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, also collectively referred to as “oil wells”), steel pipes called OCTG (Oil Country Tubular Goods) are used to mine underground resources. Steel pipes are sequentially connected. A threaded joint is used for the connection.
[0003]
　The types of threaded joints for steel pipes are roughly classified into coupling type and integral type. In the case of a coupling type screw joint, one pipe member is a steel pipe and the other pipe member is a coupling among a pair of pipe members to be connected. In this case, male screw portions are formed on the outer circumferences of both ends of the steel pipe, and female screw portions are formed on the inner circumferences of both end portions of the coupling. Then, the steel pipe and the coupling are connected. In the case of integral type threaded joints, the pair of pipe members to be connected are both steel pipes, and separate couplings are not used. In this case, a male screw portion is formed on the outer circumference of one end of the steel pipe, and a female screw portion is formed on the inner circumference of the other end. Then, one steel pipe and the other steel pipe are connected.
[0004]
　The joint portion of the pipe end formed with the external thread is called a pin because it includes an element to be inserted into the internal thread. On the other hand, the joint portion of the pipe end portion formed with the female thread portion is referred to as a box because it includes an element for receiving the male thread portion. These pins and boxes are tubular because they are the ends of the tubing.
[0005]
　The thread of the threaded joint for steel pipes is a taper thread. Therefore, a tapered male screw portion is formed on the pin. The box is formed with a taper female screw part that meshes with the taper male screw part.
[0006]
　As a basic performance, threaded joints are required to have sealing performance. Moreover, seizure resistance is required for threaded joints. This is because if seizure occurs when the pin is screwed into the box during fastening, the meshing of the screw parts becomes insufficient and the desired sealing performance cannot be obtained. Furthermore, in recent years, threaded joints are required to have fatigue resistance. This is because a large bending load is repeatedly applied to the threaded joint due to the severer oil well environment and the adoption of new drilling technology.
[0007]
　Techniques for improving the fatigue resistance of threaded joints are disclosed in, for example, Japanese Patent Laid-Open No. 2005-221038 (Patent Document 1) and Japanese Patent Publication No. 2010-537135 (Patent Document 2). In the techniques disclosed in Patent Documents 1 and 2, shot peening is performed on a specific part of the surface of the screw part (male screw part and female screw part). The hardness of the specific portion subjected to shot peening increases, and compressive residual stress is introduced into the surface layer of the specific portion. As a result, fatigue strength increases.
Prior art documents
Patent literature
[0008]
Patent Document 1: Japanese Patent Laid-Open No. 2005-221038
Patent Document 2: Japanese Patent Publication No. 2010-537135
Summary of the invention
Problems to be Solved by the Invention
[0009]
　In the case of the threaded joints disclosed in Patent Documents 1 and 2, it is necessary to apply shot peening to only specific portions of the threaded portion (male threaded portion and female threaded portion). This is for the following reason. Generally, the surface roughness of the area subjected to shot peening is large. Therefore, if shot peening is applied to a portion other than the specific portion, seizure may occur during fastening. However, it is extremely difficult to uniformly perform shot peening only on a specific portion of a threaded portion having a complicated shape.
[0010]
　An object of the present disclosure is to provide a threaded joint for steel pipes having the following characteristics:
　to improve seizure resistance and fatigue resistance.
Means for solving the problems
[0011]
　A threaded joint for steel pipes according to an embodiment of the present disclosure includes a tubular pin and a tubular box. The pin includes a tapered external thread and the box includes a tapered internal thread that meshes with the tapered external thread. The taper male screw part is divided into a first region on the tip end side of the pin and a second region on the pipe body side of the pin along the pipe axis direction. The taper rate of the first region is larger than the taper rate of the second region. The taper ratio of the first region is larger than the taper ratio of the taper female screw portion. The taper ratio of the second region is the same as or larger than the taper ratio of the taper female thread portion.
Effect of the invention
[0012]
　The threaded joint for steel pipes according to the embodiment of the present disclosure has the following remarkable effects: It is
　possible to improve fatigue resistance while ensuring seizure resistance.
Brief description of the drawings
[0013]
FIG. 1 is a vertical cross-sectional view showing a typical example of a threaded joint for steel pipes.
FIG. 2 is an enlarged vertical cross-sectional view of the threaded region of the threaded joint shown in FIG. 1.
FIG. 3 is a diagram schematically showing a threaded joint in which the taper ratio of the male screw portion is smaller than the taper ratio of the female screw portion.
FIG. 4 is a diagram schematically showing a threaded joint in which the taper ratio of the male screw portion is the same as the taper ratio of the female screw portion.
FIG. 5 is a diagram schematically showing a threaded joint in which the taper of the male screw portion is a one-step taper and the taper ratio of the male screw portion is larger than the taper ratio of the female screw portion.
FIG. 6 is a diagram schematically showing a threaded joint in which the taper of the male screw part is a two-step taper and the taper ratio of the male screw part is larger than the taper ratio of the female screw part.
MODE FOR CARRYING OUT THE INVENTION
[0014]
　In order to achieve the above object, the present inventors paid attention to the shape of the threaded portion and made various studies. As a result, the following findings were obtained.
[0015]
　FIG. 1 is a vertical sectional view showing a typical example of a threaded joint for steel pipes. FIG. 2 is an enlarged vertical cross-sectional view of the threaded region of the threaded joint shown in FIG. FIG. 1 shows a coupling type threaded joint. The threaded joint is composed of a pin 10 and a box 20. The thread of the threaded joint is a taper thread.
[0016]
　The pin 10 includes a tapered male screw portion (hereinafter, also simply referred to as “male screw portion”) 11. The box 20 includes a taper female screw portion (hereinafter, also simply referred to as “female screw portion”) 21 corresponding to the male screw portion 11 of the pin 10.
[0017]
　The male screw portion 11 of the pin 10 includes a thread crest surface 11a, a thread root surface 11b, an insertion flank surface 11c, and a load flank surface 11d. On the other hand, the female thread portion 21 of the box 20 includes a thread crest surface 21a, a thread root surface 21b, an insertion flank surface 21c, and a load flank surface 21d. The thread crest surface 11 a of the male thread portion 11 faces the thread root bottom surface 21 b of the female thread portion 21. The bottom surface 11 b of the thread root of the male threaded portion 11 faces the crest surface 21 a of the female threaded portion 21. The insertion flank surface 11 c of the male threaded portion 11 faces the insertion flank surface 21 c of the female threaded portion 21. The load flank surface 11d of the male screw portion 11 faces the load flank surface 21d of the female screw portion 21. The flank angles of the load flanks 11d and 21d are negative angles, and the load flanks 11d and 21d are strongly pressed against each other in the fastening state. The flank angles of the insertion flanks 11c and 21c are regular angles.
[0018]
　In the fastened state, the male screw portion 11 and the female screw portion 21 mesh with each other. That is, the bottom surface 11 b of the thread root of the male threaded portion 11 and the crest surface 21 a of the thread crest 21 of the female threaded portion 21 come into contact with each other. The load flank surface 11d of the male screw portion 11 and the load flank surface 21d of the female screw portion 21 contact each other. A gap is formed between the thread crest surface 11 a of the male thread portion 11 and the thread root 21 b of the female thread portion 21. A gap is formed between the insertion flank surface 11 c of the male threaded portion 11 and the insertion flank surface 21 c of the female threaded portion 21. The lubricant is filled in these gaps. As a result, a screw seal portion is formed by the engagement of the male screw portion 11 and the female screw portion 21. In the fastened state, the male screw portion 11 and the female screw portion 21 are meshed with each other with a predetermined amount of interference.
[0019]
　In the threaded joint shown in FIG. 1, the shoulder surface 12 is provided at the tip of the pin 10. The box 20 is provided with a shoulder surface 22 corresponding to the shoulder surface 12 of the pin 10. In the tightened state, the shoulder surface 12 of the pin 10 and the shoulder surface 22 of the box 20 are in contact with each other. As a result, a tightening axial force is applied to the load flank surface 11d of the male screw portion 11 of the pin 10.
[0020]
　In the threaded joint shown in FIG. 1, a seal surface 13 is provided between the shoulder surface 12 of the pin 10 and the male screw portion 11. A sealing surface 23 is provided between the shoulder surface 22 of the box 20 and the female screw portion 21. In the tightened state, the sealing surface 13 of the pin 10 and the sealing surface 23 of the box 20 are in contact with each other. As a result, a seal portion is formed by the contact between the seal surface 13 and the seal surface 23.
[0021]
　Here, with respect to the threaded joints shown in FIGS. 1 and 2, the present inventors examined four types of threaded joints having different threaded portions. 3 to 6 schematically show the shape of the threaded portion of each threaded joint. In any of the threaded joints shown in FIGS. 3 to 6, the taper rate Tb of the female threaded portion 21 is constant over the entire threaded portion. That is, the taper of the female screw portion 21 is a one-step taper.
[0022]
　In the threaded joints shown in FIGS. 3 to 5, the taper rate Tp of the male threaded portion 11 is constant over the entire area of ​​the threaded portion. That is, the taper of the male screw portion 11 is a one-step taper. On the other hand, in the threaded joint shown in FIG. 6, the male threaded portion 11 is divided into two regions 15 and 16 along the pipe axis CL direction, and the taper rate Tp (Tp1 and Tp2) of the male threaded portion 11 changes midway. ing. That is, the male screw portion 11 is divided into the first region 15 on the tip end side of the pin and the second region 16 on the tube body side of the pin, and the taper of the male screw portion 11 (the first region 15 and the second region 16) is 2. It has a step taper. 3 to 6, the total length of the male screw portion 11 in the tube axis CL direction is indicated by L. In FIG. 6, the distance in the tube axis CL direction from the end of the male threaded portion 11 on the tip side of the pin to the boundary between the first region 15 and the second region 16 is indicated by x.
[0023]
　More specifically, in the threaded joint shown in FIG. 3, the taper rate Tp of the male threaded portion 11 is smaller than the taper rate Tb of the female threaded portion 21. Hereinafter, the threaded joint shown in FIG. 3 is also referred to as a PSBF (Pin-Slow Box-Fast) taper threaded joint. In the general threaded joint shown in FIG. 4, the taper rate Tp of the male threaded portion 11 is the same as the taper rate Tb of the female threaded portion 21. Hereinafter, the threaded joint shown in FIG. 4 is also referred to as a parallelell taper threaded joint. In the threaded joint shown in FIG. 5, the taper rate Tp of the male threaded portion 11 is larger than the taper rate Tb of the female threaded portion 21. Hereinafter, the threaded joint shown in FIG. 5 is also referred to as a PFBS (Pin-Fast Box-Slow) one-step taper threaded joint. On the other hand, in the threaded joint shown in FIG. 6, the taper rate Tp (Tp1 and Tp2) of the male screw part 11 (the first region 15 and the second region 16) is larger than the taper ratio Tb of the female screw part 21, and The taper rate Tp1 of the region 15 is larger than the taper rate Tp2 of the second region 16. Hereinafter, the threaded joint shown in FIG. 6 is also referred to as a PFBS two-step taper threaded joint.
[0024]
　Samples were prepared for each of the PSBF taper, parallel taper, PFBS one-step taper, and PFBS two-step taper threaded joints, and a bending fatigue test was performed. A fatigue test can evaluate the fatigue resistance of a threaded joint. The main characteristics regarding the material and dimensions of the sample used in this test are as follows.
-Dimensions: 9-5/8 [inch], 47 [lb/ft] (outer diameter 244.48 mm, wall thickness 11.99 mm)-Material
grade: carbon steel with a yield stress of 80 ksi (552 MPa) to 95 ksi (655 MPa) ( API standard L80)
[0025]
　Here, in the PSBF taper threaded joint shown in FIG. 3, the taper rate Tp of the male threaded portion 11 is set to 5.4%, and the taper rate Tb of the female threaded portion 21 is set to 5.7%. In the parallel taper threaded joint shown in FIG. 4, the taper ratio Tp of the male screw portion 11 and the taper ratio Tb of the female screw portion 21 are 5.55%. In the PFBS single-step taper threaded joint shown in FIG. 5, the taper rate Tp of the male threaded portion 11 is set to 5.7% and the taper rate Tb of the female threaded portion 21 is set to 5.4%. In the PFBS two-step taper screw joint shown in FIG. 6, the taper ratio Tp1 of the first region 15 is 5.7%, the taper ratio Tp2 of the second region 16 is 5.55%, and the taper ratio Tb of the female screw portion 21 is It was set to 5.4%. Further, in the male screw portion 11 including the first region 15 and the second region 16, x is set to 45% of L.
[0026]
　Samples were prepared for threaded joints of PSBF taper, parallel taper, PFBS one-step taper, and PFBS two-step taper, and a make/break test in which fastening and removal were repeated was performed. The seizure resistance can be evaluated by a make/break test. The main characteristics regarding the material and size of the sample used in this test are as follows. In this test, the tightening torque at the time of tightening was set to 23,650 (-1,000/+0) [ft-lbs] (32,065 (-1,356/+0) Nm). -Dimensions
: 7 [inch], 35 [lb/ft] (outer diameter 177.8 mm, wall thickness 12.65 mm)
-Material grade: nickel-based alloy with yield stress of 110 ksi (758 MPa) to 140 ksi (965 MPa)
[0027]
　As a result of the fatigue test, the fatigue strength of the PFBS one-step taper screw joint (see FIG. 5) and the fatigue strength of the PFBS two-step taper screw joint (see FIG. 6) were the same, and these fatigue strengths were the highest. The fatigue strength of the PSBF taper threaded joint (see FIG. 3) was the lowest. In the threaded joints with high fatigue strength such as parallel taper, PFBS one-step taper, and PFBS two-step taper, cracks occurred at the thread roots in the incomplete thread area in the area of ​​the male thread. The incompletely threaded area of ​​the male thread is the area of ​​the pin on the tube body side. On the other hand, in the PSBF taper threaded joint having the lowest fatigue strength, in the area of ​​the male threaded portion, a crack was generated due to stress concentration at the corners of the thread root and the load flank in the completely threaded area.
[0028]
　On the other hand, as a result of the make/break test, the number of make/breaks of the threaded joint of the PFBS single-step taper (see FIG. 5) was the smallest. The PSBF taper threaded joint (see FIG. 3) had the highest number of make/breaks. The number of make/breaks of the PFBS two-stage taper screw joint (see FIG. 6) was equal to the number of make/breaks of the parallel taper screw joint (see FIG. 4). In particular, the number of make/breaks of the threaded joint of the PFBS two-step taper was 5 times the number of make/breaks of the threaded joint of the PFBS one-step taper.
[0029]
　From the above test results, the following is shown. The PFBS single-stage taper threaded joint has superior fatigue resistance performance compared to the parallel taper threaded joint and the PSBF taper threaded joint. The PFBS two-step taper threaded joint has seizure resistance equivalent to that of a parallel taper threaded joint, and further has excellent fatigue resistance compared to a parallel taper threaded joint and a PSBF taper threaded joint.
[0030]
　Even if the taper of the male threaded portion 11 is a one-step taper and the taper of the female threaded portion 21 is a two-step taper, the fatigue resistance and seizure resistance equivalent to those of the PFBS two-step taper (see FIG. 6) are to be achieved. May be possible. However, in this case, the following problems occur. The internal thread portion cuts up quickly, and a sufficient effective length of the internal thread portion cannot be secured. Therefore, when a tensile load is applied to the threaded joint, a phenomenon in which the screw slips off, that is, jump-out is likely to occur. The sealing performance of the screw part is also reduced. It is also difficult to thread.
[0031]
　The threaded joint for steel pipes of the present disclosure has been completed based on the above findings.
[0032]
　A threaded joint for steel pipes according to an embodiment of the present disclosure includes a tubular pin and a tubular box. The pin includes a tapered external thread and the box includes a tapered internal thread that meshes with the tapered external thread. The taper male screw part is divided into a first region on the tip end side of the pin and a second region on the pipe body side of the pin along the pipe axis direction. The taper rate of the first region is larger than the taper rate of the second region. The taper ratio of the first region is larger than the taper ratio of the taper female screw portion. The taper ratio of the second region is the same as or larger than the taper ratio of the taper female thread portion.
[0033]
　According to such a threaded joint, the taper ratio of the first region of the male screw portion is larger than the taper ratio of the female screw portion, and the taper ratio of the second region of the male screw portion is equal to or larger than the taper ratio of the female screw portion. Fatigue performance can be improved. Furthermore, since the taper rate of the first region is larger than the taper rate of the second region, seizure resistance can be ensured.
[0034]
　As a typical example, the threaded joint of the present embodiment is a coupling type threaded joint. However, the type of the threaded joint is not particularly limited, and may be an integral type.
[0035]
　As a typical example, the taper screw including the male screw portion and the female screw portion is a buttress type taper screw. Buttress type taper screws include a simple trapezoidal screw whose load flank surface is inclined at a positive angle and a special trapezoidal screw whose load flank surface is inclined at a negative angle. These trapezoidal screws include taper screws defined by the API standard.
[0036]
　In the case of a threaded joint to which a buttress type taper screw is applied, the bottom surface of the thread root of the male thread portion (first and second regions) and the thread crest surface of the female thread portion are in contact with each other in the fastening state. The load flank surface of the male screw portion contacts the load flank surface of the female screw portion. A gap is formed between the thread crest surface of the male threaded portion and the thread root bottom of the female threaded portion. A gap is formed between the insertion flank surface of the male screw portion and the insertion flank surface of the female screw portion.
[0037]
　However, the meshing state of the male screw portion and the female screw portion is not particularly limited as long as the load flank surfaces contact each other. For example, instead of contacting the bottom surface of the male thread portion with the thread crest surface of the female thread portion, the thread crest surface of the male thread portion may contact with the bottom surface of the female thread portion. In short, it is sufficient that the male screw portion and the female screw portion are engaged with each other with a predetermined interference amount in the fastening state.
[0038]
　In the above threaded joint, when the taper ratio of the first region is Tp1, the taper ratio of the second region is Tp2, and the taper ratio of the taper female thread portion is Tb, Tp1, Tp2, and Tb are expressed by the formula (1), ( It is preferable that the conditions 2) and 3) are satisfied.
　1.00