DESCRIPTION
TITLE OF INVENTION
THREADED JOINT FOR STEEL PIPES
TECHNICAL FIELD
[000 r ]
The present invention relates to a threadedjoint for use in connecting steel pipes
or tubes (hereinafter also referred to as "steel pipes").
BACKGROUND ART
[0002]
ln 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) are used
for extraction ofunderground resources. The steel pipes are sequentially connected to
each other, and threaded joints are used for the connection.
100031
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 connected to each otheq ofwhich one is a steel pipe and the other
is a coupling. In this case, the steel pipe includes male threaded portions fonned 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 steelpipes as tubular goods that are connected to each other, without a
separated 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 porlion
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]
Also, threaded joints for steelpipes are sometimes classified into types such as a
flLrsh type, semi-flush type, and slirn type based on the outside diameter of the joint
portion. A slirn type threaded joint is defined using, as a measure, a coupling in which
the area of the critical cross section is substantially equalto that of the pipe body
(heleirrafter also referred to as a "standard coupling"). Specifically, a slim type
threacled joint refers to one in which the maximum outside diarneter of the joint portion
is smaller than the outside diarneter of the standard coupling. A flush type threadecl
joint and a semi-flush type threaded joint are defined using, as a lneasure, the pipe body.
Specifically, a flush type threaded joint refers to one in which the maximum outside
diameter of the joint portion is substantially equalto the outside diameter of the pipe
body. A semi-flush type threaded joint refers to one in which the maximum outside
diameter of the joint portion falls between that of a flush type threaded joint and that of
a slirn type threaded joint. In short, regarding the joint portion,-flush type threaded
joints have the smallest maximutn outside diameter, senri-flush type threaded joints
have the next smallest maximum outside diameter, and the slim type threaded joints
have the largest outside diameter.
[000s]
The term "critical cross section" as used herein refers to a cross section at an end
of the engagernent region of the threaded portion, i.e., ata position where the areaof the
cross section for bearing tensile loads is a minimum. The position of the critical cross
section and the cross-sectional area thereof are factors in determining tlre tensile
strength of the threaded joint. There are no definitions for clearly distinguishing
among the flush type, serni-flush type, and slirn type. However, as a general rule for
tlireaded joints for steel pipes currently on the market, those in which the maximum
outside diameter of the joint portion is up to about 104Yo of that of the pipe body are
referred to as the flush type or the semi-flush type, and those in which the maximum
outside diameter of the joint portion is up to about 108% of that of the pipe body are
referred to as the slim type.
[0006]
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 tlie other hand, the joint portion at the tLlbular end where
a female threaded porlion is disposed is referred to as a box because it includes an
element that receives a male threaded poftion. Pins ancl boxes both have a tubular
g
/
shape because they are constituted by end portions oftubular goods.
[0007]
In service environrnents, threaded joints for steel pipes are subjected to high
pressLrre frorn fluids (e.g., gas or liquid) present in the exterior and the interior.
Hereinafter, pressLlre from the exterior is also referred to as external pressure and
pressure from the interior is also referred to as internal pressure. Under these
circumstances, threaded joints for steel pipes are required to exhibit high sealing
performance against external pressure and internal pressure.
[0008]
Threaded joints for steel pipes ernploy a tapered threacled poftiorl corrstituted by a
male threaded portion of the pin and a female threaded portion of the box. ln threaded
joints having a tapered threaded porlion, tlie male threaded portion of the pin and the
female threaded porlion of the box engage in intimate contact with each other. Thus, a
thread seal is formed. In addition, in order to supplement the sealing performance of
the thread seal, an external seal may be provided and an internalseal may be provided,
separately frorn the thread seal.
[000e]
An extemal seal is constituted by a sealing surface formed on the inner periphery
of the end regiort of the box and a sealing surface correspondingly formed on the outer
periphery of the pin. When these sealing surfaces come into interference contact with
each other at high contact pressure, the external seal is formed. The external seal
primarily serves to prevent external fluids from accidentally invading the region of the
thread seal so as to contribute to ensuring sealing performance against external pressure.
[0010]
An internal seal is constituted by a sealing surface formed on the outer periphery
of the end region of the pin and asealingsurface correspondingly forrned on the inner
periphery of the box. When these sealing surfaces come into interference contact with
each other at high contact pressure, the internal seal is formed. The internal seal
primarily serves to prevent internal fluids from accidentally invading the region of the
thread seal so as to contribute to ensuring sealing performance against internal pressure.
[00 r l]
Typically, in the region of the externalseal, the wallthickness of the box is
/
thinner than that of the pin. Thus, in the case where high internal pressure has been
loaded to the threaded joint and the threadecljoint as a whole has swollen radially, the
region of the external seal in the box easily expands radially to plastically defonn. [n
this case, the regiou of the extemal seal in the box rernair.rs radially expanded after
rernoval of the internal pressure, and as a result, the contact pressure between the
sealing surfaces ofthe external seal is significantly decreased. Ifa high external
pressure is loaded to the threaded joint in this state, the sealing surfaces ofthe external
seal easily become separated. When this occllrs, the extemal fluid invades the region
of the thread seal beyond the externalsealand finally invades the interior of the
threaded joint. This probler¡ is referred to as an external pressure leak.
[00 r2]
In the past, various proposals have been made on techniques to ensure the sealing
perfonnance of the external seal. For example, techniques of arnplifying contact
pressure at the external seal are discloied in United States Patent No. 7,506,900 (Patent
Literature l), European PatentApplication Publication No.2325435 (Patent Literature
2), International Publication No. WO2009/083523 (Patent Literature 3), International
Publication No. WO20ll1044690 (Patent Literature 4), United States Patent Application
Publication No. 201 0/l 81763 (Patent Literature 5), and United States Patent Application
Publication No. 2008/265575 (Patent Literature 6).
[0013]
Patent Literatures 1 and2 disclose a technique in which a nose portion is
provided in the end region of the box. The nose portion extends alongthe pipe axis
from an end ofthe sealing surface ofthe external seal, adjacent the end, and does not
contact the pin. The stiffness of the nose portion itself increases deformation
resistance of the region of the external seal. As a result, radially outward plastic
deformation is inhibited in the region of the external sealof the box, whereby a
suffìcient contact pressure between the sealing surfaces is ensured.
[0014]
Patent Literatures 3 to 5 disclose a technique in which a shoulder surface is
provided on the end ofthe box. The pin has a shoulder surface corresponding to the
shoulder surface ofthe box. The shoLllder surfaces ofthe box and the pin are brought
into contact and pressed against each other by the screwing of the pin and serve as a
5
stopper for restricting the screwing of the pin. In addition, in a made up state(also
referred to as "fastened state"), the shoulder surfaces ofthe pin and the box serve to
impart the so-calleclthread tightening axiaì force to the load flanks of the male threaded
portion of the pin and the load flanks of the female threadecl portion of the box. The
shoLllder surfaces of the box and the pin are inclined with respect to a plane
perpendicular to the pipe axis toward the direction in which screwing of the pin
advances and they come into pressure contact with each other in a hooked manner.
Because the pressure contact between the shoulder surfaces is in a hooked manner, the
regiotr of the external seal of the box receives a reaction force that acts in a direction to
cause it to shrink radially. As a result, radially outward plastic deformation is inhibited
in the region of the externalsealof tlre box, whereby asufficient'contact pressure
between tlre sealing surfaces is ensured.
[001s]
TIre technique of Patent Literature 6 ernploys both the technique of Patent
Literatures I and2, in which anose portion is provided in the end region of the box, and
the techniqLre of Patent Literatures 3 to 5, in which a shoLllder surface is provided on the
end of the box so as to come into pressure contact with the pin in a hooked manner.
CITATION LIST
PATENT LITERATURE
[00 r 6]
Patent Literature I : United States Patent No. 7,506,900
Patent Literature 2: European Patent Appl ication Publ ication No. 232543 5
Patent Literature 3: International Publication No. WC)20091083523
Patent Literature 4: International Publication No. WO20ll1044690
Patent Literature 5: United States PatentApplication Publication No.
20101t8n63
Patent Literature 6: United States Patent Application Publication No.
2008126s57 s
SUMMARY OF INVENTION
TECHNICAL PROBLEM
ø
[00 r 7]
The techniques of Patent Literatures I to 6 provide the advantages effectively in
the case of threaded joints in which the wall thickness of the box is large relative to the
wallthickness of the pin, e.g., coupling-type threaded joints, threaded joints of the slim
type but for use in snall diarneter steel pipes, etc. However', in the case of certain
types of threaded joints for which expanding of the box outside diameter is limitecl and
which thus have a box oL¡tside diameter substantially equal to that of the pin, e.g.,
threaded joints of the flush type, of the semi-flusli type, or of the slim type, which are
widely employed in large diameter steel pipes, the advantages of the techniques of
Patent Literatures I to 6 as described above cannot be sufficiently provided and
therefore it is impossible to prevent exter¡ral pressure leak. This is because, in those
threaded joints, the wallthickness of the box is significantly thin with respect to that of
the pin, and accordingly the wallthickness of the nose portion in the end region of the
box is tlrin anclthe wiclth of the shoLrldersurface on tlie end of the box is small.
[001 8]
An object of the present invention is to provide a tlireaded joint for steel pipes
having the following characteristics: reliably ensuring sealing performance of the
external seal,
SOLUTION TO PROBLEM
[00 r e]
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 made
up(also referred to as "fastened") by screwing the pin onto the box, the box having an
outsidediameterlessthan 104o/oof an outsidediameterof atubularbodyhavingthepin.
The box includes, in order from an end of the box toward atubular body: a lip portion
and a tapered female threaded portion, the lip portiorr including a shoulder surface and a
sealing surface. The pin includes: a shoulder surface; a sealing surface; and a tapered
rnale threaded portion, the shoulder surface being in contact with the shoulder surface of
the box in a made up state, the sealing surface being in contact wìth the sealing surface
of the box in a rnade up state, the tapered male threaded portion engaging with the
tapered female threaded porlion in a made up state. The slroulder surface of the box
+
/
and the shoulder surface of the pin are inclined with respect to a plane perpendicular to
the pipe axis toward a direction in which the screwing of the pin advances. The lip
portion of the box has: a nose portion disposed between the shoulder surface and the
sealing surface; and an annular poftion disposed between the sealing surface and the
fernale threaclecl portion. The nose portion and the annular portion are not in contact
with the pin in a made up state, and the annular porlion has a length along the pipe axis,
the length being longer than a thread pitch of the female threaded porlion.
[0020]
The above threaded joint rnay be configured such that the pin includes a
complemental sealing surface disposed on an end region of the pin and the box includes
a complemerrtal sealing surface that is in contact with the cornplemental sealing surface
of the pin in a made up state.
[002 r]
The above threaded joint may be configurecl such that the pin includes a
cornplementalshouldersurface disposed on an end of the pin and tlie box iricludes a
complemental shoulder surface that is in contact with the complemental shoulder
surface of the pin in a made up state.
100221
The above threaded joint rnay be configured such that the male threaded portion
of the pin and the female threaded portion of the box each comprise two-step threads
composed of two separated portions alongthe pipe axis. In this threaded joint, the pin
may include an intermediate sealing surface disposed between a first-step male threaded
portion and a second-step male threaded portion, and the box may include an
intennediate sealing surface that is in contact with the intermediate sealing surface of
the pin in a made up state. [n addition, the pin may include an intennediate shoulder
surface disposed between the first-step male threadecl portiou and the second-step male
threaded portion, and the box may include an intermecliate shoulder surface that is ilt
contact with the intermediate shoulder surface of the pin in a made up state.
[0023]
The above threaded joint may preferably be confÌgured such that a
circumferentialtensile yield strengths of the box in aregion includingthe nose portion
and in a region including the sealing surface is at least 1050/o of a tensile yield strength
8
of the box in a region including the tubular body having the box.
100241
The above threaded.joint rnay be configured such that, in a longitudinal section
along the pipe axis, slrapes of the sealing surfaces of the box and the pin are each one of
the following or a combirration of two or lrore of the following: a straigllt line; a
circular are1, an elliptical arc; and a quadratic curve.
[002s]
The above threadedjoint rnay be configured such that the tubular body having the
pin has an outside diameter of at least 170 mm.
ADVANTACEOUS EFFECTS OF INVENTION
[0026]
A threaded joint for steel pipes according to tlre present invention has the
following signifìcant advantages: being capable of reliably ensuring sealing
performance of the external seal.
BRIEF DESCRIPTION OF DRAWINGS
100271
IFIG. l] FIG. I is a longitudinalsectionalview of athreaded jointforsteel pipes
according to an embodiment of the present invention.
lFIG.2] FIG.2 is an enlarged longitudinalsectional view of an end region of the
box in the threaded joint for steel pipes shown in FIG, l.
IFIG. 3] FIG. 3 is an enlarged longitudinal sectional view of a region of the
threaded portion in the threaded joint for steel pipes shown in FIG. l.
[FIG. 4] FIG. 4 is a longitudinal sectional view illustrating another example of the
tapered threaded portion applicable to the tlireaded joint for steel pipes according to an
embodiment of the present invention.
IFlG, 5] FIG. 5 is a lorrgitr-rdinal sectional view illustrating still another example
of the tapered threaded portion applicable to the threadecl joint for steel pipes according
to an embodiment of the present invention.
[FIG. 6] FIG. 6 is a longitudinal sectional view schematically showing a region
including a surface-to-surface seal.
1
,-;.-1,-,..^-:-:^: , !'- i-1.:
/
[FIG. 7] FIG. 7 is a longitudinal sectional view schematically showing a region
including a surface-to-surface seal in a threaded joint having a different configuration
from that of FIG. 6.
DESCRIPTION OF' EMBODIM ENTS
[0028]
As described above, in threaded joints for steel pipes, expanding of the box
outside diatneter is restricted. Thus, the wall thickness of the box, pafticularly of the
end region thereof, is thin, naturally, ln the case of threaded joints in which a nose
portion is provided in the end region of the box, the wallthickness of the nose portion is
thin because of the thin thickness of the end region of the box. As a result, the
stiffness of the nose portion itself cannot be sufficiently ensured. In the case of
threaded joints in which a shoulder surface is provided on the end of the box so as to
corne into pressLlre contact with the pìn in a hookecl ulauner, the width of the shoulder
surface is small because of the thin thickness of the end region of the box, As a result,
the radial component of the reaction force of the shoulder surface on the end of the box
that amplifies the contact pressure of the external seal is limited.
Thus, in either case, the effect of arnplifyirrg the contact pressure at the external seal
peaks out.
[002e]
In view ofthe above, the present inventors considered taking advantage ofthe
thin thickness of the end region of the box. Specifically, the present inventors
conceived that, ifthe region ofthe external seal in the box can be pressed against the
region of the external seal in the pin when a high external pressure has been Ioaded to
the threaded joint, it may be possible to amplify the contact pressure at the external seal.
After intense research, they have found that the following configurations are effective.
[0030]
The end region of the box is confìgurecl as follows. An annular groove is
formed contiguous with the sealing surface of the extemal seal at a side adjacent the
female threaded portion. The forrnation of the annular groove creates a thin-walled
annular portion between the sealing surface and the female threaded portion. The
length of the annular portion along the pipe axis is long to some extent. With the
lo
presence of the annular poftion, the length fi'om the female threaded portion to the
sealing surface ofthe external seal is elongated.
[003 r]
With this configuration, when a high external pressure has been loaded to the
threaded joint, the annular poftion and the region of the external sealcontinuous with
the annular portion shrink radially. This causes the region of the externalsealof the
box to be pressed agairrst the region of the extenralseal of the pin, thereby making it
possible to arnplify the contact pressure at the external seal.
[0032]
In short, the box is provided with a nose poftion disposed on the end region and a
shoulder surface disposed on the end so as to come into pressure contact with the pin in
a hooked lnarlner. Furthermore, the box is provided with an annular portion disposed
between the region of the extenral seal and the female threaded portion. These
produce a synergistic effect, thereby achieving significant arnplification of contact
pressure at the extemal seal. Consequently, it is possible to reliably ensure sealing
perfonnance of the extemal seal.
[0033]
The threaded joint for steel pipes of the present invention has been rnade based
on the above findings, E,rnbodiments of the threaded joint for steel pipes according to
the present invention are described below.
[0034]
FIG. I is a longitLrdinal sectional view of a threaded joint for steel pipes according
to an embodiment of the present invention. FIG. 2 is an enlarged longitudinalsectional
viewoftheendregionoftheboxinthethreadedjointforsteel pipesshowninFlG. 1.
FlG, 3 is an enlarged longitudinalsectionalview of a region of the threaded portion in
the threaded joint for steel pipes shown in FIG. l. As shown in FIGS. 1 to 3, the
threaded joint of the present ernbodiment is an integral-type threaded joint and is
constructed ofa pin l0 and a box 20. It is to be noted that the threadedjoint ofthe
present embodiment rnay be employed as a coupling-type threaded joint.
[003s]
Tlie threaded joint of the present embodiment is intended for configurations in
which the wall thickness of the end region of the box 20 is thin. Thus, the outside
tt
diameter of the box 20 is in the range of more than 100% to less than 104% of the
outside diameter of the tubular body having the pin 10. The size of steel pipes to be
connected by the threaded joint of the present embodiment is not particr-rlarly limited,
but the threaded joint is particularly suitable for connection of large diameter steel pipes,
whiclr have a thin wall thickness in the end region of the box 20. "Large diameter steel
pipes" refer to those in which the tubular body havirrg the pin l0 has an outside
diameter of at least 170 mm.
[0036]
The box 20 inclLldes, in order fi'om the end of the box 20 toward the tubular body:
a lip portion 24 and a female threaded portion 2l , the lip portion 24 including a shoulder
surface 22 and a sealing surface 23, The lip portion 24 extends along the pipe axis CL
from an end of the female threaded portion 21, adjacent the end of the box. The lip
portion has a nose portion 25 disposed between the shoulder surface 22 and the sealing
surface 23 arÅ an annr¡lar portion 26 clisposed between tlre sealing surface 23 and the
female threaded portion 21. The female threacled portion 2l is not formed in the
annular portion 26.
[0037]
The annular portion 26 extends along the pipe axis CL frorn an end of the female
threaded portion 21, adjacenl the end of the box, to connect with the sealing surface 23.
This annLrlar portion 26 nay be created, for example, by forming an annular groove
contiguous with a side of the sealing surface 23, adjacent the female threaded portion 21.
That is, as shown in FIC. 2, the inside diameter of the box 20 atthe annular porlion 26 is
largel than tlre minimum diameter at the sealing surface 23 and the maximum diameter
at the roots 2l b of the female threaded portion 2l ,
[003 8]
The nose portion 25 extends along the pipe axis C[, from an end of the sealing
surface 23, adjacerfi. the end ofthe box. A shoulder surface 22 is provided on the end
of the nose portion 25 (corresponding to the end of the Iip portion 24 or the end of the
box 20).
[003e]
The shoulder surface 22 is an annular surface inclined with respect to a plane
perpendicular to the pipe axis CL toward a direction in which the screwing of the pin l0
tL
advances (a direction toward the end of the pin l0). In other words, the shoulder
surface 22 is inclined in such a manner that the outer circumference side is closer to the
end of the pin 10. The shape of the shoulder surface 22 in a longitudinal section along
the pipe axis CL is a straight line. It is necessary that, in a longitudinal section along
the pipe axis CL, the shape of the shoulder surface 22 conform to the shape of the
shoulder surface l2 of the pin 10 described below. As long as this condition is met,
tlre shape of the shoulder surface 22 nay be a curved line.
[0040]
The sealing surface 23 is a tapered surface and provided on the inner periphery of
the lip portion 24. The sealingsurface 23 has ashape correspondingto ashape of the
peripheral surface of a truncated cone increasing in diameter toward the end (toward tlie
shoulder surface 22), or a shape corresponding to a combined shape ofthe peripheral
surfàce ofsuch a truncated cone and the peripheral surface ofrevolution that can be
obtained by rotatirig a curved line such as arì arc aboLrt the pipe axis CL. In other
words, the shape of the sealing surface 23 in a longitudinal section along the pipe axis
CL is one of the following or a combinatiorr of two or lrìore of the f'ollowing: a straight
line; a circular arc; an elliptical arc; and a quadratic cLlrve.
[004 r ]
The pin 10 includes, in order from the encl adjoining the tubular body toward the
end: a shoulder surface 12; a sealing surface 13; and a male threaded portion 1 I . The
shouldersurface l2,sealingsurface l3,andmalethreadedportion ll ofthepin 10are
provided to correspond to the shoLllder surface 22, sealing surface 23, and female
threaded poftion 21 of the box 20, r'espectively.
100421
The male threaded portion ll of the pin i0 and the female threaded portion 21 of
the box 20 are tapered threaded portions and constitute a threaded porlion in which tliey
engage with each other. As shown in FIG. 3, the male threaded porlion 1l of the pin
10 includes crests 1la, roots 11b, stabbingflanks 1lc which are in leadingpositions in
screwing, and load flanks lld located opposite from the stabbing flanks. The fernale
threaded portion 21 of the box 20 includes crests 21 a facing the roots I I b of the rnale
threaded portion I 1 , roots Zlb facing the crests I 1 a of the male threaded portion I I ,
stabbing flanks Zlc facing the stabbing flanks I I c of the male threaded portion I 1, and
\3
load flanks 2ld facing the load flanks lld of the male threaded portion 11. The
tapered threaded portions of the present ernbodiment are constituted by dovetail threads.
Thus, the load flanks 11d,21d and the stabbing flanks !lc,21c each have a negative
flank angle.
[0043]
The male threaded porlion I I of the pin I 0 is able to be screwed in the female
threaded pofiion 2l of the box 20. In a made up state, the roots l lb of the male
threaded portion l1 are in intimate contact with the crests 2la of the ferrale threaded
portion 2l,and the load flanks lld are in intimate contact with the load flanks 2ld.
Furthermore, in a made up state, clearances are provided between the crests I la of the
male threaded portion 1 1 and the roots 21b of the female threaded portion 21 and
between the stabbing flanks 11c and the stabbing flanks 2lc, and a lubricant fills these
clearances, which results in fonning a thread seal. Tlie sealing surfaces 13,23 are
brought into contact with each other by the screwing of the pin 10, and in a made up
state, they intirnately contact each other in interference so as to have a shrink fit. As a
result, an external seal by surface-to-surface contact is formed. The shoulder surfaces
12,22 are brought into contact and pressed against each other in a hooked manner by
the screwing of the pin I 0, and in a made up state, they irnpart the axial tightening force
to the load flanks 1ld of the rnale threaded portion 1l of the pin 10.
[0044]
In a made up state, aclearance is provided between the nose portion 25 of the box
20 and the pin l0 so that the nose portion 25 is not in contact with the pin 10.
Furtherrnore, a clearauce is provided between the annular portion 26 of the box 20 and
the pin l 0 so that the annular portion 26 is not in contact with the pin i 0.
[004s]
In the threaded joint for steel pipes of the present enrbodiment configured in this
rïanner, the region (externalseal region) of the sealing surface 23, which is continuous
with the nose portion 25 of the box 20, has increased resistance to defonnation because
of the stiffness of the nose portion 25 itself. In addition, since the shoulder surfaces 12,
22 are in pressure contact with each other in a hooked manner, the region of the external
seal ofthe box 20 receives a reaction force that acts in a direction to cause it to slirink
radially. As a result, radially outward plastic deformation is inhibited in the region of
r+
the external seal of the box 20, whereby the contact pressure between the sealing
surfaces 13, 23 is amplified.
[0046]
Moreover, when a high external pressure has been loaded to the threaded joint,
the annular porlion 26 of the box 20 and the region of the externalseal thereof, which is
continuous with the arrnular portion 26, shrink radially. This causes the sealing silrface
23 ofthe box 20 to be pressecl against the sealing surface l 3 ofthe pin 1 0, thereby
making it possible to arnplify the contact pressLrre between the sealing surfaces 13,23,
in the region of the external seal.
[0047]
As described above, the effect of amplifying the contact pressure at the external
seal is produced synergistically. Consequently, it is possible to reliably ensure sealing
performance ofthe external seal and thus to prevent external pressure leak.
[0048]
It is noted that the threaded joint of the present embodiment secondarily inclLrdes
an internal seal. Specifically, as shown in FIC. 1, the pin l0 includes aconrplemental
sealingsurface l7 disposed on the end region of the pin 10. The box 20 includes a
complemental sealing surface 27 corresponding to the complemental sealing surface ì7
of the pin I 0. The complemental sealing surfaces 17 ,27 are brought into contact with
each other by the screwing of the pin 10, and in a made up state, they intirnately contact
each other in interference so as to have a shrink fit. As a result, an internal seal by
surface-to-surface contact is formed.
[004e]
The following are descriptions regarding preferred embodirnents of the principal
porlions.
[Nose Portion]
lf the length of the nose poftion of the box along the pipe axis is too shofi, the
stiffness of the nose poftion itself is insufficient, and therefore the effect of arnplifying
the contact pressure at the external seal is not effectively produced. On the other hand
if the length of the nose portion is too lorrg, the materialcost is increased forthe
extended ler-rgth and the manufacturing cost is increased. Moreover, the length from
the shoulder surface to the sealing surface is elongated, and therefore the reaction force
tç
from the shoulder surfaces that are in pressure contact in a hooked lrìanner does not act
effectively on the region of the external seal. Accorclingly, the length of the nose
portion is preferably 0.5 to 3.5 times the wallthickness of the nose portion. A more
preferred lower limit of the length of the nose portion is 1.5 times the wallthickness of
the nose porlion. A more preferred upper limit of the length of the nose portion is 3.0
times the wall thickness of the nose portion.
[00s0]
ISealing SLrrface]
If, in a made up state, the length of contact between the sealingsurfaces alongthe
pipe axis is too short, sufficient sealing perforrrance is not provided. On the other
hand, if the length of contact between the sealing surfaces is too-long, the contact
pressLrre on the average is decreased and, as a result, sufficient sealing performance is
not provided. Accordingly, the length of contact between the sealing surfaces is
preferably 0.5 to 5 mrn. A rnore preferred lower limit of the length of contact is I mm.
A more preferred upper lirnit of the length of contact is 3.5 mm.
[00s 1]
IShoulder Surface]
If the hook angle (angle of inclination fi'om a plane perpendicLrlar to the pipe
axis) of the shoulder surfaces is too small, the reaction force from the shoulder surfaces
that are in pressure contact in a hooked rnanner is small. Therefore, the effect of
amplifying the contact pressure at the external seal is not effectively produced. On the
other hand, if the hook angle is too large, the stiffness of the region inclLrding the
shoulder surface in the pin is decreased and the region of the shoulder surface becomes
prone to plastic deformation, which results in adverse effects on the external seal.
Accordingly, the hook angle of the shoulder surfaces is preferably 5 to 25 degrees (o).
A more preferred lower limit of the hook angle is 9 degrees. A more preferred r"rpper
limit of the hook angle is 20 degrees.
[00s2]
[Annr-rlar Portion]
As described above, the annular poftion of the box allows the region of the
external seal in the box to shrink radially when external pressure has been loaded,
thereby producing the effect of arnplifying the contact pressure at the external seal. In
tø
addition, the annular portion of the box, owing to the interior space, serves as a
threading tool relief in thread machirring of the fernale threaded porlion.
[00s3]
If the length of the annular portion along the pipe axis is too short, sufficient
radial shrinkage of the region of the external seal will not occur, and in addition, it is
diffìcult to provide tool relief for the threading tool. Accordingly, the length of the
annular portion is set to be longer than the thread pitch of the female threaded portion.
A more preferred length of the annular portion is at least 1.2 times the thread pitch of
the female threaded portion. On the other hand, if the length of the annular porlion is
too long, the rnaterial cost is increased for the extended length and the rnanufacturing
cost is increased. Accordingly, a preferred upper lirnit of the length of the annular
portion is 4 times the thread pitch of the female threaded portion, and more preferably,
2.5 times the thread pitch.
[00s4]
It is to be noted that the tlireaded joint for steel pipes of the present embodiment
is preferably configured as follows. As shown in FIGS. I and2, a circumferential
tensile yield strength of the box 20 in a region irrcluding the nose portion 25 and in a
region including the sealing surface 23 is at least i 050/o of atensile yield strength of the
box 20 in a region including the tubular body having the box. More preferably, it is at
least i 10% thereof. By increasing the tensile yield strength of the box 20 in the region
inclr"rding the nose porlion 25 and the sealing surface 23 locally as described above,
radially outward deformation of the region of the external seal in the box 20 is inhibited.
As a result, the effect of arnplifying the contact pressure between the sealing surfaces 13,
23, is produced to a greater extent.
[00ss]
One technique for locally increasing the tensile yield strength is expanding the
diameter of the end region of the box 20 by cold working before a series of machining
operations are performed on the box 20 so that the strength is increased by strain aging,
Another technique for increasing the strength is induction hardening of the end region
ofthe box 20.
[00s6]
The present invention is not limited to the embodiments described above, and
r+
varioLrs modifications may be made without departing from the spirit and scope of the
present invention. For example, the threads that constitute the tapered threaded
portion of the tlireaded joint are not limited to dovetail threads but any type of threads
may be ernployed as long as they constitute a tapered threaded porlion. For example, a
tapered threaded porlion constituted by buttress threads as shown in FIGS. 4 and 5 rnay
be employed. The tapered threaded porlion shown in FIG.4 is of the type in which the
crests lla,2laandtheroots llb,2lbareinclinedwithrespecttothepipeaxisCL.
The tapered threaded portion slrown in FIG. 5 is of the type in which the crests lla,21a
and the roots llb,2lb are parallel with respectto the pipe axis CL.
l00s7l
Furthermore, the threaded joint of the present embodiment may secondarily
include shoulder surfaces near the internal seal. Specifically, the pin includes a
complemental shoulder surface disposed on the end thereof. The box includes a
complemental shoulder surface corresponding to the complemental shoulder surface of
the pin, The cornplemental shoulder surfaces are in pressure contact with each other in
a made up state, and perform functions similar to those of the shoulder surfaces near the
external seal.
[00s 8]
Furthennore, in the threaded joint of the present embodiment, the threaded
portion constituted by the male threaded portion of the pin and the female threaded
portion of the box may comprise two-step threads composed of two separated portions
along the pipe axis. In the case where two-step threads are employed in the threaded
joint, the pin rnay include an intermediate sealing surface disposed between a first-step
male threaded portion and a second-step male threaded portion, and the box may
include an intermediate sealing surface corresponding to the intermediate sealing
surface of the pin. The intermediate sealing surfaces intirnately contact each other in
interference in a made up state, thereby forming an intermedìate seal by
surface-to-surface contact. Furthermore, in the case where two-step threads are
employed in the threaded joint, the pin rnay include an intermediate shoulder surface
disposed between the first-step male threaded portion and the second-step male threaded
portion, and the box may include an intermediate shoulder surface corresponding to the
intermediate shoulder surface of the pin. The intermediate shoulder surfaces are in
l8
pressure contact with each other in a made up state, and perform functions similar to
those ofthe shoulder surfaces near the external seal.
[00se]
The sealing surfaces, which constitute the above-described surface-to-surface
seals (external seal, internal seal, and intermediate seal), are formed together with the
other portions in a series of machining operations. In the operations, the tool feed rate
for rnachining the seal surfaces is set to be lower than the feed rate for machining the
other portions. This urakes the sealing surfaces much slnoother than the rnachined
surfaces ofthe other portions,
[0060]
FIGS. 6 andT are longitudinal sectional views each scliematically showing a
region including the surface-to-surface seal. FIGS. 6 and 7 illustrate the external seal.
As shown in FIGS. 6 and 7, the sealing surfaces 13,23 are not limited to the region Sa
where they contact each other in a made up state but are constituted by the entireties of
regions Sb, Sc that have been machined to smooth surfaces as sealing surfaces. That is,
the sealing surfaces 13,23 are regions where they contact and slide relative to each
other (including regions where the sliding may occur) during making up thread, and
they include the entireties of the regions Sb, Sc rnachined to a surface roughness
comparable to that of the region Sa where they contact each other in a made up state.
The same applies to the internal seal and the intennediate seal.
[0061 ]
The seal portions can be identified by removing the pin l0 from the box 20 and
observing the sealing surfaces. This is because there are marks (portions where strong
sliding occurred) of the seal portions (the contact region Sa in a made up state) left on
the sealing surfaces.
EXAMPLES
100621
To verify the advantages of the present invention, a numerical simulation and
analysis using the elasto-plastic finite element rnethod (FEM analysis) was carried out.
[0063]
Test Conditions
I
r1
In the FEM analysis, models of the threaded joint for steelpipes shown in FIG. I
were prepared, Amongthese models, the length of the nose poftion, the hook angle of
the shoulder surface, and the length of the annular poftion, in the box, were varied.
Fufthermore, for one of the models, cold working was applied to its pipe end region in
the simulation so that the tensile yield strength of the box in the region including the
nose portion and the sealing surface was increased to i 10% of the tensile yield strength
thereof in the region including the tubular body having the box. The varied conditions
are shown in Table I below.
[0064]
[Table I]
TABLE I
Remarks:
- Minimum values of average contact pressure are relative values to the vaìue of No. 4, which is
assumed to be l.
- Symbol "*" indicates that the value does not satis! the condition specified by the present
invention.
[006s]
Common properties regarding the material and dimensions of the steel pipes are
as follows.
- Steel pipe size: l4 [inch], 112.6 Ublftl (outside diameter of 355.6 mm and wall
thickness of 20.32 mm).
No
Length of
nose
porlion
Imm]
Hook angle
of shoulder
surface
ldegl
Length of
annular
porlion
Imm]
Cold
Working on
pipe end
Minimum value
of average
contact pressure
at external seal
Classification
I l5 t2 No 4.3 Inv. Example
2 9 l5 l2 Yes 4.6 Inv. Example
J 0tÉ l5 l2 No t.3 Comp. Example
4 0* l5 3.5 * No 1.0 Cornp. ExarnpÌe
5 9 l5 i.) ^ No 3.3 Comp. Example
6 9 0* t2 No 2.9 Comp. Example
2ß
- Steel pipe grade:API (American Petroleurn Institute) Standard Q125 (carbon
steel for oil country tLrbular goods specified in API 5CT having a tensile yield strength
of 125 ksi (862 N/rnm2);.
- Thread form: dovetail type tapered threads; clearances provided between crests
of male threaded portion and roots of female threaded portion, and between stabbing
flanks; thread height of about 2 mm; and thread pitch of 8.47 mm.
- Sealing surface: taper of 10%o and length of contact of 4 mm.
- Width of shoulder surface: 5 mm.
[0066]
Evaluation Method
In the FEM analysis, for each model, the pin was tightened to the point of
shouldering (abutrnent of shoulder surfaces against each other) and furlher tightened
0.01 turns. In this state, load steps which simulated those in the ISO 13679:2002
Series A test (a test in which internal and external pressures are repeatedly loaded at
ambient temperatures) were sequentially perforrned. The sealing performance of the
external seal was evaluated by comparing the minimum values of the average contact
pressures at the sealing surfaces ofthe external seal, in the internal pressure cycle (the
first and second qLradrants) and the external pressure cycle (the third aud foufth
qLradrants) in the load step sequence. (lt is noted that the higher the minimurn value of
average contact pressure, the better the sealing performance ofthe sealing surfaces.)
[0067]
Evaluations of the sealing performance of the external seal were made by
assuming the value of Test No. 4, which exhibited the lowest minimum value of the
contact pressure at the external seal, to be I and detennining, as indices, relative values
to this value of Test No. 4. Models having an index of 4 or rnore were evaluated as
being good. The results are shown in Table I above.
[0068]
Test Results
The results shown in Table 1 indicate that the inventive examples of Test Nos. 1
and2, which satisfy all the conditions specifìed by the present invention, each exhibited
a greater minimum value of the contact pressure at the external seal than the
comparative examples of Test Nos. 3 to 6, which do not satisfy each of the conditions
LI
7
specified by the present invention. This demonstrates that the threaded joint for steel
pipes of the present embodiment is advantageous in the sealing perfonnance of the
external seal.
INDUSTRIAL APPLICAB I LITY
[006e]
Threaded joints of the present invention are capable of being effectively utilized
in connecting steel pipes that are used as oil country tubular goods.
REFERENCE SIGNS LIST
[0070]
l0: pin, I I : rnale threaded poftion,
1 1 a: crest of male threaded poftion, I I b: root of male threaded pofiion,
1 1c: stabbing flank of male threaded poftion, 1 1d: load flank of male threaded
portion,
I 2: shoLrlder surface, l3: sealing surface, 1 7: complemental sealing surface,
20: box, 2l: female threaded poftion,
2la: cresf of fe¡nale threaded poftion, 2lb: root of female threaded poftion,
2lc: stabbing flank of female threaded porlion, 2ld: load flank of female
threaded portion,
22: shoulder surface, 23: sealing surface, 24: lip portion,
25: nose portion, 26: annular poftion, 27: complernental sealing surface,
CL: pipe axis.

We claim:
I . A threadecl-joint for steel pipes, comprising: a tubular pin ancl a tubular box, the
pin and the box being rnade up by screwing the pin onto the box, the box having an
outside diameter less than 104%o of an outside diameter of a tubular body having the pin,
characterized in that,
the box cornprises, in order froln an end of the box toward a tubular body: a lip
portion and a tapered female threaded portion, the lip portion including a shoulder
surface ancl a sealing surface,
the pin comprises: a shoulder surface; a sealing surface; and a tapered rnale
threaded portion, the shoulder surface being in contact with the shoulder surface of the
box in a made up state, the sealing surface being in contact with the sealing surface of
the box in a made up state, the taperedmale threaded portion engaging with the tapered
female threaded portion in a made up state,
the shoulder surface of the box and the shoulder surface of the pin are inclined
with respect to a plaue perpendicular to the pipe axis toward a direction in which the
screwing of the pirr aclvances,
the lip portion of the box has: a nose portion disposed between the shoulder
surface and the sealing surface; and an annular portion disposed between the sealing
surface and the female threaded portion,
the nose portion and the annular portion are not in contact with the pin in a rnade
up state, and
the annular portion has a length along the pipe axis, the length being longer than
a thread pitch of the female threaded portiorr.
2. The threaded .ioint for steel pipes according to claim 1, characterized in that,
the pin includes a complemental sealing surface disposed on an end region of the
piu and the box includes a complemental sealing surface that is in contact with the
complemental sealiug surface of the pin in a made up state.
3. The threadecl joint for steel pipes according to claim I or 2, characterized in that,
L3
the pin iricludes a complernental shoulder surface disposed on an end of the pin
and the box includes a complemental shoulder surface that is in contact with the
complemental shoulder surface of the pin in a made up state.
4. The threaded joint for steel pipes according to any one of claims I to 3,
characferized in that,
the male threaded portion of the pin and the female threaded portion of the box
each comprise two-step threads composed of two separated portions along the pipe axis,
5. The threaded joint for steel pipes accorcling to clairn 4, characterized in that,
the pin inclr¡des an intermediate sealing surface disposed between a first-step
male threaded portion and a second-step male threaded portion, and the box includes an
intermediate sealing surface that is in contact with the intermediate sealing surface of
the pin in a made up state,
6. The threaded joint for steel pipes according to claim 4 or 5, characterized in that,
the pin includes an intermediate shoulder surface disposed between a first-step
male threaded porlion and a second-step male threaded portion, and the box includes an
intermediate shoulcler surface that is in contact with the intermediate shoulder surface of
the pin in a made up state.
7. The threaded joint for steel pipes according to any one of claims 1 to 6,
characterized in that,
a circumferential tensile yield strengths of the box in a region including the nose
portion and in a region including the sealing surface is at least 105%o of a tensile yield
strength of the box in a region inclLrdingthe tubular body havingthe box.
8. The threaded joint fbr steel pipes according to any one of claims I to 7,
characterized in that,
in a longitudinal section along the pipe axis, shapes of the sealing surfaces of the
box and the pin are each one of the following or a combination of two or more of the
following: a straight lirre; a circular arc; an elliptical arc; and a quadratic curve.
L+
9. The threaded joint for steel pipes according to any one of claims I to 8,
characterized in that,
the tLrbular body having the piu has an outsicle diameter of at Ieast 170 rnm.