Patent Publication Number: US-2021164593-A1

Title: Threaded connection for steel pipes

Description:
TECHNICAL FIELD 
     The present invention relates to a threaded connection used for connecting steel pipes. 
     BACKGROUND ART 
     In an oil well, natural gas well, and the like (hereafter, collectively referred to as an “oil well”), steel pipes called oil country tubular goods (OCTG) are used to mining underground resources. The steel pipes are connected one by one. To connect the steel pipes, threaded connections are used. 
     Types of the threaded connection for steel pipes are roughly categorized into a coupling type and an integral type. In a case of a threaded connection of the coupling type, one of tubes in a pair to be connected is a steel pipe, and the other one is a coupling. In this case, male thread parts are formed on outer circumferences of both end portions of the steel pipe, and female thread parts are formed on inner circumferences of both end portions of the coupling. The steel pipe and the coupling are then connected with each other. In a case of a threaded connection of the integral type, tubes in a pair to be connected are both steel pipes, and a coupling is not used separately. In this case, a male thread part is formed on an outer circumference of one end portion of each steel pipe, and a female thread part is formed on an inner circumference of the other end portion. One and the other of the steel pipes are then connected with each other. 
     A joint portion of a tube leading end portion on which a male thread part is formed includes an element to be inserted into a female thread part, and thus is called a pin. In contrast, a joint portion of a tube leading end portion on which a female thread part is formed includes an element to receive a male thread part, and thus is called a box. The pin and the box are end portions of a tube and are thus both tubular. 
       FIG. 1  is a longitudinal sectional view illustrating a typical threaded connection for steel pipes as conventionally used. The threaded connection illustrated in  FIG. 1  is a threaded connection of the coupling type and includes a pin  10  and a box  20  (e.g., see Japanese Patent Application Publication No. 10-096489 (Patent Literature 1)). 
     The pin  10  includes, in order from a front end of the pin  10  toward a pipe body  11  of the pin  10 , an annular shoulder surface  12 , an annular sealing surface  13 , and a male thread part  14 . In the pin  10 , the sealing surface  13  is located adjacent to the shoulder surface  12 . The box  20  includes, in order from a pipe body  21  of the box  20  toward a front end of the box  20 , an annular shoulder surface  22 , an annular sealing surface  23 , and a female thread part  24 . In the box  20 , the sealing surface  23  is located adjacent to the shoulder surface  22 . 
     When the pin  10  and the box  20  are connected to each other, by screwing the pin  10  into the box  20 , the shoulder surface  12  of the pin  10  comes into contact with the shoulder surface  22  of the box  20 . When rotation of the pin  10  is continued by a predetermined amount, a fastening axial tension is generated between the male thread part  14  and the female thread part  24  meshing with each other, and fastening is thereby completed. In a state where the fastening is completed (hereafter, referred to also as a “fastening state”), the sealing surface  13  of the pin  10  comes in contact with the sealing surface  23  of the box  20  while interfering with the sealing surface  23 , forming a seal part by metal contact. This seal part ensures a sealing ability of the threaded connection. 
     In recent years, as oil wells are increasingly shifted at great depths and to hadal zones, oil well environments become harsh with high temperatures and high pressures. In such oil well environments, a compressive load, a tensile load, and pressures from the outside (hereafter, also referred to as “external pressures”) and pressures from the inside (hereafter, also referred to as “internal pressures”) applied to oil country tubular goods are extremely high. For that reason, heavy-wall steel pipes are used particularly as oil country tubular goods for casing or tubing. In a case where a threaded connection illustrated in  FIG. 1  is used for connecting such heavy-wall steel pipes, the threaded connection is required to have a comparable level of strength to that of bodies of the heavy-wall steel pipes and high sealing ability. In particular, the sealing ability against an external pressure is required. 
     Here, the compressive load is received by a contact surface between a shoulder surface  12  of a pin  10  and a shoulder surface  22  of a box  20  (hereafter, also referred to as a “shoulder contact surface”). Referring to  FIG. 1 , in a conventional threaded connection, to ensure an area of the shoulder contact surface to the maximum, the shoulder surface  12  of the pin  10  is brought into contact with the shoulder surface  22  of the box  20  over their entire areas. Specifically, a diameter Dpi of an inner circumferential edge of the shoulder surface  12  of the pin  10  is substantially the same as a diameter Dbi of the inner circumferential edge of the shoulder surface  22  of the box  20 . 
     CITATION LIST 
     Patent Literature 
     PATENT LITERATURE 1: Japanese Patent Application Publication No. 10-096489 
     SUMMARY OF INVENTION 
     Technical Problem 
     An objective of the present invention is to provide a threaded connection for steel pipes that is capable of ensuring sufficient sealing ability against external pressures even in a case of using a heavy-wall steel pipe. 
     Solution to Problem 
     A threaded connection for steel pipes according to an embodiment of the present invention includes a tubular pin and a tubular box. The pin includes, in order from a front end of the pin toward a pipe body of the pin, an annular shoulder surface, an annular sealing surface located adjacent to the shoulder surface of the pin, and a male thread part. The box includes, in order from a pipe body of the box toward a front end of the box, an annular shoulder surface, an annular sealing surface located adjacent to the shoulder surface of the box, and a female thread part. The shoulder surface of each of the pin and the box inclines from a plane perpendicular to a pipe axis in a direction of screwing the pin. A diameter of an inner circumferential edge of the shoulder surface of the pin is smaller than a diameter of an inner circumferential edge of the shoulder surface of the box. 
     Advantageous Effect of Invention 
     With the threaded connection for steel pipes according to an embodiment of the present invention, a sealing ability against external pressure can be sufficiently ensured even in a case of using a heavy-wall steel pipe. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a longitudinal sectional view illustrating a typical threaded connection for steel pipes as conventionally used. 
         FIG. 2  is a longitudinal sectional view illustrating a threaded connection for steel pipes in the present embodiment. 
         FIG. 3  is a longitudinal sectional view illustrating a vicinity of a front end of a pin of the threaded connection illustrated in  FIG. 2 , in an enlarged manner. 
         FIG. 4  is a longitudinal sectional view illustrating a vicinity of a front end of a pin of the threaded connection illustrated in  FIG. 2 , in an enlarged manner. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     To solve the above problem, the present inventors conducted various analyses and tests, and conducted intensive studies. Consequently, the following findings were obtained. 
     To increase sealing ability of a threaded connection used for a heavy-wall steel pipe, the following techniques are conceivable. As a first technique, referring to  FIG. 1 , it is conceivable to increase a wall thickness of a portion  15  including a sealing surface  13  of a pin  10  (hereafter, also referred to as a “pin seal part”). 
     The first technique is derived from the following inference. When the wall thickness of the pin seal part  15  is large, a rigidity of the pin seal part  15  is increased in a radial direction. This large wall thickness improves an elastic resilience of the pin seal part  15 , which leads to an increase in a contact force between the sealing surface  13  of the pin  10  and a sealing surface  23  of a box  20  in a fastening state (hereafter, also referred to as a “seal contact force”), and thus an increase in the sealing ability against internal pressures and external pressures. In addition, the large wall thickness prevents radially-contractive deformation of the pin seal part  15  when an external pressure is applied to the pin  10 . For that reason, it is possible to minimize the decrease in the seal contact force even when an external pressure is applied to the pin  10 . Therefore, it may be said that a large wall thickness of the pin seal part  15  can prevent the sealing ability against external pressures from decreasing. 
     In the first technique, since the wall thickness of the pin seal part  15  is increased, a shoulder surface  12  of the pin  10  has a large area. In a conventional threaded connection, the shoulder surface  12  of the pin  10  comes into contact with the shoulder surface  22  of the box  20  over their entire areas. Therefore, the shoulder surface  22  of the box  20  also has a large area. In other words, a shoulder contact surface has a large area. 
     However, if the shoulder contact surface has an excessively large area, a contact force between the shoulder surface  12  of the pin  10  and the shoulder surface  22  of the box  20  (hereafter, also referred to as a shoulder contact force) becomes uneven in the shoulder contact surface. The unevenness of the shoulder contact force has a significant influence on a seal part, which is adjacent to the shoulder contact surface. For that reason, contact of the seal part actually becomes unstable, resulting in a decrease in the sealing ability. 
     In contrast to the first technique described above, as a second technique, it is conceivable to decrease the wall thickness of the pin seal part  15 . In such a case, the decrease in the wall thickness of the pin seal part  15  makes the area of the shoulder surface  12  of the pin  10  small, and also makes the area of the shoulder surface  22  of the box  20  small. For that reason, the shoulder contact surface has a small area. Therefore, the shoulder contact force can be made uniform. 
     However, in the second technique, since the wall thickness of the pin seal part  15  is decreased, the rigidity of the pin seal pan  15  is low in the radial direction. This makes the pin seal part  15  susceptible to radially-contractive deformation when an external pressure is applied to the pin  10 . For that reason, the sealing ability against external pressures is actually decreased. 
     Briefly, it is not possible to ensure the sealing ability against external pressures by both of the first and second techniques. 
     Thus, in consideration of the problems with the first and second techniques, the present inventors paid attention to the pin seal part and the shoulder contact surface. Specifically, the wall thickness of the pin seal part is increased, and at the same time, the area of the shoulder contact surface is decreased. This increases the rigidity of the pin seal part in the radial direction and also makes the shoulder contact force uniform in the shoulder contact surface. Therefore, stabilization of the contact of the seal part can be achieved. Consequently, it is possible to ensure the sealing ability against external pressures. 
     The threaded connection for steel pipes according to the present invention is completed based on the above findings. 
     A threaded connection for steel pipes according to an embodiment of the present invention includes a tubular pin and a tubular box. The pin includes, in order from a front end of the pin toward a pipe body of the pin, an annular shoulder surface, an annular sealing surface located adjacent to the shoulder surface of the pin, and a male thread part. The box includes, in order from a pipe body of the box toward a front end of the box, an annular shoulder surface, an annular sealing surface located adjacent to the shoulder surface of the box, and a female thread part. The shoulder surface of each of the pin and the box inclines from a plane perpendicular to a pipe axis in a direction of screwing the pin. A diameter of an inner circumferential edge of the shoulder surface of the pin is smaller than a diameter of an inner circumferential edge of the shoulder surface of the box. 
     In a typical example, the threaded connection in the present embodiment is used for connecting heavy-wall steel pipes used as casing pipes or tubing pipes. Wall thicknesses of the heavy-wall steel pipes are more than 1 inch (25.4 mm). 
     In the threaded connection in the present embodiment, an area of the shoulder surface of the pin is large, and an area of the shoulder surface of the box is small. This is because the diameter of the inner circumferential edge of the shoulder surface of the pin is smaller than the diameter of the inner circumferential edge of the shoulder surface of the box. This makes a wall thickness of the pin seal part large, which increases a rigidity of the pin seal part in the radial direction. In addition, the area of the shoulder contact surface being small makes the shoulder contact force uniform across the shoulder contact surface. 
     In addition, in the threaded connection in the present embodiment, when the threaded connection is in a fastening state, the shoulder surface of the pin and the shoulder surface of the box are in pressing contact with each other in a hooked form. This is because the shoulder surface of each of the pin and the box inclines from a plane perpendicular to a pipe axis in a direction of screwing the pin. This causes the pin seal part to receive a reaction force all the time in a direction in which the pin seal part radially expands. The pin seal part thus resists radially-contractive deformation when an external pressure is applied to the pin. 
     From the above reason, the contact between the seal parts is stabilized even in a case of using a heavy-wall steel pipe. Consequently, it is possible to ensure the sealing ability against external pressures sufficiently. 
     In the above threaded connection, a feed angle of the shoulder surface of each of the pin and the box with respect to the plane perpendicular to the pipe axis is preferably 5° to 20°. When the feed angle of the shoulder surfaces are 5° or more, the shoulder surface of the pin and the shoulder surface of the box are in an effective pressing contact with each other in the hooked form in the fastening state. The feed angle of the shoulder surface is preferably 10° or more. At the same time, when the feed angle of shoulder surface is 20° or less, deformation of a shoulder part of the box is small even when a compressive load is repeatedly applied. Therefore, the pressing contact between the shoulder surfaces in the hooked form is effectively maintained. 
     In the above threaded connection, a thickness tp of an annular pin shoulder region that appears when the shoulder surface of the pin is projected onto the plane perpendicular to the pipe axis is preferably 60% or more of a wall thickness t of the pipe body of the pin. When the thickness tp of the pin shoulder region is 60% or more of the wall thickness t of the pipe body of the pin, the wall thickness of the pin seal part is effectively large. 
     In contrast, an upper limit of the thickness tp of the pin shoulder region is not limited to a particular thickness. However, an excessively large thickness tp of the pin shoulder region makes it difficult to secure a length of the male thread part. Thus, the thickness tp of the pin shoulder region is preferably 80% or less of the wall thickness t of the pipe body of the pin. 
     In the above threaded connection, a thickness tb of an annular box shoulder region that appears when the shoulder surface of the box is projected onto the plane perpendicular to the pipe axis is preferably 20% or more to 55% or less of the wall thickness t of the pipe body of the pin. The box shoulder region is equivalent to an annular shoulder contact surface region that appears when the shoulder contact surface is projected onto the plane perpendicular to the pipe axis. 
     When the thickness tb of the box shoulder region (shoulder contact surface region) is 20% or more of the wall thickness t of the pipe body of the pin, in a case where an excessively heavy compressive load is applied to the threaded connection, plastic deformation of the shoulder surface and the sealing surface adjacent to the shoulder surface can be prevented, which can stabilizes a contact state of the sealing surface. Consequently, it is possible to ensure the seal contact force. More preferably, the thickness tb of the box shoulder region is 30% or more of the wall thickness t of the pipe body of the pin. In contrast, when the thickness tb of the box shoulder region is 55% or less of the wall thickness t of the pipe body of the pin, an area of the shoulder contact surface is made significantly small. More preferably, the thickness tb of the box shoulder region is 45% or less of the wall thickness t of the pipe body of the pin. 
     To be exact, there is a convex corner part between the shoulder surface and the sealing surface seen in a longitudinal section of the pin. This convex corner part connects the shoulder surface and the sealing surface of the pin seamlessly. Similarly, there is a concave corner part between the shoulder surface and the sealing surface seen in a longitudinal section of the box. This concave corner part connects the shoulder surface and the sealing surface of the box seamlessly. Radii of these convex corner part and concave corner part seen in the longitudinal section are at most about 1.5 mm. In this case, the thickness tp of the pin shoulder region does not include a region of the convex corner part. Similarly, the thickness tb of the box shoulder region does not include a region of the concave corner part. 
     Hereafter, a specific example of a threaded connection for steel pipes according to the present embodiment will be described with reference to the accompanying drawings. 
       FIG. 2  is a longitudinal sectional view illustrating the threaded connection for steel pipes in the present embodiment.  FIG. 3  and  FIG. 4  are longitudinal sectional views each illustrating a vicinity of a front end of the pin of the threaded connection illustrated in  FIG. 2 , in an enlarged manner.  FIG. 3  illustrates the fastening state.  FIG. 4  illustrates a state where the pin  10  is separated from the box  20 , for convenience of description. Solid-white arrows in  FIG. 2  to  FIG. 4  illustrate a direction of screwing the pin  10  with respect to the box  20 . In the present specification, a longitudinal section refers to a cross section including a pipe axis CL of the threaded connection (see  FIG. 2 ). 
     Referring to  FIG. 2  to  FIG. 4 , the threaded connection in the present embodiment is a threaded connection of the coupling type and includes the pin  10  and the box  20 . The pin  10  is a heavy-wall steel pipe. 
     The pin  10  includes, in order from a front end of the pin  10  toward a pipe body  11  of the pin  10 , an annular shoulder surface  12 , an annular sealing surface  13 , and a male thread part  14 . Hereinafter, the shoulder surface  12  of the pin  10  will be referred to also as a “pin shoulder surface”. The sealing surface  13  of the pin  10  will be referred to also as a “pin sealing surface”. 
     The pin shoulder surface  12  is an annular surface forming a front end surface of the pin  10  and inclines from the plane perpendicular to the pipe axis CL in the direction of screwing the pin  10 . This causes an outer circumferential edge  12   b  of the pin shoulder surface  12  (an edge farthest from the pipe axis CL) to protrude from an inner circumferential edge  12   a  of the pin shoulder surface  12  (an edge closest to the pipe axis CL) in the direction of screwing the pin  10 . The pin sealing surface  13  is located adjacent to the pin shoulder surface  12 . That is, the pin sealing surface  13  is connected to the outer circumferential edge  12   b  of the pin shoulder surface  12 . The pin sealing surface  13  is a tapered annular surface. Note that the pin sealing surface  13  may have a shape formed by combining the tapered annular surface and a surface equivalent to a circumferential surface of a solid of revolution obtained by rotating a curve such as an arc about the pipe axis CL. A diameter of the pin sealing surface  13  decreases as the pin  10  extends toward its front end side (close to the pin shoulder surface  12 ). 
     The box  20  includes, in order from a pipe body  21  of the box  20  toward a front end of the box  20 , an annular shoulder surface  22 , an annular sealing surface  23 , and a female thread part  24 . Hereinafter, the shoulder surface  22  of the box  20  will be referred to also as a “box shoulder surface”. The sealing surface  23  of the box  20  will be referred to also as a “box sealing surface”. 
     The box shoulder surface  22  is an annular surface corresponding to the pin shoulder surface  12  and inclines from the plane perpendicular to the pipe axis CL in the direction of screwing the pin  10 . This causes an inner circumferential edge  22   a  of the box shoulder surface  22  (an edge closest to the pipe axis CL) to protrude from an outer circumferential edge  22   b  of the box shoulder surface  22  (an edge farthest from the pipe axis CL) in an opposite direction to the direction of screwing the pin  10 . The box sealing surface  23  is located adjacent to the box shoulder surface  22 . That is, the box sealing surface  23  is connected to the outer circumferential edge  22   b  of the box shoulder surface  22 . This box sealing surface  23  is a tapered annular surface corresponding to the pin sealing surface  13 . Note that the box sealing surface  23  may have a shape formed by combining the tapered annular surface and a surface equivalent to a circumferential surface of a solid of revolution obtained by rotating a curve such as an arc about the pipe axis CL. 
     The male thread part  14  of the pin  10  corresponds to the female thread part  24  of the box  20 . Each of the male thread part  14  and the female thread part  24  includes crests, roots, stabbing flanks, and load flanks. 
     In the present embodiment, a diameter Dpi of the inner circumferential edge  12   a  of the pin shoulder surface  12  is smaller than a diameter Dbi of the inner circumferential edge  22   a  of the box shoulder surface  22 . An area of the pin shoulder surface  12  is large, and an area of the box shoulder surface  22  is small. For that reason, a shoulder contact surface  30  has a small area. The diameter Dpi of the inner circumferential edge  12   a  of the pin shoulder surface  12  is the same as an inner diameter of the pipe body  11  of the pin  10 . That is, the inner diameter of the pin  10  is constant. 
     In the present embodiment, feed angles θp and θb of the pin shoulder surface  12  and the box shoulder surface  22  with respect to the surface perpendicular to the pipe axis CL are 5° to 20°. The thickness tp of the annular pin shoulder region that appears when the pin shoulder surface  12  is projected onto the plane perpendicular to the pipe axis CL is 60% or more of the wall thickness t of the pipe body  11  of the pin  10 . The thickness tb of the annular box shoulder region that appears when the box shoulder surface  22  is projected onto the plane perpendicular to the pipe axis CL is 20% or more to 55% or less of the wall thickness t of the pipe body  11  of the pin  10 . 
     When the pin  10  and the box  20  are connected to each other, by screwing the pin  10  into the box  20 , the male thread part  14  meshes with the female thread part  24 . A part of the pin shoulder surface  12  comes in contact with a whole area of the box shoulder surface  22 . That is, the pin shoulder surface  12  comes in contact with the box shoulder surface  22  within a range of the shoulder contact surface  30 . When rotation of the pin  10  is continued by a predetermined amount, the part of the pin shoulder surface  12  and the whole area of the box shoulder surface  22  are in pressing contact with each other in the hooked form. This generates a fastening axial tension between the male thread part  14  and the female thread part  24  meshing with each other, and fastening is thereby completed. In the fastening state, the pin sealing surface  13  comes in contact with the box sealing surface  23  while interfering with the box sealing surface  23 , forming a seal part by metal contact. This seal part ensures the sealing ability of the threaded connection. 
     Referring to  FIG. 2  to  FIG. 4 , in the threaded connection in the present embodiment, the area of the pin shoulder surface  12  is large, and the area of the box shoulder surface  22  is small. This makes a wall thickness of the pin seal part  15  large, which increases a rigidity of the pin seal part  15  in the radial direction. In addition, the area of the shoulder contact surface  30  being small makes the shoulder contact force uniform across the shoulder contact surface  30 . 
     In addition, in the threaded connection in the present embodiment, when the threaded connection is in the fastening state, the part of the pin shoulder surface  12  and the whole area of the box shoulder surface  22  are in the pressing contact with each other in the hooked form. This causes the pin seal part  15  to receive a reaction force all the time in a direction in which the pin seal part radially expands. The pin seal part  15  thus resists radially-contractive deformation when an external pressure is applied to the pin  10 . 
     From the above reason, the contact between the seal parts is stabilized even in a case of using a heavy-wall steel pipe. Consequently, it is possible to ensure the sealing ability against external pressures sufficiently. 
     EXAMPLES 
     To confirm effects brought by the present embodiment, the present inventors conducted numerical simulations and analyses using the elasto-plastic finite element method (FEM analysis). 
     Example 1 
     [Test Conditions] 
     In the FEM analysis, a model was used of a coupling-type threaded connection in which a diameter Dpi of an inner circumferential edge of a pin shoulder surface and a diameter Dbi of an inner circumferential edge of a box shoulder surface were variously changed. The followings are common conditions.
         Dimensions of steel pipe (pin body): 7⅝ inches×1.06 inches (outer diameter was 193.7 mm, wall thickness was 27.0 mm)   Grade of steel pipe: P110 according to the API standard (a carbon steel having a nominal yield stress of 110 ksi)   Diameter Do of outer circumferential edge of shoulder surface (pin shoulder surface and box shoulder surface): 179.9 mm   Feed angle of shoulder surface: 15°   Thread pitch: 5.08 mm   Flank angle of load flank: −3°   Flank angle of stabbing flank: 10°   Clearance in stabbing flank: 0.15 mm       

     Changed dimensional conditions were according to Table 1 shown below. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Sealing ability 
                   
               
               
                   
                 [Dimensionless] 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                 External 
                   
                   
               
               
                   
                   
                   
                   
                   
                   
                 pressure + 
                 Only 
               
               
                   
                 Dpi 
                 Dbi 
                 Dpi − Dbi 
                 tp/t 
                 tb/t 
                 compressive 
                 external 
               
               
                 No. 
                 [mm] 
                 [mm] 
                 [mm] 
                 [%] 
                 [%] 
                 load 
                 pressure 
                 Category 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 1 
                 146.67 
                 154.32 
                 −7.65 
                 62 
                 47 
                 1.17 
                 1.18 
                 Inventive 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 2 
                 146.67 
                 146.67 
                 0 
                 62 
                 63 
                 1.00 
                 1.00 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 (Reference) 
               
               
                 3 
                 146.67 
                 150.20 
                 −3.53 
                 62 
                 55 
                 1.10 
                 1.08 
                 Inventive 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 4 
                 146.67 
                 158.45 
                 −11.78 
                 62 
                 40 
                 1.22 
                 1.25 
                 Inventive 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 5 
                 146.67 
                 162.58 
                 −15.91 
                 62 
                 32 
                 1.20 
                 1.21 
                 Inventive 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 6 
                 146.67 
                 166.71 
                 −20.04 
                 62 
                 24 
                 1.11 
                 1.13 
                 Inventive 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 7 
                 150.20 
                 150.20 
                 0 
                 55 
                 56 
                 0.95 
                 0.70 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 8 
                 154.32 
                 154.32 
                 0 
                 47 
                 48 
                 0.86 
                 0.32 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 9 
                 158.45 
                 158.45 
                 0 
                 40 
                 41 
                 0.70 
                 0.00 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                   
               
               
                 (Note) 
               
               
                 Meanings of the signs in Table 1 are as follows. 
               
               
                 Dpi: Diameter of inner circumferential edge of pin shoulder surface [mm] 
               
               
                 Dbi: Diameter of inner circumferential edge of box shoulder surface [mm] 
               
               
                 tp: Thickness of pin shoulder region [mm] 
               
               
                 t: Wall thickness of pipe body of pin [mm] 
               
               
                 tb: Thickness of box shoulder region [mm] 
               
            
           
         
       
     
     In the FEM analysis, a material of the threaded connection was assumed to be an isotropic-hardening elasto-plastic body. A modulus of elasticity of the elasto-plastic body was set at 210 GPa, and a yield strength of the elasto-plastic body as a 0.2% yield stress was set at 110 ksi (758.3 MPa). Tightening was performed until the pin shoulder surface came into contact with the box shoulder surface and a 1.0/100 turn was made. 
     Test Nos. 1 and 3 to 6 were Inventive Examples of the present invention intended to represent threaded connections in the present embodiment, in each of which the diameter Dpi of the inner circumferential edge of the pin shoulder surface was smaller than the diameter Dbi of the inner circumferential edge of the box shoulder surface. Test No. 2 was Comparative Example as a reference intended to represent a conventional threaded connection, in which the diameter Dpi of the inner circumferential edge of the pin shoulder surface was the same as the diameter Dbi of the inner circumferential edge of the box shoulder surface. Test Nos. 7 to 9 were Comparative Examples, in each of which the diameter Dpi of the inner circumferential edge of the pin shoulder surface was the same as the diameter Dbi of the inner circumferential edge of the box shoulder surface. 
     [Evaluation Method] 
     In the FEM analysis, load steps (combinations of internal pressure, external pressure, tensile load, and compressive load) that simulate Series A test according to ISO13679 2011 were applied to the model in a fastening state. Of load points in an external pressure cycle in a loading step history, attention was paid to a load point of an external pressure and a compressive load and a load point of only an external pressure, and at each of the load points, sealing ability of a seal part was evaluated. Here, a seal contact force [N/mm] under the external pressure and compressive load, and a seal contact force [N/mm] under only the external pressure were investigated. The seal contact force mentioned here refers to a value of [average contact interfacial pressure between sealing surfaces]×[contact width], and means that the higher the value, the better the sealing ability. 
     A specific evaluation of the sealing ability was conducted with reference to Test No. 2. Specifically, the seal contact force under the external pressure and compressive load, and the seal contact force under only the external pressure in Test No. 2 were regarded as references (1.00), and a ratio of seal contact forces in each Test No were compared with the respective seal contact forces in Test No. 2. 
     [Test Result] 
     The test results are shown in the above Table 1. From the results shown in Table 1, the followings are indicated. In each of Test Nos. 1 and 3 to 6 being Inventive Example of the present invention, the sealing ability was improved as compared with Test No. 2 being Comparative Example as the reference. This was attributable to such a condition that the diameter Dpi of the inner circumferential edge of the pin shoulder surface was smaller than the diameter Dbi of the inner circumferential edge of the box shoulder surface. In particular, in each of Test Nos. 1 and 3 to 6, the sealing ability was further improved. This was attributable to such a condition that a thickness of tp of a pin shoulder region was 60% or more of a wall thickness t of a pipe body of the pin, so that the wall thickness of the pin seal part was effectively large. In addition, this further improvement in the sealing ability was attributable to such a condition that a thickness tb of a box shoulder region was 55% or less of the wall thickness t of the pipe body of the pin, so that the area of the shoulder contact surface was effectively small. 
     In contrast, in each of Test Nos. 7 to 9 being Comparative Examples, the sealing ability was decreased as compared with Test No. 2 as the reference. This was attributable to such a condition that the diameter Dpi of the inner circumferential edge of the pin shoulder surface was the same as the diameter Dbi of the inner circumferential edge of the box shoulder surface. In particular, this decrease in the sealing ability was also attributable to such a condition that the thickness tp of the pin shoulder region did not reach 60% of wall thickness t of the pipe body of the pin, so that the wall thickness of the pin seal part was small. 
     Example 2 
     [Test Conditions] 
     In Example 2, the same FEM analysis as that in the above Example 1 was conducted. In particular, in Example 2, the feed angle of the shoulder surface was set at 5°. The otherwise common conditions were the same as those in the above Example 1. Changed dimensional conditions (diameter Dpi of the inner circumferential edge of the pin shoulder surface, and diameter Dbi of the inner circumferential edge of the box shoulder surface) were according to Table 2 shown below. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Sealing ability 
                   
               
               
                   
                 [dimensionless] 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                 External 
                   
                   
               
               
                   
                   
                   
                   
                   
                   
                 pressure + 
                 Only 
               
               
                   
                 Dpi 
                 Dbi 
                 Dpi − Dbi 
                 tp/t 
                 tb/t 
                 compressive 
                 external 
               
               
                 No. 
                 [mm] 
                 [mm] 
                 [mm] 
                 [%] 
                 [%] 
                 load 
                 pressure 
                 Category 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 10 
                 146.67 
                 154.32 
                 −7.65 
                 62 
                 47 
                 1.28 
                 1.17 
                 Inventive 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 11 
                 146.67 
                 146.67 
                 0 
                 62 
                 63 
                 1.00 
                 1.00 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 (Reference) 
               
               
                 12 
                 146.67 
                 150.20 
                 −3.53 
                 62 
                 55 
                 1.14 
                 1.10 
                 Inventive 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 13 
                 146.67 
                 158.45 
                 −11.78 
                 62 
                 40 
                 1.38 
                 1.22 
                 Inventive 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 14 
                 146.67 
                 162.58 
                 −15.91 
                 62 
                 32 
                 1.46 
                 1.25 
                 Inventive 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 15 
                 146.67 
                 166.71 
                 −20.04 
                 62 
                 24 
                 1.41 
                 1.21 
                 Inventive 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 16 
                 150.20 
                 150.20 
                 0 
                 55 
                 56 
                 0.91 
                 0.69 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 17 
                 154.32 
                 154.32 
                 0 
                 47 
                 48 
                 0.74 
                 0.28 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 18 
                 158.45 
                 158.45 
                 0 
                 40 
                 41 
                 0.52 
                 0.00 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                   
               
               
                 (Note) 
               
               
                 Meanings of the signs in Table 2 are as follows. 
               
               
                 Dpi: Diameter of inner circumferential edge of pin shoulder surface [mm] 
               
               
                 Dbi: Diameter of inner circumferential edge of box shoulder surface [mm] 
               
               
                 tp: Thickness of pin shoulder region [mm] 
               
               
                 t: Wall thickness of pipe body of pin [mm] 
               
               
                 tb: Thickness of box shoulder region [mm] 
               
            
           
         
       
     
     Test Nos. 10 and 12 to 15 were Inventive Examples of the present invention intended to represent a threaded connection in the present embodiment, in each of which the diameter Dpi of the inner circumferential edge of the pin shoulder surface was smaller than the diameter Dbi of the inner circumferential edge of the box shoulder surface. Test No. 11 was Comparative Example as a reference intended to represent a conventional threaded connection, in which the diameter Dpi of the inner circumferential edge of the pin shoulder surface was the same as the diameter Dbi of the inner circumferential edge of the box shoulder surface. Test Nos. 16 to 18 were Comparative Examples, in each of which the diameter Dpi of the inner circumferential edge of the pin shoulder surface was the same as the diameter Dbi of the inner circumferential edge of the box shoulder surface. 
     [Evaluation Method] 
     As in the above Example 1, the sealing ability was evaluated. Specifically, the evaluation of the sealing ability was conducted with reference to Test No. 11. Specifically, the seal contact force under the external pressure and compressive load, and the seal contact force under only the external pressure in Test No. 11 were regarded as references (1.00), and ratios of seal contact forces in each Test No were compared to the respective seal contact forces in Test No. 11. 
     [Test Result] 
     The test results are shown in the above Table 2. From the results shown in Table 2, the followings are indicated. In each of Test Nos. 10 and 12 to 15 being Inventive Example of the present invention, the sealing ability was improved as compared with Test No. 11 being Comparative Example as the reference. This was attributable to such a condition that the diameter Dpi of the inner circumferential edge of the pin shoulder surface was smaller than the diameter Dbi of the inner circumferential edge of the box shoulder surface. 
     In contrast, in each of Test Nos. 16 to 18 being Comparative Examples, the sealing ability was decreased as compared with Test No. 11 as the reference. This was attributable to such a condition that the diameter Dpi of the inner circumferential edge of the pin shoulder surface was the same as the diameter Dbi of the inner circumferential edge of the box shoulder surface. 
     The present invention is not limited to the embodiments described above, and various modifications may be made without departing from the spirit and scope of the present invention. For example, the threaded connection may be of any one of a coupling type and an integral type. 
     INDUSTRIAL APPLICABILITY 
     The threaded connection according to the present invention can be effectively used for connecting steel pipes used as oil country tubular goods. 
     REFERENCE SIGNS LIST 
     
         
           10  pin 
           11  pipe body 
           12  shoulder surface 
           12   a  inner circumferential edge 
           12   b  outer circumferential edge 
           13  sealing surface 
           14  male thread part 
           15  pin seal part 
           20  box 
           21  pipe body 
           22  shoulder surface 
           22   a  inner circumferential edge 
           22   b  outer circumferential edge 
           23  sealing surface 
           24  female thread part 
           30  shoulder contact surface 
         Do diameter of outer circumferential edge of shoulder surface 
         Dpi diameter of inner circumferential edge of pin shoulder surface 
         Dbi diameter of inner circumferential edge of box shoulder surface 
         tp thickness of pin shoulder region 
         tb thickness of box shoulder region 
         t wall thickness of pipe body of pin 
         CL pipe axis