Specification
Title of Invention : Threaded Joint for Pipes
Technical field
[0001]
　TECHNICAL FIELD The present disclosure relates to a threaded joint for pipes used for connecting steel pipes and the like.
Background technology
[0002]
　In oil wells, natural gas wells, etc. (hereinafter also collectively referred to as "oil wells"), in order to extract underground resources, a casing that constructs a multi-stage well wall, and a casing that is arranged in the casing and contains oil and gas Tubing is used that produces These casings and tubings are made up of a large number of steel pipes connected in sequence, and threaded joints for pipes are used for the connections. Steel pipes used in oil wells are also called oil well pipes.
[0003]
　Types of threaded joints for pipes are broadly classified into an integral type and a coupling type. Integral threaded joints for pipes are disclosed, for example, in Patent Document 1 below and FIGS. It is disclosed in Patent Document 3 and the like.
[0004]
　In the integral type, oil country tubular goods are directly connected to each other. Specifically, one end of the oil country tubular good is provided with a female threaded portion, and the other end thereof is provided with a male threaded portion. concatenated.
[0005]
　In the coupling type, oil country tubular goods are connected to each other via tubular couplings. Specifically, both ends of the coupling are provided with female threads, and both ends of the oil well pipe are provided with male threads. One male threaded portion of one oil country tubular good is screwed into one female threaded portion of the coupling, and one male threaded portion of the other oil country tubular good is screwed into the other female threaded portion of the coupling, whereby the coupling is formed. The oil country tubular goods are connected to each other through the joint. That is, in the coupling type, one of a pair of directly connected pipe members is an oil well pipe and the other is a coupling.
[0006]
　In general, the end of an oil country tubular good having a male thread is called a pin because it includes an element that is inserted into a female thread formed in the oil country tubular good or coupling. The end of a female-threaded oil country tubular good or coupling is called a box because it includes an element that receives the male-threaded portion formed on the end of the oil country tubular good.
[0007]
　The latest standard for OCTG joints, such as API 5C5 RP CAL-IV 2017,
has increased each load of tension, compression, internal pressure, and external pressure in the combined load ellipse in the Series A test compared to the old standard, such as ISO-CAL IV 2002. ing. Furthermore, in recent years, the development of high-temperature, high-pressure deep wells has progressed, and the usage environment for threaded joints for oil country tubular goods has become increasingly severe. improvement is required.
[0008]
　On the other hand, in deep wells, it is necessary to increase the number of casing stages due to the complexity of the depth distribution of formation pressure. A comparable threaded joint is required. A threaded joint whose box outer diameter is approximately equal to the outer diameter of the main body of the oil well pipe is also called a flush-type threaded joint. Further, a threaded joint whose box outer diameter is approximately less than 108% of the outer diameter of the main body of the oil well pipe is also called a semi-flush type threaded joint. These flush-type and semi-flush-type threaded joints are required not only to have high strength and sealing performance, but also to place the threaded structure and seal structure within the limited wall thickness of pipes. Dimensional constraints are imposed.
[0009]
　Flush-type and semi-flush-type threaded joints, which have large dimensional restrictions, are
provided with an intermediate shoulder surface in the axial middle of the joint, as disclosed in Patent Document 1. Joint designs are often employed in which the male and female threads are formed by two-stage threads arranged in a . A joint design with a two-step thread structure can ensure a larger risk cross-sectional area.
[0010]
　The critical cross-section (CCS) is the longitudinal cross-section (in the cross-section perpendicular to the pipe axis) of the joint where the maximum stress occurs when a tensile load is applied in the fastened state. When an excessive tensile load is applied, there is a high possibility of breakage near the dangerous cross section.
[0011]
　In the threaded joint for oil country tubular goods, the propagation of the tensile load from the pin to the box is distributed axially over the entire threaded engagement range. Therefore, the cross-sectional portion of the pin where the entire tensile load acts is closer to the pipe body of the pin than the screw fitting range, and the cross-sectional portion of the box where all the tensile load acts is closer to the pipe body of the box than the thread fitting range. becomes. The critical cross-section is the one with the smallest cross-sectional area among the cross-sections on which all the tensile loads act. That is, of the meshing ends of the male and female threads in the fastened state, the vertical cross section of the box (on the cut plane perpendicular to the pipe axis) that includes the root position of the female thread corresponding to the tip end of the male thread. is the box critical cross section (BCCS). In addition, of the engagement ends of the male and female threads in the fastened state, the vertical cross section of the pin including the root position of the male thread corresponding to the engagement end of the male thread on the side of the pipe body (the cut surface perpendicular to the pipe axis ) becomes the pin critical cross section (PCCS). The smaller of the box critical cross section and the pin critical cross section is the critical cross section (CCS) of the threaded joint. The ratio of the dangerous cross-sectional area to the cross-sectional area of ​​the main body of the oil country tubular good is called the joint efficiency, and 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 country tubular good.
[0012]
　A threaded joint having a two-step thread structure also has the above-described box critical section and pin critical section. Furthermore, in a threaded joint having a two-step thread structure, as described above, there is also a portion in the axially intermediate portion of the joint portion where the joint cross-sectional area that can withstand the tensile load becomes small. That is, in a threaded joint having a two-step thread structure, there is a section with no threaded engagement in the middle in the axial direction. In this non-threaded section, the tensile load shared by the pin and box propagates axially without increasing or decreasing. Therefore, the cross section of the pin with the smallest cross-sectional area in the section without threaded engagement is the Pin Intermediate Critical Cross Section (PICCS), and the cross section of the box with the smallest cross-sectional area in the section without threaded engagement is the Box Intermediate Critical Cross Section (PICCS). BICCS). In order to prevent the occurrence of breakage in the intermediate portion of the joint, it is preferable to make the sum of the pin intermediate critical cross section area and the box intermediate critical cross section area larger than the area of ​​the critical cross section (CCS) of the threaded joint.
prior art documents
patent literature
[0013]
Patent Document 1: Japanese Patent Publication No. 2018-536818 (International Publication No. 2017/097700)
Patent Document 2: JP-A-57-186690
Patent Document 3: International Publication No. 2014/045973
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
[0014]
　In the semi-flush type integral screw joint with a two-step screw structure disclosed in Patent Document 1, it is necessary to form the two-step screw structure within the wall thickness range of the base pipe of the oil country tubular goods. The strength of the joint portion is lower than that of a coupling-type threaded joint that uses a coupling having a large outer diameter, and it is difficult to ensure strength that can withstand a high compressive load.
[0015]
　Generally speaking, in order to improve compression performance, it is effective to increase the contact area between the pin and the box, which bear the compressive load. That is, securing a large radial contact width (that is, the radial width of the contacting portion) between the intermediate shoulder surfaces that function as a torque stopper during fastening is thought to contribute to the improvement of compression performance.
[0016]
　However, if the contact width between the intermediate shoulder surfaces is increased, the wall thickness of the threaded section of the pin and box and the wall thickness of the seal section are sacrificed, resulting in a decrease in sealing performance, as well as an intermediate dangerous section of the pin and the box. This also leads to a reduction in the sum of the areas of the critical cross-sections, leading to a reduction in the tensile strength of the threaded joint.
[0017]
　In addition, in the threaded joint described in Patent Document 1, the intermediate shoulder surface functions as a torque stopper during fastening, and fastening is completed in a state in which the pin tip (20) and the end shoulder (30) of the box are spaced apart. Patent Document 1 makes no reference to the behavior when an axial compressive load is applied in this fastened state.
[0018]
　Therefore, in the case of the conventional threaded joint described in Patent Document 1, it is necessary to bear the compressive load only by the contact between the intermediate shoulder surfaces (26, 28), and it is difficult to greatly improve the compressive performance.
[0019]
　An object of the present disclosure is to further improve compression resistance performance in a threaded joint for pipes having a two-step thread structure.
Means to solve problems
[0020]
　A threaded joint for pipes according to the present disclosure includes a tubular pin and a tubular box, and the pin and the box are fastened by screwing the pin into the box.
[0021]
　The pin is provided between a first male screw, a second male screw provided on the tip end side of the first male screw and having a smaller diameter than the first male screw, and the first male screw and the second male screw. A pin intermediate shoulder surface, a pin end shoulder surface provided at the tip of the pin, and a pin seal surface provided between the second male thread and the pin end shoulder surface.
[0022]
　The box includes a first female thread into which the first male thread is fitted in a fastened state, a second female thread into which the second male thread is fitted in a fastened state, and a box intermediate shoulder that contacts the pin intermediate shoulder surface in a fastened state. a surface, a box end shoulder surface provided corresponding to the pin end shoulder surface, and a box end shoulder surface provided between the second female thread and the box end shoulder surface so as to cover the entire circumference of the pin seal surface in a fastened state. a box seal surface in contact over the .
[0023]
　In the threaded joint for pipes according to the present disclosure, the axial distance LB between the box intermediate shoulder surface of the box before fastening and the box end shoulder surface is equal to the pin intermediate shoulder surface of the pin before fastening. greater than the axial distance LP between said pin end shoulder surface . As a result, the pin end shoulder surface and the box end shoulder surface do not contact each other when the pin intermediate shoulder surface and the box intermediate shoulder surface start to contact each other during fastening. That is, the pin intermediate shoulder surface and the box intermediate shoulder surface function as torque stoppers. On the other hand, when fastening is completed, the pin end shoulder surface and the box end shoulder surface are separated from each other. Alternatively, the pin tip portion is pulled toward the tip side with respect to the intermediate shoulder surface of the pin due to fastening, and the shoulder surface of the pin end portion is elastically deformed toward the pin tip side. At this point, the pin end shoulder surface and the box end shoulder surface may be in light contact with each other.
[0024]
　Further, in the threaded joint for pipes according to the present disclosure, when an axial compressive load is applied in a fastened state,
before the threaded joint for pipes yields, the pin end shoulder surface starts contacting the box end shoulder surface. A difference in said axial distance (L B −L P ) is defined. As a result, when an axial compressive load is applied, the pin end shoulder surface and the box end shoulder surface come into contact with each other, and part of the axial compressive load is borne by the pin end shoulder surface and the box end shoulder surface. It is possible to obtain the yield compressive strength required for the joint structure as a whole. Furthermore, the contact between the end shoulder surfaces can suppress the amount of axial displacement of the pin seal surface with respect to the box seal surface, thereby reducing the damage accumulated in the vicinity of the pin seal surface and the box seal surface when a large compressive load is applied. Therefore, it is possible to maintain the internal pressure sealing performance after the compression load disappears.
[0025]
Preferably, the axial distances LB and LP 　are determined such that a gap is formed between the pin end shoulder surface and the box end shoulder surface when no compressive load is applied in the axial direction in a fastened state . . According to this, the pin end shoulder surface and the box end shoulder surface do not function as torque shoulders, but function as "pseudo shoulder surfaces" that bear part of the axial compressive load.
[0026]
　More preferably, the second male thread and the second female thread are configured such that a gap is formed between the insertion surfaces of the second male thread and the second female thread when no compressive load is applied in the axial direction in a fastened state. , in the process in which the applied axial compressive load gradually increases, the insertion surfaces first start to come into contact with each other, and then the pin end shoulder surface and the box end shoulder surface start to come into contact, A size of the gap between the insertion surfaces is defined. According to this, part of the axial compressive load can be borne by the second male thread and the second female thread, and the compression resistance performance can be further improved. In addition, since contact between the end shoulder surfaces begins after contact between the insertion surfaces of the second male screw and the second female screw begins, the magnitude of the compressive load borne by the pin end shoulder surface and the box end shoulder surface is can be suppressed. Therefore, even if the contact area between the end shoulder surfaces is small, excellent compression resistance performance can be exhibited as a whole.
[0027]
　Preferably, the gradient of the straight line connecting both axial ends of the pin seal surface is 5% or more and 25% or less. If the gradient is greater than 25%, it becomes difficult to secure a sufficient amount of seal interference. If the gradient is less than 5%, the risk of seizure during fastening increases. More preferably, the slope can be between 10% and 17%. The box seal surface can also have a similar slope to the pin seal surface, preferably the box seal surface slope is equal to the pin seal surface slope. The taper generatrix of the pin seal surface and the box seal surface may be straight, slightly curved in a convex shape, or partially including a convex curve and a straight line.
[0028]
　In this specification, the term "at the completion of fastening" means the time at which neither axial load nor internal/external pressure is applied to the threaded joint after the pin has been fastened to the box. On the other hand, the "fastened state" means a state in which the pin and the box are fastened regardless of whether an axial load and internal/external pressure are applied. Even after applying an axial load and internal and external pressure within the range where the threaded joint is not destroyed or the contact surface pressure of the pin and box seal surfaces is not lost, more preferably within the elastic range, the pin and box If it is concluded, it is in a "fixed state".
[0029]
In addition, since the difference (L B - L P ) between 　the axial distances L P and L B may be substantially uniquely determined, it is sufficient to measure the axial distance L P , LB themselves need not be strictly defined individually. For example, the axial distance LB may be the axial distance between the radially inner edge of the box intermediate shoulder surface and the radially inner edge of the box end shoulder surface, where the axial distance LP is the portion of the pin intermediate shoulder surface corresponding to the radially inner end of the box intermediate shoulder surface (that is, the portion that contacts the radially inner end of the box intermediate shoulder surface) and the diameter of the box end shoulder surface. It is the axial distance between the portion of the pin intermediate shoulder surface corresponding to the direction inner end (ie, the portion that contacts the radially inner end of the box end shoulder surface). Also, the axial distance L P may be the axial distance between the radially outer edge of the pin intermediate shoulder surface and the radially outer edge of the pin end shoulder surface, in which case the axial distance LB _
is the portion of the box intermediate shoulder surface corresponding to the radially outer edge of the pin intermediate shoulder surface (i.e., the portion that contacts the radially outer edge of the pin intermediate shoulder surface) and the radially outer edge of the pin end shoulder surface. It is the axial distance between the portion of the box intermediate shoulder surface corresponding to the end (that is, the portion that contacts the radially outer end of the pin end shoulder surface).
Effect of the invention
[0030]
　According to the present disclosure, when fastening is completed, the pin end shoulder surface and the box end shoulder surface are not in contact, or even if they are in contact, there is a gap between the pin intermediate shoulder surface and the box intermediate shoulder surface. The contact pressure between the pin end shoulder surface and the box end shoulder surface is less than the contact pressure. Therefore, when the fastening of the pin and the box is completed, a large compressive stress does not occur in the vicinity of the tip of the pin having the shoulder surface of the pin end, and the axial compressive load that can be borne by the shoulder surface of the pin end has a margin. can have. Furthermore, when a relatively large axial compressive load is applied to the threaded joint for pipes in the fastened state, the pin end shoulder surface contacts the box end shoulder surface and bears part of the axial compressive load. Therefore, it is possible to prevent excessive compressive stress from acting on the intermediate shoulder surface that functions as a torque shoulder, thereby improving compression resistance.
Brief description of the drawing
[0031]
1] Fig. 1 is a vertical cross-sectional view of a fastened state of an oil country tubular goods threaded joint according to an embodiment. [Fig.
[Fig. 2A] Fig. 2A is an enlarged view of the vicinity of the tip of the pin when no compression load is applied in a fastened state.
[Fig. 2B] Fig. 2B is an enlarged view of the vicinity of the tip of the pin when a compressive load of a certain magnitude is applied in the fastened state.
[Fig. 2C] Fig. 2C is an enlarged view of the vicinity of the tip of the pin when a large compressive load (however, a compressive load that does not yield the intermediate shoulder surface and each screw) is applied in the fastened state.
3] Fig. 3 is an enlarged view of the vicinity of a pin seal surface of a threaded joint for oil country tubular goods according to another embodiment. [Fig.
4 is a diagram showing paths of combined load conditions used in the analysis; FIG.
5 is a comparison graph of seal contact pressure under three load conditions (Load Points) of a simple internal pressure load state; [0031]FIG.
MODE FOR CARRYING OUT THE INVENTION
[0032]
　As illustrated in FIG. 1 , a threaded joint 1 for pipes according to the present embodiment comprises a tubular pin 2 and a tubular box 3 . The pin 2 and the box 3 are fastened together by screwing the pin 2 into the box 3 . The pin 2 is provided at the tube end of the first tube P1 and the box 3 is provided at the tube end of the second tube P2. The first pipe P1 may be a long pipe such as an oil well pipe. The second pipe may be a coupling for connecting long pipes, but is preferably a long pipe such as an oil well pipe. That is, the threaded joint 1 for pipes according to the present embodiment is preferably an integral threaded joint for pipes. Oil country tubular goods and couplings are typically made of steel, but may be made of metal such as stainless steel or nickel-based alloy.
[0033]
　The pin 2 may be formed at one end of the first oil country tubular good P1 whose diameter has been reduced. The box 3 may be formed at the diameter-expanded one end of the second oil country tubular good P2. Preferably, the pin 2 can be formed at one end of each of the oil country tubular goods P1 and P2, and the box 3 can be formed at the other end. More specifically, the first oil country tubular good P1 is formed by reducing the diameter of one end of a raw pipe made of a long pipe, and then
cutting the outer circumference of the diameter-reduced one end so as to form a component of the pin 2 . Manufactured by cutting. The second oil country tubular good P2 is formed by enlarging the diameter of one end of the base pipe made of a long pipe, and then cutting the inner circumference of the one end that has been diameter-enlarged so as to form a constituent element of the box 3. Manufactured by As a result, the thickness of the pin 2 and the box 3 can be ensured in the semi-flush type integral threaded joint.
[0034]
　In this specification, the portion other than the pin 2 and the box 3 of the oil country tubular goods P1 and P2, which is neither diameter-reduced nor diameter-enlarged, is referred to as a "tube body". The tube end side of the pin 2 means the direction from the tube main body of the pin 2 toward the tube end of the pin 2, and is sometimes referred to as the "tip side". The tube main body side of the pin 2 means the direction from the tube end of the pin 2 toward the tube main body of the pin 2, and is sometimes referred to as the "base end side". The open end side of the box 3 means the direction from the pipe body of the box 3 toward the open end of the box 3 .
[0035]
　The pin 2 includes a first male screw 21, a second male screw 22 provided closer to the tube end side of the pin 2 than the first male screw 21 and having a diameter smaller than that of the first male screw 21, and the first male screw 21 and the second male screw 22. a pin intermediate shoulder surface 23 provided between; a pin end shoulder surface 24 provided at the tube end of the pin 2; and a pin seal provided between the second external thread 22 and the pin end shoulder surface 24. a surface 25; The first male thread 21 and the second male thread 22 may be axially spaced apart with a pin intermediate shoulder surface 23 therebetween.
[0036]
　Preferably, the first and second external threads 21, 22 each comprise a tapered thread. Preferably, the first and second external threads 21, 22 have the same thread taper angle and the same thread pitch. Preferably, the tapered generatrix of the tapered thread forming the second male thread 22 is located radially inwardly of the taper generatrix of the tapered thread forming the first male thread 21 . The pin intermediate shoulder surface 23 can be constituted by a side surface of a step formed on the outer circumference of the pin between the first male thread 21 and the second male thread 22 . The pin intermediate shoulder surface 23 is directed toward the tube end of the pin 2 . The first and second external threads 21, 22 may each be trapezoidal threads, API round threads, API buttress threads, wedge threads, or the like.
[0037]
　The box 3 includes a first female thread 31 into which the first male thread 21 is fitted at the completion of fastening, a second female thread 32 into which the second male thread 22 is fitted at the completion of fastening, and a pin intermediate shoulder surface 23 at the completion of fastening. The contacting box intermediate shoulder surface 33, the box end shoulder surface 34 provided corresponding to the pin end shoulder surface 24, and the second female thread 32 and the box end shoulder surface 34 are provided to complete the fastening. and a box seal surface 35 which is in contact with the pin seal surface 25 over the entire circumference. These pin seal surface 25 and box seal surface 35 can function as seals for internal pressure to exhibit sealing performance against internal pressure. Preferably, the box 3 may further include an external pressure box sealing surface 36 provided closer to the opening end side of the box 3 than the first female thread 31, and the pin 2 will close to the external pressure box sealing surface 36 at the time of completion of fastening. An external pressure pin seal surface 26 may be further provided that contacts the entire circumference of the . This external pressure pin seal surface 26 is provided closer to the proximal end of the pin than the first male thread 21 .
[0038]
　The first female thread 31 and the second female thread 32 are axially spaced apart and a box intermediate shoulder surface 33 may be provided therebetween. Preferably, the first and second internal threads 31,32 comprise tapered threads that match the first and second external threads 21,22, respectively. The box intermediate shoulder surface 33 can be configured by a stepped side surface formed on the inner circumference of the box 3 between the first female thread 31 and the second female thread 32 . The box intermediate shoulder surface 33 faces the open end of the box 3 and faces the pin intermediate shoulder surface 23 . The box intermediate shoulder surface 33 contacts the pin intermediate shoulder surface 23 at least at the completion of fastening, and these intermediate shoulder surfaces 23, 33 function as torque shoulders for exhibiting torque performance. The first
and second internal threads 31, 32 may be trapezoidal threads, API round threads, API buttress threads, wedge threads, or the like that match the first and second external threads 21, 22, respectively.
[0039]
　Preferably, the vertical cross-sectional shape of the crest and root of each thread 21, 22, 31, 32 is linear extending parallel to the tube axis.
[0040]
　Preferably, when the fastening of the pin 2 and the box 3 is completed, the load surfaces 21L and 31L of the first male screw 21 and the first female screw 31 are in contact with each other, and the load surfaces 22L and 32L of the second male screw 22 and the second female screw 32 are in contact with each other. contact, a gap is formed between the insertion surfaces 21S, 31S of the first male screw 21 and the first female screw 31, and a gap is formed between the insertion surfaces 22S, 32S of the second male screw 22 and the second female screw 32. .
[0041]
　Preferably, the size of the gap formed between the insertion surfaces 21S, 31S of the first male screw 21 and the first female screw 31 is uniform over the entire axial length of the fitting range of these screws 21, 31. A larger gap may be formed in a small area of ​​. Preferably, the size of the gap formed between the insertion surfaces 22S, 32S of the second male screw 22 and the second female screw 32 is uniform over the entire axial length of the fitting range of these screws 22, 32. A larger gap may be formed in a small area of ​​. Preferably, the size of the gap formed between the insertion surfaces 21S and 31S is equal to the size of the gap formed between the insertion surfaces 22S and 32S.
[0042]
　Preferably, the gap formed between the insertion surfaces 21S, 31S of the first male screw 21 and the first female screw 31 at the time of completion of fastening is loaded with a predetermined axial compressive load smaller than the yield compressive load of the pin 2 and the box 3. When the pin 2 and the box 3 are deformed, the insertion surfaces 21S and 31S start contacting each other so as to bear part of the axial compressive load. The contact state between the insertion surfaces 21S and 31S at the start of contact may vary. The contact area between the insertion surfaces 21S and 31S may gradually expand as the size increases, or the entire insertion surfaces 21S and 31S may start contacting at the same time. The size of the gap formed between the insertion surfaces 21S and 31S at the completion of fastening may be, for example, 0.15 mm or less in the direction along the pipe axis direction. From the viewpoint of preventing seizure during fastening, the size of the gap is preferably 0.06 mm or more.
[0043]
　Preferably, the gap formed between the insertion surfaces 22S, 32S of the second male thread 22 and the second female thread 32 at the completion of fastening is loaded with a predetermined axial compressive load smaller than the yield compressive load of the pin 2 and the box 3. When the pin 2 and the box 3 are deformed, the insertion surfaces 22S and 32S start to contact each other so as to bear part of the axial compressive load. The contact state between the insertion surfaces 22S and 32S at the start of contact may vary. The contact area between the insertion surfaces 22S and 32S may be gradually expanded as the distance increases, or the entire insertion surfaces 22S and 32S may start contacting at the same time. Also, the axial compression load at which the insertion surfaces 22S and 32S start contacting each other may be different from the axial compression load at which the insertion surfaces 21S and 31S start contacting each other. The size of the gap formed between the insertion surfaces 22S and 32S at the completion of fastening may be, for example, 0.15 mm or less in the direction along the tube axis direction. From the viewpoint of preventing seizure during fastening, the size of the gap is preferably 0.06 mm or more.
[0044]
　The box end shoulder surface 34 is formed of a tapered surface in which the radially inner end inclines toward the opening end side of the box 3 with respect to the radially outer end. The pin end shoulder surface 24
may contact the box end shoulder surface 34 at the completion of fastening, or may be gapped with the box end shoulder surface 34 at the completion of fastening as shown in FIG. 2A. may At least when a predetermined axial compressive load smaller than the yield compressive load of the threaded joint is applied, elastic deformation of the pin 2 and the box 3 causes the end shoulder surfaces 24, 34 of the pin 2 and the box 3 to come into contact with each other. bearing a portion of the axial compressive load.
[0045]
　The radial width of the contact area between the pin end shoulder surface 24 and the box end shoulder surface 34 (see FIG. 2C) may be less than 1 mm. By narrowing the contact width between the end shoulder surfaces 24 and 34 in this manner, it becomes easier to ensure the thickness of other portions. The contact area between the end shoulder surfaces 24 and 34 of the pin 2 and the box 3 is the contact area in which the pin 2 and the box 3 are deformed in the radial direction due to the amount of seal interference. That is, it is smaller than the overlapping range of the pin 2 and the end shoulder surface 34 of the box 3 before deformation when viewed in the tube axial direction.
[0046]
　More preferably, as shown in FIG. 2B, the insertion surfaces 22S, 32S of the second male screw 22 and the second female screw 32 first start contact with each other, and when the applied compressive load further increases, the end portions are separated as shown in FIG. 2C. The size of the gap between the insertion surfaces 22S, 32S and the size of the gap between the end shoulder surfaces 24, 34 at the completion of fastening can be determined such that contact between the shoulder surfaces 24, 34 begins. According to this, even if the contact width between the end shoulder surfaces 24, 34 is small, the compressive load borne by the end shoulder surfaces 24, 34 when an axial compressive load is applied can be reduced. Plastic strain in the vicinity of the shoulder surfaces 24, 34 can be reduced. In addition, part of the compressive load acting on the insertion surface 22S of the second male thread 22 of the pin 2 is expected to act to expand the diameter of the pin tip side like a trumpet. The effect of maintaining the contact pressure between them can also be expected.
[0047]
　Alternatively, the end shoulder surfaces 24, 34 may initiate contact first, and the insertion surfaces 22S, 32S may initiate contact when the applied compressive load is further increased. According to this, by bringing the end shoulder surfaces 24 and 34 into contact with each other more reliably, it is possible to suppress diameter-reducing deformation in the vicinity of the tip of the pin.
[0048]
The end shoulder angle θ sh of the 　box end shoulder surface 34 is preferably greater than 5°, more preferably greater than 10°. Also, the end shoulder angle θ sh is preferably 45° or less, more preferably 25° or less. The end shoulder angle of the pin end shoulder surface 24 is preferably equal to the end shoulder angle θ sh of the box end shoulder surface 34 .
[0049]
　The intermediate shoulder surfaces 23 and 33 of the pin 2 and the box 3 are flat surfaces orthogonal to the tube axis, but the radially outer end is inclined toward the tube end of the pin 2 rather than the radially inner end. It may be configured by a tapered surface.
[0050]
　Moreover, the longitudinal cross-sectional shape of each of the seal surfaces 25, 35, 26, and 36 may be arbitrary, and in the threaded joint 1 shown in FIGS. It is composed of Alternatively, one of the sealing surfaces that contact each other may be configured with a convex curved surface, or both sealing surfaces may be configured with a convex curved surface. In any case, each sealing surface is configured such that the farther the pin 2 is pushed into the box 3, the greater the seal interference amount. The gradient of the straight line connecting both ends in the axial direction of each seal face is preferably 5% (10% as taper ratio) or more, more preferably 10% (20% as taper ratio). In addition, the gradient of the straight line connecting both ends in the axial direction of each seal face is preferably 25% (50% as a taper ratio) or less, more preferably 17% (34% as a taper ratio) or less. .
[0051]
　In the embodiment shown in FIG. 3, the box seal surface 35 is composed of a tapered surface that is linearly inclined in the vertical section, while the pin seal surface 25 is composed of a tapered surface 25b that is linearly inclined in the vertical section, and a tapered surface 25b that is linearly inclined in the vertical section. is composed of a convex curved surface 25a that is convex toward the box seal surface 35. As shown in FIG. The tapered surface 25b is formed so as to smoothly continue to the distal end of the convex curved surface 25a. In this embodiment, the pin seal surface 25 is formed so as to function as a seal point where the convex surface 25a is strongly pressed against the box seal surface 35 during fastening. In this embodiment, it is possible to ensure that the seal point of the pin seal surface 25 is located away from the pin end shoulder 24, and the stress generated in the pin end shoulder surface 24 when a large axial compressive load is applied causes the pin seal surface 25 to seal. The influence on the vicinity of the point can be reduced.
[0052]
　FIG. 3 is also shown as an example of a slow tapered pin seal surface 25 and box seal surface 35 . In the illustrated example, the gradient of the box seal surface 35 with respect to the pipe axis is 10% (20% in terms of taper ratio), and the gradient of the straight line connecting both ends of the convex curved surface 25a of the pin seal surface 25 in the axial direction is the gradient of the box seal surface 35. The gradient of the tapered surface 25b of the pin seal surface 25 is 17.5% (35% as a taper ratio). 6+17.5)/(100+60)≈15%. By slow-tapering the pin seal surface 25 and the box seal surface 35 in this way, it is possible to increase the radial widths of the pin end shoulder surface 24 and the box end shoulder surface 34, thereby further improving compression resistance. can be improved. In addition, since more compression load can be borne by these end shoulders 24, 34, the middle shoulders 23, 33 and the screws 21, 31, 22, 32 can be designed with more room to withstand compression. Other improvements to improve performance can also be made.
[0053]
　In the threaded joint 1 for pipes of this embodiment, when the pin 2 is fastened to the box 3 , the intermediate shoulder surface 23 of the pin 2 comes into contact with the intermediate shoulder surface 33 of the box 3 . The fastening torque at this time is also called shouldering torque. As the pin 2 is further tightened against the box 3, the sliding contact between the intermediate shoulder surfaces 23 and 33 causes a rapid increase in the tightening torque. The intermediate shoulder surfaces 23, 33 thus function as torque shoulders. If the tightening torque exceeds the yield torque, the vicinity of the intermediate shoulder surfaces 23, 33, the male threads 21, 22 and the female threads 31, 32 are destroyed, and the tightening torque does not increase even if the amount of tightening rotation is increased. Therefore, the tightening should be completed before the tightening torque reaches the yield torque.
[0054]
　In the threaded joint 1, when the fastening is completed, as shown in FIG. A minute gap is formed, and a minute gap is also formed between the end shoulder surfaces 24 and 34 .
[0055]
　When the axial compressive load applied to the threaded joint 1 in the fastened state gradually increases, the compressive strain caused by the compressive load causes the portion of the pin 2 closer to the pipe main body than the intermediate shoulder surface 23 of the pin and the intermediate portion of the box. A portion of the box 3 closer to the pipe main body than the shoulder surface 33 is slightly compressed in the axial direction. When the compressive load increases to a certain magnitude, as shown in FIG. 2B, the insertion surfaces 22S and 32S of the second male thread 22 and the second female thread 32 come into contact with each other before the end shoulder surfaces 24 and 34 start coming into contact with each other. From that point on, these insertion surfaces 22S, 32S also bear part of the compressive load. It is not necessary for the entire spiral insertion surfaces 22S, 32S to come into contact with each other, and it is only necessary that portions of the insertion surfaces 22S, 32S in the axial direction and the circumferential direction start to contact each other.
[0056]
　When the compressive load further increases, the end shoulder surfaces 24 and 34 begin to come into contact with each other before the yield compressive load is reached. The amount of relative displacement between the pin seal surface 25 and the box seal surface 35 can be regulated. If the amount of axial displacement of the pin seal surface 25 with respect to the box seal surface 35 is
large, the tapered shape of these seal surfaces 35 generates a large pressure in the vicinity of the pin seal surface 25 and the box seal surface 35, which accumulates damage and then compressive load. Even if is removed and elastically restored, the initial seal contact pressure cannot be obtained, and in particular, the internal pressure seal performance is lowered. According to the threaded joint 1 of this embodiment, since the amount of relative displacement between the pin seal surface 25 and the box seal surface 35 due to the compressive load is suppressed, the damage accumulated in the vicinity of the pin seal surface 25 and the box seal surface 35 is reduced. It is possible to maintain the internal pressure sealing performance after the compressive load disappears.
[0057]
　The present disclosure may be applied not only to integral type threaded joints but also to coupling type threaded joints. In addition, the present disclosure is not limited to the above embodiments, and various modifications are possible within the scope of the technical ideas of the present disclosure.
Example
[0058]
　In order to confirm the effect of the threaded joint 1 for oil country tubular goods according to the present embodiment, an example in which the end shoulders 24 and 34 were in contact with each other when an axial compressive load was applied and a comparative example in which the end shoulders were not in contact with each other were conducted. A numerical analysis simulation was performed using the elasto-plastic finite element method to evaluate the internal pressure sealing performance.
[0059]
　The internal pressure seal performance was evaluated by sequentially applying the combined load conditions (1) to (52) that trace the combined load ellipse that simulates the Series A test based on the 2017 version of API5C5 CAL IV shown in FIG. In the figure, "Compression" is compressive load, "Tension" is tensile load, "IP" is internal pressure, "EP" is external pressure, and "VME 100% for pipe" is oil country tubular goods. Yield curve of the pipe body, "CYS" (Connection Yield Strength) is the strength of the threaded joint, "CYS 100%" is the yield curve of the threaded joint, "CYS 95%" is the 95% yield curve for CYS 100%, "High collapse for connection" is a collapse curve due to external pressure on a screw joint
. "CYS 100%" is a curve obtained by multiplying the axial force (compression or tension) of "VME 100% for pipe" by the
joint efficiency JE.
[0060]
　FIG. 5 compares the seal contact forces of the pin seal surface 25 and the box seal surface 35 under three load conditions (7), (27), and (45) when a simple internal pressure is applied. It should be noted that the example is "with a false shoulder" and the comparative example is "without a false shoulder".
[0061]
　LP7 indicates the seal contact force under load condition (7) in which simple internal pressure is first applied in paths (1) to (52) of the repeated combined load, and LP27 indicates load condition (27 ), and LP45 indicates the seal contact force under load condition (45) when the third simple internal pressure is applied.
[0062]
　As is clear from the figure, the decrease in the seal contact force is remarkable in the case of "without false shoulder" as compared with "with false shoulder". From this, it was confirmed that according to the present disclosure, it is possible to suppress deterioration in internal pressure sealing performance after repeated application of a combined load.
Code explanation
[0063]
1: threaded joint for pipes
2: pin, 21: first male thread, 22: second male thread
23: intermediate shoulder surface, 24: end shoulder surface, 25: pin seal surface
3: box, 31: first female thread, 32: second 2 internal thread
33: intermediate shoulder surface, 34: end shoulder surface, 35: box seal surface

The scope of the claims

[Claim 1]
　A threaded joint for pipes comprising a tubular pin and a tubular box, wherein the pin is screwed into the box to fasten the pin and the box,
　wherein the pin comprises a first male thread and a first male thread. a second male screw provided on the tip end side and having a smaller diameter than the first male screw; a pin intermediate shoulder surface provided between the first male screw and the second male screw; A pin end shoulder surface and a pin seal surface provided between the second male thread and the pin end shoulder surface, and the
　box includes a first female thread into which the first male thread is fitted in a fastened state. a second female thread into which the second male thread is fitted in a fastened state; a box intermediate shoulder surface in contact with the pin intermediate shoulder surface in the fastened state; and a box end provided corresponding to the pin end shoulder surface. a shoulder surface and a box seal surface provided between the second female thread and the box end shoulder surface and in contact with the pin seal surface over the entire circumference in a fastened state
　; an axial distance LB between a shoulder surface and the box end shoulder surface is greater than an axial distance LP between the pin intermediate shoulder surface and the pin end shoulder surface of the pin before fastening; ,
　the difference in the axial distance (L B −L P ) is defined,
　Threaded fittings for pipes.
[Claim 2]
　2. The threaded joint for pipes according to claim 1, wherein the axial distance LB is such that a gap is formed between the pin end shoulder surface and the box end shoulder surface when no axial compressive load is applied in a fastened
　state. , LP are defined 　.
[Claim 3]
　3. The threaded joint for pipes according to claim 2,
　wherein the second male thread and the second male thread are arranged such that a gap is formed between the insertion surfaces of the second male thread and the second female thread when no compressive load is applied in the axial direction in a fastened state. In the process in which the internal thread is formed and the
　applied axial compressive load gradually increases, the insertion surfaces first start to come into contact with each other, and then the pin end shoulder surface and the box end shoulder surface come into contact with each other.
　a threaded joint for pipes , wherein the gap between the insertion surfaces is sized such that
[Claim 4]
　4. The threaded joint for pipes according to claim 1, 2 or 3,
　wherein the gradient of a straight line connecting both axial ends of said pin seal surface is more than 5% and less than 25%.