Note: Descriptions are shown in the official language in which they were submitted.
In recent years, the -tre~d has been toward drilling
deeper gas wells with higher pressuxe and as a resul-t demand
has risen for a joint for oi.l and gas well pipes having
performance properties superior to those employing ~PI
threads (round, buttress) and haviny high tensile joint
strength and excellent gas-leak-tightness as well as resis-
tance to corrosion and erosion. Among the desired properties
of such joints, gas-leak-tightness is especially important.
This is because the leakage of an inflammable gas might lead
1~ to fire or explosion while the leakage of a poisonous gas
such as hydrogen sulfide might cause a disas-trous accident.
As conventional ways to enhance the gas-leak-
tightness of a threaded portion, it is known to make the
clearance as small as possible by strictly controlling the
machining tolerance, to plate the threaded portion with a
soft metal such as Sn having high malleability, and to apply
to the threaded portion a compound grease mixed with a
very fine metallic powder and a mineral oil thickened with a
calcium stearate or a lithium stearate. ~lso, i-t is known
to be important to control the torque and number of turns
at the time of make-up. However7 even with these means,
the leakage of gas cannot be perfectly prevented under the
conditions of high temperature and high pressure currently
being encountered in gas and oil wells. Because of this,
special types of joints have been used to improve gas-leak-
tightness. Some of these have a seal por-tion which employs
metal-to-metal contact between a pin (steel pipe) and a box
(coupling) forming the joint, while others have an annular
*
groove with a Teflon ring inserted therein.
Among the special type joints having a seal
portion, several varieties are known. To assure effective
metal-to-metal contact in these known joints, however, it
* trade mark for tetrafluoroethylene
'7~
is important that there be no clearance in said contact
portion even when the steel pipe is subjected to tensile
force.
Therefore, in this kind of joint, it is necessary
to use a large torque for make-up~but to do so causes uneven
plastic deformation at the contact portion.
In particular, this deformation tends to degrade
gas-leak-tightness in repeat make-up and is also a cause
of galling at the screw thread portion.
In general, it can be assumed that repeat make-up
will be effected three times at the most in a casing string
and ten times at the most in a tubing string, and in either
case, a high degree of seal tightness is required without
causing any damage.
This invention relates to a joint for oil and gas
well pipe having excellent seal tightness.
In the present specification, the word " box~
means a coupling with a female thread on the internal surface
thereof, and the word " pin" means a pipe end with a male
thread on the external surface thereof.
The present invention provides a revolutionary
solution to the above-mentioned problem in the form of a
joint which gives perfect sealing performance and has excel~
lent repeat make-up performance, and which was realized
through an improvement in the screw thread portion achieved
as the result of various studies which took into consideration
the fact that conventional special joints all aim at enhancing
gas-leak-tightness by the sole expedient of specially contrived
metal-to-metal contact portions. The improvement of the screw
thread portion in accordance with this invention involves
forming the joint portion with a composite thread structure
comprising a rounded triangular thread and a square thread
7~ 1
or a buttress thread.
~ s is widely lcnown, the tapered rounded triangular
thread has excellent sealing function in itself, and is
standardized as a taper thread for pipes under JIS (Japanese
Industrial Standard) and is used widely in high pressure
piping.
Now, the sealing mechanism of this tapered rounded
triangular thread will be briefly described as follows.
The leakage route of the fluid within a made-up
joint is generally considered to follow two passages in
the threaded portion. The first of these is in the axial
direction of the pipe and the second is in the helical
direction along the thread locus. As for the leakage in
the axial direction of the pipe, in a tapered thread, high
contact surface pressure is generated due to the wedge effect
of the triangular thread and, since a number of turns of the
thread are provided in the axial direction of the pipe, the
leakage cutoff function in this direction is superior to
any other sealing mechanism. The rounded triangular
thread is also superior in preventing helical leakage. When
threads are engaged, the clearance between them is concentrated
between the crest and the root of the threads when the taper
thread is tightned. Thus, leakage can be prevented by filling
the clearance with compound and then compressing the compound
by the " wedge" effect of the rounded triangular thread to
eliminate all voids.
This rounded triangular thread is, of course,
adopted as a standard for ~PI 5B but it cannot bc easily
used to obtain both superhigh pressure sealiny and high
joint strength at the same time.
More specifically, the sealing effect of the
rounded triangular thread results from -the fact that the
7~1~
clamping action of the tapered male thread into the female
thread is such as to generate high contact surface pressure
at the thread flank, the crest and the root. Thus, the wedge
effect of the rounded triangular thread works advan~ageously
insofar as sealing performance is concerned. However, since
the flank, which bears a part of the load, is inclined at
30 to the pipe axis, the axial load on the pipe proper is
transmitted from the box to the pin not only as tensile force
but also as a pipe expanding force which may give rise to the
jump-out phenomenon, the phenomenon of the pin being pulled
out of the box without breakage of the thread. Therefore,
the strength of the joint is much inferior to one employing a
buttress thread which has its flank practically perpendicular
to the pipe axis.
The object of the present invention is to provide
a joint which derives its seal performance and strength from
its threaded engagement and which takes positive advantage of
the sealing mechanism (wedge effect) peculiar to the rounded
triangular thread without reducing the strength of the joint.
Accordingly, what is broadly claimed herein is an
improvement in a threaded joint for oil and gas well pipe
wherein a box having an internal female thread and a pin having
an external male thread are screwed together, the improvement
being that the threaded joint has high gas-leak-tightness
wherein said pin is formed at its end with a number of turns of
rounded triangular thread which follow into a buttress or
square thread, and said box is provided with a thread structure
correspondlng to sald threads provided on said pln whereby a
composite thread arrangement is provided in a single thread
train in both said pin and said box.
- 4 -
7~.1
Brief description o the drawings:
Further objec-ts and additional features of the
present invention will become apparent from the following
detailed description, given in conjunction with the accom-
panying drawings, wherein:
Pig. 1 is a longitudinal sectional view of a
coupling type joint provided with the sealing structure of
the present invention.
Fig. 2 is a longitudinal sectional view showing
one part of an embodiment in which the rounded triangular
thread at the end of a pin is inclined toward the side of
the pin root.
- 4a -
3 ~ 7'~
*
Fig. 3 is a vie~ sho~iny o~e example o -the sh~pe
of the flank and -the cr~st of a rounded triangul~r thread
having a wedge eEEect.
Fig. 4*is an enlarged view o-f an inverse rounded
triangular thread practically furnished and extended into
a buttress female thread.
Fig. 5 and Fig. 6 are views showing examples of
application of the joint according to the present invention.
Description of the preferred embodiments:
Fig. 1 shows a joint wherein a tapered male
thread of a pipe body is engaged with a tapered female thread
of a box couplings 1. The threads consist of rounded trian-
gular threads 3 at the end of a pin 2 and buttress threads 4.
The rounded triangular threads 3 serve mainly to provide
sealing while the buttress threads 4 serve to sustain the
high axial load. The important feature of this thread arran-
gement is that there is a clear division of function between
threads of different shapes, with the threads of one shape
being used for sealing and those of the other shape to
~0 provide joint strength. Namely, since the joint comprises
both rounded triangular threads and buttress threads, it not
only gives improved seal tightness but also provides enhanced
reliability of joint strenght. Thus, with the arrangement
according to this invention, since there is no need to confer
sealing capability on the buttress thread portion at the
time of screw tightening, it is no necessary to give the
pin and box an excessive interference allowance.
As a result, the circumferential tensLle stress
arising in the coupling during make~up, a major problem in
conjuction with conventlonal -tapered threads, can be reduced
to a low level.
Moreover, differently from the conven-tional joint
* first sheet of drawings
-5-
which has a metallic contact portion at the end of its pin
and is unable to assure stable gas-leak-tightness because
of its easy deformation under the combination of internal
pressure with secondary and tertiary external forces (tension,
bending etc.), the threaded joint accordint to the present
invention uses rounded triangular threads to provide sealing
so that the restraining force is strong and the fit between
the threads excellent, giving stable gas-leak-tightness even
under exposure to such external forces.
Further, even if the fitting allowance of the
rounded triangular thread portion is increased in order to
obtain adequate gas-leak-sealing effect, insofar as the
increase is not so large as to cause the pin end to yield,
there is no need to worry about breaking of oil film or of
galling due to excessive contact between smooth surfaces
since the clearance between crest and root of the threads is
always filled with compound grease.
A joint of the type just described, wherein gas-
leak-tightness and high joint strength are provided by
rounded triangular threads and buttress threads arranged
in the same thread train displays a very large effect as
an oil and gas well pipe joint solely as a result of the
structural features described so far. To obtain a joint
evcn better suitcd to actual practice, however, it is ad-
visable to addi-tionally employ the following measures for
imparting cven highcr sealincJ effcct to thc rounded triangular
threads:
~1) L'rovi.sion of a shoul~lcr portioll 8 on tllc ccntral
internal surface of the box 1 and, similarly, of a
non-threaded portion at the end of the pin 2, this
non-threaded portion having an end surface 11 corres-
ponding to the shoulder portion 8 so that these
t~>7~l
two portions 8, 11 make surface-to-surface contact
a~ter make-up. sy the incorporation of this
structural feature, the interference allowance
of the rounded triangular thread portion can be
controlled to a fixed value and, at the same time,
the force of reaction from the shoulder portion 8
can be utilized to compress the end of the pin 2,
so as to cause the rounded triangular thread of the
pin to press strongly onto the rounded triangular
thread of the box, thus reinforcing the seal
tightness of the rounded triangular threads.
(2) Giving the rounded triangular thread at the
pin end such shape that it is inclined toward
the side of the pin root (generally called "
inverse angle." )
(See Fig. 2.)
There are two reasons for adopting this inverse
angle. One is to more effectively utilize the wedge effect
of the rounded triangular threads when internal pressure,
axial load or bending load act on the joint, and the other
is to prevent the pin from opening to the inside and the
box to the outside at the rounded triangular thread portion
of the pin end, the most important point as regards sealing
effect. (This type of opening is generally called the
" unzipper effect." )
Namely, when a tensile load and internal pressure
act on the joint at the same time, the force which the
male thread at thc pin cnd exerts on the female thread
on the internal surface of the box 1 is as shown in Fig. 2,
that represented by the vector 19, the resultant of the
internal pressure 18 acting on the internal surface 15 and
the tensile force 17.
7~
And, for the purpose of obtaining a rn~re effective
wedge effect, the direction of said vector 19 and that of
the crest of the rounded triangular thread are preferably
coincident. As for the unzipper effect, assuming that
in Fig. 2 the non-threaded external surface 9 of the pin
end and the non-threaded internal surface 12 of the box
are respectively subjected to perpendicular forces, it is
understandable that the thread with the inverse angle as
shown in Fig. 2 will have stronger resistance than a round
thread and that the fitting portion at the pin end of the
former will be less openable than the latter.
(3) Forming the shoulder portion with a convex conical sur-
face and forming the corresponding part of -the pin
end with an arched surface or, otherwise, forming
these with linear conical surfaces. ~ith such an
arrangement, the pin end will be pushed in the radial
direction along the shoulder portion of the box,
and as a reaction to this, the engagement between
the round thread portions will be strengthened.
The flanks of a rounded triangular thread may be
either curved or flat. However, as shown in Fig. 3, it is
preferable from the viewpoint of machining for improvement
of accuracy in finishing that the flanks 31 be made flat
and the shape of thread crest 32 conform to the arc of a
circle tangent to the flanks 31.
The foregoing is a description of sealing structure
and mechanism of the joint according to the present invention.
Its rcsistancc to strcss corrosion cracklng ~nd
erosion is, moreover, very high since, similarly to those
dcscribcd in publishcd litcraturc (Patcnt Publication 18096/
1970 and Laid-Open Patent Application 17125/1973) the internal
surface of the joint are smooth and are subjected to compres-
l~tj5~1
sive stress ~ene~ated at the time of make-up.
Next, an embodiment of the present in~ention will
be explained with reference to Fig. l and Fig. 2.
The pin 2 has a non-threaded end portion (a), a
rounded triangular thread portion (b) and a buttress
thread portion (c). The end surface 11 of the pin end is
formed so as to exert a force of reaction on the rounded
triangular thread portion (b) when it comes into pressing
contact with the surface of the box shoulder portion 8,
whereby the non-threaded surface 9 of the pin end does not
come into contact with the non-threaded portion surface 12
of the box.
The rounded triangular thread lO is given an in-
verse angle and is provided on the pin end as a perfect
thread of 3 - 4 turns. The pitch diameter of the rounded
triangular thread is an extension of the pitch diameter of
the buttress thread. The shape of the thread is determined
by inscribing the two-dot chain line shown in Fig. 2 within
the imaginary buttress female thread 13. This shape is
used to minimize interference between the male and female
threads at the time of make-up. An angle of inclination
of lO - 30 wi-th respect to a line normal to the pipe axis
is preferable in consideration of machinability.
The thread train consisting of two diEfering thread
shapes provided continuously according to this invention
can be machined either by a method wherein the boundary
portion between the two thread shapes ((c) in Fig. 2) is
first turned smaller than the diameter of the thread root
by about one pitch and the rounded triangular thread and
the buttress thread are thereafter cut separately, or by
a method wherein the entire thread train is first machined
as a buttress thread and the rounded triangular thread is
'7~1
therea~ter machined for 3 - 4 threads a-t the pi~ e~d. Like
the pin, the box has a non-threaded internal sux~ace portion
7, a rounded triangular thread portio~ 6, a buttress thread
portion 5 and a central shoulder portion 14~ The non-
threaded surface 12 is on an extension of the crest of the
rounded triangular thread~ The rounded triangular thread
and the buttress thread are shaped to correspond to the
shape of the pin, and the thread clearance is set according
to API standards.
Fig. 4 is an enlarged view of an inverse rounded
triangular male thread which is nearly internally tangent
to a buttress Eemale thread. The angle ~ between the pipe
axis 41 and the bisector 42 of the flank angle is 63 (incli-
nation angle 27), the height of the thread is about two
times that of the API buttress thread and the flank angle
of the thread is 55. Fig. 4 merely shows an example of
the inverse rounded triangular thread and other shapes may
be used instead. In short, it is sufficient if the shape
of the flanks and the tip which cause the wedge effect are
round and the clearance between the female and male threads
ls small.
The joint according to the present invention can
be applied in various shapes. Namely, when it is used for
the internal surface of a joint, it will act as a leakage
preventing mechanism for fluid from thc inside to the outside
of the joint, and when it is used for the cxtcrnal surface
of the joint, it will display a leakage preventing function
for fluid from the outside to the inside of the joint.
Fig. 5 and Fig. 6 show concrete examples of these applications.
Fig. 5 shows a joint in which pins 54 with male
threads machined thereon are clamped by a coupling 53. Portions
51 within the circles have sealing function from inside to
--10--
it~ l
outside, and po~tions 52 within the circles have sea~ing
function from outside to inside. It will be understood
that even with only one or the other of these cases, seali~g
function from inside to outside and vice versa can be
attained. However, in this example the sealing portions
are provided at both inlets of fluid permeation with the
aim of attaining perfection. The portions 55 intermediate
of the total length of the pin-coupling are provided with
buttress threads to assure strength.
Fig. 6 shows a type of joint for clamping a pin
64 and a box 65 without using a coupling. Here, similar
to the joint in Fig. 5, the portions within circles 61,
62 are round thread portions having seal tightness and the
intermediate portion 63 is provided with a buttress thread
having joint strength. In this case, either one of the
round thread portions 61, 62 can be omitted and ~ buttress
thread provided in its place without causing any problem
in actual use of the joint.
--11--
