Specification
Title of invention: Steel pipe threaded joint
Technical field
[0001]
　The present disclosure relates to a threaded joint used for connecting steel pipes.
Background technology
[0002]
　In oil wells, natural gas wells, etc. (hereinafter collectively referred to as “oil wells”), oil well pipes such as casings and tubing are used to mine underground resources. The oil country tubular goods are formed by sequentially connecting steel pipes, and a threaded joint is used for the connection.
[0003]
　The type of threaded joint for steel pipes of this kind is roughly classified into a coupling type and an integral type. In the case of the coupling type, one of the pair of pipe members to be connected is a steel pipe and the other pipe member is a coupling. In this case, male thread portions are provided on the outer circumferences of both ends of the steel pipe, and female thread portions are provided on the inner circumferences of both end portions of the coupling. Then, the male thread portion of the steel pipe is screwed into the female thread portion of the coupling, whereby the both are fastened and connected. In the case of the integral type, the pair of pipe materials to be connected are both steel pipes, and separate couplings are not used. In this case, a male screw portion is provided on the outer circumference of one end of the steel pipe, and a female screw portion is provided on the inner circumference of the other end. Then, the male screw part of one steel pipe is screwed into the female screw part of the other steel pipe, whereby both are fastened and connected.
[0004]
　Generally, a joint portion of a pipe end portion having a male thread portion is called a pin because it includes an element to be inserted into a female thread portion. On the other hand, the joint portion of the pipe end formed with the internal thread is called a box because it includes an element for receiving the external thread. These pins and boxes are tubular because they are the ends of the tubing.
[0005]
　In recent years, the development of deep wells, which become a harsh environment of high temperature and high pressure, is increasing, and the screw joints for oil country tubular goods used in these wells are required to have further improved sealing performance. Especially in deep wells, the tensile/compressive load and the external pressure are high, so it is necessary to improve the sealing performance against the external pressure under the tensile/compressive load.
[0006]
　Oil well pipe threaded joints exhibit high sealing performance at the seal. Generally, the diameter of the pin seal is larger than the diameter of the box seal. Therefore, in the fastened state, both seal parts are fitted and closely contacted with each other to be in an interference fit state, thereby forming a seal part by metal contact. The difference between the diameter of the pin seal and the diameter of the box seal is called the "seal interference". The larger the seal interference amount, the higher the seal contact force, and the better sealing performance can be obtained.
[0007]
　In order to improve the sealing performance against external pressure, it is effective to increase the wall thickness of the seal portion of the pin on which external pressure acts. As a result, the diameter reduction resistance of the seal portion of the pin when external pressure is applied to the threaded joint is increased, so that a substantial decrease in the amount of seal interference and a decrease in the seal contact force are reduced. In addition, in order to exhibit stable sealing performance even under a high tensile/compressive load, it is necessary to have a corresponding thread length and shoulder surface area.
[0008]
　International Publication No. 2015/194160 (Patent Document 1) discloses a threaded joint for steel pipes that improves sealing performance against internal pressure and external pressure (see paragraph 0020). This threaded joint includes a shoulder surface, a first sealing surface, a first male/female screw portion, an auxiliary shoulder surface, an annular portion, a second sealing surface, and a second portion in this order from the tip of the pin (pipe end of the oil country tubular goods body). A male/female screw portion is provided (see paragraph 0063 and FIG. 6 ).
[0009]
　Japanese Patent Laying-Open No. 10-89554 (Patent Document 2) discloses a slim type threaded joint for oil country tubular goods which has high strength only in the torque shoulder portion and is excellent in compression resistance and corrosion resistance (see paragraph 0016). This screw joint manages the screw part for screwing and fastening the pin part and the box part, the seal part for ensuring sealing performance against internal pressure or external pressure of the oil country tubular good, or both, and tightening torque. Is an integral type oil country tubular goods threaded joint having a torque shoulder portion, and the strength of the torque shoulder portion is higher than other portions of the joint and the oil country tubular body (see paragraph 0018 and FIG. 1 ).
[0010]
　The present specification incorporates the following prior art documents by reference.
Patent Document 1: International Publication No. 2015/194160
Patent Document 2: Japanese Patent Application Laid-Open No. 10-895554
Patent Document 3: International Publication No. 2017/104282
Summary of disclosure
[0011]
　An object of the present disclosure is to provide a threaded joint for steel pipes that improves the external pressure sealing performance while maintaining the internal pressure sealing performance.
[0012]
　In the threaded joint described above, it is not possible to obtain a predetermined sealing performance simply by arranging the seal surface, the shoulder surface, the screw and the like. The present inventors have newly found that in order to obtain a predetermined sealing performance, it is necessary to design well-balanced thickness of each part in an oil well pipe having a finite wall thickness. Invented a threaded joint for steel pipes.
[0013]
　The threaded joint for steel pipes according to the present disclosure includes a tubular pin and a tubular box into which the pin is screwed and fastened to the pin. The pin includes a nose, a tapered inner male thread, a tapered outer male thread, a pin inner seal surface, a pin middle shoulder surface, a pin middle seal surface, and a pin annular portion. The nose is formed at the tip of the pin. The inner male screw is located near the tip of the pin and is formed on the outer peripheral surface of the pin. The outer male screw is located near the main body of the steel pipe and is formed on the outer peripheral surface of the pin. The in-pin sealing surface is formed between the nose and the internal male screw. The pin intermediate shoulder surface is formed between the inner male screw and the outer male screw. The pin intermediate sealing surface is formed between the outer male screw and the pin intermediate shoulder surface. The pin annular portion is formed between the pin intermediate shoulder surface and the pin intermediate sealing surface. The box includes a recess, a tapered inner female thread, a tapered outer female thread, a box inner sealing surface, a box intermediate shoulder surface, a box intermediate sealing surface, and a box annular portion. The recess corresponds to the nose. The inner female screw corresponds to the inner male screw and is formed on the inner peripheral surface of the box. The outer female screw corresponds to the outer male screw and is formed on the inner peripheral surface of the box. The in-box sealing surface faces the in-pin sealing surface and contacts the in-pin sealing surface when the pin and the box are fastened. The box intermediate shoulder surface faces the pin intermediate shoulder surface and contacts the pin intermediate shoulder surface when the pin and the box are fastened. The box intermediate sealing surface faces the pin intermediate sealing surface and contacts the pin intermediate sealing surface when the pin and the box are fastened. The box annular portion faces the pin annular portion and is separated from the pin annular portion when the pin and the box are fastened.
[0014]
　The threaded joint for steel pipes satisfies the following equations (1) and (2).
　α1>α2 (1) In
　formula (1), α1 is the taper gradient of the inner male screw. α2 is the taper of the external male screw.
[0015]
[

　Equation 1] In the formula (2), L1 is the distance between the tip of the pin and the pin intermediate shoulder surface in the tube axis direction when the pin and the box are not fastened, and L2 is It is the distance between the box intermediate shoulder surface and the tip of the box in the tube axis direction when the pin and the box are not fastened.
Brief description of the drawings
[0016]
FIG. 1 is a vertical cross-sectional view showing a threaded joint for steel pipes according to a first embodiment.
FIG. 2 is a longitudinal sectional view showing a threaded joint for steel pipes according to a second embodiment.
FIG. 3 is a graph showing sealing performance of a threaded joint according to an experimental example.
FIG. 4 is a graph showing the external pressure sealing performance of the threaded joint shown in FIG.
MODE FOR CARRYING OUT THE INVENTION
[0017]
　The steel pipe threaded joint according to the present embodiment includes a tubular pin and a tubular box into which the pin is screwed and fastened to the pin. The pin includes a nose, a tapered inner male thread, a tapered outer male thread, a pin inner seal surface, a pin middle shoulder surface, a pin middle seal surface, and a pin annular portion. The nose is formed at the tip of the pin. The inner male screw is located near the tip of the pin and is formed on the outer peripheral surface of the pin. The outer male screw is located near the main body of the steel pipe and is formed on the outer peripheral surface of the pin. The in-pin sealing surface is formed between the nose and the internal male screw. The pin intermediate shoulder surface is formed between the inner male screw and the outer male screw. The pin intermediate sealing surface is formed between the outer male screw and the pin intermediate shoulder surface. The pin annular portion is formed between the pin intermediate shoulder surface and the pin intermediate sealing surface. The box includes a recess, a tapered inner female thread, a tapered outer female thread, a box inner sealing surface, a box intermediate shoulder surface, a box intermediate sealing surface, and a box annular portion. The recess corresponds to the nose. The inner female screw corresponds to the inner male screw and is formed on the inner peripheral surface of the box. The outer female screw corresponds to the outer male screw and is formed on the inner peripheral surface of the box. The in-box sealing surface faces the in-pin sealing surface and contacts the in-pin sealing surface when the pin and the box are fastened. The box intermediate shoulder surface faces the pin intermediate shoulder surface and contacts the pin intermediate shoulder surface when the pin and the box are fastened. The box intermediate sealing surface faces the pin intermediate sealing surface and contacts the pin intermediate sealing surface when the pin and the box are fastened. The box annular portion faces the pin annular portion and is separated from the pin annular portion when the pin and the box are fastened.
[0018]
　The threaded joint for steel pipes satisfies the following formula (1).
　α1>α2 (1)
[0019]
　In the formula (1), α1 is the taper gradient of the inner male screw. α2 is the taper gradient of the external male screw.
[0020]
　Preferably, the threaded joint for steel pipes satisfies the following expression (2).
[0021]
[Number 2]

[0022]
　In Formula (2), L1 is the distance between the tip of the pin and the pin intermediate shoulder surface in the tube axis direction when the pin and the box are not fastened. L2 is the distance between the box intermediate shoulder surface and the tip of the box in the tube axis direction when the pin and the box are not fastened.
[0023]
　Preferably, the pin further includes an in-pin shoulder surface formed on the tip of the pin. The box further includes an in-box shoulder surface opposite the in-pin shoulder surface.
[0024]
　Further, preferably, the in-pin shoulder surface contacts the in-box shoulder surface when the pin and the box are fastened.
[0025]
　Further, the threaded joint for steel pipes satisfies the following expression (3).
[0026]
[Number 3]

[0027]
　In formula (3), A0 is the cross-sectional area of ​​the main body of the steel pipe in a plane perpendicular to the pipe axis, and A1 is the projected area of ​​the box intermediate shoulder surface with respect to the plane perpendicular to the pipe axis.
[0028]
　Alternatively, the steel pipe threaded joint satisfies the following expression (4).
[0029]
[Number 4]

[0030]
　In formula (4), A0 is a cross-sectional area of ​​the main body of the steel pipe in a plane perpendicular to the pipe axis, and A2 is a projected area of ​​the shoulder surface in the pin with respect to the plane perpendicular to the pipe axis and the pin intermediate shoulder surface. Is the total projected area of.
[0031]
　Hereinafter, the present embodiment will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are designated by the same reference numerals, and the description thereof will not be repeated.
[0032]
　[Structure of Steel Pipe Threaded Joint]
　Referring to FIG. 1, the steel pipe threaded joint according to the present embodiment is a coupling type threaded joint, and includes a tubular pin 10 and a pin into which the pin 10 is screwed. 10 and a tubular box 20 fastened.
[0033]
　The pin 10 includes a nose 12, a tapered inner male thread 14, a tapered outer male thread 17, a pin inner seal surface 13, a pin intermediate shoulder surface 18, a pin intermediate seal surface 16, and a pin annular portion 15a. including.
[0034]
　The nose 12 is formed at the tip of the pin 10. The inner male screw 14 is located near the tip of the pin and is formed on the outer peripheral surface of the pin 10. The outer male screw 17 is located near the main body PM of the steel pipe and is formed on the outer peripheral surface of the pin 10. The main body PM of the steel pipe is a portion other than the pin 10 in the entire steel pipe. The pin inner seal surface 13 is formed between the nose 12 and the inner male screw 14. The pin intermediate shoulder surface 18 is formed between the inner male screw 14 and the outer male screw 17. The pin intermediate sealing surface 16 is formed between the external male screw 17 and the pin intermediate shoulder surface 18. The pin annular portion 15 a is formed between the pin intermediate shoulder surface 18 and the pin intermediate sealing surface 16.
[0035]
　The box 20 includes a concave portion 22, a tapered inner female thread 24, a tapered outer female thread 27, a box inner sealing surface 23, a box intermediate shoulder surface 28, a box intermediate sealing surface 26, and a box annular portion 25a. including.
[0036]
　The recess 22 corresponds to the nose 12. The inner female screw 24 corresponds to the inner male screw 14 and is formed on the inner peripheral surface of the box 20. The outer female screw 27 corresponds to the outer male screw 17, and is formed on the inner peripheral surface of the box 20. The in-box sealing surface 23 faces the in-pin sealing surface 13 and contacts the in-pin sealing surface 13 when the pin 10 and the box 20 are fastened. The box intermediate shoulder surface 28 faces the pin intermediate shoulder surface 18 and contacts the pin intermediate shoulder surface 18 when the pin 10 and the box 20 are fastened. The box intermediate sealing surface 26 faces the pin intermediate sealing surface 16 and contacts the pin intermediate sealing surface 16 when the pin 10 and the box 20 are fastened. The box annular portion 25a faces the pin annular portion 15a and is separated from the pin annular portion 15a when the pin 10 and the box 20 are fastened.
[0037]
　The threaded joint for steel pipes satisfies the following formula (1).
　α1>α2 (1) In
　Expression (1), α1 is the taper gradient of the inner male screw 14. α2 is the taper gradient of the external male screw 17.
[0038]
　When the screw bottom is parallel to the screw taper, the taper gradient α1 is defined by a straight line connecting the bottom surface of the inner male screw 14 and the pipe axis CL (or a straight line parallel thereto) in the longitudinal section of the pin 10 including the pipe axis CL. Is the gradient between. The taper gradient α2 is a gradient between a straight line connecting the bottom surface of the outer male screw 17 and the pipe axis CL (or a straight line parallel to the straight line) in the longitudinal section of the pin 10 including the pipe axis CL.
[0039]
　When the screw bottom is parallel to the pipe axis CL, the taper gradient α1 is defined by the straight line connecting the intersection of the extension line of the bottom surface of the inner male screw 14 and the extension line of the load surface in the longitudinal section of the pin 10 including the pipe axis CL. It is the gradient to and from the axis CL (or a straight line parallel to it). The taper gradient α2 is defined by the straight line connecting the intersection of the extension line of the bottom surface of the external male screw 17 and the extension line of the load surface and the pipe axis CL (or a straight line parallel to this) in the longitudinal section of the pin 10 including the pipe axis CL. Is a gradient between.
[0040]
　Hereinafter, the inner male screw 14 and the inner female screw 24 may be collectively referred to as “inner screw 14, 24”. The outer male screw 17 and the outer female screw 27 may be collectively referred to as "outer screw 17, 27".
[0041]
　Each of the in-pin sealing surface 13 and the pin intermediate sealing surface 16 has a shape formed by a surface corresponding to a peripheral surface of a truncated cone whose diameter is reduced toward the tip side, or a peripheral surface of the truncated cone and a curve such as an arc. It has a shape in which a surface corresponding to the peripheral surface of the rotating body obtained by rotating around the axis CL is combined.
[0042]
　The nose 12 has a tubular shape and extends in the direction of the tube axis CL in a continuous manner with the in-pin sealing surface 13. However, the outer peripheral surface of the nose 12 may have the same inclination as the taper of the in-pin sealing surface 13, or may be a small (loose) or large (abrupt) tapered surface. When the outer peripheral surface of the nose 12 is a tapered surface, strictly speaking, the outer peripheral surface has a shape corresponding to the peripheral surface of a truncated cone whose diameter is reduced toward the tip side, or the peripheral surface of the truncated cone, It has a shape in which a curved line such as an arc is combined with a surface corresponding to the peripheral surface of a rotating body obtained by rotating around the tube axis CL.
[0043]
　The pin 10 further includes an in-pin shoulder surface 11 formed at the tip of the pin 10. The box 20 further includes an in-box shoulder surface 21 opposite the in-pin shoulder surface 11. The in-pin shoulder surface 11 contacts the in-box shoulder surface 21 when the pin 10 and the box 20 are fastened.
[0044]
　The in-pin shoulder surface 11 is an annular surface that is substantially perpendicular to the tube axis CL. Strictly speaking, the outer peripheral side of the inner shoulder surface 11 slightly tilts toward the tip side of the pin 10.
[0045]
　The intermediate shoulder surface 18 is arranged between the inner male screw 14 and the pin annular portion 15a. The intermediate shoulder surface 18 is provided so as to be continuous with the pin annular portion 15a. In the present embodiment, the intermediate shoulder surface 18 is an annular surface perpendicular to the tube axis CL. However, like the inner shoulder surface 11, the outer peripheral side of the intermediate shoulder surface 18 may be slightly tilted toward the tip side of the pin 10.
[0046]
　The pin annular portion 15a continues to the front of the pin intermediate seal surface 16 and extends in the pipe axis CL direction. The inner male screw 14 is provided in series with the pin annular portion 15a. The pin annular portion 15b continues to the rear of the pin intermediate seal surface 16 and extends in the tube axis CL direction. An external male screw 17 is provided so as to be continuous with the pin annular portion 15b. The outer peripheral surface of the pin annular portion 15a may have any shape as long as its rigidity can be ensured. Good. The same applies to the outer peripheral surface of the pin annular portion 15b.
[0047]
　The box inner sealing surface 23 projects toward the pin inner sealing surface 13. However, the in-box sealing surface 23 does not have to project. In that case, the pin inner sealing surface 13 projects toward the box inner sealing surface 23.
[0048]
　The inner male screw 14 and the inner female screw 24 are taper screws and trapezoidal screws. The external male screw 17 and the external female screw 27 are also taper screws and trapezoidal screws.
[0049]
　The tapered surfaces of the inner threads 14 and 24 are closer to the tube axis CL than the tapered surfaces of the outer threads 17 and 27. This is because the intermediate shoulder surfaces 18, 28 are installed between the inner threads 14, 24 and the outer threads 17, 27. Therefore, in the pin 10, the outer diameter of the portion of the inner male screw 14 and the pin inner sealing surface 13 becomes small, and the thickness of that portion becomes thin. On the other hand, in the pin 10, the outer diameter of the portion of the pin intermediate sealing surface 16 and the outer male screw 17 becomes large, and the thickness of that portion becomes large.
[0050]
　The inner male screw 14 and the inner female screw 24 can be screwed into each other, and in a fastening state, they are fitted and closely contacted with each other to be in an interference fit state. Similarly, the outer male screw 17 and the outer female screw 27 are in an interference fit state.
[0051]
　The inner seal surfaces 13 and 23 and the intermediate seal surfaces 16 and 26 are brought into contact with each other as the pin 10 is screwed in, and are fitted and closely contacted in the tightened state to be in an interference fit state. Thereby, the inner seal surfaces 13 and 23 and the intermediate seal surfaces 16 and 26 respectively form an inner seal portion and an intermediate seal portion by metal contact.
[0052]
　In the fastened state, a gap (not shown) is formed between the nose 12 of the pin 10 and the recess 22 of the box 20. A gap is also formed between the pin annular portion 15a and the box annular portion 25a. A gap is also formed between the pin annular portion 15b and the box annular portion 25b.
[0053]
　In the tightened state, the inner shoulder surfaces 11 and 21 are pressed against each other and come into contact with each other. The pressing contact between the inner shoulder surfaces 11 and 21 mainly applies a tightening axial force to the load surface of the inner male screw 14. The intermediate shoulder surfaces 18 and 28 contact each other in the tightened state. When the intermediate shoulder surfaces 18 and 28 are in pressure contact with each other, a tightening axial force is mainly applied to the load surface of the external male screw 17.
[0054]
　The steel pipe threaded joint also satisfies the following equation (2).
[0055]
[Number 5]

[0056]
　In Formula (2), L1 is the distance between the tip of the pin 10 and the pin intermediate shoulder surface 18 in the tube axis CL direction when the pin 10 and the box 20 are not fastened. L2 is the distance between the box intermediate shoulder surface 28 and the tip of the box 20 in the tube axis CL direction when the pin 10 and the box 20 are not fastened.
[0057]
　Here, the influence of the taper gradients α1 and α2 of the inner threads 14 and 24 and the outer threads 17 and 27 will be considered. When the lengths of the inner threads 14 and 24 and the outer threads 17 and 27 are constant, when α1>α2, as compared with when α1=α2, the wall thickness of the pin 10 on the pin intermediate seal surface 18 Can be made thicker and the external pressure sealing performance is improved. On the other hand, when α1<α2, as compared with when α1=α2, the thickness of the pin 10 on the pin intermediate seal surface 18 becomes smaller, so the external pressure sealing performance deteriorates.
[0058]
　Next, the influence of the lengths of the inner threads 14 and 24 and the outer threads 17 and 27 will be considered. The wall thickness of the main body PM of the steel pipe is WT, the wall thickness of the pin 10 at the pin inner sealing surface 13 is t1, the wall thickness of the pin 10 at the pin intermediate sealing surface 16 is t2, and the pin intermediate shoulder surface 18 along the pipe radial direction is Assuming that the height is ts, the following relations (5) and (6) are obtained. However, X1 and X2 are constants that represent minute changes in wall thickness, such as internal machining of the inner seal faces 13 and 23 and changes in wall thickness near the inner seal faces 13 and 23.
[0059]
　WT=t1+L1α1+ts+L2α2+X1 (5)
　t2=t1+L1α1+ts+X2 (6)
[0060]
　Here, in order to enhance the external pressure sealing performance, it is necessary to increase the wall thickness t2 as much as possible to reduce the diameter reduction amount due to the external pressure and suppress a substantial decrease in the seal contact force when the external pressure is applied. If the inner sealing surfaces 13 and 23 to which the external pressure does not act are too thin, a sufficient fitting force cannot be obtained. Therefore, the thickness t1 needs to be a certain value or more. Further, in order to secure the compression resistance, the height ts also needs to be a certain value or more. Therefore, assuming that the wall thickness t1 and the height ts are constant, in order to maximize the wall thickness t2 from Expressions (5) and (6), L1α1 may be maximized and L2α2 may be minimized. Here, the external pressure sealing performance parameter S is defined by the following equation (7).
[0061]
[Numerical equation 6]

[0062]
　At this time, if the parameter S is larger than a certain value, a sufficient wall thickness t2 can be secured and a good external pressure sealing performance can be obtained.
[0063]
　However, if the parameter S is too large, the meshing of the outer screws 17, 27 is reduced, so that when the external pressure is applied, the diameters of the outer screws 17, 27 and the intermediate sealing surfaces 16, 26 tend to be reduced.
[0064]
　Here, in order to obtain a sufficient fitting contact at the seal portion, the wall thickness t1 is preferably 4 mm or more. Further, the height ts is preferably 4 mm or more in order to obtain sufficient compression resistance. Further, in order to prevent jump-out (disengagement of screw meshing or slip-out due to shear failure of screw) when an excessive tensile load is applied, it is preferable to satisfy the following formula (8).
　(L1+L2)/OD>1 (8)
[0065]
　In addition, if the in-pin shoulder surface 11 is provided at the tip of the pin 10, the damage due to the compressive load can be reduced, and further improvement in performance can be expected.
[0066]
　Below, the suitable aspect of the threaded joint which concerns on this Embodiment is supplemented.
[0067]
　In the pin 10, the cross-sectional area of ​​the steel pipe main body in a cross section perpendicular to the pipe axis CL is A0, and the projected area of ​​the box intermediate shoulder surface 28 on the plane perpendicular to the pipe axis CL is A1. At this time, the area ratio A1/A0 of both is preferably 0.3 or more. More preferably, A1/A0 is 0.35 or more. The reason is as follows. A1/A0 substantially depends on the area of ​​the box intermediate shoulder surface 28. When A1/A0 is small, the area of ​​the box intermediate shoulder surface 28 is small, and therefore, when an excessive compressive load is applied to the threaded joint, the box intermediate shoulder surface 28 cannot withstand the compressive load. In this case, the pin intermediate shoulder surface 18, the pin annular portion 15a and the pin intermediate sealing surface 16 connected to the pin intermediate plastic surface are plastically deformed, and the contact state between the intermediate sealing surfaces 16 and 26 becomes unstable. As a result, the contact surface pressure between the intermediate seal surfaces 16 and 26 may decrease. Therefore, the area ratio A1/A0 of the intermediate shoulder to the steel pipe body is preferably large to some extent.
[0068]
　The upper limit of the area ratio A1/A0 of the intermediate shoulder to the steel pipe body is not specified. However, if A1/A0 is too large, it becomes difficult to secure the engagement length of the inner threads 14 and 24 and the outer threads 17 and 27. Therefore, in consideration of practicality, the total shoulder area ratio A1/A0 to the steel pipe body is preferably 60% or less.
[0069]
　In the pin 10, the sectional area of ​​the steel pipe main body in the cross section perpendicular to the pipe axis CL is A0, and the total projected area of ​​each of the in-pin shoulder surface 11 and the pin intermediate shoulder surface 18 on the plane perpendicular to the pipe axis CL (total The shoulder area) is A2. At this time, the area ratio A2/A0 of both is preferably 0.3 or more. More preferably, A2/A0 is 0.35 or more. The reason is as follows. A2/A0 substantially depends on the area of ​​the in-pin shoulder 11 and the area of ​​the pin intermediate shoulder surface 18. When A2/A0 is small, the area of ​​the in-pin shoulder 11 and the area of ​​the pin intermediate shoulder surface 18 are small. Cannot withstand the compressive load. In this case, the in-pin shoulder surface 11 and the nose 12 and the in-pin seal surface 13 that are continuous with it are plastically deformed, and the contact state between the inner seal surfaces 13 and 23 becomes unstable. At the same time, the pin intermediate shoulder surface 18 and the pin annular portion 15a and the pin intermediate seal surface 16 connected to the pin intermediate plastic surface are plastically deformed, and the contact state between the intermediate seal surfaces 16 and 26 becomes unstable. As a result, the contact surface pressure between the inner sealing surfaces 13 and 23 and between the intermediate sealing surfaces 16 and 26 may decrease. Therefore, the area ratio A2/A0 of the total shoulder area to the steel pipe body is preferably large to some extent.
[0070]
　The upper limit of the area ratio A2/A0 of all the shoulders to the steel pipe body is not particularly specified. However, if A2/A0 is too large, it becomes difficult to secure the engagement length of the inner threads 14 and 24 and the outer threads 17 and 27. Therefore, in consideration of practicality, the total shoulder area ratio A2/A0 with respect to the steel pipe body is preferably 60% or less.
[0071]
　The upper limit of the outer diameter of the pin annular portion 15a is not specified. However, the outer diameter of the pin annular portion 15a needs to be a dimension that does not interfere with the box intermediate sealing surface 26 during fastening.
[0072]
　In the pin 10, the length of the pin annular portion 15a along the tube axis is preferably at least one time the screw pitch of the external male screw 17 as measured from the end portion on the front end side of the pin intermediate sealing surface 16. When the length of the pin annular portion 15a is short, when the external pressure is applied to the threaded joint, the contact surface pressure between the pin intermediate sealing surfaces 16 and 26 is the same as when the wall thickness of the pin annular portion 15a is thin. This is because there is a risk that it will decrease.
[0073]
　The upper limit of the length of the pin annular portion 15a is not particularly specified. However, if the length of the pin annular portion 15a is too long, the total length of the joint becomes long, and the manufacturing cost increases due to an increase in processing time and material cost. Further, when the length of the pin annular portion 15a becomes a certain length or more, the effect of improving the sealing performance is almost saturated. Therefore, in consideration of practicality, it is preferable that the length of the pin annular portion 15a be 5 times or less the screw pitch of the outer male screw 17.
[0074]
　In the pin 10, it is preferable that the length of the pin annular portion 15b along the tube axis is one time or more of the screw pitch of the outer male screw 17 measured from the end portion on the rear end side of the pin intermediate sealing surface 16. When the length of the pin annular portion 15b is short, a substantial interference amount between the intermediate sealing surfaces 16 and 26 is reduced due to the interference fit action of the outer screws 17 and 27, so that the contact surface pressure between the intermediate sealing surfaces 16 and 26 is reduced. Is likely to decrease.
[0075]
　The upper limit of the length of the pin annular portion 15b is not particularly specified. However, if the length of the pin annular portion 15b is too long, the overall length of the joint becomes long, and the manufacturing cost increases due to an increase in processing time and material cost. Further, when the length of the pin annular portion 15b becomes a certain length or longer, the effect of improving the sealing performance is almost saturated. Therefore, in consideration of practicality, it is preferable that the length of the pin annular portion 15b be 5 times or less the screw pitch of the outer male screw 17.
[0076]
　In the pin 10, the length of the nose 12 along the tube axis CL is preferably 5 mm or more. The reason is as follows. When the length of the nose 12 is short, when the excessive tensile load is applied to the threaded joint, the elastic restoring force of the in-pin sealing surface 13 by the nose 12 becomes insufficient. In this case, the contact surface pressure between the inner seal surfaces 13 and 23 may decrease. Therefore, the length of the nose 12 is preferably long to some extent.
[0077]
　The upper limit of the length of the nose 12 is not specified. However, if the length of the nose 12 is too long, the overall length of the joint becomes long, and the manufacturing cost increases due to an increase in processing time and material cost. Further, if the length of the nose 12 exceeds a certain length, the effect of improving the sealing performance is almost saturated. Therefore, in consideration of practicality, the length of the nose 12 is preferably 5 times or less the screw pitch of the inner male screw 14.
[0078]
　In addition, the present disclosure is not limited to the above embodiments, and various modifications can be made without departing from the gist of the present disclosure.
[0079]
　For example, as shown in FIG. 2, the in-pin shoulder surface 11 and the in-box shoulder surface 21 may be separated from each other.
[0080]
　For example, when internal pressure is applied to the threaded joint, a measure may be added to limit the fitting close contact of the inner screws 14 and 24 to the region close to the inner seal surfaces 13 and 23. As a result, the area of ​​the pin inner seal surface 13 is more effectively expanded and deformed, and the contact surface pressure between the inner seal surfaces 13 and 23 is further amplified. As a measure therefor, it is possible to adopt a configuration in which the inner male screw 14 or the inner female screw 24 is an incomplete screw having an incomplete screw shape in a region close to the inner sealing surfaces 13, 23 of the inner screws 14, 24. As an example of this configuration, in the region of the incomplete thread, the thread crest surface of the inner female thread 24 of the box 20 is a cylindrical surface parallel to the tube axis CL, and the thread height is made lower than the regular thread height. As a result, a gap is provided between the top surface of the inner female screw 24 and the bottom surface of the inner male screw 14 only in the region of the incomplete screw. In this case, the length of the region of the incomplete screw is 3 to 9 times (15 to 45 mm) the screw pitch of the inner female screw 24.
[0081]
　Further, the threaded joint according to the above embodiment can be applied not only to the coupling type but also to the integral type.
Example
[0082]
　In order to confirm the effect of the threaded joint for steel pipes according to the present embodiment, a numerical simulation analysis by an elasto-plastic finite element method (FEM) was performed.
[0083]
　
　For a plurality of specimens having different external pressure sealing performance parameters S (=L1α1/L2α2), a finite element analysis was performed to compare the performance differences. Each specimen is a coupling type threaded joint having the basic structure shown in FIG. The common test conditions are shown below.
[0084]
(1) Steel pipe dimensions
　7-5/8 [inch] x 1.06 [inch] (outer diameter 193.68 [mm], wall thickness 27.0 [mm])
(2) Steel pipe grade
　API standard P110 (Carbon steel with a nominal yield stress of 110 [ksi])
(3) Screw dimensions (common to all screws)
　Screw pitch: 5.08 [mm], load surface flank angle: -3°, insertion surface flank Angle: 10°, Insert face clearance: 0.15 [mm]
[0085]
　In the finite element analysis, the material is an isotropic hardened elasto-plastic body, the elastic modulus is 210 [GPa], and the yield strength is 110 [ksi] (=758.3 [MPa]) with 0.2% proof stress. A model of each specimen was used.
[0086]
　
[0087]
　After analyzing the tightening of the screws for each test piece, a load simulating the ISO 13679 2011 CAL4 Series A test was applied to evaluate the sealing performance against external pressure and internal pressure. Sealing performance against inner and outer pressures is the contact force per unit length in the circumferential direction of the inner seal in the inner pressure cycle of load history (first and second quadrants) and the outer pressure cycle of load history (third and fourth quadrants), respectively. The evaluation was made by the minimum value of the contact force per unit length in the circumferential direction of the intermediate seal. The larger the contact force, the better the sealing performance. In addition, the sealing performance was evaluated by the following two levels, using a relative value where the performance of the specimen #1 was "1".
　・○: Excellent: The contact force of the intermediate seal is 1.2 or more.
　×: Impossible: The contact force of the intermediate seal is less than 1.2 or the contact force of the inner seal is 0.9 or less.
[0088]
　Table 1 summarizes the test conditions and evaluations of each specimen.
[table 1]

[0089]
　
　Referring to FIGS. 3 and 4, since the specimen #1 has α1=α2, the wall thickness of the intermediate seal of the pin is small. Therefore, the diameter reduction resistance of the pin intermediate seal against external pressure was small, and the external pressure sealing performance was poor.
[0090]
　Since the values ​​of (L1+L2)/D are larger than 1 for Specimens #2 to 5, screw engagement is sufficient, tensile load resistance is high, α1 is larger than α2, and L1α1/L2α2 is 0. It is 5 or more and 1.2 or less. Therefore, since the thickness of the intermediate seal is sufficiently thick, the diameter reduction resistance to external pressure is particularly large, and the external pressure sealing performance is particularly good.
[0091]
　In specimens #6 to 7, α1 is larger than α2, but L1α1/L2α2 is larger than 1.2, so the external threads do not mesh well and the intermediate seal part shrinks when external pressure is applied. It's easy to do. Therefore, the external pressure sealing performance was poor.
Explanation of symbols
[0092]
10: Pin
11: Pin inner shoulder surface
12: Nose
13: Pin inner sealing surface
14: Inner male screw
15a, 15b: Pin annular portion
16: Pin intermediate sealing surface
17: External male screw
18: Pin intermediate shoulder surface
20: Box
21: In-box shoulder surface
22: Recess
23: In-box sealing surface
24: Inner female thread
25a: Box annular portion
26: Box intermediate sealing surface
27: Outer female thread
28: Box intermediate shoulder surface
The scope of the claims
[Claim 1]
　A threaded joint for steel pipes
　,
　comprising: a tubular pin; and a tubular box into which the pin is screwed and fastened to the
　pin, the
　pin including a nose formed at a tip end portion of the pin, and a tip end of the
　pin. A tapered inner male screw
　formed on the outer peripheral surface of the pin, and a tapered outer male screw formed on the outer peripheral surface of the pin, which is located closer to the main body of the steel pipe, the
　nose and the A pin inner seal surface formed between an inner male screw,
　a pin intermediate shoulder surface formed between the inner male screw and the outer male screw, and a pin intermediate shoulder surface formed between the
　outer male screw and the pin intermediate shoulder face. A
　pin intermediate sealing surface, and a pin annular portion formed between the pin intermediate shoulder surface and the pin intermediate sealing surface, the
　box
　corresponding to the nose and the
　inner male screw. A tapered inner female thread
　formed on the inner peripheral surface of the box, and a tapered outer female thread formed on the inner peripheral surface of the box corresponding to the outer male thread, and
　facing the pin inner sealing surface. A box inner sealing surface that contacts the pin inner sealing surface when the pin and the box are fastened together,
　The box intermediate shoulder surface facing the pin intermediate shoulder surface, the box intermediate shoulder surface contacting the pin intermediate shoulder surface when the pin and the box are fastened, and the
　pin intermediate sealing surface, the pin and the box are A box intermediate sealing surface that contacts the pin intermediate sealing surface when fastened, and a box
　annular portion that faces the pin annular portion and is separated from the pin annular portion when the pin and the box are fastened. A
　threaded joint for steel pipes, including and satisfying the following expressions (1) and (2).
　α1>α2 (1) In
　formula (1), α1 is the taper gradient of the inner male screw, and α2 is the taper gradient of the outer male screw.
[

　Equation 1] In the formula (2), L1 is the distance between the tip of the pin and the pin intermediate shoulder surface in the tube axis direction when the pin and the box are not fastened, and L2 is It is the distance between the box intermediate shoulder surface and the tip of the box in the tube axis direction when the pin and the box are not fastened.
[Claim 2]
　The steel pipe threaded joint according to claim 1,
　wherein the pin further
　includes an in-pin shoulder surface formed at a tip of the pin, and the
　box further includes
　an in-box shoulder facing the in-pin shoulder surface. Threaded joints for steel pipes, including surfaces.
[Claim 3]
　The threaded joint for steel pipes according to claim 2,
　wherein the shoulder surface in the pin contacts the shoulder surface in the box when the pin and the box are fastened.
[Claim 4]
　The threaded joint for steel pipes according to claim 1, wherein the
　threaded joint for steel pipes satisfies the following expression (3).
[

　Equation 2] In the equation (3), A0 is a cross-sectional area of ​​the main body of the steel pipe in a plane perpendicular to the pipe axis, and A1 is a projected area of ​​the box intermediate shoulder surface with respect to a plane perpendicular to the pipe axis. ..
[Claim 5]
　The threaded joint for steel pipes according to claim 2 or 3, wherein the
　threaded joint for steel pipes satisfies the following expression (4).
[

　Equation 3] In the equation (4), A0 is a cross-sectional area of ​​the main body of the steel pipe in a plane perpendicular to the pipe axis, and A2 is a projected area of ​​the shoulder surface in the pin with respect to a plane perpendicular to the pipe axis and It is the total projected area of ​​the pin intermediate shoulder surface.