Threaded connection

Abstract

In a threaded connection of a two-step thread construction including intermediate shoulder surfaces, a break is prevented near the intermediate critical cross sections upon application of a tensile load and the intermediate shoulder surfaces are easier to create, thus improving productivity. The outer periphery portion of the pin (2) located between the intermediate shoulder surface (24) of the pin (2) and the second male thread (27) is provided with a circumferential groove (25) including a curved surface (25a) smoothly contiguous to the intermediate shoulder surface (24), thereby mitigating plastic strains generated near the pin intermediate critical cross section (PICCS). The inner periphery portion of the box (3) located between the intermediate shoulder surface (35) of the box (3) and the first female thread (32) is provided with a circumferential groove (34) including a curved surface (34a) smoothly contiguous to the intermediate shoulder surface (35), thereby mitigating plastic strains generated near the box intermediate critical cross section (BICCS).

Description

Screw fittings

[0001] This disclosure relates to a threaded joint used for connecting steel pipes.

[0002] In oil wells, natural gas wells, etc. (hereinafter collectively referred to as "oil wells"), casings are used to construct multi-tiered well walls for extracting underground resources, and tubing is placed within the casing to produce oil or gas. These casings and tubing are made up of numerous steel pipes connected in sequence, and threaded joints are used for these connections. The steel pipes used in oil wells are also called oil well pipes.

[0003] Screw joints can be broadly classified into two types: integral type and coupling type.

[0004] In the integral type, oil well pipes are directly connected to each other. Specifically, one end of an oil well pipe has a female threaded section, and the other end has a male threaded section. The oil well pipes are connected by screwing the male threaded section of one oil well pipe into the female threaded section of another oil well pipe.

[0005] In the coupling type, oil well pipes are connected to each other via a tubular coupling. Specifically, the coupling has female threads at both ends, and the oil well pipes have male threads at both ends. One male thread of one oil well pipe is screwed into one female thread of the coupling, and the other male thread of another oil well pipe is screwed into the other female thread of the coupling, thereby connecting the oil well pipes. In other words, in the coupling type, one of the pair of pipes to be connected is the oil well pipe, and the other pipe is the coupling.

[0006] Generally, the end of an oil well pipe with a male threaded portion is called a pin because it includes an element that is inserted into a female threaded portion formed in the oil well pipe or coupling. The end of an oil well pipe or coupling with a female threaded portion is called a box because it includes an element that receives the male threaded portion formed in the oil well pipe.

[0007] In recent years, the development of even higher temperature and pressure deep wells has progressed. In deep wells, due to the complexity of the depth distribution of geological pressure, it is necessary to increase the number of casing stages, and for this reason, slim-type threaded fittings are sometimes used in which the maximum outer diameter of the fitting, i.e., the outer diameter of the box, is approximately the same as the outer diameter of the main body of the oil well pipe. Threaded fittings in which the outer diameter of the box is approximately equal to the outer diameter of the main body of the oil well pipe are also called flush-type threaded fittings. In addition, threaded fittings in which the outer diameter of the box is approximately less than 108% of the outer diameter of the main body of the oil well pipe are also called semi-flush-type threaded fittings. These flush-type and semi-flush-type threaded fittings require not only high strength and sealing performance, but also strict dimensional constraints on each part in order to arrange the thread structure and sealing structure within the limited pipe wall thickness.

[0008] In flush and semi-flush threaded joints with significant dimensional constraints, a joint design is often adopted in which a two-stage thread is used to construct the male and female threaded portions, with an intermediate shoulder surface located in the axial middle of the joint and threaded portions positioned before and after it. Such two-stage threaded joints are disclosed, for example, in Patent Documents 1 and 2 below. Furthermore, two-stage threaded joints with a hook-type intermediate shoulder structure that interlocks with each other instead of an intermediate shoulder surface are disclosed, for example, in Patent Documents 3 and 4 below.

[0009] In these two-stage threaded joints, as mentioned in Patent Document 1 above, there are critical cross-sections (PICCS and BICCS) in the middle of the joint.

[0010] The critical cross section (CCS) is the cross-section of a joint where the area capable of withstanding tensile load is smallest when the joint is fastened. When excessive axial tensile load is applied, there is a high probability that fracture will occur near the critical cross section.

[0011] In a typical single-stage threaded joint, the transmission of tensile load from the pin to the box is distributed axially across the entire threaded engagement range. Therefore, the cross-sectional area of ​​the pin on which all the tensile load acts is on the pipe side of the pin, and the cross-sectional area of ​​the box on which all the tensile load acts is on the pipe side of the box, on the pipe side of the box. The cross-sectional area with the smallest area among the cross-sectional areas on which all the tensile load acts is the critical section. That is, the box critical section (BCCS) is the cross-sectional area of ​​the box that encompasses the thread root position of the female thread corresponding to the tip end of the male thread at the engagement end of the male thread in the fastened state. Also, the pin critical section (PCCS) is the cross-sectional area of ​​the pin that encompasses the thread root position of the male thread corresponding to the pipe side end of the male thread at the engagement end of the male thread in the fastened state. The critical section (CCS) of the threaded joint is the one with the smaller area between the box critical section and the pin critical section. The ratio of the area of ​​the critical cross-section to the cross-sectional area of ​​the main body of an oil well pipe is called joint efficiency, and it is widely used as an indicator of the tensile strength of the joint relative to the tensile strength of the main body of the oil well pipe.

[0012] Even in screw joints with a two-stage thread structure, the box and pin critical sections exist. Furthermore, in screw joints with a two-stage thread structure, as described above, there are also parts in the axial middle of the joint where the joint cross-sectional area capable of withstanding tensile loads becomes smaller. That is, in screw joints with a two-stage thread structure, there is a section in the axial middle where there is no thread engagement. In this section without thread engagement, the tensile load distributed to the pin and box is transmitted axially without increasing or decreasing. Therefore, the pin cross-section with the smallest cross-sectional area in the section without thread engagement becomes the pin intermediate critical section (PICCS), and the box cross-section with the smallest cross-sectional area in the section without thread engagement becomes the box intermediate critical section (BICS). In order to prevent fracture in the middle of the joint, it is preferable to make the sum of the area of ​​the pin intermediate critical section and the area of ​​the box intermediate critical section as large as possible.

[0013] Japanese Patent Publication No. 2013-536339, Japanese Patent Publication No. 2002-357287, Japanese Patent Publication No. Hei 7-504483, Japanese Patent Publication No. 2013-519854

[0014] However, in two-stage threaded joints, intermediate shoulder surfaces are provided in sections without thread engagement. To improve torque resistance, it is necessary to ensure the radial dimension of the intermediate shoulder surfaces and increase the contact area between them. As a result, it is difficult to secure large cross-sectional areas for the pin and box intermediate critical sections in flush and semi-flush two-stage threaded joints.

[0015] Furthermore, the intermediate hazard sections of the pin and box are located close to the intermediate shoulder surface. Because the cross-sectional area of ​​the pin and box changes abruptly before and after the intermediate shoulder surface, large distortions are likely to occur in the intermediate hazard sections of the pin and box located nearby.

[0016] Furthermore, in conventional threaded joints, in order to ensure the contact area between intermediate shoulder surfaces and to ensure the pipe wall thickness near the intermediate shoulder surfaces, the diameter of the chamfer at the base of the intermediate shoulder surfaces is made as small as possible, as shown in Patent Documents 1 and 2. Therefore, machining the intermediate shoulder surfaces is difficult.

[0017] Furthermore, Patent Document 2 discloses a screw joint in which an annular relief is formed to provide flexibility against expansion or bending of the screw connection. This annular relief is formed by an annular groove provided on the outer surface of the pin and the inner surface of the box near the intermediate shoulder. This annular groove is provided separately from the intermediate shoulder surface and is not related to the machinability of the intermediate shoulder. In addition, Patent Document 2 does not mention fracture at the intermediate critical section when a tensile load is applied.

[0018] Furthermore, the hook-shaped intermediate shoulder structures disclosed in Patent Documents 3 and 4 have a complex structure, which further worsens their processability.

[0019] This disclosure aims to prevent fracture near the pin intermediate critical section and / or box intermediate critical section when a tensile load is applied to a screw joint with a two-stage thread structure including an intermediate shoulder surface, and to improve productivity by making the intermediate shoulder surface easier to process.

[0020] The threaded joint according to this disclosure comprises a tubular pin and a tubular box, wherein the pin is screwed into the box to fasten the pin and the box together. The outer circumference of the pin is provided with a first male thread, a second male thread spaced apart from the first male thread toward the axial tip and having a smaller diameter than the first male thread, an intermediate shoulder surface formed by a stepped portion located between the first and second male threads, and a first circumferential surface formed by the outer circumferential surface of the pin between the intermediate shoulder surface and the second male thread. The inner circumference of the box is provided with a first female thread into which the first male thread fits when fastened, a second female thread into which the second male thread fits when fastened, an intermediate shoulder surface formed by a stepped portion located between the first and second female threads and in contact with the intermediate shoulder surface of the pin when fastened, and a second circumferential surface formed by the inner circumferential surface of the box between the intermediate shoulder surface and the first female thread. One of the first and second circumferential surfaces is provided with a first circumferential groove having a curved surface as at least a part of the groove's inner wall surface that is smoothly continuous with the intermediate shoulder surface related to that circumferential surface.

[0021] According to this disclosure, in a screw joint with a two-stage thread structure including an intermediate shoulder surface, it is possible to prevent fracture near the intermediate critical section of the pin or the intermediate critical section of the box when a tensile load is applied, and to improve productivity by making the intermediate shoulder surface easier to process.

[0022] Figure 1 is a longitudinal cross-sectional view showing the fastening state of a threaded joint for steel pipes according to an embodiment. Figure 2 is an enlarged longitudinal cross-sectional view of the main part of the threaded joint for steel pipes. Figure 3 is a cross-sectional view for explaining the profile near the intermediate shoulder of the pin and box. Figure 4 is a side view of the cutting edge of a cutting tip for machining the intermediate shoulder surface. Figure 5 is a process diagram showing the machining process for the intermediate shoulder surface. Figure 6A is a graph showing the equivalent plastic strain that occurs near the critical section in the middle of the pin when a simple tensile load is applied to specimens #1 and #2. Figure 6B is a graph showing the equivalent plastic strain that occurs near the critical section in the middle of the pin when repeated loads are applied to specimens #1 and #2. Figures 6A and 6B show a cross-sectional view of the thread root of the second male thread near the critical section in the middle of the pin, with the strain measurement positions at each point on the horizontal axis of the graphs. Figure 7A is a graph showing the equivalent plastic strain that occurs near the critical section in the middle of the box when a simple tensile load is applied to specimens #1 and #2. Figure 7B is a graph showing the equivalent plastic strain that occurs near the critical section in the middle of the box when repeated loads are applied to specimens #1 and #2. The graphs in Figures 7A and 7B show the strain measurement positions at each point on the horizontal axis, and are cross-sectional views of the thread root of the first female thread near the critical section in the middle of the box. Figure 8A is a graph showing the equivalent plastic strain that occurs near the critical section in the middle of the pin when a simple tensile load is applied to specimens #3 and #4. Figure 8B is a graph showing the equivalent plastic strain that occurs near the critical section in the middle of the pin when repeated loads are applied to specimens #3 and #4. Figure 9A is a graph showing the equivalent plastic strain that occurs near the critical section in the middle of the box when a simple tensile load is applied to specimens #3 and #4. Figure 9B is a graph showing the equivalent plastic strain that occurs near the critical section in the middle of the box when repeated loads are applied to specimens #3 and #4.

[0023] The threaded joint according to this embodiment consists of a tubular pin and a tubular box. The pin and the box are fastened together by screwing the pin into the box. The pin may be provided at the end of a steel pipe such as an oil well pipe. The box may be provided at the end of a coupling or at the end of another steel pipe. The oil well pipe and coupling may be made of metal such as stainless steel or a nickel-based alloy.

[0024] The outer circumference of the pin may be provided with a first male thread, a second male thread spaced apart from the first male thread toward the axial tip and having a smaller diameter than the first male thread, an intermediate shoulder surface formed by a stepped portion located between the first and second male threads, and a first circumferential surface formed by the outer surface of the pin between the intermediate shoulder surface and the second male thread. The intermediate shoulder surface may be formed by the side surface of the stepped portion (i.e., the end face facing toward the axial tip). The first and second male threads can be made of tapered threads that gradually decrease in diameter towards the axial tip of the pin. In this case, the second male thread having a smaller diameter than the first male thread means that the diameter of the base end of the second male thread is smaller than the diameter of the tip end of the first male thread.

[0025] Furthermore, an external pin sealing surface can be provided on the outer circumference of the pin at the base end of the first male thread, and an internal pin sealing surface can be provided on the outer circumference of the pin at the tip end of the second male thread. An intermediate pin sealing surface may be provided on the outer circumference of the pin between the intermediate shoulder surface and the first male thread, or between the intermediate shoulder surface and the second male thread. One or more of these sealing surfaces may be provided depending on the required sealing performance and joint structure.

[0026] The inner circumference of the box may be provided with a first female thread into which the first male thread fits when fastened, a second female thread into which the second male thread fits when fastened, an intermediate shoulder surface formed by a stepped portion located between the first and second female threads, and a second circumferential surface formed by the inner circumferential surface of the box between the intermediate shoulder surface and the first female thread. The intermediate shoulder surface of the box may be formed by the side surface of the stepped portion. The intermediate shoulder surface of the box contacts the intermediate shoulder surface of the pin when fastened. The first and second female threads may be tapered threads having a thread profile that matches the thread profiles of the first and second male threads, and which gradually decrease in diameter towards the axial tip side of the box (i.e., the axial base side of the pin).

[0027] Incidentally, the male screw and the female screw may be buttress threads, or may be wedge-shaped threads whose longitudinal cross-sectional shape is a double-tail shape and whose thread width gradually narrows toward the tip side along the spiral direction of the threads, or may be other appropriate threads.

[0028] Further, on the inner periphery of the box, a sealing surface corresponding to the sealing surface provided on the outer periphery of the pin can be provided. That is, when providing an outer-pin sealing surface, an outer-box sealing surface that contacts the outer-pin sealing surface in the fastened state can be provided. When providing an inner-pin sealing surface, an inner-box sealing surface that contacts the inner-pin sealing surface in the fastened state can be provided. When providing an intermediate-pin sealing surface, an intermediate-box sealing surface that contacts the intermediate-pin sealing surface in the fastened state can be provided. A predetermined radial interference amount is set between the mutually contacting sealing surfaces, and when they are fitted together, they are all closely adhered by the elastic restoring force that each tries to return to its original diameter, thereby exhibiting the sealing performance.

[0029] The intermediate shoulder surfaces of the pin and the box may be flat surfaces orthogonal to the tube axis, or may be tapered surfaces having tapered generatrices that are slightly inclined with respect to a straight line orthogonal to the tube axis in the cross section. Also, the intermediate shoulder surfaces of the pin and the box may be a single surface extending substantially over the entire radial width of the step.

[0030] One of the first and second circumferential surfaces described above can be provided with a first circumferential groove having a curved surface as at least a part of the groove's inner wall surface that smoothly continues to the intermediate shoulder surface associated with that circumferential surface. The first circumferential groove has a radial groove depth. In conventional structures without this circumferential groove, the rigidity of the parts near the intermediate shoulder surfaces of the pin and box is relatively high, while the rigidity of the intermediate critical sections of the pin and box located near the intermediate shoulder surfaces is low. As a result, large strains exceeding the elastic range occur near the intermediate critical sections of the pin and box, making them prone to fracture. On the other hand, in the structure of this embodiment, by providing a first circumferential groove in the part where the pipe wall thickness suddenly thins due to the stepped portion constituting the intermediate shoulder surface, the extensibility under tensile load in that part can be improved, and the amount of strain occurring in the intermediate critical section of the pin or box can be suppressed. Furthermore, by smoothly continuing the curved surface forming the first circumferential groove to the corresponding intermediate shoulder surface, stress concentration in the first circumferential groove can be alleviated. Furthermore, even if the radius of curvature of the curved surface is relatively large, the contact area between the intermediate shoulder surfaces can be secured by accommodating all or part of the curved surface within the groove depth. In addition, by increasing the radius of curvature of the curved surface, it is possible to use cutting inserts with a larger radius of curvature than conventional ones for machining the intermediate shoulder surfaces and each threaded portion, thereby increasing the axial advance per revolution of the lathe. This improves the life of the cutting inserts and enhances the productivity of the pins and boxes.

[0031] A second circumferential groove can be provided on the other of the first and second circumferential surfaces described above, having a curved surface as at least a part of the groove's inner wall surface that is smoothly continuous with the intermediate shoulder surface associated with that circumferential surface. The second circumferential groove has a radial groove depth. By providing a circumferential groove continuous with the intermediate shoulder surface on both the pin and the box, the above-mentioned advantages can be obtained in both the intermediate hazard section of the pin and the intermediate hazard section of the box. The cross-sectional shape and size of the second circumferential groove may be the same as those of the first circumferential groove.

[0032] Preferably, the first circumferential groove and / or the second circumferential groove can be an annular groove extending over the entire circumference of the pin or the box. This can further improve productivity.

[0033] Preferably, the radius of curvature of the curved surface of the first circumferential groove is equal to the radius of curvature of the curved surface of the second circumferential groove. Thereby, the first and second circumferential grooves can be machined using a common cutting tool.

[0034] The first circumferential groove may be provided on the first circumferential surface. According to this, it is possible to prevent breakage at the intermediate dangerous section of the pin and improve the workability of the intermediate shoulder surface of the pin.

[0035] When providing the first circumferential groove on the pin, a threadless portion can be further provided on the first circumferential surface, which is located between the first circumferential groove and the second male thread and has an axial length. The threadless portion may have an outer peripheral surface having an outer diameter larger than the diameter of the thread bottom of the second male thread adjacent to the threadless portion. According to this, the rigidity of the portion between the first circumferential groove and the intermediate dangerous section of the pin can be ensured by the threadless portion, and the resistance to compressive load and bending load can be ensured.

[0036] The first circumferential groove provided on the pin may further have, as a part of the groove inner wall surface, a tapered surface that is continuous with the end portion on the side opposite to the intermediate shoulder surface among the axially both end portions of the curved surface, and the curved surface and the outer peripheral surface of the threadless portion can be connected via the tapered surface. According to this, the cutting edge of the cutting tool can be inserted along the tapered surface to the position where the curved surface of the first circumferential groove is formed, and the intermediate shoulder surface can be machined continuously after machining the curved surface, further improving productivity. In the case of the above configuration, the first circumferential surface includes the outer peripheral surface of the threadless portion, the curved surface and the tapered surface of the first circumferential groove, and the first circumferential groove is defined by the shape of such first circumferential surface.

[0037] The pin includes a pin intermediate critical cross-section PICCS (see FIG. 3) located near the end portion closer to the intermediate shoulder surface among both end portions of the second male thread. Preferably, the cross-sectional area A of the cross-section of the pin including the bottom of the first circumferential groove provided on the pin Ｐ１ is the cross-sectional area A of the pin intermediate critical cross-sectionＰ２ It is larger than this. According to this, the strength of the part where the first circumferential groove is provided can be ensured.

[0038] Preferably, the connection point P between the curved surface and the intermediate shoulder surface of the first circumferential groove provided on the pin. １ The radial inner end P of the intermediate shoulder surface of the box. ２ It can be positioned radially inward (see Figure 3). This allows for a large contact area between intermediate shoulder surfaces even if the radius of curvature of the curved surface is relatively large. Note that the radial inner end P of the intermediate shoulder surface of the box. ２ This refers to the inner end of the straight portion in the longitudinal section of the intermediate shoulder surface, and does not include the R portion provided at the corner.

[0039] The first circumferential groove may be provided on the second circumferential surface. This prevents fracture in the critical section of the box and improves the machinability of the intermediate shoulder surface of the box.

[0040] When a first circumferential groove is provided in the box, a threadless portion can be further provided on the second circumferential surface, located between the first circumferential groove and the first female thread and having an axial length. This threadless portion may have an inner circumferential surface with an inner diameter smaller than the diameter of the thread root of the first female thread adjacent to the threadless portion. This ensures rigidity in the area between the first circumferential groove and the critical section in the middle of the box, thereby ensuring resistance to compressive and bending loads.

[0041] The first circumferential groove provided in the box may further have tapered surfaces as part of the inner wall surface of the groove, which are continuous with the ends of the axial surface of the curved surface that are opposite to the intermediate shoulder surface, and the curved surface and the inner circumferential surface of the unthreaded portion can be connected via these tapered surfaces. This allows the cutting edge of a cutting chip to be inserted along the tapered surface to the position where the curved surface is formed in the first circumferential groove, and the intermediate shoulder surface can be machined continuously after the curved surface has been machined, thereby further improving productivity. In the above configuration, the first circumferential surface includes the inner circumferential surface of the unthreaded portion and the curved surface and tapered surface of the first circumferential groove, and the shape of this first circumferential surface defines the first circumferential groove.

[0042] The box includes a box intermediate dangerous cross-section located near the end closer to the intermediate shoulder surface among both ends of the first female thread (see FIG. 3). Preferably, the cross-sectional area A of the cross-section of the box including the bottom of the first circumferential groove provided in the box Ｂ１ is larger than the cross-sectional area A of the box intermediate dangerous cross-section BICCS. According to this, the strength of the part where the first circumferential groove is provided can be ensured. Ｂ２

[0043] Preferably, the connection point P between the curved surface and the intermediate shoulder surface of the first circumferential groove provided in the box ３ can be positioned radially outward of the radial outer end P of the intermediate shoulder surface of the pin (see FIG. 3). According to this, even if the radius of curvature of the curved surface is made relatively large, a large contact area between the intermediate shoulder surfaces can be ensured. Note that the radial outer end P of the intermediate shoulder surface of the pin ４ is the outer end of the straight part in the longitudinal section of the intermediate shoulder surface and does not include the R part provided at the corner part. ３

[0044] The screw joint according to the present embodiment can be suitably implemented as a semi-flush type screw joint in which the maximum outer diameter of the box is less than 108% of the pipe diameter of the steel pipe having the pin, and particularly can be suitably implemented as an integral type semi-flush screw joint in which the box is provided at the end of another steel pipe.

[0045] In the present disclosure, each of the above dangerous cross-sections does not include the cross-section of the thread, and the cross-sectional area of each dangerous cross-section is defined as the area of the cross-section part excluding the threads of the complete thread part and the incomplete thread part of each tapered thread.

[0046] ​​[Configuration of threaded joint for oil well pipes] Referring to Figure 1, the threaded joint 1 for oil well pipes according to this embodiment is an integral type threaded joint comprising a tubular pin 2 and a tubular box 3 into which the pin 2 is screwed and fastened. The pin 2 is provided at one end of a pair of oil well pipes that are connected to each other, and the box 3 is provided at the other end of the oil well pipe. Although the structure is such that the pin 2 formed at the end of one oil well pipe is fitted into the box 3 formed at the end of the other oil well pipe, in order to ensure that the pipe wall thickness of the pin 2 and the box 3 is as large as possible, the end of the oil well pipe into which the pin 2 is formed is subjected to diameter reduction processing, and the end of the oil well pipe into which the box 3 is formed is subjected to diameter expansion processing, and the pin 2 and the box 3 are formed by turning after these diameter reduction processing or diameter expansion processing.

[0047] Pin 2 extends axially from one end of the main body of the oil well pipe toward the outer end (right side in Figure 1). The outer circumference of pin 2 is provided with, in order from the main body side toward the outer end, an outer pin sealing surface 21, a first male thread 22 made of tapered threads, an unthreaded portion 23 having an outer circumference connected to the bottom of the thread root of the first male thread 22, a stepped portion including an intermediate torque shoulder surface 24, a circumferential groove 25, an unthreaded portion 26 having an outer circumference connected to the top of the thread crest of the second male thread 27, a second male thread 27 made of tapered threads with a smaller diameter than the first male thread 22, and an inner pin sealing surface 28. The circumferential groove 25 is provided on the outer circumference of pin 2 between the intermediate shoulder surface 24 and the second male thread 27.

[0048] Box 3 extends axially from the end of the main body of the other oil well pipe toward the outer end (left side in Figure 1). The outer circumference of box 3 is provided with, in order from the outer end toward the main body, an outer box sealing surface 31, a first female thread 32 made of tapered threads, an unthreaded portion 33 having an inner circumferential surface connected to the crest surface of the first male thread 32, a circumferential groove 34, a stepped portion including an intermediate torque shoulder surface 35, an unthreaded portion 36 connected to the bottom of the thread valley of a second female thread 37, a second female thread 37 made of tapered threads with a smaller diameter than the first female thread 32, and an inner box sealing surface 38. The circumferential groove 34 is provided on the inner circumferential surface of box 3 between the intermediate shoulder surface 35 and the first female thread 32.

[0049] As shown in Figure 1, the two-stage threaded joint 1 of this embodiment has a pin-critical cross section PCCS near the end of the pin tube body side within the meshing range of the first male thread 22 and the first female thread 32, and a box-critical cross section BCCS near the end of the box tube body side within the meshing range of the second male thread 27 and the second female thread 37. Furthermore, it has a box-intermediate critical cross section BICCS near the end of the intermediate shoulder side within the meshing range of the first male thread 22 and the first female thread 32, and a pin-intermediate critical cross section PICCS near the end of the intermediate shoulder side within the meshing range of the second male thread 27 and the second female thread 37.

[0050] With the pin 2 and box 3 fastened, the outer sealing surface 21 of the pin and the outer sealing surface 31 of the box are in contact around their entire circumference to form a metal-to-metal seal, the first male thread 22 is fitted into the first female thread 32, the intermediate shoulder surfaces 24 and 35 are in contact with each other to bear the fastening torque, the second male thread 27 is fitted into the second female thread 37, and the inner sealing surface 28 of the pin is in contact with the inner sealing surface 38 of the box around its entire circumference to form a metal-to-metal seal.

[0051] Furthermore, in the fastened state, the threadless portion 23 of the pin 2 is inserted into the threadless portion 33 of the box 3, and the threadless portion 26 of the pin 2 is inserted into the threadless portion 36 of the box 3. Gaps are formed between the threadless portion 23 and the threadless portion 33, and between the threadless portion 26 and the threadless portion 36.

[0052] The circumferential grooves 25 and 34 are each composed of annular grooves that are continuous around the entire circumference. The circumferential groove 25 of the pin 2 has a curved surface 25a that is smoothly continuous with the intermediate shoulder surface of the pin 2 and has a constant radius of curvature, and tapered surfaces 25b that are continuous with the ends of the curved surface 25a that are opposite to the intermediate shoulder surface 24, as inner wall surfaces of the groove. The tapered surface 25b connects the curved surface 25a to the outer surface of the threadless portion 26. The circumferential groove 34 of the box 3 has a curved surface 34a that is smoothly continuous with the intermediate shoulder surface 35 of the box 3 and has a constant radius of curvature, and tapered surfaces 34b that are continuous with the ends of the curved surface 34a that are opposite to the intermediate shoulder surface 35, as inner wall surfaces of the groove. The tapered surface 34b connects the curved surface 34a to the inner circumferential surface of the threadless portion 33.

[0053] The groove depth of the circumferential groove 25, that is, the depth relative to the outer circumferential surface of the unthreaded portion 26, may be approximately the same as the thread root depth near the end of the second male thread 27 on the axial inner end side (i.e., the intermediate shoulder surface side). Referring to Figure 3, preferably, the cross-sectional area A of the cross-section of the pin 2 that encloses the bottom of the circumferential groove 25. Ｐ１ This is the cross-sectional area A of the pin intermediate hazard section PICCS. Ｐ２ It is larger than that. Also, in order to fully generate the effects of the circumferential grooves 25, a cross-sectional area A is preferable. Ｐ１ Cross-sectional area A Ｐ２ The groove depth can be set to be less than 110% of the cross-sectional area A. Ｐ１ Cross-sectional area A Ｐ２ The groove depth of the circumferential groove 25 can be set to be less than 105% of the above.

[0054] The groove depth of the circumferential groove 34, that is, the depth relative to the inner circumferential surface of the unthreaded portion 33, may be approximately the same as the thread root depth near the end of the first female thread 32 on the axial inner end side (i.e., the intermediate shoulder surface side). Referring to Figure 3, preferably, the cross-sectional area A of the cross-section of the box 3 that encloses the bottom of the circumferential groove 34. Ｂ１ This is the cross-sectional area A of the box intermediate hazard section BICCS. Ｂ２ It is larger than that. Also, in order to fully generate the effects of the circumferential grooves 34, the cross-sectional area A is preferably larger. Ｂ１ Cross-sectional area A Ｂ２The groove depth can be set to be less than 110% of the cross-sectional area A. Ｂ１ Cross-sectional area A Ｂ２ The groove depth of the circumferential groove 34 can be set to be less than 105% of the above.

[0055] The intermediate shoulder surfaces 24 and 35 are each composed of flat surfaces perpendicular to the pipe axis in the illustrated embodiment. As shown in detail in Figure 3, the radial inner end P of the intermediate shoulder surface 24 of the pin 2 １ The outer surface of the threadless portion 26 is located at approximately the same position radially as the outer surface of the intermediate shoulder surface 24, and the radial outer end P ４ The outer surface of the threadless portion 23 is located at approximately the same position radially as the outer surface of the threadless portion 23. Also, the radial inner end P of the intermediate shoulder surface 35 of the box 3. ２ The inner circumferential surface of the threadless portion 36 is located at approximately the same position radially as the outer end P of the intermediate shoulder surface 35. ３ It is located at approximately the same position radially as the inner circumferential surface of the threadless portion 33. The inner end of the intermediate shoulder surface 24 of the pin 2, that is, the connection point P between the intermediate shoulder surface 24 and the curved surface 25a of the circumferential groove 25. １ The radial inner end P of the box's intermediate shoulder surface 35. ２ It is located radially inward. The outer end of the intermediate shoulder surface 35 of box 3, i.e., the connection point P between the intermediate shoulder surface 35 and the curved surface 34a of the circumferential groove 34. ３ The radial outer end P of the pin intermediate shoulder surface 24. ４ It is located radially outward from the intermediate shoulder surfaces 24 and 35, respectively. By connecting the curved surfaces 25a and 34a to the intermediate shoulder surfaces 24 and 35 in this way, the radius of curvature of the curved surfaces 25a and 34a can be increased without reducing the contact area between the intermediate shoulder surfaces 24 and 35.

[0056] [Method for machining circumferential grooves] Figure 4 is an enlarged side view of the tip blade portion 4 of a cutting tip (cutting tool) for turning each screw 22, 27, 32, 37 and circumferential grooves 25, 34. The blade portion 4 has a tip R portion 41 with a predetermined radius of curvature R, and left and right side edges 42, 43. The cutting tool angle α is set so that the left and right side edges 42, 43 gradually widen as they move away from the tip R portion 41.

[0057] The cutting edge 4 has a gap angle K', K'' required with respect to the machined surface during cutting. That is, as shown in Figure 5, the angle θ', θ'' between the left and right side edges 42, 43 and the machined surface must be greater than the gap angle K', K'' for machining to be possible. Therefore, when machining the pin intermediate shoulder surface 24 and the circumferential groove 25, as shown in Figure 5, the circumferential groove 25 and the intermediate shoulder surface 24 can be continuously machined by inserting the cutting edge 4 into the workpiece at an angle that satisfies the relationship θ' > K' and θ'' > K''. Since it has a tapered surface 25b, the cutting edge 4 can be inserted while ensuring a gap angle K'. Furthermore, although the radius of curvature R of the tip R portion 41 needs to be smaller than the radius of curvature of the curved surface 25a to be machined, the circumferential groove 25 according to this disclosure has depth, so the radius of curvature R' of the curved surface 25a can be made relatively large without sacrificing the contact area between the intermediate shoulder surfaces, and consequently a cutting tip with a large radius of curvature R of the tip R portion 41 can be used. The same applies to machining the circumferential groove 34 and the box intermediate shoulder surface 35.

[0058] This disclosure is applicable not only to integral-type threaded joints but also to coupling-type threaded joints. Furthermore, each thread may be a trapezoidal thread, API round thread, API buttress thread, or wedge-type thread, etc. In addition, this disclosure is not limited to the embodiments described above, and various modifications are possible without departing from the spirit of this disclosure.

[0059] To confirm the effectiveness of the threaded joint for steel pipes according to this embodiment, a numerical analysis simulation using the elastoplastic finite element method was performed.

[0060] <Test Conditions> For the integral-type oil well pipe threaded joints shown in Figures 1 to 3, specimens #2 and #4 (analysis models) with circumferential grooves 25 and 34 were prepared for two different pipe diameter sizes, and specimens #1 and #3 without any circumferential grooves were prepared. Elastoplastic finite element analysis was performed on each specimen to compare the equivalent plastic strain generated at the bottom of the thread root of the second male thread, which includes a critical cross section in the middle of the pin, and the equivalent plastic strain generated at the bottom of the thread root of the first female thread, which includes a critical cross section in the middle of the box.

[0061] <Dimensions of the oil well pipe> The size of the oil well pipes for specimens #1 and #2 is 9-5 / 8” 47.0# (outer diameter of pipe body: 244.48 mm, inner diameter of pipe body: 220.50 mm), and the joint efficiency (ratio of the cross-sectional area of ​​the box hazard section BCCS to the cross-sectional area of ​​the pipe body) is 67.3%.

[0062] The oil well pipes for specimens #3 and #4 were 13-3 / 8” 72.0# (outer diameter of pipe body: 339.73 mm, inner diameter of pipe body: 313.61 mm), and the joint efficiency was 70.5%.

[0063] In all test specimens, the thread taper angle, thread height, and thread pitch were standardized to 1.591°, 1.3 mm, and 5.08 mm respectively.

[0064] Furthermore, the shapes of the circumferential grooves 25 and 34 were standardized, with the radius of curvature of the curved surfaces 25a and 34a being 1.7 mm, the groove depth being 1.2 mm, and the angle between the tapered generatrix of the tapered surfaces 25b and 34b and the tube axis being 15°.

[0065] <Materials for oil well tubular goods> API standard oil well tubular goods material Q125 (Nominal yield strength YS = 862 MPa (125 kSi))

[0066] <Evaluation Method> First, an analysis simulating fastening was performed on each specimen. Then, a repeated combined load simulating the Series A test specified in API 5C5 2017 CAL-IV was applied to the model of the fastened state. The equivalent plastic strain immediately after the application of the simple tensile load in the initial stage of the combined load, and the equivalent plastic strain after the completion of the entire repeated combined load process, were compared for each specimen.

[0067] <Evaluation Results> The comparison results between the small-diameter specimens #1 and #2 are shown in the graphs in Figures 6A, 6B, 7A, and 7B. Figure 6A shows the equivalent plastic strain distribution near the critical cross section in the middle of the pin immediately after simple tensile load application, Figure 6B shows the equivalent plastic strain distribution near the critical cross section in the middle of the pin after the completion of the entire process of repeated compound load application, Figure 7A shows the equivalent plastic strain distribution near the critical cross section in the middle of the box immediately after simple tensile load application, and Figure 7B shows the equivalent plastic strain distribution near the critical cross section in the middle of the box after the completion of the entire process of repeated compound load application. The "axial coordinate" on the horizontal axis is the distance in the axial direction of the pipe from the intermediate shoulder surface, with positive values ​​indicating the pipe body side of the pin from the intermediate shoulder surface and negative values ​​indicating the pin tip side from the intermediate shoulder surface. The points where the equivalent plastic strain was measured are indicated by dots in Figures 6C and 7C.

[0068] As is clear from these graphs, it was confirmed that in both the vicinity of the critical cross-section in the middle of the pin and the vicinity of the critical cross-section in the middle of the box, specimen #2, which has circumferential grooves 25 and 34, exhibits a reduction in equivalent plastic strain over the entire area compared to specimen #1, which does not have circumferential grooves. In the graphs of Figures 6A and 6B, the peak on the right corresponds to the R portion at the bottom of the load surface of the second male screw 27, and the peak on the left corresponds to the R portion at the bottom of the insertion surface.

[0069] Furthermore, the difference between specimens #1 and #2 is more pronounced immediately after applying a simple tensile load than after the entire process is completed. This is thought to be due to the accumulation of plastic strain in each part caused by repeated combined loading.

[0070] Similarly, the comparison results between large-diameter specimens #3 and #4 are shown in the graphs in Figures 8A, 8B, 9A, and 9B. As is clear from these graphs, the same trend is observed for large-diameter specimens as for small-diameter specimens, confirming that regardless of pipe diameter, the provision of circumferential grooves can reduce the plastic strain generated at the bottom of the thread root near the intermediate critical section.

[0071] 1: Threaded joint, 2: Pin, 22: First male thread, 24: Intermediate shoulder surface, 25: Circumferential groove, 25a: Curved surface, 25b: Tapered surface, 26: Unthreaded section, 27: Second male thread, 3: Box, 32: First female thread, 33: Unthreaded section, 34: Circumferential groove, 34a: Curved surface, 34b: Tapered surface, 35: Intermediate shoulder surface, 37: Second female thread, PICCS: Pin intermediate hazardous section, BICCS: Box intermediate hazardous section

Claims

1. A threaded joint comprising a tubular pin and a tubular box, wherein the pin is screwed into the box to fasten the pin and the box, wherein the outer periphery of the pin is provided with a first male thread, a second male thread spaced axially toward the tip end of the first male thread and having a smaller diameter than the first male thread, an intermediate shoulder surface formed by a step located between the first male thread and the second male thread, and a first circumferential surface formed by the outer periphery of the pin between the intermediate shoulder surface and the second male thread, and the inner periphery of the box is provided with a first female thread with which the first male thread fits in a fastened state, a second female thread with which the second male thread fits in a fastened state, an intermediate shoulder surface formed by a step located between the first female thread and the second female thread and in contact with the intermediate shoulder surface of the pin in a fastened state, and a second circumferential surface formed by the inner periphery of the box between the intermediate shoulder surface and the first female thread, a first circumferential groove is provided in one of the first circumferential surface and the second circumferential surface, the first circumferential groove having, as at least a part of an inner wall surface of the groove, a curved surface that smoothly continues onto the intermediate shoulder surface associated with that circumferential surface.

2. A threaded joint as set forth in claim 1, wherein the other of the first and second circumferential surfaces is provided with a second circumferential groove having, as at least a part of the groove inner wall surface, a curved surface that smoothly continues to the intermediate shoulder surface associated with that circumferential surface.

3. A threaded joint as set forth in claim 2, wherein the radius of curvature of the curved surface of the first circumferential groove is equal to the radius of curvature of the curved surface of the second circumferential groove.

4. A threaded joint as set forth in claim 1, 2 or 3, wherein the first circumferential groove is provided on the first circumferential surface.

5. A threaded joint as set forth in claim 4, wherein the first circumferential surface is further provided with an unthreaded portion located between the first circumferential groove and the second male thread and having an axial length, the unthreaded portion having an outer peripheral surface with an outer diameter larger than the diameter of the thread root of the second male thread adjacent to the unthreaded portion, and the first circumferential groove further has a tapered surface as part of the groove inner wall surface that continues to one of the axial end portions of the curved surface on the side opposite the intermediate shoulder surface, and the curved surface and the outer peripheral surface of the unthreaded portion are connected via the tapered surface.

6. A threaded joint as set forth in claim 4 or 5, wherein the pin includes a pin intermediate danger cross section located near one of the two end portions of the second male thread that is closer to the intermediate shoulder surface, and the cross-sectional area of ​​the pin's transverse section including the bottom of the first circumferential groove is larger than the cross-sectional area of ​​the pin intermediate danger cross section.

7. A threaded joint as set forth in claim 4, 5 or 6, wherein the connection point between the curved surface of the first circumferential groove and the intermediate shoulder surface is located radially inward of the radial inner end of the intermediate shoulder surface of the box.

8. A threaded joint as set forth in claim 1, 2 or 3, wherein the first circumferential groove is provided on the second circumferential surface.

9. A threaded joint as set forth in claim 8, wherein the second circumferential surface is further provided with an unthreaded portion located between the first circumferential groove and the first female thread and having an axial length, the unthreaded portion having an inner circumferential surface with an inner diameter smaller than the diameter of the root of the first female thread adjacent to the unthreaded portion, and the first circumferential groove further has a tapered surface as part of the groove inner wall surface that continues to one of the axial ends of the curved surface on the side opposite the intermediate shoulder surface, and the curved surface and the inner circumferential surface of the unthreaded portion are connected via the tapered surface.

10. A threaded joint as set forth in claim 8 or 9, wherein the box includes a box intermediate danger cross section located near one of the two end portions of the first female thread that is closer to the intermediate shoulder surface, and the cross-sectional area of ​​the box transverse section including the bottom of the first circumferential groove is larger than the cross-sectional area of ​​the box intermediate danger cross section.

11. A threaded joint as set forth in claim 8, 9 or 10, wherein the connection point between the curved surface of the first circumferential groove and the intermediate shoulder surface is located radially outward of the radial outer end of the intermediate shoulder surface of the pin.