Note: Descriptions are shown in the official language in which they were submitted.
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PIPE CENTRALIZER AND METHOD OF ATTACHMENT
Field of the Invention
The present invention relates to centralizers attached to pipe placed in
boreholes.
The invention discloses centralizers and methods of attachment to enable
transfer of
structurally significant axial and torsional loads between the centralizer and
pipe.
Background of the Invention
The processes of drilling and completing well bores in earth materials using
tubular
strings are frequently benefited if the tubular string is prevented from fully
eccentering and generally contacting or laying against the borehole wall.
Numerous
devices, typically referred to as centralizers, are employed to provide this
function of
reducing eccentricity, or centralizing, the tubular string within the
borehole. These
devices are configured to economically meet a variety of drilling and
completion
applications.
Within the context of petroleum drilling and well completions, wells are
typically
constructed by drilling the well bore using one tubular string, largely
comprised of
drill pipe, then removing the drill pipe string and completing by installing a
second
tubular string, referred to as casing, which is subsequently permanently
cemented in
place. The requirements for centralizers historically used on these two types
of
strings is thus significantly different.
Drilling places the severest structural demands on centralizers since they
must
survive extended periods of time in rotating contact with the borehole wall.
Centralizers suitable for drilling must therefore be rugged and may be reused.
To
meet these requirements drilling centralizers are typically integral with the
drill string,
and may be relatively expensive since they are reused.
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In. contrast, centralizers used for casing are not typically required to
withstand
significant rotation, are typically optimized to improve cementing quality and
are only
used once. These requirements have led to casing centralizers that attached to
the
exterior of the connection by means having little or no torsional and limited
axial load
transfer capacity. As a single use item, they are constructed for lowest cost
not
durability. With this historic method of well construction, both the drill
pipe and
casing centralizes designs are separately optimised for the different
performance
requirements of the drilling and completion operations respectively.
Recent advances in drilling technology have enabled wells to be drilled and
completed with a single casing string, eliminating the need to 'trip' the
drill pipe in
and out of the hole to service the bit and make room for the casing upon
completion
of drilling. This change is motivated by potential cost savings arising from
reduced
drilling time and the expense of providing and maintaining the drill string,
plus
various technical advantages, such as reduced risk of well caving before
installation
of the casing.
However, using casing to both drill and complete the well changes the
performance
requirements of the casing centralizers employed. Casing centralizers, as
employed
in the prior art, typically rotate relative to the casing body under
application of
extended rotation required for drilling, causing wear of the centralizes,
casing or both,
leading to potential failure of the centralizes or casing. Adapting the
integral
centralizes architecture employed for drill string centralizers, while
providing a
technically feasible means to centralize casing for drilling, is costly and
more
complex to implement than simply attaching to the casing exterior. What is
required
are inexpensive casing centralizers that are rugged, comparatively easy to
attach to
the casing and able to withstand drilling rotation sufficient to complete at
least one
well.
Summary of the Invention
A crimped centralizes has been invented for installation on metal pipe, such
as would
be useful in well bore drilling and casing operations. The present invention
provides
a centralizes having a cylindrical body which when coaxially placed over a
metal pipe
and substantially radially inwardly displaced at a plurality of points (i.e.
crimped)
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about the circumference of a section of the cylindrical body, attaches to the
pipe to
create a connection having structurally significant axial and torque load
transfer
capacity. When crimped according to the methods of the present invention, the
load
transfer capacity of the connection between the centralizer and the pipe can
be
arranged to substantially prevent significant relative movement of the
centralizer on
the pipe under loads that may be encountered when using one or more of the
metal
pipes as components of a tubular string used for drilling or completing well
bores.
The metal pipe on which the centralizer of the present invention is installed
must be
capable of accepting the hoop stresses of crimping without becoming unstable,
for
example, without buckling or crumpling. This generally requires that the pipe
be
thick-walled, for example, having an external diameter to thickness ratio
("D/t") less
than 100 and preferably less than 50.
To be most generally useful for these applications, the centralizer should be
amenable to rapid field installation on joints of pipe having at least one non-
upset
end. In addition, the centralizer, once installed should not substantially
reduce the
minimum diameter (drift diameter) through the pipe.
Thus, in accordance with a broad aspect of the present invention, there is
provided a
metal centralizer comprising: a body having an outer facing surface and an
inner
bore therethrough sufficiently large to allow insertion therethrough of a
selected
metal pipe having an external diameter, at least one tubular section on the
body, the
portion of the inner bore extending through the tubular section having an
internal
diameter capable of loosely fitting about the external diameter of the metal
pipe and
a centralizing section on the body including a plurality of bearing surfaces
extending
outwardly from the outer facing surface.
The tubular section can be cylindrical or largely cylindrical with some radial
variations
to the internal diameter or outer surface. The tubular section should be
circumferentially continuous such that a hoop stress can be set up by radially
inwardly displacement (i.e. crimping) at a plurality of points about the
circumference
of the outer surface of the section. The tubular section should be capable of
accepting the hoop stresses of crimping without becoming unstable, for
example,
without buckling or crumpling. This generally requires that the section be
thick-
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walled, for example, having an external diameter to thickness ratio ("D/t")
less than
100 and preferably less than 50.
The loose fit of the section about the pipe must be sufficient to accommodate
the
variations of the outer diameter of the metal pipe intended to be used.
The bearing surfaces can be for example ribs, lines of weldments etc.
In accordance with the present invention there is also provided, a method to
attach a
centralizer to a metal pipe by crimping, the metal pipe having an outer
surface, such
method comprising the steps of: providing a metal pipe; providing a
centralizer
having a body with an inner bore therethrough sufficiently large to allow
insertion
therethrough of the metal pipe, a plurality of outward facing bearing surfaces
on the
body and at least one tubular section on the body having an internal diameter
capable of fitting about the outer surface of the metal pipe; inserting the
metal pipe
through the inner bore of the centralizer, applying an inward, substantially
radially-
directed force to a plurality of points about an outer circumference of the
tubular
section causing it to plastically deform inwardly and come into contact with
the outer
surface of the pipe, applying such additional inward, substantially radially
directed
force as required to force both the centralizer and the outer surface of the
metal pipe
to displace inwardly an amount at least great enough so that when the force is
released, an interference fit is created between the centralizer and the metal
pipe.
Preferably, the inward, substantially radially directed force is not so great
that the
drift diameter of the metal pipe is excessively reduced. Frictional forces
enabled by
the interference fit at the inwardly displaced section provide the mechanism
by which
structurally significant axial and torsional load may be transferred between
the
centralizer and metal pipe without slippage therebetween.
The ability of the crimping method to thus ensure a residual interference fit
without
compromising the drift diameter is dependent on appropriate selection of
various
parameters as will be apparent to one skilled in the art. Where the
application
permits, from the point where plastic deformation of the centralizer induced
during
crimping has reduced the original loose fit to come into contact with the
metal pipe of
the method, differential temperature may be used to control interference
according to
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the well known methods of shrink fitting, whereby the differential temperature
is
obtained by heating the centralizer, cooling the metal pipe, or both, prior to
crimping.
However for the present application it is preferable to avoid the requirement
to either
heat the centralizer or cool the metal pipe as required to obtain interference
by shrink
fitting. An additional purpose of the present invention is, therefore, to
provide a
method of obtaining sufficient interference in the crimped connection through
purely
mechanical means, without requiring a significant temperature differential
between
the centralizer and metal pipe at the time of crimping. This purpose is
realized by
selecting the elastic limit of the centralizer material, in the section to be
crimped, to
be less than that of the pipe on which the centralizer is to be installed. In
this
context, the elastic limit generally refers to the strain at which the metal
of the parts
yields. Having the material properties thus selected, it will be apparent to
one skilled
in the art, that when the radial displacement applied during crimping is
sufficient to
force the hoop strain of the metal pipe to be at least equal to its elastic
limit, upon
release of the load causing the radial displacement, the metal pipe will tend
to
radially 'spring back' an amount greater than the centralizer, were both parts
separated. Since the parts are not separated, the difference in this amount of
spring
back is manifest as interference and fulfills the desired purpose of creating
interference by purely mechanical means.
While a purely mechanical method of obtaining interference through crimping is
desirable for most applications, the present invention also anticipates
applications
where thermal and mechanical methods can be combined.
A further purpose of the present invention is to facilitate the frictional
engagement of
the crimped centralizer to the thick-wall pipe. To meet this purpose, in one
embodiment of the present invention the inside surface of centralizer, at
least over
the section to be crimped, is provided with a roughened surface finish. In a
further
embodiment, a friction enhancing material such as a grit epoxy mixture is
disposed
in the interfacial region of the crimped section. Similarly, various bonding
materials
may be disposed in the interfacial region prior to crimping to act as glues
augmenting
the frictional aspects of the connection once their shear strength is
developed after
setting.
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Brief Description of the Drawings
A further, detailed, description of the invention, briefly described above,
will follow by
reference to the following drawings of specific embodiments of the invention.
These
drawings depict only typical embodiments of the invention and are therefore
not to
be considered limiting of its scope. In the drawings:
Figure 1 is a perspective view of a centralizer according to the present
invention;
Figure 2 is a perspective view of the centralizer shown in Figure 1 placed on
a joint
of casing as it might appear before crimping;
Figure 3 is a partial sectional schematic view through the wall of a
centralizer
positioned coaxially on a casing joint and inside a collet crimping tool prior
to
application of radial crimping displacement; and
Figure 4 is the partial sectional schematic view of the assembly shown in
Figure 3 as
it would appear after application of radial crimping displacement.
Description of the Preferred Embodiment
According to the present invention, a centralizer is provided as shown in
Figure 1,
and a method of crimping it to a thick-wall metal pipe when placed on the pipe
as
shown in Figure 2.
Referring to Figure 1, the centralizer is provided having a metal body 1
containing an
internal bore 2, a cylindrical end 3 forming a section suitable for crimping,
and a
centralizing section 4 on which ribs 5 are placed.
The cylindrical end and the centralizing section are formed integral on the
body and
the internal bore passes through both of them. While the crimpable section in
the
illustrated embodiment is cylindrical end, it is to be noted that the
crimpable section
can be formed intermediate a pair of centralizing sections, if desired, rather
than on
an end. Also, it is to be noted that more than one crimpable section and more
than
one centralizing section can be provided on the centralizer, as desired.
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Ribs 5 are evenly spaced around the centralizing section. There are at least
three
ribs spaced about the circumference of the centralizing section. Preferably,
each rib
is helically shaped and the number, length and pitch of the rib helixes are
arranged
to ensure the starting circumferential position of each rib overlaps the
ending
circumferential position of at least one adjacent rib. The ribs may be placed
on the
centralizer body by a variety of methods including milling, casting, welding
or
hydroforming.
The internal bore 2 of the centralizer body is selected to loosely fit over at
least one
end of a thick-wall metal pipe 6, shown as a threaded and coupled casing joint
in
Figure 2. As shown, this allows the centralizer to be readily inserted over an
end of
the pipe 6 and placed somewhere along the length of the pipe joint prior to
crimping.
Thus placed, the crimping method of the present invention in its preferred
embodiment provides a means to obtain a significant interference fit after
crimping
even where the centralizer and casing material are at similar temperatures
prior to
crimping. In applications where significant heating of the pipe and
centralizer, after
centralizer installation, is anticipated, the centralizer is preferably
selected to have a
thermal expansion coefficient that is equal to or less than that of the
casing. Similarly
in applications where cooling subsequent to crimping is anticipated, the
opposite
relationship between thermal expansion coefficients is preferred.
Radial displacement required to crimp the centralizer cylindrical end 3 to the
casing
joint 6, on which it is placed, may be accomplished by various methods,
however a
fixture employing a tapered 'collet in housing' architecture was found to work
well in
practice. This well known method of applying uniform radial displacement, and
consequently radial force when in contact with the exterior of a cylindrical
work piece
surface, employs a device as shown schematically in Figure 3. The device
retains
the externally tapered fingers or jaws 7 of a collet (segments of an
externally conical
sleeve) inside a matching internally tapered solid housing 8. Application of
axial
setting force to the housing 8, as shown by vector F, which is reacted at the
face 7a
of the collet jaws 7, as shown by vector R, tends to induce the collet jaws 7
to
penetrate into the collet housing 8 along the angle of its conical bore. This
causes
the jaws 7 to move radially inwardly and engage the work piece to be gripped,
in the
present case, shown as the cylindrical end 3 of a centralizer. (Alternately,
the action
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of the collet may be described in terms of setting displacement, understood as
axial
displacement of the collet housing 8 with respect to the collet jaws 7. In
this case the
setting force is understood to arise correlative with the setting
displacement.) The
axial force F and reaction R are readily applied by, for example, a hollow
bore
hydraulic actuator (not shown), arranged with an internal bore greater than
the
casing 6 outside diameter.
With this arrangement, upon application of sufficient force (F), the jaws may
be
forced inward to first cause sufficient radial displacement to plastically
deform the
centralizer cylindrical end 3 and bring it into contact with the casing 6.
This amount of
radial displacement removes the annular clearance of the loose fit initially
required
for placing and positioning the centralizer on the casing 6. Application of
additional
setting force then forces both the centralizer cylindrical end 3, and the
underlying
wall of the casing 6, inward. In the preferred embodiment, the setting
displacement is
preferably applied until the hoop strain in the casing wall at the crimp
location equals
or slightly exceeds its elastic limit. It will be apparent to one skilled in
the art that
radial displacement beyond this point will cause little increase in residual
interference
but will have the undesirable effect of reducing the drift diameter of the
casing joint 6.
Figure 4 schematically shows the collet, centralizer and casing as they might
appear
in the fully crimped position. After the desired radial displacement is
achieved, the
setting displacement of the collet is reversed which releases it from the
centralizer
allowing the collet to be removed, leaving the centralizer crimped to the
casing.
To ensure that this method of cold crimping (i.e., mechanical crimping
unassisted by
thermal effects) results in sufficient residual interference between the
centralizer
cylindrical end 3 and the casing 6, in its preferred embodiment the
centralizer
material at the cylindrical end 3 has an elastic limit less than that of the
casing 6. As
is typically the case, the centralizer and casing material are both made from
carbon
steel having nearly the same elastic modulii. Therefore, the elastic limit may
be
expressed in terms of yield strength, since elastic limit is generally given
by yield
stress divided by elastic modulus.
For example, in one trial conducted to assess the torque capacity to be
obtained by
crimping a centralizer to 7inch diameter API grade L80 26ppf casing material
(minimum specified yield strength of 80,OOOpsi), steel centralizer material
having a
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measured yield strength of 47,OOOpsi was selected. The centralizer elastic
limit was
thus less than 50% that of the casing. Using this material, a centralizer
having an
outside diameter of 7.625inches, an inside diameter of 7.125inches and a
machined
inside bore, was constructed for one trial. After crimping this centralizer to
the casing
over a 3.5inch section using the method of the present invention described
above,
the axial force required to displace the centralizer was measured to be
approximately
20,000 Ibf. Had this sliding force been applied through torsion, the required
torque to
induce sliding rotation of the centralizer relative to the casing would be
5833 ftlb.
This may be compared to the maximum expected total drilling torque for this
size of
casing, which is in the order of 20,OOOftlb. Given this crimped centralizer
configuration, the torque transferred between just one such centralizer and
casing,
would need to exceed 25% of the total worst case drilling torque, to induce
slippage
of the centralizer on the casing.
However, in certain applications it may be desirable to further enhance the
load
transfer capacity of a centralizer attached to casing, without increasing the
crimped
length, by improving the frictional engagement achieved for a given level of
interference. While this may be accomplished by various means, roughening one
or
both of the cylindrical end inner wall or the casing outer surface on which
the
centralizer was to be crimped, was found to be particularly effective. In one
trial
using a centralizer configured similar to that described in the preceding
example, but
where the wall surface 9 defining the internal bore 2 of the centralizer was
roughened by grit blasting prior to crimping, the equivalent torque capacity
was
increased approximately 70%.
The length of the section crimped will in general linearly affect the load
transfer
capacity of the crimped connection. For centralizers attached to full length
casing
joints, the length of section suitable for crimping, provided by the
cylindrical end 3
may be extended almost without limit. Similarly the length of the collet jaws
7, do not
limit length that may be crimped. The collet tool may be used to apply the
required
radial displacement at multiple axial locations to incrementally crimp an
extended
length cylindrical end 3. Increased load transfer capacity may thus be readily
achieved by increasing the crimped section length.
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It will be apparent that these and many other changes may be made to the
illustrative embodiments, while falling within the scope of the invention, and
it is
intended that all such changes be covered by the claims appended hereto.
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