Threaded joints for steel pipes

Abstract

By setting the curvature radius r1 of the bottom of the load surface of the first thread (111) at the top end of the external thread (11) to be greater than the thread height Th×0.14, more preferably greater than Th×0.16, the shear resistance of the first thread (111) is improved. In addition, by reducing the pitch to increase the number of threads and increase the contact area of ​​the load surface, and by increasing the curvature radius of the bottom of the load surface of the first thread at the top end of the external thread and setting the curvature radius of other parts to be smaller to increase the contact area of ​​the load surface, high torque resistance is maintained.

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, steel pipes known as oil country tubular goods are used in the exploration and production of oil and natural gas wells (hereinafter collectively referred to as "oil wells"), the development of unconventional resources such as oil sands and shale gas, the recovery and storage of carbon dioxide (CCS), geothermal power generation, and hot springs. Threaded joints are used to connect steel pipes. These threaded joints are broadly categorized into combination and integral types.

[0003] In the case of a combination type, steel pipes are connected using a tubular pipe joint. Typically, the inner circumference of each end of the pipe joint is provided with internal threads, and the outer circumference of each end of the steel pipe is provided with external threads. The steel pipes are then connected by screwing one end of one steel pipe into one end of the pipe joint, and one end of the other steel pipe into the other end of the pipe joint. In other words, in a combination type, one of the directly connected pair of pipes is a steel pipe, and the other is a pipe joint.

[0004] In the case of the integral type, steel pipes are directly connected to each other without the use of additional pipe joints. Specifically, the inner circumference of one end of the steel pipe is provided with an internal thread, and the outer circumference of the other end is provided with an external thread. The steel pipes are connected by screwing the one end of the steel pipe with the internal thread into the other end of the steel pipe with the external thread.

[0005] Typically, the end of an externally threaded steel pipe includes an element that is inserted into the end of an internally threaded steel pipe or pipe fitting, and is therefore called a "pin." The end of an internally threaded steel pipe or pipe fitting includes an element that receives the end of the externally threaded steel pipe, and is therefore called a "box." These pins and boxes are the ends of pipes and are therefore tubular.

[0006] In recent years, the widespread use of well development technologies such as DwC (Drilling with Casing) and horizontal excavation has led to a dramatic increase in demand for high-torque joints. The applicant has previously manufactured threaded joints using tapered threads with a dovetail-shaped cross-section, also known as wedge threads, as high-torque joints for relatively small-diameter steel pipes. Such high-torque joints are disclosed, for example, in Patent Document 1 listed below.

[0007] Wedge threads have a thread profile in which the pitch of the insertion surface is smaller than the pitch of the load surface. Consequently, the thread width of the male thread of the pin gradually narrows as it progresses toward the tip. Similarly, the thread groove width of the female thread of the corresponding box gradually narrows toward the tip. Furthermore, both the load surface and the insertion surface of the male and female threads have negative pitch angles. When the pin and box are fastened, the load surfaces and the insertion surfaces contact each other, ensuring a secure fit between the male and female threads. This structure enables threaded joints employing wedge threads to exhibit high torque resistance.

[0008] In addition, in the following patent document 1 Figure 5 Paragraph 0065 discloses the following technology: a curved portion consisting of two arcs with different curvature radii is provided in the connection portion between the load surface of the thread tooth and the top surface of the thread tooth and the connection portion between the load surface of the thread tooth and the bottom surface of the thread groove, thereby reducing the concentrated load factor at the base of the load surface and improving the fatigue performance of the connection portion.

[0009] Prior art literature

[0010] Patent Literature

[0011] Patent Document 1: Japanese Patent Application No. 2007-504420 Summary of the Invention

[0012] Problems to be solved by the invention

[0013] The applicant of the present application has developed a high-torque threaded joint for steel pipes with larger diameters. However, if a threaded joint with a diameter of 9-5 / 8" or larger is designed according to the same design criteria as in the past, in a composite load test of a prototype according to ISO13679:2011 Series A, the thread teeth of the external thread portion of the pin will shear when the maximum tensile load is applied. In addition, in the applicant's previous design criteria, the curvature radius of the arc (23) connected to the load surface of the two arcs constituting the above-mentioned curved portion disclosed in Patent Document 1 is 0.125 mm, and the curvature radius of the arc (24) connected to the bottom surface of the thread groove or the top surface of the thread teeth is 0.875 mm.

[0014] An object of the present disclosure is to enable a threaded joint for large-diameter steel pipes to exhibit high torque resistance and shear resistance corresponding to the size of the steel pipe to be connected.

[0015] Solutions for solving problems

[0016] The present inventors conducted extensive research to investigate the causes of thread fracture in the male thread of a high-torque threaded joint for large-diameter steel pipes. They discovered that the shear strength of the threads at the tip of the male thread is insufficient for the tensile strength, or in other words, shear resistance, required for large-diameter steel pipes. Consequently, shear failure occurs starting from the bottom of the threads at the tip of the male thread. Specifically, when shear failure first occurs in the threads around the tip of the male thread (hereinafter referred to as the "first thread"), the load concentrates on the second thread located one side inward of the thread in the cross section (i.e., on the pipe body side), causing shear failure of the second thread. When shear failure occurs in the second thread, the load concentrates on the third thread located one side inward, causing shear failure, and the shear failure propagates one by one. This is believed to be the result of shear failure occurring over a wide area of ​​the male thread.

[0017] Furthermore, while conventional male threads with a trapezoidal cross-section may deform radially and pop out when subjected to excessive tensile loads, the threads rarely break widely. On the other hand, in the high-torque threaded joint described above, the male and female threads, with their dovetail cross-sections, firmly engage with each other, preventing disengagement.

[0018] Therefore, in a high-torque threaded joint in which the male and female threads are formed by wedge threads having a dovetail cross-section, the rigidity of the first thread where shear failure first occurs is important in order to ensure anti-bounce performance and shear resistance.

[0019] As a means of improving the shear resistance of the male thread tip, the inventors focused on the curvature radius of the boundary between the load-bearing surface at the tip of the male thread and the bottom surface of the thread groove. They believe that shear failure in male threads is caused by stress concentration at the boundary between the load-bearing surface at the tip of the male thread and the bottom surface of the thread groove. Therefore, they believe that increasing the curvature radius of this boundary can alleviate stress concentration and improve shear resistance.

[0020] Furthermore, it is believed that as the thread height increases, the bending moment acting on the boundary portion increases, and the equivalent plastic strain at the boundary portion increases, even if the total uniformly distributed load acting on the load surface remains the same. Therefore, it is believed that the boundary portion needs to be formed using a curved surface portion with an appropriate curvature radius corresponding to the thread height.

[0021] On the other hand, if the thread height becomes too large, the cutting depth of the thread groove increases, and workability deteriorates. Therefore, even for threaded joints for large-diameter steel pipes, the thread height is preferably 3.0 mm or less. In addition, to achieve sufficient torque resistance, it is necessary to ensure a sufficient contact area between the load surface and the insertion surface, so the thread height is preferably 1.8 mm or more.

[0022] The threaded joint for steel pipes disclosed herein was discovered through a comprehensive study of the above-mentioned technical insights. That is, the threaded joint for steel pipes disclosed herein includes a tubular pin provided at the top end of a steel pipe and a tubular box into which the pin is screwed and fastened. The pin has an external thread formed on the outer periphery of the pin. The box has an internal thread formed on the inner periphery of the box and engaged with the external thread when fastened. The external thread and the internal thread have a load surface, an insertion surface, a thread crest surface, and a thread groove bottom surface. The insertion surface pitch of the external thread and the internal thread is smaller than the load surface pitch of the external thread and the internal thread. In the fastened state, the load surface of the external thread contacts the load surface of the internal thread, and the insertion surface of the external thread contacts the insertion surface of the internal thread. Preferably, the load surface and the insertion surface of the external thread and the internal thread have a negative thread profile half angle.

[0023] Furthermore, the load surface of the male thread and the thread groove bottom surface within a predetermined range in the thread helix direction from the tip of the male thread may be connected via a first curved surface portion having a curvature radius r1 in a longitudinal section satisfying the following formula (1).

[0024] r1≥Th×0.14…(1)

[0025] Wherein, Th is the thread height on the load-bearing side of the external thread within a predetermined range from the tip of the external thread in the thread helix direction. Preferably, the thread height Th satisfies 1.8 mm ≤ Th ≤ 3.0 mm. More preferably, r1 ≥ Th × 0.16.

[0026] Effects of the Invention

[0027] According to the present disclosure, by forming the external thread and the internal thread with wedge threads, high torque resistance can be exhibited, and the shear resistance required for large-diameter steel pipes can be imparted to the external thread tip of the pin. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a longitudinal sectional view of the threaded joint for steel pipes according to the first embodiment, taken along the pipe axis direction.

[0029] Figure 2 It is an enlarged longitudinal sectional view of the range including the first thread and the second thread of the male thread.

[0030] Figure 3 This is an enlarged longitudinal sectional view of the vicinity of the first thread of the external thread.

[0031] Figure 4 This is an enlarged perspective view of the connection between the first and second threads of a male thread.

[0032] Figure 5 It is a longitudinal sectional view of a threaded joint for steel pipes according to a second embodiment, taken along the pipe axis direction.

[0033] Figure 6 This is a graph showing the evaluation results of equivalent plastic strain obtained by FEM analysis.

[0034] Figure 7 This is a graph showing the evaluation results of the torque resistance performance obtained by FEM analysis. DETAILED DESCRIPTION

[0035] The threaded joint for steel pipes of this embodiment includes a tubular pin attached to the distal end of a steel pipe and a tubular box into which the pin is screwed and fastened. The pin has external threads formed on its outer circumference. The box has internal threads formed on its inner circumference that engage with the external threads during fastening.

[0036] Preferably, the external thread is a tapered thread that gradually tapers as it reaches the pin tip. The external thread may include a fully threaded portion with a constant thread height and an incomplete threaded portion with a thread height less than the fully threaded portion. The external thread is formed by machining the outer circumference of a steel pipe. The thread groove cut depth gradually deepens from zero to the fully threaded portion's thread height as it moves from the pipe body to the pin tip. However, the incomplete threaded portion of the external thread primarily consists of a portion where the thread groove cut depth is less than the fully threaded portion's thread height. In this structure, in the fully threaded portion, both the thread crest and the thread groove bottom gradually taper as they progress along the thread helix to the pin tip. In the incomplete threaded portion, the thread groove bottom gradually tapers as it progresses along the thread helix to the pin tip, but the thread crest of the incomplete threaded portion maintains a constant diameter.

[0037] Preferably, the internal thread is a tapered thread that gradually expands in diameter as it reaches the top side of the box (i.e., the main body side of the steel pipe). The internal thread can have a fully threaded portion with a constant thread height and an incomplete threaded portion with a thread height lower than the thread height of the fully threaded portion. Alternatively, the thread height of the fully threaded portion of the internal thread can be slightly greater than the thread height of the fully threaded portion of the external thread. In this case, when the pin and box are tightened, the top surface of the thread of the internal thread contacts the bottom surface of the thread groove of the external thread, but a gap is formed between the top surface of the thread of the external thread and the bottom surface of the thread groove of the internal thread. By providing this gap, it is possible to prevent adhesion and heat welding when the external and internal threads are fitted, and the above-mentioned gap can be appropriately utilized as a lubricant discharge flow path.

[0038] The internal thread is formed by machining the inner circumference of the pipe fitting or steel pipe that constitutes the box. Preferably, when the pin and box are fastened, the first thread groove of the internal thread, into which the first thread crest at the distal end of the fully threaded portion of the male thread engages, has a groove depth such that the radial dimension of the contact area between the load-bearing surfaces of the male and female threads is at least 60%, more preferably at least 70%, of the thread height of the first thread crest of the male thread.

[0039] External and internal threads have a load surface, an insertion surface, a thread crest, and a thread groove bottom surface. The load surface and insertion surface of the external and internal threads have negative thread profile angles. That is, the external and internal threads have a dovetail-shaped thread groove and thread teeth in a longitudinal cross-section. The external and internal threads exhibit multiple thread crests and multiple thread groove bottom surfaces in a longitudinal cross-section. The shape of each thread crest and each thread groove bottom surface in the longitudinal cross-section can be parallel to the pipe axis or inclined relative to the pipe axis along the taper angle of the tapered thread. The thread profile angle of the load surface can be, for example, a predetermined value in the range of -10° to -1°, more preferably, a predetermined value in the range of -4° to -6°. Furthermore, the thread profile angle of the insertion surface can be, for example, a predetermined value in the range of -10° to -1°, more preferably, a predetermined value in the range of -4° to -6°. Furthermore, the cross-sectional shape of the load surface and insertion surface of each thread of the external and internal threads in a longitudinal cross-section can be linear.

[0040] In the threaded joint for steel pipes of this embodiment, the insertion surface pitch of the male and female threads is smaller than the load surface pitch of the male and female threads. As a result, the male thread forms a wedge-shaped thread, with the thread width decreasing and the thread groove width increasing as it reaches the pin tip, while the female thread forms a wedge-shaped thread, with the thread width decreasing and the thread groove width increasing as it reaches the box tip. Furthermore, the load surface pitch can be a predetermined value within the range of, for example, 8.0 mm to 11.0 mm, while the insertion surface pitch can be a predetermined value within the range of, for example, 7.5 mm to 10.5 mm. The pitch difference Δp between the load surface pitch and the insertion surface pitch can be, for example, 0.3 mm to 0.6 mm.

[0041] In the present disclosure, the term "external thread" refers to a portion where the load surface of the external thread contacts the load surface of the internal thread and the insertion surface of the external thread contacts the insertion surface of the internal thread, regardless of whether the portion is a complete thread portion or an incomplete thread portion, in the state where the pin and the box are fastened. Furthermore, the term "internal thread" refers to a portion where the load surface of the internal thread contacts the load surface of the external thread and the insertion surface of the internal thread contacts the insertion surface of the external thread, regardless of whether the portion is a complete thread portion or an incomplete thread portion, in the state where the pin and the box are fastened. Furthermore, for example, Figure 4As shown, an incomplete thread 15 may be provided on the pin tip side of the fully threaded portion of the external thread 11, in which at least one of the insertion surface and the load surface does not contact the internal thread. However, since such an incomplete thread does not contribute to the torque resistance performance, it is not considered a portion constituting the "external thread" in the present disclosure. Alternatively, an incomplete thread may be provided on the box tip side of the fully threaded portion of the internal thread, in which at least one of the insertion surface and the load surface does not contact the external thread. However, since such an incomplete thread does not contribute to the torque resistance performance, it is not considered a portion constituting the "internal thread" in the present disclosure.

[0042] In the threaded joint for steel pipes of this embodiment, the load-bearing surface of the male thread and the bottom surface of the thread groove within a predetermined range in the thread helix direction from the tip of the male thread are connected via a first curved portion having a curvature radius r1 in a longitudinal cross-section that satisfies the following equation (1). Furthermore, it is preferred that the radially inner end of the first curved portion smoothly connects to the bottom surface of the thread groove of the male thread. Furthermore, it is preferred that the radially outer end of the first curved portion smoothly connects to the load-bearing surface of the male thread.

[0043] r1≥Th×0.14…(1)

[0044] Here, Th is the thread height on the load-bearing side of the external thread within a predetermined range from the tip of the external thread in the thread helix direction, satisfying 1.8 mm ≤ Th ≤ 3.0 mm. Preferably, the thread within the predetermined range from the tip of the external thread in the thread helix direction is a portion of the fully threaded portion of the external thread. If the thread height Th is less than 1.8 mm, the contact area between the load-bearing surfaces of the external and internal threads decreases, and the required torque resistance cannot be achieved. On the other hand, if the thread height Th is greater than 3.0 mm, the cutting depth increases, increasing cutting time and processing costs.

[0045] When the thread height Th is 1.8 mm, the curvature radius r1 becomes 0.252 mm or more. When the thread height is 3.0 mm, the curvature radius r1 becomes 0.42 mm or more. However, the present inventors have first proposed a method of connecting the load surface of the external thread and the bottom surface of the thread groove with such a large curvature radius.

[0046] The first curved portion may be provided across the entire length of the external thread.

[0047] One method of improving torque resistance is to reduce the load surface pitch and the insertion surface pitch, thereby increasing the number of threads visible in a longitudinal cross-section. Preferably, the thread profiles of the male and female threads are specified so that, in the tightened state, the load surface and insertion surface of the thread portion extending from the tip of the male thread along the thread helix direction for at least eight turns, and more preferably for at least nine turns, contact the load surface and insertion surface of the female thread. More preferably, the load surface pitch can be set to 8.50 mm or less, the insertion surface pitch to 8.10 mm or less, the pitch difference between the load surface pitch and the insertion surface pitch to 0.35 mm or more and 0.45 mm or less, and the minimum thread width at the bottom of the thread at the tip of the male thread is set to 2.0 mm or more. Furthermore, the minimum thread width at the bottom of the thread at the tip of the female thread box is preferably 2.1 mm or more. Furthermore, an incomplete thread may be formed closer to the box tip than the tip of the internal thread, where at least one of the load surface and the insertion surface does not contact the external thread. However, since such an incomplete thread does not contribute to torque resistance, the thread width of the incomplete thread may be less than 2.00 mm depending on cutting conditions. Such an incomplete thread does not constitute an "internal thread" in the present disclosure.

[0048] Alternatively, the first curved portion having a larger radius of curvature may be provided not across the entire length but rather extending from the tip of the external thread along the thread helix direction for at least half a turn, more preferably at least one turn. This allows the first curved portion to enhance the shear stiffness at the base of the first thread of the external thread, where the thread width is the narrowest, thereby preventing shear failure originating from the first thread.

[0049] In addition, for example, Figure 4 As shown, the external thread 11 may include a first thread portion 111 having a first curved surface portion 111A, and a second thread portion 112 continuous with the first thread portion 111 in the thread helical direction. The load-bearing surface and the thread groove bottom surface of the second thread portion 112 may be connected via a second curved surface portion 112A having a smaller curvature radius r2 than that of the first curved surface portion 111A. Furthermore, it is preferable that the first curved surface portion 111A and the second curved surface portion 112A be smoothly connected, with no step formed at the boundary between the first curved surface portion 111A and the second curved surface portion 112A.

[0050] The first curved surface portion may be provided from the tip of the male thread to extend over at least x turns along the thread helical direction, where x satisfies the following formula (2).

[0051] x=(r1-r2) / Δp…(2)

[0052] Here, Δp is the pitch difference between the load surface pitch and the insertion surface pitch of the external thread.

[0053] By forming the external thread in this manner, in the second thread portion having a relatively small radius of curvature r2, the radius of curvature of the boundary between the load surface of the portion corresponding to the radius of curvature r2 in the internal thread of the box 20 and the thread crest surface is also made relatively small, thereby ensuring a large contact area between the load surfaces of the external and internal threads, which is advantageous in terms of torque resistance. Furthermore, the tip of the second thread portion becomes the portion with the narrowest thread width in the second thread portion, but by satisfying the above formula (2), the minimum thread width in the second thread portion becomes equal to the thread width at the tip of the first thread portion, which increases the thread width at the thread base by the first curved portion having a larger radius of curvature r1, thereby preventing shear failure starting from the tip of the second thread portion.

[0054] That is, for example, Figure 3 As shown in FIG, if the tooth profile half angle of the load surface is about -10° to -1°, the first curved portion 111A connecting the load surface and the bottom surface of the thread groove is roughly a quarter arc in the longitudinal section. Therefore, the distance between the radial inner end Pi and the radial outer end Po of the first curved portion 111A in the pipe axis direction is roughly equal to the curvature radius r1 of the first curved portion 111A. Figure 3 In order to compare the thread widths, the second curved surface portion 112A' in the case where the tip of the second thread portion is present at a position approximately half a turn along the thread helix direction from the tip of the first thread portion 111 and the second curved surface portion 112A" in the case where the tip of the second thread portion is present at a position approximately 3 / 4 of a turn along the thread helix direction from the tip of the first thread portion 111 are overlapped with the tip of the first thread portion 111 and are represented by imaginary lines. The axial distance between the radial inner end and the radial outer end of the second curved surface portion is also approximately the same as the curvature radius r2 of the second curved surface portion. Here, if one turn is made along the thread helix direction, the thread width changes by the pitch difference Δp between the load surface pitch and the insertion surface pitch. Therefore, if x turns are made, the thread width increases by Δp×x. When the tip of the second thread portion is present at a position around x turns, if the thread width at the radial inner end of the first curved surface portion at the tip of the first thread portion (at Figure 3 The thread width at the position of Pi in the middle is set as W1, and the thread width W2 at the radial inner end of the second curved portion (at Figure 3 The thread width at the position corresponding to Pi in can be expressed by the following formula (3).

[0055] W2=W1-r1+Δp×x+r2…(3)

[0056] Here, W1-r1 approximately represents the position of Po. (W1-r1)+Δp×x approximately represents the position of the outer end portion of the second curved surface portion 112A', 112A" corresponding to Po. Furthermore, the above formula (3) approximately represents the position of the inner end portion of the second curved surface portion 112A', 112A" corresponding to Pi. The longer the length of the first threaded portion 111 in the spiral direction, the larger W2. Therefore, depending on the length of the first threaded portion 111, there is a case where W2 is smaller than W1, and there is also a case where W2 is larger than W1.

[0057] When W2 is smaller than W1, the second curved surface portion 112A' rises with a smaller curvature radius r2, so the thread width of the second thread portion in the second curved surface portion 112A' is smaller than the bottom of the first thread portion 111 (i.e., Figure 3 As shown, curved surface portion 112A' is sunken into the surface of curved surface portion 111A. The area where the thread width of the second thread portion decreases may become a weak point in terms of strength. Therefore, it is preferable that W2 is approximately equal to W1 or W2 is larger than W1. Therefore, based on the above formula (3) and the condition of W2 ≥ W1, x ≥ (r1 - r2) / Δp is derived. In other words, when defined as x = (r1 - r2) / Δp, it is preferable that the first curved surface portion be provided from the tip of the external thread to at least x turns along the thread helical direction.

[0058] For example, if the load surface pitch is 9.845 mm, the insertion surface pitch is 9.400 mm, the curvature radius r1 of the first curved surface portion is 0.4 mm, and the curvature radius r2 of the second curved surface portion is 0.1 mm, it is preferable to provide the first curved surface portion over a radius of (0.4 - 0.1) / (9.845 - 9.400) ≈ 2 / 3 of the circumference. If the curvature radius r2 of the second curved surface portion is 0.2 mm and all other conditions are the same as above, it is preferable to provide the first curved surface portion over a radius of approximately half the circumference.

[0059] Furthermore, by providing the male thread with a second curved portion having a smaller radius of curvature, the radius of curvature between the load-bearing surface and the thread crest of the thread portion of the female thread that meshes with the second curved portion is made smaller than that of the third curved portion described later. This ensures a larger overall contact area between the load-bearing surfaces and enables greater torque resistance to be achieved.

[0060] In the threaded joint for steel pipes of this embodiment, it is preferable that the load-bearing surface and the thread crest of the internal thread can be connected via a third curved surface portion that, in the tightened state, faces the first curved surface portion of the external thread and has a larger radius of curvature than the first curved surface portion. This prevents the corner portion between the load-bearing surface and the thread crest of the internal thread from interfering with the first curved surface portion.

[0061] Furthermore, a radial gap may be provided between the thread crest surface of the portion of the internal thread that contacts the load-bearing surface of the tip of the external thread in the tightened state and the thread groove bottom surface of the external thread opposite the thread crest surface. More preferably, the thread crest surface of the internal thread may be the thread crest surface of the internal thread end located at the innermost portion of the box. Furthermore, the thread crest surface of the internal thread may have the same diameter as the inner circumferential surface of a tubular unthreaded extension located further inward of the box than the internal thread. Furthermore, preferably, the pin may include a tubular unthreaded extension corresponding to the unthreaded extension of the box, wherein the outer circumferential surface of the unthreaded extension of the pin does not contact the inner circumferential surface of the unthreaded extension of the box in the tightened state. Furthermore, preferably, the pin includes a pin sealing surface located closer to the pin tip than the unthreaded extension of the pin, and preferably, the box includes a pin sealing surface located further inward of the box than the unthreaded extension of the box. The pin and box sealing surfaces form a metal-to-metal seal, which, when the pin and box are fastened, contacts each other, providing sealing performance against both external and internal pressures. The unthreaded extension prevents strain on the pin and box sealing surfaces due to the compressive-tensile loads acting on the male and female threads. Furthermore, by creating the gap by making the thread height of the local internal thread smaller than the thread height of the fully threaded portion of the female thread, the pin wall thickness near the pin tip can be increased compared to creating the gap by increasing the thread groove depth at the pin tip. Furthermore, the presence of the gap prevents direct contact between the female thread crest and the load-bearing root of the male thread tip, where the thread width is smallest, thereby reducing direct damage to the male thread's weakest point.

[0062] In addition, a sealing portion for external pressure and a sealing portion for internal pressure can be set separately. In this case, the sealing portion for internal pressure can be set at a position closer to the top end of the male buckle than the external thread and the internal thread, and the sealing portion for external pressure can be set at a position closer to the pipe body than the external thread and the internal thread.

[0063] Preferably, the radially outer end of the first curved portion is positioned radially outward relative to the radially inner end of the third curved portion that faces the first curved portion, and the radially outer end of the first curved portion is positioned radially inward relative to the radially outer end of the third curved portion that faces the first curved portion. This minimizes the distance between the radially inner end of the contact area between the load-bearing surfaces of the male and female threads and the radially outer end of the first curved portion, further improving torque resistance.

[0064] Hereinafter, a threaded joint for steel pipes according to the present embodiment will be described with reference to the accompanying drawings. In the drawings, identical or corresponding components are denoted by identical reference numerals, and identical descriptions will not be repeated.

[0065] Reference Figure 1 The threaded joint 1 for steel pipes of this embodiment includes a tubular pin 10 and a tubular box 20. The pin 10 is formed at the end of the steel pipe 2. The box 20 is formed at the end of the pipe joint 3 and is inserted into and fastened to the pin 10. In this specification, the portion of the steel pipe 2 other than the end portion is sometimes referred to as the "pipe body."

[0066] The threaded joint for steel pipes of this embodiment can be appropriately implemented when the outer diameter OD of the pipe body of the steel pipe 2 is 240 mm or more, more preferably when the outer diameter OD of the pipe body of the steel pipe 2 is 245 mm or more, and further preferably when the outer diameter OD of the pipe body of the steel pipe 2 is 270 mm or more. It can be appropriately implemented when the outer diameter OD of the pipe body of the steel pipe 2 is 400 mm or less, more preferably when the outer diameter OD of the pipe body of the steel pipe 2 is 350 mm or less, and further preferably when the outer diameter OD of the pipe body of the steel pipe 2 is 310 mm or less. Preferably, the pipe body of the steel pipe 2 has a substantially uniform wall thickness over the entire axial length. In addition, preferably, the pipe body of the steel pipe 2 has a substantially uniform outer diameter OD and inner diameter ID over the entire axial length. The male buckle is provided at the end of the pipe body of the steel pipe 2. In addition, at Figure 1 The pipe axis CL of the steel pipe 2 is also shown.

[0067] The pin 10 has an external thread 11 composed of a tapered thread whose diameter gradually decreases as it reaches the top end of the pin, and a lip 12. The external thread 11 is composed of thread teeth formed in a spiral shape on the outer peripheral surface of the pin 10. The external thread 11 is composed of a wedge-shaped thread whose thread tooth width gradually narrows as it reaches the top end of the pin 10. The thread teeth and thread grooves of the external thread 11 have a dovetail-shaped cross-sectional shape. The lip 12 is connected to the external thread 11 via a non-threaded extension portion extending toward the top end relative to the top end of the external thread 11. A pin sealing surface 13 is provided on the outer peripheral surface of the lip 12. In the example shown in the figure, the pin sealing surface 13 is composed of a cylindrical sealing surface with an arc-shaped cross section, but the cross-sectional shape of the pin sealing surface 13 can also be a straight line or a shape composed of a combination of a straight line and an arc.

[0068] The box 20 has an open end for receiving the pin 10. The box 20 includes an internal thread 21, formed on the inner circumference of the box 20 and comprising a tapered thread that gradually tapers toward the top end of the box, and a box sealing surface 22. The internal thread 21 comprises threads helically formed on the inner circumference of the box 20, corresponding to the external thread 11. The internal thread 21 comprises a wedge-shaped thread whose thread width gradually widens as it moves from the open end of the box 20 toward the inner side. The threads and grooves of the internal thread 21 have a dovetail-shaped cross-section. The box sealing surface 22 comprises a tapered surface located farther inward from the internal thread 21. The box sealing surface 22 can be a cylindrical sealing surface with an arc-shaped cross section, or a cross-sectional shape combining straight lines and arcs. A predetermined amount of interference is set between the box sealing surface 22 and the pin sealing surface 13 , and in the fastened state, the sealing surfaces 13 , 22 are in contact with each other without a gap over the entire circumference, thereby forming a metal seal.

[0069] like Figure 1 and Figure 2 As shown, the external thread 11 of this embodiment has a fully threaded portion and an incompletely threaded portion. The fully threaded portion of the external thread 11 has a predetermined thread height Th, with threads formed at predetermined load surface pitch LP and insertion surface pitch SP. In the illustrated embodiment, the thread height Th of the external thread 11 is set to 2.2 mm.

[0070] The incomplete thread portion of the male thread 11 is a portion where the imaginary tapered surface defining the tapered shape of the tapered thread intersects with the outer surface of the steel pipe 2, resulting in an insufficient cutting depth on the outer surface of the steel pipe 2 and a failure to form the predetermined thread height Th. In the male thread 11 of this embodiment, both the male thread portion and the male incomplete thread portion are in contact with the female thread 21 at both the load surface and the insertion surface. Figure 1 In the threaded joint 1 shown, the load surface pitch LP is set to 9.845 mm, the insertion surface pitch SP is set to 9.400 mm, and the minimum thread width at the bottom of the thread height direction of the top end of the male thread 11 is set to approximately 2.8 mm.

[0071] The internal thread 21 also has a fully threaded portion and an incompletely threaded portion. The fully threaded portion of the internal thread 21 extends from the open end of the box 20 to near the second thread of the external thread 11 of the pin 10. When the pin 10 and box 20 are fastened, the threads of the incompletely threaded portion of the internal thread 21 engage with the first thread 11A of the external thread 11 of the pin 10. In the present embodiment, both the fully threaded portion and the incompletely threaded portion of the box contact the external thread 11 on both the load surface and the insertion surface. Furthermore, in the illustrated embodiment, threads 23 and 24 are formed continuously with the internal thread 21 at a position closer to the open end of the box 20 than the fully threaded portion of the internal thread 21, which do not contact the external thread 11 on at least one of the load surface and the insertion surface. However, in the present embodiment, threads 23 and 24 are not included in the internal thread 21.

[0072] The thread height of the fully threaded portion of the external thread 11 is slightly smaller than the thread height of the fully threaded portion of the internal thread 21. Figure 2 As shown, a slight gap (e.g., approximately 0.1 mm) is formed between the top surface of the thread teeth of the external thread 11 and the bottom surface of the thread groove of the internal thread 21, and the top surface of the thread teeth of the internal thread 21 contacts the bottom surface of the thread of the external thread 11. In the tightened state, the range in which the external thread 11 and the internal thread 21 are engaged, i.e., the range in which the load surface of the external thread 11 contacts the load surface of the internal thread 21 and the insertion surface of the external thread 11 contacts the insertion surface of the internal thread 21, preferably has an axial length of 60 to 100 mm.

[0073] In addition, if Figure 2 As shown, the load side and insertion side of the threads of the external thread 11 and internal thread 21 each have a negative thread profile half-angle θ. The thread profile half-angle θ of the load side and insertion side can be the same or different. In the example shown, the thread profile half-angle θ of both the load side and insertion side is -5.0°. Furthermore, in the example shown, the thread taper of the external thread 11 and internal thread 21 is set to 1 / 16.

[0074] By locking the pin 10 to the box 20 with the insertion surface and load surface of the thread teeth of the external thread 11 in contact with the insertion surface and load surface of the thread teeth of the internal thread 21 respectively when the pin 10 and the box 20 are tightened, the pin 10 is locked to the box 20, thereby exerting high torque resistance, and the pin seal portion 13 is engaged with the box seal portion 22 in an interference fit state, thereby exerting high sealing performance.

[0075] like Figures 2 to 4As shown, the external thread 11 of this embodiment includes a first thread portion 111 having a first curved surface portion 111A with a relatively large curvature radius r1, and a second thread portion 112 that is continuous with the first thread portion 111 in the thread helical direction. The second thread portion 112 constitutes the entire remaining portion of the external thread 11, excluding the first thread portion 111. The load-bearing surface of the second thread portion 112 and the bottom surface of the thread groove are connected via a second curved surface portion 112A having a smaller curvature radius r2 than the first curved surface portion 111A. In the illustrated example, the curvature radius r1 of the first curved surface portion 111A is 0.4 mm, and the curvature radius R2 of the second curved surface portion 112A is 0.1 mm.

[0076] like Figure 4 As shown, the first thread portion 111 is provided over approximately one turn from the tip of the male thread 11 in the thread helical direction. This is equal to or greater than (0.4-0.1) / (9.845-9.400)=0.674 turns obtained from the above formula (2). Therefore, the thread width at the base of the tip of the second thread portion 112 is substantially greater than the thread width at the base of the tip of the first thread portion 111.

[0077] Furthermore, in this embodiment, the load-bearing surface and the thread crest of the internal thread 21 are connected via a third curved surface portion 21A having a curvature radius r3 that is larger than the curvature radius r1 of the first curved surface portion 111 of the external thread 11. In the illustrated example, the curvature radius r3 of the third curved surface portion 21A is set to 0.5 mm. Furthermore, in the illustrated example, the third curved surface portion 21A is provided throughout the entire internal thread 21. However, the third curved surface portion 21A having a relatively large curvature radius r3 may be provided only in the portion that faces the first curved surface portion 111A when the pin 10 and the box 20 are tightened. A curved surface portion having a curvature radius smaller than the curvature radius r3, for example, 0.5 to 2.0 mm, may be provided between the load-bearing surface and the thread crest of the internal thread 21 in other locations.

[0078] In addition, if Figure 2 and Figure 3 As shown, a radial gap is provided between the portion of the thread crest surface of the first thread 211 of the internal thread 21 that contacts the load-bearing surface of the first thread portion 111 at the top end of the external thread 11 in the tightened state, the portion of the thread crest surface that contacts the load-bearing surface, and the bottom surface of the thread groove of the external thread 11 opposite the thread crest surface. The dimension of this gap is smaller than the curvature radius r1 of the first curved surface portion 111A, and is approximately 0.3 mm in the illustrated embodiment.

[0079] Furthermore, the radially outer end portion of the first curved surface portion 111 is located radially outward from the radially inner end portion of the third curved surface portion 21A, and is located radially inward from the radially outer end portion of the third curved surface portion 21A.

[0080] Figure 5 The second embodiment of the threaded joint for steel pipes has a smaller load-face pitch and insertion-face pitch than the first embodiment, thereby increasing the number of threads visible in a longitudinal cross-section. Specifically, the load-face pitch is set to 8.466 mm, and the insertion-face pitch is set to 8.084 mm. This results in a minimum thread width of approximately 2.1 mm at the bottom of the thread at the tip of the male thread 11.

[0081] The present disclosure is applicable not only to combined threaded joints but also to integral threaded joints. In addition, the present disclosure is not limited to the above-mentioned embodiments, and various modifications can be made within the scope of the present disclosure.

[0082] Example

[0083] In order to confirm the effects of the threaded joint for steel pipes according to the present embodiment, shear resistance and torque resistance were evaluated by numerical analysis simulation based on the elastic-plastic finite element method.

[0084] <Test conditions>

[0085] In finite element analysis (FEM analysis), a plurality of test specimens (analysis models) with varying thread profiles are prepared, and elastic-plastic finite element analysis is performed on each test specimen to compare performance differences.

[0086] Samples #1 to #4 are based on the threaded joint of the first embodiment described above, with a first curved portion extending along the entire length of the male thread. Sample #1 has a first curved portion with a radius of curvature of 0.1 mm, Sample #2 has a radius of curvature of 0.2 mm, Sample #3 has a radius of curvature of 0.3 mm, and Sample #4 has a radius of curvature of 0.4 mm.

[0087] Samples #5 to #8 are based on the threaded joint of the second embodiment described above, with a first curved surface portion provided along the entire length of the male thread. Sample #1 has a first curved surface portion with a curvature radius of 0.1 mm, Sample #2 has a first curved surface portion with a curvature radius of 0.2 mm, Sample #3 has a first curved surface portion with a curvature radius of 0.3 mm, and Sample #4 has a first curved surface portion with a curvature radius of 0.4 mm.

[0088] The materials used are all API standard oil well pipe material Q125 (nominal yield strength YS=862 MPa (125 ksi)).

[0089] Furthermore, for comparison with existing products, a comparative model having two curvature radii (0.125 mm and 0.875 mm) at the boundary between the load surface of the male thread and the thread bottom surface was prepared, and the existing product was also evaluated in the same manner.

[0090] [Shear resistance]

[0091] Regarding the shear resistance, 100% of the tensile load at which the steel pipe body yields was applied, and the equivalent plastic strain at the first curved portion of the thread bottom on the load side of the external thread tip, which is the starting point of shear failure of the external thread, was calculated. The smaller the value, the better the shear resistance. The evaluation results are shown in Figure 6 .

[0092] [Evaluation of torque resistance]

[0093] Regarding the torque resistance performance, the value MTV (Maximum Torque Value) at which the tightening torque line graph begins to yield is defined as the yield torque. The higher the value, the better the torque resistance performance is. The evaluation results are shown in Figure 7 .

[0094] [Evaluation results]

[0095] like Figure 6 As shown, in the conventional product with two curvature radii, the strain concentrated in the portion with a curvature radius of 0.125 mm is relaxed in the portion with a curvature radius of 0.875 mm, indicating a relatively low equivalent plastic strain. In samples #1 to #4 and #5 to #8, which have a first curved portion, it can be assessed that the equivalent plastic strain generated in the first curved portion decreases as the curvature radius increases. In samples #1 to #4, the equivalent plastic strain is lower than that of conventional products when the curvature radius exceeds 0.30 mm, and in samples #5 to #8, the equivalent plastic strain is lower than that of conventional products when the curvature radius exceeds 0.35 mm. Since the thread height Th of the above-mentioned specimens is 2.2 mm, it can be evaluated that: in the case of specimens #1 to #4, when the curvature radius r1 ≥ Th × 0.14 is satisfied, the equivalent plastic strain is smaller than that of the existing product. In addition, in the case of specimens #5 to #8, when the curvature radius r1 ≥ Th × 0.16 is satisfied, the equivalent plastic strain is smaller than that of the existing product.

[0096] Meanwhile, torque resistance was largely unaffected by changes in the radius of curvature of the first curved surface at the bottom of the male thread's load-bearing surface, with samples #1 to #4 and samples #5 to #8 showing comparable values. For samples #1 to #4, it is believed that the reduction in load-bearing surface contact area caused by increasing the radius of curvature of the female thread's load-bearing surface top compared to existing products resulted in lower torque resistance compared to existing products. On the other hand, samples #5 to #8, which had the load-bearing surface pitch and the insertion surface pitch narrowed to increase the load-bearing surface contact area, showed higher torque resistance compared to existing products.

[0097] If the load surface pitch and the insertion surface pitch are reduced, the thread width at the top of the external thread will be reduced, and there is a concern that the shear resistance will be significantly reduced. However, if Figure 6 As shown, the increase in the curvature radius also resulted in a better evaluation result of the shear resistance than that of the existing product. It is concluded that the present disclosure can provide a threaded joint having better shear resistance and torque resistance than the existing product.

[0098] For samples #1 to #4, the torque resistance is considered to be lower than that of existing products. However, by increasing the curvature radius of the first curved surface portion to greater than 0.3 mm, better shear resistance is achieved compared to existing products, which has utility in applications that do not require high torque resistance. In addition, by applying other means to improve torque resistance, it can also be used in products that require torque resistance.

[0099] The same evaluation was also conducted on samples in which only the curvature radius of the bottom of the load surface of the first thread of the male thread, considered to be the starting point of shear failure, was increased. The results showed that the change in the curvature radius of the first thread alone had a slight effect on the torque resistance and shear resistance.

[0100] However, it is thought that the torque resistance can be significantly improved by reducing the curvature radius of the top of the load surface of the female thread in the portion not meshing with the first thread of the male thread to increase the load surface contact area.

[0101] Description of Reference Numerals

[0102] 1. Threaded joint for steel pipe; 2. Steel pipe; 10. Pin; 20. Box; 11. External thread; 111. First threaded portion; 111A. First curved portion; r1. Radius of curvature; 112. Second threaded portion; 112A. Second curved portion; r2. Radius of curvature; 21. Internal thread; 21A. Third curved portion; r3. Radius of curvature; 2. Steel pipe.

Claims

1. A threaded joint for a steel pipe, comprising a tubular pin provided at the top end of a steel pipe and a tubular box into which the pin is screwed and fastened with the pin, the pin having an external thread formed on the outer periphery of the pin, the box having an internal thread formed on the inner periphery of the box and fitted into the external thread when fastened, the external thread and the internal thread having a load surface, an insertion surface, a thread crest surface, and a thread groove bottom surface, the insertion surface pitch of the external thread and the internal thread being smaller than the load surface pitch of the external thread and the internal thread, in a fastened state, the load surface of the external thread contacts the load surface of the internal thread, and the insertion surface of the external thread contacts the insertion surface of the internal thread, the load surface and the insertion surface of the external thread and the internal thread having a negative thread profile half angle, wherein The load surface of the external thread and the thread groove bottom surface within a predetermined range in the thread helix direction from the top end of the external thread are connected via a first curved portion having a curvature radius r1 in a longitudinal section that satisfies the following formula (1): r1≥Th×0.14…(1) Wherein, Th is the thread height on the load surface side of the external thread within a predetermined range from the top end of the external thread in the thread helix direction, and satisfies 1.8 mm ≤ Th ≤ 3.0 mm. The external thread includes: a first thread portion having the first curved surface portion; and a second thread portion continuous with the first thread portion in the thread helical direction, wherein the load surface and the thread groove bottom surface of the second thread portion are connected via a second curved surface portion having a smaller curvature radius r2 than that of the first curved surface portion. The first curved portion is provided from the top end of the external thread along the spiral direction of the thread over at least x turns. Where x satisfies the following formula (2): x=(r1-r2) / Δp…(2) Here, Δp is the pitch difference between the load surface pitch and the insertion surface pitch of the external thread.

2. The threaded joint for steel pipes according to claim 1, wherein: The first curved surface portion is provided from the distal end of the male thread over at least 1 / 2 of the turn along the spiral direction of the thread.

3. The threaded joint for steel pipes according to claim 1, wherein: The first curved portion is provided across the entire length of the external thread.

4. The threaded joint for steel pipes according to claim 1, wherein: The load surface and the thread crest surface of the internal thread are connected via a third curved surface portion that faces the first curved surface portion in a tightened state and has a larger curvature radius than the first curved surface portion. A radial gap is provided between the thread crest surface of a portion of the internal thread that contacts the load surface of the top end of the external thread in a tightened state and the thread groove bottom surface of the external thread opposite to the thread crest surface. The radially outer end of the first curved portion is located radially outward from the radially inner end of the third curved portion facing the first curved portion, and is located radially inward from the radially outer end of the third curved portion facing the first curved portion.

5. The threaded joint for steel pipes according to any one of claims 1 to 4, wherein The thread profiles of the male and female threads are specified so that, in a tightened state, the load surface and the insertion surface of a portion extending from the tip of the male thread over at least eight turns in the thread helical direction come into contact with the load surface and the insertion surface of the female thread.

6. The threaded joint for steel pipes according to claim 5, wherein: The load surface pitch is less than 8.50 mm, the insertion surface pitch is less than 8.10 mm, the pitch difference between the load surface pitch and the insertion surface pitch is greater than or equal to 0.35 mm and less than or equal to 0.45 mm, and the minimum thread width at the bottom of the thread at the top end of the external thread is greater than or equal to 2.1 mm.

7. The threaded joint for steel pipes according to any one of claims 1 to 4, wherein The outer diameter of the steel pipe is greater than 240 mm.

8. A threaded joint for a steel pipe, comprising a tubular pin provided at the top end of a steel pipe and a tubular box into which the pin is screwed and fastened with the pin, the pin having an external thread formed on the outer periphery of the pin, the box having an internal thread formed on the inner periphery of the box and engaged with the external thread when fastened, the external thread and the internal thread having a load surface, an insertion surface, a thread top surface and a thread groove bottom surface, the insertion surface pitch of the external thread and the internal thread being smaller than the load surface pitch of the external thread and the internal thread, in a fastened state, the load surface of the external thread contacts the load surface of the internal thread, and the insertion surface of the external thread contacts the insertion surface of the internal thread, the load surface and the insertion surface of the external thread and the internal thread having a negative thread profile half angle, wherein The load surface of the external thread and the thread groove bottom surface within a predetermined range in the thread helix direction from the top end of the external thread are connected via a first curved portion having a curvature radius r1 in a longitudinal section that satisfies the following formula (1): r1≥Th×0.14…(1) Wherein, Th is the thread height on the load surface side of the external thread within a predetermined range from the top end of the external thread in the thread helix direction, and satisfies 1.8 mm ≤ Th ≤ 3.0 mm. The load surface and the thread crest surface of the internal thread are connected via a third curved surface portion that faces the first curved surface portion in a tightened state and has a larger curvature radius than the first curved surface portion. A radial gap is provided between the thread crest surface of a portion of the internal thread that contacts the load surface of the top end of the external thread in a tightened state and the thread groove bottom surface of the external thread opposite to the thread crest surface. The radially outer end of the first curved portion is located radially outward from the radially inner end of the third curved portion facing the first curved portion, and is located radially inward from the radially outer end of the third curved portion facing the first curved portion.

9. The threaded joint for steel pipes according to claim 8, wherein: The first curved surface portion is provided from the distal end of the male thread over at least 1 / 2 of the turn along the spiral direction of the thread.

10. The threaded joint for steel pipes according to claim 8, wherein: The first curved portion is provided across the entire length of the external thread.

11. The threaded joint for steel pipes according to any one of claims 8 to 10, wherein The thread profiles of the male and female threads are specified so that, in a tightened state, the load surface and the insertion surface of a portion extending from the tip of the male thread over at least eight turns in the thread helical direction come into contact with the load surface and the insertion surface of the female thread.

12. The threaded joint for steel pipes according to claim 11, wherein: The load surface pitch is less than 8.50 mm, the insertion surface pitch is less than 8.10 mm, the pitch difference between the load surface pitch and the insertion surface pitch is greater than or equal to 0.35 mm and less than or equal to 0.45 mm, and the minimum thread width at the bottom of the thread at the top end of the external thread is greater than or equal to 2.1 mm.

13. The threaded joint for steel pipes according to any one of claims 8 to 10, wherein The outer diameter of the steel pipe is greater than 240 mm.

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