Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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WEAR RESISTANT TUBULAR CONNECTION
Field of the Invention
The present invention relates to tubular connections provided with features
that enhance their performance in applications where drilling is conducted
with joints of casing joined by such tubular connections. In particular, this
invention relates to wellbore casing connections having enhanced wear
resistance over at least some portion of their exterior surfaces.
Background of the Invention
Lengths of tubulars used to drill and complete bore holes in earth materials,
referred to as joints, are typically joined by threaded connections to form a
long assembly referred to as a drill string. Numerous threaded connection
geometries are employed to provide sealing and load carrying capacities to
meet drilling, installation and operating requirements. Of these geometries,
connections having an external diameter greater than the pipe body are the
most widely used. Thus the majority length of a typical drill string is
comprised
of alternating long lengths of generally cylindrical pipe separated by
relatively
short externally upset intervals at the connections.
Within the context of petroleum drilling and well completion, 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
the
well by installing a second tubular string, referred to as casing, which is
subsequently permanently cemented in place. The tubular strings are formed
by connecting joints of pipe with threaded connections. With this historic
method of well construction, both the drill pipe and casing joint designs are
separately optimized for the different performance requirements of the
drilling
and completion operations respectively. More specifically, the drill pipe
connections must typically accommodate more torque to drill, than is required
during completion, and must resist wear that occurs where the connection is
in contact with the abrasive borehole wall during extended periods of drilling
rotation. The tendency toward wear is strongly dependent on the lateral forces
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that arise at the points of contact between the drill string and borehole.
These
contact forces result from the interaction of several variables, but may be
generally attributed to: inertia loads required to react the tendency of the
rotating drill string to whirl, reaction of lateral load induced by the axial
load
transferred along the string through intervals of curvature and gravity loads
in
deviated intervals. Concentration of all or a majority of the wall contact
load
over the short upset interval containing the connection tends to exacerbate
wear at these locations. This wear has the effect of generally reducing the
diameter of the connections. For that reason, it is common industry practice
to
apply bands or zones of abrasion resistant coatings around the circumference
of the drill pipe connections, referred to as hardbanding or hardfacing, to
build
up the diameter of the connection and thus provide a sacrificial layer of slow
wearing material. US patents 4665996 and 6375395 are two examples
describing the materials and application methods used to apply such surface
preparations to drill pipe tool joints.
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 technology employs a wireline retrievable
bottom hole drilling assembly capable of deployment on the distal end of
casing strings. Development of the technology was initially 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. In
addition to drilling, this technology finds utility in casing running
applications
where reaming is required to resize the borehole.
The established performance requirements for casing are only those required
to meet the needs of historic well construction methods. The new use of
casing to drill, naturally changes the performance requirements of the casing
string. Such changes include increased torque capacity required to drill with
the casing connections, but did not initially anticipate the need for
increased
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wear resistance particularly in relatively straight wells where lateral forces
arising from curvature and gravity are minimal. This expectation was based on
the shorter exposure time to conditions of rotating wear likely for casing
strings compared to drill pipe. (Drill pipe is used to drill many wells,
resulting in
extended exposure of drill pipe connections to conditions of rotating wear. In
contrast, the application using a casing string to drill, deliberately only
intends
to expose the connections to rotating wear conditions for the time required to
drill the single well interval to be cased by that string.)
However, it has been discovered that drilling. with casing strings using
industry
standard threaded and coupled buttress (BTC) connections, having tapered
pipe thread geometries specified by the American Petroleum Institute (API)
and equipped with shoulder rings such as, for example, those described in
Canadian Patent Application 2,311,156, frequently causes eccentric wear in
the region of the connection. This wear may locally reduce the coupling side
wall thickness until the coupling radius, in the region of wear, is little
more
than the pipe body. This amount of wear may occur during even a fraction of
the relatively short period required to drill a single well interval in a
nearly
vertical well. As will be appreciated by one skilled in the art, this wear
substantially compromises the load and sealing capacity of the connection.
This eccentric wear mechanism arises because the straightness of these
connections are not as tightly controlled as in drill pipe, since the historic
use
of casing only contemplates the requirements of running, cementing and well
access and not drilling. Thus a small bend in the string axis often occurs
across the connections. Such bends tend to preferentially force the
connections against the borehole wall at the 'outside' of the bends. The
lateral
wall contact force arising at these points of contact is strongly dependent on
whether or not the lateral deflection imposed by the bend angles in the
axially
loaded casing is sufficient to interfere with the confining bore hole. This
lateral
interference acts to displace the casing string from its neutral position at
the
points of contact with the borehole, the casing string behaving as a long beam
bent at the connections and restrained by the borehole. Particularly, where
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such lateral interference occurs between connections spaced one joint apart,
the lateral load and hence wear rate is much greater than occurs over
comparably 'straight' intervals.
For example, the connection bend angles were inferred a sample of typical
7inch (178mm) API buttress threaded and coupled (BTC) casing joints. These
magnitudes were used to calculate the possible maximum lateral load arising
from this load mechanism, were such casing joints assembled into a casing
string and placed in a borehole drilled with a bit size of 8.5inches (216mm).
It
was found that, with negligible axial load, a lateral force of at least 1000
Ibf
(4450N) could be present if the casing string were so confined in an interval.
As described above, this lateral load mechanism is not normally present in
drill pipe strings placed in a bore hole because the connections in those
strings are typically straighter and the tube bodies flexurally less rigid
than the
same respective components of a casing string assembly. Furthermore, unlike
the other lateral loading mechanisms which result in relatively axi-symmetric
wear of the connection, wear resulting from the connection bend angle is non-
axi-symmetric or eccentric.
This eccentric wear could be mitigated by providing connections with
increased straightness. In certain applications this alternative may be
preferable. However in general this will increase manufacturing cost and
prevent the use of readily available tubulars. Furthermore, the presence of
this new lateral wall contact load, while discovered to produce an unfavorable
tendency toward excess wear, was simultaneously discovered to have a
beneficial effect by improving borehole wall stability and reducing the risk
of
lost circulation when compared to drilling with straight drill pipe strings.
Excess wear can be avoided by use of a separate device, termed a wear
band, as disclosed in Cdn. Patent App 2,353,249. The disclosed wear band
includes a band of wear resistant material and is structurally attached to the
casing adjacent the connection by crimping. This solution is effective and
provides a readily implemented means enhance the usefulness of casing
joints having standard non-wear resistant connections for casing drilling or
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reaming. However, the method requires additional handling and operations to
crimp the wear bands to the casing joints with associated labour, capital and
logistical overburden costs, plus introducing a longer upset interval length
in
the region of the connection, which longer interval must be accommodated by
the pipe handling, running and drilling equipment.
Summary of the Invention
A wear resistant connection has been invented for joining lengths of casing
tubulars into assemblies referred to as strings. The wear resistant connection
of the present invention provides a means to substantially prevent loss of
material from the exterior surface of the tube wall, in the region of the
connection, caused by rotating wear mechanisms present where such strings
are placed in boreholes and rotated. In one embodiment, this wear resistant
connection provides resistance to eccentric rotating wear mechanisms arising
from the bend angle either accidentally or deliberately present in casing
connections.
For the purpose of this invention, a connection is understood broadly to mean
any arrangement or device that joins the ends of casing tubulars to create a
section over which a structural union is arranged so that the axes of the
joined
tubulars is substantially continuous across the connection interval, and while
generally straight, may have a small bend either accidentally or deliberately
introduced. Understood thus, the connection of the present invention includes
but is not limited to welded connections, integral connections and threaded
and coupled connections. Where an upset interval is associated with such a
connection, references to the connection are understood to include this upset
interval. Where the connection is made without an upset interval, i.e., an
externally flush connection, reference to the connection is understood to
include a section of the joined casing tubulars having a length of at least 10
casing diameters on each side of the actual joint (i.e. the weld) between the
casing tubulars.
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Thus in accordance with a broad aspect of the present invention, a casing
connection is provided having an exterior surface, at least some portion of
which includes a wear resistant material.
The casing connection is preferably selected to be useful for drilling with
casing.
The wear resistant material can be arranged to at least overlap the
circumferentially oriented location forming the outside of any bend that may
be accidentally or intentionally imposed across the connection.
The wear resistant material may be integral to the connection, obtained by
surface hardness treatment such as boronizing, nitriding or case hardening or
applied thereto such as by use of a coating such as hardfacing. The relatively
high cost of the applying, working with and forming wear resistant materials
encourages a reduction in the size of the area covered and thickness of
material.
The vast majority of well bores are lined with metal casing strings having
threaded connections. Therefore to be most readily implemented, wear
resistance of metal casing connections is best provided in a manner which
accommodates existing thread-forms, sealing geometries and bend
magnitude tolerances. Such existing threaded connections include the thread-
forms and sealing geometries comprising so called premium connections, in
addition to both integral and threaded and coupled American Petroleum
Institute (API) specified geometries. (Reference herein to a 'thread-form' is
generally understood to include the seal geometry if present, unless these two
components of the connection geometry are specifically separated in the
context.) This accommodation of existing geometry extends to the connection
diameter where it is preferable to provide wear resistance without a
significant
increase in outside diameter to avoid correlatively increasing the annular
flow
resistance, where such a wear resistant connection is deployed in a casing
string within a well bore.
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It is advantageous to adapt existing threaded connection geometries to
provide locations where wear resistant materials can be most economically
and least invasively applied to the connection, i.e., without significantly
altering the existing connections with respect to seal and structural
performance, while providing adequate protection against wear from rotation
while drilling. In particular, preferably the wear resistant material is
provided at
the lower or leading end of the coupling (leading is defined with respect to
the
axial direction of travel while drilling), as the upset diameter change from
the
pipe body to the coupling occurring at this location tends to promote
preferential wear while drilling with casing.
Threaded and coupled connections according to the present invention can
include an internally threaded coupling having an upper end, a lower end and
generally cylindrical exterior surface, as typically provided for such
couplings,
where wear resistant surface treatment or coating material is disposed axi-
symmetrically on said external surface over one or more axial intervals to
form
one or more hardbands of diameter somewhat greater than the diameter of
the generally cylindrical exterior surface. Said axial interval length and
coating
thickness are chosen, based on application requirements, to provide sufficient
volume of material to resist wearing through to the base metal. Wear resistant
surface treatment or coating material is axi-symmetrically distributed to
accommodate the random distribution of bend angle and hence
circumferential location of connection contact with the well bore.
For most of these geometries, wear resistance can be provided by applying
coatings resistant to abrasive wear to the exterior surface of the connection.
Such coatings are commonly referred to as hardfacing. These coatings are
applied using a variety of techniques and materials, but typically the bond
chemistry and mechanics require heat input to obtain the elevated
temperature required to create a strong bond between the coating and metal
substrate. It is therefore necessary to consider the effects of this heat
input
and bond chemistry on the metal substrate, and in particular to allow for any
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changes in structural or mechanical performance the heat input and bond
chemistry might have.
In addition, the choice of axial interval location where wear resistant
surface
treatment or coating is provided is preferably selected to occur at locations
where stresses induced by structural and pressure loads are lowest. Such
choice of location reduces the risk of connection failure due to crack
initiation
within the typically brittle coating material.
However such a suitable region of low stress is often not available for many
of
the threaded and coupled connection geometries employed by industry. It is
therefore a further purpose of the present invention to provide such a
suitable
region of low stress at one or both ends of the coupling by more preferably
providing a coupling having its length and interval of internal threading
arranged so that the end hardband interval does not overlap with the internal
threaded interval of the coupling. Otherwise stated, relative to the
'standard'
non-wear resistant coupling geometry a coupling is provided where at least
one end and preferably the lower end is modified to provide a generally
cylindrical extension which extension or extensions having external and
internal surfaces without load bearing threads on which said external surface
or surfaces wear resistant surface treatment or coating material such as
hardfacing is applied to create a hardband or hardbands of upset diameter.
Where only one hardband is required, the lower end is preferred as this end
forms the leading edge of the coupling while drilling with casing and protects
this region from preferential wear.
Application of these teachings for placement of wear resistant surface
treatment or coating material on the couplings of threaded and coupled
connections may be extended to integral connections and externally upset
integral connections. As commonly understood in the industry, an integral
connection is comprised of an externally threaded pin formed on the end of
one tubular screwed into a mating internally threaded box formed on the end
of a second tubular. Said internally threaded box having an external largely
cylindrical surface and proximal end. Particularly where the connection design
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is arranged to shoulder on said proximal end when made up to the pin, the
stress state in this region is less prone to crack initiation and propagation.
To
best serve the purposes of the present invention a wear resistant integral
connection is therefore provided having a hardband of wear resistant surface
treatment or coating material disposed on its proximal end. Relative to the
'standard' non-wear resistant geometry of an integral connection box it is
more preferable if the proximal end of the box is modified to provide a
generally cylindrical extension which extension having external and internal
surfaces without load bearing threads on which said external surface wear
resistant surface treatment or coating material such as hardfacing is applied
axi-symmetrically to create a hardband or hardbands of upset diameter.
Where the integral connection is formed on externally upset tubulars, such
externally upset interval typically extends beyond the depth required to carry
the box or pin threaded connection geometry, and in certain applications it
may be preferable to provide a hardband on the connection exterior surface at
or near the leading end of the upset interval either separately or in
combination with a hardband placed at the proximal end of the box. The
leading end of the upset interval, thus carrying the hardband, occurs at a
location of significantly greater thickness than the pipe body and therefore
of
significantly reduced stress, but having the further advantage of being
positioned at the location of preferential wear. It is therefore an additional
purpose of the present invention to provide a wear resistance externally upset
tubular connection having an externally upset interval with leading and
trailing
ends comprising the connection, and having at least one hardband positioned
on said leading end.
The bend magnitude occurring across the connection interval is a function of
the pipe end straightness and combined thread axis angle alignment with the
pipe axes for integral connections and additionally the coupling thread axes
with respect to the coupling for threaded and coupled connections. For
industry typical casing connections, the bend magnitude or axis misalignment
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is not tightly controlled, as for example described in the API Specification
5CT
and Standard 5B. Furthermore the bend direction is randomly oriented.
The wear resistant casing connections of the present invention enjoy further
utility when also deliberately provided with a small bend in tubular axis
across
the connection interval. Where such connections are employed to assemble a
plurality of tubular joints to form at least one interval of a casing string
placed
in a borehole, the bend angle and direction controls the local lateral stess
of
the casing string within the confines of the borehole. The bend angle and
direction may thus be arranged to deflect some or all of the connections into
generally radially opposed contact with the borehole wall over an interval of
several joints. As will be apparent to one skilled in the art, the lateral
forces
arising from this contact will tend to increase with increasing bend angle. It
will
also be apparent that control of the bend direction provides a further means
to
control this force compared to random orientation of connection bend
direction. When such a string is rotated within the confining borehole, the
region of connection contact rotates with respect to the borehole causing an
axi-symmetric 'wiping' action on the interior of the borehole wall, but does
not
rotate with respect to the connection causing the associated wear mechanism
to be non-axi-symmetric, i.e., eccentric. In certain applications, the wiping
action thus provided results in axi-symmetric consolidation of the near well
bore earth material, reducing risk of sloughing and lost circulation. The
degree
of consolidation and associated benefits depends on the lateral force
generated by the casing as it bears against the borehole wall. Control of the
connection bend magnitude, and preferably also the bend direction, enables
control of said lateral force exerted and is thus a means to balance the
benefits gained by wiping action on the borehole wall against the eccentric
wear rate of the connection. This then is the basis for the further utility
obtained for the present invention of a wear resistant connection having a
controlled small bend.
In accordance with this further purpose, in one embodiment of the present
invention, a wear resistant casing connection is provided having at least some
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portion of its exterior surface provided with a wear resistant coating or
surface
treatment and arranged to provide a controlled bend in the axes of the
tubulars joined by the connection. Said bend magnitude is selected such that
when said bent wear resistant connections are employed to assemble at least
some portion of a plurality of tubular joints to form at least one interval of
a
casing string placed in a borehole, the resulting local directional variations
introduced by the bend magnitudes will induce some or all of the bent wear
resistant connections to at least contact the borehole wall and induce
generally radially opposed contact forces.
As a means to more predictably control said radially opposed contact forces,
in a further embodiment, said wear resistant casing connection of controlled
bend is provided having the circumferential direction of the bend controlled
with respect to a casing string assembled from such connections. Such
control of circumferential direction is preferably selected to provide a
repeating pattern between bent connections comprising an interval of an
assembled casing string.
As will be apparent to one skilled in the art, the teachings of the present
invention with respect to placement of wear resistant surface treatment or
coatings on typical threaded connection geometries to form wear resistant
connection where the bend angle and direction is allowed to vary randomly
according to existing industry practice apply equally well to connections
where
the bend angle is controlled. However, where the bend angle is introduced
deliberately in the manufacturing process the circumferential location
corresponding to the outside of the bend may be readily identified. Since
contact with the borehole must occur at this location wear resistant surface
treatment or coating material need only be disposed over this region and need
not be disposed axi-symmetrically, thus requiring less volume of wear
resistant material with consequent opportunity for cost saving.
In accordance with another aspect of the present invention, there is provided
a casing string including an interval over which the bend angle is selected to
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control the lateral reaction force of the casing string against the borehole
wall
in which the casing string is intended to extend.
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 wear resistant connection according to one
embodiment of the present invention;
Figure 2 is a sectional view through the sidewall of the connection shown in
Figure 1 wherein a shoulder ring is included to provide improved torque
capacity;
Figure 3 is a sectional view through the sidewall of another connection
wherein improved torque capacity is provided without a shoulder ring;
Figure 4 is a front elevation of a pair of connected casing joints showing the
bend angle formed by the connection shown in Figure 1; and
Figure 5 is a partially cut away view through another connection, where the
coupling bend angle is controlled.
Description of the Preferred Embodiment
According to the present invention, a wear resistant casing connection is
provided for joining two lengths or joints of tubulars suitable for drilling
with
casing. In its preferred embodiment, the wear resistant casing connection is
generally of a threaded and coupled nature and more preferably employs a
thread-form geometry compatible with a buttress connection as specified by
the American Petroleum Institute (API).
Referring to Figures 1 and 2, an assembled threaded and coupled wear
resistant connection 1 is shown according to one embodiment of the invention
including a lower joint 2 with threaded ends 5a, 5b, an internally threaded
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coupling 3 and an upper joint 4 with threaded ends 7a, 7b. As commonly
understood in the industry, the connection is assembled or 'made up' by
screwing the externally threaded mill end pin 5a of lower joint 2, into the
mill
end box 6 of coupling 3 and screwing the field end pin 7b of upper joint 4
into
the field end box 8 of coupling 3 to form a sealing structural union. The
generally cylindrical coupling 3 includes an upper end 9, a lower end 10 and a
hardband 13 formed from application of hardfacing axi-symmetrically about
the circumference of the coupling on the exterior surface 11 adjacent lower
end 10. In the illustrated embodiment, the hardfacing is applied in a
substantially uniform thickness to form the hardband.
The main body of coupling 3 is arranged to generally match the thread-form
geometry, tolerancing and length of an API specified buttress connection,
where the lower end 10 is formed as a generally cylindrical extension of the
main body. The extension extends out beyond the threads 6a of the mill box
end a sufficient length to carry the hardband 13 such that the hardband does
not radially overlap the threaded interval of the mill end box 6.
The outer diameter of the coupling at hardband 13 is preferably selected to be
greater than the diameter of the coupling outer surface 11 to such that the
hardband preferentially contacts the borehole wall when connection 1 is
employed in a casing string. However, when selecting the outer diameter of
the hardband, care should be taken, with consideration as to the borehole
diameter in which the coupling is to be used to reduce adverse effects on
annular flow.
A multi-lobe shoulder ring 15 is disposed in the coupling centre region,
between the mill and field end pins 5a, 7b. Under application of sufficient
torque the mill and field end pins 5a, 7b are caused to abut ring 15 to thus
increase torque capacity in support of drilling with casing as described in
Canadian Patent Application 2,311,156.
The illustrated embodiment of Figures 1 and 2, thus provides a wear resistant
connection where the manufacturing of the pin and box thread-forms is
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compatible with existing industry practice with respect to geometry, tolerance
and make-up practice.
Referring now to Figure 3, an alternate embodiment of a wear resistance
connection is shown where the geometry of the coupling 3 is arranged to
support direct abutment of the field and mill end pins 5a, 7b, under
application
of sufficient torque, eliminating the need to use a torque ring. To support
this
alternate embodiment, the thread form geometry and tolerancing of the
coupling is adjusted, relative to the API specified standards, to accommodate
pin ends made according the API specified standards for geometry and
tolerancing. The coupling is adjusted by reducing the length of the thread-
form
of the coupling main body to eliminate the pin end standoff and by adjusting
the diameter and taper tolerance of boxes 6, 8 to ensure that the smallest API
allowable field or mill end pins, when made up to the centre of the coupling
main body, will result in sufficient radial interference to create the
normally
intended thread seal. Thus configured, the connection is preferably made up
using position control to ensure the pin ends 5a, 7b are brought into abutment
at generally the center of the coupling. The embodiment of Figure 3 thus
offers compatibility with standard forms of casing joints with threaded pin
ends, but is shorter than a coupling according to Figure 2 and achieves
increased torque capacity over a standard non-shouldering API connection
without requiring a shoulder ring, thus reducing cost and complexity.
The bend angle and direction formed across the assembled connection 1
depends on the cumulative effect of the thread axis angle misalignments and
the relative direction of the misalignments for the pins 5a, 7b and boxes 6, 8
after make-up. With reference now to Figure 4, the bend angle a is defined as
the angle change between a first line 2a extending though the center points
Sax, 5bx at the ends of the lower joint 2 and a second line extending though
the center points lax, 7bx at the ends of the upper joint 4 in the connection.
The bend angle or connection straightness is dependant on variables
generally controlled by specifications known to the industry such as: pipe
straightness, pin geometry parameters such as imperfect thread limits for
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buttress threads, coupling thread angular misalignment and make-up position.
Prevalent industry practice for control of these variables results in randomly
controlled casing connection bend magnitudes, where a significant number of
connection bend angles are greater than allowed by comparable drill pipe
specifications. Therefore, when a plurality of such connections are employed
to form a tubular casing string placed in a bore hole, joint to joint local
directional variations interfering with the borehole confinement are likely.
As
noted hereinbefore, this interference is frequently great enough to cause
large
radial or lateral reaction loads between the connection outside bend surface
16 and the confining borehole wall and, thus, there is a need to protect the
connections against excess rates of wear under conditions of extended
rotation, such as in drilling with said tubular casing string.
While the wear resistant connections shown in Figures 2 and 3 are useful for
applications where the bend angle a is allowed to vary randomly in
accordance with typical industry practice for manufacture and assembly of
threaded and coupled casing connections, in certain applications it is
desirable to control the magnitude of said lateral reaction force in at least
one
interval of an assembled casing string, which lateral reaction force is
dependent on several design variables including: casing flexural stiffness,
spacing between contacting bent connections, axial load, relative radial
orientation of connection bends and radial interference of local bent section
as
controlled by the magnitude of the connection bend angle a.
To control of lateral load arising in an interval of a casing string, it is
useful to
control the bend angle geometry and spacing along that string interval. This
can be done by surveying couplings and casing joints to determine the bend
angle magnitude at a connection of selected ones of the couplings and casing
joints and selecting the couplings and casing joints to be used in the string
interval.
Referring now to Figure 5, in an alternate embodiment of the present
invention a bent wear resistant connection 101 largely as shown in Figure 2 is
provided, but where the center axis 6x of the mill end box 6 and the center
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axis 8x of the field end box 8 are offset out of alignment to form a bent
coupling 103 having an angle ~i between axes 6x and 8x. A wear pad 113 is
positioned on the outer surface of the coupling about the circumferential
location defined by the outside bend 16 of bent coupling 103. Coupling 103
accommodates a shoulder ring 115 which substantially conforms to the bend
of the coupling. In particular, shoulder ring 115 includes end faces 115a,
115b defining planes that are not parallel, such that the width of the ring
varies from a narrow wall 115c to a long wall 115d. The ring is set within the
coupling bore having its long wall 115d positioned radially inwardly of
outside
bend 16 of the bent coupling 103. The planes of end faces 115a, 115b
therebetween define an angle selected to be similar to that of angle a.
In use, the bent coupling can be employed to achieve further control of said
lateral force arising from confinement within a borehole, by selecting the
frequency of bent connections and, thereby the spacing therebetween, and by
controlling the relative orientation of outside bend position 16 between
sequential bent couplings employed to connect a plurality of tubular joints
forming an interval in a casing string. To conveniently select the bend
orientation of the connection during make up of a string, means, such as a
power tong, can be used to apply torque to the coupling for control of mill
end
make-up position. Final mill end make-up position may then be selected to
align the outside bend position of sequential connections at, for example,
positions 180° apart or other similar pattern as required.
In a ,further embodiment, the casing joint pin ends used can have the
misalignment tolerance of their thread axes reduced from typical industry
practice to further improve control of their bend angle.
It will be apparent that 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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