in until a predetermined tightening torque is generated, the load surface and insertion surface of the internal thread portion 11a of the male screw both contact the load surface and insertion surface of the internal thread portion 21a of the female screw, and the load surface and insertion surface of the external thread portion 11b of the male screw both contact the load surface and insertion surface of the external thread portion 21b of the female screw, thereby providing a locking effect when tightening is complete.

[0049] On the other hand, when the fastening is completed with a predetermined tightening torque, a small gap z is formed between the shoulder surface 12 of the pin 10 and the shoulder surface 22 of the box 20, as shown in Figure 3, so that no compressive load is generated on the shoulder surfaces 12 and 22 due to the tightening torque. This stabilizes the tightening torque when the tightening operation is performed until the insertion surfaces of the wedge-shaped screws come into contact not only with each other's load surfaces but also with each other. The size of the small gap z is less than 0.2 mm in one example, and more preferably less than 0.1 mm.

[0050] This disclosure is applicable not only to integral-type threaded joints but also to coupling-type threaded joints. Furthermore, this disclosure is not limited to the embodiments described above, and various modifications are possible without departing from the spirit of this disclosure. [Examples]

[0051] To confirm the effectiveness of the threaded joint for steel pipes according to this embodiment, the sealing performance was evaluated by numerical analysis simulation using the elastoplastic finite element method.

[0052] <Test Conditions> In the FEM analysis, multiple specimens (analysis models) with different thread profiles were created, and elastoplastic finite element analysis was performed on each specimen to compare the performance differences. Each specimen was an integral type threaded joint with the basic structure shown in Figures 1 to 3, and the following steel pipes were used.

[0053] Steel pipe size: 9-5 / 8″ 58.4# (Nominal outer diameter of pipe body: 244.48mm, Nominal inner diameter of pipe body: 214.25mm) Material: API standard oil well tubular material Q125 (nominal yield strength YS = 862 MPa (125 ksi))

[0054] The dimensions of each threaded portion 11a, 11b, 21a, and 21b are as follows. Total screw length L1 + L2 (see Figure 1): 139.47 mm Thread taper angle: 1.79° Screw height: 2.00mm Load surface pitch: 7.00 mm Insertion surface pitch: 6.71 mm

[0055] Furthermore, for each specimen, the cross-sectional area of ​​the box-type hazardous section BCCS is smaller than the cross-sectional area of ​​the pin-type hazardous section PCCS, and the sum of the cross-sectional areas of the pin-intermediate hazardous section PICCS and the box-intermediate hazardous section BICCS is greater than the cross-sectional area of ​​the box-type hazardous section BCCS. The joint efficiency JE of the threaded joint relating to the specimen is the ratio of the cross-sectional area of ​​the box-type hazardous section BCCS to the cross-sectional area A of the steel pipe body, which is 72% in this embodiment. Therefore, the yield compressive load Lc of the threaded joint under no internal or external pressure is 72% of the yield compressive load of the pipe body under no internal or external pressure in the yield curve of the steel pipe body shown in Figure 4 with VME 100%.

[0056] Table 1 shows the dimensions of the threaded joints used in the analysis. [Table 1]

[0057] In Table 1, specimens #1 to #4 are variations in the axial distance x between the shoulder surface 22 of the box 20 and the outer seal portion 24, and the axial distance y between the shoulder surface 12 of the pin 10 and the inner seal portion 13. Specimen #5 is based on specimen #1, with slightly reduced outer and inner seal interference. Specimen #6 is based on specimen #1, with slightly increased outer and inner seal interference. Sub-numbers indicate variations in the gap between the shoulder surfaces of the pin and the box for each specimen #1 to #6.

[0058] For the sealing performance evaluation, as shown in Figure 4, a composite load simulating the actual test specified in the 2011 version of the ISO 13679 standard was applied. The minimum sealing contact force was defined as the value at which the sealing contact force was smallest along the load path, and the sealing performance was evaluated based on the magnitude of this value.

[0059] In many of the test specimens, the internal seal contact force was found to be at its minimum value around the load step (22), i.e., when the second internal pressure + tensile load was applied.

[0060] [Evaluation Results] As shown in Table 1, in all specimens #1 to #6, when the gap z between shoulder surfaces 12 and 22 was between 0 and 0.2 mm, the minimum internal seal contact force tended to decrease as the gap z increased. However, when the gap z was 0.2 mm or greater, the minimum internal seal contact force remained almost constant. From this, it can be concluded that when the yield compressive load Lc of the threaded joint 1 is applied, if the gap z between shoulder surfaces 12 and 22 is greater than 0 and less than 0.2 mm, the pin 10 and box 20 undergo elastic deformation as a whole, causing the shoulder surfaces 12 and 22 to come into contact with each other. This suppresses the axial displacement of the pin 10 and box 20 thereafter, and reduces damage to each threaded portion 11a, 11b, 21a, and 21b when the yield compressive load Lc is applied. Furthermore, a detailed analysis of the displacement between shoulder surfaces at load steps (10), (12), and (29), where there is no internal or external pressure load and the yield compressive load Lc of the threaded joint is applied, confirmed that the shoulder surfaces come into contact with each other at the first load step (10), the second load step (12), and the third load step (29).

[0061] On the other hand, when the gap exceeds 0.2 mm, the shoulder surfaces no longer come into contact with each other at any load step, and the axial displacement between the pin and the box remains constant. As a result, the amount of damage accumulated in each threaded section becomes constant, and the minimum internal seal contact force can be considered to remain almost unchanged. Note that 0.2 mm is approximately 0.082% of the outer diameter of the steel pipe body, which is 244.48 mm.

[0062] Based on these test results, it can be inferred that by designing the screw joint so that the shoulder surfaces do not come into contact with each other depending on the tightening torque of the pin and box, and that the shoulder surfaces come into contact with each other under a compressive load smaller than the yield compressive load Lc of the screw joint, it is possible to suppress the amount of damage accumulated in each threaded part under repeated combined loads, even in a screw joint with a two-stage threaded wedge-type thread.

[0063] Furthermore, since the axial distance y between the shoulder surface 12 of the pin 10 and the inner seal portion 13 is 19.7 to 25.2 mm, which is greater than twice the thickness of the pin's inner seal (approximately 8 mm), it was confirmed that the strain near the shoulder surface caused by the compressive load acting on the shoulder surface 12 does not significantly affect the inner seal portion 13, and the internal pressure sealing performance by the inner seal can be maintained. In addition, the axial distance x between the shoulder surface 22 of the box 20 and the outer seal portion 24 is 19.7 mm, which is greater than twice the thickness of the box's outer seal (approximately 7.4 mm), so it was confirmed that the strain near the shoulder surface caused by the compressive load acting on the shoulder surface 22 does not significantly affect the outer seal portion 14, and the external pressure sealing performance by the outer seal can be maintained. Furthermore, the sum of the distance y between the inner seal shoulders and the distance x between the outer seal shoulders (x+y) is kept to a maximum of 25.2 + 19.7 = 44.9 mm, which is less than three times the sum of the wall thickness of the inner pin seal and the wall thickness of the outer box seal, approximately 15.4 mm. Therefore, the axial length of the joint portion does not become excessively long relative to the pipe size, thereby reducing manufacturing costs. [Explanation of Symbols]

[0064] 1: Threaded fitting for steel pipes, 2: Steel pipe, 10: Pin, 20: Box 11a: Internal thread portion of male screw, 11b: External thread portion of male screw 21a: Internal thread portion of female screw, 21b: External thread portion of female screw 12: Pin shoulder side, 22: Box shoulder side 13: Inner sealing part of the pin, 23: Inner sealing part of the box 14: Outer seal portion of the pin, 24: Outer seal portion of the box

Claims

1. A threaded joint for steel pipes, comprising 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 comprises a male thread including an internal thread portion and an external thread portion spaced apart in the axial direction, a shoulder surface provided between the internal thread portion and the external thread portion of the male thread, an internal seal portion provided on the outer circumferential surface of the pin on the tip side of the steel pipe beyond the shoulder surface, and an external seal portion provided on the outer circumferential surface of the pin on the pipe body side of the steel pipe beyond the shoulder surface. The box comprises a female thread including an internal thread portion into which the internal thread portion of the male screw fits when fastened and an external thread portion into which the external thread portion of the male screw fits; a shoulder surface provided between the internal thread portion and the external thread portion of the female thread and facing the shoulder surface of the pin in the axial direction; an internal seal portion provided corresponding to the internal seal portion of the pin and in contact with the internal seal portion of the pin when fastened; and an external seal portion provided corresponding to the external seal portion of the pin and in contact with the external seal portion of the pin when fastened. A gap is formed between the tip surface of the pin and the box. The internal and external threads of the male screw and the internal and external threads of the female screw are all made of wedge-shaped threads, and in the fastened state, at least a portion of the male screw has a thread profile such that its insertion surface and load surface both contact the insertion surface and load surface of the female screw. In the fastened state, the shoulder surfaces of the pin and the box are arranged such that a gap is formed between them when there is no axial compressive load. The size of the gap is determined such that the shoulder surfaces of the pin and the box come into contact when a load Lc, represented by the following formula (1), is applied to a steel pipe threaded joint. Lc=Lp×JE...(1) Here, the load Lc is the compressive load applied from the outside to the steel pipe threaded joint via the pipe body of the steel pipe when fastened, Lp is the yield compressive load of the pipe body of the steel pipe when there is no internal or external pressure load, and JE is the joint efficiency expressed as the ratio of the smaller of the cross-sectional area of ​​the pin hazard section of the pin and the cross-sectional area of ​​the box hazard section of the box to the cross-sectional area of ​​the cross section of the steel pipe body.

2. In the threaded joint for steel pipes according to claim 1, A threaded joint for steel pipes, wherein the cross-sectional shape of the threads of the male and female threads is dovetail.

3. In the threaded joint for steel pipes according to claim 1 or 2, A threaded fitting for steel pipes, wherein the size of the gap is less than 0.082% of the outer diameter of the steel pipe body.

4. In the threaded joint for steel pipes according to claim 1 or 2, A threaded joint for steel pipes, wherein the internal seal portion of the pin is provided between the shoulder surface of the pin and the internal thread portion, and the axial distance between the shoulder surface of the pin and the internal seal portion is greater than twice the pipe wall thickness of the pin in the cross-section including the internal seal portion.

5. In the threaded joint for steel pipes according to claim 1 or 2, The outer seal portion of the box is provided between the shoulder surface of the box and the external thread portion, and the axial distance between the shoulder surface of the box and the outer seal portion is greater than twice the pipe wall thickness of the box in the cross-section including the outer seal portion, in a threaded joint for steel pipes.

6. In the threaded joint for steel pipes according to claim 1 or 2, The internal seal portion of the pin is provided between the shoulder surface of the pin and the internal thread portion, and the axial distance between the shoulder surface of the pin and the internal seal portion is greater than twice the pipe wall thickness of the pin in the cross-section including the internal seal portion. The outer seal portion of the box is provided between the shoulder surface of the box and the external thread portion, and the axial distance between the shoulder surface of the box and the outer seal portion is greater than twice the wall thickness of the box in the cross-section including the outer seal portion. A threaded joint for steel pipes, wherein the sum of the axial distance between the shoulder surface of the pin and the inner seal portion and the axial distance between the shoulder surface of the box and the outer seal portion is less than three times the sum of the pipe wall thickness of the pin in a cross-section including the inner seal portion and the pipe wall thickness of the box in a cross-section including the outer seal portion.

7. In the threaded joint for steel pipes according to claim 1 or 2, A threaded fitting for steel pipes, wherein the maximum outer diameter of the box is between 100% and 105% of the outer diameter of the steel pipe body.

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