EA044818B1 - THREADED CONNECTION FOR STEEL PIPE - Google Patents

Description

Field of technology to which the invention relates

The invention relates to a threaded connection for a steel pipe used for connecting steel pipes.

BACKGROUND OF THE INVENTION

Steel pipes, called oil well pipes, are used, for example, for exploration or production of oil, natural gas wells, etc. (hereinafter collectively referred to as oil wells or the like), for the development of unconventional resources such as oil sands or shale gas, for the extraction or storage of carbon dioxide (carbon capture and storage (CCS)), for geothermal power generation, or in hot springs. A threaded connection is used to connect steel pipes. These steel pipe threaded connections are generally classified as coupling type and integral type.

In the case of coupling type connection, steel pipes are connected using a tubular coupling. Generally, an internal thread is provided on the inner periphery of each of the ends of the coupling, while an external thread is provided on the outer periphery of each of the ends of each steel pipe. Then, one end of one steel pipe is screwed into one end of the coupling, and one end of the other steel pipe is screwed into the other end of the coupling, so that the steel pipes are connected. That is, a coupling type connection directly connects a pair of pipes, one of which is a steel pipe, while the other is a coupling.

The integral type connection directly connects the steel pipes and does not use a separate coupling. Specifically, an internal thread is provided on the inner periphery of one end of each steel pipe, while an external thread is provided on the outer periphery of the other end of each steel pipe; into one end of one steel pipe equipped with an internal thread, the other end of another steel pipe equipped with an external thread is screwed in, so that the steel pipes are connected.

The male threaded end of a steel pipe pipe includes a member that is to be inserted into the internal threaded end of a steel pipe or coupling, and is thus commonly referred to as a nipple. The pipe end of a steel pipe or coupling on which an internal thread is provided includes a member for receiving the pipe end of a steel pipe on which an external thread is provided, and is thus called a coupling. The nipple and coupling form the ends of the pipes and are thus tubular in shape.

In recent years, well development techniques such as DwC (drilling with casing) and horizontal drilling have become popular, leading to a rapid increase in the demand for high torque connections. The inventor of the present invention has manufactured high torque connections for relatively small diameter steel pipes using a tapered dovetail thread which may be called a wedge thread. Patent Document 1 below, for example, discloses such a high torque connection.

A wedge thread has a thread profile with a pitch of the embedded side smaller than the pitch of its supporting side, so that the width of the thread crest of the male thread profile of the pin gradually decreases as it moves toward the end portion along the helix thread line, and similarly, the width of the thread groove of the internal thread profile of the opposite coupling gradually decreases as you move towards the end part. Additionally, each of the supporting and embedded sides of each external and internal threads has a negative engagement angle; when the screwing of the pin and the coupling is completed, the supporting sides are in contact and the inserting sides are in contact, so that the threads with the male thread profile and the female thread profile are firmly engaged with each other. Thanks to this design, a threaded connection using a tapered thread can exhibit high torque resistance.

Patent Document 1 in FIG. 5 and in the description text discloses a method in which each thread portion connecting the support side and the crest surface, and the thread portion connecting the support side and the groove/groove surface, includes a curved portion consisting of two arcs with different radii of curvature to reduce load concentration at the base of the supporting side, thereby improving the fatigue performance of these connecting sections.

Prior Art Documents

Patent documents

Patent Document 1. Japan Application JP 2007-504420 A

The essence of the invention

Problems to be solved by invention

The present inventor is developing high torque threaded connections for even larger diameter steel pipes. When they designed larger diameter threaded connections of 9-5/8" or larger to the same design standards,

- 1 044818 that both conventional and complex load tests were carried out on prototypes complying with ISO 13679: 2011 series A, shear failure occurred in the male pin thread profile when maximum tensile load was applied. According to the usual design standards adopted by the present applicant, one of the two arcs forming the above-mentioned curved portion disclosed in Patent Document 1, which is adjacent to the support side (23), has a radius of curvature of 0.125 mm, and the arc adjacent to the surface of the groove/thread root or surface (24) of the thread tip, has a radius of curvature of 0.875 mm.

It is an object of the present disclosure to provide a threaded connection for large diameter steel pipe that exhibits high torque resistance as well as shear resistance that is commensurate with the size of the steel pipes being connected.

Problem solver.

The inventors of the present invention have conducted extensive research to determine the causes of thread failure with a male thread profile of a high torque threaded connection for large diameter steel pipes, and have found that with respect to the tensile strength or in other words, the shear resistance required for a large diameter steel pipe diameter, the shear strength of the thread measured at the end of the male thread profile associated with the pin end portion is insufficient so that shear failure occurs, with the origin point being the root of the thread at the end of the male thread profile associated with the pin end portion. That is, if shear failure first occurs in the thread section for one thread starting at the end of the male thread profile associated with the pin end (hereinafter referred to as the first thread), the load is concentrated on the second thread, i.e. the next turn of the thread, located inward (i.e., in addition to the body of the steel pipe), when viewed in cross-section, which causes a shift in the second turn of the thread; after shear failure in the second thread, the load is concentrated in the third thread, i.e. the next turn of the thread, located inward, which causes shear failure here; Thus, shear failures propagate in the threads, one thread after another. This is believed to be how shear failure occurs in threads over a wide area of a male thread profile.

In a typical trapezoidal male thread profile, the application of excessive tensile load may cause radial deformation of the pin and box and thus cause pop-out, but will rarely cause thread failure over a wide area. On the other hand, in a high torque threaded connection, as discussed above, the male and female thread profiles do not disengage because the threads of the male and female dovetail thread profiles are tightly engaged.

Accordingly, in a high-torque threaded connection with male and female thread profiles, each of which is a wedge thread with a dovetail section shape, the stiffness of the first thread where shear failure occurs first is critical to providing resistance to pop-out and shear resistance.

To improve the shear resistance of the end of the male thread profile associated with the pin end portion, the present inventors have focused on the radius of curvature of the boundary portion between the support side and the thread groove/root surface in the end portion of the male thread profile associated with the pin end portion. Shear failure in a male thread profile, as discussed above, is believed to be caused by stress concentration at the interface between the bearing side and the thread groove/groove surface at the end portion of the male thread profile associated with the pin end portion; thus, increasing the radius of curvature of this boundary region is expected to reduce the stress concentration and improve the shear strength.

Additionally, it is considered that the larger the thread height, the larger the bending moment acting on the boundary portion becomes, even if the total number of uniformly distributed loads acting on the support side is the same, which means higher equivalent plastic strains in the boundary portion; Thus, it is assumed that the boundary portion should be formed by a curved surface with a suitable radius of curvature, which depends on the height of the thread.

Another consideration is that an excessively large thread height means a large cutting depth of the thread flute, which means poor machinability; Therefore, it is preferable that even in a threaded connection for steel pipes with large diameters, the thread height should not exceed 3.0 mm. Additionally, in order for the connection to exhibit sufficient torque resistance, it is necessary to provide sufficient contact areas for the support and embedded sides; thus, the thread height is preferably not less than 1.8 mm.

The inventors have comprehensively considered these technical data, and have arrived at a threaded connection for a steel pipe according to the present disclosure. That is, the threaded connection for a steel pipe according to the present disclosure includes: a tubular nipple provided on an end portion of the steel pipe; and a tubular coupling configured to be screwed onto the nipple as the nipple is screwed thereon. The nipple includes a male thread profile formed

- 2 044818 on the outer periphery of the nipple. The coupling includes an internal thread profile formed on the inner periphery of the coupling and configured to engage with the male thread profile when the connection is screwed together. Each of the male thread profile and the internal thread profile includes a support side, an embedded side, a thread crest surface and a thread groove/bottom surface, where the pitch of the embedded side of each of the male thread profile and the internal thread profile is less than the pitch of the support side of each of the profile. male thread and female thread profile and, when the connection is screwed together, the seat side of the male thread profile is in contact with the seat side of the female thread profile, and the embedded side of the male thread profile is in contact with the embedded side of the female thread profile. It is preferable that each of the support sides and the embedded sides of the male thread profile and the internal thread profile have a negative engagement angle.

The support side and the thread groove/groove surface of a portion of a male thread profile for a given range in the helix direction of the thread, starting from the end of the male thread profile associated with the end portion of the pin, can be connected by a first curved surface having a radius of curvature r1 when viewed longitudinally section satisfying the following expression (1):

ri > Th x 0.14 · ·· (1),

Here, Th is the thread height measured at the support side in the male thread profile measured over a predetermined range in the direction of the thread helix starting from the end of the male thread profile associated with the end portion of the pin. Preferably, the thread height Th satisfies the requirement of 1.8 mm < Th < 3.0 mm and, more preferably, satisfies the requirement of r1 > Thx0.16.

Results of the invention

According to the present disclosure, each male and female thread profile is formed by a tapered thread and thus exhibits high torque resistance, and the end portion of the male thread profile associated with the pin end portion can be provided with the shear resistance required of a large diameter steel pipe .

Brief description of drawings

Fig. 1 is a longitudinal sectional view of a threaded connection for a steel pipe according to a first embodiment along its pipe axis.

Fig. 2 is an enlarged longitudinal sectional view of an area of a male thread profile including first and second threads.

Fig. 3 is an enlarged longitudinal sectional view of a portion of a male thread profile including the first thread and adjacent the first thread.

Fig. 4 is an enlarged perspective view of the portions of the male thread profile that connect the first and second threads.

Fig. 5 is a longitudinal sectional view of a threaded connection for a steel pipe according to the second embodiment along its pipe axis.

Fig. 6 is a graph showing the results of calculation of equivalent plastic strain by finite element analysis.

Fig. 7 is a graph showing results of calculation of torque resistance by finite element analysis.

Embodiments for Carrying Out the Invention

The threaded connection for a steel pipe according to the present embodiment includes: a pipe nipple provided on an end portion of the steel pipe; and a tubular coupling configured to be screwed onto the nipple as the nipple is screwed thereon. The pin includes a male thread profile formed on the outer periphery of the pin. The coupling includes an internal thread profile formed on the inner periphery of the coupling and configured to engage with the male thread profile when the connection is screwed together.

Preferably, the external thread profile is formed by a tapered thread, the diameter of which gradually decreases towards the end portion of the pin. The male thread profile may include a full thread portion with a constant thread height, and a partial thread portion with a lower thread height than the full thread portion. The external thread profile is formed through a cutting process on the outer peripheral surface of the steel pipe, and the cutting depth of the thread groove gradually increases as it moves towards the end part, starting from the end of the thread profile associated with the pipe body of the steel pipe, ranging from zero to the thread height of the section with full-profile thread; Moreover, the partial thread section of the external thread profile is mainly composed of sections of the thread profile in which the cutting depth of the thread groove is less than the thread height of the section with a full-profile thread. In this design, the diameter of each of the thread crest surface and the thread groove/bottom surface of a male thread profile, measured at the full thread section, gradually decreases as it moves toward the end thread.

- 3 044818 parts of the pin in the direction of the helix of the thread, while the thread crest surface in the partly threaded section has a constant diameter, while the diameter of the thread groove/bottom surface measured in the partly threaded section gradually decreases as it moves towards the end parts of the nipple along the direction of the helix thread.

Preferably, the internal thread profile is composed of a tapered thread, the diameter of which gradually increases as it moves towards the end portion of the coupling (ie the pipe body of the steel pipe). The internal thread profile may include a full thread portion with a constant thread height, and a partial thread portion with a lower thread height than the full thread portion. Additionally, the thread height of the full thread portion of the internal thread profile may be slightly larger than the thread height of the full thread portion of the male thread profile. In such implementations, when the pin and box are screwed together, the thread crest surface of the internal thread profile is in contact with the thread groove/valley surface of the male thread profile, while a gap is formed between the thread crest surface of the male thread profile and the thread groove/valley surface of the internal thread profile. threads. This clearance prevents galling or galling during engagement of the male and female thread profiles and can also be advantageously used as a lubricant release passage.

The profile shape of the internal thread is formed by the cutting process on the inner peripheral surface of the coupling or steel pipe that forms the coupling. Preferably, the first thread groove of the internal thread profile, which engages when the pin and box are screwed together, with the first thread of the male thread profile, i.e. of the male thread in the full-profile portion located furthest to the end portion has a groove depth such that the radial size of the contact area for the support side of the male thread profile and the support side of the internal thread profile is not less than 60%, more preferably, not less than 70% of the height thread of the first thread of the external thread profile.

Each of the male and female thread profiles includes a support side, a pitch side, a thread crest surface, and a thread groove/bottom surface, and each of the support and base sides of the male and female thread profiles has a negative engagement angle. That is, each of the male and female thread profiles includes a dovetail thread groove and a dovetail thread thread when viewed in longitudinal section. In a longitudinal section of each of the external and internal thread profiles, a plurality of thread crest surfaces and a plurality of thread groove/groove surfaces appear, where each of the thread crest surfaces and each of the thread groove/groove surface may be formed when viewed in a longitudinal section so as to be parallel to the pipe axis of the steel pipe, or may be inclined relative to the pipe axis of the steel pipe at the tapering angle of the tapered thread. The support side engagement angle may, for example, have a predetermined value in the range of -10° to -1°, and more preferably, a predetermined value in the range of -4° to -6°. The engagement angle of the embedded side may, for example, have a predetermined value in the range of -10° to -1°, and more preferably, a predetermined value in the range of -4° to -6°. Each of the supporting and embedded sides of each turn of external and internal thread profiles can have a sectional shape that looks like a straight line in longitudinal section.

In the threaded connection for a steel pipe according to the present embodiment, the pitch of the embedded side of the external and internal thread profiles is smaller than the pitch of the supporting side of the external and internal thread profiles. Thus, the external thread profile is made up of a wedge thread having a thread width that decreases and the thread groove width that increases towards the end of the pin, and the internal thread profile is made up of a wedge thread having a thread width that decreases and a groove width thread, which increases as it moves towards the end of the coupling. The support side pitch may be, for example, a predetermined value in the range of 8.0 mm to 11.0 mm, and the embedded side pitch may be, for example, a predetermined value in the range of 7.5 mm to 10.5 mm. The difference in pitch between the pitch of the support side and the embedded side, Ap, can be, for example, from 0.3 mm to 0.6 mm.

As used herein, the term male thread profile means a portion of a male thread with a seat side in contact with the support side of a female thread, and a embedded side in contact with the embedded side of a female thread when the pin and box are screwed together, whether or not it is part of a full thread portion. or a section of incomplete thread. The term internal thread profile means a portion of an internal thread with the seat side in contact with the seat side of the male thread, and the embedded side in contact with the embedded side of the male thread when the pin and box are screwed together, regardless of whether it is part of a full thread portion or a partial thread portion. threads. It is possible to provide, located in addition to the end portion of the pin than the full-threaded portion of the male thread profile 11, a partially threaded portion 15 in which at least one of the insert and support sides is not in contact with the internal thread, as shown, for example, in FIG. 4; however, since such a section is incomplete

- 4 044818 thread does not contribute to torque resistance, it does not form part of the male thread in this opening. Additionally, it is possible to provide, located in addition to the end part of the coupling than the section with full-profile threads of the internal thread profile, a section with incomplete threads, in which at least one of the embedded and supporting sides is not in contact with the external thread; however, since such a portion of partial thread does not contribute to torque resistance, it does not constitute part of the internal thread in this disclosure.

In the threaded joint for a steel pipe according to the present embodiment, the support side and the thread groove/groove surface of the male thread profile portion for a predetermined range in the thread helix direction starting from the end of the male thread profile associated with the pin end portion are connected by a first curved surface, having a radius of curvature r1, when viewed in longitudinal section, satisfying expression (1) below. Preferably, the radially inner end of the first curved surface abuts smoothly the surface of the thread groove/groove of the male thread. Additionally, it is preferable that the radially outer end of the first curved surface smoothly abuts the support side of the male thread.

ri > Th x 0.14 · ·· (1).

Here, Th is the thread height measured on the reference side of the male thread profile, measured in a specified range in the direction of the thread helix, starting from the end of the male thread profile associated with the pin end, and satisfying the condition 1.8 mm < Th < 3, 0 mm. It is preferable that the thread(s) of the male thread profile located within a predetermined range in the direction of the helix of the thread, starting from the end of the male thread profile associated with the end portion, is/are part of the full thread portion of the male thread profile. If the thread height Th is less than 1.8 mm, this means that the contact area between the supporting sides of the male and female thread profile is small, which makes it impossible to obtain the required torque resistance. On the other hand, if the thread height Th is greater than 3.0 mm, it means a larger cutting depth, which in turn means an increase in cutting time and process costs.

For a thread height Th of 1.8 mm, the radius r1 of curvature is at least 0.252 mm, while for a thread height of 3.0 mm, the radius r1 of curvature is at least 0.42 mm; The inventors of the present invention have proposed for the first time the connection of the support side and the surface of the groove/thread root in a male thread profile with such large radii of curvature.

A first curved surface may be provided along the entire length of the male thread.

One way to improve the torque resistance is to make the bearing side pitch and the embedded side pitch relatively small, so as to make the number of threads appearing in the longitudinal section relatively large. Preferably, the male and female thread profiles may have thread profiles such that, when screwed together, the supporting and backing sides of that portion of the thread of the male thread profile that covers at least 8 threads, more preferably 9 threads, in the direction of the thread helix starting from the end of the external profile, connected to the end part of the nipple, are in contact with the support side and the embedded side of the internal thread profile. More preferably, the support side pitch can be no more than 8.50 mm, the embedded side pitch can be no more than 8.10 mm, the corresponding pitch difference can be no less than 0.35 mm and no more than 0.45 mm, and the minimum width threads, measured at the end of the male thread profile associated with the end of the pin, and measured at the root of the thread, may not be less than 2.0 mm. Additionally, it is preferable that the minimum thread width measured at the end of the internal thread profile associated with the end portion of the coupling and measured at the root of the thread is also not less than 2.1 mm. It is possible to provide, located in addition to the end part of the coupling than the end of the internal thread profile associated with the end part of the coupling, a section of incomplete thread, in which at least one of the embedded and supporting sides is not in contact with the external thread; however, since such a partial thread portion does not contribute to torque resistance, the thread width of the partial thread portion may be less than 2.00 mm depending on the cutting process conditions and other factors. The portion of the partial thread does not form part of the internal thread in this opening.

The first curved surface with a large radius of curvature may not be provided along the entire length of the male thread profile, but may be provided for at least 1/2 turn, more preferably at least one turn in the direction of the helix thread starting from the end of the external thread profile associated with the end part of the nipple. Thus, the base of the first thread of a male thread profile, at which the thread width is minimal, can be reinforced by the first curved surface in terms of shear stiffness, thereby allowing shear failure to occur at the first thread as its initiation point.

Additionally, as shown in FIG. 4, for example, the male thread profile 11 may include a first thread portion 111 having a first curved surface 111A and a second thread portion 112 adjacent to the first thread portion 111, which portions are located in the direction of the helix thread. Support side and thread groove/groove surface of the second section

- 5 044818

The threads 112 may be connected by a second curved surface 112A having a smaller radius of curvature r2 than the first curved surface 111A. It is preferable that the first and second curved surfaces 111A and 112A smoothly touch without a stepped portion at the boundary therebetween.

The first curved surface may be provided along at least x turns in the direction of the helix thread, starting from the end of the male thread profile associated with the end portion of the pin. Here x satisfies the following expression (2):

x=(rl - r2) / Ar · · · (2).

Here, Ap represents the difference in pitch between the pitch of the supporting side and the pitch of the embedded side of the external thread profile.

Since the male thread profile is configured in this way, in the second thread portion with a relatively small radius of curvature r2, the radius of curvature of the boundary portion between the support side and the thread crest surface of the portions of the internal thread profile of the coupling 20, which has a radius of curvature r2, is also relatively small, thereby providing a large contact area between the supporting sides of the male and female thread profiles, which is advantageous in terms of torque resistance. Additionally, the end portion of the second thread portion associated with the end portion of the pin is a portion of the second thread portion that has the smallest thread width; however, since expression (2) is satisfied, the smallest thread width in the second thread portion is approximately equal to the thread width in the end portion of the first thread portion associated with the end portion of the pin, in which the thread width measured at the root of the thread is increased by the first curved surface, having a large radius of curvature r1, thereby preventing shear failure with the initiation point at the end portion of the second thread portion associated with the end portion of the pin.

That is, if the support side engagement angle is about -10° to -1°, for example, as shown in FIG. 3, the first curved surface 111A connecting the support side with the thread groove/groove surface generally has a quarter-arc shape when viewed in a longitudinal section. Thus, the distance in the pipe axis direction between the radially inner end Pi and the radially outer end Po of the first curved surface 111A is substantially equal to the radius of curvature r1 of the first radius of curvature 111A. Fig. 3 shows imaginary lines that represent the second curved surface 112A' created when the end of the second thread portion associated with the pin end portion is located approximately half a turn in the helix thread direction from the end of the first thread portion 111 associated with the pin end portion. and a second curved surface 112A'' created when the end of the second thread portion associated with the pin end portion is located at a position of about 3/4 of a thread from the end of the first thread portion superimposed on the end portion of the first thread portion 111 associated with the end of the nipple to show the comparison between the width of the threads. Also, the axial distance between the radially inner end and the radially outer end of the second curved surface is substantially equal to the radius of curvature r2 of the second curved surface. The thread width changes by the pitch difference Ap between the pitches of the supporting side and the embedded side for one turn in the direction of the thread helix. Thus, for x turns, the thread width increases by Apxx. If the end of the second thread portion associated with the pin end portion is located at the x thread position, the thread width W2 measured using the radially inner end of the second curved surface (ie, the thread width measured using the position Pi in FIG. 3) may be represented by the following expression (3):

W2=W1 -rl+Apxx+r2 ·· (3) where W1 is the thread width measured at the end of the first thread portion associated with the pin end portion and measured using the radially inner end of the first curved surface (i.e., the width thread measured using the Pi position in Fig. 3). Here W1-r1 indicates the approximate position of Po. W1-r1+Apxx indicates a suitable position of the outer end of the second curved surface 112A', 112A'', designated Po. Expression (3) indicates the corresponding position of the inner end of the second curved surface 112', 112A'', designated Pi. The longer the screw length of the first threaded portion 111, the larger W2 becomes. Thus, depending on the length of the first threaded portion 111, W2 may be smaller than W1, or W2 may be larger than W1.

If W2 is smaller than W1, the second curved surface 112A' rises with a radius of curvature r2 that is small; As such, the width of the threads of those portions of the second threads that are associated with the second curved surface 112A' may be less than the width of the base of the first thread portion 111 (that is, as shown in Fig. 3, the curved surface 112A' may cut into the threads along compared to the surface of the curved surface 111A) and portions of the second thread portion with smaller thread widths may present weak points in terms of strength. In view of this, it is preferable that W2 is substantially equal to or greater than W1. Thus, from expression (3) and the condition W2 > W1, we can obtain x > (r1-r2)/Ap. That is, if x is defined as x=(r1-r2)/Ap, it is preferable that the first curved surface is provided at least x turns in the thread helix direction starting from the end of the male thread profile associated with the end portion nipple

For example, if the pitch of the supporting side is 9.845 mm, the pitch of the embedded side is 9.400 mm, the radius of curvature r1 of the first curved surface is 0.4 mm, and the radius of curvature r2 of the second curved surface is 0.1 mm, then it is preferable that the first curved surface be made along (0.4-0.1)/(9.845-9.400) ® 2/3 turns or longer. If the radius of curvature r2 of the second curved surface is 0.2 mm and other conditions are the same as above, it is preferable that the first curved surface is made at half a turn or longer.

In addition, since the male thread profile includes a second thread portion having a second curved surface with a small radius of curvature, and the radius of curvature between the abutment side and the thread crest surface in those thread portions of the internal thread profile that exist to engage the second thread portion is smaller, than the third curved surface discussed below, a larger contact area can be provided between the supporting sides as a whole, so that the connection can exhibit greater torque resistance.

Preferably, in a threaded connection for a steel pipe according to the present embodiment, the support side and the thread crest surface of the internal thread profile may be connected by a third curved surface that faces the first curved surface of the male thread profile when the connection is screwed, and has a radius of curvature larger than first curved surface. This will prevent the boundary/angle between the support side and the thread crest surface of the internal thread profile from interfering with the first curved surface.

In addition, a radial clearance may be provided between the thread crest surface of the internal thread profile portion that is in contact with the support side in the male thread profile end portion associated with the pin end portion when the joint is screwed on one side and the groove surface/ the root of the thread in the portion of the external thread profile that faces the surface of the thread crest. More preferably, the thread crest surface in the above-mentioned internal thread profile portion may be a thread crest surface in an end portion of the internal thread profile located at the innermost position with respect to the box on the internal thread profile. Additionally, the thread crest surface in the above-mentioned internal thread profile portion may have the same diameter as the inner peripheral surface of the tubular threadless extension located within the internal thread profile in relation to the coupling. Additionally, the pin may include a tubular threadless extension corresponding to the threadless coupling extension, wherein preferably the outer peripheral surface of the threadless pin extension is not in contact with the inner peripheral surface of the threadless coupling extension when the connection is screwed together. In addition, the pin preferably includes a pin sealing surface located additional to the end portion of the pin than the threadless pin extension, and the box preferably includes a box sealing surface located additional to the interior of the box than the threadless box extension. When the pin and coupling are screwed together, these sealing surfaces of the pin and coupling are in contact with each other, forming a metal-to-metal seal that can provide a seal under external and internal pressure. Threadless extensions are useful in preventing deformation caused by compressive and tensile loads acting on male and female thread profiles from the influence of pin and coupling sealing surfaces. Additionally, since the clearance described above is created by reducing the thread height of the profile portion of the internal thread compared to the thread height of the full thread portion of the internal thread, it can be ensured that the wall thickness of the pin portion including and near the end portion of the pin is greater than in implementations where The clearance is created by increasing the depth of the thread groove at and near the end of the pin. Additionally, the presence of the gap prevents direct contact of the thread crest of the internal thread profile with the thread base portion of the male thread profile support side at the end of the thread profile associated with the pin end portion where the thread width is minimal, thereby reducing direct damage to the weakest portion of the male thread profile .

In some implementations, separate external pressure seals and internal pressure seals may be provided, wherein the internal pressure seal may be located in addition to the pin end portion of the male and female thread profile, and the external pressure seal may be additionally provided to the pipe body, which is the profile of the external and internal threads.

Preferably, the radially outer end of the first curved surface is located radially outward from the radially inner end of the third curved surface portion that faces the first curved surface, and may be located radially inward from the radially outer end of the third curved surface portion that faces the first curved surface. This will minimize the distance between the radially inner end of the contact area between the support sides of the male and female threads and the radially outer end of the first curved surface, thereby further improving torque resistance.

A threaded connection for a steel pipe according to the present embodiment will now be described with reference to the drawings. Like and corresponding components in different drawings are designated by the same reference symbols, and the same description will not be repeated.

Referring to FIG. 1, a threaded connection for a steel pipe according to the present embodiment, designated 1, includes a tubular nipple 10 and a tubular coupling 20. The nipple 10 is formed on the end of the steel pipe 2. The coupling 20 is formed on the end of the coupling 3 into which the nipple 10 is inserted , so that the coupling is screwed onto the nipple 10. Sections of the steel pipe 2 other than the end section of the pipe may be referred to here as the body of the pipe.

The threaded connection for a steel pipe according to the present embodiment can be suitably used in implementations in which the outer diameter of the steel pipe body 2 is not less than 240 mm, and more preferably not less than 245 mm, and even more preferably not less than 270 mm. The connection can be suitably used in implementations in which the outer diameter of the steel pipe body 2 does not exceed 400 mm, more preferably does not exceed 350 mm, and even more preferably does not exceed 310 mm. It is preferable that the body of the steel pipe 2 has a wall thickness that is substantially constant along its entire axial length. Further, it is preferable that the steel pipe body 2 has an outer diameter OD and an inner diameter ID, each of which is substantially constant along the entire axial length. The nipple is located at the end of the pipe body of the steel pipe 2. FIG. 1 also shows the pipe CL axis of steel pipe 2.

The pin 10 includes a male thread profile 11 formed by a tapered thread with a diameter that gradually decreases as it moves toward the end portion of the pin and the edge 12. The male thread profile 11 is formed by a helical thread on the outer periphery of the pin 10. The male thread profile 11 The thread section is formed by a wedge-shaped thread with a thread width that gradually decreases as it moves toward the end portion of the pin 10. The external thread profile 11 has a dovetail cross-sectional shape formed by the crest of the thread and the groove of the thread. The edge 12 is adjacent to the male thread profile 11 with a threadless extension disposed therebetween, the threadless extension extending toward the pin end portion from the end of the male thread profile 11 associated with the pin tip portion. A pin sealing surface 13 is provided on the outer periphery of the edge 12. In the embodiment shown, the pin sealing surface 13 is composed of a cylindrical sealing surface that has an arcuate cross-section; alternatively, the pin sealing surface 13 may have a cross-section that appears as a straight line, or may have a combination of a straight line and an arc.

The coupling 20 includes an opening end for receiving a pin 10. The coupling 20 includes an internal thread profile 21 formed at its inner periphery by a tapered thread with a diameter that gradually decreases toward the end portion of the coupling, and a coupling sealing surface 22. The internal thread profile 21 is formed by a helix thread on the inner periphery of the coupling 20 to be complementary to the male thread profile 11. The internal thread profile 21 is formed by a wedge-shaped thread with a thread width that gradually increases as it moves from the opening end of the coupling 20 to its interior. The internal thread profile 21 has a dovetail cross-sectional shape formed by its thread crest and thread groove. The coupling sealing surface 22 is formed by a tapered surface located within the internal thread profile 21 relative to the coupling 20. The sealing surface of the coupling 22 may be formed by a cylindrical sealing surface that has an arcuate cross-section, or may have the shape of a combination of a straight line and an arc when viewed in cross-section. A predetermined amount of interference is established between the sealing surfaces 22 and 13 of the coupling and nipple, and when screwed together, the sealing surfaces 13 and 22 are in contact with each other around the entire circumference without play, to form a metal seal.

As shown in FIG. 1 and 2, the male thread profile 11 of the present embodiment includes a full thread portion and a partial thread portion. The section of the full-profile thread of the external thread profile 11 is a section with a given thread height Th, in which the thread is formed with a given pitch LP of the load support side and a given pitch SP of the embedded side. The thread height Th of the external thread profile of the embodiment shown is 2.2 mm.

The incompletely threaded portion of the male thread profile 11 is a portion in which a predetermined thread height Th is not provided due to the fact that the imaginary tapering surface forming the tapering shape of the tapered thread intersects the outer surface of the steel pipe 2 so that the cutting depth on the outer surface 2 steel pipe is insufficient. According to

- 8 044818 In this embodiment, both the pin section with a full thread and the pin section with a partial thread of the external thread profile 11 are in contact with the internal thread profile 21 on both their supporting and embedded sides. In the threaded connection 1 shown in FIG. 1, the pitch LP of the supporting side is 9.845 mm, the pitch SP of the embedded side is 9.400 mm, and the minimum thread width in the end portion of the male thread profile 11 associated with the pin end portion measured at the base as determined along the thread height direction is about 2 .8 mm.

Likewise, the internal thread profile 21 includes a full thread portion and a partial thread portion. The fully threaded portion of the internal thread profile 21 begins adjacent to the opening end of the coupling 20 and ends adjacent to the second thread of the male thread profile 11 of the pin 10. When pin 10 and coupling 20 are screwed together, the partially threaded portion of the internal thread profile 21 engages with the first turn 11A of the profile 11 of the external thread of the pin 10. According to the present embodiment, both the section of the coupling with a full thread and the section of the coupling with a partial thread of the profile 21 of the internal thread are in contact with the profile 11 of the external thread on both their supporting and embedded sides. In the embodiment shown, located in addition to the opening end of the coupling 20, which is the full-threaded portion of the internal thread profile 21, turns 23 and 24 are provided to abut the internal thread profile 21 and not be in contact with the male thread profile 11, at least at least on their supporting sides or embedded sides; however, in the present embodiment, the turns 23 and 24 are not included in the internal thread profile 21.

The thread height of the full thread portion of the male thread profile 11 is slightly larger than the thread height of the full thread portion of the internal thread profile 21. Thus, when the connection is screwed together as shown in FIG. 2, a small gap (for example, about 0.1 mm) is formed between the crest surface of the thread profile 11 of the male thread and the surface of the groove/thread root of the internal thread profile 21, while the crest surface of the thread profile 21 of the internal thread, and the surface of the root thread of the profile 11 external threads are in contact with each other. The area in which the external and internal thread profiles 11 and 21 are engaged during screwing, i.e. the region in which the supporting sides of the male thread profile 11 are in contact with the supporting sides of the female thread profile 21, and the embedded sides of the male thread profile 11 are in contact with the embedded sides of the internal thread profile 21, preferably has an axial length of 60 mm to 100 mm.

As shown in FIG. 2, the supporting and embedded sides of the thread turns of profiles 11 and 21 of the external and internal threads have a negative engagement angle θ. The engagement angles θ of the supporting and embedded sides can be the same or different engagement angles can be specified. In the shown implementation, the engagement angle θ of the support and embedded sides is -5.0°. Additionally, in the embodiment shown, the thread taper of the external and internal thread profiles 11 and 21 is 1/16.

When the pin 10 and the coupling 20 are screwed together, the supporting and anchoring sides of the threads of the male thread profile 11 are in contact with the embedded and supporting sides of the threads of the internal thread profile 21, so that the pin 10 is locked in the coupling 20 to provide high torque resistance, while The nipple seal 13 fits into the coupling seal 22 with an interference fit to ensure high tightness.

As shown in FIG. 2-4, the male thread profile 11 of the present embodiment includes a first thread portion 111 having a first curved surface 111A with a relatively large radius of curvature r1, and a second thread portion 112 adjacent to the first thread portion 111, wherein the first and second thread portions are located in the direction of the helix thread. The second thread section 112 includes all those sections of the external thread profile 11 except the first thread section 111. The support side and the thread groove/groove surface of the second thread portion 112 are connected by a second curved surface 112A having a radius of curvature r2 smaller than the first curved surface 111A. In the embodiment shown, the radius of curvature r1 of the first curved surface 111A is 0.4 mm, and the radius of curvature r2 of the second curved surface 112A is 0.1 mm.

As shown in FIG. 4, the first thread portion 111 covers approximately one turn in the direction of the thread helix, starting from the end of the male thread profile 11 associated with the pin end portion. This is not less than (0.4-0.1)/(9.845-9.400)=0.674 turns calculated from the above expression (2), and the width of the thread in the end section of the second thread section 112 associated with the end part of the pin, measured at its base, clearly greater than the width of the thread in the end portion of the first thread portion 111 associated with the end portion of the pin, measured at its base.

Additionally, in the present embodiment, the support side and the thread crest surface of the internal thread profile 21 are connected by a third curved surface 21A having a radius of curvature r3 larger than the radius of curvature r1 of the first curved surface 111A of the male thread profile 11. In the embodiment shown, the radius of curvature r3 of the third curved surface 21A is 0.5 mm. In the embodiment shown, the third curved surface 21A is

- 9 044818 viewed along the entire profile 21 of the internal thread; alternatively, a third curved surface 21A having a relatively large radius of curvature r3 may be provided only on those portions of the internal thread profile that face the first curved surface 111A when the pin 10 and sleeve 20 are screwed together, and a curved surface having a radius of curvature less than the radius r3 curvature, i.e. 0.5 to 2.0 mm may be provided between the support side and the thread crest surface on other portions of the internal thread profile 21.

Additionally, as shown in FIG. 2 and 3, a radial clearance is provided between the thread crest surface of the first turn 211 of the internal thread profile 21, which is in contact with the supporting side of the first thread portion 111 on the end of the male thread profile 11 associated with the end portion of the pin on one side, and those portions the surfaces of the thread groove/base in the external thread profile 11 that face those portions of the thread crest surface when the connection is screwed together. The size of this gap is less than the radius of curvature r1 of the first curved surface 111A and is about 0.3 mm in the illustrated implementation.

In addition, the radially outer end of the first curved surface 111A is located radially outward from the radially inner end of the third curved surface 21A and is located radially inward from the radially outer end of the third curved surface 21A.

Fig. 5 shows a threaded connection for a steel pipe according to the second embodiment, in which the pitch of the support side and the embedded side is slightly smaller than that of the threaded connection according to the first embodiment, which increases the number of threads appearing in the longitudinal section. Specifically, the pitch of the support side is 8.466 mm, and the pitch of the embedded side is 8.084 mm. As a result, the minimum thread width in the end portion of the male thread profile 11 associated with the pin end portion, measured at the root of the thread, is about 2.1 mm.

The present disclosure can be applied not only to a coupling type threaded connection, but also to an integral type threaded connection. Otherwise, the present disclosure is not limited to the embodiments illustrated above, and various modifications are possible without deviating from the spirit of the present disclosure.

Examples

To confirm the effects of a threaded connection for a steel pipe according to the present embodiment, a numerical simulation analysis was carried out using an elastoplastic finite element method to calculate its shear resistance and torque resistance.

Test conditions.

In finite element analysis (i.e. FEM analysis), a number of samples (i.e. analysis models) with different thread profiles were created and an elastoplastic finite element analysis was performed on each of the samples and a comparison was made between the different values productivity.

Each of samples No. 1-No. 4 had a basic design corresponding to the design of the threaded connection according to the first embodiment, where the first curved surface was provided along the entire external thread profile. For sample No. 1, the radius of curvature of the first curved surface was 0.1 mm; for sample No. 2 - 0.2 mm; for sample No. 3 - 0.3 mm; and for sample No. 4 it is 0.4 mm.

Each of samples No. 5-No. 8 had a basic design corresponding to the design of the threaded connection according to the second embodiment, where the first curved surface was provided along the entire external thread profile. For sample No. 5, the radius of curvature of the first curved surface was 0.1 mm; for sample No. 6 - 0.2 mm; for sample No. 7 - 0.3 mm; and for sample No. 8 it is 0.4 mm.

The material of each sample was Q125 oil well pipe material according to API standards (nominal yield strength YS=862 MPa (125 ksi)).

To allow comparison with conventional products, comparative models were created, each of which had two radii of curvature (i.e., 0.125 mm radius of curvature and 0.875 mm radius of curvature) at the interface between the bearing side and the thread root surface of the male thread profile, and were assessed in the same way.

Shear resistance.

To evaluate the shear strength, a tensile load of 100% of the level at which the steel pipe body deforms plastically was applied, and the equivalent plastic deformation at the point of shear failure initiation in the male thread profile was calculated, i.e. position on the first curved surface on the base section of the thread of the supporting side in the end section of the outer profile associated with the end part of the nipple, and it was determined that the smaller this value, the better the shear resistance. The calculation results are shown in Fig. 6.

Calculation of torque resistance.

To evaluate torque resistance, the value at which yielding begins on the make-up torque diagram, i.e. the maximum torque value (MTV) was taken to determine the yield torque, and it was determined that the higher the value, the better the torque resistance. The calculation results are shown in Fig. 7.

- 10 044818

Calculation results.

As shown in FIG. 6, the equivalent plastic strains in conventional two-radius curvature products were relatively low, presumably because the strains concentrated in the 0.125 mm radius of curvature region were mitigated in the 0.875 mm radius of curvature region. The evaluation results of samples No. 1-4 and No. 5-8 with the first curved surface are as follows: as the radius of curvature increases, the equivalent plastic deformation occurring on the first curved surface decreases; for samples No. 1-No. 4 with radii of curvature of more than 0.30 mm, the equivalent plastic deformation was lower than for conventional products; and in samples No. 5 to No. 8, the equivalent plastic strain was lower than that of conventional products for radii of curvature greater than 0.35 mm. Since these samples had a thread height Th of 2.2 mm, the following evaluation results are available: the equivalent plastic strains in samples No. 1-No. 4 were lower than in conventional products if the radius of curvature r1 satisfied the condition r1 > Thx 0.14, and the equivalent plastic strains in samples No. 5-No. 8 were lower than in conventional products if the radius r1 of curvature satisfied r1 > Thx0.16.

On the other hand, the torque resistance was almost unaffected by changes in the radius of curvature of the first curved surface of the external thread profile located at the base of the supporting side: samples No. 1-No. 4 had generally the same values, and samples No. 5-No. 8, in general, had the same meaning. It is assumed that the torque resistance in samples No. 1 to No. 4 was lower than in conventional products due to the reduction in the contact area between the supporting sides of the load caused by the increased radius of curvature of the upper portion of the supporting side of the internal thread profile above the conventional products. In contrast, samples No. 5 to No. 8, in which the support side pitch and embedded side pitch were reduced to increase the contact area between the support sides, exhibited higher torque resistance than the conventional products.

Concerns have been expressed that reducing the support side pitch and the embedding side pitch may result in a reduction in the thread width in the end portion of the male thread profile associated with the pin end portion, and thus a significant reduction in shear strength; however, the evaluation results show that, as shown in FIG. 6, due to the increase in the radius of curvature, the shear resistance was also higher than that of conventional products, which shows that the present disclosure provides a threaded connection that is better in terms of both shear resistance and torque resistance than conventional products.

As for samples No. 1 to No. 4, although the torque resistance was lower than that of conventional products, the shear resistance was better than that of conventional products because the radius of curvature of the first curved surface was greater than 0.3 mm, which suggests that these implementations are useful in applications that do not require high torque resistance, and the use of other means to improve torque resistance can make these implementations applicable in products that require torque resistance.

Similar estimates were made for samples with an increased radius of curvature of the base section of the supporting side of only the first turn of the external thread profile, which is considered the point of onset of shear failure; it was found that torque resistance and shear resistance are practically independent of changes in the radius of curvature only in the first turn of the thread.

However, it is believed that reducing the radius of curvature of the upper portion of the support side on portions of the internal thread profile that do not engage the first thread of the male thread profile, and thus increasing the contact area between the support sides, can significantly increase the torque resistance.

Decoding links

1: threaded connection for steel pipe;

2: steel pipe;

10: nipple;

20: coupling;

11: external thread profile;

111: first thread section;

111A: first curved surface;

r1: radius of curvature;

112: second thread section;

112A: second curved surface;

r2: radius of curvature;

21: internal thread profile;

21A: third curved surface;

r3: radius of curvature.

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Claims (12)

CLAIM 1. A threaded connection for a steel pipe, comprising: a tubular nipple provided at the end of the steel pipe; and a tubular coupling configured to screw into the nipple as the pin is screwed into it, the pin including a male thread profile formed on an outer periphery of the nipple, the coupling including a female thread profile formed on an inner periphery of the coupling and configured to engage an external thread profile when screwing together a joint, wherein each of the external thread profile and the internal thread profile includes a support side, an embedded side, a thread crest surface and a thread groove/bottom surface, wherein the pitch of the embedded side of each of the external thread profile and the internal thread profile is smaller, than the pitch of the supporting side of each of the male thread profile and the internal thread profile, and when the connection is screwed together, the supporting side of the male thread profile is in contact with the supporting side of the female thread profile, and the embedded side of the male thread profile is in contact with the embedded side of the female thread profile , and wherein each of the supporting sides and embedded sides of the male thread profile and the internal thread profile has a negative engagement angle, wherein the supporting side and the surface of the groove/groove of the thread in the portion of the external thread profile for a given range in the direction of the helical line of the thread, starting from the end the external thread profile associated with the end part of the nipple are connected by a first curved surface having a radius of curvature r1, when viewed in a longitudinal section, satisfying the following expression (1): ri > Th x 0.14 · ·· (1) where Th is the thread height, measured at the reference side, in the male thread profile, when measured over a specified range in the direction of the thread helix, starting from the end of the male thread profile associated with the end part of the nipple, and satisfies the condition: 1.8 mm < Th < 3.0 mm, and the external thread profile includes a first thread section having a first curved surface, and a second thread section adjacent to the first thread section, and these the portions are located in the direction of the helix of the thread, and wherein the support side and the thread groove/groove surface of the second thread portion are connected by a second curved surface having a smaller radius of curvature r2 than the first curved surface, wherein the first curved surface is provided along at least x turns in direction of the helix thread, starting from the end of the external thread profile associated with the end part of the pin, where x satisfies the following expression (2): x=(rl - r2) / Δρ · · · (2), where Δρ represents the difference in pitch between the pitch of the supporting side and the pitch of the embedded side of the external thread profile. 2. The threaded connection for a steel pipe according to claim 1, wherein the first curved surface is provided at least 1/2 turn in the direction of the helix thread starting from the end of the male thread profile associated with the end portion of the pin. 3. A threaded connection for a steel pipe according to claim 1, wherein a first curved surface is provided along the entire length of the male thread profile. 4. The threaded joint for a steel pipe according to claim 1, wherein the support side and the thread crest surface of the internal thread profile are connected by a third curved surface that faces the first curved surface when the joint is screwed and has a larger radius of curvature than the first curved surface, wherein a radial clearance is provided between the surface of the crest of the thread in the portion of the internal thread profile that is in contact with the support side in the end portion of the external thread profile associated with the end portion of the pin when the connection is screwed, on the one hand, and the surface of the groove/groove of the thread in a portion of a male thread profile that faces that thread crest surface, and wherein a radially outer end of the first curved surface is located radially outward from a radially inner end of a portion of a third curved surface that faces the first curved surface, and is located radially inward from a radially outer end of a portion of the third curved surface that faces the first curved surface. 5. A threaded connection for a steel pipe according to any one of claims 1 to 4, wherein the external and internal thread profiles have thread profiles such that when the connection is screwed together, the supporting and embedded sides of that portion of the thread of the external thread profile that covers at least 8 turns in the direction of the helical thread line starting from the end of the male thread profile associated with the end portion of the pin are in contact with the seat side and the embedded side of the female thread profile. 6. Threaded connection for a steel pipe according to claim 5, in which the pitch of the supporting side is not - 12 044818 more than 8.50 mm, the pitch of the embedded side is not more than 8.10 mm, while the associated pitch difference is not less than 0.35 mm and not more than 0.45 mm, and the minimum thread width measured at the end of the external profile thread associated with the end of the nipple and measured at the base of the thread is not less than 2.1 mm. 7. Threaded connection for a steel pipe according to any one of claims 1 to 4, in which the steel pipe has an outer diameter greater than 240 mm. 8. A threaded connection for a steel pipe, comprising: a tubular nipple provided at an end of the steel pipe; and a tubular coupling configured to screw into the nipple as the pin is screwed into it, the pin including a male thread profile formed on an outer periphery of the nipple, the coupling including a female thread profile formed on an inner periphery of the coupling and configured to engage an external thread profile when screwing together a joint, wherein each of the external thread profile and the internal thread profile includes a support side, an embedded side, a thread crest surface and a thread groove/bottom surface, wherein the pitch of the embedded side of each of the external thread profile and the internal thread profile is smaller, than the pitch of the supporting side of each of the male thread profile and the internal thread profile, and when the connection is screwed together, the supporting side of the male thread profile is in contact with the supporting side of the female thread profile, and the embedded side of the male thread profile is in contact with the embedded side of the female thread profile , and wherein each of the supporting sides and embedded sides of the male thread profile and the internal thread profile has a negative engagement angle, wherein the supporting side and the surface of the groove/groove of the thread in the portion of the external thread profile for a given range in the direction of the helical line of the thread, starting from the end the external thread profile associated with the end part of the nipple are connected by a first curved surface having a radius of curvature r1, when viewed in a longitudinal section, satisfying the following expression (1): rl > Th x 0.14 ·· (1) where Th is the thread height, measured on the reference side, in the male thread profile, when measured over a specified range in the direction of the thread helix, starting from the end of the male thread profile associated with end part of the nipple, and satisfies the condition: 1.8 mm < Th < 3.0 mm, and the supporting side and the surface of the thread tip of the internal thread profile are connected by a third curved surface, which faces the first curved surface when the connection is screwed, and has a larger radius curvature than the first curved surface, wherein a radial clearance is provided between the thread crest surface in a portion of the internal thread profile that is in contact with the support side in the end portion of the male thread profile associated with the end portion of the pin when the connection is screwed on one side, and a thread groove/groove surface in a portion of the male thread profile that faces that thread crest surface, and wherein a radially outer end of the first curved surface is located radially outward from the radially inner end of a portion of the third curved surface that faces the first curved surface, and is located radially inwardly from a radially outer end of the third curved surface portion that faces the first curved surface. 9. The threaded connection for a steel pipe according to claim 8, wherein the male thread profile includes a first thread portion having a first curved surface and a second thread portion adjacent to the first thread portion, these portions being located in a direction of the helix of the thread, and wherein the support side and the thread groove/groove surface of the second thread portion are connected by a second curved surface having a smaller radius of curvature r2 than the first curved surface, wherein the first curved surface is provided along at least x turns in the direction of the helix of the thread, starting from the end external thread profile associated with the end part of the nipple, where x satisfies the following expression (2): x=(rl - r2) / Δρ · · · (2), where Δρ represents the difference in pitch between the pitch of the supporting side and the pitch of the embedded side of the external thread profile. 10. The threaded connection for a steel pipe according to claim 8, wherein the first curved surface is provided at least 1/2 turn in a direction of the helix thread starting from the end of the male thread profile associated with the end portion of the pin. 11. A threaded connection for a steel pipe according to claim 8, wherein a first curved surface is provided along the entire length of the male thread profile. 12. A threaded connection for a steel pipe according to any one of claims 8 to 11, in which the external and internal thread profiles have thread profiles such that when the connection is screwed together, the supporting and embedded -