THREADED CONNECTION FOR STEEL PIPE

Abstract

A high-torque threaded connection is provided with male and female thread forms consisting of wedge threads with high shear strength and high torsional strength. The radius of curvature r1 of the base of the load flank of the first thread 111, located at the end of the male thread form 11 associated with the tip, is not less than the thread height Th×0.14, and more preferably not less than Th×0.16, to improve the shear strength of the first thread 111.In addition, the thread pitch can be reduced to increase the number of threads to increase the contact area for load flanks, or only the radius of curvature of the base of the load flank of the first thread located at the end of the male thread form associated with the tip can be increased, and a smaller curvature diameter can be provided for the other portions in order to increase the contact area for the load flanks, thus maintaining high resistance to torsional stress.

Description

Steel pipe threaded joints

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

[0002] For example, steel pipes known as oil country tubular goods (OCTG) are used in the exploration and production of oil wells, natural gas wells, etc. (hereinafter collectively referred to as "oil wells"), the development of unconventional resources such as oil sands and shale gas, carbon dioxide capture and storage (CCS), geothermal power generation, hot springs, etc. Threaded joints are used to connect steel pipes together. These types of threaded joints for steel pipes can be broadly divided into coupling and integral types.

[0003] In the case of the coupling type, steel pipes are connected to each other via a tubular coupling. Typically, a female thread is provided on the inner periphery of each end of the coupling, and a male thread is provided on the outer periphery of each end of the steel pipe. The steel pipes are then connected to each other by screwing one end of one steel pipe into one end of the coupling and screwing one end of another steel pipe into the other end of the coupling. In other words, in the coupling type, of a pair of pipes that are directly connected, one pipe is a steel pipe and the other pipe is a coupling.

[0004] In the case of integral type, steel pipes are directly connected to each other without using a separate coupling. Specifically, a female thread is provided on the inner periphery of one end of the steel pipe and a male thread is provided on the outer periphery of the other end, and the steel pipes are connected by screwing one end of one steel pipe with the female threaded portion into the other end of the other steel pipe with the male threaded portion.

[0005] Generally, the end of a male-threaded steel pipe is called a "pin" because it includes an element that is inserted into the end of a female-threaded steel pipe or coupling. The end of a female-threaded steel pipe or coupling is called a "box" because it includes an element that receives the end of a male-threaded steel pipe. Both pins and boxes are tubular because they are the ends of pipe material.

[0006] In recent years, well development technologies such as Drilling with Casing (DwC) and horizontal drilling have become more widespread, resulting in a rapid increase in demand for high-torque fittings. The applicant of the present application has previously manufactured threaded fittings that employ tapered threads with a dovetail cross-sectional shape, also known as wedge threads, as high-torque fittings for relatively small-diameter steel pipes. Such high-torque fittings are disclosed, for example, in Patent Document 1 listed below.

[0007] A wedge thread has a thread profile in which the stabbing flank pitch is smaller than the load flank pitch, so that the thread width of the male thread of the pin gradually narrows as it approaches the tip along the thread helix, and the thread groove width of the mating female thread of the box similarly gradually narrows as it approaches the tip. Furthermore, both the load flanks and stabbing flanks of the male and female threads have negative flank angles, so that when the pin and box are tightened together, the load flanks and stabbing flanks come into contact, causing the threads of the male and female threads to tightly fit together. With this configuration, a threaded joint using a wedge thread can exhibit high torque resistance.

[0008] Furthermore, Figure 5 and paragraph 0065 of Patent Document 1 below disclose a technology in which curved portions consisting of two arcs with different radii of curvature are provided at the connection portion between the load flanks and the crest surface of a thread, and at the connection portion between the load flanks and the root surface of a thread groove, thereby reducing the factor of concentrated load at the base of the load flank and improving the fatigue performance of the connection portion.

[0009] Special Publication No. 2007-504420

[0010] The applicant has been developing high-torque threaded joints for larger diameter steel pipes, but when threaded joints with diameters of 9-5 / 8" or larger are designed in accordance with the same conventional design standards, a problem has arisen in which the threads of the male thread portion of the pin suffer shear fracture when the maximum tensile load is applied in a combined load test of a prototype in accordance with ISO 13679:2011 Series A. Note that, according to the applicant's previous design standards, of the two arcs that make up the curved portion disclosed in Patent Document 1, the radius of curvature of the arc (23) that connects to the load surface is 0.125 mm, and the radius of curvature of the arc (24) that connects to the thread groove root or crest is 0.875 mm.

[0011] An object of the present disclosure is to provide a threaded joint for large diameter steel pipes that can exhibit high torque resistance and shear resistance according to the size of the steel pipes to be connected.

[0012] The inventors conducted extensive research to determine the cause of thread fracture in male threads in high-torque threaded joints for large-diameter steel pipes. As a result, they discovered that the shear strength of the threads at the tip of the male thread is insufficient relative to the tensile strength, or shear resistance, required for large-diameter steel pipes, and that shear fracture occurs starting from the root of the thread at the tip of the male thread. That is, when the thread (hereinafter referred to as the "first thread") from the tip of the male thread first undergoes shear fracture, load is concentrated on the second thread located one thread inward in the cross section (i.e., toward the main body of the steel pipe), causing shear fracture of the second thread. After the second thread shear fractures, load is concentrated on the third thread located one thread inward, causing shear fracture, and the shear fracture propagates successively. It is believed that shear fracture of the male thread thread occurs over a wide area in this manner.

[0013] Furthermore, in a typical male thread with a trapezoidal cross section, the pin and box may deform radially and jump out when an excessive tensile load is applied, but the threads rarely break over a wide area. On the other hand, in the above-mentioned high-torque threaded joint, the threads of the male and female threads, which have a dovetail cross section, are firmly interlocked, so the male and female threads do not come out of engagement.

[0014] Therefore, in a high-torque threaded joint in which the male and female threads are formed by wedge-shaped threads with a dovetail cross section, the rigidity of the first thread, where shear fracture occurs first, is important for ensuring jump-out resistance and shear resistance.

[0015] The inventors focused on the radius of curvature of the boundary between the load flank and the thread groove root at the tip of a male thread as a means for improving the shear resistance of the tip of a male thread. Since the shear fracture of the male thread described above is thought to be caused by stress concentration at the boundary between the load flank and the thread groove root at the tip of the male thread, it is thought that by increasing the radius of curvature of the boundary, stress concentration can be alleviated and shear resistance can be improved.

[0016] Furthermore, it is believed that the greater the thread height, the greater the bending moment acting on the boundary, even if the sum of the uniformly distributed loads acting on the load surface is the same, and the greater the equivalent plastic strain at the boundary. Therefore, it is believed that the boundary needs to be constructed by a curved surface portion with an appropriate radius of curvature according to the thread height.

[0017] On the other hand, if the thread height is too great, the cutting depth of the thread groove increases and workability deteriorates, so even in threaded joints for large diameter steel pipes, the thread height is preferably 3.0 mm or less. Furthermore, in order to exhibit sufficient torque resistance performance, it is necessary to ensure sufficient contact area between the load flanks and the stabbing flanks, so the thread height is preferably 1.8 mm or greater.

[0018] The threaded joint for steel pipes according to the present disclosure was discovered by comprehensively examining the above-mentioned technical knowledge. That is, the threaded joint for steel pipes according to the present 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 has a male thread formed on its outer periphery. The box has a female thread formed on its inner periphery and which engages with the male thread when fastened. The male thread and the female thread have load flanks, stabbing flanks, thread crests, and thread groove roots, and the stabbing flank pitch of the male thread and the female thread is smaller than the load flank pitch of the male thread and the female thread, so that in a fastened state, the load flanks of the male thread contact the load flanks of the female thread and the stabbing flanks of the male thread contact the stabbing flanks of the female thread. It is preferable that the load flanks and the stabbing flanks of the male thread and the female thread have negative flank angles.

[0019] Furthermore, the load flank and the thread groove bottom surface of the male thread within a predetermined range from the tip end of the male thread in the helical direction of the thread may be connected via a first curved surface portion whose radius of curvature r1 in a vertical cross section satisfies the following formula (1): r1≧Th×0.14 (1)

[0020] Here, Th is the thread height on the load flank side of the male thread within a predetermined range from the tip of the male thread in the helical direction of the thread. Preferably, the thread height Th satisfies 1.8 mm≦Th≦3.0 mm. More preferably, r1≧Th×0.16.

[0021] According to the present disclosure, by forming the male and female threads using wedge-shaped threads, high torque resistance performance can be achieved, and the shear resistance performance required for large-diameter steel pipes can be imparted to the tip of the male thread of the pin.

[0022] Fig. 1 is a longitudinal sectional view, taken along the pipe axis direction, of a threaded joint for steel pipes according to a first embodiment. Fig. 2 is an enlarged longitudinal sectional view of an area including the first thread thread and the second thread thread of the male thread. Fig. 3 is an enlarged longitudinal sectional view of the vicinity of the first thread thread of the male thread. Fig. 4 is an enlarged perspective view of the connection between the first thread thread and the second thread thread of the male thread. Fig. 5 is a longitudinal sectional view, taken along the pipe axis direction, of a threaded joint for steel pipes according to a second embodiment. Fig. 6 is a graph showing the results of an evaluation of equivalent plastic strain by FEM analysis. Fig. 7 is a graph showing the results of an evaluation of torque resistance performance by FEM analysis.

[0023] A threaded joint for steel pipes according to this embodiment comprises a tubular pin provided at the tip of a steel pipe, and a tubular box into which the pin is screwed and fastened. The pin has a male thread formed on its outer periphery. The box has a female thread formed on its inner periphery that mates with the male thread when fastened.

[0024] Preferably, the male thread is a tapered thread whose diameter gradually decreases toward the tip of the pin. The male thread may have a complete thread portion with a constant thread height and an incomplete thread portion whose thread height is lower than that of the complete thread portion. The male thread is formed by cutting the outer surface of a steel pipe, and the cutting depth of the thread groove gradually increases from 0 to the thread height of the complete thread portion as it moves from the pipe body side to the tip side of the steel pipe, while the incomplete thread portion of the male thread is mainly composed of portions where the cutting depth of the thread groove is smaller than the thread height of the complete thread portion. In this configuration, in the complete thread portion, both the crest and root surfaces of the male thread gradually decrease in diameter as they move toward the tip of the pin along the thread spiral direction, while in the incomplete thread portion, the root surface of the thread groove gradually decreases in diameter as it moves toward the tip of the pin along the thread spiral direction, but the crest surfaces of the incomplete thread portion have a constant diameter.

[0025] Preferably, the female thread is a tapered thread whose diameter gradually increases toward the tip of the box (i.e., toward the main body of the steel pipe). The female thread may have a complete thread portion with a constant thread height and an incomplete thread portion with a lower thread height than the complete thread portion. The complete thread portion of the female thread may have a thread height slightly greater than that of the complete thread portion of the male thread. In this case, when the pin and the box are fastened together, the crest surface of the female thread contacts the bottom surface of the thread groove of the male thread, but a gap is formed between the crest surface of the male thread and the bottom surface of the thread groove of the female thread. By providing this gap, galling and seizing during engagement of the male thread and the female thread can be prevented, and the gap can be suitably used as a dope discharge passage.

[0026] The female thread is formed by cutting the inner peripheral surface of the coupling or steel pipe that constitutes the box. Preferably, when the pin and the box are fastened together, the first thread groove of the female thread, into which the first thread ridge at the tip end side of the fully threaded portion of the male thread mates, has a groove depth such that the radial dimension of the contact area between the load flank of the male thread and the load flank of the female thread is 60% or more, more preferably 70% or more, of the thread height of the first thread ridge of the male thread.

[0027] The male and female threads have load flanks, stabbing flanks, thread crests, and thread groove roots, and the load flanks and stabbing flanks of the male and female threads have negative flank angles. That is, the male and female threads have dovetail-shaped thread grooves and threads in longitudinal cross section. The male and female threads have multiple thread crests and multiple thread groove roots in longitudinal cross section, and the shape of each thread crest and each thread groove root in the longitudinal cross section may be parallel to the pipe axis of the steel pipe or may be inclined with respect to the pipe axis of the steel pipe so as to follow the taper angle of the tapered thread. The flank angle of the load flank may 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°. The flank angle of the stabbing flank may 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 shapes of the load flanks and stabbing flanks of each thread of the male and female threads in the longitudinal cross section may be linear.

[0028] In the threaded joint for steel pipes of this embodiment, the stabbing flank pitch of the male thread and the female thread is smaller than the load flank pitch of the male thread and the female thread. As a result, the male thread is formed as a wedge-shaped thread in which the thread width decreases and the thread groove width increases toward the pin tip, and the female thread is formed as a wedge-shaped thread in which the thread width decreases and the thread groove width increases toward the box tip. The load flank pitch may be a predetermined value in the range of, for example, 8.0 mm to 11.0 mm, and the stabbing flank pitch may be a predetermined value in the range of, for example, 7.5 mm to 10.5 mm. The pitch difference Δp between the load flank pitch and the stabbing flank pitch may be, for example, 0.3 mm to 0.6 mm.

[0029] In this disclosure, a "male thread" refers to a portion where, when the pin and box are fastened together, the load flank of the male thread contacts the load flank of the female thread and the stabbing flank of the male thread contacts the stabbing flank of the female thread, regardless of whether the portion is a complete thread or an incomplete thread. Furthermore, a "female thread" refers to a portion where, when the pin and box are fastened together, the load flank of the female thread contacts the load flank of the male thread and the stabbing flank of the female thread contacts the stabbing flank of the male thread, regardless of whether the portion is a complete thread or an incomplete thread. Note that, for example, as shown in FIG. 4 , an incomplete thread 15 in which at least one of the stabbing flank and the load flank does not contact the female thread may be provided on the pin tip side of the complete thread portion of the male thread 11. However, since such an incomplete thread does not contribute to torque resistance performance, it is not considered to be a portion constituting a "male thread" in this disclosure. Furthermore, a incomplete thread in which at least one of the stabbing flank and the load flank does not contact the male thread may be provided on the box tip side of the complete thread portion of the female thread. However, since such an incomplete thread does not contribute to torque resistance performance, it is not considered to be a portion constituting a "female thread" in this disclosure.

[0030] In the threaded joint for steel pipes of this embodiment, the load flanks and thread groove roots of the male thread within a predetermined range from the tip of the male thread in the helical direction of the thread are connected via a first curved surface portion whose radius of curvature r1 in a longitudinal cross section satisfies the following formula (1): It is preferable that the radially inner end of the first curved surface portion smoothly continues with the thread groove root of the male thread. It is also preferable that the radially outer end of the first curved surface portion smoothly continues with the load flank of the male thread: r1 ≧ Th × 0.14 (1)

[0031] Here, Th is the thread height on the load flank side of the male thread within a predetermined range from the tip of the male thread in the screw helix direction, and satisfies 1.8 mm≦Th≦3.0 mm. Preferably, the thread within a predetermined range from the tip of the male thread in the screw helix direction is a part of the complete thread portion of the male thread. If the thread height Th is less than 1.8 mm, the contact area between the load flanks of the male thread and the female thread becomes small, and the required torque resistance performance cannot be obtained. On the other hand, if the thread height Th is greater than 3.0 mm, the cutting depth becomes large, increasing the cutting time and processing cost.

[0032] When the thread height Th is 1.8 mm, the radius of curvature r1 is 0.252 mm or more, and when the thread height is 3.0 mm, the radius of curvature r1 is 0.42 mm or more. Connecting the load flank and the root surface of the thread groove of a male thread with such a large radius of curvature was proposed for the first time by the present inventors.

[0033] The first curved surface portion may be provided over the entire length of the male thread.

[0034] One method for improving torque resistance is to relatively increase the number of threads appearing in a longitudinal section by making the load flank pitch and stabbing flank pitch relatively small. Preferably, the thread profiles of the male thread and the female thread can be determined so that, in a fastened state, the load flanks and stabbing flanks of the thread portions extending from the tip of the male thread for at least eight laps, more preferably nine laps or more, in the helical direction of the thread, contact the load flanks and stabbing flanks of the female thread. More preferably, the load flank pitch can be 8.50 mm or less, the stabbing flank pitch can be 8.10 mm or less, the pitch difference between these can be 0.35 mm to 0.45 mm, and the minimum thread width at the thread root at the tip of the male thread can be 2.0 mm or more. It is also preferable that the minimum thread width at the thread root at the box tip of the female thread is 2.1 mm or more. An incomplete thread may be formed closer to the box tip than the tip of the female thread, in which at least one of the load flanks and the stabbing flanks does not contact the male thread, but since such an incomplete thread is a portion that does not contribute to torque resistance performance, the thread width of this incomplete thread may be less than 2.00 mm for reasons of cutting processing, etc. The above-mentioned incomplete thread does not constitute a "female thread" in the present disclosure.

[0035] The first curved surface portion having a large radius of curvature may be provided not over the entire length but over at least half a circumference, more preferably at least one circumference, from the tip of the male thread along the helical direction of the thread. In this way, the shear rigidity at the base of the first thread of the male thread, where the thread width is narrowest, is reinforced by the first curved surface portion, and it is possible to prevent shear fracture from occurring starting from the first thread.

[0036] 4, the male thread 11 may include a first thread portion 111 having a first curved surface portion 111A and a second thread portion 112 that is continuous with the first thread portion 111 in the helical direction of the thread. The load flank and the thread groove bottom surface of the second thread portion 112 may be connected via a second curved surface portion 112A that has a smaller radius of curvature r2 than the first curved surface portion 111A. It is preferable that the first curved surface portion 111A and the second curved surface portion 112A are smoothly connected so that no step is formed at the boundary between them.

[0037] The first curved surface portion may be provided over at least x number of revolutions in the helical direction from the tip end of the male thread, where x satisfies the following formula (2): x = (r1 - r2) / Δp (2), where Δp is the pitch difference between the load flank pitch and the stabbing flank pitch of the male thread.

[0038] By configuring the male 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 flank and the crest of the female thread of the box 20 corresponding to the radius of curvature r2 is also made relatively small, thereby ensuring a large contact area between the load flanks of the male thread and the female thread, which is advantageous in terms of torque resistance. Furthermore, the tip of the second thread portion is the portion with the narrowest thread width in the second thread portion, but by satisfying the above formula (2), the smallest thread width in the second thread portion is equivalent to the thread width at the tip of the first thread portion, where the thread width at the root of the thread is increased by the first curved surface portion having a large radius of curvature r1, and shear fracture originating from the tip of the second thread portion can be avoided.

[0039] 3, if the load flank angle is approximately −10° to −1°, the first curved surface portion 111A connecting the load flank and the thread groove bottom surface will be approximately a quarter-circular arc in longitudinal section. Therefore, the axial distance between the radially inner end Pi and the radially outer end Po of the first curved surface portion 111A will be approximately equal to the radius of curvature r1 of the first curved surface portion 111A. In FIG. 3, a second curved surface portion 112A′ when the tip end of the second thread portion is located at a position approximately halfway around from the tip end of the first thread portion 111 in the screw helix direction, and a second curved surface portion 112A″ when the tip end of the second thread portion is located at a position approximately 3 / 4 of the way around, are shown by imaginary lines superimposed on the tip end of the first thread portion 111 for comparison of thread width. The axial distance between the radially inner end and the radially outer end of the second curved surface portion is also approximately the same as the radius of curvature r2 of the second curved surface portion. Here, the thread width is When the thread is rotated one full turn in the helical direction, it changes by the pitch difference Δp between the load flank pitch and the stabbing flank pitch. Therefore, when the thread is rotated x times, the thread width increases by Δp × x. When the tip end of the second thread portion is located at a position rotated x times, the thread width W2 at the radially inner end of the second curved surface portion (thread width at a position corresponding to Pi in FIG. 3 ) can be expressed by the following equation (3), where W1 is the thread width at the radially inner end of the first curved surface portion at the tip end of the first thread portion (thread width at position Pi in FIG. 3 ). W2 = W1 - r1 + Δp × x + r2 (3) Here, W1 - r1 approximately represents the position of Po. (W1 - r1) + Δp × x approximately represents the position of the outer end of the second curved surface portion 112A', 112A" corresponding to Po. The above formula (3) approximately represents the position of the inner end of the second curved surface portion 112A′, 112A″ corresponding to Pi. The longer the length of the first thread portion 111 in the spiral direction, the larger W2 becomes. Therefore, depending on the length of the first thread portion 111, W2 may be smaller than W1 in some cases, or W2 may be larger than W1 in other cases.

[0040] If W2 is smaller than W1, the second curved surface portion 112A' rises with a small radius of curvature r2, and the thread width of the second thread portion at this second curved surface portion 112A' is smaller than the bottom of the first thread portion 111 (i.e., as shown in FIG. 3, the curved surface portion 112A' is in a state where it is embedded in the surface of the curved surface portion 111A). This may result in the portion of the second thread portion where the thread width is narrow becoming a weak point. Therefore, it is preferable that W2 be approximately equal to or larger than W1. Therefore, from the above formula (3) and the condition W2≧W1, x≧(r1−r2) / Δp is derived. That is, if x=(r1−r2) / Δp is defined, it is preferable that the first curved surface portion be provided over at least x circumferences in the helical direction from the tip of the male thread.

[0041] For example, if the load flank pitch is 9.845 mm, the stabbing flank pitch is 9.400 mm, the radius of curvature r1 of the first curved surface portions is 0.4 mm, and the radius of curvature r2 of the second curved surface portions is 0.1 mm, it is preferable that the first curved surface portions be provided over at least (0.4-0.1) / (9.845-9.400) ≈ 2 / 3 of the circumference. If the radius of curvature r2 of the second curved surface portions is 0.2 mm and the other specifications are the same as above, it is preferable that the first curved surface portions be provided over at least approximately half the circumference.

[0042] In addition, by providing a second threaded portion having a second curved surface portion with a small radius of curvature on the male thread, and making the radius of curvature between the load surface and the crest surface of the thread portion of the female thread that meshes with the second threaded portion smaller than that of the third curved surface portion described below, it is possible to ensure a large overall contact area between the load surfaces, and to achieve great torque resistance performance.

[0043] In the threaded joint for steel pipes of this embodiment, the load flanks and crests of the female thread can preferably be connected via a third curved surface portion that faces the first curved surface portion of the male thread in the make-up state and has a larger radius of curvature than the first curved surface portion, thereby preventing the boundary corner between the load flanks and crests of the female thread from interfering with the first curved surface portion.

[0044] Furthermore, a radial gap may be provided between the crest surface of a portion of the female thread that contacts the load surface at the tip of the male thread in a fastened state and the bottom surface of the male thread groove that faces the crest surface. More preferably, the crest surface of the portion of the female thread may be the crest surface of the female thread end located at the innermost part of the box of the female thread. Furthermore, the crest surface of the portion of the female thread may have the same diameter as the inner peripheral surface of a tubular unthreaded extension located deeper than the female thread in the box. The pin may also have a tubular unthreaded extension that corresponds to the unthreaded extension of the box, and it is preferable that the outer peripheral surface of the unthreaded extension of the pin does not contact the inner peripheral surface of the unthreaded extension of the box in a fastened state. Furthermore, the pin preferably has a pin seal surface located closer to the pin tip than the unthreaded extension of the pin, and the box preferably has a pin seal surface located deeper than the unthreaded extension of the box. The pin seal surface and the box seal surface contact each other when the pin and box are fastened, forming a metal-to-metal seal that seals against external and internal pressures. The unthreaded extension helps prevent the pin seal surface and box seal surface from being distorted by compressive and tensile loads acting on the male and female threads. Furthermore, by forming the gap by making the thread height of a portion of the female thread smaller than the thread height of the fully threaded portion of the female thread, it is possible to ensure a larger pin wall thickness near the pin tip compared to when the gap is formed by deepening the thread groove depth on the pin tip side. Furthermore, the presence of the gap prevents the crest of the female thread from directly contacting the thread root of the load flank at the tip of the male thread, where the thread width is smallest, thereby reducing direct damage to the weakest part of the male thread.

[0045] In addition, the external pressure seal and the internal pressure seal can be provided separately. In this case, the internal pressure seal can be provided closer to the pin tip than the male and female threads, and the external pressure seal can be provided closer to the pipe body than the male and female threads.

[0046] Preferably, the radially outer end of the first curved surface portion is positioned radially outward from the radially inner end of the third curved surface portion facing the first curved surface portion, and is positioned radially inward from the radially outer end of the third curved surface portion facing the first curved surface portion. This makes it possible to minimize the distance between the radially inner end of the contact area between the load surfaces of the male thread and the female thread and the radially outer end of the first curved surface portion, thereby further improving torque resistance.

[0047] A threaded joint for steel pipes according to this embodiment will now be described with reference to the drawings. In the drawings, the same or corresponding components are given the same reference numerals and the same description will not be repeated.

[0048] With reference to Figure 1, a threaded joint for steel pipes 1 according to this embodiment comprises a tubular pin 10 and a tubular box 20. The pin 10 is formed at the end of a steel pipe 2. The box 20 is formed at the end of a coupling 3, into which the pin 10 is inserted and fastened. In this specification, the portion of the steel pipe 2 other than the end may be referred to as the "pipe main body".

[0049] The threaded joint for steel pipes of this embodiment can be suitably used when the outer diameter OD of the pipe body of the steel pipe 2 is 240 mm or more, more preferably 245 mm or more, and even more preferably 270 mm or more. It can be suitably used when the outer diameter OD of the pipe body of the steel pipe 2 is 400 mm or less, more preferably 350 mm or less, and even more preferably 310 mm or less. The pipe body of the steel pipe 2 preferably has a substantially uniform wall thickness over its entire axial length. Furthermore, the pipe body of the steel pipe 2 preferably has a substantially uniform outer diameter OD and inner diameter ID over its entire axial length. The pin is provided at the end of the pipe body of the steel pipe 2. The pipe axis CL of the steel pipe 2 is also shown in Figure 1.

[0050] The pin 10 has a male thread 11, which is a tapered thread whose diameter gradually decreases toward the tip of the pin, and a lip 12. The male thread 11 is composed of a thread formed in a spiral shape on the outer peripheral surface of the pin 10. The male thread 11 is composed of a wedge-shaped thread whose thread width gradually narrows toward the tip of the pin 10. The threads and grooves of the male thread 11 have a dovetail-shaped cross-sectional shape. The lip 12 is connected to the male thread 11 via an unthreaded extension that extends further toward the tip than the tip of the male thread 11. A pin seal surface 13 is provided on the outer peripheral surface of the lip 12. In the illustrated example, the pin seal surface 13 is composed of a cylindrical seal surface with an arc-shaped cross section, but the cross-sectional shape of the pin seal surface 13 may be linear or a shape that combines a linear line and an arc.

[0051] The box 20 has an open end that receives the pin 10. The box 20 includes a female thread 21, which is provided on its inner circumferential surface and is a tapered thread that gradually reduces in diameter toward the tip of the box, and a box seal surface 22. The female thread 21 is composed of a spiral thread formed on the inner circumferential surface of the box 20 to correspond to the male thread 11. The female thread 21 is composed of a wedge-shaped thread whose thread width gradually increases from the open end of the box 20 toward the rear. The threads and grooves of the female thread 21 have a dovetail cross-sectional shape. The box seal surface 22 is composed of a tapered surface provided on the rear side of the box 20 relative to the female thread 21. The box seal surface 22 may be composed of a cylindrical seal surface with an arc-shaped cross section, or may have a cross-sectional shape that combines straight lines and arcs. A predetermined interference is set between the box seal surface 22 and the pin seal surface 13, and when fastened, the seal surfaces 13 and 22 contact each other without gaps around the entire circumference, forming a metal seal.

[0052] 1 and 2, the male thread 11 of this embodiment has a complete thread portion and an incomplete thread portion. The complete thread portion of the male thread 11 has a predetermined thread height Th, and is a portion where the thread is formed with a predetermined load flank pitch LP and stabbing flank pitch SP. The thread height Th of the male thread 11 in the illustrated example is 2.2 mm.

[0053] The incomplete thread portion of the male thread 11 is a portion where the predetermined thread height Th is not formed because the imaginary tapered surface that defines 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. The male thread 11 of this embodiment contacts the female thread 21 on both the load flanks and the stabbing flanks, in both the pin complete thread portion and the pin incomplete thread portion. In the threaded joint 1 shown in Figure 1, the load flank pitch LP is 9.845 mm, the stabbing flank pitch SP is 9.400 mm, and the minimum thread width at the bottom in the thread height direction at the tip of the male thread 11 is approximately 2.8 mm.

[0054] The female thread 21 also has a complete thread portion and an incomplete thread portion. The complete thread portion of the female thread 21 extends from the open end of the box 20 to the vicinity of the second thread of the male thread 11 of the pin 10. The threads of the incomplete thread portion of the female thread 21 engage with the first thread 11A of the male thread 11 of the pin 10 when the pin 10 and the box 20 are fastened together. The female thread 21 of this embodiment contacts the male thread 11 on both the load flank and the stabbing flank in both the box complete thread portion and the box incomplete thread portion. In the illustrated embodiment, threads 23, 24 that do not contact the male thread 11 on at least either the load flank or the stabbing flank are formed continuously with the female thread 21 closer to the open end of the box 20 than the complete thread portion of the female thread 21, but in this embodiment, the threads 23, 24 are not included in the female thread 21.

[0055] The thread height of the fully threaded portion of the male thread 11 is slightly smaller than the thread height of the fully threaded portion of the female thread 21. As a result, in the fastened state, as shown in Fig. 2, a small gap (for example, about 0.1 mm) is formed between the crest surfaces of the male thread 11 and the thread groove root surfaces of the female thread 21, and the crest surfaces of the female thread 21 come into contact with the thread root surfaces of the male thread 11. The range in which the male thread 11 and the female thread 21 mesh in the fastened state, i.e., the range in which the load flanks of the male thread 11 come into contact with the load flanks of the female thread 21 and the stabbing flanks of the male thread 11 come into contact with the stabbing flanks of the female thread 21, preferably has an axial length of 60 to 100 mm.

[0056] 2, the load flanks and stabbing flanks of the threads of the male thread 11 and the female thread 21 each have a negative flank angle θ. The flank angles θ of the load flanks and the stabbing flanks may be the same, or different flank angles may be set. In the illustrated example, the flank angles θ of the load flanks and the stabbing flanks are both -5.0°. In the illustrated example, the thread taper of the male thread 11 and the female thread 21 is 1 / 16.

[0057] When the pin 10 and the box 20 are fastened together, the stabbing flank and load flank of the thread of the male thread 11 come into contact with the stabbing flank and load flank of the thread of the female thread 21, respectively, thereby locking the pin 10 to the box 20, thereby providing high torque resistance, and the pin seal 13 is fitted into the box seal 22 in an interference fit, providing high sealing performance.

[0058] 2 to 4, the male thread 11 of this embodiment includes a first thread portion 111 having a first curved surface portion 111A with a relatively large radius of curvature r1, and a second thread portion 112 that is continuous with the first thread portion 111 in the helical direction of the thread. The second thread portion 112 constitutes the entire remaining portion of the male thread 11 other than the first thread portion 111. The load flank and the thread groove root of the second thread portion 112 are connected via a second curved surface portion 112A that has a radius of curvature r2 smaller than that of the first curved surface portion 111A. In the illustrated example, the radius of curvature r1 of the first curved surface portion 111A is 0.4 mm, and the radius of curvature R2 of the second curved surface portion 112A is 0.1 mm.

[0059] 4, the first thread portion 111 extends over approximately one full turn from the tip end of the male thread 11 in the helical direction of the thread. This is equal to or greater than (0.4-0.1) / (9.845-9.400)=0.674 turns, as calculated by the above formula (2), and the thread width at the base of the tip end of the second thread portion 112 is significantly larger than the thread width at the base of the tip end of the first thread portion 111.

[0060] Furthermore, in this embodiment, the load flank and the crest surface of the female thread 21 are connected via a third curved surface portion 21A having a radius of curvature r3 larger than the radius of curvature r1 of the first curved surface portion 111 of the male thread 11. In the illustrated example, the radius of curvature r3 of the third curved surface portion 21A is 0.5 mm. Note that, in the illustrated example, the third curved surface portion 21A is provided over the entire female thread 21; however, the third curved surface portion 21A having a relatively large radius of curvature r3 may be provided only in a portion that faces the first curved surface portion 111A when the pin 10 and the box 20 are fastened, and curved surface portions having a radius of curvature smaller than the radius of curvature r3, for example, 0.5 to 2.0 mm, may be provided in other portions between the load flank and the crest surface of the female thread 21.

[0061] 2 and 3, a radial gap is provided between the crest surface of the first thread 211 of the female thread 21, which contacts the load surface of the first thread portion 111 at the tip of the male thread 11 in the fastened state, and the bottom surface of the thread groove of the male thread 11, which faces the crest surface of the first thread 211. The dimension of this gap is smaller than the radius of curvature r1 of the first curved surface portion 111A, and is approximately 0.3 mm in the illustrated embodiment.

[0062] The radially outer end of the first curved surface portion 111 is located radially outward from the radially inner end of the third curved surface portion 21A, and is also located radially inward from the radially outer end of the third curved surface portion 21A.

[0063] Figure 5 shows a threaded joint for steel pipes according to the second embodiment, in which the load flank pitch and stabbing flank pitch are slightly smaller than those of the threaded joint of the first embodiment, thereby increasing the number of threads appearing in longitudinal section. Specifically, the load flank pitch is 8.466 mm and the stabbing flank pitch is 8.084 mm. As a result, the minimum thread width at the root of the thread at the tip of the male thread 11 is approximately 2.1 mm.

[0064] The present disclosure can be applied not only to coupling-type threaded joints but also to integral-type threaded joints. In addition, the present disclosure is not limited to the above-described embodiments, and various modifications are possible without departing from the spirit and scope of the present disclosure.

[0065] In order to confirm the effects of the threaded joint for steel pipes according to this embodiment, the shear resistance performance and torque resistance performance were evaluated by a numerical analysis simulation using the elastic-plastic finite element method.

[0066] <Test Conditions> In the finite element analysis (FEM analysis), a plurality of test specimens (analysis models) with different thread profiles were prepared, and elastic-plastic finite element analysis was performed on each test specimen to compare the differences in performance.

[0067] Specimens #1 to #4 had the basic structure of the threaded joint according to the first embodiment described above, with a first curved surface portion provided over the entire length of the male thread. Specimen #1 had a first curved surface portion with a radius of curvature of 0.1 mm, specimen #2 had a first curved surface portion with a radius of curvature of 0.2 mm, specimen #3 had a first curved surface portion with a radius of curvature of 0.3 mm, and specimen #4 had a first curved surface portion with a radius of curvature of 0.4 mm.

[0068] Specimens 5 to 8 had the basic structure of the threaded joint according to the second embodiment described above, with a first curved surface portion provided over the entire length of the male thread. Specimen 1 had a first curved surface portion with a radius of curvature of 0.1 mm, specimen 2 with a radius of curvature of 0.2 mm, specimen 3 with a radius of curvature of 0.3 mm, and specimen 4 with a radius of curvature of 0.4 mm.

[0069] The materials used were all API standard oil country tubular goods material Q125 (nominal yield strength YS = 862 MPa (125 ksi)).

[0070] For comparison with conventional products, a comparative model was created with two radii of curvature (radii of curvature of 0.125 mm and 0.875 mm) at the boundary between the load flank and the bottom surface of the male thread, and this conventional product was also evaluated in the same manner.

[0071] [Shear resistance] For shear resistance, 100% of the tensile load at which the pipe body of the steel pipe yields was applied, and the equivalent plastic strain at the first curved surface at the thread root on the load flank side of the tip of the male thread, which is the starting point of shear fracture of the male thread, was calculated. The smaller this value, the better the shear resistance. The evaluation results are shown in Figure 6.

[0072] [Evaluation of Torque Resistance Performance] The maximum torque value (MTV) at which the make-up torque diagram begins to yield was defined as the yield torque, and the higher this value, the better the torque resistance performance was evaluated. The evaluation results are shown in Figure 7.

[0073] [Evaluation Results] As shown in Figure 6, in the conventional product having two curvature radii, the strain concentrated in the part with a curvature radius of 0.125 mm is alleviated in the part with a curvature radius of 0.875 mm, which is considered to indicate a relatively low equivalent plastic strain. In the test specimens #1 to #4 and #5 to #8 having a first curved surface portion, the equivalent plastic strain generated in the first curved surface portion decreases as the curvature radius increases, and it can be evaluated that the equivalent plastic strain of test specimens #1 to #4 becomes smaller than that of the conventional product once the curvature radius exceeds about 0.30 mm, and that the equivalent plastic strain of test specimens #5 to #8 becomes smaller than that of the conventional product once the curvature radius exceeds about 0.35 mm. Since the thread height Th of these test specimens is 2.2 mm, it can be evaluated that in the case of test specimens #1 to #4, the equivalent plastic strain will be smaller than that of the conventional product when the radius of curvature r1 ≧ Th × 0.14 is satisfied, and in the case of test specimens #5 to #8, the equivalent plastic strain will be smaller than that of the conventional product when the radius of curvature r1 ≧ Th × 0.16 is satisfied.

[0074] On the other hand, torque resistance was hardly affected by changes in the radius of curvature of the first curved surface portion at the bottom of the load flanks of the male thread, and specimens #1 to #4 and #5 to #8 each showed similar values. It is believed that specimens #1 to #4 had lower torque resistance than the conventional product due to the reduced load flank contact area caused by the larger radius of curvature of the load flank top of the female thread compared to the conventional product. On the other hand, specimens #5 to #8, in which the load flank pitch and stabbing flank pitch were narrowed to increase the load flank contact area, showed higher torque resistance than the conventional product.

[0075] It was feared that reducing the load flank pitch and stabbing flank pitch would result in a decrease in the thread width at the tip of the male thread, resulting in a significant decrease in shear resistance. However, as shown in Figure 6, an increase in the radius of curvature resulted in evaluation results that were better than those of conventional products in terms of shear resistance, and it was found that the present disclosure can provide a threaded joint that is better than conventional products in both shear resistance and torque resistance.

[0076] Although the torque resistance of specimens #1 to #4 was lower than that of conventional products, by making the radius of curvature of the first curved surface portion larger than 0.3 mm, better shear resistance was obtained than that of conventional products, making them useful in applications that do not require high torque resistance, and it is also thought that they can be applied to products that require torque resistance by applying other means to improve torque resistance.

[0077] In addition, a similar evaluation was performed on a test specimen in which the radius of curvature at the bottom of the load surface of only the first thread of the male screw, which is thought to be the starting point of shear fracture, was increased.The effect of the change in the radius of curvature of only the first thread on the torque resistance performance and shear resistance performance was found to be negligible.

[0078] However, it is believed that torque resistance can be significantly improved by reducing the radius of curvature of the load flank apex of the female thread in the portion that does not mesh with the first thread of the male thread and increasing the load flank contact area.

[0079] 1: Steel pipe threaded joint, 2: Steel pipe, 10: Pin, 20: Box 11: Male thread, 111: First threaded portion, 111A: First curved surface portion, r1: Radius of curvature 112: Second threaded portion, 112A: Second curved surface portion, r2: Radius of curvature 21: Female thread, 21A: Third curved surface portion, r3: Radius of curvature 2: Steel pipe

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, wherein the pin has a male thread formed on its outer periphery, and the box has a female thread formed on the inner periphery of the box that fits over the male thread when fastened, the male thread and the female thread have load flanks, stabbing flanks, thread crests and thread groove roots, the stabbing flank pitch of the male thread and the female thread is smaller than the load flank pitch of the male thread and the female thread, the load flanks of the male thread contact the load flanks of the female thread and the stabbing flanks of the male thread contact the stabbing flanks of the female thread when fastened, and the load flanks and the stabbing flanks of the male thread and the female thread have negative flank angles, A threaded joint for steel pipes, in which the load flank and the thread groove root of the male thread, within a predetermined range from the tip of the male thread in the thread helix direction, are connected via a first curved surface portion whose radius of curvature r1 in longitudinal cross section satisfies the following formula (1): r1≧Th×0.14 (1) where Th is the thread height on the load flank side of the male thread within the predetermined range from the tip of the male thread in the thread helix direction, and satisfies 1.8 mm≦Th≦3.0 mm.

2. A threaded joint for steel pipes as set forth in claim 1, wherein the male thread comprises a first thread portion having the first curved surface portion and a second thread portion contiguous with the first thread portion in the direction of the thread spiral, the load flanks and thread groove root surfaces of the second thread portion being connected via a second curved surface portion having a smaller radius of curvature r2 than that of the first curved surface portion, and the first curved surface portion is provided over at least x circumferences in the direction of the thread spiral from the tip of the male thread, where x satisfies the following formula (2): x = (r1 - r2) / Δp (2) where Δp is the pitch difference between the load flank pitch and the stabbing flank pitch of the male thread.

3. A threaded joint for steel pipes as set forth in claim 1, wherein the first curved surface portion is provided over at least half a circumference from the tip of the male thread in the direction of the thread spiral.

4. A threaded joint for steel pipes as set forth in claim 1, wherein the first curved surface portion is provided over the entire length of the male thread.

5. A threaded joint for steel pipes as set forth in claim 1, wherein the load flank and the thread crest of the female thread are connected via a third curved surface portion that faces the first curved surface portion in a fastened state and has a larger radius of curvature than the first curved surface portion, and a radial gap is provided between the thread crest surface of a part of the female thread that contacts the load flank of the tip of the male thread in a fastened state and the bottom surface of the thread groove of the male thread that faces the thread crest surface, and the radial outer end of the first curved surface portion is located radially outward of the radial inner end of the third curved surface portion that faces the first curved surface portion, and is also located radially inward of the radial outer end of the third curved surface portion that faces the first curved surface portion.

6. A threaded joint for steel pipes as set forth in any one of claims 1 to 5, wherein the thread profiles of the male thread and the female thread are determined so that, in a made-up state, the load flanks and the stabbing flanks of the male thread over a portion extending for at least eight revolutions from the tip of the male thread in the direction of the thread spiral are in contact with the load flanks and the stabbing flanks of the female thread.

7. A threaded joint for steel pipes as set forth in claim 6, wherein the load flank pitch is 8.50 mm or less, the stabbing flank pitch is 8.10 mm or less, the difference between these pitches is 0.35 mm or more and 0.45 mm or less, and the minimum thread width at the root of the thread at the tip of the male thread is 2.1 mm or more.

8. A threaded joint for steel pipes as set forth in any one of claims 1 to 7, wherein the outer diameter of the steel pipe is greater than 240 mm.

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