0001] The present disclosure relates to a threaded connection for steel
pipe and, more particularly, to a coupling-type threaded connection for
connecting two steel pipe.
5
BACKGROUND ART
[0002] Steel pipes called oil-well pipes are used, for example, for
prospecting or production in oil wells, natural-gas wells etc.
(hereinafter collectively referred to as “oil wells” or the like), for
10 developing non-conventional resources such as oil sand or shale gas, for
retrieving or storing carbon dioxide (Carbon dioxide Capture and
Storage (CCS)), for geothermal power generation, or in hot springs. A
threaded connection is used to connect steel pipes.
[0003] Such threaded connections for steel pipe are generally
15 categorized as coupling type and integral type. A coupling-type
connection connects a pair of pipes, one of which is a steel pipe and the
other one is a coupling. In this case, a male thread is provided on the
outer periphery of each of the ends of the steel pipe, while a female
thread is provided on the inner periphery of each of the ends of the
20 coupling. Then, a male thread of a steel pipe is screwed into a female
thread of the coupling such that they are made up and connected. An
integral connection connects a pair of pipes that are both steel pipes,
and does not use a separate coupling. In this case, a male thread is
provided on the outer periphery of one end of each steel pipe, while a
25 female thread is provided on the inner periphery of the other end.
Then, the male thread of one steel pipe is screwed into the female
thread of another steel pipe such that they are made up and connected.
[0004] A connection portion of a pipe end on which a male thread is
provided includes an element to be inserted into a female thread, and
30 thus is usually referred to as “pin”. A connection portion of a pipe end
on which a female thread is provided includes an element for receiving
a male thread, and thus is referred to as “box”. Such a pin and a box
constitute ends of pipes and thus are tubular in shape.
[0005] An oil well is drilled along while its side wall is reinforced by
35 oil-well pipes to prevent the side wall from collapsing during digging,
3
which results in multiple oil-well pipes arranged in one another. In
recent years, both on-land and offshore oil wells have become deeper
and deeper; in such environments, to connect oil-well pipes, threaded
connections in which the inner and outer diameters of the connection
portions are generally equal to, or slightly larger than, 5 the inner and
outer diameters of the steel pipes are often used, in order to improve
efficiency in developing oil wells. The use of such threaded
connections minimizes the gaps between the oil-well pipes arranged in
one another, which improves efficiency in developing a deep oil well
10 without significantly increasing the diameter of the well. A threaded
connection is required to have good sealability against pressure fluid
from the inside (hereinafter also referred to as “internal pressure”) and
pressure fluid from the outside (hereinafter also referred to as “external
pressure”) under the above-described restrictions as to the inner and
15 outer diameters. Further, if oil-well pipes are used in an oil well with
great depth, for example, thermal expansion of a pipe may apply large
tensile loads or compression loads to the associated threaded connection.
In such environments, too, a threaded connection is required to have
good sealability.
20 [0006] Known threaded connections that ensure sealability include
ones having a seal that uses metal-to-metal contact (hereinafter
referred to as “metal seal”). A metal seal is constructed such that the
pin has a sealing surface with a diameter that is slightly larger than the
pin has a sealing surface with a diameter of a sealing surface of the box
25 and, when the threaded connection is made up and the sealing surfaces
are fitted together, the diameter of the sealing surface of the pin
decreases and the diameter of the sealing surface of the box increases,
which produces elastic recovery forces in each of the sealing surfaces
with which they try to return to their original diameters, thus
30 generating contact pressures on the sealing surfaces, which now adhere
to each other along the entire periphery to provide sealability. Other
known threaded connections that ensure sealability include ones
constructed to provide sealability by means of their threaded portions,
without a metal seal or using a metal seal in addition to the threaded
35 portions. Specifically, a connection is constructed in such a manner
4
that, in regions of the threaded portions with a predetermined length or
more as measured when the connection is made up, the clearance
between the thread faces of the pin and box is small and a viscous
lubricant called dope is present inside this clearance, and an
interference in thread diameter between the pin 5 and box provides
contact pressures on the thread faces (this construction will be
hereinafter referred to as “thread seal (construction)”. Threaded
connections that provide sealability against internal and external
pressures by virtue of such a seal are also known.
10 [0007] The following prior art documents are incorporated herein by
reference.
[0008] [Patent Document 1] WO 2018/180218 A1
[Patent Document 2] WO 2018/052141 A1
[Patent Document 3] US 5233742 A
15 [Patent Document 4] WO 2001/086185 A1
[Patent Document 5] WO 2008/029957 A1
[Patent Document 6] WO 2005/040657 A1
[Patent Document 7] US 4641410 A
20 SUMMARY OF THE DISCLOSURE
[0009] An object of the present disclosure is to provide a threaded
connection for steel pipe that can be made up quickly and appropriately.
[0010] A threaded connection for steel pipe according to the present
disclosure includes: a first steel pipe; a second steel pipe; and a coupling
25 adapted to connect the first and second steel pipes. The first steel pipe
includes a first pipe body and a first pin, the first pin being tubular in
shape. The first pin is formed to be contiguous to the first pipe body
and located at a tip of the first steel pipe. The second steel pipe
includes a second pipe body and a second pin, the second pin being
30 tubular in shape. The second pin is formed to be contiguous to the
second pipe body and located at a tip of the second steel pipe. The
coupling includes a first box and a second box, the first and second
boxes being tubular in shape. The first box is made up on the first pin
as the first pin is inserted therein. The second box is located to be
35 opposite to the first box, the second box being made up on the second
5
pin as the second pin is inserted therein. Each of the first and second
pins includes a male thread formed on an outer periphery of the pin.
Each of the first and second boxes includes a female thread formed on
an inner periphery of the box to correspond to the associated one of the
male threads. The male and female threads are trapezoidal 5 threads
and tapered threads. At least a portion of each of the male and female
threads constitutes a thread seal when the connection is made up. The
male thread includes a male thread crest, a male thread root, a male
thread stabbing flank located closer to a tip of the pin, and a male
10 thread load flank located farther from the tip of the pin. The male
thread stabbing flank includes a first male thread stabbing stage
located farther from a pipe axis of the steel pipe and having a stabbing
angle of −10 to 15 degrees and a second male thread stabbing stage
located closer to the pipe axis and having a stabbing angle of 20 to 60
15 degrees. The second male thread stabbing stage has a height of 20 to
60 % of a height of the male thread. The female thread includes a
female thread crest adapted to face the male thread root, a female
thread root adapted to face the male thread crest, a female thread
stabbing flank adapted to face the male thread stabbing flank, and a
20 female thread load flank adapted to face the male thread load flank.
The female thread stabbing flank includes a first female thread
stabbing stage located farther from the pipe axis and having a stabbing
angle equal to the stabbing angle of the first male thread stabbing stage,
and a second female thread stabbing stage located closer to the pipe
25 axis and having a stabbing angle equal to the stabbing angle of the
second male thread stabbing stage. The first pin further includes a
first pin shoulder surface on the tip of the first pin. The second pin
further includes a second pin shoulder surface on the tip of the second
pin, the second pin shoulder surface being adapted to be in contact with
30 the first pin shoulder surface when the connection is made up. The
first pipe body includes a first marking groove formed on an outer
periphery of the first pipe body, the first marking groove being annular
in shape.
35 BRIEF DESCRIPTION OF THE DRAWINGS
6
[0011] [FIG. 1] FIG. 1 is a longitudinal cross-sectional view of a
threaded connection for steel pipe according to an embodiment, taken
along the pipe-axis direction.
[FIG. 2] FIG. 2 is an enlarged longitudinal cross-sectional view
of the connection, depicting the shapes of the male 5 and female threads
shown in FIG. 1.
[FIG. 3] FIG. 3 is an enlarged longitudinal cross-sectional view
of the connection, depicting the male and female threads shown in FIG.
1.
10
EMBODIMENTS FOR CARRYING OUT THE INVENTION
[0012] A threaded connection for steel pipe according to the present
embodiment includes: a first steel pipe; a second steel pipe; and a
coupling adapted to connect the first and second steel pipes. The first
15 steel pipe includes a first pipe body and a first pin, the first pin being
tubular in shape. The first pin is formed to be contiguous to the first
pipe body and located at a tip of the first steel pipe. The second steel
pipe includes a second pipe body and a second pin, the second pin being
tubular in shape. The second pin is formed to be contiguous to the
20 second pipe body and located at a tip of the second steel pipe. The
coupling includes a first box and a second box, the first and second
boxes being tubular in shape. The first box is made up on the first pin
as the first pin is inserted therein. The second box is located to be
opposite to the first box, the second box being made up on the second
25 pin as the second pin is inserted therein. Each of the first and second
pins includes a male thread formed on an outer periphery of the pin.
Each of the first and second boxes includes a female thread formed on
an inner periphery of the box to correspond to the associated one of the
male threads. The male and female threads are trapezoidal threads
30 and tapered threads. At least a portion of each of the male and female
threads constitutes a thread seal when the connection is made up. The
male thread includes a male thread crest, a male thread root, a male
thread stabbing flank located closer to a tip of the pin, and a male
thread load flank located farther from the tip of the pin. The male
35 thread stabbing flank includes a first male thread stabbing stage
7
located farther from a pipe axis of the steel pipe and having a stabbing
angle of −10 to 15 degrees and a second male thread stabbing stage
located closer to the pipe axis and having a stabbing angle of 20 to 60
degrees. The second male thread stabbing stage has a height of 20 to
60 % of a height of the male thread. The female 5 thread includes a
female thread crest adapted to face the male thread root, a female
thread root adapted to face the male thread crest, a female thread
stabbing flank adapted to face the male thread stabbing flank, and a
female thread load flank adapted to face the male thread load flank.
10 The female thread stabbing flank includes a first female thread
stabbing stage located farther from the pipe axis and having a stabbing
angle equal to the stabbing angle of the first male thread stabbing stage,
and a second female thread stabbing stage located closer to the pipe
axis and having a stabbing angle equal to the stabbing angle of the
15 second male thread stabbing stage. The first pin further includes a
first pin shoulder surface on the tip of the first pin. The second pin
further includes a second pin shoulder surface on the tip of the second
pin, the second pin shoulder surface being adapted to be in contact with
the first pin shoulder surface when the connection is made up. The
20 first pipe body includes a first marking groove formed on an outer
periphery of the first pipe body, the first marking groove being annular
in shape.
[0013] This embodiment allows the first and second steel pipes to be
made up on the coupling quickly and appropriately.
25 [0014] The second pipe body may include a second marking groove
formed on an outer periphery of the second pipe body, the second
marking groove being annular in shape. The first marking groove may
have a width smaller than a width of the second marking groove.
[0015] Such an implementation makes it easier to appropriately make
30 up the first steel pipe on the coupling and allows the second steel pipe to
be quickly made up on the coupling.
[0016] The first box further includes a first recess, the first recess
being tubular in shape. The first recess is located at an open end of the
first box and has an inner peripheral surface facing an outer peripheral
35 surface of the first pipe body and spaced apart from the outer
8
peripheral surface of the first pipe body. The second box may further
include a second recess, the second recess being tubular in shape. The
second recess is located at an open end of the second box and has an
inner peripheral surface facing an outer peripheral surface of the
second pipe body and spaced apart from the outer peripheral 5 surface of
the second pipe body.
[0017] In such an implementation, dope does not reach the marking
groove through the open end of the coupling.
[0018] The portion of each of the male thread of the second pin and the
10 female thread of the second box that constitutes the thread seal may
have an amount of interference in thread diameter smaller than an
amount of interference in thread diameter of the portion of each of the
male thread of the first pin and the female thread of the first box that
constitutes the thread seal.
15 [0019] Such an implementation reduces the amount of rotation of the
first pin caused by corotation during the process of screwing in the
second steel pipe.
[0020] The male thread of the first pin and the female thread of the
first box may include a perfect-thread portion and an imperfect-thread
20 portion formed between the first pipe body and the perfect-thread
portion. The first pin and the first box may be bonded together along
the entire imperfect-thread portion or part thereof when the connection
is made up.
[0021] Such an implementation reduces the amount of rotation of the
25 first pin caused by corotation during the process of screwing in the
second steel pipe.
[0022] The steel pipe may have an outer diameter exceeding 16 inches.
[0023] A pipe assembly according to the present embodiment includes:
a first steel pipe; and a coupling adapted to connect the first steel pipe
30 to a second steel pipe. The first steel pipe includes a first pipe body
and a first pin, the first pin being tubular in shape. The first pin is
formed to be contiguous to the first pipe body and located at a tip of the
first steel pipe. The coupling includes a first box and a second box, the
first and second boxes being tubular in shape. The first box is made up
35 on the first pin as the first pin is inserted therein. The second box is
9
located to be opposite to the first box, the second box being made up on a
second pin of the second steel pipe as the second pin is inserted therein.
The first pin includes a male thread formed on an outer periphery of the
pin. The first box includes a female thread formed on an inner
periphery of the box to correspond to the male thread. 5 The male and
female threads are trapezoidal threads and tapered threads. At least
a portion of each of the male and female threads constitutes a thread
seal when the connection is made up. The male thread includes a male
thread crest, a male thread root, a male thread stabbing flank located
10 closer to the tip of the pin, and a male thread load flank located farther
from the tip of the pin. The male thread stabbing flank includes a first
male thread stabbing stage located farther from a pipe axis of the steel
pipe and having a stabbing angle of −10 to 15 degrees and a second
male thread stabbing stage located closer to the pipe axis and having a
15 stabbing angle of 20 to 60 degrees. The second male thread stabbing
stage has a height of 20 to 60 % of a height of the male thread. The
female thread includes a female thread crest adapted to face the male
thread root, a female thread root adapted to face the male thread crest,
a female thread stabbing flank adapted to face the male thread
20 stabbing flank, and a female thread load flank adapted to face the male
thread load flank. The female thread stabbing flank includes a first
female thread stabbing stage located farther from the pipe axis and
having a stabbing angle equal to the stabbing angle of the first male
thread stabbing stage, and a second female thread stabbing stage
25 located closer to the pipe axis and having a stabbing angle equal to the
stabbing angle of the second male thread stabbing stage. The first pin
further includes a first pin shoulder surface on the tip of the first pin.
The first pipe body includes a first marking groove formed on an outer
periphery of the first pipe body, the first marking groove being annular
30 in shape. An open end of the first box is positioned within a width of
the first marking groove.
[0024] Now, an embodiment of the threaded connection for steel pipe
will be described with reference to the drawings. The same or
corresponding portions in drawings are labeled with the same
35 characters, and the same description will not be repeated.
10
[0025] Referring to FIG. 1, the threaded connection 10 is a
coupling-type connection that includes a steel pipe 20m, a steel pipe 20f,
and a coupling 50 for connecting the steel pipes 20m and 20f. The steel
pipes 20m and 20f may be any steel pipes having an outer diameter
exceeding 16 inches. The steel pipe 20m includes a 5 pipe body 21m and
a tubular pin 30m. The pin 30m is formed to be contiguous to the pipe
body 21m and at the tip 22m of the steel pipe 20m. The steel pipe 20f
includes a pipe body 21f and a tubular pin 30f. The pin 30f is formed
to be contiguous to the pipe body 21f and at the tip 22f of the steel pipe
10 20f. The coupling 50 includes a tubular box 40m and a tubular box 40f.
The box 40m is made up on the pin 30m as the pin 30m is inserted
therein. The box 40f is located opposite to the box 40m and is made up
on the pin 30f as the pin 30f is inserted therein. Each of the pins 30m
and 30f includes a male thread 31m, 31f formed on the outer periphery
15 of the pin 30m, 30f. Each of the boxes 40m and 40f includes a female
thread 41m, 41f formed on the inner periphery of the box 40m, 40f to
correspond to the male thread 31m, 31f. The male threads 31m and
31f and female threads 41m and 41f are trapezoidal threads and
tapered threads.
20 [0026] The pin 30m is made up on the box 40m at the factory in
advance, and thus may be referred to as “mill end”. The pin 30f is
made up on the box 40f at the oil well, and thus may be referred to as
“field end”.
[0027] Each of the male threads 31m and 31f is formed on the outer
25 periphery of the pin 30m, 30f and is helical in shape, where the
diameter of the helix decreases toward the tip of the pin 30m, 30f (or
pin shoulder surface 24m, 24f). Each of the female threads 41m and
41f is formed on the inner periphery of the box 40m, 40f and is helical in
shape, where the diameter of the helix increases toward the open end
30 51m, 51f of the box 40m, 40f. A preferred taper ratio of the tapered
threads is 6.0 to 18.0 %. The taper ratio is set such that the length of
the resulting thread portions is appropriate in view of the wall
thicknesses of the steel pipes 20m and 20f. The taper ratio may be
constant, while it is preferable that the taper ratio of each of the male
35 threads 31m and 31f decrease as it goes away from the tip of the pin
11
30m, 30f.
[0028] When the connection is made up, at least portions of the male
threads 31m and 31f and female threads 41m and 41f constitute a
thread seal. The portions of the male threads 31m and 31f and female
threads 41m and 41f that constitute the thread 5 seal have an axial
length three times as large as the wall thickness of the steel pipes 20m
and 20f or more. The portions of the male threads 31m and 31f and
female threads 41m and 41f constituting the thread seal are perfect
threads. The larger the length of the thread seal, the better the
10 sealability. Still, if the length of the thread seal is excessively large,
this requires costs and work for thread-cutting, and may also raise the
possibility of galling occurring during make-up. The length of the
thread seal is preferably not larger than eight times the wall thickness.
While the threaded connection 10 includes such a thread seal, it
15 includes no metal seal. Alternatively, a Teflon (registered trademark)
ring seal may be used together with a thread seal. Dope is present in
the clearance between the thread-seal surfaces. The presence of dope
between the surfaces improves sealability.
[0029] The portions of the male thread 31m of the pin 30m and the
20 female thread 41m of the box 40m that constitute the thread seal
(hereinafter referred to as “thread-seal portions”) have an amount of
interference in thread diameter smaller than the amount of
interference in thread diameter of the portions of the male thread 31f of
the pin 30f and the female thread 41f of the box 40f (i.e., thread-seal
25 portions). Amount of interference in thread diameter is defined as the
difference between the outer diameter of the thread-seal portions of the
pins 30m and 30f and the inner diameter of the corresponding
thread-seal portions of the boxes 40m and 40f. The outer diameter of
the male threads 31m and 31f of the pins 30m and 30f, at the
30 thread-seal portions, is larger than the inner diameter of the female
threads 41m and 41f of the boxes 40m and 40f at the corresponding
portions. That is, the amount of interference in thread diameter takes
a positive value throughout the entire thread-seal portions.
[0030] The pin 30m further includes a pin shoulder surface 24m on the
35 tip of the pin 30m. The pin 30f further includes a pin shoulder surface
12
24f on the tip of the pin 30f, which is in contact with the pin shoulder
surface 24m when the connection is made up. The threaded
connection 10 has a so-called “pin-to-pin” construction. Typically, in
the threaded connection 10 according to the present embodiment, the
pin 30m at the mill end is first made up on the box 40m. 5 Thereafter, at
the oil well, the pin 30f at the field end is made up on the box 40f. As a
result, the pin shoulder surface 24f of the pin 30f is in contact with the
pin shoulder surface 24m of the pin 30m.
[0031] The pipe body 21m includes an annular marking groove 23m.
10 The marking groove 23m is formed on the outer periphery of the pipe
body 21m. The pipe body 21f includes an annular marking groove 23f.
The marking groove 23f is formed on the outer periphery of the pipe
body 21f. The marking groove 23m has a width wm smaller than the
width wf of the marking groove 23f (wmα1). The stabbing angle α1 is −10 to 15
degrees. The upper limit for the stabbing angle α1 is preferably 14
20 degrees, more preferably 13 degrees, and still more preferably 12
degrees. The smaller α1, the higher the compression resistance. The
lower limit for the stabbing angle α1 is preferably 0 degrees, and more
preferably 8 degrees. On the other hand, the larger α1, the easier the
cutting for forming the threads. The stabbing angle α1 is about 10
25 degrees, for example. The stabbing angle α2 is 20 to 60 degrees. The
upper limit for the stabbing angle α2 is preferably 50 degrees, more
preferably 40 degrees, and still more preferably 32 degrees. The
smaller α2, the higher the composition resistance. The lower limit for
the stabbing angle α2 is preferably 23 degrees, more preferably 26
30 degrees, and still more preferably 28 degrees. The larger α2, the less
likely a cross thread is to be produced. The stabbing angle α 2 is about
30 degrees, for example. As such, a portion of the male thread
stabbing flank 34 located about halfway up is depressed.
[0036] The height of the male thread stabbing stage 342 (i.e., distance
35 from the male thread root 33 up to the border between the male thread
14
stabbing stages 341 and 342) is 25 to 60 %, for example 35 %, of the
height of the male thread.
[0037] Each of the female threads 41m and 41f (hereinafter collectively
labeled “41”) includes a female thread crest 42, a female thread root 43,
a female thread stabbing flank 44, and a female thread 5 load flank 45.
The female thread crest 42 faces the male thread root 33. The female
thread root 43 faces the male thread crest 32. The female thread
stabbing flank 44 faces the male thread stabbing flank 34. The female
thread load flank 45 faces the male thread load surface 35.
10 [0038] The female thread stabbing flank 44 includes two female thread
stabbing stages 441 and 442. The female thread stabbing stage 441 is
located farther from the pipe axis X and has a stabbing angle α1 equal
to the stabbing angle α1 of the male thread stabbing stage 341. The
female thread stabbing stage 442 is located closer to the pipe axis X and
15 has a stabbing angle α2 equal to the stabbing angle α2 of the male
thread stabbing stage 342. As such, a portion of the female thread
stabbing flank 44 located about halfway up is bulged. The stabbing
angles α1 and α2 of the male thread stabbing stages 341 and 342 may
not be exactly equal to the stabbing angles α1 and α2 of the female
20 thread stabbing stages 441 and 442, respectively, and they are only
required to be substantially equal. In other words, the stabbing angles
α1 and α2 may have errors resulting from the cutting step.
[0039] It is preferably that the female thread stabbing stage 442 have a
height equal to the height of the male thread stabbing stage 342. This
25 prevents the clearance between the thread faces of the pin and box from
becoming unnecessarily large, allowing the thread seal construction to
exhibit good sealability. The height of the male thread stabbing stage
342 and the height of the female thread stabbing stage 442 may not be
exactly equal, and they are only required to be substantially equal. In
30 other words, these heights may have errors resulting from the cutting
step.
[0040] The male thread 31 further includes male thread round faces 36
to 39. The male thread round face 36 is formed at the corner between
the male thread crest 32 and male thread stabbing flank 34. The male
35 thread round face 37 is formed at the corner between the male thread
15
crest 32 and male thread load flank 35. The male thread round face 38
is formed at the corner between the male thread root 33 and male
thread stabbing flank 34. The male thread round face 39 is formed at
the corner between the male thread root 33 and male thread load flank
5 35.
[0041] The female thread 41 includes female thread round faces 46 to
49. The female thread round faces 46 is formed at the corner between
the female thread crest 42 and female thread stabbing flank 44. The
female thread round face 47 is formed at the corner between the female
10 thread crest 42 and female thread load flank 45. The female thread
round face 48 is formed at the corner between the female thread root 43
and female thread stabbing flank 44. The female thread round face 49
is formed at the corner between the female thread root 43 and female
thread load flank 45.
15 [0042] The round faces 36 to 39 and 46 to 49 are so-called “R” surfaces
(i.e., round chamfered surfaces), and have a predetermined radius of
curvature. The radius of curvature is 0.1 to 1.2 mm, and preferably 0.3
to 0.8 mm.
[0043] The male thread load flank 35 has a load angle β. Load angle β
20 is defined as the angle at which the male thread load angle 35 is
inclined from a plane Y perpendicular to the pipe axis X. If the load
flank 35 overhangs. the load angle β is negative. The load angle β is
−10 to 3 degrees, and preferably −5 to −1 degree, for example about −3
degrees. The female thread load flank 45 has a load angle β equal to
25 the load angle β of the male thread load flank 35. The load angle β of
the male thread load flank 35 and the load angle β of the female thread
load flank 45 may not be exactly equal, and they are only required to be
substantially equal. In other words, the load angle β may have errors
resulting from the cutting step.
30 [0044] The male thread crest 32, male thread root 33, female thread
crest 42 and female thread root 43 are parallel to the pipe axis X.
Specifically, the lines representing the faces 32, 33, 42 and 43 in a
longitudinal cross section containing the pipe axis X are parallel to the
pipe axis X.
35 [0045] As shown in FIG. 3, the male thread stabbing flank 34 and
16
female thread stabbing flank 44 form clearances with a dimension of 60
to 120 μm when the connection is made up. Further, the male thread
crest 32 and female thread root 43 form clearances sized at 0 to 50 μm
when the connection is made up. The male thread root 33 and female
thread crest 42 also form clearances sized at 0 5 to 50 μm when the
connection is made up.
[0046] Now, an exemplary method of manufacturing the threaded
connection 10, i.e., a method of making up two steel pipes 20m and 20f
using a coupling 50 will be explained.
10 [0047] At the factory, the mill-end pin 30m of the steel pipe 20m is
screwed into the box 40m. At this time, the pin 30m is screwed in to
such an extent that the open end 51m of the box 40m is within the
range represented by the width wm of the marking groove 23m. Thus,
the mill-end steel pipe 20m is made up on the coupling 50. The
15 mill-end steel pipe 20m and the coupling 50 constitute a pipe assembly,
and such a pipe assembly is shipped from the factory to an oil well.
[0048] Then, at the oil well, the field-end pin 30f of the steel pipe 20f is
screwed into the box 40f. At this time, the pin 30f is screwed in while
the steel pipes 20m and 20f are gripped, without the coupling 50 being
20 gripped. Further, the pin 30f is screwed in to such an extent that the
open end 51f of the box 40f is within the range represented by the width
wf of the marking groove 23f. Thus, the field-end steel pipe 20f is
made up on the coupling 50. As a result of this make-up of the two
steel pipes 20m and 20f by the coupling 50, the threaded connection 10
25 is manufactured.
[0049] While the mill-end pin 30m of the steel pipe 20m is screwed into
the box 40m, torque gradually increases as the screw-in advances, but
no rapid torque increases occurs. Thus, unlike during screw-in for the
field end discussed below, the appropriate make-up position cannot
30 easily be recognized from changes in torque. However, since the
threaded connection 10 includes a marking groove 23m, for the mill end,
one may visually observe the positional relationship between the
marking groove 23m and the open end 51m of the box 40m while
making up the steel pipe 20m on the coupling 50 at a desired position.
35 [0050] For the field end, as the steel pipe 20f is screwed in, the pin
17
shoulder surface 24f of the pin 30f abuts the pin shoulder surface 24m
of the mill end. At this moment, make-up torque rapidly increases.
Thus, one may screw in the steel pipe 20f while monitoring these
changes in torque to make up the steel pipe 20f on the coupling 50 to an
appropriate torque. Still, for verification purposes, 5 monitoring is
required to determine the increase in torque each time one steel pipe
has been made up.
[0051] To address this, a marking groove 23f is formed on the steel pipe
20f as well to allow one to visually observe the positional relationship
10 between the open end 51f of the box 40f and the marking groove 23f
after the steel pipe 20f has been tightened to a predetermined
appropriate torque. This allows one to make up the steel pipe 20f on
the coupling 50 quickly and appropriately without meticulously
performing the above-mentioned step of monitoring torque changes.
15 [0052] Whether the steel pipe 20f can be appropriately made up
significantly depends on whether the steel pipe 20m, which is made up
first, has been appropriately made up. In view of this, the make-up of
the mill-end steel pipe 20m can be controlled more precisely if the
groove width wm of the marking groove 23m at the mill end is smaller
20 than the groove width wf of the marking groove 23f at the field end.
[0053] This will enable make-up without exactly monitoring make-up
torque at the oil well field. This will eliminate the necessity for
equipment for monitoring make-up torque, facilitating running and
improving efficiency.
25 [0054] Further, the recesses 52m and 52f are provided on the open ends
of the boxes 40m and 40f. As such, even when dope in the clearances
between the thread seal surfaces seeps out, it remains in the clearance
between the recess 52m, 52f and the pipe body 21m, 21f. Thus, dope
does not reach the marking groove 23m, 23f through the open end 51m,
30 51f of the coupling 50. As a result, visually observing the positional
relationship between the marking groove 23m, 23f and open end 51m,
51f is always possible.
[0055] Further, the amount of interference in thread diameter at the
field end is smaller than the amount of interference in thread diameter
35 at the mill end. As such, the contact pressure between the pin 30f and
18
box 40f during screw-in of the steel pipe 20f is smaller than the contact
pressure between the pin 30m and box 40m until the pin shoulder
surface 24f contacts the pin shoulder surface 24m. Thus, the amount
of rotation at the mill end caused by corotation during make-up at the
field end may be zero or below an acceptable level. 5 “Corotation” is a
phenomenon in which, when the field-end steel pipe 20f is rotated to
screw the pin 30f into the box 40f, the coupling 50 rotates together with
the steel pipe 20f, i.e., rotates relative to the mill-end steel pipe 20m.
The amount of interference in thread diameter at the mill end is
10 preferably below a predetermined value to prevent galling. The
amount of interference in thread diameter at the field end is preferably
above a predetermined level to increase the sealability of the thread
seal.
[0056] The mill-end pin 30m and the box 40m include zones Z1 to Z3,
15 arranged in this order starting from the tip of the pin 30m. The male
thread 31m of the mill-end pin 30m and the female thread 41m of the
box 40m may include a perfect-thread portion and an imperfect-thread
portion formed between the pipe body 21m and the perfect-thread
portion. The perfect-thread portion is located in the zone Z2.
20 Imperfect-thread portions are located in the zones Z1 and Z3. The
mill-end pin 30m and box 40m may be bonded together along the
imperfect-thread portion in the zone Z3 when the connection is made up.
Specifically, the pin 30m and box 40m may be bonded together along the
entire imperfect-thread portion in the zone Z3 or part thereof. In
25 addition, the pin 30m and box 40m may be bonded together along
portions of the perfect-thread portion adjacent to the imperfect-thread
portions. If the mill-end pin 30m and box 40m are bonded together,
the amount of rotation of the mill end due to corotation can be zero or
smaller than a tolerance during the process of screwing in the field-end
30 steel pipe 20f.
[0057] Prior to make-up of the mill end, an adhesive is applied to the
zone Z3, along which the pin 30m and box 40m are to be bonded
together, and dope is applied to the zone Z2. This prevents galling and
makes it easier to provide a certain level of sealability to the thread seal.
35 The adhesive may be applied to the entire zone Z3, or may be applied to
19
part of the zone Z3. Further, the adhesive may be applied so as to even
cover a portion of the zone Z2 adjacent to the zone Z3. Further, the
adhesive may be applied to the male thread 31m only or the female
thread 41m only, or both.
[0058] While an embodiment has been described, the 5 present invention
is not limited to the above-illustrated embodiment, and various
modifications are possible without departing from the spirit of the
invention.
10 EXPLANATION OF CHARACTERS
[0059] 10: threaded connection
20m, 20f: steel pipes
21m, 21f: pipe bodies
23m, 23f: marking grooves
15 24m, 24f: pin shoulder surfaces
30m, 30f: pins
31, 31m, 31f: male threads
32: male thread crest
33: male thread root
20 34: male thread stabbing flank
341, 342: male thread stabbing stages
35: male thread load flank
40m, 40f: boxes
41, 41m, 41f: female thread
25 42: female thread crest
43: female thread root
44: female thread stabbing flank
441, 442: female thread stabbing stages
45: female thread load flank
30 50: coupling
51m, 51f: open ends
52m, 52f: recesses

WE CLAIMS

1. A threaded connection comprising:
a first steel pipe;
a second 5 steel pipe; and
a coupling adapted to connect the first and second steel pipes,
the first steel pipe including:
a first pipe body; and
a first pin, the first pin being tubular in shape, the first pin
10 being formed to be contiguous to the first pipe body and located at a tip
of the first steel pipe,
the second steel pipe including:
a second pipe body; and
a second pin, the second pin being tubular in shape, the second
15 pin being formed to be contiguous to the second pipe body and located at
a tip of the second steel pipe,
the coupling including:
a first box, the first box being tubular in shape, the first box
being adapted to be made up on the first pin as the first pin is inserted
20 therein; and
a second box, the second box being tubular in shape, the second
box being located to be opposite to the first box, the second box being
adapted to be made up on the second pin as the second pin is inserted
therein,
25 each of the first and second pins including a male thread formed
on an outer periphery of the pin,
each of the first and second boxes including a female thread
formed on an inner periphery of the box to correspond to the associated
one of the male threads,
30 the male and female threads being trapezoidal threads and
tapered threads,
at least a portion of each of the male and female threads
constituting a thread seal when the connection is made up,
the male thread including:
35 a male thread crest;
21
a male thread root;
a male thread stabbing flank located closer to a tip of the pin;
and
a male thread load flank located farther from the tip of the pin,
the male thread stabbing 5 flank including:
a first male thread stabbing stage located farther from a pipe
axis of the steel pipe and having a stabbing angle of −10 to 15 degrees;
and
a second male thread stabbing stage located closer to the pipe
10 axis and having a stabbing angle of 20 to 60 degrees,
the second male thread stabbing stage having a height of 20 to
60 % of a height of the male thread,
the female thread including:
a female thread crest adapted to face the male thread root;
15 a female thread root adapted to face the male thread crest;
a female thread stabbing flank adapted to face the male thread
stabbing flank; and
a female thread load flank adapted to face the male thread load
flank,
20 the female thread stabbing flank including:
a first female thread stabbing stage located farther from the
pipe axis and having a stabbing angle equal to the stabbing angle of the
first male thread stabbing stage; and
a second female thread stabbing stage located closer to the pipe
25 axis and having a stabbing angle equal to the stabbing angle of the
second male thread stabbing stage,
the first pin further including a first pin shoulder surface on the
tip of the first pin,
the second pin further including a second pin shoulder surface
30 on the tip of the second pin, the second pin shoulder surface being
adapted to be in contact with the first pin shoulder surface when the
connection is made up,
the first pipe body including a first marking groove formed on
an outer periphery of the first pipe body, the first marking groove being
35 annular in shape.
22
2. The threaded connection according to claim 1, wherein:
the second pipe body includes a second marking groove formed
on an outer periphery of the second pipe body, the second marking
groove being annular 5 in shape; and
the first marking groove has a width smaller than a width of the
second marking groove.
3. The threaded connection according to claim 2, wherein:
10 the first box further includes a first recess, the first recess being
tubular in shape, the first recess being located at an open end of the
first box and having an inner peripheral surface facing an outer
peripheral surface of the first pipe body and spaced apart from the
outer peripheral surface of the first pipe body; and
15 the second box further includes a second recess, the second
recess being tubular in shape, the second recess being located at an
open end of the second box and having an inner peripheral surface
facing an outer peripheral surface of the second pipe body and spaced
apart from the outer peripheral surface of the second pipe body.
20
4. The threaded connection according to any one of claims 1 to 3,
wherein the portion of each of the male thread of the second pin and the
female thread of the second box that constitutes the thread seal has an
amount of interference in thread diameter smaller than an amount of
25 interference in thread diameter of the portion of each of the male thread
of the first pin and the female thread of the first box that constitutes
the thread seal.
5. The threaded connection according to any one of claims 1 to 4,
30 wherein:
the male thread of the first pin and the female thread of the first
box include a perfect-thread portion and an imperfect-thread portion
formed between the first pipe body and the perfect-thread portion; and
the first pin and the first box are bonded together along the
35 entire imperfect-thread portion or part thereof when the connection is
23
made up.
6. The threaded connection according to any one of claims 1 to 5,
wherein the first and second steel pipes have an outer diameter
exceeding 5 16 inches.
7. A pipe assembly comprising:
a first steel pipe; and
a coupling adapted to connect the first steel pipe to a second
10 steel pipe,
the first steel pipe including:
a first pipe body; and
a first pin, the first pin being tubular in shape, the first pin
being formed to be contiguous to the first pipe body and located at a tip
15 of the first steel pipe,
the coupling including:
a first box, the first box being tubular in shape, the first box
being adapted to be made up on the first pin as the first pin is inserted
therein; and
20 a second box, the second box being tubular in shape, the second
box being located to be opposite to the first box, the second box being
adapted to be made up on a second pin of the second steel pipe as the
second pin is inserted therein,
the first pin including a male thread formed on an outer
25 periphery of the pin,
the first box including a female thread formed on an inner
periphery of the box to correspond to the male thread,
the male and female threads being trapezoidal threads and
tapered threads,
30 at least a portion of each of the male and female threads
constituting a thread seal when the connection is made up,
the male thread including:
a male thread crest;
a male thread root;
35 a male thread stabbing flank located closer to the tip of the pin;
24
and
a male thread load flank located farther from the tip of the pin,
the male thread stabbing flank including:
a first male thread stabbing stage located farther from a pipe
axis of the steel pipe and having a stabbing angle of 5 −10 to 15 degrees;
and
a second male thread stabbing stage located closer to the pipe
axis and having a stabbing angle of 20 to 60 degrees,
the second male thread stabbing stage having a height of 20 to
10 60 % of a height of the male thread,
the female thread including:
a female thread crest adapted to face the male thread root;
a female thread root adapted to face the male thread crest;
a female thread stabbing flank adapted to face the male thread
15 stabbing flank; and
a female thread load flank adapted to face the male thread load
flank,
the female thread stabbing flank including:
a first female thread stabbing stage located farther from the
20 pipe axis and having a stabbing angle equal to the stabbing angle of the
first male thread stabbing stage; and
a second female thread stabbing stage located closer to the pipe
axis and having a stabbing angle equal to the stabbing angle of the
second male thread stabbing stage,
25 the first pin further including a first pin shoulder surface on the
tip of the first pin,
the first pipe body including a first marking groove formed on
an outer periphery of the first pipe body, the first marking groove being
annular in shape, and
30 an open end of the first box being positioned within a width of
the first marking groove.
8. The pipe assembly according to claim 7, wherein the first box
further includes a first recess, the first recess being tubular in shape,
35 the first recess being located at an open end of the first box and having
25
an inner peripheral surface facing an outer peripheral surface of the
first pipe body and spaced apart from the outer peripheral surface of
the first pipe body.
9. The pipe assembly according to claim 5 7 or 8, wherein:
the male thread of the first pin and the female thread of the first
box include a perfect-thread portion and an imperfect-thread portion
formed between the first pipe body and the perfect-thread portion; and
the first pin and the first box are bonded together along the
10 entire imperfect-thread portion or part thereof when the connection is
made up.
10. The pipe assembly according to any one of claims 7 to 9, wherein
the first steel pipe has an outer diameter exceeding 16 inches.