The present invention relates to a threaded joint for use in connecting steel pipes
or tubes (hereinafter also referred to as "steel pipes").
BACKGROTIND ART
[0002]
In oil wells, natural gas wells, and the like (hereinafter also collectively referred
to as "oil wells"), oil well tubular (ocrc : oil country tubular goods), such as casings
and tubings, are used for extraction ofunderground resources. Oil country tubular
goods are sequentially connected to each other, and threadedjoints are used for the
connection.
[0003]
Threaded joints for steel pipes are classified into two types: coupling-type joints
and integral{ype joints. A coupling-type threaded joint is constituted by a pair of
tubular goods that are to be cor¡rected 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 the¡eof. The male threaded
portion of the steel pipe is screwed into the female threaded portion of the coupling,
thereby fastening the tbread to complete connecting. An integral-type threaded joint is
constituted by a pair ofsteel 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 irurer periphery at the other end thereof. The male
threaded portion of one of the steel pipes is screwed into the female threaded portion of
the other of the steel pipes, thereby fastening the thread to complete connecting.
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[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 lnale tlueaded portion. Pins and boxes both have a tubular
shape because they are constituted by end portions oftubular goods.
[000s]
FIG. 1 is a longitudinal sectional view showing an example of a typical
conventional threaded joint for steel pipes. The threaded joint shown in FIG. 1 is a
coupling-type threaded joint and is constructed of a pin 110 and a box 120.
[0006]
The pin 110 includes, in order from the free end thereof toward the tubular body,
a shoulder surface 111, a sealing surface 113, and a male threaded portion l14. The
box 120 includes, in order from the tubular body side toward the free end, a shoulder
surface l2l, a sealing surface 123, and a female threaded pofüon I24. The shoulder
surface 12I,the sealing surface 123, and the female threaded portion 124 of thebox 120
are provided corresponding to the shoulder surface 111, the sealing surface 113, and the
male threaded portion 1 1 4 of the pin 1 1 0, respectively. The male threaded portion 1 14
of the pin I 10 and the female threaded portion 124 of thebox 120 are configured to
engage with each other and constitute a tapered threaded portion with trapezoidal
threads.
[0007]
The male threaded portion 114 and the female threaded portion 724 are
threadedly engageable with each other, and in a fastened state, they engage in intimate
contact with each other to have an interference fit. The sealing surfaces 773,723 are
brought into contact with each other by the screwing of the pin 110, and in a fastened
state, they engage in intimate contact with each other to have an interference fit. With
this, the sealing surfaces 113, I23 form a seal portion therebetween with metal-to-metal
contact. The shoulder surfaces 717,127 are brought into contact and pressed against
each other by the screwing of the pin 110, and serye as stoppers for restricting the
screwing of the pin 110. The shoulder surfaces l7l, 72l,in a fastened state, serve to
i,:
impart the so-called thread tightening axial force to load flanks of the male threaded
portion 1 14 of the pin 1 I 0.
100081
With a threaded joint having this configuration, the sealing performance is
ensured by the engagement and intimate contact between the sealing surfaces I73,123
as well as the engagement and intimate contact between the male threaded portion 114
and the female threaded portion 124.
[000e]
In recent years, oil well environments have increasingly become
deep-underground or ultra deep-water environments, and accordingly have become
harsh environments with high temperatures, high pressures, and high corrosivity. For
application in such harsh environments, heavy wall steel pipes are widely used as oil
country tubular goods. Threaded joints used to connect such steel pipes are required to
provide excellent sealing perfonnance against internal pressure and external pressure.
[0010]
The fotrlowing are examples of conventional techniques that have attempted to
irnprove the sealing performance of threaded joints for heavy wall steel pipes.
[00i 1]
FIG. 2 is a longitudinal sectional view showing an example of a conventional
threaded joint for steel pipes which was designed to provide improved sealing
performance. The threaded joint shown in FIG. 2 includes two metal-to-metal seal
portions (see, for example, International Publication No. WOO1/029416 (Patent
Literature 1)).
[0012]
Specifically, as shown in FIG. 2, the pin 210 includes, in order from the free end
thereof toward the tubular body, a shoulder surface 217, a first sealing surface 2I3, a
first male th¡eaded portion 214, a second sealing surface 276, and a second male
threaded pofüon2l7. The box 220 includes, in order from the tubular body side
toward the free end, a shoulder surface 221, af,rst sealing surface 223, afirst female
threaded portion 224, a second sealing surface 226, and a second female threaded
pofüon227 . A fust threaded portion constituted by the first male threaded portion2I4
and the fust female threaded portion 224 is a tapered threaded portion with trapezoidal
threads. A second threaded portion constituted by the second male th¡eaded portion
271 and the second female threaded portion 227 is also a tapered th¡eaded portion with
trapezoidal threads.
[0013]
FIG. 3 is a longitudinal sectional view showing another example of a
conventional threaded joint for steel pipes which was designed to provide improved
sealing performance. The threaded joint shown in FIG. 3 is similar to the threaded
joint shown in FIG. 2 inlha| it includes two metal-to-metal seal portions, but differs in
that the locations of the shoulder surfaces have been modified (see, for example, U. S.
Patent No.4,662,659 (Patent Literature 2)).
[0014]
Specifically, as shown in FIG. 3, the pin 210 includes, in order from the free end
thereof toward the tubular body, a first sealing surface 2I3, aflrst male threaded portion
2I4, a shoulder surface 211, a second sealing surface 276, and a second male threaded
portion 217. The box 220 includes, in order from the tubular body side toward the free
end, a first sealing surface 223, a first female threaded portion 224, a shoulder surface
221, a second sealing surface 226, and a second female threaded porIion227. A first
threaded portion constituted by the first male threaded portion 2I4 and the first female
threaded portion 224 and a second threaded portion constituted by the second male
threaded portion 277 and.the second female threaded portion 227 are each a tapered
threaded portion with trapezoidal threads.
[0015]
Úr the case of the threaded joint shown in FIG. 2, the tapered surface defined by
the fust threaded portion and the tapered surface defined by the second threaded portion
are the same. In this configuration, the second sealing surfaces 216,226 are simply
added between the first threaded portion and the second threaded portion. In contrast,
in the case of the threaded joint shown in FIG. 3, the tapered surface defined by the first
threaded portion is closer to the pipe axis CL than the tapered surface defined by the
second threaded portion. h this configuration, the shoulder surface 2I1,227 are
provided between the first threaded portion and the second threaded portion.
[0016]
kr both the threaded joints shown in FIGS. 2 and3,the rnale tfueaded portion 214
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and the female threaded portion 224 are threadedly engageable with each other, and in a
fastened state, they engage in intimate contact with each other to have an interference fit.
Likewise, the second male threaded portion 277 and the second female threaded portion
227 have an interference fit between them. The f,rst sealing surfaces 213,223 and the
second sealing surfaces 276,226 are each brought into contact with each other by the
screwing of the pin 2I0 and,,in a fastened state, they engage in intimate contact'ù/ith
each other to have an interference fit. The shoulder surfaces 217,221 are brought into
contact and pressed against each other by the screwing of the pin 210.
[0017]
With a threaded joint having this configuration, sealing performance primarily
against internal pressure is ensured by the engagement and intimate contact between the
first sealing surfaces 213 , 223. Also, sealing performance primarily against extemal
pressure is ensured by the engagement and intimate contact between the second sealing
surfaces 216,226.
CITATIONLIST
PATENT LITERATURE
[00i 8]
Patent Literature 1: Írternational Publication No. WO011029476
Patent Literature 2: United States Patent No. 4,662,659
SUMMARY OF INTV'ENTION
TECHNICALPROBLEM
[001e]
Threaded joints for use in harsh environments of recent years, particularly
threaded joints for heavy wall steel pipes, are required to provide further improved
sealing performance against internal and external pressures.
[0020]
An object of the present invention is to provide a threaded joint for steel pipes
having the following characteristics: improved sealing performance against internal
pressure and external pressure.
SOLUTION TO PROBLEM
[0021]
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 including: in order from an end thereof, a
shoulder surface; a first sealing surface; a tapered fnst male threaded portion; a second
sealing surface; and a tapered second male threaded portion, the box including: a
shoulder surface; a flrst sealing surface; a tapered flirst female threaded portion; a
second sealing surface; and a tapered second female threaded portion, corresponding to
the shoulder surface, the first sealing surface, the first male threaded portion, the second
sealing surface, and the second male threaded portion, of the pin, respectively.
Further, the pin includes: a nose portion disposed between the shoulder surface
and the first sealing surface, the nose portion being continuous with the first sealing
surface; and an annular portion disposed between the f,rst male threaded portion and the
second sealing surface, the annular portion being continuous with the second sealing
surface, the box includes: a recessed portion corresponding to the nose portion of the
pin; and an annular portion corresponding to the annular portion of the pin.
This threaded joint is configured such that, in a fastened state: the shoulder
surfaces are in contact with each other; the f,rst sealing surfaces are in contact with each
other; the second sealing surfaces are in contact with each other; a clearance is provided
between the nose portion of the pin and the recessed portion of the box; a clearance is
provided between the annular portion of the pin and the annular portion of the box; the
first male threaded portion engages with the frst female threaded portion; and the
second male threaded portion engages with the second female threaded portion.
la022l
The above threaded joint may have the following configuration. The pin
includes an auxiliary shoulder surface disposed between the first male threaded portion
and the annular portion, the auxiliary shoulder surface being continuous with the
annular portion; the box includes an auxiliary shoulder surface corresponding to the
auxiliary shoulder surface of the pin; and, in a fastened state, the auxiliary shoulder
surfaces are in contact with each other.
[0023]
This threaded joint may preferably have the following configuration. During
fastening thread, the contact between the shoulder surfaces and the contact between the
auxiliary shoulder surfaces occur simultaneously, or the contact between the auxiliary
shoulder surfaces occurs prior to the contact between the shoulder surfaces.
100241
Furthermore, the above threaded joint may preferably have the following
configuration. The pin is configured such that, when a tubular body having the pin has
a cross-sectional area Ao in a plane perpendicular to the pipe axis, and the shoulder
surface has a projected area Al in a plane perpendicular to the pipe axis, an area ratio
ArlAo is at least 30ol0.
[002s]
Furthermore, the above threaded joint may preferably have the following
configuration. The pin is configured such that, when a tubular body having the pin has
a cross-sqctional area Ao in a plane perpendicular to the pipe axis, and the shoulder
surface and the auxiliary shoulder surface have projected areas in a plane perpendicular
to the pipe axis with a total of the projected areas being designated as Az, anarea ratio
Az/Ao is at least 30olo.
[0026]
Furthermore, the above threaded joint may preferably have the following
configuration. The pin is configured such that, when the shoulder surface and the
auxiliary shoulder surface have projected areas in a plane perpendicular to the pipe axis,
with a total of the projected areas being designated as Az and the projected area of the
shoulder surface being designated as ,A'1, aî area ratio ü/Az is at ieast 35%.
100271
In addition, the above threaded joint may preferably have the following
configuration. The annular portion of the pin has an outside diameter greater than a
diameter ofa reference tapered surface, the reference tapered surface being a tapered
surface having an outside diarneter srnaller than an outside diameter of an extension of a
tapered surface defined by roots of the second male threaded portion by twice a height
of threads of the second male threaded portion.
[0028]
In addition, the above threaded joint may preferably have the following
configuration. The armular portion of the pin has a length along the pipe axis, the
length being equal to or greater than a thread pitch of the second male threaded portion.
[002e]
In addition, the above threaded joint may preferably have the following
configuration. The nose portion of the pin has a iength along the pipe axis, the length
being at least 5 mm.
AD\¿ANTAGEOUS EFFECTS OF INVENTION
[0030]
A threaded joint for steel pipes of the present invention has the following
significant advantages: improved sealing performance against internal pressure and
external pressure.
BRIEF DESCRIPTION OF DRAWINGS
[0031]
[FIG. 1] FIG. I is a longitudinal sectional view showing an example of a t¡pical
conventional threaded joint for steel pipes.
[FIG. 2] FrG.2 is a longitudinal sectional view showing an example of a
conventional threaded joint for steel pipes which was designed to provide improved
sealing performance.
IFIG. 3l FIG. 3 is a longitudinal sectional view showing another example of a
conventional threaded joint for steel pipes which was designed to provide improved
sealing performance.
[FIG. 4Al FIG. 4A is a longitudinal sectional view of a threaded joint for steel
pipes according to a f,rst embodiment of the present invention.
[FIG. 48] FIG. 4B is an enlarged longitudinal sectional view of the threaded joint
for steel pipes according to the f,rst embodiment, showing a region near the free end of
the pin.
[FIG' 5] FIG. 5 is an enlarged longitudinal sectional view of the threaded joint for
steel pipes according to the first embodiment of the present invention, showing a region
in the vicinity of a second seal portion, which is the outer one of the seal portions.
[FIG' 6] FIG 6 is a longitudinal sectional view of a threaded joint for steel pipes
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according to a second embodiment of the present invention.
[FIG. 7] FIG. 7 is an enlarged longitudínal sectional view of the threaded joint for
steel pipes according to the second embodiment of the present invention, showing a
region in the vicinity of a second seal portion, which is the outer one of the seal
portions.
DESCRIPTION OF EMBODIMENTS
[0032]
The present inventors conducted intense research for a configuration that enables
the seal portion to provide sealing performance to the fullest extent. Their intended
threaded joint was basically of the type that includes a shoulder surface disposed on the
free end of the pin and two metal-to-metal seal portions. Specifically, the intended
threaded joint was basically of the type that includes a first seal portion, which is the
inner one that primarily contributes to sealing performance against internal pressure,
and a second seal portion, which is the outer one that primarily contributes to sealing
performance against external pressure. Consequently, they have made the following
f,rndings.
[0033]
One method for improving sealing performance against external pressure is to
increase the wall thickness of the pin. Application of external pressure to the threaded
joint can cause radially inward deformation of the pin, which can result in loosening the
contact between the outer, second sealing surfaces, but, by increasing the wall thickness
of the pin, its resistance to radial contraction is enhanced. However, the pin inside
diameter is regulated byAPI (American Petroleum Institute) standards while the
enlargement of the pin outside diameter is restricted by structural limitations of the oil
well. Thus, there are lirnitations to increasing the wall thickness of the pin,
particularly in the region of the second sealing surface.
[0034]
In view of this, the pin is provided with an annular portion disposedbetween the
second sealing surface and the inner, first male threaded portion, such that the annular
portion is continuous with the second sealing surface and extends in the pipe axis
direction. As a result of tlre presence of,the annular portion, the pin exhibits increased
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resistance to radial contraction in the region ofthe second sealing surface.
Consequently, a decrease in contact pressure between the second sealing surfaces is
inhibited, resulting in improved sealing performance against external pressure. In this
case, the box is provided with an annular portion that is continuous with the second
sealing surface thereof, the annular portion corresponding to the region ofthe annular
portion of the pin. In a fastened state, a clearance is provided between the annular
portion of the box and the annular portion of the pin. This clearance allows an excess
of the grease compound (hereinafter also referred to as "dope"), applied for fastening
thread, to accumulate therein, and therefore makes it possible to avoid an inadvertent
decrease in contact pressure between the second sealing surfaces due to an increased
pressure ofthe dope.
[0035]
Furthermore, the pin and the box may be provided with armular portions different
from the above annular portions disposed between the second sealing surfaces and the
outer, second threaded portions, such that the different annular portions are continuous
with the respective second sealing surfaces and extend in the pipe axis direction. In a
fastened state, a clearance is provided between these amular portions. This clearance
inhibits a decrease in substantial interference between the second sealing surfaces that
may otherwise result from the interference engagement of the second th¡eaded portions.
Consequently, a decrease in contact pressure between the second sealing surfaces is
inhibited, resulting in improved sealing performance against extemal pressure.
[0036]
For improvement of sealing performance against internal pressure, the following
two approaches may be possible. The flirst approach is as follows. The pin is
provided with a cylindrical nose portion disposed between the inner, f,rst sealing surface
and the shoulder surface, such that the nose portion is continuous with the flirst sealing
surface and extends in the pipe axis direction. The box is provided with a recessed
portion corresponding to the nose portion of the pin. In a fastened state, a clearance is
provided between the recessed portion and the nose portion. Thus, even if the
shoulder surface of the pin becomes separated from the shoulder surface of the box,
which can cause loosening of the contact between the first sealing surfaces, owing to the
presence of the nose portion of the pin and the recessed portion of the box, the first
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sealing surface of the pin recovers.elastically together with the nose portion.
Consequently, a decrease in contact pressure between the first sealing surfaces is
ínhibited, so that high sealing performance against internal pressure can be achieved
even while an excessive tensile load is being applied.
[0037]
The second approach is to reduce the wall thickness of the pin in the region of the
inner, first male threaded portion and first sealing surface, in addition to employing the
above first approach. The reduced wall thickness of the pin allows for an effective
radíally outward deformation of the pin when internal pressure has been applied to the
threaded joint, which results in amplifþg the contact pressure between the inner first
sealing surfaces. However, since reduction in the wall thickness of the pin involves a
decrease in its stiffness, the contact between the f,rrst sealing surfaces can become
loosened when internal pressure is not applied to the threaded joint. This situation can
be addressed by the presence ofthe nose portion according to the above flrst approach,
by which a decrease in contact pressure between the first sealing surfaces is inhibited.
[0038]
The second approach may be accomplished by providing the pin with an
auxiliary shoulder surface disposed between the first male threaded portion and the
annular portion, such that the auxiliary shoulder surface is continuous with the annular
portion, and providing the box with an auxiliary shoulder surface disposed between the
first female threaded portion and the annular portion, the auxiliary shoulder surface
corresponding to the auxiliary shoulder surface of the pin. When this conf,rguration is
employed, because of the presence of the auxiliary shoulder surfaces, it is possible to
reduce the outside diameter of the pin in the region of the f,rst male threaded portion
and the first sealing surface and accordingly reduce the inside diameter of the box in the
region of the f,irst female threaded portion and the frst sealing surface. As a result, it
is easier to ensure a suff,rcient cross-sectional area ofthe critical section ofthe box (the
cross section of the femaie threaded portion at a position closest to the tubular body, i.e.,
the cross section of the first female threaded portion at a position closest to the tubular
body), whereby a reduction of the box outside diameter can be achieved.
[003e]
The threaded joint for steel pipes of the present invention has been made based
12
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on the above findings. Embodiments of the threaded joint for steel pipes according to
the present invention are described below.
[0040]
[First Embodiment]
FIG. 4A is a longitudinal sectional view of a threaded joint for steel pipes
according to a first embodiment of the present invention. FIG 48 is an enlarged
longitudinal sectional view of the threaded joint for steel pipes, showing a region near
the free end of the pin. FIG. 5 is an enlarged longitudinal sectional view of the
threaded joint for steel pipes according to the frst embodiment of the present invention,
showing a region in the vicinity of a second seal portion, which is the outer one of the
seal portions. As shown in FIG. 4A and FIG. 5, the th¡eaded joint according to the first
embodiment is a coupling-type threaded joint and is constructed of a pin l0 and a box
20.
[0041]
The pin 10 includes, in order from the free end thereof toward the tubular body, a
shoulder surface 11, a nose portion 12, a first sealing surface 13, a first male threaded
portion 14, a first annularportion 15a, a second sealing surface 16, a second annular
portion 15b, and a second male threaded portion 17. Both the first sealing surface 13
and the second sealing surface 16 are tapered surfaces. Technically, the frst sealing
surface 13 and the second sealing surface 16 each have a shape corresponding to a
shape of the peripheral surface of a truncated cone having a diameter decreasing toward
the end, or a shape corresponding to a combined shape of the peripheral surface of a
trtrncated cone and the 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.
100421
The nose portion 12, which has a cylindrical shape, is continuous with the inner
first sealing surface 13 and extends in the pipe axis direction. It is to be noted that the
outer peripheral surface of the nose portion 12 may instead be a surface having a taper
equal to that of the first sealing surface 13, smaller (gentler) than that, or greater
(steeper) than that. Technically, it has a shape corresponding to a shape of the
peripheral surface of a truncated cone having a diarneter decreasing toward the end, or a
shape corresponding to a combined shape of the peripheral surface of a truncated cone
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and the 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.
[0043]
The shoulder surface 11 is provided on an end ofthe nose portion I2. The
shoulder surface 1l is an annular surface generally perpendicular to the pipe axis CL.
Technically, the shoulder surface 11 is slightly inclined in such a manner that the outer
circumferential side thereof is closer to the free end of the pin I 0. The first annular
portion 15a is continuous with the inner side of the outer, second sealing surface 16 and
extends in the pipe axis direction. The inner, f,irst male threaded portion 14 is disposed
continuous with the first annular portion 15a. The second annular portion 15b is
continuous with the outer side of the outer second sealing surface 16 and extends in the
pipe axis direction. The outer, second male threaded portion 17 is disposed continuous
with the second annular portion 15b. The outer peripheral surface of the f,ust annular
portion l5a may be of any shape as long as its stiffrress can be ensured. For example,
it may be a cylindrical surface or a surface having a taper smaller (gentler) than that of
the tapered surface defined by the first male threaded portion 14, or may be a curved
surface. The same applies to the outer peripheral surface of the second annular portion
15b.
[0044]
The box 20 includes, in order from the tubular body side toward the free end, a
shoulder surface 21 , a recessed portion 22, a first sealing surface 23, a ftrst female
threaded portion 24, afts| annular portion 25a, a second sealing surface 26, a second
annular portion 25b and a second female threaded portion?T. The shoulder surface 21,
recessed portion 22, ftst sealing surface 23, f,rst female threaded portion 24, ftrst
annular portion 25a, second sealing surface 26, second annular portion 25b and second
female threaded portion 27, of thebox20, are provided corresponding to the shoulder
surface 11, nose portion 12, f,ust sealing surface 13, first male threaded portion 14, first
annular portion 15a, second sealing surface 16, second annular portion 15b, and second
male threaded portion 17, of the pin lQ respectively.
[0045]
FIGS. 4A and 4B show a case in which the first sealing surface 23 of the box 20
protrudes toward the fnst sealing surface 13 of the pin 10. It is to be noted that the
st
T4
first sealing surface 23 of the box 20 may not protrude. In that case, the first sealing
surface 13 of the pin 10 protrudes toward the first sealing surface 23 of the box 20.
[0046]
The first male threaded portion 14 of the pin 10 and the first female threaded
portion 24 of the box 20 are tapered threaded portions with trapezoidal threads that
mesh with each other, and constitute a f,irst threaded portion, which is the inner one.
The second male threaded portion 1 7 of the pin I 0 and the second female threaded
portion 21 of lhe box 20 are also tapered threaded portions with trapezoidal threads that
mesh with each other, and constitute a second threaded portion, which is the outer one.
In the case of the th¡eaded joint of the f,irst embodiment, the tapered surface defured by
the first threaded portion and the tapered surface defrned by the second threaded portion
are the same. ln this configuration, the second sealing surfaces 16,26 are simply
added between the first threaded portion and the second threaded portion.
10047J
The first male threaded portion 14 and the frst female threaded portion 24 are
threadedly engageable with each other, and in a fastened state, they engage in intimate
contact with each other to have an interference fit. Likewise, the second male threaded
portion l7 and the second female threaded portion 27 have an interference f,rt between
them. The first sealing surface 13,23 and the second sealing surface 16,26 are each
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. Thus,
the first sealing surface 13,23 andthe second sealing surface 16,26 form a first seal
portion and a second seal portion, respectively, by metal-to-metal contact. The
shoulder surfaces 71,21 are brought into contact and pressed against each other by the
screwing of the pin 10, and serve as stoppers for restricting the screwing of the pin 10.
Furthermore, in a fastened state, the shoulder surfaces 1 1 , 21 serve to impart the
so-called th¡ead tightening axial force to the load flanks of the first male threaded
portion 14 and the second male threaded portion 17 of the pin 10. I¡n a fastened state, a
clearance is provided between the nose portion 72 of the pin 10 and the recessed portion
22 of the box, a clearance is also provided between the f,rst annular portion 15a of the
pin 10 and the fust annular portion 25a of the box 20, and a clearance is also provided
between the second annular portion 15b of the pin 10 and the second annutar portion
:!
15
25b ofthe box 2O.
[0048]
With the threaded joint of the first embodiment having this configuration, the
sealing performance primarily against internal pressure is ensured by the engagement
and intimate contact between the inner first sealing surfaces 73, 23 . Also, sealing
performance primarily against external pressure is ensured by the engagement and
intimate contact between the outer second sealing surfaces 16,26.
[004e]
In particular, since the pin 10 includes the first annular portion 15a, which is
continuous with the inner side of the outer second sealing surface 16, the pin 10 exhibits
increased resistance to radial contraction in the region ofthe second sealing surface 16
owing to the stiffness of the first annular portion 15a. Thus, even in the case where
external pressure is applied to the threaded joint, radially inward deformation of the pin
10 is inhibited, so that a decrease in contact pressure between the second sealing
surfaces 16,26 is inhibited. As a result, sealing performance against external pressure
is improved. In the box 20, a flrrst annular portion 25a is provided corresponding to the
first annular portion 15a of the pin 10, and in a fastened state, a clearance is provided
between the flrst annular portions ISa,25a. Thus, an excess of dope that has been
applied for fastening thread is allowed to accumulate in the clearance. As a result, it is
possible to avoid an inadvertent decrease in contact pressure between the second sealing
surfaces 16,26 due to an increased pressure ofthe dope.
[0050]
The pin l0 and the box 20 are provided with the second annular portion 75b,25b,
respectively, which are continuous with the outer side of the outer second sealing
surface 76,26. In a fastened state, a clearance is provided between the second annular
portions 15b,25b. By virtue of this, it is possible to inhibit a decrease in contact
pressure between the second sealing surfaces 16,26 due to a decrease in substantial
interference therebetween that may otherwise result from the interference engagement
of the second threaded portions. Consequently, sealing performance against external
pressure is improved
[0051]
Furthermore, the pin 10 is provided with the nose portion 12 that is continuous
t6
v/ith the inner first sealing surface 13. The box 20 is provided with a recessed portion
22 corresponding to the region of the nose portion 12 of the pin I 0. ln a fastenei state
a clearance is provided between the nose portion 72 and the recessed porIion22. kt
the case where an excessive tensile load is applied to the threaded joint, for example, the
shoulder surface 11 of the pin 10 can become separated from the shoulder surface 21 of
the box 20, causing a decrease in substantial interference between the f,irst sealing
surface 13,23 and therefore loosening of the contact therebetween. Even in such a
case, the nose portion 12 elastically recovers and thereby produces the effect of
amplifying the contact pressure between the frst sealing surfaces 13,23. As a result,
the decrease in contact pressure can be inhibited as a whole, and consequently, high
sealing performance against internal pressure can be achieved even while an excessive
tensile load is being applied.
[00s2]
The following are additional descriptions of preferred embodiments of the
threaded joint according to the first embodiment.
[00s3]
With regard to the pin 10, Ao is defined as a cross-sectional area of the tubular
body in a plane perpendicular to the pipe axis CL, and A1 is defined as a projected area
of the shoulder surface 11 in a plane perpendicular to the pipe axis CL. Preferably, the
area ratio Ar/Ao (hereinafter also referred to as the "ratio of the shoulder surface area lo
the tubular body area") is at least 30olo. More preferably, the area ratio AtlAo is at least
35%. The reason for this is as follows. The area ratio Ar/Ao is actually dependent on
the area of the shoulder surface 11. Ír the case where the area ratio Al/Ao is smaller,
because of the smaller area of the shoulder surface 11, the shoulder surface I I carurot
withstand an excessive compressive load when it has been applied to the threaded joint.
In such a case, the shoulder surface 11, and the nose portion 12 and the flirst sealing
surface 13 extending continuously from the shoulder surface 1 l, undergo plastic
deformation, so that the contact between the first sealing surfaces 13, 23 becomes
unstable. As a result, the contact pressure between the f,rst sealing surfaces 13,23
rnay be decreased. Fo¡ thisreason, the ratio ofthe shoulder surface area to the tubular
body area Al/Ao is preferably suitably large.
[00s4]
l7
il.
ii*T:
ì¡
i¡
:
ù'
í1
The upper limit of the ratio of the shoulder surface area to the tubular body area
Al/Ao is not particularly limited. However, too large an area ratio ArlAo means the
area of the shoulder surface 11 is too large, i.e., the outside diameter of the shoulder
surface I 1 of the pin 10 is too large. Accordingly, the box 20 has a larger inside
diameter in the region of the first female threaded portion 24 and the first sealing
surface 23. This results in the need to increase the outside diameter of the box 20 in
order to ensure that the critical section ofthe box 20 has a sufficient cross-sectional area.
Moreover, it becomes difflicult to ensure sufficient lengths of engagement of the f,Íst
threadedportion andthe secondth¡eadedportion. Thus, forpractical applications, the
ratio of the shoulder surface area to the tubular body area Al/Ao is preferably up to 60%.
[0055]
In the pin 10, the first annular portion 15a preferably has a minimum outside
diameter greater than the diameter of a reference tapered surface 19b as shown in FIG. 5.
The reference tapered surface 19b is a tapered surface having an outside diameter
smaller than an outside diameter of an extension 19a of the tapered surface defined by
the roots of the second male threaded portion 17 by twice the height of threads of the
second male threaded portion 17. The reason for the above is as follows. The
outside diameter of the f,rrst annular portion 15a is actually dependent on the wall
thickness of the first annular portion 15a. When the outside diameter of the fi¡st
annular portion 15a is smaller, which means the wall thickness of the first annular
portion l5a is thinner, the resistance to radial contraction in the region of the second
sealing surface 16 which results from the stiftress of the frst annular portion 15a will
be insufficient in the case where extemal pressure is applied to the threaded joint.
When this occurs, the contact pressure between the second sealing surfaces 16,26 may
be decreased. For this reason, the outside diameter of the first annular portion l5a is
preferably suitably large.
[00s6]
The upper limit of the outside diameter of the fust annular portion 15a is not
particularly limited. However, it is necessary that the outside diameter of the first
annular portion 15a be sized so as not to interfere with the second sealing surface 26 of
the box 20 during fastening thread.
[00s7]
18
In the pin i0; the first annular portion 15a preferably has a length along the pipe
axis of equal to or greater than the thread pitch of the second male threaded portion 1 7,
with the length being measured from the irxrer end of the second sealing surfàce 16.
This is because, if the length of the flrst annular portion 15a is shorter, the contact
pressure between the second sealing surfaces 16,26 may be decreased in the case where
external pressure is applied to the threaded joint, for the same reason as that for the case
where the wall thickness of the first annular portion 15a is thin.
[00s8]
The upper limit of the length of the first annular portion l5a is not particularly
limited. It is to be noted, though, that if the length of the fîrst annular portion 15a is
too long, the overall length of the joint is accordingly long, which results in increased
manufacfuring costs due to increased machining time and increased material costs.
Also, when the length of the first annular portion 15a exceeds a certain length, the effect
of improving the sealing performance will substantially no longer be increased. Thus,
for practical applications, the length of the flrst annular portion 15a is preferably at most
flrve times the thread pitch of the second male threaded portion 17.
[005e]
In the pin 10, the second annular portion 1 5b preferably has a length along the
pipe axis of equal to or greater than the thread pitch of the second male threaded portion
17, with the length being measured from the outer end of the second sealing surface 16.
The reason is that, if the length of the second annular portion l5b is shorter, the
interference engagement of the second threaded portions may cause a decrease in
substantial interference between the second sealing surfaces 76,26, and therefore the
contact pressure between the second sealing surfaces 16,26 may be decreased.
[0060]
The upper limit of the length of the second annular portion 15b is not particularly
limited. It is to be noted, though, that if the length of the second annular portion 15b is
too long, the overall length of the joint is accordingly long, which results in increased
manufacturing costs due to increased machining time and increased material costs.
Also, when the length of the second annular portion 15b exceeds a certain length, the
effect of improving the sealing perforrnance will substantially no longer be increased.
Thus, for practical applications, the length of the second annular portion 15b is
T9
preferably at most five times the thread pitch of the second male threaded portion 17.
[0061]
h the pin 10, the nose portion 12 preferably has a length along the pipe axis of at
least 5 mm. The reason for this is as follows. If the length of the nose portion 12 is
shorter, the elastic recovery of the f,rst sealing surface 13 owing to the nose portion l2
will be insufficient in the case where an excessive tensile load is applied to the threaded
joint. 'When this occurs, the contact pressrrre between the first sealing surfaces 13,23
may be decreased. For this reason, the length of the nose portion 12 is preferably
suitably long.
[0062]
The upper limit of the length of the nose portion 12 is not particularly limited. It
is to be noted, though, that if the length of the nose portion 12 is too long, the overall
length of the joint is accordingly long, which results in increased manufacturing costs
due to increased machining time and increased material costs. Also, when the length
of the nose portion 12 exceeds a certain length, the effect of improving the sealing
performance will substantially no longer be increased. Thus, for practical applications,
the length of the nose portion 12 is preferably at most five times the thread pitch of the
first male threaded portion 14.
[0063]
fSecond Embodiment]
FIG 6 is a longitudinal sectional view of a threaded joint for steel pipes according
to a second embodiment of the present invention. FIG. 7 is an enlarged longitudinal
sectional view of the threaded joint for steel pipes according to the secoqd embodiment
of the present invention, showing a region in the vicinity of a second seal portion, which
is the outer one of the seal portions. The threaded joint according to the second
embodiment shown in FIGS. 6 andT is a variation of the th¡eaded joint according to the
first embodiment shown in FIGS. 4 and,5,and therefore descriptions redundant to those
given in the first embodiment will not be repeated where appropriate.
[0064]
As shown in FIGS. 6 anö7, the threaded joint of the second embodiment is
similar to the threaded joint of the first embodirnent in that a shoulder surface 1l is
disposed on the free end of the pin 10 and two metal-to-metal seal portions are provided,
20
but differs in that an additional, auxiliary shoulder surface 18 is included.
[006s]
Specifically, the pin 10 includes an auxiliary shoulder surface 18 disposed
between the inner, first male threaded portion 14 and first annular portion 15a, such that
the auxiliary shoulder surface 18 is continuous with the first annular portion 15a. The
auxiliary shoulder surface 18 is an ar¡rular surface perpendicular to the pipe axis CL.
It is noted that the auxiliary shoulder surface 18 may be slightly inclined in such a
maïìner that the outer circumferential side thereof is closer to the free end of the pin 10,
similarly to the shoulder surface 11 of the pin 10. The box 20 includes an auxiliary
shoulder surface 28 corresponding to the auxiliary shoulder surface 1 B of the pin 10.
[0066]
In the case of the threaded joint of the second embodiment, the tapered surface
defined by the first threaded portion is closer to the pipe axis CL than the tapered
surface def,rned by the second threaded portion. This is because of the auxiliary
shoulder surfaces 18, 28 provided between the frst threaded pofion (the first male
threaded portion 14 and the first female threaded portion 24) and the second threaded
portion (the second male threaded portion 17 and the second female threaded portion
27). Thus, in the threaded joint of the second embodiment, the pin 10 has a smaller
outside diameter in the region of the inner f,irst male threaded portion 14 and first
sealing surface 13 and therefore has a thir¡rer wall thickness therein, than in the
threaded joint of the first embodiment.
[0067]
In a fastened state, the shoulder surfaces 11,21 arein pressure contact with each
other and the auxiliary shoulder surfaces 18, 28 are also in pressure contact with each
other. The pressure contact between the shoulder surfaces ll,2l imparts the axial
tightening force primarily to the load flanks of the first male threaded portion 14 of the
pin 10. The pressure contact between the auxiliary shoulder surfaces 18, 28 imparts
the axial tightening force primarily to the load flanks of the second male threaded
portion 17 of the pin 10.
[0068]
With regard to the time at which the shoulder surfaces 1I,2tr are brought into
contact with each other and the tfune at which the auxiliary shoulder surface 18, 28 are
2t
brought into contact with each other in the course of fastening thread, it is preferred that
the two contacts occur simultaneously or the contact between the auxiliary shoulder
surfaces 18, 28 occur prior to the contact between the shoulder surfaces 11,21. V/ith
the contacts timed in such a manner, the balance between the contact force of the
shoulder surfaces 17,2I and the contact force ofthe auxiliary shoulder surfaces 18,28
is maintained. As a result, damage to the pin l0 and the box 20 is reduced.
[006e]
The threaded joint of the second embodiment also produces advantageous effects
similar to those of the first embodiment described above. In the second embodiment,
particularly, because of the presence of the auxiliary shoulder surfaces 18,28, the pin 10
has a reduced wall thickness in the region of.the inner first male th¡eaded portion 14 and
first sealing surface 13. As a result, the region having a reduced wall thickness
effectively undergoes radially outward deformation when internal pressure has been
applied to the threaded joint. Consequently, the contact pressure between the first
sealing surfaces 13,23 is amplif,red.
[0070]
The following are additional descriptions of preferred embodiments of the
threaded joint according to the second embodiment.
[0071]
'With regard to the pin 10, Ao is defined as a cross-sectional area of the tubular
body in a plane perpendicular to the pipe axis CL, and Az is defined as a total of
projected areas ofthe shoulder surface 11 and the auxiliary shoulder surface 18 in a
plane perpendicular to the pipe axis CL. Preferably, the area ratio AzlAo (hereinafter
also referred to as the "ratio ofthe total shoulder surface area to the tubular body area")
is at least 300/o. More preferably, the area ratio AzlAo is at least 35%. The reason for
this is as follows. The area ratio AzlÀo is actually dependent on the areas of the
shoulder surface 11 and the auxiliary shoulder surface 18. In the case where the area
ratio AzlAo is smaller, because of the srnaller areas of the shoulder surface 11 and the
auxiliary shoulder surface 18, the shoulder surface 11 and the auxiliary shoulder surface
18 cannot withstand an excessive compressive load when it has been applied to the
threaded joint. In such a case, the shoulder surface 11, and the nose portion 12 and the
flrst sealing surface 13 extending continuouslSz from the shoulder surface 1 I , undergo
.,i
22
plastic deformation, so that the contact between the fust sealing surfaces 13, 23
becomes unstable. In addition to this, the auxiliary shoulder surface 18, and the first
annular portion l5a and the second sealing surface 16 extending continuously from the
auxiliary shoulder surface 18, undergo plastic deformation, so that the contact between
the second sealing surfaces 16,26 becomes unstable. As a result, the contact pressure
between the first sealing surfaces 13,23 and the contact pressure between the second
sealing surfaces 16,26 may be decreased. For this reason, the ratio of the total
shoulder surface area to the tubular body area AzlAo is preferably suitably large.
100721
The upper limit of the ratio of the total shoulder surface area to the tubular body
area Azl&o is not particularly limited. However, too large an area ratio AzlAo means
the areas of the shoulder surface 11 and the auxiliary shoulder surface 18 are actually
too large, i.e., the outside diameters of the shoulder surface 11 and the auxiliary
shoulder surface 18 of the pin 10 are too large. Accordingly, the box 20 has larger
inside diameter in the region of the first female threaded portion 24 andthe first sealing
surface 23. This results in the need to increase the outside diameter of the box in order
to ensure that the critical section ofthe box 20 has a suffrcient cross-sectional area.
Moreover, it becomes difficult to ensure sufficient lengths of engagement of the flrst
threaded portion and the second threaded portion. Thus, for practical applications, the
ratio of the shoulder surface area to the tubular body area Az/Ao is preferably tp Io 600/o.
[0073]
With regard to the pin 10, Az is defined as a total of projected areas of the
shoulder surface 11 and the auxiliary shoulder surface 18 in a plane perpendicular to the
pipe axis CL, and Ar is defined as the projected area of the shoulder surface 11 in a
plane perpendicular to the pipe axis CL. Preferably, the area ratio Ar/Az (hereinafter
also referred to as the "ratio ofthe shoulder surface area to the total shoulder surface
area") is atleast 35%o. More preferably, the area ratio AtlAz is at least 40%. The
reason for this is as follows. The area ratio ùlAz is actually dependent on the area of
the shoulder surface 11 relative to the auxiliary shoulder surface 18. In the case where
the area ratio At/Az is smaller, because of the smaller area of the shoulder surface I I ,
the shoulder surface 11 cannot withstand an excessive cornpressive load when it has
been applied to the threaded joint. In such a case, the shoulder surface 11, and the
'*
: ì:
:
¿J
',1,
É.
:Y.
l¿'
:i
ii.
tì
tqr-.
f¿
':t
nose portion 12 and the first sealing surface 13 extending continuously from the
shoulder surface 11, undergo plastic deformation, so that the contact between the first
sealing surfaces 13,23 becomes unstable. As a result, the contact pressure between
the first sealing surfaces 73, 23 may be decreased. For this reason, the ratio of the
shoulder surface area to the total shoulder surface area At/Az is preferably suitably
large.
100141
The upper limit of the ratio of the shoulder surface area to the total shoulder
surface area AtlAz is not particularly limited. However, too large an arearatio Al/Az
means the area of the shoulder surface 1 1 relative to the auxiliary shoulder surface 1 8 is
actually too large, i.e., the wall thickness of the nose portion 12 continuous with the
shoulder surface 11 and the region of the fnst male threaded portion 14 and the first
sealing surface 13, of the pin 10, are too thick. As a result, the effective radially
outward deformation does not occur in the region when internal pressure has been
applied to the threaded joint, and therefore it is not possible to produce the effect of
amplifying the contact pressure between the flrst sealing surfaces 13,23. When this is
the case, the contact pressure between the first sealing surfaces 73,23 may be decreased.
Thus, for practical applications, the ratio of the shoulder surface area to the total
shoulder surface areaAt/Az is preferablyup to 55%.
[007s]
Preferred outside diameters and lengths of the first annular portion 15a and
preferred lengths of the nose portion 12, of the pin 10, are similar to those of the first
embodiment.
[0076]
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, additional techniques may be used so that the
engagement and intimate contact in the inner first threaded portion can be loosened
exclusively in a region near the flrst sealing portion when internal pressure has been
applied to the threaded joint. This allows the region of the inner first sealing surface
13 of the pin 10 to more effectively undergo radially outward deformation, so that the
contact pressure between the first sealing surfaces 13,23, is further amplifred. One
24
possible approach is to employ a configuration in which, in the f,rrst threaded portion,
the first male threaded portion 14 or the flrrst female threaded portion 24 is provided
with incomplete threads of imperfect thread form, in a region near the flrrst sealing
surface. As an example of this conf,rguration, the area of the incomplete threads may
be such that the first female threaded portion 24 of thebox 20 has crests defining a
cylindrical periphery parallel to the pipe axis CL and therefore has thread heights that
are lower than the normal thread height. Thus, in the incomplete thread area,
clearances are provided between the crests of the first female threaded portion 24 and,
the roots of the first male threaded portion 14. h this case, the length of the
incomplete thread area is three to nine times the thread pitch of the first female threaded
portion 24 (about 15 to 45 mm).
100771
The threaded joints of the above embodiments may be employed either as an
integral-type threaded joint or as a coupling.type threaded joint.
EXAMPLES
[0078]
To verify the advantages of the present invention, numerical simulation and
analysis was carried out using the elasto-plastic f,rnite element method.
[007e]
Test Conditions
In the FEM analysis, models of coupling-type threaded joints for oil country
tubular goods were prepared with the dimensions of the first ar¡rular portion, the nose
portion, and the shoulder surface (including the auxiliary shoulder surface) of the pin
being varied. For the principal dimensions, the following two types of steel pipe (pin)
and coupling (box) were specified.
(1) Steel pipe size: 10-1/8 finch] x 0.8 finch] (outside diameter of 257.2mm and
wall thickness of 20.3 mm), Coupling outside diameter: 276.6 mm.
(2) Steel pipe size: 7-518 [inch] x i.2 [inch] (outside diameter of 193.7 mm and
wall thickness of 30.5 mm), Coupling outside diameter: 217.g mm.
[0080]
The comrnon conditions were as follows.
25
- Grade of steel pipe and coupling: API standard Q125 (carbon steel having a
yield stress of 125 [ksi]).
- Thread form: taper of l/10 (for 10-1/B [inch] pipes) or taper of 1/11 (for 7-518
finchJ pipes); thread height of 1.575 [mm]; thread pitch of 5.08 [mmJ; load flank angle
of -3o; stabbing flank angle of 10"; and stabbing flank clearance of 0.15 [mm].
[0081]
h the FEM analysis, the material was specified as isotropic hardening
elasto-plastic, and the models used were prepared to have a modulus of elasticity of 270
[GPa], and a nominal yield strength, expressed as O.2o/oproof stress , of 125 [ksi] (:862
[MPa]). The tightening was carried out to the point where the shoulder surfaces of the
pin and the box were brought into contact with each other, and fuither continued up to a
point of l.5i 100 turns.
[0082]
The varied dimensional conditions are shown in Table 1 below.
[0083]
[Table 1]
26
Remarks) The symbol "*" indicates that the value is outside the range specified by the
present invention.
È,Ì
Ë
TABLE 1
No.
Pin
Size
First annular
Portion
Nose
portion
Area ratio [%]
Minimum of
average contact
pressure at sealing
surface [MPal
11'
Ê¡
Ê)
o5
(l
Þ
oÞ
o
Length
INumber
of
Pitchesl
o.D.
IRelative
to
reference
tapered
surface]
Total
shoulder
surface/
tubular
body
A1/AO,
A2/AO
Shoulder
surface/
total
shoulder
surface
AlIA2
Intemal
pressure
cycle
(fìrst
sealing
surface)
External
Pressure
Cycle
(second
sealing
surface)
1
l 0- l/8"
x0.8 0* Absent
{<
45 44 119 168 NA
Comp.
Example.
4a 1 0-l/8"
x0.8
2.4 Small
Absent
* 4l 150 257 NA
Comp.
Example
#3
10-1/8"
x0.8 2.4 Large
Absent
t<
41 138 3t2 NA
Comp.
Examole
#4
I 0-1/8"
x0.8 2.4 Large Present 35 266 313 G
Inv.
Example
#5
l0-l/8"
x0.8' I Large Present 58 40 264 307 G
Inv.
Example
+6
7-519"
x1.2
2 Large Present 43 45 303 400 E
Inv.
Examole
7-518"
xl.2 Large Present 43 45 305 38'l E
Inv.
Examnle
T8
l-5/8',
x7.2
J Large Present 43 45 312 401 E
Inv.
Example
+9
7-5/8
xl.2' J Large Present 43 35 243 449 A
Inv.

We claim:
L 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 cornprising: in order from an end thereof, a slioulder surface; a first
sealing surface; a tapered first male threaded portion; a second sealhg surface; and a
tapered second male tlueaded portion,
the box comprising: a shoulder surface; a first sealing surface; a tapered first
female tlueaded portion; a second sealing surface; and a tapered second fernale tlueaded
portion, corresponding to the shoulder surface, the flrst sealing surface, the f,rrst male
threaded portion, the second sealing surface, and the second male threaded portion, of
the pin, respectively,
wherein,
the pin further includes: a nose portion disposed between the shoulder surface
and the first sealing surface, the nose portion being continuous with the first sealing
surface; and an arurular portion disposed between the f,rst rnale tlueaded portion and tlie
second sealing surface, the annular portion being continuous with the second sealing
surface,
the box further includes: a recessed portion corresponding to the nose portion of
the pin; and an annular portion corresponding to the annular portion of the pin, and
in a fastened state: the shoulder surfaces are in contact with each other, the first
sealing surfaces are in contact with each other; the second sealing surfaces are in contact
with each other; a clearance is provided between the nose portion of the pin and the
recessed portion of the box; a clearance is provided between the annular portion of the
pin and the annular portion of the box; the f,rst male threaded portion engages with the
f,irst female threaded portion; and the second male threaded portion engages with the
second fernale threaded portion.
2. The threaded joint for steel pipes according to claim 1, wherein:
the pin includes an auxiliary shoulder surface disposed between the first rnale
threaded portion and the annular portion, the auxiliary shoulder surface being
continuous with the annular portion; the box includes an auxiliary shoulder surface
31
coresponding to the auxiliary shoulder surface of the pin; and, in a fastened state, the
auxiliary shoulder surfaces are in contact with each other.
3. The threaded joint for steel pipes according to claim?-, wherein:
during fastening thread, the contact between the shoulder surfaces and the contact
between the auxiliary shoulder surfaces occur simultaneously, or the contact between
the auxiliary shoulder surfaces occurs prior to the contact between the shoulder
surfaces.
4. The threaded joint for steel pipes according to claim i, wherein:
the pin is configured such that, when a tubular body having the pin has a
cross-sectional areaAo in a plane perpendicular to the pipe axis, and the shoulder
surface has a projected areaAl in a plane perpendicular to the pipe axis, an area ratio
Ar/Ao is atleast3}o/o.
5. The threaded joint for steel pipes according to claim2 or 3, wherein:
the pin is conflrgured such that, when a tubular body having the pin has a
cross-sectional areaAo in a plane perpendicular to the pipe axis, and the shoulder
surface and the auxiliary shoulder surface have projected areas in a plane perpendicular
to the pipe axis with a total of the projected areas being designated as Az, an area ratio
AzlAo is at least 30%.
6. The threaded joint for steel pipes according to any one of claims 2, 3 , and. 5 ,
wherein:
the pin is configured such that, when the shoulder surface and the auxiliary
shoulder surface have projected areas in a plane perpendicular to the pipe axis, with a
total of the projected areas being designated as Az and the projected area of the shoulder
surface being designated as 41, an area ratio Atlfuz is at least 35olo.
7. The threaded joint for steel pipes according to any one of claims 1 to 6, wherein:
the annular portion of the pin has a minimum outside diameter greater than a
diameter of a reference tapered surface, the reference tapered surface being a tapered
32
fr:..
fÈ
1.,
lj
ì:
surface having an outside diameter smaller than an outside diameter of an extension of a
tapered surface defired by roots of the second male th¡eaded portion by twice a height
of threads of the second male threaded portion.
B. The th¡eaded joint for steel pipes according to any one of claims I to 7, wherein:
the annular portion of the pin has a length along the pipe axis, the length being
equal to or greater than a th¡ead pitch of the second male threaded portion.
9. The threaded joint for steel pipes according to any one of claims 1 to 8, wherein:
the nose portion of the pin has a length along the pipe axis, the length being at
least 5 mm.