The present invention relates to a threaded joint for use in connecting steel pipes.
BACKGROUND ART
[0002]
In oil wells, natural gas wells, and the like (hereinafter also collectively referred
to as "oil wells"), steel pipes referred to as oil country tubular goods (OCTG) such as
casings and tubings are used for extraction of underground resources. The steel pipes
are sequentially connected to each other, and threaded joints are used for the connection.
[0003]
Threaded joints for steel pipes are classified into two types: coupling-type joints
and integral-type joints. A coupling-type threaded joint is constituted by a pair of
tubular goods that are to be connected to each other, of which one is a steel pipe and the
other is a coupling. In this case, the steel pipe includes male threaded portions formed
on the outer peripheries at both ends thereof, and the coupling includes female threaded
portions formed on the inner peripheries at both ends thereof. Thus, the steel pipe and
the coupling are connected to each other. An integral-type threaded joint is constituted
by a pair of steel pipes as tubular goods that are to be connected to each other, without a
separate coupling being used. In this case, each steel pipe includes a male threaded
portion formed on the outer periphery at one end thereof and a female threaded portion
formed on the inner periphery at the other end thereof. Thus, the one steel pipe and the
other steel pipe are connected to each other.
[0004]
In general, the joint portion at the tubular end where a male threaded portion is
disposed is referred to as a pin because it includes an element that is inserted into a
female threaded portion. On the other hand, the joint portion at the tubular end where
a female threaded portion is disposed is referred to as a box because it includes an
element that receives a male threaded portion. Pins and boxes both have a tubular
shape because they are constituted by end portions of tubular goods.
[0005]
Threaded joints for steel pipes are configured such that the male threaded portion
of the pin is screwed onto the female threaded portion of the box and accordingly the
male threaded portion and the female threaded portion, each being a tapered threaded
portion, engage in intimate contact with each other. Basically, this thread seal
produced by the engagement and intimate contact of the male threaded portion and the
female threaded portion ensures the sealing performance of threaded joints.
[0006]
In recent years, some drilling techniques such as horizontal drilling and
directional drilling have been increasingly employed in oil wells, so that threaded joints
are more likely to be subjected to high torque loads. For this reason, there is an
increasing demand for threaded joints capable of providing high torque resistance
performance. Furthermore, oil well environments in which the above-mentioned
drilling techniques are applied are harsh environments with high temperatures and high
pressures. To address such harsh environments, threaded joints need to be capable of
providing not only high torque resistance performance but also high sealing
performance.
[0007]
In general, coupling-type threaded joints are widely used because they exhibit
better sealing performance than integral-type threaded joints.
[OOOS]
An example of threaded joints capable of providing high torque resistance
performance is a threaded joint employing tapered threads of the dovetail type also
referred to as wedge threads. Threaded joints employing wedge threads are configured
as follows. The thread width of the male threaded portion gradually decreases along
the thread helix in the right-hand screw direction, and the groove width of the
corresponding female threaded portion also gradually decreases along the thread helix
in the right-hand screw direction. Both the load flanks and the stabbing flanks have
negative flank angles, and at the completion of fastening thread, the stabbing flanks are
in contact with each other and the load flanks are in contact with each other, whereby
the threaded portions as a whole firmly engage with each other. Furthermore, in the
fastened state, crests and roots of the threaded portions are in intimate contact with each
other. Thus, threaded joints employing wedge threads are capable of providing high
torque resistance performance while ensuring sealing performance.
[0009]
However, in the case of the thread seal using wedge threads, a sufficient seal
cannot be formed unless the male threaded portion and the female threaded portion
engage in intimate contact with each other by strictly setting manufacturing tolerances
such as the radius of curvature for each of the threaded portions. Furthermore, the
thread seal can be formed substantially only in the region of complete threads, and
therefore, if a threaded joint is designed to have a relatively long incomplete thread
region, sealing performance as desired may not be obtained. In particular, when an
internal pressure or external pressure is excessively applied, the highly pressurized fluid
may penetrate into gaps at the thread seal, thereby incurring the risk of leakage.
[OO 1 01
As described above, there is a limit to sealing performance that can be ensured
solely by the thread seal. For this reason, attempts have hitherto been made to ensure
sealing performance against internal and external pressures by providing an internal seal
and an external seal separately from the thread seal. The internal seal is formed by
contact between a sealing surface of the pin disposed on a free end region thereof,
forward of the male threaded portion of the pin, and a sealing surface of the box
corresponding thereto. The external seal is formed by contact between a sealing
surface of the pin disposed on a region rearward of the male threaded portion thereof,
and a sealing surface of the box corresponding thereto.
[OOl 11
For example, United States Patent Application Publication No. 20 1010 1 8 1763
(Patent Literature 1) and International Publication No. W020041106797 (Patent
Literature 2) each disclose a threaded joint having an internal seal and an external seal
in addition to a thread seal. Of these threaded joints of the patent literatures, the
threaded joint of Patent Literature 2 is configured such that, in a fastened state,
clearances are provided between the crests and the roots of the threaded portions.
These clearances allow an excess of the lubricant (hereinafter also referred to as "dope"),
applied for fastening thread, to accumulate therein, thereby contributing to prevention of
an abnormal increase in the dope pressure.
[OO 121
It is true that the conventional threaded joints disclosed in Patent Literatures 1
and 2 exhibit improved sealing performance against pressures. However, threaded
joints having an external seal have a reduced cross sectional area of the critical section
of the pin (the cross section in the rearmost end region of the male threaded portion, i.e.,
the cross section in the male threaded portion closest to the tubular body), and therefore
have a reduced resistance to tensile forces. For this reason, it is not desirable to
provide an external seal from the standpoint of the threaded joint strength.
[00 1 31
In the case of threaded joints having an internal seal, when an excessively high
external pressure has been applied thereto, plastic deformation occurs in the region of
the sealing surface of the pin in the free end region thereof because the region of the
sealing surface does not have a sufficient wall thickness. As a result, there is a risk
that sealing performance may not be exhibited.
CITATION LIST
PATENT LITERATURE
[0014]
Patent Literature 1 : United States Patent Application Publication No.
201010181763
Patent Literature 2: International Publication No. W020041106797
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[00 1 51
One serious problem that a threaded joint may experience is the occurrence of
fluid leaks when an excessively high pressure (internal pressure or external pressure)
has been applied. In the case of threaded joints employing wedge threads, in particular,
it is important to reliably ensure the sealing performance against pressures.
[0016]
An object of the present invention is to provide a threaded joint for steel pipes
having the following characteristics: being capable of reliably providing high sealing
performance while maintaining high torque resistance performance of wedge threads,
i,e., dovetail-shaped tapered threads.
SOLUTION TO PROBLEM
[0017]
A threaded joint for steel pipes according to an embodiment of the present
invention includes a tubular pin and a tubular box, the pin and the box being fastened by
screwing the pin onto the box.
The pin includes, in order from a tubular body having the pin toward a free end
thereof: a tapered male threaded portion with dovetail threads; and a lip portion
including a sealing surface.
The box includes: a tapered female threaded portion with dovetail threads
corresponding to the male threaded portion of the pin; and a recessed portion
corresponding to the lip portion, the recessed portion including a sealing surface.
The lip portion includes, in order from the male threaded portion toward the free
end of the pin: a neck portion; and a sealing head portion including the sealing surface.
The sealing surface is disposed on a region in the sealing head portion, the region
having a maximum outside diameter that is larger than an outside diameter of the neck
portion at a boundary between the neck portion and the male threaded portion.
[00 1 81
The above threaded joint may be configured such that the sealing head portion
has an inside diameter that is smaller than an inside diameter of the tubular body.
When this configuration is employed, it is preferred that an inner peripheral surface of
the lip portion includes, in order from the free end of the pin: a tapered surface
increasing in diameter toward the free end; and a cylindrical surface that is continuous
with the tapered surface, and that the cylindrical surface has a length along the pipe axis
of at least 3 mm, the length extending from a boundary between the tapered surface and
the cylindrical surface. Alternatively, the above threaded joint may be configured such
that the sealing head portion has an inside diameter that is larger than an inside diameter
of the tubular body.
[OO 191
The above threaded joint may preferably be configured as follows. In a fastened
state, clearances are provided in at least one of the following: between crests of the male
threaded portion and roots of the female threaded portion; and between roots of the male
threaded portion and crests of the female threaded portion.
[0020]
The above threaded joint may preferably be configured as follows. The region
of the sealing surface in the sealing head portion has a wall thickness at a position of the
maximum outside diameter, the wall thickness being in a range of 55% to 80% of a wall
thickness of the tubular body, and the neck portion has a minimum wall thickness in a
range of 45% to 70% of the wall thickness of the tubular body.
[0021]
The above threaded joint may preferably be configured as follows. In a fastened
state, a clearance between an end face of the lip portion and an end of the recessed
portion on a tubular body side of the box is in a range of 0.1 mm to 3.0 mm.
[0022]
The above threaded joint may preferably be configured as follows. The male
threaded portion and the female threaded portion each have a taper angle in a range of
lo to 5' with respect to the pipe axis.
[0023]
The above threaded joint may preferably be configured as follows. The neck
portion has an outer peripheral surface, the outer peripheral surface being a cylindrical
surface, and the sealing head portion has an outer peripheral surface, the outer
peripheral surface including: a tapered surface that is continuous with the outer
peripheral surface of the neck portion and increases in diameter at an angle in a range of
5" to 20" with respect to the pipe axis; a cylindrical surface that is continuous with the
tapered surface; and the sealing surface, the sealing surface including a curved surface
that is continuous with the cylindrical surface and a tapered surface that decreases in
diameter toward the free end of the pin.
[0024]
The above threaded joint may preferably be configured as follows. The sealing
surface of the lip portion includes a tapered surface, the tapered surface having a taper
angle in a range of 3" to 10" with respect to the pipe axis.
[0025]
The above threaded joint may preferably be configured as follows. The male
threaded portion and the female threaded portion each have a thread height in a range of
1.0 mm to 3.0 mm.
ADVANTAGEOUS EFFECTS OF INVENTION
[0026]
A threaded joint for steel pipes according to the present invention has the
following significant advantages: being capable of reliably providing high sealing
performance while maintaining high torque resistance performance of wedge threads,
i.e., dovetail-shaped tapered threads.
BRIEF DESCRIPTION OF DRAWINGS
[0027]
[FIG. 11 FIG. 1 is a longitudinal sectional view of a threaded joint for steel pipes
according to a first embodiment.
[FIG. 21 FIG. 2 is a longitudinal sectional view showing a threaded portion in the
threaded joint for steel pipes according to the first embodiment.
[FIG. 31 FIG. 3 is a longitudinal sectional view showing an example of a free end
region of the pin in the threaded joint for steel pipes according to the first embodiment.
[FIG. 41 FIG. 4 is a longitudinal sectional view showing an example of a free end
region of the pin in a threaded joint for steel pipes according to a second embodiment.
[FIG. 51 FIG. 5 is a diagram showing the results of evaluation of torque
resistance performance in Example 1.
[FIG. 61 FIG. 6 is a diagram showing the results of evaluation of sealing
performance in Example 1.
[FIG. 71 FIG. 7 is a schematic diagram showing a model of an FEM analysis in
Example 2.
[FIG. 81 FIG. 8 is a diagram showing the results of evaluation of sealing
performance in Example 2.
[FIG. 91 FIG. 9 is a schematic diagram showing a model of an FEM analysis in
Example 3.
[FIG. 101 FIG. 10 is a diagram showing the results of evaluation of sealing
performance in Example 3.
DESCRIPTION OF EMBODIMENTS
[0028]
The present inventors had realized that conventional threaded joints that employ
the thread seal using wedge threads but do not have an internal or external seal pose a
problem with regard to sealing performance particularly against external pressures.
This is based on numerical simulations and analyses using the elasto-plastic finite
element method, performed by the present inventors, and evaluations of the results. In
view of the above, the present inventors firstly contemplated improving the sealing
performance against external pressure by providing an external seal and further an
internal seal as with the threaded joints disclosed in Patent Literatures 1 and 2.
[0029]
However, as described above, providing an external seal results in decreasing the
tensile force resistance of the threaded joint. In view of this, the present inventors
conceived of the idea of actively utilizing the internal seal so that it can provide the
sealing performance against external pressure as well while maintaining its sealing
performance against internal pressure, and they contemplated embodiments thereof.
[0030]
The pin is provided with a lip portion which extends from the male threaded
portion along the axis of the pipe. A sealing surface is provided on the outer periphery
of the lip portion to constitute an internal seal. External pressure entering from outside
of the threaded joint penetrates through the threaded portions and reaches a location
near the sealing surface in the lip portion of the pin. The external pressure that has
reached the lip portion of the pin acts to induce radial contraction of the lip portion and
thus to loosen the contact between the sealing surfaces.
[003 11
The present inventors appreciated that an effective way to prevent the
phenomenon is to enhance the stiffness of the lip portion of the pin so as to inhibit the
radially inward deformation of the lip portion, and therefore they directed their attention
to the geometry of the lip portion.
[0032]
One simple method for enhancing the stiffness of the lip portion is to enlarge the
outside diameter of the lip portion so that the lip portion has a larger wall thickness.
Certainly, a larger wall thickness of the lip portion leads to improvement in the sealing
performance against external pressure. However, in the pin, if the outside diameter of
the lip portion is excessively enlarged and the wall thickness thereof is excessively
increased, then the start point for thread machining of the male threaded portion will be
shifted to a position closer to the outer periphery of the pin as a result of the
enlargement of the outside diameter of the lip portion. Consequently, the thread height
will have to be lowered or the length of the threaded portion will have to be shortened
from the length as initially designed. As a result, the high torque resistance
performance that can be achieved by wedge threads may be lost.
[0033]
Furthermore, if it is desired to maintain the wall thickness of the region of the
sealing surface in the box when the outside diameter of the lip portion is enlarged, then
the outside diameter of the box (e.g., a coupling) has to be increased. When this is the
case, the clearance between an inner threaded joint and an outer threaded joint in the
well, which has a multiple wall structure, will be significantly reduced. Thus, it is
preferred that the outside diameter of the box not be increased.
[0034]
If the outside diameter of the box is maintained when the outside diameter of the
lip portion of the pin is enlarged to increase the wall thickness of the lip portion, then
the wall thickness of the region of the sealing surface in the box has to be reduced. In
such a case, even the sealing performance against internal pressure may be decreased.
[0035]
In view of the above, the present inventors have come to the conclusion that
optimizing the geometry of the lip portion of the pin and the wall thickness thereof is
effective at maintaining the high torque resistance performance of wedge threads while
reliably ensuring high sealing performance.
[0036]
The threaded joint of the present invention has been completed on the basis of
results of studies based on the above concepts. Embodiments of the threaded joint for
steel pipes according to the present invention are described below.
[0037]
[First Embodiment]
FIG. 1 is a longitudinal sectional view of a threaded joint for steel pipes
according to a first embodiment. FIG. 2 is a longitudinal sectional view showing a
threaded portion in the threaded joint for steel pipes. FIG. 3 is a longitudinal sectional
view showing an example of a free end region of the pin in the threaded joint for steel
pipes. As shown in FIGS. 1 to 3, the threaded joint of the first embodiment is a
coupling-type threaded joint employing wedge threads (dovetail-shaped tapered threads)
and is constructed of a pin 10 and a box 20.
[0038]
The pin 10 includes, in order from the tubular body having the pin 10 toward the
free end thereof: a male threaded portion 11; and a lip portion 12. The lip portion 12
extends continuously from the male threaded portion 11 along the pipe axis CL. The
lip portion 12 is composed of, in order toward the free end of the pin 10: a neck portion
14; and a sealing head portion 15. The sealing head portion 15 includes a sealing
surface 1 3.
[0039]
The box 20 includes, in order from the free end of the box 20 toward the tubular
body thereof: a female threaded portion 2 1 ; and a recessed portion 22. The female
threaded portion 2 1 is provided to correspond to the male threaded portion 1 1 of the pin
10. The recessed portion 22 is provided to correspond to the lip portion 12 of the pin
10. The recessed portion 22 includes a sealing surface 23 corresponding to the sealing
surface 13 of the pin 10.
[0040]
The neck portion 14 of the pin 10 connects the male threaded portion 1 1 to the
sealing head portion 15. The outer peripheral surface 14a of the neck portion 14 is a
cylindrical surface (hereinafter also referred to as "neck outer peripheral cylindrical
surface") whose central axis is the pipe axis CL. In the threaded joint shown in FIG. 3,
the inner peripheral surface 14b of the neck portion 14 includes a cylindrical surface
14ba (hereinafter also referred to as "neck inner peripheral cylindrical surface") and
tapered surface 14bb (hereinafter also referred to as "neck inner peripheral tapered
surface"), with the former being located closer to the male threaded portion 11 and
defining an inside diameter identical with that of the tubular body and the latter being
located closer to the sealing head portion 15 and decreasing in diameter toward the free
end of the pin 10. Alternatively, the neck inner peripheral cylindrical surface 14ba
may not be provided.
[004 11
The outer peripheral surface 15a of the sealing head portion 15 includes a tapered
surface 15aa (hereinafter also referred to as a "head outer peripheral inside tapered
surface") which is continuous with the outer peripheral surface 14a of the neck portion
14 and increases in diameter toward the free end of the pin 10. A cylindrical surface
15ab (hereinafter also referred to as "head outer peripheral cylindrical surface") whose
central axis is the pipe axis CL is continuous with the head outer peripheral inside
tapered surface 15aa. The sealing surface 13 is continuous with the head outer
peripheral cylindrical surface 15ab. The sealing surface 13 is composed of, for
example, a curved surface 13a (hereinafter also referred to as "seal curved surface") and
a tapered surface 13b (hereinafter also referred to as "seal tapered surface"). The seal
curved surface 13a is a surface that corresponds to a peripheral surface of a solid of
revolution that can be obtained by rotating a curved line such as an arc about the pipe
axis CL. The seal tapered surface 13b is a tapered surface which decreases in diameter
toward the free end of the pin 10.
[0042]
In the threaded joint shown in FIG. 3, the inner peripheral surface 15b of the
sealing head portion 15 includes a tapered surface 15ba (hereinafter also referred to as
"head inner peripheral inside tapered surface") that is continuous with the neck inner
peripheral tapered surface 14bb at the same taper angle. A cylindrical surface 15bb
(hereinafter also referred to as "head inner peripheral cylindrical surface") whose central
axis is the pipe axis CL is continuous with the head inner peripheral inside tapered
surface 15ba. A tapered surface 15bc (hereinafter also referred to as "head inner
peripheral foremost tapered surface") which slightly increases in diameter toward the
free end of the pin 10 is continuous with the head inner peripheral cylindrical surface
15bb. Alternatively, the head inner peripheral inside tapered surface 15ba may not be
provided. In such a case, the head inner peripheral cylindrical surface 15bb extends
into the region of the neck portion 14 to be continuous with the neck inner peripheral
tapered surface 14bb.
[0043]
The maximum outside diameter Dl of the region of the sealing surface 13 in the
sealing head portion 15 is larger than an outside diameter D2 of the neck portion 14 at a
boundary between the neck portion 14 and the male threaded portion 11. The
boundary between the neck portion 14 and the male threaded portion 1 1 corresponds to
the thread machining start point of the male threaded portion 11. By virtue of the
aforementioned configuration, in the lip portion 12 of the pin 10, it is possible to
increase the wall thickness of the sealing head portion 15, which has the sealing surface
13, without the need to shift the thread machining start point of the male threaded
portion 1 1 to a position closer to the outer periphery of the pin 10.
[0044]
In the threaded joint of the first embodiment in particular, the sealing head
portion 15 has an inside diameter dl smaller than the inside diameter do of the tubular
body. The inside diameter dl of the sealing head portion 15 is smallest in the region of
the head inner peripheral cylindrical surface 15bb. This can be readily accomplished
by swaging the free end region of the pin 10 in advance. The head inner peripheral
foremost tapered surface 15bc and the head inner peripheral cylindrical surface 15bb are
formed by machining after swaging. The neck inner peripheral tapered surface 14bb is
an as-swaged surface or a machined surface depending on the specifications. Since the
inside diameter dl of the sealing head portion 15 is smaller than the inside diameter do
of the tubular body, the wall thickness of the sealing head portion 15 can be sufficiently
increased without the need to enlarge the outside diameter of the sealing head portion 15
to a great extent.
[0045]
The male threaded portion 1 1 of the pin 10 and the female threaded portion 21 of
the box 20 are tapered threaded portions with dovetail threads (wedge threads) that can
engage with each other. The load flanks 11 c of the male threaded portion 11 and the
load flanks 21c of the female threaded portion 2 1, and the stabbing flanks 1 Id of the
male threaded portion 1 1 and the stabbing flanks 21 d of the female threaded portion 21,
each have a negative flank angle 0 in a range of about 1 " to 10" with respect to the pipe
axis CL. The male threaded portion 11 and the female threaded portion 21 are
threadedly engageable with each other, and in a fastened state, the load flanks 1 1 c and
the load flanks 2 1 c are in intimate contact with each other, and the stabbing flanks 1 1 d
and the stabbing flanks 2 1 d are in intimate contact with each other, so that the threaded
portions as a whole firmly engage with each other. The sealing surface 13 of the pin
10 and the sealing surface 23 of the box 20 are brought into contact with each other by
the screwing of the pin 10 and, in a fastened state, they engage in intimate contact with
each other to have an interference fit, so as to form an internal seal by surface-to-surface
contact.
[0046]
Thus, with the threaded joint according to the first embodiment, it is possible to
maintain high torque resistance performance because it employs wedge threads.
[0047]
Moreover, the sealing head portion 15, which has the sealing surface 13, has an
increased wall thickness, and therefore the region of the sealing surface 13 has increased
stiffness. As a result, even if external pressure has reached a location near the sealing
surface 13 of the lip portion 12, the internal seal effected by the sealing surface 13
provides high sealing performance. This is due to the fact that radially inward
deformation is inhibited in the region of the sealing surface 13. It is noted that the
internal seal provides, as its inherent function, high sealing performance against internal
pressure as well.
[0048]
Furthermore, there is no need to shift the thread machining start point of the male
threaded portion 11 to a position closer to the outer periphery of the pin 10, and
therefore it is possible to ensure a sufficient length of the threaded portion while
maintaining the high thread height. As a result, the high torque resistance performance
of wedge threads is sufficiently ensured.
[0049]
The following are additional descriptions of preferred embodiments of the
threaded joint according to the first embodiment.
[0050]
In the threaded joint of the first embodiment, clearances 3 1 are provided between
the crests 1 1 a of the male threaded portion 1 1 and the roots 2 1 b of the female threaded
portion 21 in a fastened state. These clearances 3 1 prevent an abnormal increase in the
dope pressure. On the other hand, the roots 1 1 b of the male threaded portion 1 1 are in
contact with the crests 21 a of the female threaded portion 2 1. It is to be noted that
clearances may be provided both between the crests 1 la of the male threaded portion 1 1
and the roots 2 1 b of the female threaded portion 2 1 and between the roots 1 1 b of the
male threaded portion 1 1 and the crests 2 1 a of the female threaded portion 2 1.
Alternatively, clearances may be provided solely between the roots 1 1 b of the male
threaded portion 1 1 and the crests 21 a of the female threaded portion 21. A preferred
range of each clearance is from 0.05 to 0.5 mm. Within this range, a sufficient height
of engagement between the male threaded portion 1 1 and the female threaded portion 2 1
can be ensured, so that high torque performance can be provided.
[005 11
The taper angles of the male threaded portion 11 and the female threaded portion
21 are each preferably in a range of 1" to 5" with respect to the pipe axis CL. If the
taper angles of the threaded portions are too large, the lengths of the threaded portions
are excessively shortened and therefore the torque resistance performance will be
decreased. On the other hand, if the taper angles of the threaded portions are too small,
the lengths of the threaded portions are excessively elongated and therefore the cost of
manufacturing will be increased. A more preferred lower limit of the taper angles is
1.5" and a still more preferred lower limit thereof is 2". A more preferred upper limit
of the taper angles is 4".
[0052]
The thread heights of the male threaded portion 11 and the female threaded
portion 21 are each preferably in a range of 1.0 mm to 3.0 mm. If the thread heights
are too high, the cost of manufacturing will be increased, and in addition, the sealing
performance against internal pressure will be decreased because of the reduced wall
thickness of the box 20. On the other hand, if the thread heights are too low, the torque
resistance performance will be decreased. A more preferred lower limit of the thread
heights is 1.2 mm and a still more preferred lower limit thereof is 1.5 mm.
[0053]
In the sealing head portion 15 which constitutes the lip portion 12 of the pin 10,
the wall thickness of the region of the sealing surface 13 at the position of the maximum
outside diameter is preferably in a range of 55% to 80% of the wall thickness of the
tubular body. If the wall thickness of the region of the sealing surface 13 is too thin,
the sealing performance against external pressure cannot be ensured because the
stiffness is decreased. On the other hand, if the wall thickness of the region of the
sealing surface 13 is too large, the stiffness is increased. However, if the increase of
the wall thickness of the region of the sealing surface 13 is to be accomplished by
enlarging the outside diameter of the sealing head portion 15, then the diameter of the
sealing surface 23 of the box 20 is enlarged, and the resulting decrease in the wall
thickness of the box 20 may cause a decrease in sealing performance against internal
pressure. If the increase of the wall thickness of the region of the sealing surface 13 is
to be accomplished by reducing the inside diameter of the sealing head portion 15, then
it is necessary to limit the inside diameter of the sealing head portion 15, particularly the
inside diameter of the head inner peripheral cylindrical surface 15bb, to the inside
diameter required by API specifications.
[0054]
In the meantime, the neck portion 14 which constitutes the lip portion 12 of the
pin 10 preferably has a minimum wall thickness in a range of 45% to 70% of the wall
thickness of the tubular body. If the minimum wall thickness of the neck portion 14 is
too thin, the sealing performance against external pressure cannot be ensured because
the stiffness of the neck portion 14 is decreased. On the other hand, if the minimum
wall thickness of the neck portion 14 is too large, the thread machining start point is to
be shifted to a position closer to the outer periphery of the pin 10, and therefore the
thread height will have to be lowered or the lengths of the threaded portions will have to
be shortened, with the result that the torque resistance performance of wedge threads is
not exhibited.
[0055]
Of all portions of the outer peripheral surface 15a of the sealing head portion 15,
the head outer peripheral inside tapered surface 15aa, which is continuous with the outer
peripheral surface 14a of the neck portion 14, preferably has a taper angle in a range of
5" to 20" with respect to the pipe axis CL. If the taper angle of the head outer
peripheral inside tapered surface 15aa is larger, the stiffness of the sealing surface 13 is
increased as a result of the increased wall thickness of the sealing head portion 15, but
the sealing performance against internal pressure will be decreased because of the
reduced wall thickness of the box 20. On the other hand, if the taper angle of the head
outer peripheral inside tapered surface 15aa is too small, the sealing performance
against external pressure will be decreased because a sufficient wall thickness of the
region of the sealing surface 13 cannot be provided.
[0056]
Of all portions of the inner peripheral surfaces 14b and 15b of the lip portion 12,
the head inner peripheral cylindrical surface 15bb (including the case in which it
extends into the region of the neck portion 14) preferably has a length LI along the pipe
axis CL of at least 3 mm. The length LI is a length extending from the boundary
between the head inner peripheral foremost tapered surface 15bc and the head inner
peripheral cylindrical surface 15bb. The head inner peripheral foremost tapered
surface 15bc has a length L2 along the pipe axis CL of about 8 to 12 mm. If the length
LI of the head inner peripheral cylindrical surface 15bb is too short, the stiffness of the
sealing head portion 15 is not sufficiently increased, and therefore the sealing
performance against external pressure cannot be ensured. A preferred lower limit of
the length LI is 4 mm. In the meantime, the length LI is preferably as long as possible.
However, the length LI depends on the degree of swaging. Thus, a preferred upper
limit of the length LI is 15 mm.
[0057]
The seal tapered surface 13b which constitutes the sealing surface 13 of the lip
portion 12 preferably has a taper angle in a range of 3" to 10" with respect to the pipe
axis CL. If the taper angle of the seal tapered surface 13b is larger, the stiffness is
decreased in an end portion of the region of the sealing surface 13 as a result of the
reduced wall thickness therein, and therefore the sealing performance against external
pressure will be decreased. On the other hand, if the taper angle of the seal tapered
surface 13b is too small, the sealing surfaces 13,23 slide relative to each other when a
tensile load is applied to the threaded joint, so that the contact between them may be
lost.
[0058]
In a fastened state, a clearance 30 is provided between the end face of the lip
portion 12 of the pin 10 and the tubular body-side end of the recessed portion 22 of the
box 20 (see FIG. 1). The clearance 30 is preferably in a range of 0.1 mm to 3.0 mm.
Without the clearance 30, the end face of the pin 10 would inadvertently come into
contact with the recessed portion 22 of the box 20 during fastening thread. As a result,
the engagement between the male threaded portion 11 and the female threaded portion
21 becomes insufficient, and therefore the torque resistance performance cannot be
obtained. On the other hand, if the clearance 30 is too large, turbulence of fluids
flowing through the threaded joint can occur in the vicinity of the clearance 30, which
can induce erosion.
[0059]
[Second Embodiment]
FIG. 4 is a longitudinal sectional view showing an example of a free end region
of the pin in the threaded joint for steel pipes according to the second embodiment.
The threaded joint according to the second embodiment shown in FIG. 4 is a variation
of the threaded joint according to the first embodiment shown in FIGS. 1 to 3, and
therefore descriptions redundant to those given in the first embodiment will not be
repeated where appropriate.
[0060]
In the threaded joint according to the second embodiment, the neck inner
peripheral tapered surface 14bb, of the inner peripheral surface 14b of the neck portion
14 which constitutes the lip portion 12, is a tapered surface that increases in diameter
toward the free end of the pin 10. Thus, the inside diameter dl of the sealing head
portion 15 is larger than the inside diameter do of the tubular body. It is noted however
that, in the second embodiment as well similarly to the first embodiment, the maximum
outside diameter DI of the region of the sealing surface 13 in the sealing head portion
15 is larger than an outside diameter D2 of the neck portion 14 at a boundary between
the neck portion 14 and the male threaded portion 11. Consequently, the threaded joint
of the second embodiment also produces advantageous effects similar to those of the
first embodiment described above.
[006 11
However, in the case of the second embodiment, an increase of the wall thickness
of the region of the sealing surface 13 results in an enlargement of the outside diameter
of the region of the sealing surface 13. As a result, the diameter of the sealing surface
23 of the box 20 is enlarged, and the resulting decrease in the wall thickness of the box
20 may cause a decrease in sealing performance against internal pressure. In this
regard, the first embodiment is more advantageous.
[0062]
The present invention is not limited to the embodiments described above, and
various modifications may be made without departing from the spirit and scope of the
present invention. For example, the threaded joints according to the above
embodiments may be employed not only as a coupling-type threaded joint but also as an
integral-type threaded joint.
EXAMPLES
[0063]
To verify the advantages of the present invention, numerical simulations and
analyses were carried out using the elasto-plastic finite element method.
[0064]
EXAMPLE 1
Test Conditions
~ o d e lfso r the FEM analysis were prepared based on coupling-type threaded
joints for oil country tubular goods employing wedge threads. For the inventive
example, a model of the threaded joint of the first embodiment shown in FIGS. 1 to 3
was prepared. For a comparative example, a model of a typical conventional threaded
joint, in which the maximum outside diameter of the region of the sealing surface in the
sealing head portion is smaller than the outside diameter of the neck portion at the
thread machining start point, was prepared. The representative dimensions of each
model are shown in Table 1.
[0065]
[Table 11
TABLE 1
[0066]
The common conditions were as follows.
- Size of threaded joint (steel pipe): 7-518 [inch] x 55,3[lb./ft.]
- Grade of steel pipe (pin) and coupling (box): API standard T95 (carbon steel
having a yield stress of 95 [ksi] (655 [MPa])).
[0067]
Evaluation Method
In the FEM analysis, a load sequence which simulated that in an IS0 13679
Series A test was applied to each model in a fastened state. In the analysis, the torque
Inventive Ex.
193.68 [mm]
155.58 [mm]
182.9 [mm]
1 84.4 [mm]
7.0 ["I
14.3 [mm]
1 5 5.5 [mm]
2.39 ["I
155.4 [mm]
2.5 [mm]
222.4 [mm]
155.21 [mm]
183.8 [mm]
2.9 ["I
19.3 [mm]
2.39 ["I
155.4 [mm]
2.6 [mm]
2.0 [mm]
Comparative Ex.
194.8 [mm]
156.8 [mm]
171.7 [mm]
17 1.4 [mm]
2.9 ["I
4.6 [mm]
163.0 [mm]
3.58 ["I
150 [mm]
2.8 [mm]
2 15.3 [mm]
161.9 [mm]
170.6 [mm]
2.9 ["I
13.5 [mm]
3.58 ["I
150 [mm]
2.8 [mm]
5.45 [mm]
Object
Pin
(Steel Pipe)
Box
(Coupling)
Joint
Item
Outside diameter of tubular body
Inside diameter of tubular body
Diameter at thread machining start point
Diameter of seal portion
Taper angle of seal portion
Wall thickness of seal portion
Inside diameter of sealing head Portion
Taper angle of male threaded Portion
Length of male threaded portion
Height of male threads
Outside diameter of tubular body
Inside diameter of tubular body
Diameter of seal portion
Taper angle of seal portion
Wall thickness of seal portion
Taper angle of female threaded Portion
Length of female threaded portion
Height of female threads
Clearance between pin end face and box
resistance performance was evaluated by using the values at the point of yielding (yield
torque) in the torque chart and comparing the values. Furthermore, the sealing
performance was evaluated by comparing the minimum values of the seal contact force
of the sealing surfaces (the product of the average seal contact pressure and the seal
contact width, of the sealing surfaces) in the internal pressure cycle (the first and second
quadrants) and the external pressure cycle (the third and fourth quadrants) in the load
sequence. (It is noted that the higher the minimum value of the average contact pressure,
the better the sealing performance of the sealing surfaces.)
[0068]
Test Results
FIG. 5 is a diagram showing the results of evaluation of torque resistance
performance in Example 1. FIG. 6 is a diagram showing the results of evaluation of
sealing performance in Example 1. As shown in FIG. 5, high torque resistance
performance was exhibited in both Inventive Example and Comparative Example. In
Comparative Example, the contact between the sealing surfaces was lost as shown in
FIG. 6. In contrast, Inventive Example showed markedly improved sealing
performance.
[0069]
EXAMPLE 2
Test Conditions
Models for the FEM analysis were prepared based on coupling-type threaded
joints for oil country tubular goods employing wedge threads, with the geometries of
the lip portions varied based on the threaded joints of the first and second embodiments
shown in FIGS. 1 to 4. Specifically, as shown in FIG. 7, by varying the inside
diameter of the sealing head portion, the wall thickness of the region of the sealing
surface at a position of the maximum outside diameter in the sealing head portion
relative to the wall thickness of the tubular body (hereinafter also referred to as the
"wall thickness of the seal portion relative to the wall thickness of the tubular body")
was varied to provide five different levels of wall thickness, namely, 50%, 56%, 6 I%,
74% and 83%. The representative dimensions of each model are shown in Table 2.
[0070]
[Table 21
TABLE 2
[0071]
The common conditions were as follows.
- Size of threaded joint (steel pipe): 7 [inch] x 35.0 [lb./ft.]
- Grade of steel pipe (pin) and coupling (box): API standard L80 (carbon steel
having a yield stress of 80 [ksi] (552 [MPa])).
[0072]
The models of Test Nos. 1 to 3 are based on the threaded joint of the second
Object
Pin
(Steel Pipe)
Box
(Coupling)
Joint
Item
Outside diameter of tubular body [mm]
Inside diameter of tubular body [mm]
Diameter at thread machining start point
[mml
Diameter of seal portion [mm]
Taper angle of seal portion ["I
Wall thickness of seal portion relative to wall
thickness of tubular body [%]
Inside diameter of sealing head portion [mm]
Taper angle of male threaded portion ["I
Length of male threaded portion [mm]
Height of male threads [mm]
Outside diameter of tubular body [mm]
Inside diameter of tubular body [mm]
Diameter of seal portion [mm]
Taper angle of seal portion ["I
Wall thickness of seal portion [mm]
Taper angle of female threaded portion ["I
Length of female threaded portion [mm]
Height of female threads [mm]
Clearance between pin end face and box
[mml
Test No.
1 2
177.8
152.5
169.8
170.9
7.0
3
50
158.2
4
1.79
151.9
2.2
198.2
151.5
170.3
2.9
13.9
1.79
151.9
2.3
2.0
56
156.8
61
155.4
5
74
152.0
83
149.8
embodiment shown in FIG. 4, each including a sealing head portion having an inside
diameter larger than the inside diameter of the tubular body. The models of Test Nos.
4 and 5 are based on the threaded joint of the first embodiment shown in FIG. 3, each
including a sealing head portion having an inside diameter smaller than the inside
diameter of the tubular body.
[0073]
Evaluation Method
As in Example I , a load sequence which simulated that in an IS0 13679 Series A
test was applied to each model in a fastened state, and the sealing performance was
evaluated by comparing the minimum values of the seal contact force of the sealing
surfaces.
[0074]
Test Results
FIG. 8 is a diagram showing the results of evaluation of sealing performance in
Example 2. The results shown in FIG. 8 demonstrate that, as the wall thickness of the
seal portion relative to the wall thickness of the tubular body increases, i.e., the wall
thickness of the lip portion increases, the sealing performance is improved. With
regard to the extent of the increase of the wall thickness of the lip portion, in particular,
it is seen that, when the wall thickness of the seal portion relative to the wall thickness
of the tubular body is at least 55%, sufficient sealing performance is ensured.
[0075]
EXAMPLE 3
Test Conditions
Models for the FEM analysis were prepared based on coupling-type threaded
joints for oil country tubular goods employing wedge threads, with the geometries of
the lip portions varied based on the threaded joint of the first embodiment shown in
FIGS. 1 to 3. Specifically, as shown in FIG. 9, the length of the head inner peripheral
cylindrical surface was varied to provide four different levels of length, namely, 2 mm,
3 mm, 4 mm, and 5mm. The representative dimensions of each model are shown in
Table 3.
[0076]
[Table 31
TABLE 3
[0077]
The common conditions were as follows.
- Size of threaded joint (steel pipe): 3-112 [inch] x 9.2 [lb./ft.]
- Grade of steel pipe (pin) and coupling (box): API standard L80 (carbon steel
having a yield stress of 80 [ksi] (552 [MPa])).
[0078]
Object
Pin
Box
(Coupling)
Joint
Item
Outside diameter of tubular body [mm]
Inside diameter of tubular body [mm]
Diameter at thread machining start point [mm]
Diameter of seal portion [mm]
Taper angle of seal portion ["I
Wall thickness of seal portion [mm]
Length of head inner peripheral cylindrical
surface [mm]
Inside diameter of sealing head portion [mm]
Taper angle of male threaded portion [O]
Length of male threaded portion [mm]
Height of male threads [mm]
Outside diameter of tubular body [mm]
Inside diameter of tubular body [mm]
Diameter of seal portion [mm]
Taper angle of seal portion ["I
Wall thickness of seal portion [mm]
Taper angle of female threaded portion ["I
Length of female threaded portion [mm]
Height of female threads [mm]
Clearance between pin end face and box [mm]
Test No.
11 12
88.9
76.0
82.6
83.3
7.0
4.5
13
2
14
74.2
2.39
87.9
1.25
98.1
74.1
82.9
2.9
7.6
2.39
87.9
1.35
2.0
3 4 5
Evaluation Method
As in Examples 1 and 2, a load sequence which simulated that in an IS0 13679
Series A test was applied to each model in a fastened state, and the sealing performance
was evaluated by comparing the minimum values of the seal contact force of the sealing
surfaces.
[0079]
Test Results
FIG. 10 is a diagram showing the results of evaluation of sealing performance in
Example 3. The results shown in FIG. 10 demonstrate that, as the length of the head
inner peripheral cylindrical surface increases, the sealing performance is improved. In
particular, it is seen that, when the length of the head inner peripheral cylindrical surface
is at least 3 mm, sufficient sealing performance is ensured.
INDUSTRIAL APPLICABILITY
[OOSO]
A threaded joint according to the present invention is capable of being effectively
utilized in connecting steel pipes that are used for extraction, production, or transport of
underground resources. Examples of underground resources include fossil fuels such
as crude oil, natural gas, shale gas, and methane hydrate, and also include gas or liquid
underground resources such as ground water and hot springs.
REFERENCE SIGNS LIST
[OOSl]
10: pin, 1 1 : male threaded portion,
1 1 a: crest, 1 1 b: root,
1 1 c: load flank, 1 1 d: stabbing flank,
12: lip portion, 13: sealing surface,
14: neck portion, 15 : sealing head portion,
20: box, 21 : female threaded portion,
2 1 a: crest, 2 1 b: root,
21c: load flank, 2 Id: stabbing flank,
22: recessed portion, 23: sealing surface,
30, 3 1 : clearance,
Dl: maximum outside diameter of region of sealing surface in sealing head
portion,
D2: outside diameter of neck portion at boundary with male threaded portion,
dl: inside diameter of sealing head portion,
do: inside diameter of tubular body,
CL: pipe axis,
LI: length of sealing head inner peripheral cylindrical surface along pipe axis,
L2: length of sealing head inner peripheral foremost tapered surface along pipe
axis.

We claim:
1. A threaded joint for steel pipes, comprising: a tubular pin and a tubular box, the
pin and the box being fastened by screwing the pin onto the box,
the pin comprising, in order from a tubular body having the pin toward a free end
thereof: a tapered male threaded portion with dovetail threads; and a lip portion
including a sealing surface,
the box comprising: a tapered female threaded portion with dovetail threads
corresponding to the male threaded portion of the pin; and a recessed portion
corresponding to the lip portion, the recessed portion including a sealing surface,
characterized in that,
the lip portion includes, in order from the male threaded portion toward the free
end of the pin: a neck portion; and a sealing head portion including the sealing surface,
and
the sealing surface is disposed on a region in the sealing head portion, the region
having a maximum outside diameter that is larger than an outside diameter of the neck
portion at a boundary between the neck portion and the male threaded portion.
2. The threaded joint for steel pipes according to claim 1, characterized in that,
the sealing head portion has an inside diameter that is smaller than an inside
diameter of the tubular body.
3. The threaded joint for steel pipes according to claim 2, characterized in that,
the lip portion has an inner peripheral surface, the inner peripheral surface
including, in order from the free end of the pin: a tapered surface increasing in diameter
toward the free end; and a cylindrical surface that is continuous with the tapered surface,
the cylindrical surface having a length along the pipe axis of at least 3 mm, the length
extending from a boundary between the tapered surface and the cylindrical surface.
4. The threaded joint for steel pipes according to claim 1, characterized in that,
the sealing head portion has an inside diameter that is larger than an inside
diameter of the tubular body.
5. The threaded joint for steel pipes according to any one of claims 1 to 4,
characterized in that,
in a fastened state, clearances are provided in at least one of the following:
between crests of the male threaded portion and roots of the female threaded portion;
and between roots of the male threaded portion and crests of the female threaded
portion.
6. The threaded joint for steel pipes according to any one of claims 1 to 5,
characterized in that,
the region of the sealing surface in the sealing head portion has a wall thickness
at a position of the maximum outside diameter, the wall thickness being in a range of
55% to 80% of a wall thickness of the tubular body, and
the neck portion has a minimum wall thickness in a range of 45% to 70% of the
wall thickness of the tubular body.
7. The threaded joint for steel pipes according to any one of claims 1 to 6,
characterized in that,
in a fastened state, a clearance between an end face of the lip portion and the
recessed portion of the box is in a range of 0.1 mm to 3.0 mm.
8. The threaded joint for steel pipes according to any one of claims 1 to 7,
characterized in that,
the male threaded portion and the female threaded portion each have a taper angle
in a range of lo to 5' with respect to the pipe axis.
9. The threaded joint for steel pipes according to any one of claims 1 to 8,
characterized in that,
the neck portion has an outer peripheral surface, the outer peripheral surface
being a cylindrical surface, and
the sealing head portion has an outer peripheral surface, the outer peripheral
surface including: a tapered surface that is continuous with the outer peripheral surface
of the neck portion and increases in diameter at an angle in a range of 5" to 20" with
respect to the pipe axis; a cylindrical surface that is continuous with the tapered surface;
and the sealing surface, the sealing surface including a curved surface that is continuous
with the cylindrical surface and a tapered surface that decreases in diameter toward the
free end of the pin.
10. The threaded joint for steel pipes according to any one of claims 1 to 9,
characterized in that,
the sealing surface of the lip portion includes a tapered surface, the tapered
surface having a taper angle in a range of 3" to 10" with respect to the pipe axis.
11. The threaded joint for steel pipes according to any one of claims 1 to 10,
characterized in that,
the male threaded portion and the female threaded portion each have a thread
height in a range of 1.0 mm to 3.0 mm.