Threaded connection for pipe

Abstract

In a threaded connection for pipe (1) with a two-step thread construction, including intermediate shoulders (23, 33), during stabbing, the internal-pressure sealing surface (25) of the pin near its tip is prevented from contacting the intermediate shoulder (33) of the box (3) and thus receiving damage. Lp, LB, hp and hB shown in FIG. 2 satisfy Expressions (1) and (2), provided below. 8seal is the angle of slope of the straight line connecting the two ends, as determined along the axial direction, of the internal-pressure sealing surface of the pin. Expression (3), provided below, is also satisfied. LP < LB...(1) hB < hP + (LB-LP) X tan θseal....(2) LSP < LB−LP ... (3) (Typical Drawing: Fig. 2)

Description

Pipe threaded joints

[0001] The present disclosure relates to a tubular threaded joint used to connect steel pipes and the like.

[0002] In oil wells, natural gas wells, and the like (hereinafter collectively referred to as "oil wells"), casings are used to construct multiple stages of well walls to extract underground resources, and tubing is placed inside the casings to produce oil and gas. These casings and tubing are made up of a number of steel pipes connected in series, and pipe thread joints are used to connect them. Steel pipes used in oil wells are also called oil country tubular goods.

[0003] Tubular threaded joints are broadly divided into integral and coupling types. Integral-type tubular threaded joints are disclosed, for example, in Patent Documents 1 and 2 listed below, and coupling-type tubular threaded joints are disclosed, for example, in Patent Document 3 listed below.

[0004] In the integral type, oil well pipes are directly connected to each other. Specifically, an internal thread portion is provided at one end of the oil well pipe and an external thread portion is provided at the other end, and the oil well pipes are connected to each other by screwing the external thread portion of one oil well pipe into the internal thread portion of another oil well pipe.

[0005] In the coupling type, oil well pipes are connected to each other via a tubular coupling. Specifically, female threads are provided at both ends of the coupling, and male threads are provided at both ends of the oil well pipes. One male thread of one oil well pipe is screwed into one female thread of the coupling, and one male thread of another oil well pipe is screwed into the other female thread of the coupling, thereby connecting the oil well pipes to each other via the coupling. That is, in the coupling type, one of a pair of pipe materials that are directly connected is an oil well pipe, and the other is a coupling.

[0006] Generally, the end of an oil country tubular good on which a male thread is formed is called a pin because it includes an element that is inserted into a female thread formed on the oil country tubular good or a coupling, and the end of an oil country tubular good or a coupling on which a female thread is formed is called a box because it includes an element that receives the male thread formed on the end of the oil country tubular good.

[0007] In recent years, the development of even higher-temperature, higher-pressure, deeper wells has progressed. In deep wells, the complexity of the depth distribution of geological formation pressures necessitates an increased number of casing stages. Therefore, threaded joints are sometimes used in which the maximum outer diameter of the joint, i.e., the outer diameter of the box, is approximately the same as the outer diameter of the OCTG body. A threaded joint in which the outer diameter of the box is approximately the same as the outer diameter of the OCTG body is also called a flush-type threaded joint. A threaded joint in which the outer diameter of the box is approximately less than 108% of the outer diameter of the OCTG body is also called a semi-flush-type threaded joint. These flush-type and semi-flush-type threaded joints not only require high strength and sealing performance, but also have strict dimensional constraints on each part in order to accommodate the thread and seal structures within the limited pipe wall thickness.

[0008] Flush and semi-flush threaded joints, which have significant dimensional constraints, often employ a joint design in which an intermediate shoulder is provided in the axial middle of the joint, with threaded portions located before and after the shoulder to form the male and female threads using a two-stage thread.In threaded joints with such a two-stage thread structure, as disclosed in Patent Documents 1 and 2, in order to ensure sealing performance against internal pressure and sealing performance against external pressure under strict dimensional constraints, an internal pressure seal is provided near the tip of the pin, and an external pressure seal is provided near the open end of the box.

[0009] It should be noted that both Patent Documents 1 and 2 are applications filed by the applicants of the present application, and the drawings attached to these applications are schematic diagrams for clearly explaining the features of the respective inventions, and the dimensions of the parts unrelated to the features are not accurate. Therefore, it should be noted that technical features not explicitly disclosed in the specifications of these applications should not be extracted from the dimensions on the drawings of parts not explained in the specifications.

[0010] Patent Publication No. 2018-536818 International Publication No. 2018 / 211873 Japanese Patent Application Laid-Open No. 2012-149760

[0011] In the case of a threaded joint with a two-stage thread structure, a box intermediate shoulder is present near the axial center of the box, and when inserting the pin into the box or when aligning the pin just before the male thread begins to be threaded into the female thread (i.e., before thread interference begins), there is a risk that the pin internal seal surface (pin seal surface for internal pressure) provided near the tip of the pin will collide with the corner of the box intermediate shoulder and be damaged.

[0012] During actual drilling site work, the pin is stabbed after the stabbing guide is attached to the open end of the box, so the box outer seal surface (external pressure box seal surface) located near the open end of the box is protected by the stabbing guide and is unlikely to be damaged by impact with the pin intermediate shoulder.However, even when a stabbing guide is used, there is a risk that the pin inner seal surface will come into contact with the box intermediate shoulder.

[0013] Generally, the internal pressure in the environment in which oil well tubular goods are used is higher than the external pressure. Therefore, in threaded joints with a two-stage thread structure in which an outer seal seals out external pressure and an inner seal seals out internal pressure, it is important to avoid damage to the inner seal.

[0014] An object of the present disclosure is to prevent the box intermediate shoulder from hitting the pin internal seal surface during stabbing in a tubular threaded joint having a two-stage thread structure, thereby making the pin internal seal less likely to be damaged.

[0015] A tubular threaded joint according to the present disclosure is composed of a tubular pin and a tubular box, and the pin and the box are fastened together by threading the pin into the box.

[0016] The pin includes a first male thread, a second male thread that is provided distally of the first male thread and has a smaller diameter than the first male thread, a pin intermediate shoulder that is provided between the first male thread and the second male thread, a pin internal seal surface that is provided distally of the second male thread, and a pin external seal surface that is provided proximal to the first male thread.

[0017] The box includes a first female thread into which the first male thread fits in a fastened state, a second female thread into which the second male thread fits in a fastened state, a box intermediate shoulder provided between the first female thread and the second female thread and contacting the pin intermediate shoulder in the fastened state, a box inner seal surface that contacts the pin inner seal surface in the fastened state, and a box outer seal surface that contacts the pin outer seal surface in the fastened state.

[0018] A tubular threaded joint according to the present disclosure satisfies the following expressions (1) and (2).

[0019]

[0020]

[0021] Here, L Ｐ is the axial distance between the end of the pin inner seal surface on the pin intermediate shoulder side and the radially outer end of the pin intermediate shoulder, L Ｂ is the axial distance between the end of the box outer seal surface on the box intermediate shoulder side and the radially inner end of the box intermediate shoulder, h Ｐ is the radial distance between the end of the pin inner seal surface on the pin intermediate shoulder side and the radially outer end of the pin intermediate shoulder, h Ｂ is the radial distance between the end of the box outer seal surface on the box intermediate shoulder side and the radially inner end of the box intermediate shoulder, θ seal is the gradient angle of the line connecting both axial ends of the pin inner seal surface.

[0022] Preferably, the slope angle θ of a line connecting the end of the pin inner seal surface on the pin intermediate shoulder side and the radially outer end of the pin intermediate shoulder is Ｐ The gradient angle θ of the straight line connecting the end of the box outer seal surface on the box intermediate shoulder side and the radially inner end of the box intermediate shoulder Ｂ More preferably, the gradient angle θ Ｐ is less than 6°.

[0023] Also, L Ｐ L Ｂ The present disclosure is particularly useful when the ratio is greater than 94%.

[0024] Also, according to the present disclosure, h Ｂ Ga h Ｐ Even if the pressure is greater than h, damage to the seal point SP where the seal contact pressure is the greatest on the seal surface inside the pin can be avoided. Ｂ Since there is a design margin for the first female thread 31 provided within the radial range and the corresponding first male thread 21, this margin can be allocated to various performances.

[0025] In order to more reliably protect the seal point SP, the seal point SP where the seal contact force is highest due to contact with the seal surface inside the box at the time of completion of fastening, and the end P of the seal surface inside the pin on the pin middle shoulder side are １ The axial distance L between ＳＰ But, L Ｂ and L Ｐ It is preferable that the difference between

[0026] According to the present disclosure, it is possible to prevent the pin inner seal surface from colliding with the box intermediate shoulder and being damaged during stabbing, and to prevent a decrease in internal pressure sealing performance after make-up.

[0027] Fig. 1 is a longitudinal sectional view of a tubular threaded joint according to an embodiment. Fig. 2 is a diagram illustrating the dimensions of characteristic portions of a pin and a box. Fig. 3 is an enlarged view of the vicinity of the tip of the pin. Fig. 4 is a sectional view illustrating the state during stabbing.

[0028] As illustrated in Figures 1 to 4, a tubular threaded joint 1 according to this embodiment is composed of a tubular pin 2 and a tubular box 3. The pin 2 and box 3 are fastened together by threading the pin 2 into the box 3. The pin 2 is provided at the end of the first pipe P1, and the box 3 is provided at the end of the second pipe P2. The first pipe P1 may be a long pipe such as an oil country tubular good. The second pipe may be a coupling for connecting long pipes together, but is preferably a long pipe such as an oil country tubular good. In other words, the tubular threaded joint 1 according to this embodiment is preferably an integral type tubular joint. The oil country tubular good and the coupling are typically made of steel, but may also be made of metal such as stainless steel or a nickel-based alloy.

[0029] The pin 2 may be formed on one end of the first oil country tubular good P1 that has been reduced in diameter. The box 3 may be formed on one end of the second oil country tubular good P2 that has been expanded in diameter. Preferably, the pin 2 can be formed on one end of each of the oil country tubular good P1, P2, and the box 3 can be formed on the other end. More specifically, the first oil country tubular good P1 can be manufactured by reducing the diameter of one end of a mother pipe made of a long pipe, and then cutting the outer periphery of the reduced end to form a component of the pin 2. The second oil country tubular good P2 can be manufactured by expanding the diameter of one end of a mother pipe made of a long pipe, and then cutting the inner periphery of the expanded end to form a component of the box 3. This allows the wall thicknesses of the pin 2 and the box 3 to be ensured in a semi-flush type integral threaded joint.

[0030] In this specification, the portions of the oil country tubular goods P1, P2 other than the pins 2 and the boxes 3 that have not been subjected to diameter reduction or diameter expansion are referred to as the "pipe main body." The pipe end side of the pin 2 refers to the direction from the pipe main body of the pin 2 toward the pipe end of the pin 2, and is sometimes referred to as the "tip side." The pipe main body side of the pin 2 refers to the direction from the pipe end of the pin 2 toward the pipe main body of the pin 2, and is sometimes referred to as the "base side." The open end side of the box 3 refers to the direction from the pipe main body of the box 3 toward the open end of the box 3.

[0031] The pin 2 may include a first male thread 21, a second male thread 22 that is provided closer to the pipe end of the pin 2 than the first male thread 21 and has a smaller diameter than the first male thread 21, a pin intermediate shoulder 23 provided between the first male thread 21 and the second male thread 22, a pin end shoulder 24 provided at the pipe end of the pin 2, a pin internal seal surface 25 provided between the second male thread 22 and the pin end shoulder 24, and a pin external seal surface 26 provided between the first male thread 21 and the pipe body of the pin 2. The first male thread 21 and the second male thread 22 are spaced apart in the axial direction, and the pin intermediate shoulder 23 may be provided between them.

[0032] Preferably, the first and second male threads 21, 22 are each tapered threads. Preferably, the first and second male threads 21, 22 have the same thread taper angle and the same thread pitch. Preferably, the taper generatrix of the tapered thread that constitutes the second male thread 22 is located radially inward of the taper generatrix of the tapered thread that constitutes the first male thread 21. The pin intermediate shoulder 23 can be formed by a side surface of a step formed on the outer periphery of the pin between the first male thread 21 and the second male thread 22. The pin intermediate shoulder 23 has a pin intermediate shoulder surface facing the pipe end side of the pin 2. The first and second male threads 21, 22 may each be a trapezoidal thread, an API round thread, an API buttress thread, a wedge thread, or the like.

[0033] The box 3 may include a first female thread 31 into which the first male thread 21 mates when fastening is complete, a second female thread 32 into which the second male thread 22 mates when fastening is complete, a box intermediate shoulder 33 that contacts the pin intermediate shoulder 23 when fastening is complete, a box end shoulder 34 provided corresponding to the pin end shoulder 24, a box internal seal surface 35 provided between the second female thread 32 and the box end shoulder 34 and that contacts the pin internal seal surface 25 over its entire circumference when fastening is complete, and a box external seal surface 36 provided between the first female thread 31 and the box open end and that contacts the pin external seal surface 26 over its entire circumference when fastening is complete. The pin internal seal surface 25 and the box internal seal surface 35 can function primarily as internal pressure seals to provide sealing performance against internal pressure. The pin external seal surface 26 and the box external seal surface 36 can function primarily as external pressure seals to provide sealing performance against external pressure.

[0034] The first female thread 31 and the second female thread 32 are spaced apart in the axial direction, and a box intermediate shoulder 33 may be provided therebetween. Preferably, the first and second female threads 31, 32 are tapered threads that fit the first and second male threads 21, 22, respectively. The box intermediate shoulder 33 may be configured by a step formed on the inner circumference of the box 3 between the first female thread 31 and the second female thread 32. The box intermediate shoulder 33 has a box intermediate shoulder surface facing the open end of the box 3 and faces the pin intermediate shoulder surface of the pin intermediate shoulder 23. The box intermediate shoulder 33 contacts the pin intermediate shoulder 23 at least when make-up is complete, and these intermediate shoulders 23, 33 function as torque shoulders for demonstrating torque performance. The first and second female threads 31, 32 may be trapezoidal threads, API round threads, API buttress threads, wedge threads, or the like that match the first and second male threads 21, 22, respectively.

[0035] 1 at the time of completion of make-up, the pin end shoulder 24 may be spaced apart from the box end shoulder 34, or may be in contact with the box end shoulder 34. If these end shoulders 24, 34 are spaced apart at the time of completion of make-up, when a predetermined axial compressive load smaller than the yield compressive load of the threaded joint is applied, the end shoulders 24, 34 may come into contact with each other due to elastic deformation of the pin 2 and the box 3, and a configuration may be adopted in which they bear part of the axial compressive load.

[0036] The intermediate shoulders 23, 33 of the pin 2 and the box 3 are formed by flat surfaces perpendicular to the tube axis, but may also be formed by tapered surfaces inclined more toward the tube end of the pin 2 at their radially outer ends than at their radially inner ends.

[0037] The longitudinal cross-sectional shape of each seal surface 25, 35, 26, 36 may be any suitable shape. In the threaded joint 1 shown in Figures 1 to 4, each seal surface is configured as a tapered surface that slopes linearly in the longitudinal cross section. Alternatively, one of the seal surfaces that contact each other can be configured as a convex curved surface, or both seal surfaces can be configured as convex curved surfaces. Each seal surface can also be configured by combining a convex curved surface with a tapered surface having a linear generatrix. In either case, each seal surface is configured so that the amount of seal interference increases as the pin 2 is pushed further into the box 3. That is, both the pin internal seal surface 25 and the box external seal surface 26 are tapered, with the diameter gradually decreasing toward the pin tip. The box internal seal surface 35 is tapered to fit the pin internal seal surface 25, and the box external seal surface 36 is tapered to fit the pin external seal surface 26.

[0038] The gradient of the line connecting both axial ends of each seal surface is preferably 5% or more (taper ratio of 10%), more preferably 10% (taper ratio of 20%). The gradient of the line connecting both axial ends of each seal surface is preferably 25% or less (taper ratio of 50%), more preferably 17% or less (taper ratio of 34%). The gradient angle θ of the line connecting both axial ends of the pin internal seal surface 25 seal is approximately 2.9° for a 5% gradient, approximately 5.7° for a 10% gradient, approximately 9.7° for a 17% gradient, and approximately 14.0° for a 25% gradient.

[0039] The end P of the pin inner seal surface 25 on the pin intermediate shoulder 23 side １ and the radially outer end P of the pin intermediate shoulder 23 ２ The axial distance L between Ｐ Although it depends on the pipe diameter size, it may be, for example, 65 to 95 mm. Ｐ The ratio may be between 20 and 45%.

[0040] The end P of the box outer seal surface 36 on the box intermediate shoulder 33 side ２ and the radially inner end P of the box intermediate shoulder 33４ The axial distance LB between the box intermediate shoulder 33 and the inner end of the box intermediate shoulder 33 may be, for example, 70 to 100 mm, depending on the pipe diameter size, etc. Ｂ The ratio of L may be 20 to 45%. Ｂ L for Ｐ The ratio may be between 94% and 98%.

[0041] The end P of the pin inner seal surface 25 on the pin intermediate shoulder 23 side １ and the radially outer end P of the pin intermediate shoulder 23 ２ The radial distance h between Ｐ Although it depends on the pipe diameter size, it may be, for example, 5.0 to 8.5 mm.

[0042] The end P of the box outer seal surface 36 on the box intermediate shoulder 33 side ３ and the radially inner end P of the box intermediate shoulder 33 ４ The radial distance h between Ｂ Although it depends on the pipe diameter size, it may be, for example, 5.0 to 8.5 mm. Ｐ and Ｂ is equal to.

[0043] The end P of the pin inner seal surface 25 on the pin intermediate shoulder 33 side １ and the radially outer end P of the pin intermediate shoulder 33 ２ The slope angle θ of the line connecting Ｐ is preferably less than 6°, and more preferably less than 5.5°. ３ and the radially inner end P of the box intermediate shoulder 33 ４ The slope angle θ of the line connecting Ｂ is preferably less than 5.5°, more preferably less than 5.3°.

[0044] 3 shows the seal contact pressure distribution generated on the pin internal seal surface 25 in the fastened state and the seal point SP where the seal contact pressure peaks. The shape of the pin 2 before fastening is shown by phantom lines, and the fastened state in which the pin 2 is slightly deformed radially inward due to interference between the seal surfaces 25, 35 is shown by solid lines.

[0045] Since the internal pressure sealing performance is mainly exhibited at the seal point SP, it is important to avoid damage to the pin internal seal surface 25, especially in the vicinity of the seal point SP, in order to maintain the internal pressure sealing performance. On the other hand, at a position away from the seal point SP, i.e., the end P of the pin internal seal surface on the pin intermediate shoulder 23 side, １ The closer the end P is to the seal contact pressure, the less the seal contact pressure increases, so some surface damage can be tolerated. １ It is believed that the internal pressure sealing performance can be maintained by designing the valve so as to avoid damage to the seal point SP located radially inward of the valve.

[0046] When the pin 2 is inserted into the box 3, as shown in FIG. ＰＩＮ is the tube axis CL of box 3 ＢＯＸ The radially outer end P of the pin intermediate shoulder 23 is misaligned with respect to the center of the pin. ２ is the end P of the box outer sealing surface 36 ３ In this state, there is a high possibility that the pin internal seal surface 25 will interfere with the box intermediate shoulder 33. In order to avoid damage to at least the seal point SP of the pin internal seal surface 25 in this state, first, １ must be positioned closer to the open end of the box than the radially inner end of the box intermediate shoulder 33. This leads to the above formula (1). Although a stabbing guide is not shown in FIG. 4, a stabbing guide can be used if necessary.

[0047] The pin inner seal surface 25 has an end P １ 4, the end P of the pin seal surface 25 is １ and the radially inner end P of the box intermediate shoulder 33 ４ the axial distance between the pin and the inclination angle θ of the pin inner seal surface 25, i.e., (LB-LP) seal Using this, it can be deduced that if the above formula (2) is satisfied, the box intermediate shoulder 33 is less likely to hit the pin internal seal surface 25.

[0048] To more reliably avoid damage to the seal point SP, it is sufficient that the seal point SP is located closer to the box opening end than the box intermediate shoulder 33 in the situation shown in Figure 4. From this, the seal point SP, where the seal contact force is highest due to contact with the box inner seal surface 35 among the pin inner seal surfaces 25 at the time of completion of fastening, and the end P １ The axial distance L between ＳＰ But, L Ｂ and L Ｐ This leads to the condition that the difference between

[0049] If the above conditions are met, h Ｂ Ga h Ｐ Even if the distance between the box intermediate shoulder 33 and the seal point SP of the pin internal seal surface 25 is larger than the above, it is possible to prevent the box intermediate shoulder 33 from coming into contact with the seal point SP of the pin internal seal surface 25 during stabbing.

[0050] In addition, the end P of the pin inner seal surface 25 １ and the radially outer end P of the pin intermediate shoulder 23 ２ The slope angle θ of the line connecting Ｐ is the end P of the box outer sealing surface 36 ３ and the radially inner end P of the box intermediate shoulder 33 ４ The slope angle θ of the line connecting Ｂ 4, the end P of the pin inner seal surface 25 is preferably larger than １ the end P of the box outer sealing surface 36 ３ and the radially inner end P of the box intermediate shoulder 33 ４ and the box intermediate shoulder 33 can be positioned radially inward from the line connecting the pin inner seal surface 25, thereby further reducing the possibility of the box intermediate shoulder 33 coming into contact with the pin inner seal surface 25.

[0051] Furthermore, the gradient angle θ Ｐ is preferably less than 6°. This makes it possible to avoid damage to the pin internal seal surface 25 while limiting the range of thickness in which the second male thread 22 and the pin intermediate shoulder 23 are provided.

[0052] In addition, L Ｐ Is L ＢThe thread engagement state and strength of each of the two-stage thread structures can be made uniform, and a threaded joint 1 with a good overall balance can be provided.

[0053] The present disclosure may be applied to not only integral-type but also coupling-type threaded joints. Furthermore, the present disclosure is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims.

[0054] 1: Pipe threaded joint 2: Pin, 21: First male thread, 22: Second male thread, 23: Pin intermediate shoulder, 25: Pin inner seal surface, 26: Pin outer seal surface 3: Box, 31: First female thread, 32: Second female thread, 33: Box intermediate shoulder, 35: Box inner seal surface, 36: Box outer seal surface

Claims

1. A pipe screw joint comprising a tubular pin and a tubular box, wherein the pin is screwed into the box and the pin and the box are fastened. The pin includes a first male thread, a second male thread provided on the tip side of the first male thread and having a smaller diameter than the first male thread, a pin intermediate shoulder provided between the first male thread and the second male thread, a pin inner seal surface provided on the tip side of the second male thread, and a pin outer seal surface provided on the base end side of the first male thread. The box includes a first female thread into which the first male thread fits in the fastened state, a second female thread into which the second male thread fits in the fastened state, a box intermediate shoulder provided between the first female thread and the second female thread and contacting the pin intermediate shoulder in the fastened state, a box inner seal surface contacting the pin inner seal surface in the fastened state, and a box outer seal surface contacting the pin outer seal surface in the fastened state. The pipe screw joint satisfies the following formulas (1) and (2). Here, L Ｐ is the axial distance between the end of the pin inner seal surface on the pin intermediate shoulder side and the radially outer end of the pin intermediate shoulder, L Ｂ is the axial distance between the end of the box outer seal surface on the box intermediate shoulder side and the radially inner end of the box intermediate shoulder, h Ｐ is the radial distance between the end of the pin inner seal surface on the pin intermediate shoulder side and the radially outer end of the pin intermediate shoulder, h Ｂ is the radial distance between the end of the box outer seal surface on the box intermediate shoulder side and the radially inner end of the box intermediate shoulder, θ seal is the angle of inclination of the straight line connecting both axial ends of the pin inner seal surface.

2. In the pipe screw joint according to claim 1, the angle of inclination θ of the straight line connecting the end on the pin intermediate shoulder side of the pin inner seal surface and the radially outer end of the pin intermediate shoulder Ｐ is larger than the angle of inclination θ of the straight line connecting the end on the box intermediate shoulder side of the box outer seal surface and the radially inner end of the box intermediate shoulder. Ｂ Pipe screw joint.

3. In the pipe screw joint according to claim 2, the hook angle θ Ｐ is less than 6°, the pipe screw joint.

4. In the pipe screw joint according to claim 1, 2 or 3, L Ｐ is greater than 94% of L Ｂ The pipe screw joint.

5. In the pipe screw joint according to any one of claims 1 to 4, h Ｂ where h Ｐ is larger than h, a pipe screw joint.

6. In the pipe screw joint according to any one of claims 1 to 5, at the time of completion of fastening, a seal point where the seal contact force is highest due to contact between the seal surface inside the pin and the seal surface inside the box, and an axial distance L between the seal point and an end portion of the seal surface inside the pin on the side of the pin intermediate shoulder ＳＰ is such that L Ｂ and L Ｐ is smaller than the difference between them, the pipe screw joint.