Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to insulated tubular structures
for fluid transfer, and is concerned more particularly,
hut not exclusively, with downhole tubulars of the type
employed in oil well steam injection. Downhole tubular
systems are intended to be inserted deep into the ground
for downhole fluid transfer, and normally comprise a
string of insulated component tubular structures coupled
together in vertical end to end alignment, the assembly
forming a rigid and leakproof structure~
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As is well known, downhole tubular systems of the type
referred to are subjected to extremely arduous operating
conditions and encounter several potentially destructive
forces besides being exposed to the corrosive and
erosive effects of the steam and other fluids. These
conditions can, and from time to time do, result in
structural failure. Since the recovery of a failed
system is a very expensive and time consuming procedure,
it is important that they be designed so as to minimize
the risk of structural failure. One might in principle
increase structural reliability by making the stress
bearing components of the s~stem of a heavier gauge or
thickness, but this introduces its own disadvantages.
Apart from the additional cost of the metal that would
be rec~uired in making the component str~ctures more
stress resistant, the heavier gauge of the components
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would increase their weight and so actually add to the
axial stresses in the other components from which they
are suspended.
The problem of constructing a downhole tubular system
which is both structurally and functional]y reliable is
greatly increased by the need to insulate the system to
prevent heat loss from the steam. Conventionally
insulated pipes cannot readily be adapted for oil well
steam injection purposes. Damage to the insulation
would result in loss of insulatiny properties and
consequent damage to structural components due to
overstressingO On the other hand, if the insulation is
to be encased using a double-walled tubular structure,
it is necessary also to allow for differential expansion
and contraction of the inner and outer walls of the
structure since otherwise additional stresses tending to
cause failure would be set up. However, this must be
accomplished without loss of structural rigidity of the
system as a whole.
In United States Patent No. 4,332,401, Stephenson et al,
- issued June 1, 1982, there is disclosed an insulated
tubular assembly for fluid transfer which is designed so
as to reduce the problems mentioned above. The assembly
comprises a plurality of component tubular structures
2~ coupled together in end to end alignment, each component
structure comprising radially spaced inner and outer
pipe sections defining an annular space which is filled
with thermally insulating material, the ends of the pipe
sections being interconnected by axially extending
tubular bellows which, in effect, hermetically seal the
ends of the annular space containing the insulation.
The outer pipe sections of the component tubular
structures àre rigidly coupled together end to end by
threaded couplings. In this way not only is the
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insulation protected, but the assernbly provides an inner
fluid-carrying section which is free to expand and
contract in response to temperature changes while the
outer load-bearing section of the structure remains
essentially rigid.
However, the arrangement described in ~nited States
Patent No. 4,332,401 has several potential short-
comings. In the first place the tubular bellows are
exposed to the fluid being transferred. It is true that
they are shielded from direct exposure by diffuser
sleeves which interconnect the ends of the inner pipe
sections, but should a diffuser sleeve fail the tubular
bellows are directly exposed and likely to fail
mechanically. The mechanical failure of a bellows may
have serious consequences. First, since the thermally
insulating material will become exposed to the fluid
being transferred, it will become impregnated with con-
sequent loss of thermal efficiency. Even more serious
is the fact that mechanical failure of a bellows will
result in loss of mechanical integrity of the string of
inner pipe sections, and in consequence it will become
- practically impossible to recover the balance of the
assembly below the failed component. Another short-
coming is that the entire weight of the tubular assembly
is borne by the outer pipe sections and their couplings,
which means that the wall thickness of these components
must be sufficient to withstand the heavy load incurr-
ed. But the inner pipe sections must in any case be of
substantial wall thickness to withstand the high intern-
al fluid pressure and therefore, for a given diameter,the space available to accommodate insulation is limited
by the structural considerations for the inner and outer
walls. The consequence of this is loss of thermal
efficiency or, in the alternative, reliance upon special
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types o~ insulation which are far more costly than would
otherwise be necessary.
By contrast, in a downhole tubular system according to
the present invention the entire weight of the system is
borne by the inner pipe sect.ions which are rigidly
interconnected at their ends, the outer pipe sections
being decoupled from stresses transferred via the inner
pipe sections by expansion joints which are not
subjected to load.
According to one aspect of the present invention there
is provided an insulated tubular structure for fluid
transfer which comprises: inner and outer pipe sections
arranged concentrically one within the other to define
an annular cavity therebetween; thermally insulating
material substantially filling sai.d cavity; sealing
means at both ends of each pipe section hermetically
sealing the ends of said annular cavity; said outer pipe
section compri.sing a main tubular member having a pair
of end tubular members extending therefrom; said end
tubular members being rigidly connected to respective
ends of said inner pipe section by said sealing means;
and one said end tubular member being interconnected
with the main tubular member by a gas-tight expansion
joint.
Preferably the main tubular member and one end tubular
member of the outer pipe section have respective free
end portions telescoping one within the other, one such
free end portion carrying an annular seal which slidably
engages the other to provide the expansion joint.
Alternatively the expansion joint may be provided
by a tubular bellows rather than the annular sliding
seal, although the latter is usually to be preferred on
account of the cost oE the bello~s and risk of their
mechanical failure.
According to another aspect o~ the invention there is
provided a downhole tubular system comprising a
plurality of insulated tubu:Lar structures coupled
together in vertical end to end alignment each said
tubular structure comprising inner and outer pipe
sections arranged concentrically one within the other to
define an annular cavity therebetween; thermally
insulating material substantially filling said cavity;
sealing means at both ends of each pipe section
hermetically sealing the ends of said annular cavity;
said outer plpe section comprising a main tubular member
having a pair of end tubular members extending therefrom
said end tubular members being rigidly connected to
respective ends of said inner pipe section by said
sea].ing means and one end tubular member being
interconnected with the main tubular member by an
expansion joint providing a gas-tight seal between said
members said tubular structures being coupled together
by rigid coupling means rigidly interconnecting the
inner pipe sections thereof so as to decouple the outer
pipe sections from stresses transferred via the inner
pipe sections.
~part from the mechanical integrity ensured by rigidly
interconnecting the inner pipe sections while disposing
the expansion joints so that they will not be subjected
to load stresses, the arrangement has the considerable
advantage that the outer pipe sections can be relatively
thin-walled. This means that, for a given diameter of
the tubular structure, more space can be made available
to accommodate the insulation and hence the thermal
properties of the design are greatly improved. Thus,
~or a given insulating material the insulation can be
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made more effective. On the other hand, it is also
feasible to use a simpler and less expensive insulating
material than would otherwise be required. In
a preferred embodiment of the present invention the
thermally insulating material is a gas-permeable solid
material such as cerarnic wool which is permeated with
low conductivity gas such as argon.
Although the present invention is primarily concerned
with the structural reliability of downhole tubing of
the type discussed above, it has general application to
the design of insulated pipe structures used for fluid
transfer, wherein expansion joints required to
accommodate differential thermal expansion of the inner
and outer pipe sections must be shielded from load
stresses.
In order that the invention may be readily understood
one embodLment thereof will now be described, by way of
e~ample, with reference to the accompanying drawings.
In the drawings, which show details of a downhole
tubular system comprising a plurality of component
tubular structures coupled together in vertical end to
end alignment;
Figure 1 is a fragmentary half-sectional elevational
view showing one of the component tubular structures and
part of an adjacent tubular structure to which it is
coupled;
Figure 2 is an exploded fragmentary perspective view of
the component elements shown in Figure 1;
Figure 3 is an enlarged half-sectional elevational view
showing the coupling in greater detail; and
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Figure 4 is an enlarged half-sectional elevational view
showing the expansion joint in greater detail.
Referring to the drawings, each of the component tubular
structures is essentially a double-walled structure
comprising an inner pipe section 10 and an outer pipe
section 11, the pipe sections being arranged
concentrically one within the other to define an annular
space therebetween. The outer pipe section 11 comprises
a main tubular casing member 12, which extends for the
greater part of the length of the structure, and a pair
of end tubular casing members 13, 14 extending from it
at each end. As shown in Figure 1, the end tubular
members 13, 14 are of smaller diameter than the main
tubular member 12. The end member 13 is received at one
end within one end of the main tubular member 12, to
which it is welded as shown at 15. The other end member
14, on the other hand, has a free end portion which
telescopes within a free end portion of the main tubular
member 12, being interconnected with the latter by an
expansion joint 17 as hereinafter described. The distal
ends of the end members 13, 14 are rigidly connected to
respective ends of the inner pipe section by means o
steel rings 18 which are welded to the ends of the inner
pipe section 10 and to which they in turn are welded at
their ends. Thus the steel rings 18 hermetically seal
the ends of the annular cavity defined by the inner and
outer pipe sections.
The annular cavity is itself filled with a thermally
insulating material for minimizing heat losses rom the
fluid being transferred. This material may comprise a
gas permeable solid material permeated with a low
conductivity ~as. For example, the insulation may be
made up o~ a multilayered insulation as described in
~nited States Patent No. 4,332,~01 permeated with a very
low conductivity gas such as krypton. However, since
the present design permits space economy without loss of
structural integrity, it is feasible to use a less
elaborate and less expensive insulation system without
loss of thermal efficiency owing to the additional space
made available for its accommodation. Thus, in the
present example the insulation comprises a ceramic wool
19 substantiall~ filling the annular cavity and
back-filled with a low conductivity gas such as argon.
For this purpose a hole is drilled in the main tubular
member 12 to permit evacuation of air from the annular
cavity in which the ceramic wool has been laid, and then
argon gas is introduced via the hole at atmospheric
pressure. The hole is then sealed by a weld 20.
As previously mentionedr the end tubular casing member
14 is rigidly connected to one end of the inner pipe
section 10 by means of the steel ring 18 to which it is
welded, and has a free end portion which telescopes
within the cooperating free end portion of the tubular
member 12. The expansion joint 17 is formed by an
annular sliding seal arrangement which is best
illustrated in Figure 4. This consists essentially of a
pair of short cylindrical members 21, 22 of the same
diameters as the casing members 12, 14 and welded
thereto in end ~o end alignment as shown by the welds
23, 24. The cylindrical members 21, 22 telescope one
within the other and thus constitute, in effect, the
free end portions of the casing members 14 and 12
respectively. The inside surface of the member 22 is
machined to provide an annular abutment stop 25 which is
positioned to engaye an abutment flanye 26 of the inner
member 21 for limiting the extent of slidiny movement
one relative to the other. It will be observed that by
limiting the extent of sliding movement of the member 22
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within the member 21, the deslgn provides additional
protection in the event of structural failure of one
component.
Also machined in the inside surface of the outer
cylindrical member 22 are a pair of annular slots for
accor~odating a pair of retaining C-rings 27, 28, and an
annular step 29. A pair of circular mechanical seals
30, 31 located by the retaining rings and the step 29
slidably engage the surface of the inner member 21 to
provi~e a gas-tight seal while permitting relative
movement of the inner and outer members in response to
differential thermal expansion and contraction of the
pipe sections. For this purpose fluorocarbon seals are
well known in the art.
The component tubular structures are coupled end to end
to form a rigid string, and details of the coupling
between a pair of casing structures 32, 33 are best
shown in Figure 3. This figure clearly shows the steel
rings 18, 18' by which the end tubular members 14, 13'
of the component structures 32, 33 are rigidly connected
to the inner pipe sections 10, 10'. The external
surfaces of the members 14, 13' are tapered as shown and
provided with buttress threads 34, 34'. An internally
threaded collar 35 engaging these threads provides the
coupling by which the adjacent ends of the casing
structures 32, 33 are rigidly coupled together. The
inner pipe sections 10, 10' are aligned to form a
continuous duct for the fluid to be transferred. A
filler ring or gasket 36 is interposed between the end
rings 18l 18', which abut against it, and extends
between the pipe ends 37 so as to minimize fluid leakage
therefrom. The filler ring is firmly wedged in
position within the collar 35 and is frictionally held
by the internal th~eads thereof. The purpose of the
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filler ring 36, which is made of a tough, heat resistant
and steam resistant insulating material, is to prevent a
heat short at the join between the two inner pipe
sections, as well as to break the flow of steam which
would otherwise be damaging to the coupling.
As will be apparent from the construction described, the
entire weight of the suspended tubular system is borne
by the inner pipe sectons 10, the axial load being
transferred from each section to the next via the welded
steel end rings 18, to which the tubular casing members
13, 1~ are welded, and the threaded collars 35 by which
the cooperating ends of the component tubular structures
are coupled together. The inner pipe sections 10 must
therefore be of sufficient strength to carry the axial
lS load, as must the couplings also, but this requirement
is conveniently met since the pipe sections must in any
case be strong enough to withstand the internal fluid
pressure. On the other hand the outer pipe sections are
decoupled from the axial load.
In the embodiment described above the outside diameter
of the tubular system is 5.56 inches and each component
casing structure is 30 feet long~ The inner pipe
sections are of 2 7/8 inch outside diameter seamless
steel tubing API-J55. The main tubular members 12 of
the outer pipe sections are of standard wall-5 inch
diameter line pipe. The end casing members 13, 14,
which are specifically designed to withstand the grip of
a service rig, are typically 3 feet long and are of 5
inch diameter API-J55 or API-N80 steel casing with API-8
Round Long Threaded and coupled ends. The internally
threaded coupling collars 35, which are also of API-J55
or API-~8n steel casing. The specifications of these
components are given by way of example only, of course,
and for any given application the materials and
dimensions of the components must be selected according
to the working requirements. Thus, for example, the
wall thickness of the outer pipe sections is very much
less than that of the inner pipe sections and the load
bearing components.
