Patent Description:
In an oil well, natural gas well, and the like (hereafter, collectively referred to as an "oil well"), steel pipes called oil country tubular goods (OCTG) are used to mining underground resources. The steel pipes are connected one by one. To connect the steel pipes, threaded connections are used.

Types of the threaded connection for steel pipes are roughly categorized into a coupling type and an integral type. In a case of a threaded connection of the coupling type, one of tubes in a pair to be connected is a steel pipe, and the other one is a coupling. In this case, male thread parts are formed on outer circumferences of both end portions of the steel pipe, and female thread parts are formed on inner circumferences of both end portions of the coupling. The steel pipe and the coupling are then connected with each other. In a case of a threaded connection of the integral type, tubes in a pair to be connected are both steel pipes, and a coupling is not used separately. In this case, a male thread part is formed on an outer circumference of one end portion of each steel pipe, and a female thread part is formed on an inner circumference of the other end portion. One and the other of the steel pipes are then connected with each other.

A joint portion of a tube leading end portion on which a male thread part is formed includes an element to be inserted into a female thread part, and thus is called a pin. In contrast, a joint portion of a tube leading end portion on which a female thread part is formed includes an element to receive a male thread part, and thus is called a box. The pin and the box are end portions of a tube and are thus both tubular.

<FIG> is a longitudinal sectional view illustrating a typical threaded connection for steel pipes as conventionally used. The threaded connection illustrated in <FIG> is a threaded connection of the coupling type and includes a pin <NUM> and a box <NUM> (e.g., see <CIT> (Patent Literature <NUM>)).

The pin <NUM> includes, in order from a front end of the pin <NUM> toward a pipe body <NUM> of the pin <NUM>, an annular shoulder surface <NUM>, an annular sealing surface <NUM>, and a male thread part <NUM>. In the pin <NUM>, the sealing surface <NUM> is located adjacent to the shoulder surface <NUM>. The box <NUM> includes, in order from a pipe body <NUM> of the box <NUM> toward a front end of the box <NUM>, an annular shoulder surface <NUM>, an annular sealing surface <NUM>, and a female thread part <NUM>. In the box <NUM>, the sealing surface <NUM> is located adjacent to the shoulder surface <NUM>.

When the pin <NUM> and the box <NUM> are connected to each other, by screwing the pin <NUM> into the box <NUM>, the shoulder surface <NUM> of the pin <NUM> comes into contact with the shoulder surface <NUM> of the box <NUM>. When rotation of the pin <NUM> is continued by a predetermined amount, a fastening axial tension is generated between the male thread part <NUM> and the female thread part <NUM> meshing with each other, and fastening is thereby completed. In a state where the fastening is completed (hereafter, referred to also as a "fastening state"), the sealing surface <NUM> of the pin <NUM> comes in contact with the sealing surface <NUM> of the box <NUM> while interfering with the sealing surface <NUM>, forming a seal part by metal contact. This seal part ensures a sealing ability of the threaded connection.

In recent years, as oil wells are increasingly shifted at great depths and to hadal zones, oil well environments become harsh with high temperatures and high pressures. In such oil well environments, a compressive load, a tensile load, and pressures from the outside (hereafter, also referred to as "external pressures") and pressures from the inside (hereafter, also referred to as "internal pressures") applied to oil country tubular goods are extremely high. For that reason, heavy-wall steel pipes are used particularly as oil country tubular goods for casing or tubing. In a case where a threaded connection illustrated in <FIG> is used for connecting such heavy-wall steel pipes, the threaded connection is required to have a comparable level of strength to that of bodies of the heavy-wall steel pipes and high sealing ability. In particular, the sealing ability against an external pressure is required.

Here, the compressive load is received by a contact surface between a shoulder surface <NUM> of a pin <NUM> and a shoulder surface <NUM> of a box <NUM> (hereafter, also referred to as a "shoulder contact surface"). Referring to <FIG>, in a conventional threaded connection, to ensure an area of the shoulder contact surface to the maximum, the shoulder surface <NUM> of the pin <NUM> is brought into contact with the shoulder surface <NUM> of the box <NUM> over their entire areas. Specifically, a diameter Dpi of an inner circumferential edge of the shoulder surface <NUM> of the pin <NUM> is substantially the same as a diameter Dbi of the inner circumferential edge of the shoulder surface <NUM> of the box <NUM>. <CIT> discloses a threaded connection. <CIT> discloses a threaded tubular connection comprising a first tubular component and a second tubular component. The first tubular component includes a female portion defined on an interior surface of the first tubular component. The female portion includes an inner threaded portion and an outer threaded portion which are offset radially with respect to a longitudinal axis of the first tubular component by a first shoulder. The second tubular component includes a male portion defined on an exterior surface of the second tubular component. The male portion is to be inserted into the female portion, and includes an inner threaded portion and an outer threaded portion which are offset radially with respect to a longitudinal axis of the second tubular component by a second shoulder. The second shoulder is to abut the first shoulder once the male portion is connected to the female portion. <CIT> discloses a cylindrical threaded connector which is provided with a small amount of taper to produce a controlled radial interference to prevent inadvertent thread disengagement. The threaded connector is especially suited for use with radial clearance cylindrical threads subjected to tension load such as tubular goods used in wells. The taper can be used on either or both threads of two step cylindrical threads. <CIT> discloses a first axial abutment surface formed at the free end of an annular lip of a threaded male element and a second axial abutment surface formed at the end of a housing of a threaded female element have a same axial half-sections the inclination of which varies progressively in the radial direction. Such a configuration of the axial abutment surfaces optimizes the radial thrust exerted on the end of the lip depending on its thickness, in order to obtain an increased radial interference between the respective sealing surfaces of both elements.

<CIT> discloses a threaded connection in accordance with a precharacterising section of claim <NUM>.

An objective of the present invention is to provide a threaded connection for steel pipes that is capable of ensuring sufficient sealing ability against external pressures even in a case of using a heavy-wall steel pipe.

A threaded connection for steel pipes according to an embodiment of the present invention includes a tubular pin and a tubular box. The pin includes, in order from a front end of the pin toward a pipe body of the pin, an annular shoulder surface, an annular sealing surface located adjacent to the shoulder surface of the pin, and a male thread part. The box includes, in order from a pipe body of the box toward a front end of the box, an annular shoulder surface, an annular sealing surface located adjacent to the shoulder surface of the box, and a female thread part. The shoulder surface of each of the pin and the box inclines from a plane perpendicular to a pipe axis in a direction of screwing the pin. A diameter of an inner circumferential edge of the shoulder surface of the pin is smaller than a diameter of an inner circumferential edge of the shoulder surface of the box. A thickness of an annular pin shoulder region that appears when the shoulder surface of the pin is projected onto the plane perpendicular to the pipe axis is <NUM>% or more of a wall thickness of the pipe body of the pin, a thickness of an annular box shoulder region that appears when the shoulder surface of the box is projected onto the plane perpendicular to the pipe axis is <NUM>% or more and <NUM>% or less of a wall thickness of the pipe body of the pin.

With the threaded connection for steel pipes according to an embodiment of the present invention, a sealing ability against external pressure can be sufficiently ensured even in a case of using a heavy-wall steel pipe.

To solve the above problem, the present inventors conducted various analyses and tests, and conducted intensive studies. Consequently, the following findings were obtained.

To increase sealing ability of a threaded connection used for a heavy-wall steel pipe, the following techniques are conceivable. As a first technique, referring to <FIG>, it is conceivable to increase a wall thickness of a portion <NUM> including a sealing surface <NUM> of a pin <NUM> (hereafter, also referred to as a "pin seal part").

The first technique is derived from the following inference. When the wall thickness of the pin seal part <NUM> is large, a rigidity of the pin seal part <NUM> is increased in a radial direction. This large wall thickness improves an elastic resilience of the pin seal part <NUM>, which leads to an increase in a contact force between the sealing surface <NUM> of the pin <NUM> and a sealing surface <NUM> of a box <NUM> in a fastening state (hereafter, also referred to as a "seal contact force"), and thus an increase in the sealing ability against internal pressures and external pressures. In addition, the large wall thickness prevents radially-contractive deformation of the pin seal part <NUM> when an external pressure is applied to the pin <NUM>. For that reason, it is possible to minimize the decrease in the seal contact force even when an external pressure is applied to the pin <NUM>. Therefore, it may be said that a large wall thickness of the pin seal part <NUM> can prevent the sealing ability against external pressures from decreasing.

In the first technique, since the wall thickness of the pin seal part <NUM> is increased, a shoulder surface <NUM> of the pin <NUM> has a large area. In a conventional threaded connection, the shoulder surface <NUM> of the pin <NUM> comes into contact with the shoulder surface <NUM> of the box <NUM> over their entire areas. Therefore, the shoulder surface <NUM> of the box <NUM> also has a large area. In other words, a shoulder contact surface has a large area.

However, if the shoulder contact surface has an excessively large area, a contact force between the shoulder surface <NUM> of the pin <NUM> and the shoulder surface <NUM> of the box <NUM> (hereafter, also referred to as a shoulder contact force) becomes uneven in the shoulder contact surface. The unevenness of the shoulder contact force has a significant influence on a seal part, which is adjacent to the shoulder contact surface. For that reason, contact of the seal part actually becomes unstable, resulting in a decrease in the sealing ability.

In contrast to the first technique described above, as a second technique, it is conceivable to decrease the wall thickness of the pin seal part <NUM>. In such a case, the decrease in the wall thickness of the pin seal part <NUM> makes the area of the shoulder surface <NUM> of the pin <NUM> small, and also makes the area of the shoulder surface <NUM> of the box <NUM> small. For that reason, the shoulder contact surface has a small area. Therefore, the shoulder contact force can be made uniform.

However, in the second technique, since the wall thickness of the pin seal part <NUM> is decreased, the rigidity of the pin seal part <NUM> is low in the radial direction. This makes the pin seal part <NUM> susceptible to radially-contractive deformation when an external pressure is applied to the pin <NUM>. For that reason, the sealing ability against external pressures is actually decreased.

Briefly, it is not possible to ensure the sealing ability against external pressures by both of the first and second techniques.

Thus, in consideration of the problems with the first and second techniques, the present inventors paid attention to the pin seal part and the shoulder contact surface. Specifically, the wall thickness of the pin seal part is increased, and at the same time, the area of the shoulder contact surface is decreased. This increases the rigidity of the pin seal part in the radial direction and also makes the shoulder contact force uniform in the shoulder contact surface. Therefore, stabilization of the contact of the seal part can be achieved. Consequently, it is possible to ensure the sealing ability against external pressures.

The threaded connection for steel pipes according to the present invention is completed based on the above findings.

A threaded connection for steel pipes according to an embodiment of the present invention includes a tubular pin and a tubular box. The pin includes, in order from a front end of the pin toward a pipe body of the pin, an annular shoulder surface, an annular sealing surface located adjacent to the shoulder surface of the pin, and a male thread part. The box includes, in order from a pipe body of the box toward a front end of the box, an annular shoulder surface, an annular sealing surface located adjacent to the shoulder surface of the box, and a female thread part. The shoulder surface of each of the pin and the box inclines from a plane perpendicular to a pipe axis in a direction of screwing the pin. A diameter of an inner circumferential edge of the shoulder surface of the pin is smaller than a diameter of an inner circumferential edge of the shoulder surface of the box.

In a typical example, the threaded connection in the present embodiment is used for connecting heavy-wall steel pipes used as casing pipes or tubing pipes. Wall thicknesses of the heavy-wall steel pipes are more than <NUM> inch (<NUM>).

In the threaded connection in the present embodiment, an area of the shoulder surface of the pin is large, and an area of the shoulder surface of the box is small. This is because the diameter of the inner circumferential edge of the shoulder surface of the pin is smaller than the diameter of the inner circumferential edge of the shoulder surface of the box. This makes a wall thickness of the pin seal part large, which increases a rigidity of the pin seal part in the radial direction. In addition, the area of the shoulder contact surface being small makes the shoulder contact force uniform across the shoulder contact surface.

In addition, in the threaded connection in the present embodiment, when the threaded connection is in a fastening state, the shoulder surface of the pin and the shoulder surface of the box are in pressing contact with each other in a hooked form. This is because the shoulder surface of each of the pin and the box inclines from a plane perpendicular to a pipe axis in a direction of screwing the pin. This causes the pin seal part to receive a reaction force all the time in a direction in which the pin seal part radially expands. The pin seal part thus resists radially-contractive deformation when an external pressure is applied to the pin.

From the above reason, the contact between the seal parts is stabilized even in a case of using a heavy-wall steel pipe. Consequently, it is possible to ensure the sealing ability against external pressures sufficiently.

In the above threaded connection, a feed angle of the shoulder surface of each of the pin and the box with respect to the plane perpendicular to the pipe axis is preferably <NUM>° to <NUM>°. When the feed angle of the shoulder surfaces are <NUM>° or more, the shoulder surface of the pin and the shoulder surface of the box are in an effective pressing contact with each other in the hooked form in the fastening state. The feed angle of the shoulder surface is preferably <NUM>° or more. At the same time, when the feed angle of shoulder surface is <NUM>° or less, deformation of a shoulder part of the box is small even when a compressive load is repeatedly applied. Therefore, the pressing contact between the shoulder surfaces in the hooked form is effectively maintained.

In the above threaded connection, a thickness tp of an annular pin shoulder region that appears when the shoulder surface of the pin is projected onto the plane perpendicular to the pipe axis is <NUM>% or more of a wall thickness t of the pipe body of the pin. When the thickness tp of the pin shoulder region is <NUM>% or more of the wall thickness t of the pipe body of the pin, the wall thickness of the pin seal part is effectively large.

In contrast, an upper limit of the thickness tp of the pin shoulder region is not limited to a particular thickness. However, an excessively large thickness tp of the pin shoulder region makes it difficult to secure a length of the male thread part. Thus, the thickness tp of the pin shoulder region is preferably <NUM>% or less of the wall thickness t of the pipe body of the pin.

In the above threaded connection, a thickness tb of an annular box shoulder region that appears when the shoulder surface of the box is projected onto the plane perpendicular to the pipe axis is preferably <NUM>% or more to <NUM>% or less of the wall thickness t of the pipe body of the pin. The box shoulder region is equivalent to an annular shoulder contact surface region that appears when the shoulder contact surface is projected onto the plane perpendicular to the pipe axis.

When the thickness tb of the box shoulder region (shoulder contact surface region) is <NUM>% or more of the wall thickness t of the pipe body of the pin, in a case where an excessively heavy compressive load is applied to the threaded connection, plastic deformation of the shoulder surface and the sealing surface adjacent to the shoulder surface can be prevented, which can stabilizes a contact state of the sealing surface. Consequently, it is possible to ensure the seal contact force. In the invention, the thickness tb of the box shoulder region is <NUM>% or more of the wall thickness t of the pipe body of the pin. In contrast, when the thickness tb of the box shoulder region is <NUM>% or less of the wall thickness t of the pipe body of the pin, an area of the shoulder contact surface is made significantly small. In the invention, the thickness tb of the box shoulder region is <NUM>% or less of the wall thickness t of the pipe body of the pin.

To be exact, there is a convex corner part between the shoulder surface and the sealing surface seen in a longitudinal section of the pin. This convex corner part connects the shoulder surface and the sealing surface of the pin seamlessly. Similarly, there is a concave corner part between the shoulder surface and the sealing surface seen in a longitudinal section of the box. This concave corner part connects the shoulder surface and the sealing surface of the box seamlessly. Radii of these convex corner part and concave corner part seen in the longitudinal section are at most about <NUM>. In this case, the thickness tp of the pin shoulder region does not include a region of the convex corner part. Similarly, the thickness tb of the box shoulder region does not include a region of the concave corner part.

Hereafter, a specific example of a threaded connection for steel pipes according to the present embodiment will be described with reference to the accompanying drawings.

<FIG> is a longitudinal sectional view illustrating the threaded connection for steel pipes in the present embodiment. <FIG> are longitudinal sectional views each illustrating a vicinity of a front end of the pin of the threaded connection illustrated in <FIG>, in an enlarged manner. <FIG> illustrates the fastening state. <FIG> illustrates a state where the pin <NUM> is separated from the box <NUM>, for convenience of description. Solid-white arrows in <FIG> illustrate a direction of screwing the pin <NUM> with respect to the box <NUM>. In the present specification, a longitudinal section refers to a cross section including a pipe axis CL of the threaded connection (see <FIG>).

Referring to <FIG>, the threaded connection in the present embodiment is a threaded connection of the coupling type and includes the pin <NUM> and the box <NUM>. The pin <NUM> is a heavy-wall steel pipe.

The pin <NUM> includes, in order from a front end of the pin <NUM> toward a pipe body <NUM> of the pin <NUM>, an annular shoulder surface <NUM>, an annular sealing surface <NUM>, and a male thread part <NUM>. Hereinafter, the shoulder surface <NUM> of the pin <NUM> will be referred to also as a "pin shoulder surface". The sealing surface <NUM> of the pin <NUM> will be referred to also as a "pin sealing surface".

The pin shoulder surface <NUM> is an annular surface forming a front end surface of the pin <NUM> and inclines from the plane perpendicular to the pipe axis CL in the direction of screwing the pin <NUM>. This causes an outer circumferential edge 12b of the pin shoulder surface <NUM> (an edge farthest from the pipe axis CL) to protrude from an inner circumferential edge 12a of the pin shoulder surface <NUM> (an edge closest to the pipe axis CL) in the direction of screwing the pin <NUM>. The pin sealing surface <NUM> is located adjacent to the pin shoulder surface <NUM>. That is, the pin sealing surface <NUM> is connected to the outer circumferential edge 12b of the pin shoulder surface <NUM>. The pin sealing surface <NUM> is a tapered annular surface. Note that the pin sealing surface <NUM> may have a shape formed by combining the tapered annular surface and a surface equivalent to a circumferential surface of a solid of revolution obtained by rotating a curve such as an arc about the pipe axis CL. A diameter of the pin sealing surface <NUM> decreases as the pin <NUM> extends toward its front end side (close to the pin shoulder surface <NUM>).

The box <NUM> includes, in order from a pipe body <NUM> of the box <NUM> toward a front end of the box <NUM>, an annular shoulder surface <NUM>, an annular sealing surface <NUM>, and a female thread part <NUM>. Hereinafter, the shoulder surface <NUM> of the box <NUM> will be referred to also as a "box shoulder surface". The sealing surface <NUM> of the box <NUM> will be referred to also as a "box sealing surface".

The box shoulder surface <NUM> is an annular surface corresponding to the pin shoulder surface <NUM> and inclines from the plane perpendicular to the pipe axis CL in the direction of screwing the pin <NUM>. This causes an inner circumferential edge 22a of the box shoulder surface <NUM> (an edge closest to the pipe axis CL) to protrude from an outer circumferential edge 22b of the box shoulder surface <NUM> (an edge farthest from the pipe axis CL) in an opposite direction to the direction of screwing the pin <NUM>. The box sealing surface <NUM> is located adjacent to the box shoulder surface <NUM>. That is, the box sealing surface <NUM> is connected to the outer circumferential edge 22b of the box shoulder surface <NUM>. This box sealing surface <NUM> is a tapered annular surface corresponding to the pin sealing surface <NUM>. Note that the box sealing surface <NUM> may have a shape formed by combining the tapered annular surface and a surface equivalent to a circumferential surface of a solid of revolution obtained by rotating a curve such as an arc about the pipe axis CL.

The male thread part <NUM> of the pin <NUM> corresponds to the female thread part <NUM> of the box <NUM>. Each of the male thread part <NUM> and the female thread part <NUM> includes crests, roots, stabbing flanks, and load flanks.

In the present embodiment, a diameter Dpi of the inner circumferential edge 12a of the pin shoulder surface <NUM> is smaller than a diameter Dbi of the inner circumferential edge 22a of the box shoulder surface <NUM>. An area of the pin shoulder surface <NUM> is large, and an area of the box shoulder surface <NUM> is small. For that reason, a shoulder contact surface <NUM> has a small area. The diameter Dpi of the inner circumferential edge 12a of the pin shoulder surface <NUM> is the same as an inner diameter of the pipe body <NUM> of the pin <NUM>. That is, the inner diameter of the pin <NUM> is constant.

In the present embodiment, feed angles θp and θb of the pin shoulder surface <NUM> and the box shoulder surface <NUM> with respect to the surface perpendicular to the pipe axis CL are <NUM>° to <NUM>°. The thickness tp of the annular pin shoulder region that appears when the pin shoulder surface <NUM> is projected onto the plane perpendicular to the pipe axis CL is <NUM>% or more of the wall thickness t of the pipe body <NUM> of the pin <NUM>. The thickness tb of the annular box shoulder region that appears when the box shoulder surface <NUM> is projected onto the plane perpendicular to the pipe axis CL is <NUM>% or more to <NUM>% or less of the wall thickness t of the pipe body <NUM> of the pin <NUM>.

When the pin <NUM> and the box <NUM> are connected to each other, by screwing the pin <NUM> into the box <NUM>, the male thread part <NUM> meshes with the female thread part <NUM>. A part of the pin shoulder surface <NUM> comes in contact with a whole area of the box shoulder surface <NUM>. That is, the pin shoulder surface <NUM> comes in contact with the box shoulder surface <NUM> within a range of the shoulder contact surface <NUM>. When rotation of the pin <NUM> is continued by a predetermined amount, the part of the pin shoulder surface <NUM> and the whole area of the box shoulder surface <NUM> are in pressing contact with each other in the hooked form. This generates a fastening axial tension between the male thread part <NUM> and the female thread part <NUM> meshing with each other, and fastening is thereby completed. In the fastening state, the pin sealing surface <NUM> comes in contact with the box sealing surface <NUM> while interfering with the box sealing surface <NUM>, forming a seal part by metal contact. This seal part ensures the sealing ability of the threaded connection.

Referring to <FIG>, in the threaded connection in the present embodiment, the area of the pin shoulder surface <NUM> is large, and the area of the box shoulder surface <NUM> is small. This makes a wall thickness of the pin seal part <NUM> large, which increases a rigidity of the pin seal part <NUM> in the radial direction. In addition, the area of the shoulder contact surface <NUM> being small makes the shoulder contact force uniform across the shoulder contact surface <NUM>.

In addition, in the threaded connection in the present embodiment, when the threaded connection is in the fastening state, the part of the pin shoulder surface <NUM> and the whole area of the box shoulder surface <NUM> are in the pressing contact with each other in the hooked form. This causes the pin seal part <NUM> to receive a reaction force all the time in a direction in which the pin seal part radially expands. The pin seal part <NUM> thus resists radially-contractive deformation when an external pressure is applied to the pin <NUM>.

To confirm effects brought by the present embodiment, the present inventors conducted numerical simulations and analyses using the elasto-plastic finite element method (FEM analysis).

In the FEM analysis, a model was used of a coupling-type threaded connection in which a diameter Dpi of an inner circumferential edge of a pin shoulder surface and a diameter Dbi of an inner circumferential edge of a box shoulder surface were variously changed. The followings are common conditions.

Changed dimensional conditions were according to Table <NUM> shown below.

In the FEM analysis, a material of the threaded connection was assumed to be an isotropic-hardening elasto-plastic body. A modulus of elasticity of the elasto-plastic body was set at <NUM> GPa, and a yield strength of the elasto-plastic body as a <NUM>% yield stress was set at <NUM> ksi (<NUM> MPa). Tightening was performed until the pin shoulder surface came into contact with the box shoulder surface and a <NUM>/<NUM> turn was made.

<NUM> to <NUM> were Inventive Examples of the present invention intended to represent threaded connections in the present embodiment, in each of which the diameter Dpi of the inner circumferential edge of the pin shoulder surface was smaller than the diameter Dbi of the inner circumferential edge of the box shoulder surface. Test No. <NUM> was Comparative Example as a reference intended to represent a conventional threaded connection, in which the diameter Dpi of the inner circumferential edge of the pin shoulder surface was the same as the diameter Dbi of the inner circumferential edge of the box shoulder surface. <NUM> to <NUM> were Comparative Examples, in each of which the diameter Dpi of the inner circumferential edge of the pin shoulder surface was the same as the diameter Dbi of the inner circumferential edge of the box shoulder surface.

In the FEM analysis, load steps (combinations of internal pressure, external pressure, tensile load, and compressive load) that simulate Series A test according to ISO13679 <NUM> were applied to the model in a fastening state. Of load points in an external pressure cycle in a loading step history, attention was paid to a load point of an external pressure and a compressive load and a load point of only an external pressure, and at each of the load points, sealing ability of a seal part was evaluated. Here, a seal contact force [N/mm] under the external pressure and compressive load, and a seal contact force [N/mm] under only the external pressure were investigated. The seal contact force mentioned here refers to a value of [average contact interfacial pressure between sealing surfaces] × [contact width], and means that the higher the value, the better the sealing ability.

A specific evaluation of the sealing ability was conducted with reference to Test No. <NUM>. Specifically, the seal contact force under the external pressure and compressive load, and the seal contact force under only the external pressure in Test No. <NUM> were regarded as references (<NUM>), and a ratio of seal contact forces in each Test No were compared with the respective seal contact forces in Test No. <NUM>.

The test results are shown in the above Table <NUM>. From the results shown in Table <NUM>, the followings are indicated. In each of Test Nos. <NUM> to <NUM> being Inventive Example of the present invention, the sealing ability was improved as compared with Test No. <NUM> being Comparative Example as the reference. This was attributable to such a condition that the diameter Dpi of the inner circumferential edge of the pin shoulder surface was smaller than the diameter Dbi of the inner circumferential edge of the box shoulder surface. In particular, in each of Test Nos. <NUM> to <NUM>, the sealing ability was further improved. This was attributable to such a condition that a thickness of tp of a pin shoulder region was <NUM>% or more of a wall thickness t of a pipe body of the pin, so that the wall thickness of the pin seal part was effectively large. In addition, this further improvement in the sealing ability was attributable to such a condition that a thickness tb of a box shoulder region was <NUM>% or less of the wall thickness t of the pipe body of the pin, so that the area of the shoulder contact surface was effectively small.

In contrast, in each of Test Nos. <NUM> to <NUM> being Comparative Examples, the sealing ability was decreased as compared with Test No. <NUM> as the reference. This was attributable to such a condition that the diameter Dpi of the inner circumferential edge of the pin shoulder surface was the same as the diameter Dbi of the inner circumferential edge of the box shoulder surface. In particular, this decrease in the sealing ability was also attributable to such a condition that the thickness tp of the pin shoulder region did not reach <NUM>% of wall thickness t of the pipe body of the pin, so that the wall thickness of the pin seal part was small.

In Example <NUM>, the same FEM analysis as that in the above Example <NUM> was conducted. In particular, in Example <NUM>, the feed angle of the shoulder surface was set at <NUM>°. The otherwise common conditions were the same as those in the above Example <NUM>. Changed dimensional conditions (diameter Dpi of the inner circumferential edge of the pin shoulder surface, and diameter Dbi of the inner circumferential edge of the box shoulder surface) were according to Table <NUM> shown below.

<NUM> and <NUM> were Inventive Examples of the present invention intended to represent a threaded connection in the present embodiment, in each of which the diameter Dpi of the inner circumferential edge of the pin shoulder surface was smaller than the diameter Dbi of the inner circumferential edge of the box shoulder surface. Test No. <NUM> was Comparative Example as a reference intended to represent a conventional threaded connection, in which the diameter Dpi of the inner circumferential edge of the pin shoulder surface was the same as the diameter Dbi of the inner circumferential edge of the box shoulder surface. <NUM> to <NUM> were Comparative Examples, in each of which the diameter Dpi of the inner circumferential edge of the pin shoulder surface was the same as the diameter Dbi of the inner circumferential edge of the box shoulder surface.

As in the above Example <NUM>, the sealing ability was evaluated. Specifically, the evaluation of the sealing ability was conducted with reference to Test No. <NUM>. Specifically, the seal contact force under the external pressure and compressive load, and the seal contact force under only the external pressure in Test No. <NUM> were regarded as references (<NUM>), and ratios of seal contact forces in each Test No were compared to the respective seal contact forces in Test No. <NUM>.

The test results are shown in the above Table <NUM>. From the results shown in Table <NUM>, the followings are indicated. In each of Test Nos. <NUM> and <NUM> being Inventive Example of the present invention, the sealing ability was improved as compared with Test No. <NUM> being Comparative Example as the reference. This was attributable to such a condition that the diameter Dpi of the inner circumferential edge of the pin shoulder surface was smaller than the diameter Dbi of the inner circumferential edge of the box shoulder surface.

In contrast, in each of Test Nos. <NUM> to <NUM> being Comparative Examples, the sealing ability was decreased as compared with Test No. <NUM> as the reference. This was attributable to such a condition that the diameter Dpi of the inner circumferential edge of the pin shoulder surface was the same as the diameter Dbi of the inner circumferential edge of the box shoulder surface.

The present invention is not limited to the embodiments described above. For example, the threaded connection may be of any one of a coupling type and an integral type.

Claim 1:
A threaded connection for steel pipes comprising a tubular pin (<NUM>) and a tubular box (<NUM>), wherein
the pin (<NUM>) includes, in order from a front end of the pin (<NUM>) toward a pipe body (<NUM>) of the pin (<NUM>), an annular shoulder surface (<NUM>), an annular sealing surface (<NUM>) located adjacent to the shoulder surface (<NUM>) of the pin (<NUM>), and a male thread part (<NUM>),
the box (<NUM>) includes, in order from the pipe body (<NUM>) of the box (<NUM>) toward the front end of the box (<NUM>), an annular shoulder surface (<NUM>), an annular sealing surface (<NUM>) located adjacent to the shoulder surface (<NUM>) of the box (<NUM>), and a female thread part (<NUM>),
the shoulder surface (<NUM>, <NUM>) of each of the pin (<NUM>) and the box (<NUM>) inclines from a plane perpendicular to a pipe axis (CL) in a direction of screwing the pin (<NUM>),
a diameter (Dpi) of an inner circumferential edge (12a) of the shoulder surface (<NUM>) of the pin (<NUM>) is smaller than a diameter (Dbi) of an inner circumferential edge (22a) of the shoulder surface (<NUM>) of the box (<NUM>), and
a thickness (tp) of an annular pin shoulder region that appears when the shoulder surface (<NUM>) of the pin (<NUM>) is projected onto the plane perpendicular to the pipe axis (CL) is <NUM>% or more of a wall thickness (t) of the pipe body (<NUM>) of the pin (<NUM>), and
characterised in that:
a thickness (tb) of an annular box shoulder region that appears when the shoulder surface (<NUM>) of the box (<NUM>) is projected onto the plane perpendicular to the pipe axis (CL) is <NUM>% or more and <NUM>% or less of a wall thickness (t) of the pipe body (<NUM>) of the pin (<NUM>).