Title of invention: Threaded joint for steel pipe
Technical field
[0001]
　The present disclosure relates to threaded joints for steel pipes.
Background technology
[0002]
　For example, exploratory drilling or production of oil wells and natural gas wells (hereinafter collectively referred to as "oil wells"), development of non-conventional resources such as oil sands and shale gas, carbon capture and storage (CCS (Carbon)). In dioxide capture and storage)), geothermal power generation, hot springs, etc., steel pipes called oil well pipes are used. Threaded joints are used to connect the steel pipes.
[0003]
　The types of threaded joints for steel pipes of this type are roughly classified into coupling type and integral type. In the case of the coupling type, of the pair of pipe materials to be connected, one pipe material is a steel pipe and the other pipe material is a coupling. In this case, male threads are formed on the outer circumferences of both ends of the steel pipe, and female threads are formed on the inner circumferences of both ends of the coupling. Then, the male screw of the steel pipe is screwed into the female screw of the coupling, whereby 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 is formed on the outer circumference of one end of the steel pipe, and a female screw is formed on the inner circumference of the other end. Then, the male screw of one steel pipe is screwed into the female screw of the other steel pipe, whereby both are fastened and connected.
[0004]
　Generally, the joint portion of the pipe end portion on which the male thread is formed is referred to as a "pin" because it contains an element to be inserted into the female thread. On the other hand, the joint portion of the pipe end portion on which the female thread is formed is referred to as a "box" because it contains an element that accepts the male thread. Since these pins and boxes are the ends of the pipe material, they are both tubular.
[0005]
　For example, for a threaded joint used in a deep oil well, a large tensile load due to the own weight of the oil well pipe is applied in the shallow part, and a large compressive load due to thermal expansion is applied in the deep part.
[0006]
　US Reissue Patent No. 30647 (Patent Document 1), US Patent No. 6158785 (Patent Document 2), and International Publication WO2015 / 194193 (Patent Document 3) provide threaded joints using wedge-shaped threads. Disclose. The thread width of the wedge-shaped screw gradually changes along the spiral direction. The wedge-shaped screw is also called a dovetail type, and high torque performance can be obtained. However, none of Patent Documents 1 to 3 describes the rate of change in the thread width of the wedge-shaped screw.
[0007]
　Japanese Patent Application Laid-Open No. 2012-512347 (Patent Document 4) also discloses a threaded joint using a wedge-shaped screw. Near both ends of the male screw region, the reed between the male stubing flanks and the reed between the male road flanks are both constant. Similarly, the reeds between the female stubing flanks and the reeds between the female load flanks are both constant near both ends of the female thread region. Therefore, the thread width is constant near both ends of the threaded region. It is acknowledged that there is a difference between the lead between the road flanks and the lead between the stubing flanks, but no specific numerical value for the difference is given.
[0008]
　The present specification is incorporated by reference from the following prior art documents.
[0009]
Patent Document 1: US Reissue Patent No. 30647,
Patent Document 2: US Patent No. 6158785,
Patent Document 3: International Publication WO 2015/194193
Patent Document 4: Japanese Patent Application Laid-Open No. 2012-512347
Summary of disclosure
[0010]
　Since the load surface and the insertion surface of the wedge-shaped screw have a negative flank angle, the wedge-shaped screw exhibits high torque performance by caulking at the time of fastening. Also, wedge threads may be narrowed as the thread width approaches the tip of the pin or box to facilitate fastening. In other words, there is a difference between the load surface pitch and the insertion surface pitch. This pitch difference is called "delta reed". The delta lead determines the thread width near the tip of the pin and box.
[0011]
　Considering the influence of the absolute value of the screw pitch, the "Wedge Ratio" may be used instead of the delta lead. The wedge ratio is the delta lead divided by the load surface pitch and is expressed as a percentage as the ratio of the delta lead to the load surface pitch.
[0012]
　A large wedge ratio means that the rate of decrease in thread width is also large. When the wedge ratio is large, the thread width becomes narrow near the tip of the pin and the box. If the thread width is narrow, the wedge-shaped screw cannot withstand a large tensile load, and the thread itself may be destroyed. Therefore, care must be taken when setting the wedge ratio. Hereinafter, the performance that a wedge-shaped screw can withstand a tensile load is referred to as "tensile performance".
[0013]
　The above-mentioned Patent Document 4 (Japanese Patent Laid-Open No. 2012-512347) discloses the optimization of the wedge ratio. However, there is no literature that evaluates the effect of wedge ratio on torque performance in addition to tensile performance.
[0014]
　An object of the present disclosure is to provide a threaded joint for steel pipes that can achieve both high torque performance and high tensile performance.
[0015]
　As a result of diligent studies on an appropriate wedge ratio that improves both torque performance and tensile performance, the present inventors have found that high torque performance and high tensile performance can be achieved at the same time by changing the wedge ratio.
[0016]
　The threaded joint for steel pipe according to the present disclosure includes a tubular pin and a tubular box. The tubular pin is formed at one end of the steel pipe. The tubular box is fastened to the pin by inserting the pin. The pin includes a male screw. The male screw is formed on the outer circumference of the pin and is composed of a wedge-shaped screw. The box contains a female screw. The female thread corresponds to the male thread and is formed on the inner circumference of the box and is composed of a wedge-shaped thread. The threaded joint satisfies the following equation (1).
[0017]
　3% ≤ (LP-SP) / LP ≤ 8% (1)
[0018]
　In equation (1), LP is the pitch between the load planes of the male screw. SP is the pitch between the insertion surfaces of the male screw.
A brief description of the drawing
[0019]
FIG. 1 is a vertical cross-sectional view of the threaded joint for steel pipe according to the embodiment along the pipe axis direction.
FIG. 2 is an enlarged vertical sectional view of a male screw and a female screw in FIG. 1.
FIG. 3 is a graph showing the relationship between the wedge ratio and the yield torque when the load surface pitch is 8.64 mm.
FIG. 4 is a graph showing the relationship between the wedge ratio and the yield torque when the load surface pitch is 10.8 mm.
FIG. 5 is a graph showing the relationship between the wedge ratio and the yield torque when the load surface pitch is 7.2 mm.
FIG. 6 is a graph showing the relationship between the wedge ratio and the equivalent plastic strain when the load surface pitch is 8.64 mm.
FIG. 7 is a graph showing the relationship between the wedge ratio and the equivalent plastic strain when the load surface pitch is 10.8 mm.
FIG. 8 is a graph showing the relationship between the wedge ratio and the equivalent plastic strain when the load surface pitch is 7.2 mm.
Mode for carrying out the invention
[0020]
　The threaded joint for steel pipe according to the present embodiment includes a tubular pin and a tubular box. The tubular pin is formed at one end of the steel pipe. The tubular box is fastened to the pin by inserting the pin. The pin includes a male screw. The male screw is formed on the outer circumference of the pin and is composed of a wedge-shaped screw. The box contains a female screw. The female thread corresponds to the male thread and is formed on the inner circumference of the box and is composed of a wedge-shaped thread. The threaded joint satisfies the following equation (1).
[0021]
　3% ≤ (LP-SP) / LP ≤ 8% (1)
[0022]
　In equation (1), LP is the pitch between the load planes of the male screw. SP is the pitch between the insertion surfaces of the male screw.
[0023]
　Preferably, the threaded joint satisfies the following equation (2).
[0024]
　4% ≤ (LP-SP) / LP ≤ 7% (2)
[0025]
　The threaded joint may satisfy the following equation (3).
[0026]
　-10 degrees ≤ α ≤ -1 degree (3)
[0027]
　In the formula (3), α is the flank angle of the load surface and the insertion surface of the male screw.
[0028]
　The male and female threads may include a complete threaded portion composed of a complete thread. The fully threaded portion may have a length of 40 to 60 mm in the axial direction of the steel pipe.
[0029]
　Hereinafter, the threaded joint for steel pipes according to the present embodiment will be described with reference to the drawings. The same and corresponding configurations are designated by the same reference numerals in the drawings, and the same description is not repeated.
[0030]
　With reference to FIG. 1, the steel pipe threaded joint 1 according to the present embodiment includes a tubular pin 10 and a tubular box 20. The pin 10 is formed at one tip of the steel pipe 2. The box 20 is fastened to the pin 10 by inserting the pin 10. Hereinafter, a portion other than the tip end portion of the steel pipe 2 may be particularly referred to as a “steel pipe body”.
[0031]
　The pin 10 includes a male screw 11. The male screw 11 is formed on the outer circumference of the pin 10. The box 20 includes a female screw 21. The female screw 21 corresponds to the male screw 11 and is formed on the inner circumference of the box 20. More specifically, the male screw 11 is spirally formed on the outer circumference of the pin 10. The female screw 21 is spirally formed on the inner circumference of the box 20. The male screw 11 and the female screw 21 are composed of tapered threads. The male thread 11 and the female thread 21 are also composed of wedge-shaped threads.
[0032]
　With reference to FIG. 2, the load surface 111 of the male screw 11 and the load surface 211 of the female screw 21 have a flank angle α. The insertion surface 112 of the male screw 11 and the insertion surface 212 of the female screw 21 have a flank angle β. The flank angle α is the angle of the load surfaces 111, 211 with respect to the plane VP perpendicular to the pipe axis (axis of the steel pipe 2) TA. The flank angle β is the angle of the insertion surfaces 112 and 212 with respect to the plane VP perpendicular to the tube axis TA. When the load surfaces 111, 211 or the insertion surfaces 112, 212 are parallel to the plane VP, the flank angle is 0 degrees. When the load surface 111 of the male screw 11 is tilted toward the tip of the pin 10 with respect to the flat surface VP (in other words, when the load surface 211 of the female screw 21 is tilted toward the tip of the box 20 with respect to the flat surface VP). The flank angles α of the load planes 111 and 211 are positive. On the contrary, when the load surface 111 of the male screw 11 is tilted toward the steel pipe body of the pin 10 with respect to the flat surface VP (in other words, the load surface 211 of the female screw 21 is tilted toward the steel pipe body of the box 20 with respect to the flat surface VP). ), The flank angle α of the load planes 111, 211 is negative. Further, when the insertion surface 112 of the male screw 11 is tilted toward the steel pipe body side of the pin 10 with respect to the flat surface VP (in other words, the insertion surface 212 of the female screw 21 is tilted toward the pipe body side of the box 20 with respect to the flat surface VP). If yes), the flank angles of the insertion surfaces 112 and 212 are positive. On the contrary, when the insertion surface 112 of the male screw 11 is tilted toward the tip end side of the pin 10 with respect to the flat surface VP (in other words, the insertion surface 212 of the female screw 21 is tilted toward the tip end side of the box 20 with respect to the flat surface VP). Case), the flank angles of the insertion surfaces 112 and 212 are negative. The flank angles α and β of the wedge-shaped screw are both negative.
[0033]
　Although not particularly limited, it is preferable that the male screw 11 and the female screw 21 are all composed of complete threads and that there are no incomplete threads. If all the screws 11 and 21 are composed of complete screws, the contact area between the male screw 11 and the female screw 21 increases, and the torque performance is improved. The length of the completely threaded portion (male screw 11 and female screw 21 composed of perfect threads) is, for example, 40 to 60 mm.
[0034]
　The threaded joint 1 for steel pipe satisfies the following equation (1).
[0035]
　3% ≤ (LP-SP) / LP ≤ 8% (1)
[0036]
　Preferably, the threaded joint 1 for steel pipe satisfies the following equation (2).
[0037]
　4% ≤ (LP-SP) / LP ≤ 7% (2)
[0038]
　In the formulas (1) and (2), LP is a pitch between the load surfaces 111 of the male screw 11 (hereinafter, referred to as “load surface pitch”). SP is the pitch between the insertion surfaces 112 of the male screw 11 (hereinafter, referred to as “insertion surface pitch”). (LP-SP) / LP represents the wedge ratio. The load surface pitch LP is equal to the pitch between the load surfaces 211 of the female screw 21. The insertion surface pitch SP is equal to the pitch between the insertion surfaces 212 of the female screw 21.
[0039]
　That is, the upper limit of the wedge ratio is 8%, preferably 7%. The lower limit of the wedge ratio is 3%, preferably 4%.
[0040]
　The threaded joint 1 for steel pipe satisfies the following equation (3).
[0041]
　-10 degrees ≤ α ≤ -1 degree and -10 degrees ≤ β ≤ -1 degree (3)
[0042]
　In the formula (3), α is the flank angle of the load surface 111 of the male screw 11. β is the flank angle of the insertion surface 112 of the male screw 11. The flank angle α of the load surface 111 of the male screw 11 may be the same as or different from the flank angle β of the insertion surface 112 of the male screw 11. The flank angle α of the load surface 111 of the male screw 11 is substantially the same as the flank angle α of the load surface 211 of the female screw 21. The flank angle β of the insertion surface 112 of the male screw 11 is substantially the same as the flank angle β of the insertion surface 212 of the female screw 21.
[0043]
　Strictly speaking, the values ​​before fastening are used for the load surface pitch LP, the insertion surface pitch SP, and the flank angles α and β.
[0044]
　In the present embodiment, since the male screw 11 and the female screw 21 are composed of wedge-shaped screws and the wedge ratio thereof is set to 3 to 8%, both high torque performance and high tensile performance can be achieved at the same time.
[0045]
　The screw joint 1 may be a coupling type or an integral type. The coupling type threaded joint comprises two pins and a coupling. One pin is formed at the tip of one steel pipe. The other pin is formed at the tip of the other steel pipe. The coupling includes two boxes. One box is formed at one end of the coupling. The other box is formed at the other end of the coupling. One box is inserted with one pin and fastened to the other pin. The other box is formed on the opposite side of one box and the other pin is inserted and fastened to the other pin. On the other hand, the integral type threaded joint is for connecting two steel pipes to each other, and includes a pin and a box. In an integral type threaded joint, one steel pipe has a pin and the other steel pipe 2 has a box.
[0046]
　Although the embodiments have been described above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the spirit of the embodiments.
Example
[0047]
　In order to verify the effect of this embodiment, the torque performance and the tensile performance were evaluated by the finite element method (FEM). The evaluation target was a wedge-shaped threaded joint, and the following steel pipes were used.
[0048]
　Size: 9-5 / 8 inch (Pipe body outer diameter: 244.48 mm, Pipe body inner diameter: 216.8 mm)
　Material: API standard oil well pipe material L80 (Nominal load capacity YS = 552 MPa (80 ksi))
　Thread taper: 1 / 12
　Thread length: 50 mm (pin), 60 mm (box)
　Thread height: 1.8 mm
　Frank angle: -5 degrees (both load surface and insertion surface)
　Load surface pitch: 7.2 mm, 8.64 mm, or 10.8mm
　Wedge ratio: 2-10%
　Insertion surface pitch: Back calculation according to wedge ratio
[0049]
　As shown in FIG. 1, the threaded joint to be evaluated is composed of only male threads 11 and female threads 21. The male thread 11 and the female thread 21 are all composed of wedge-shaped threads and complete threads.
[0050]
　Table 1 shows the dimensions and the like of 27 types of threaded joints (samples) used in the analysis.
[table 1]

[0051]
　At the time of analysis, the dimensions of the male screw 11 and the female screw 21 were changed based on the threaded joint 1 shown in FIG. 1, and the torque performance and the tensile performance were evaluated.
[0052]
　[Evaluation of
　torque performance ] Regarding the torque performance, the value MTV (Maximum Torque Value) at which the fastening torque starts to yield in the fastening torque diagram is defined as the yield torque and evaluated by that value.
[0053]
　[Evaluation of
　tensile performance ] Regarding the tensile performance, a load equivalent to the tensile load at which the threaded joint 1 yields is applied to the fastened threaded joint, and the load of the thread located on the most tip side of the male thread 11 and the female thread 21 is applied. The maximum value of the equivalent plastic strain generated at the bases of the surfaces 111, 211 and the insertion surfaces 112, 212 was evaluated. From the experience of the present inventors from the actual tube test, when the equivalent plastic strain is about 0.08, the risk of thread breakage increases. Therefore, the threshold value of the equivalent plastic strain was set to 0.08, and if the value was lower than this, it was evaluated that the tensile performance was excellent. However, the threshold value of the equivalent plastic strain may be set to 0.070 with a margin on the safe side.
[0054]
　[Analysis Results]
　FIGS. 3 to 5 show the yield torque values ​​obtained by the finite element analysis. The wedge ratio is plotted on the horizontal axis, and the corresponding MTV value is plotted on the vertical axis. Regardless of the thread pitch, MTV increased with wedge ratio, especially in the 2-3% range. As can be confirmed in FIGS. 3 and 5, the MTV reached its maximum when the wedge ratio was around 9%, and then turned downward.
[0055]
　The following points can be considered as factors that increased the torque performance. It is considered that when the wedge ratio is high, the thread width becomes narrow near the tip of the pin 10, and a high contact pressure is generated by tightening the pin 10 having a narrow thread width with the box 20 having a wide thread width.
[0056]
　As described above, FIGS. 6 to 8 are graphs showing the relationship between the maximum value of the equivalent plastic strain generated when a tensile load is applied to the fastened screw joint 1 and the wedge ratio. This equivalent plastic strain occurs at the bases of the load surfaces 111, 211 and the insertion surfaces 112, 212 of the screws located on the most tip side of the male screw 11 and the female screw 21.
[0057]
　As shown in FIG. 6, when the load surface pitch LP = 8.64 mm and the wedge ratio is 9% or more, the maximum value of the equivalent plastic strain generated in the male screw exceeds 0.070 and the wedge ratio becomes 10%. Then, it was found that the maximum value of the equivalent plastic strain exceeds 0.080.
[0058]
　As shown in FIG. 7, when the load surface pitch LP = 10.8 mm, the equivalent plastic strain did not reach 0.070 even when the wedge ratio became 10%. However, as the wedge ratio increased, the equivalent plastic strain generated in the male screw tended to increase sharply.
[0059]
　As shown in FIG. 8, when the load surface pitch LP = 7.2 mm, when the wedge ratio is 9% or more, the maximum value of the equivalent plastic strain generated in the male screw exceeds 0.080, and the wedge ratio becomes 10%. Then, it was found that both the male screw and the female screw exceeded 0.080, and the possibility that the screw was broken was high.
[0060]
　From the above results, the higher the wedge ratio, the better in order to improve the torque performance. However, as mentioned above, if the wedge ratio is too high, there is a high risk that the thread near the tip of the pin (male thread) and / or box (female thread) will be destroyed, so the wedge ratio should be set to 8% or less. good. Further, a decrease in the thread width is equal to an increase in the thread bottom width, which leads to an increase in the number of passes during thread cutting and a decrease in the life of the insert. Therefore, an extremely high wedge ratio is not desirable from the viewpoint of manufacturing. From the above, the appropriate wedge ratio was 3 to 8%.
Code description
[0061]
1: Threaded joint for steel pipe
10: Pin
11: Male thread
20: Box
21: Female thread
111, 211: Load surface
112, 212: Insertion surface
LP: Load surface pitch
SP: Insertion surface pitch
The scope of the claims
[Claim 1]
　A threaded joint for steel pipes,
　and the pin of the tubular formed on one end portion of the steel pipe,
　and a tubular box which pin is fastened to the pin is inserted,
　said pins,
　the outer periphery of the pin The
　box includes a male screw formed and composed of a wedge-shaped screw, and the box
　corresponds to the male screw and includes a female screw formed on the inner circumference of the box and composed of a wedge-shaped screw,
　and satisfies the following equation (1). , Threaded joints for steel pipes.
　3% ≤ (LP-SP) / LP ≤ 8% (1) In
　equation (1), LP is the pitch between the load surfaces of the male screw, and SP is the pitch between the insertion surfaces of the male screw.
[Claim 2]
　The threaded joint for steel pipes according to claim 1,
　which satisfies the following formula (2).
　4% ≤ (LP-SP) / LP ≤ 7% (2)
[Claim 3]
　The threaded joint for steel pipe according to claim 1 or 2,
　which satisfies the following formula (3).
　-10 degrees ≤ α ≤ -1 degree and -10 degrees ≤ β ≤ -1 degree (3) In
　equation (3), α is the flank angle of the load surface of the male screw, and β is the insertion surface of the male screw. Frank angle.
[Claim 4]
　The threaded joint for a steel pipe according to any one of claims 1 to 3,
　wherein the male thread and the female thread include a complete threaded portion composed of a complete thread, and the
　complete threaded portion is a shaft of the steel pipe. A threaded joint for steel pipe having a length of 40 to 60 mm in the direction.