DESCRIPTION
TITLE OF INVENTION
THREADED JOINT FOR STEEL PIPES
TECHNICAL FIELD
[OOOl]
The present invention relates to a threaded joint for use in connecting steel pipes.
BACKGROUND ART
[OOOZ]
In oil wells, natural gas wells, and the like (hereinafter also collectively referred
to as "oil wells"), oil country tubular goods for casings, tubings or the like, which are
steel pipes and are sequentially connected to one another by threaded joints, are used.
Generally, threaded joints for steel pipes for such use are classified into two types:
coupling-type joints and integral-type joints.
[0003]
A coupling-type threaded joint is constituted by a pair of tubular goods that are to
be connected to each other, of which one is a steel pipe and the other is a coupling. In
this case, the steel pipe is provided with a male threaded portion formed on the outer
periphery at each end thereof, and the coupling is provided with a female threaded
portion formed on the inner periphery at each end thereof. The male threaded portion
of the steel pipe is screwed into the female threaded portion of the coupling, thereby
making up a joint and connecting them. An integral-type threaded joint is constituted
by a pair of steel pipes as tubular goods that are to be connected to each other, without a
separate coupling being used. In this case, each steel pipe is provided with a male
threaded portion formed on the outer periphery at one end thereof and a female threaded
portion formed on the inner periphery at the other end thereof. 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 making up a joint and connecting them.
[0004]
In general, the joint portion at the tubular end where a male threaded portion is
formed 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 formed is referred to as a box because it includes an
element that receives a male threaded portion. A pin and a box both have a tubular
shape because they are constituted by end portions of tubular goods.
[0005]
FIG. 1 is a sectional view of a threaded joint for steel pipes, showing an example
of its overall configuration. The threaded joint illustrated in FIG. 1 is a coupling-type
threaded joint and is constructed of a pin 10 and a box 20.
[0006]
The pin 10 includes, in order from a free end of the pin toward the tubular body, a
shoulder surface 17, a seal surface 16, and a male threaded portion 11. The seal
surface 16 is a tapered surface. To be exact, the seal surface 16 is a surface constituted
by the peripheral surface of a truncated cone having a diameter decreasing toward the
end, or a surface constituted by a combination of the peripheral surface of the truncated
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. The shoulder surface 17 is an
annular surface extending radially substantially perpendicular to the pipe axis CL. To
be exact, it is a slightly inclined surface with the outer circumferential side being closer
to the end of the pin 10.
[0007]
The box 20 includes, in order from the body of the box toward an end of the box,
a shoulder surface 27, a seal surface 26, and a female threaded portion 21. The
shoulder surface 27, the seal surface 26, and the female threaded portion 21 are located
so as to correspond to the shoulder surface 17, the seal surface 16, and the male
threaded portion 11 of the pin 10. The male threaded portion 11 of the pin 10 and the
female threaded portion 21 of the box 20 are tapered threaded portions with trapezoidal
threads that mate with each other.
[OOOS]
The male threaded portion 11 and the female threaded portion 21 are threadedly
engageable with each other, and in a made-up state, they mate in intimate contact with
each other and have an interference fit. The seal surfaces 16,26 are brought into
contact with each other by the screwing of the pin 10, and in a made-up state, they mate
in intimate contact with each other to have an interference fit, thereby forming a seal
therebetween with metal-to-metal contact. The shoulder surfaces 17,27 are brought
into contact and pressed against each other by the screwing of the pin 10 onto the box
20, and serve as stoppers for restricting the screwing of the pin 10. Furthermore, in a
made-up state, the shoulder surfaces 17,27 serve to impart, to the male threaded portion
11 of the pin 10, a load in a direction opposite (backward) to the screwing direction
(forward), i.e., so-called thread tightening axial force.
[0009]
With a threaded joint having this configuration, good sealing performance is
ensured because of the seal provided by the mating and intimate contact between the
seal surfaces 16,26.
[OOlO]
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
such harsh environments, steel pipes of the heavy wall type are mostly used. A
threaded joint for connecting such steel pipes is required to have high joint strength
such as resistance to tensile forces, resistance to compressive forces, and the like, and in
addition, required to provide excellent sealing performance against internal pressure and
external pressure.
[OOll]
One method for enhancing the sealing performance of a threaded joint is to
generate high contact pressure between the seal surfaces. Conventionally, in order to
increase contact pressure between the seal surfaces, the technique of increasing the
interference fit between the seal surfaces is utilized. In addition, in order to prevent
the mating and intimate contact of the threads from causing a decrease in contact
pressure between the seal surfaces, the technique of relieving the mating and intimate
contact of the threads exclusively in regions near the seal surfaces is utilized (see, for
example, U.S. Patent No. 2062407 (Patent Literature l), Japanese Patent Application
Publication No. H02-80886 (Patent Literature 2), Japanese Patent Application
Publication No. S62-196488 (Patent Literature 3), and Japanese Patent Application
Publication No. H10-89555 (Patent Literature 4)).
[0012]
FIG. 2 is a sectional view of a conventional threaded joint for steel pipes
disclosed in Patent Literatures 1 and 2, showing a configuration of regions near its seal
surfaces. In the conventional threaded joint shown in FIG. 2, in a made-up state, the
male threaded portion 11 of the pin 10 and the female threaded portion 21 of the box 20
mate in intimate contact with each other, and the load flanks 15 of the male threaded
portion 11 are in contact with the load flanks 24 of the female threaded portion 21 and
receive the axial tightening force while the roots 13 of the male threaded portion 11 are
in contact with the crests 22 of the female threaded portion 21. However, in regions
near the seal surfaces 16,26 in the threaded portions, clearances are provided between
the roots 13 of the male threaded portion I I and the crests 22 of the female threaded
portion 21, so that the mating and intimate contact of the threads is relieved therein.
[00 131
The load flank 15 of the male threaded portion 11 as referred to herein is the
flank, of the leading and trailing flanks that constitute each thread, which is on the
opposite side from the stabbing flank 14, which is in a leading position in the screwing
of the male threaded portion 11 into the female threaded portion 21. The load flank 24
of the female threaded portion 21 is the flank, of the leading and trailing flanks that
constitute each thread, which faces the load flank 15 of the male threaded portion 11.
[OO 141
In the threaded joint shown in FIG. 2, the mating and intimate contact of the
threads is relieved in regions near the seal surfaces 16, 26 in the threaded portions, and
therefore, when internal pressure is applied, the region near the seal surface 16 in the
threaded portion of the pin 10 is expanded radially outward from the inside to cause
enlargement of the diameter, whereby the contact pressure between the seal surfaces 16,
26 is amplified.
[0015]
It is to be noted that, in the conventional threaded joint shown in FIG. 2, the flank
angle 0 of the load flanks 15 and the load flanks 24 is greater than 0 degrees. The
flank angle 0 refers to the angle formed by a flank with respect to a plane perpendicular
to the pipe axis CL. Herein, when referring to the flank angle of load flanks,
clockwise angles are designated as positive angles, and conversely, when referring to
the flank angle of stabbing flanks, counterclockwise angles are designated as positive
angles. When the load flank angle 0 is greater than 0 degrees, application of intemal
pressure causes reaction force at the load flanks 15 of the pin 10 acting in a direction to
contract the pin 10 radially inward from the load flank 24 of the box 20. As a result,
the radially outward expansion of the pin 10 is not sufficiently caused, and therefore the
amplification of the contact pressure between the seal surfaces 16,26 cannot be
satisfactorily achieved when intemal pressure is applied.
[00 161
FIG. 3 is a sectional view of a conventional threaded joint for steel pipes
disclosed in Patent Literature 3, showing a configuration of regions near its seal
surfaces. In the conventional threaded joint shown in FIG. 3 as well, in regions near
the seal surfaces 16,26 in the threaded portions, clearances are provided between the
roots 13 of the male threaded portion 11 and the crests 22 of the female threaded portion
21, so that the mating and intimate contact of the threads is relieved therein. Its load
flanks 15,24 have a flank angle 0 of less than 0 degrees.
[0017]
In the threaded joint shown in FIG. 3, because of the load flank angle 0 that is
less than 0 degrees, application of internal pressure does not cause reaction force at the
load flanks 15 of the pin 10 acting in a direction to contract the pin 10 radially inward.
As a result, the radially outward expansion of the pin 10 is sufficiently caused, and
therefore amplification of contact pressure between the seal surfaces 16, 26 can be
achieved when internal pressure is applied.
[00 181
In the threaded joint shown in FIG. 3, it is to be noted that, in regions near the
seal surfaces 16, 26 in the threaded portions, the height of crests 12 in the male threaded
portion 11 of the pin 10 decreases toward the end of the pin 10 with a steep taper angle,
so that the thread height is sharply decreased toward the end. Because of this, the
stiffness of the pin 10 is reduced. This causes a decrease in the deformation resistance
of the pin 10 against external pressure, which results in reduced contact pressure
between the seal surfaces 16,26 when external pressure is applied. In addition, the
low thread height of the male threaded portion 11 in the region near the end of the pin
10 results in reduced thread mating at the time of insertion of the pin 10 into the box 20,
and this causes increased eccentricity of the pin 10. Thus, at the start of screwing of
the pin 10 onto the box 20, the thread comer regions of the male threaded portion 11 of
the pin 10 locally contact the female threaded portion 21 of the box 20. In the regions
where such local contact occurs, the contact pressure is increased and therefore galling
is more likely to occur.
[00 191
Furthermore, the threaded joint shown in FIG. 3 is configured such that the roots
23 of the female threaded portion 21 of the box 20 have different taper angles. This
configuration requires a complex manufacturing process, which leads to a longer
manufacturing time and shorter tool life, and therefore a further problem of increased
manufacturing costs arises.
[0020]
The threaded joints shown in FIGS. 2 and 3 are both configured to relieve the
mating and intimate contact of the threads in regions near the seal surfaces in the
threaded portions, by providing clearances between the roots of the male threaded
portion and the crests of the female threaded portion. As another technique for
relieving the mating and intimate contact of the threads in regions near the seal surfaces,
Patent Literature 4 discloses a technique of providing a circumferential groove between
the female threaded portion of the box and the seal surface thereof.
[002 I]
However, in the threaded joint disclosed in Patent Literature 4, thread mating is
reduced in regions near the seal surfaces because of the circumferential groove provided
in the box. Because of this, when external pressure is applied, radial contraction of the
pin easily occurs, resulting in a decrease in sealing performance against external
pressure.
CITATION LIST
PATENT LITERATURE
[0022]
Patent Literature 1: U.S. Patent No. 2062407
Patent Literature 2: Japanese Patent Application Publication No. H02-80886
Patent Literature 3: Japanese Patent Application Publication No. S62-196488
Patent Literature 4: Japanese Patent Application Publication No. H10-89555
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0023]
As described above, conventional threaded joints have been constructed only in
view of the contact pressure between the seal surfaces in a made-up state. That is, the
change in contact pressure due to application of internal and external pressures is not
sufficiently considered.
[0024]
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 with the sealing
performance against external pressure being maintained.
SOLUTION TO PROBLEM
[0025]
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
by screwing the pin onto the box.
The pin includes, in order from an end of the pin, a shoulder surface, a seal
surface, and a male threaded portion, the male threaded portion being a tapered threaded
portion with trapezoidal threads.
The box includes a shoulder surface corresponding to the shoulder surface of the
pin, a seal surface corresponding to the seal surface of the pin, and a female threaded
portion corresponding to the male threaded portion of the pin, the female threaded
portion being a tapered threaded portion with trapezoidal threads.
The male threaded portion includes crests, roots, stabbing flanks, and load flanks,
the stabbing flanks being in a leading position in the screwing of the male threaded
portion into the female threaded portion, the load flanks being located opposite from the
stabbing flanks.
The female threaded portion includes roots facing the crests of the male threaded
portion, crests facing the roots of the male threaded portion, stabbing flanks facing the
stabbing flanks of the male threaded portion, and load flanks facing the load flanks of
the male threaded portion.
The load flanks of the male threaded portion and the load flanks of the female
threaded portion have a flank angle that is less than 0 degrees.
The female threaded portion is composed of, in order from near the seal surface
of the box, an incomplete thread section and a complete thread section.
In the female threaded portion, over an entire area thereof including the
incomplete thread section and the complete thread section, the roots collectively define
a single tapered periphery.
The incomplete thread section has a length along the pipe axis, the length being at
least three times a thread pitch of the female threaded portion, and the incomplete thread
section has a thread height lower than a thread height of the complete thread section.
In the threaded joint having such a configuration, in a made-up state: the shoulder
surfaces are in contact with each other, and the seal surfaces are in contact with each
other; in the complete thread section, the roots of the male threaded portion are in
contact with the crests of the female threaded portion, and the load flanks of the male
threaded portion are in contact with the load flanks of the female threaded portion; and
in the incomplete thread section, clearances are provided between the roots of the male
threaded portion and the crests of the female threaded portion, and the load flanks of the
male threaded portion are in contact with the load flanks of the female threaded portion.
[0026]
In the above threaded joint, the length of the incomplete thread section along the
pipe axis is preferably at most eight times the thread pitch of the female threaded
portion.
[0027]
In the above threaded joint, the incomplete thread section may have one of the
following configurations: a configuration in which the crests, starting from a boundary
between the complete thread section and the incomplete thread section, collectively
define a cylindrical periphery that is parallel to the pipe axis; and a configuration in
which the crests, starting from the boundary, collectively define a tapered periphery
inclined with respect to the pipe axis.
[0028]
Alternatively, in the above threaded joint, the incomplete thread section may have
a configuration in which the crests, starting from a boundary between the complete
thread section and the incomplete thread section, collectively define a tapered periphery
that is parallel to a tapered periphery collectively defined by the crests in the complete
thread section.
[0029]
Further, the above threaded joint may be configured such that, in the male
threaded portion, over an entire area thereof corresponding to the incomplete thread
section and the complete thread section, the crests collectively define a single tapered
periphety.
[0030]
Further, the above threaded joint may be configured such that:
the pin includes a nose portion disposed between the seal surface and the
shoulder surface,
the box includes a recessed portion corresponding to the nose portion of the pin,
and
in a made-up state, the nose portion of the pin is not in contact with the recessed
portion of the box.
[0031]
Furthermore, the above threaded joint may be configured such that:
the box includes a circumferential groove disposed between the incomplete
thread section and the seal surface, the circumferential groove having a length along the
pipe axis, the length being at most three times the thread pitch of the female threaded
portion; and
the male threaded portion of the pin extends to a location corresponding to the
circumferential groove.
[0032]
Furthermore, the above threaded joint may be configured such that:
the pin includes a seal surface for external pressure, the seal surface for external
pressure being disposed on at least one of an end region of the male threaded portion
and an intermediate region thereof, and
the box includes a seal surface for external pressure corresponding to the seal
surface for external pressure of the pin.
ADVANTAGEOUS EFFECTS OF INVENTION
[0033]
A threaded joint for steel pipes of the present invention has the following
significant advantages:
Ability to exhibit improved sealing perfonnance against internal pressure while
the sealing performance against external pressure is maintained.
BRIEF DESCRIPTION OF DRAWINGS
[0034]
[FIG. 11 FIG. 1 is a sectional view of a threaded joint for steel pipes, showing an
example of its overall configuration.
[FIG. 21 FIG. 2 is a sectional view of a conventional threaded joint for steel pipes
disclosed in Patent Literatures 1 and 2, showing a configuration of regions near its seal
surfaces.
[FIG. 31 FIG. 3 is a sectional view of a conventional threaded joint for steel pipes
disclosed in Patent Literature 3, showing a configuration of regions near its seal
surfaces.
[FIG. 41 FIG. 4 is a sectional view of a threaded joint for steel pipes according to
an embodiment of the present invention, showing a configuration of regions near its seal
surfaces.
[FIG. 51 FIG. 5 is a sectional view of the threaded joint for steel pipes according
to an embodiment of the present invention, showing an example configuration of the
female threaded portion of the box.
[FIG. 61 FIG. 6 is a sectional view of the threaded joint for steel pipes according
to an embodiment of the present invention, showing another example configuration of
the female threaded portion of the box.
[FIG. 71 FIG. 7 is a sectional view of the threaded joint for steel pipes according
to an embodiment of the present invention, showing yet another example configuration
of the female threaded portion of the box.
[FIG. 81 FIG. 8 is a sectional view of the threaded joint for steel pipes according
to an embodiment of the present invention, showing a variant configuration of the
regions near the seal surfaces.
[FIG. 91 FIG. 9 is a sectional view of the threaded joint for steel pipes according
to an embodiment of the present invention, showing another variant configuration of the
regions near the seal surfaces.
[FIG. 101 FIG. 10 is a sectional view of a variation of the threaded joint for steel
pipes according to an embodiment of the present invention.
[FIG. 1 I] FIG. 11 is a sectional view of another variation of the threaded joint for
steel pipes according to an embodiment of the present invention.
[FIG. 121 FIG. 12 is a graph showing test results of Examples.
DESCRIPTION OF EMBODIMENTS
[0035]
The present inventors conceived of the idea that contact pressure between the seal
surfaces can be increased by taking advantage of the radially outward expansion of the
pin due to internal pressure, and based on this idea, they conducted finite element
analysis for various modes of mating and intimate contact of the threads in the regions
near seal surfaces and evaluated the sealing performances against internal pressure and
external pressure. As a result, they have found that it is advantageous that, of the male
threaded portion of the pin and the female threaded portion of the box that mate with
each other, the female threaded portion of the box exclusively be configured to have a
lower thread height in the region near the seal surface thereof to relieve the mating and
intimate contact of the threads. Hereinafter, preferred embodiments of the threaded
joint for steel pipes according to the present invention will be described.
[0036]
FIG. 4 is a sectional view of a threaded joint for steel pipes according to an
embodiment of the present invention, showing a configuration of regions near its seal
surfaces. The threaded joint of the present embodiment has a configuration based on
those of the threaded joints shown in FIGS. 1 to 3, while having a flank angle 8 of less
than 0 degrees at the load flanks 15 in the male threaded portion 11 and the load flanks
24 in the female threaded portion 21. That is, the tapered threads of the threaded joint
of the present embodiment are tapered trapezoidal threads having load flanks that are
inclined to form a hook shape.
[0037]
The female threaded portion 21 of the box 20 is divided into, in order from near
the seal surface 26, an incomplete thread section 21b and a complete thread section 21a.
The incomplete thread section 21b has a length along the pipe axis CL that is at least
three times the thread pitch of the female threaded portion 21. In the incomplete
thread section 21b, the female threaded portion 21 has a thread height lower than the
thread height of the complete thread section 2la. Roots 23 in the incomplete thread
section 21b, together with roots 23 in the complete thread section 21a, collectively
define a single tapered periphery. That is, the roots 23 in the female threaded portion
21, over the two thread sections 21a, 21b, collectively define a single tapered periphery.
[0038]
In a made-up state, the shoulder surfaces 17,27 are in contact with each other,
and the seal surfaces 16,26 are in contact with each other. In both the complete thread
section 21a and the incomplete thread section 21b, the load flanks 15 of the male
threaded portion 11 are in contact with the load flanks 24 of the female threaded portion
21. It is to be noted that, while in the complete thread section 21a, the roots 13 of the
male threaded portion 11 are in contact with the crests 22 of the female threaded portion
21, in the incomplete thread section 21b, which is closer to the seal surfaces 16,26,
clearances are provided between the roots 13 of the male threaded portion 11 and the
crests 22 of the female threaded portion 21, so that the mating and intimate contact of
the threads is relieved therein.
[0039]
In the threaded joint according to this embodiment, the flank angle 9 of the load
flanks is less than 0 degrees, and therefore application of internal pressure does not
cause reaction force at the load flanks 15 of the pin 10 acting in a direction to contract
the pin 10 radially inward. As a result, the radially outward expansion of the pin 10 is
sufficiently caused, and therefore the amplification of the contact pressure between the
seal surfaces 16, 26 can be achieved when internal pressure is applied.
[0040]
Regarding the male threaded portion 11 of the pin 10, its configuration in the area
corresponding to the incomplete thread section 21b of the box 20 is similar to that in the
area corresponding to the complete thread section 21a thereof. That is, the crests 12 in
the male threaded portion 11, over the entire area thereof corresponding to the entire
area of the female threaded portion 21 including the incomplete thread section 21h and
the complete thread section 21a, collectively define a single tapered periphery. By
virtue of this, in the incomplete thread section 21b area, the stiffness of the pin 10 is
ensured and the mating of the threads is sufficiently obtained. This results in
increasing the deformation resistance of the pin 10 against radial contraction due to
application of external pressure, thus making it possible to maintain the sealing
performance against external pressure. Moreover, since a sufficient thread height is
ensured in the male threaded portion 11 of the pin 10, even in the incomplete thread
section 21b area, stable thread mating is achieved during insertion of the pin 10 into the
box 20, so that galling is less likely to occur.
[0041]
The length of the incomplete thread section 21 b of the box 20 is at least three
times the thread pitch ofthe female threaded portion 21. The reason for this is as
follows. If the length of the incomplete thread section 21b is shorter than three times
the thread pitch, the area on which the radially outward expansion of the pin I0 due to
internal pressure can act will be smaller. Thus, it will be impossible to cause sufficient
radially outward expansion to amplify the contact pressure between the seal surfaces 16,
26.
[0042]
However, the longer the length of the incomplete thread section 21b is, the
further the stiffness of the box 20 is reduced so that the deformation of the box 20 is
more likely to occur. Thus, the contact pressure between the seal surfaces 16, 26 under
combined loading is decreased. In addition, the substantial total mating area in the
threads is reduced. This leads to the risk of the occurrence of the phenomenon in
which the pin 10 inadvertently becomes disengaged from the box 20 (jump-out).
Accordingly, the length of the incomplete thread section 21b is preferably not more than
eight times the thread pitch. The length is more preferably not more than six times the
thread pitch, and even more preferably not more than five times the thread pitch.
[0043]
Furthermore, in the threaded joint of the present embodiment, as described above,
the flank angle 6 of the load flanks 15 in the male threaded portion 11 and the load
flanks 24 in the female threaded portion 21 is less than 0 degrees so as to ensure that the
radially outward expansion of the pin 10 due to internal pressure is not interfered with.
The flank angle 8, in view of the ease of thread cutting, is preferably -15 degrees or
greater, and more preferably -10 degrees or greater although the lower limit therefor is
not particularly specified.
[0044]
For the configuration of the female threaded portion 21 (complete thread section
21a and incomplete thread section 21b) to be provided for the box 20, any of the
following configurations (I) to (3) may be employed.
[0045]
(1) FIG. 5 is a sectional view of the threaded joint for steel pipes according to an
embodiment of the present invention, showing an example configuration of the female
threaded portion of the box. In the female threaded portion 21 shown in FIG. 5, the
crests 22 in the incomplete thread section 21 b, starting from the boundary between the
complete thread section 21a and the incomplete thread section 21h, collectively define a
cylindrical periphery that is parallel to the pipe axis CL. Such configuration of the
female threaded portion 21 can be easily obtained by appropriately specifying the shape
of the prepared hole which is cut in the box 20 in advance prior to thread cutting.
[0046]
Specifically, as a prepared hole for the complete thread section 21a, a tapered
prepared hole that corresponds to the crests 22 of the complete thread section 21a is cut.
Then, as a prepared hole for the incomplete thread section 21h, a prepared hole having a
cylindrical periphery parallel to the pipe axis CL is cut, starting from the boundary
between the thread sections 21a and 21 b. Thread cutting may be performed on the
thus formed prepared holes in such a manner that the thread taper angle and the thread
pitch are maintained constant over the entire area from the complete thread section 21a
to the incomplete thread section 21b.
[0047]
(2) FIG. 6 is a sectional view of the threaded joint for steel pipes according to an
embodiment of the present invention, showing another example configuration of the
female threaded portion of the box. In the female threaded portion 21 shown in FIG. 6,
the crests 22 in the incomplete thread section 21 b, starting from the boundary between
the complete thread section 21a and the incomplete thread section 21b, collectively
define a tapered periphery inclined with respect to the pipe axis CL. This
configuration of the female threaded portion 21 can be obtained in the following manner.
As a prepared hole for the complete thread section 21a, a tapered prepared hole that
corresponds to the crests 22 of the complete thread section 21a is cut. Then, as a
prepared hole for the incomplete thread section 21b, a tapered prepared hole having a
diameter increasing toward the tubular body is cut, starting from the boundary between
the thread sections 21a and 21b. Thread cutting may be performed on the thus formed
prepared holes in such a manner that the thread taper angle and the thread pitch are
maintained constant over the entire area from the complete thread section 21a to the
incomplete thread section 2 1 b.
[0048]
(3) FIG. 7 is a sectional view of the threaded joint for steel pipes according to an
embodiment of the present invention, showing yet another example configuration of the
female threaded portion of the box. In the female threaded portion 21 shown in FIG. 7,
the crests 22 in the incomplete thread section 21b, starting from the boundary between
the complete thread section 21a and the incomplete thread section 21b, collectively
define a tapered periphery that is parallel to the thread taper collectively defined by the
crests in the complete thread section 21a. This configuration of the female threaded
portion 21 can be obtained in the following manner. As a prepared hole for the
complete thread section 21a, a tapered prepared hole that corresponds to the crests 22 of
the complete thread section 21a is cut. Then, as a prepared hole for the incomplete
thread section 21 b, a tapered prepared hole having a periphery parallel to the tapered
periphery that corresponds to the crests 22 of the complete thread section 21a is cut at a
radially outwardly stepped position, starting from the boundary between the thread
sections 21a and 21 b. Thread cutting may be performed on the thus formed prepared
holes in such a manner that the thread taper angle and the thread pitch are maintained
constant over the entire area from the complete thread section 21a to the incomplete
thread section 2 1 b.
[0049]
The above configurations (1) and (2) are advantageous in that a step is not
formed in the threads at the boundary between the complete thread section 21a and the
incomplete thread section 21 b. In the meantime, the above configuration (3) is
advantageous in that, although a step is formed in the threads at the boundary between
the complete thread section 21a and the incomplete thread section 21b, the clearances
between the roots 13 of the male threaded portion 11 and the crests 22 of the female
threaded portion 21 are constant throughout the incomplete thread section 21b, and
therefore regulation of the clearances is facilitated.
[OOSO]
In the incomplete thread section 21b, the clearances between the roots 13 ofthe
male threaded portion 11 and the crests 22 of the female threaded portion 21 are
configured so that the radially outward expansion of the pin 10 due to internal pressure
is not interfered with, depending on the thread taper angle and the thread position along
the pipe axis CL. The clearances may be of a height generally equal to or greater than
the difference in thread heights (about 0.1 to 0.2 mm, in general) between the male
threaded portion 11 of the pin 10 and the female threaded portion 21 of the box 20.
[OOSl]
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. As shown in FIG. 8, for example, the pin 10 may include, between
the seal surface 16 and the shoulder surface 17, a nose portion 18 extending toward the
end of the pin, and the box 20 may include a recessed portion 28 that corresponds to the
nose portion 18 of the pin 10. In this case, in a made-up state, the nose portion 18 of
the pin 10 is not in contact with the recessed portion 28 of the box 20. In a threaded
joint having this configuration, the pin 10 exhibits improved stiffness because of the
nose portion 18 provided therein. As a result, the sealing performance against external
pressure is significantly improved.
[0052]
Furthermore, as shown in FIG. 9, the box 20 may include a circumferential
groove 29 disposed between the incomplete thread section 21b and the seal surface 26,
and the male threaded portion 11 of the pin 10 may extend to a location corresponding
to the circumferential groove 29. When an excessive amount of dope (lubricant) has
been applied to the threaded portions, the circumferential groove 29 allows the dope to
accumulate therein, thereby inhibiting an abnormal increase in the dope pressure, which
results in the advantageous effect of preventing the decrease in the contact pressure
between the seal surfaces 16, 26. In this instance, the length of the circumferential
groove 29 along the pipe axis CL is at most three times the thread pitch of the female
threaded portion 21. The reason for this is as follows.
[0053]
In addition to the incomplete thread section 21b as described above, the longer
the length of the circumferential groove 29 is, the more the area on which the radially
outward expansion of the pin 10 due to internal pressure can act is increased, and
accordingly the contact pressure between the seal surfaces 16,26 is amplified.
However, when the length of the circumferential groove 29 is too long, the total mating
area in the threads is reduced and, as a result, the sealing performance against external
pressure tends to decrease. As long as the length of the circumferential groove 29 is
not more than three times the thread pitch, almost no adverse effects are produced on
the sealing performance against external pressure.
[0054]
The threaded joint of the present embodiment is configured to have the seal
surfaces 16, 26 only on end regions of the threaded portions at the pin free end side.
Optionally, aside from the seal surfaces 16, 26, seals for external pressure may be
provided on the other end regions of the threaded portions at the opposite end side or on
intermediate regions thereof so that the sealing performance against external pressure
can be ensured.
[0055]
Specifically, as shown in FIG. 10, the pin 10 may include a seal surface 16A for
external pressure disposed on the opposite end region of the male threaded portion 11,
and the box 20 may include a seal surface 26A for external pressure corresponding to
the seal surface 16A for external pressure. Alternatively, as shown in FIG. I I, the pin
10 may include a seal surface 16B for external pressure disposed on an intermediate
region of the male threaded portion 11, and the box 20 may include a seal surface 26B
for external pressure corresponding to the seal surface 16B for external pressure.
Alternatively, the seal surfaces 16A, 26A for external pressure on the opposite end
regions shown in FIG. 10 may be provided in combination with the seal surfaces 168,
26B for external pressure on the intermediate regions shown in FIG. 11. In the
threaded joint having such seal surfaces 16A, 26A, 16B, and 26B for external pressure,
it may occur that, for some reason, external pressure passes through the seal surfaces
16A, 26A, 16B, and 26B and reaches the seal surface 16,26 for internal pressure.
However, even in such a case, the seal surface 16,26 for internal pressure alternatively
provide sealing performance against external pressure.
[0056]
The threaded joint of the present embodiment may be employed either as an
integral-type threaded joint or as a coupling-type threaded joint.
EXAMPLES
[0057]
To verify the advantages of the present invention, numerical simulation and
analysis was carried out using the elasto-plastic finite element method.
[0058]
[Test Conditions]
In the FEM analysis, models of coupling-type threaded joints, with varied lengths
of threaded portions, were prepared. The common conditions were as follows.
- Steel pipe size: 8-518 [inch], 64 [lblft] (outside diameter of 219.1 mm and wall
thickness of 19.05 mm).
- Coupling outside diameter: 235.8 mm.
- Grade of steel pipe (pin) and coupling (box): API (American Petroleum
Institute) standard 4125 (carbon steel having a yield stress of 125 [ksi]).
- Thread form: taper of 111 8, thread height of 1.575 [mm], thread pitch of 5.08
[mm], load flank angle of -3 degrees, stabbing flank angle of I0 degrees, and stabbing
flank clearance of 0.15 [mm].
[0059]
In the FEM analysis, the material properties were as follows: isotropic hardening
elasto-plastic, a modulus of elasticity of 210 [GPa], and a nominal yield strength,
expressed as 0.2% proof 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 further continued up to a point of 1.01100 turns.
[0060]
The varied dimensional conditions are shown in Table 1 below.
[0061]
[Table I]
TABLE 1
Remarks) The symbol "*" indicates that the value is outside the range specified by the present
invention.
The values in the sealing performance and total thread mating area sections are
values relative to values of No. 1, which are assumed to be 1.
I
[0062]
Models of Test Nos. 1 to 5 were prepared based on the threaded joint as shown in
12 30.8 10.7 1.31 0.96 0.63 Inv. Ex. 0
FIG. 8 and had no circumferential groove in the box. Among them, models of Test
Nos. 1 to 3 are comparative examples in which the length of the incomplete thread
section of the box does not satisfy the range specified by the present invention (at least
three times the thread pitch). Models of Test Nos. 4 and 5 are inventive examples in
which the length of the incomplete thread section satisfies the range specified by the
present invention.
[0063]
Models of Test Nos. 6 to 12 were prepared based on the threaded joint as shown
in FIG. 9 and had a circumferential groove in the box. Among them, models of Test
Nos. 6 and 7 are comparative examples in which the length of the incomplete thread
section of the box does not satisfy the range specified by the present invention.
Models of Test Nos. 8 to 12 are inventive examples in which the length of the
incomplete thread section satisfies the range specified by the present invention.
[0064]
[Evaluation Method]
In the FEM analysis, load sequence which simulated that in IS0 13679 SeriesA
test was applied to the models in a made-up state. To evaluate the sealing performance
of the seal surfaces, the minimum values of average contact pressures at the seal
surfaces, in the internal pressure cycle (the first and second quadrants) and the external
pressure cycle (the third and fourth quadrants) in the load sequence, were compared. (It
is noted that the higher the minimum value of average contact pressure, the better the
sealing performance of the seal surfaces.) The evaluations were made in the following
manner: provided that the sealing performance against internal pressure and the sealing
performance against external pressure of the model of Test No. 1 were each represented
by the value 1, evaluations ofthe remaining models were made by determining values
relative to the values of Test No. 1. As the evaluation criterion, the required sealing
performance against internal pressure was 1.2 or more, and the required sealing
performance against external pressure was 0.85 or more.
[0065]
Furthermore, the total mating areas in the threads were calculated to evaluate the
resistance to jump-out by comparing the total mating areas. The evaluations were
made in the following manner: provided that the total mating area of the model of Test
No. 1 was represented by the value 1, evaluations of the remaining models were made
by determining values relative to the value of Test No. 1. As the evaluation criterion,
the required total mating area was at least 0.55, and preferably at least 0.7.
[0066]
Models that satisfied the evaluation criterion for the sealing performance of the
seal surfaces and had a total mating area of 0.55 or more were evaluated as being good
(indicated by the symbol "OM), and among them, those having a total mating area of 0.7
or more are evaluated as being excellent (indicated by the symbol "@"). Models that
did not satisfy the criterion either for the sealing performance of the seal surfaces or for
the total mating area were evaluated as being poor (indicated by the symbol "A").
[0067]
[Test Results]
The test results are shown in Table 1 above and FIG. 12 below.
[0068]
The models of Test Nos. 2, 3, and 6, which are comparative examples, had a
shorter incomplete thread section, and therefore sufficient radially outward expansion of
the pin was not caused. Thus, there was no improvement in their sealing performances
against internal pressure.
[0069]
The models of Test Nos. 4 and 5, which are inventive examples, exhibited a 20%
or more increase in sealing performance against internal pressure because sufficient
radially outward expansion of the pin was caused, and also, their sealing performances
against external pressure were maintained to he at a high level.
[0070]
The model of Test No. 7, which is a comparative example, exhibited improved
sealing performance against internal pressure. However, it underwent increased radial
contraction of the pin when external pressure was applied because its circumferential
groove had an excessively long length and thus there was no thread mating in regions
near the seal surfaces. Thus, its sealing performance against external pressure
decreased by as much as 25%.
[0071]
The models of Test Nos. 8 to 12, which are inventive examples, exhibited
improved sealing performance against internal pressure, and in addition produced the
effect of inhibiting an abnormal increase in the dope pressure by virtue of the
circumferential grooves.
[0072]
All of the models of Test Nos. 4, 5, and 8 to 12, which are inventive examples,
had, in their boxes, an incomplete thread section having a length in the range of three
times the thread pitch to six times the thread pitch, and consequently they had a total
mating area in the threads of 0.55 or more. Thus, they are expected to exhibit high
resistance to jump-out. Among them, the models ofTest Nos. 4, 5, 8, and 9 were most
suitable because they had a larger total mating area due to the length of the incomplete
thread sections in the box that falls within the range from three times the thread pitch to
five times the thread pitch.
[0073]
The results described above demonstrate that a threaded joint for steel pipes
according to the present invention exhibits excellent sealing performance against
internal pressure as well as external pressure.
INDUSTRlAL APPLICABILITY
[0074]
A threaded joint according to the present invention is capable of being effectively
utilized in connection ofheavy wall steel pipes that are used as oil well pipes in harsh
environments.
REFERENCE SIGNS LIST
[0075]
10: pin, I1 : male threaded portion,
12: crest of male threaded portion, 13: root of male threaded portion,
14: stabbing flank of male threaded portion,
15: load flank of male threaded portion,
16: pin seal surface,
16A, 16B: pin seal surfaces for external pressure,
17: pin shoulder surface, 18: nose portion,
20: box, 21: female threaded portion,
22: crest of female threaded portion, 23: root of female threaded portion,
24: load flank of female threaded portion,
25: stabbing flank of female threaded portion,
21a: complete thread section, 21 b: incomplete thread section,
26: box seal surface,
26A, 26B: box seal surfaces for external pressure,
27: box shoulder surface, 28: recessed portion, 29: circumferential groove,
8: flank angle of load flanks, CL: pipe axis
We claim:
1. A threaded joint for steel pipes, comprising: a tubular pin and a tubular box, the
pin and the box being made up by screwing the pin onto the box,
the pin comprising: in order from an end of the pin, a shoulder surface; a seal
surface; and a male threaded portion, the male threaded portion being a tapered threaded
portion with trapezoidal threads,
the box comprising: a shoulder surface corresponding to the shoulder surface of
the pin; a seal surface corresponding to the seal surface of the pin; and a female
threaded portion corresponding to the male threaded portion of the pin, the female
threaded portion being a tapered threaded portion with trapezoidal threads,
the male threaded portion including: crests; roots; stabbing flanks; and load
flanks, the stabbing flanks being in a leading position in the screwing of the male
threaded portion into the female threaded portion, the load flanks being located opposite
from the stabbing flanks,
the female threaded portion including: roots facing the crests of the male
threaded portion; crests facing the roots of the male threaded portion; stabbing flanks
facing the stabbing flanks of the male threaded portion; and load flanks facing the load
flanks of the male threaded portion,
the load flanks of the male threaded portion and the load flanks of the
female threaded portion having a flank angle that is less than 0 degrees,
the female threaded portion being composed of: in order from near the seal
surface of the box, an incomplete thread section; and a complete thread section,
wherein:
in the female threaded portion, over an entire area thereof including the
incomplete thread section and the complete thread section, the roots collectively define
a single tapered periphery,
the incomplete thread section has a length along the pipe axis, the length being at
least three times a thread pitch of the female threaded portion, and the incomplete thread
section has a thread height lower than a thread height ofthe complete thread section,
and
in a made-up state: the shoulder surfaces are in contact with each other, and the
seal surfaces are in contact with each other; in the complete thread section, the roots of
the male threaded portion are in contact with the crests of the female threaded portion,
and the load flanks of the male threaded portion are in contact with the load flanks of
the fernale threaded portion; and in the incomplete thread section, clearances are
provided between the roots of the male threaded portion and the crests of the female
threaded portion, and the load flanks of the male threaded portion are in contact with the
load flanks of the female threaded portion.
2. The threaded joint for steel pipes according to claim 1, wherein:
the length of the incomplete thread section along the pipe axis is at most eight
times the thread pitch of the female threaded portion.
3. The threaded joint for steel pipes according to claim 1 or 2, wherein:
the incomplete thread section has one of the following configurations: a
configuration in which the crests, starting from a boundary between the complete thread
section and the incomplete thread section, collectively define a cylindrical periphery
that is parallel to the pipe axis; and a configuration in which the crests, starting from the
boundary, collectively define a tapered periphery inclined with respect to the pipe axis.
4. The threaded joint for steel pipes according to claim 1 or 2, wherein:
the incomplete thread section has a configuration in which the crests, starting
from a boundary between the complete thread section and the incomplete thread section,
collectively define a tapered periphery that is parallel to a tapered periphery collectively
defined by the crests in the complete thread section.
5. The threaded joint for steel pipes according to any one of claims 1 to 4, wherein:
in the male threaded portion, over an entire area thereof corresponding to the
incomplete thread section and the complete thread section of the female threaded
portion, the crests collectively define a single tapered periphery.
6. The threaded joint for steel pipes according to any one of claims 1 to 5, wherein:
the pin includes a nose portion disposed between the seal surface and the
shoulder surface,
the box includes a recessed portion corresponding to the nose portion of the pin,
and
in a made-up state, the nose portion of the pin is not in contact with the recessed
portion of the box.
7. The threaded joint for steel pipes according to any one of claims 1 to 6, wherein:
the box includes a circumferential groove disposed between the incomplete
thread section and the seal surface, the circumferential groove having a length along the
pipe axis, the length being at most three times the thread pitch of the female threaded
portion; and
the male threaded portion of the pin extends to a location corresponding to the
circumferential groove.
8. The threaded joint for steel pipes according to any one of claims 1 to 7, wherein:
the pin includes a seal surface for external pressure, the seal surface for extemal
pressure being disposed on at least one of an end region of the male threaded portion
and an intermediate region thereof, and
the box includes a seal surface for extemal pressure corresponding to the seal
surface for external pressure of the pin.