Note: Descriptions are shown in the official language in which they were submitted.
69~
The present invention relates yenerally -to threaded
connections for tubular goods, and more particularly
to a threaded connection for use on tubular yoo~s of
bimetallic construction as used in the production of
oil and gas.
It has become increasingl~ necessary to produce
oil and gas from reservoirs containing significant amounts
of H2S and/or CO2. Serious corrosion and embrittlement
problems can occur when conventional carbon steel tubular
goods are used in such environments. Corrosion inhibitors
have been used, but these are costly and not always effec-
tive. The industry has considered using tubulars made
of entirely of stainless steel, nickel alloys, or even
titanium alloys. However, such materials, while generallv
having high performance characteristics, are very expensive
and may require special handling techniques.
The most satisfactory solution developed so far
involves the use of bimetallic tubes, which include a
high strength outer pipe or shell with an inner liner
or pipe of a corrosion resistant material. Such tubes
offer the advantages of having load-bearing and rugged-
handling characteristics of conventional steel tubes
with the corrosion resistance of high-alloy materials.
Since oil field tubular goods come in discrete lengths,
it is necessary to connect them together to form strings.
The connections must be likewise protected against corrosion
or embrittlement. There have been manufactured for use
with bimetallic pipe couplings of solid corrosion resistant
material, such as stainless steel. However, such stainless
steel couplings lack the strength and rugged handling
characteristics OL ordinary couplings. There have been
developed for use with bimetallic pipe threaded connections
.
of ordinary steel but with means for protecing the connec~ion
. .
','" ' '- ' ~','. ~;' '.' ,~ ' ' '
.
: -- ~ ' , :.
.
~3
- 2 -
material. Examples of such connections are disclosed
in U.S. Patent No. 3,307,860; U.S. Patent No. 3,336,054;
U.S. Patent No. 4,026,583; and U.S. Patent No. 4,373,750.
In addition to beiny able to withstand the corrosive
effects of sour gas environments, pipes and -their connections
must be able to withstand service in deep high-pressure
wells. Wells in excess of 15,000 feet in depth and 20,000
pounds per square inch in pressure are not uncornmon.
Accordingly, pipes and connections must have high tensile
strength and the connections must provide effective leak
proof seals. Additionally, oil field tubular goods are
often subjected to rough treatment when they are connected
toghether on site. The threaded connections rnust be
resistant to galling, jamming, cross threading, and mechanial
damage to sealing surfaces.
It is therefore an object of the present invention
to connect together bimetallic pipe in such way that
the internal structure presents a continuous metalurgical
barrier while the external coupling maintains the mechanical
properties of the pipe. It is a further object of the
present invention to provide a threaded connection for
use in sour gas environments that has a tensile strength
approaching that of the pipes being interconnected.
It is a further object of the present invention to provide
a threaded connection for use in sour gas environments
that is substantially leak proof at all operating pressure.
It is a further object of the present invention to provide
a threaded connection that is resistant to galling, jamming,
cross threading, and mechanical damage.
The tubular connection of the present invention
includes first and second externally threaded male members
and a an internally threaded coupling threadedly engaging
the males members. Each male member includes an outer
pipe and an inner pipe or liner formed of a corrosion
- ~:
,
.
-- 3
resistant metal. The coupling is made of the same rnaterial
as the outer pipes of the male members, but does not
present itself to the inside of the pipe.
The threaded interconnection between each rQa1e member
and the coupling is provided by a continuous helical
thread having a first straight, untapered, portion and
a second tapered portion. The thread has a form including
stabbing and load flanks and an axially extending crest
and root. Throughout the thread, lead as measured along
the load flanks is constant and the length of the crest
and the depth of the load flank are constant.
The ends of the male members are cut so as to form
a metal-to-metal substantially impenetrable seal across
the inside of the connection. In one embodiment, the
ends of the male members abut each other at a seal rim
formed on the inner periphery of the male member. In
a second embodiment, the ends of the male memb~rs abu-t
and seal on a tubular metal ring manufactured of the
same material as the metallic tube liner. When a preload
is applied to male members, the end of each male member
seats itself against the other end or the metallic seat,
due to controlled plastic deformation, and forms the
primary seal for the connection.
The coupling includes internal of the threads a
pair of opposed outwardly directed frusto-conical sealing
surfaces and each male member includes an external frusto-
conical sealing surface sealingly mateable with the coupling.
The frusto-conical sealing surfaces provide a secondary
seal that is ~self-energizing in that as ~ressure inside
the pipe increases, the metal is forced together for
an improved seal. The frusto-conical sealing surfaces
on the male members are spaced axially a substantial
distance apart from the threaded portion. Such spacing
serves as a pilot during makeup which prevents or reduces
.
` ' '.... : ' ~ ~ ' .'' .
:: ,
.
9~
the likelihood of cross threading. Additionally, the
spacing assists in the self-energizing fea-ture of the
secondary seal and causes a bending moment which enhances
the primary seal. As weight or tensile force is applied
against the primary seal, the initial preload preserves
the seal. EIowever, should excessive weigh-t be applied
and the primary seal break, the secondary seal remains
valid and will, in fact, be improved.
Fig. 1 is a sectional view of a preferred embodiment
of the tubular joint of the present invention.
Fig. 2 is an enlarged cross section view showing
details of the seal arrangement in the embodiment of
Fig. 1.
Fig. 3 is a sectional view showing details of the
seal area in an alternative embodiment of the present
invention.
Figs. 4A and 4B are schematic views showing the
method of threading the male member of the present invention.
Figs. 5A and 5B are schematic views showing the
method of threading the coupling of the present invention.
Referring now to the drawings, and first to Fig.
1, the tubular joint of the present invention is designated
b~ the numeral 11. Joint 11 includes, generally, a pair
of externally threaded male members or pipes 13a and
13b and an internally threaded coupling 15. Male members
13a and 13b are substantially similar to each other and
each includes an outer pipe 17 of ordinary steel and
an inner pipe or liner 19 of corrosion resistant metal,
for example, stainless steel.
,
.
. .
-: . '
' , . ' - " , ' '' ' .
.
-- 5 --
Each male member 13a and 13b includes a continuous
thread designated generally by the numeral 21. Thread
21 includes a first, nontapered portion 23 and a second
tapered portion 25. Thread 21 includes a generally
trapezoidal thread form which includes a stabbing flank
27, a load flank 29, an axially extending crest 31, and
axially extending root 33. In the preferred embodiment,
load flank 29 is reverse angled or hooked so as to enhance
the tensile strength of the joint.
Throughout the length of thread 21, in both first
nontapered portion 23 and second tapered portion 25,
thread lead, as measured along the load flank parallel
to the axis of the male member, is constant. Additionally,
the axial length of crest 31 and the depth of the thread
as measured from crest 31 to root 33 across load flank
29 are constant.
Coupling 15 is formed of tubular metal stock of
the same material as outer pipe 17. Coupling 15 includes
threaded portions which compliment and interfit with
the dual tapered threaded portions, including thread
21 of the male members.
Male members 13a and 13b each includes an external
frusto-conical sealing surface 35, which is separated
from thread 21 by a cylindrical portion 37. Coupling
15 includes a pair of internal frusto-conical sealing
surfaces, including a surface 39 which mates and seals
with surface 35. Male member frusto-conical sealing
surface 35 and cylindrical portion 37 also serve as pilot
for guiding or stabbing the male members into coupling
15. The substantial length of cylindrical portion 37
coacts with the generally cylindrical surface formed
by the crests of the straight portion of the internal
threads of coupling 15 thereby to align the male member
within the coupling to prevent cross threading. Also,
- ~
.
, .
'
, :
~X~ .3
-- 6
because of the dual taper of thread 21, first nontapered
~hread 23 stabs into the tapered portion of coupling
15, thereby to reduce the likelihood of jamming as sometimes
occurs when a tapered pin is s-tabbed in-to a box of like
taper. Additionally, -the taper of second tapered portion
25 of thread 21 is preferably substantially greater than
the standard 3/4 inch per foot on diameter. The steep
taper enables the ~oint to go from hand tight to power
tight very quickly and preferably in less than one turn,
thereby to reduce the likelihood of galling.
In the embodiment of Fig. 1, joint 11 includes a
generally tubular washer 41, which is carried within
coupling lS between the ends of male members 13a and
13b. Washer 41 is preferably formed of the same corrosion
resistant metal as that which forms inner pipe or liner
19. Washer 41 forms metal-to-metal seals with liners
19 and 19b at 43a and 43b, respectively.
Referring now to Fig. 2, there is shown an enlargement
of the seal area of the embodiment of the invention shown
in Fig. 1. Washer 41 has a generally trapezoidal cross-
section and has an inside diameter slightly smaller than
the inside diameters of male members 13a and 13b. Washer
41 has a pair of sloping faces 45a and 45b and generally
flat sealing rims 47a and 47b about its inner periphery.
Liners 19 and 19a of male members 13a and 13b, respectively,
define generally flat sealing rims 49a and 49b, respectively
which cooperate with rims 47a and 47b, respectively,
to form metal-to-metal seals. The radially outer portions
of the ends of male members 13a and 13b define frusto-
conical faces 51a and 51b, respectively. Faces 51a and
51b extend radially forward of rims 49a and 49b, thereby
to protect rims 49a and 49b from mechanical damage.
The angles of faces 51a-b and 45a-b differ so as to define
therebetween void spaces 53a and 53b when ~oint 11 is
.. . , ~
- ' ' . - ' ' , .
' ' " ' ' ~ ',
3~{,
made up hand tight. However, when joint 11 is made up
power tight, voids 53a and 53b are lost to controlled
plastic deformation.
Referring now to Fig. 3, there is shown an alternative
embodiment of the presen-t invention wherein male members
113a and 113b abut and seal on each other. Liners 119
and ll9a define sealing rims 149a and 149b respectively.
Male members 113a and 113b include faces 151a and 151b
respectively which define therebetween a void 153. In
the embodiment of Fig. 3, face 151b slopes axiall~ forward
of rim 149b and face 151a slopes axially rearward of
rim 149a. In the embodiment of Fig. 3, male member 113a
would be inserted into coupling 115 at the manufacturing
facility so that coupling 115 would provide protection
to rim 149a.
Referring now to Figs. 4 and 5 there is shown generally
the method by which the dual tapered thread of thQ present
invention is cut. Referring first to Figs. 4a and 4b,
which show the method of cutting an external thread,
a thread cutting insert 61 is provided. Thread cutting
insert 61 includes a leading edge 63 for cutting the
stabbing flank 27 a bottom edge 65 for cutting the root
33, a trailing edge 67 for cutting load flank 29, and
a topping edge 69 for cutting crest 31. To cut thread
21, the tubuIar work piece is rotated at a constant speed
of rotation about its axis and thread cutting insert
61 is moved at a substantial constant linear speed in
the direction toward and parallel to the axis of the
tubular work piece initially to cut first nontapered
portion 23. When thread cutting insert 61 reaches the
end~of first nontapered portion 23, which end is designated
by the numeral 24, thread cutting insert 61 continues
to move at~the same constant speed parallel to the axis
of the tubular work piece, but additionally begins to
move in a direction perpendicular to the axis of the
work piece thereby to form second tapered portion 25.
'.; ' ~
.
g~
-- 8 --
Since throughout the cutting of thread 21, the work
piece rotates at a constant speed of rotation and thread
cutting insert 61 moves at a constant linear speed parallel
to the axis of the work piece, lead measured along the
load flanks parallel to the axis of the work piece remains
constant. However, it will be observed that the crests
in second tapered portion 25, as for example crest 26,
are narrower than the crests in firs-t nontapered portion
lQ 23. The difference between crest length in the straight
and tapered portions causes the thread profile to be
nonuniform throughout the thread and may result in flank
to flank interference when the male and female members
are connected together. Accordingly, the method of threading
the joint of the present invention includes the further
step of shortening the crests in the first nontapered
portion of the thread. As shown in Fig. 4b, in a final
pass, thread cutting insert 61 is displaced in the direction
toward the work piece parallel to the axis thereof a
distance equal to the difference between the crests in
straight portion 23 and tapered portion 25. With thread
cutting insert 61 so displaced, the work piece is rotated
at the same constant speed of rotation and thread cutting
insert 61 is moved axially along the work piece. However,
when thread cutting insert 61 reaches the last straight
thread, thread cutting tools 61 is pulled rapidly away
from the work piece at an angle greater than thè tapered
portion so as not to affect the tapered threads.
Figs. Sa and 5b depict schematically the cutting
of the threads and coupling 15. A thread cutting insert
71, which is similar to thread cutting insert 61 is provided.
In cutting the thread, the tubular work piece is rotated
at a constant speed of rotation and thread cutting insert
71 is moved at a constant speed in the direction parallel
to the axis of the work piece and at a constant slower
: ~ .
.
` ~ . ~- ` ` `
'.' :, ' . -~ ' ' ~ ,:
.
''3~jh~i
- 9
speed in the direction perpendicular to -the axis of the
work piece to cut the tapered portion 73 when thread
cutting insert 71 reaches -the first thread 7~ of the
straight portion 75, the speed component perpendicular
to the axis of the work piece is stopped but the component
parallel to the component continues and thread cutting
insert 71 cuts the straight portion 75.
Again it will be observed that the crests in straight
portions 75 are longer than the crests and tapered portion
73. Consequently, as shown in Fig. 5b, the method of
threading coupling 15 includes the step of shortening
the crest in straight portion 75. In the crest shortening
step, thread cutting insert 71 is initially displaced
radially outwardly to correspond to the diameter of straight
portion 75 and axially a distance equal to the difference
between the lengths of the crest and straight portion
75 and tapered portion 73. With the work piece continuing
to rotate at the same constant speed of rotation, thread
cutting insert 71 is moved in the direction parallel
to the axis of the work piece at the same constant linear
speed.
From the foregoing it will be seen that this invention
is one well adapted to attain all of the ends and ob~ects
hereinabove set forth together with other advantages
which are inherent to the apparatus.
It will be understood that certain features and
subcombinations are of utility and may be employed with
reference to other features and subcombinations. This
is comtemplated by and is within the scope of the claims.
As many possible embodiments may be made of the
invention without departing from the scope thereof it
is to be understood that all matters herein set forth
as shown~in the accompanying drawings are to be interpreted
as illustrative and not in a limiting sense.
.
',: ,-,: ., ,' . : , :
:' .~ . ' ' ' '; :
.
. . .
