Note: Descriptions are shown in the official language in which they were submitted.
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TRACTOR WITH IMPROVED VALVE SYSTEM
Background of the Invention
Field of the Invention
100021 This invention relates generally to tractors for moving equipment
within
passages and, more particularly, to a hydraulically powered tractor having an
improved valve
system.
Description of the Related Art
100031 The art of moving equipment through vertical, inclined, and horizontal
passages plays an important role in many industries, such as the petroleum,
mining, and
communications industries. In the petroleum industry, for example, it is often
necessary to
move drilling, intervention, well completion, and other forms of equipment
through boreholes
drilled into the earth.
100041 One method for moving equipment through a borehole is to use rotary
drilling
equipment. In traditional rotary drilling, vertical and inclined boreholes are
commonly
drilled by the attachment of a rotary drill bit and/or other equipment
(collectively, the
"Bottom Hole Assembly" or BHA) to the end of a rigid drill string. The drill
string is
typically constructed of a series of connected links of drill pipe that extend
between ground
surface equipment and the BHA. A passage is drilled as the drill string and
drill bit are
together lowered into the earth. A drilling fluid, such as drilling mud, is
pumped from the
ground surface equipment through an interior flow channel of the drill string
to the drill bit.
The drilling fluid is used to cool and lubricate the bit, as well as for
removing debris and rock
chips from the borehole. The drilling fluid returns to the surface, carrying
the cuttings and
debris, through the annular space between the outer surface of the drill pipe
and the inner
surface of the borehole. As the drill string is lowered or raised within the
borehole. It is
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necessary to continually add or remove links of drill pipe at the surface, at
significant time
and cost.
[0005] Another method of moving equipment within a borehole
involves the use
of downhole tools commonly referred to as "tractors." A tractor is capable of
gripping onto
the borehole and thrusting both itself and other equipment through it. A self-
propelled tractor
of this type may be used for pushing and pulling adjoining equipment through
inclined or
horizontal boreholes. Tractors can be attached to rigid drill strings or may
be used in
conjunction with coiled tubing equipment.
[0006] Coiled tubing equipment generally includes a non-rigid,
compliant tube,
referred to herein simply as "coiled tubing," through which operating fluid is
delivered to the
tractor. The operating fluid can provide hydraulic power to propel the tractor
and the
equipment and, in drilling applications, to lubricate the drill bit. In such
systems, the
operating fluid may also provide the power necessary for enabling the tractor
to grip the inner
surface of the borehole. In comparison to rotary equipment, the use of coiled
tubing in
conjunction with a tractor is generally less expensive, easier to use, less
time consuming to
employ, and provides more control of speed and downhole loads. In addition,
due to its
greater compliance and flexibility, the coiled tubing permits the tractor to
negotiate sharper
turns in the borehole than rotary equipment.
[0007] Due to their versatility, self-propelled tractors may be
used in a wide
variety of applications. For example, a tractor may be used for well
completion and
production work for producing oil from an oil well, pipeline installation and
maintenance,
laying and movement of communication lines, well logging activities, washing
and acidizing
of sands and solids, retrieval of tools and debris, and the like. One type of
tractor comprises
an elongate body securable to the lower end of a drill string. The body may
include one or
more joined shafts attached to a control assembly housing or valve system.
[0008] Tractors generally include at least one anchor or
gripper assembly adapted
to grip the inner surface of the borehole. When the gripper assembly is
actuated, hydraulic
power from operating fluid may be used to propel the body axially through the
borehole. The
gripper assembly is longitudinally movably engaged with the tractor body, so
that the body
and drill string can move axially through the borehole while the gripper
assembly is anchored
=
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to the inner surface of the passage. Several embodiments of a fluid-actuated
gripper
assembly are disclosed in U.S. Patent No. 6,464,003 to Bloom et al. In one
highly effective
embodiment, the gripper assembly includes a plurality of flexible toes that
expand radially
outward by the interaction of ramps and rollers to engage, and thereby grip,
the inner surface
of the passage.
[0009] Tractors are commonly configured with two or more sets of gripper
assemblies, which provide the ability to have at least one gripper anchored to
the borehole at
all times. This configuration permits the tractor to move in a substantially
continuous
manner within the passage. Forward longitudinal motion (unless otherwise
indicated, the
terms "longitudinal" and "axial" are herein used interchangeably and refer to
the longitudinal
axis of the tractor body) is achieved by powering the tractor body forward
with respect to an
actuated first gripper assembly (a "power stroke" with respect to the first
gripper assembly),
and simultaneously moving a retracted second gripper assembly forward with
respect to the
tractor body (a "reset stroke" of the second gripper assembly). At or near the
completion of
the power stroke with respect to the first gripper assembly, the second
gripper assembly is
actuated and the first gripper assembly is retracted. Then, the tractor body
is powered
forward while the second gripper assembly is actuated (a power stroke with
respect to the
second gripper assembly), and the retracted first gripper assembly executes a
reset stroke. At
or near the completion of these respective strokes, the first gripper assembly
is actuated and
the second gripper assembly is retracted. The cycle is then repeated. Thus,
each gripper
assembly operates in a cycle of actuation, power stroke, retraction, and reset
stroke, resulting
in longitudinal motion of the tractor.
[0010] A number of highly effective tractor designs utilizing this
configuration
are disclosed in U.S. Patent No. 6,003,606 to Moore et al., which discloses
several
embodiments of a tractor known as the "Puller-Thruster Downhole Tool;" U.S.
Patent No.
6,241,031 to Beaufort et al.; and U.S. Patent No. 6,347,674 to Bloom et al.,
which discloses
an "Electrically Sequenced Tractor" ("EST").
[0011] As discussed above, the power required for actuating the gripper
assemblies, longitudinally thrusting the tractor body during power strokes,
and longitudinally
resetting the gripper assemblies during reset strokes may be provided by
pressurized
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operating fluid delivered to the tractor via the drill string. Typically, one
or more flow
control devices, such as valves, are provided within the tractor body for
distributing the
operating fluid to the tractor's gripper assemblies, thrust chambers, and
reset chambers.
[0012] Some types of tractors, including several embodiments of the Puller- ,
Thruster Downhole Tool, are entirely hydraulically powered. Pressure-
responsive valves
typically shuttle between various positions based upon the pressure of the
operating fluid in
various locations of the tractor. In one configuration, a pressure-responsive
valve may take
the form of a spool valve that is exposed on both ends to different fluid
chambers or
passages. As a result, the valve position depends on the differential pressure
between the
fluid chambers. Fluid having a higher pressure in a first chamber exerts a
greater force on the
valve than fluid having a lower pressure in a second chamber, forcing the
valve to one
extreme position. The valve moves to another extreme position when the
pressure in the
second chamber is greater than the pressure in the first chamber. Another type
of pressure-
responsive valve takes the form of a spring-biased spool valve having at least
one end
exposed to fluid. The fluid pressure force is directed opposite to the spring
biasing force, so
that the valve is opened or closed only when the fluid pressure exceeds a
threshold value.
[0013] In other configurations, tractors may be provided with one or more
valves
that are controlled by electrical signals sent from a control system at the
surface or even on
the tractor itself. For example, the aforementioned EST includes both
electrically controlled
valves and pressure-responsive valves. The electrically controlled valves are
controlled by
electrical control signals sent from a controller housed within the tractor
body. For drilling
operations, the EST may be preferred over all-hydraulic tractors because
electrical control of
the valves permits very precise control over important drilling parameters,
such as speed,
position, and thrust.
[0014] In contrast, all-hydraulic tractors, including several embodiments of
the
Puller-Thruster Downhole Tool, are generally preferred for so-called
"intervention"
operations. As used herein, the term "intervention" refers to re-entry into a
previously drilled
well for the purpose of improving well production, to thereby improve fuel
production rates.
As wells age, the rate at which fuel can be extracted therefrom diminishes for
several reasons.
This necessitates the "intervention" of many different types of tools.
Hydraulic tractors are
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generally preferred over electrically controlled tractors for intervention
operations because
hydraulic tractors are less expensive to operate and intervention operations
do not require
precise control of speed or position.
=
[0015] Tractors used in combination with coiled tubing
equipment are particularly
useful for intervention operations because, in many cases, the wells were
originally drilled
with rotary drilling equipment capable of drilling very deep holes. It is more
expensive to
bring back the rotary equipment than it is to bring in a coiled tubing unit.
However, in many
situations, the coiled tubing unit may not be capable of reaching extended
distances within
the borehole without the aid of a tractor. The tractor is particularly useful
for reaching
locations within inclined or horizontal boreholes.
[0016] Those skilled in the art appreciate that tractors of
the type generally
described above may be exposed to a wide variety of different conditions. For
example,
depending pn the particular application, the pressure, weight, and density of
the operating
fluid may vary significantly. Furthermore, the shape and angle of the borehole
may vary. In
addition, the weight of the equipment that the tractor must pull and/or push
will differ with
the particular application.
Summary of the Invention
Although tractors may be exposed to a wide variety of conditions, the
inventors have
found that existing tractors, and particularly all-hydraulic tractors, are
configured to operate
effectively within only a relatively limited range of conditions. This can be
a significant
shortcoming that increases costs and limits the effectiveness of tractors in
the field.
[0017] Therefore, an improved valve system is desired for
enabling a tractor to
operate effectively under a wider variety of conditions. In one embodiment,
such a valve
system is capable of controlling the tractor operation independently of the
tractor's load and
speed. It may also be desirable that such a valve system is not susceptible to
premature valve
shifting when exposed to fluctuations in the pressure of the operating fluid.
It may also be
desirable that such a valve system protects its internal components from
damage. It may also
be desirable that such a valve system allows the tractor to be operated
relatively
inexpensively and simplifies use of the tractor in the field by reducing or
eliminating the
steps for calibration, operation and downhole trouble-shooting. It may also be
desirable that
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such a valve system be adapted for use under a wide range of flow rates and is
compatible
with a wide variety of BHA components. It is also desirable that such a valve
system provides
for highly efficient movement by reducing unnecessary dwell times between
steps in the
operational sequence.
[0018] The pressure of the operating fluid within a tractor may fluctuate
substantially as the valve system directs fluid to actuate the grippers and/or
power the pistons
(or other similar mechanism) during advancement of the tractor through the
passage. In
certain applications, it is not uncommon for the pressure to fluctuate as much
as one thousand
psi. During field use, the inventors have found that the pressure fluctuations
can render other
tools inoperable or incompatible, particularly if the other tools are adapted
for use within a
limited range of pressure. As a result, the user's ability to use the tractor
in combination with
other tools may be limited.
[0019] Furthermore, the inventors have found that the large pressure cycles
add
undesirable fatigue cycles to the internal tractor components and/or to the
attached tools.
This may limit the design life of the tractor and/or other attached tools and
can thereby
significantly impact the operating cost of using the tractor.
[0020] Still further, the inventors have found that pressure-actuated valves
may be
susceptible to premature shifting due to pressure spikes or other large fluid
pressure
fluctuations. Similarly, testing has shown that the valves may be particularly
susceptible to
premature shifting when the tractor system is subjected to heavy loads, and/or
large dynamic
pressure waves (or "water hammer" effects) caused by the opening and closing
of other
valves within the control assembly. In certain applications, premature valve
shifting may
significantly limit the operational range and efficiency of the tractor.
[0021] In various embodiments of the present invention, there is provided an
improved valve system adapted for use with a tractor that overcomes the above-
mentioned
problems of the prior art. These embodiments represent a major advancement in
the art of
tractors, and particular in the art of well intervention tools. Compared to
the prior art, certain
embodiments of the improved valve system can provide for greater control of
tractor
movement and operate very effectively within a much larger zone of parameters.
In addition,
by providing for better control over the fluid pressure, certain embodiments
of the improved
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valve system can extend the useful life of internal components and thereby
reduce operating
costs.
[0022] In one aspect, a tractor for moving a component through a borehole
comprises an elongate body with aft and forward gripper assemblies
longitudinally movably
engaged thereon. The aft and forward gripper assemblies are preferably
hydraulically
actuated for selectively engaging an inner surface of the borehole. Aft and
forward
propulsion assemblies are provided for advancing the body through the borehole
relative to
the aft and forward gripper assemblies, respectively. A gripper control valve
is provided for
directing pressurized fluid to the aft and forward gripper assemblies. The
gripper control
valve preferably has a first position for directing pressurized fluid to the
aft gripper assembly
and a second position for directing pressurized fluid to the forward gripper
assembly. In a
significant feature, aft and forward mechanically actuated valves disposed
along the body for
detecting advancement of the body relative to said aft or forward gripper
assembly,
respectively, thereby providing a mechanism for improving the timing and
efficiency of the
tractor operation. In particular, the aft and forward mechanically actuated
valves are in fluid
communication with the gripper control valve for causing the gripper control
valve to change
positions after the body has completed an advancement stroke through the
borehole relative
to said aft or forward gripper assembly.
[0023] In another aspect, a tractor for moving a component through a borehole
comprises an elongate body having an internal passage extending therethrough
for providing
pressurized fluid to a bottom hole assembly. Aft and forward gripper
assemblies
longitudinally are slidably coupled to the elongate body. The aft and forward
gripper
assemblies are preferably hydraulically actuated for selectively engaging an
inner surface of
the borehole. Aft and forward propulsion assemblies are provided for advancing
the body
through the borehole relative to the aft and forward gripper assemblies,
respectively. A
gripper control valve is provided for directing pressurized fluid to the aft
and forward gripper
assemblies. The gripper control valve preferably has a first position for
directing pressurized
fluid to the aft gripper assembly and a second position for directing
pressurized fluid to the
forward gripper assembly. A propulsion control valve is also disposed within
the body and
has a first position for directing pressurized fluid to the aft propulsion
assembly and a second
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position for directing pressurized fluid to the forward propulsion assembly. A
supply line
provides pressurized fluid from a supply source at a location on the surface
to the gripper
control valve and the gripper control valve. A pressure relief valve is
disposed within said
body of the tractor for regulating fluid pressure in the internal passage. The
pressure relief
valve also regulates the pressure of the fluid entering through the valve
system of the tractor.
In one variation, the valve system may include a start-stop valve which
prevent fluid from
entering the gripper control valve and propulsion control valve. The outlet
from the start-
stop valve may be used to pilot the pressure relief valve, thereby providing a
mechanism for
turning off the pressure relief valve when desired.
[0024] In yet another aspect, a tractor for moving a component through a
borehole
comprises an elongate body formed with an internal passage extending
longitudinally
therethrough. Aft and forward gripper assemblies are slidably coupled to the
elongate body.
The aft and forward gripper assemblies are preferably hydraulically actuated
for selectively
engaging an inner surface of the borehole. Aft and forward propulsion
assemblies are
adapted for advancing said body through the borehole relative to the aft and
forward gripper
assemblies, respectively. A hydraulic valve system is housed within the
elongate body and is
configured for receiving a portion of the pressurized fluid from the internal
passage and
directing the fluid to the aft or forward gripper assembly in a desired
sequence for effecting
movement of the tractor through the borehole. A pressure relief valve is
provided for
limiting fluid pressure within the internal passage and the hydraulic valve
system, wherein
the pressure relief valve is adapted to vent fluid from the internal passage
to an annulus when
the fluid pressure in the internal passage exceeds a pre-selected threshold. A
first fluid path
extends from said internal passage to the hydraulic valve system. A second
fluid path
extends from the internal passage to the pressure relief valve.
[0025] In still another aspect, an apparatus for moving through a borehole
comprises an elongate body formed with an internal passage extending
longitudinally
therethrough. Aft and forward gripper assemblies are slidably coupled to the
elongate body.
The aft and forward gripper assemblies are preferably hydraulically actuated
for selectively
engaging an inner surface of the borehole. Aft and forward propulsion
assemblies are
adapted for advancing said body through the borehole relative to the aft and
forward gripper
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assemblies, respectively. A hydraulic valve system is housed within the
elongate body and is
configured for receiving a portion of the pressurized fluid from the internal
passage and
directing the fluid to the aft or forward gripper assembly in a desired
sequence for effecting
movement of the tractor through the borehole. A pressure relief valve is
provided for limiting
fluid pressure within the internal passage and the hydraulic valve system,
wherein the
pressure relief valve is adapted to vent fluid from the internal passage to an
annulus when the
fluid pressure in the internal passage exceeds a pre-selected threshold. A
first fluid path
extends from said internal passage to the hydraulic valve system. A second
fluid path
extends from the internal passage to the pressure relief valve.
10025a1 In accordance with an aspect of the present invention there is
provided a
tractor for moving a component through a borehole, comprising:
an elongate body;
aft and forward gripper assemblies longitudinally movably engaged with said
body, said aft and forward gripper assemblies each being hydraulically
actuated and
defining engagement surfaces configured to selectively engage an inner surface
of the
borehole;
aft and forward propulsion assemblies configured to advance said body
through the borehole relative to said aft and forward gripper assemblies,
respectively;
a gripper control valve having a first position in which said gripper control
valve directs pressurized fluid to said aft gripper assembly and a second
position in
which said gripper control valve directs pressurized fluid to said forward
gripper
assembly; and
aft and forward mechanically actuated valves positioned along said body and
configured to detect advancement of said body relative to said aft or forward
gripper
assembly, respectively;
wherein said aft and forward mechanically actuated valves are in fluid
communication with said gripper control valve such that fluid pressure causes
said
gripper control valve to change positions after said body has completed an
advancement stroke through the borehole relative to said aft or forward
gripper
assembly.
[0025b] In accordance with a further aspect of the present invention there is
provided
a tractor for moving a component through a borehole, comprising:
an elongate body;
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aft and forward gripper assemblies longitudinally movably engaged with said
body, said aft and forward gripper assemblies being hydraulically actuated for
selectively
engaging an inner surface of the borehole;
aft and forward propulsion assemblies adapted for advancing said body
through the borehole relative to said aft and forward gripper assemblies,
respectively;
a gripper control valve, said gripper control valve having a first position in
which said gripper control valve directs pressurized fluid to said aft gripper
assembly and a
second position in which said gripper control valve directs pressurized fluid
to said forward
gripper assembly; and
a means for detecting advancement of said body relative to said aft or forward
gripper assembly;
wherein said means for detecting advancement of said body causes said
gripper control valve to change positions after said body has completed an
advancement
stroke through the borehole relative to said aft or forward gripper assembly.
[0025c] In accordance with a further aspect of the present invention there is
provided
a method of moving a component through a borehole, comprising:
providing a tractor having an elongate body, aft and forward gripper
assemblies longitudinally movably engaged with said body, said aft and forward
gripper
assemblies being hydraulically actuated for selectively engaging an inner
surface of the
borehole, and aft and forward propulsion cylinders adapted for advancing said
body through
the borehole relative to said aft and forward gripper assemblies,
respectively;
attaching the component to the elongate body;
conveying a pressurized fluid through a supply line to said elongate body for
powering movement of the tractor;
conveying pressurized fluid through said elongate body to the component;
directing a portion of the pressurized fluid through a valve system in the
elongate body for selectively supplying fluid to said aft and forward gripper
assemblies and
said aft and forward propulsion assemblies for producing movement of the
tractor; and
mechanically detecting the completion of an advancement of said elongate
body relative to said aft and forward propulsion cylinders for causing said
aft and forward
gripper assemblies to alternately engage the inner surface in a desired
operational sequence.
[0026] These and other embodiments are intended to be within the scope of the
invention disclosed herein. These and other embodiments of the present
invention will
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become readily apparent to those skilled in the art from the following
detailed description of
the preferred embodiments having reference to the attached figures, the
invention not being
limited to any particular preferred embodiment(s) disclosed.
Brief Description of the Drawings
[0027] Figure 1 is a schematic diagram of the major components of one
embodiment
of a tractor of the present invention, utilized in conjunction with a coiled
tubing system;
[0028] Figure 2 is a front perspective view of a preferred embodiment of the
tractor;
[0029] Figure 3 is a schematic diagram illustrating a preferred embodiment of
a valve
control assembly for use with the tractor;
[0030] Figure 4 is a longitudinal sectional view illustrating a preferred
embodiment of
a pressure relief valve;
[0031] Figure 5 is an exploded view illustrating the components of a preferred
embodiment of a start-stop valve;
[0032] Figure 6 is a longitudinal sectional view illustrating a preferred
embodiment of
a vent valve assembly;
[0033] Figures 7A and 7B are exploded views of a shaft assembly for use with
the
tractor;
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[0034] Figure 8 is a longitudinal sectional view illustrating a preferred
embodiment of a piston poppet valve integrated into a piston;
[0035] Figure 9 is an exploded view of the central housing of the valve
control
assembly;
[0036] Figure 10 is an exploded view of the transition regions located at the
aft
and forward ends of the valve control assembly;
[0037] Figure 11 is a schematic diagram illustrating another preferred
embodiment of a valve control assembly for use with the reversible tractor;
[0038] Figure 12 is a perspective view of a gripper assembly having rollers
secured to its toes, shown in a retracted or non-gripping position;
[0039] Figure 13 is a longitudinal cross-sectional view of a gripper assembly
having rollers secured to its toes, shown in an actuated or gripping position;
[0040] Figure 14 is a perspective partial cut-away view of the gripper
assembly of
Figure 12;
[0041] Figure 15 is an exploded view of one set of rollers for a toe of the
gripper
assembly of Figure 14;
[0042] Figure 16 is a perspective view of a gripper assembly having rollers
secured to its slider element;
[0043] Figure 17 is a longitudinal cross-sectional view of a gripper assembly
having rollers secured to its slider element;
[0044] Figure 18 is a perspective view of a retracted gripper assembly having
toggles for causing radial displacement of the toes; and
[0045] Figure 19 is a longitudinal cross-sectional view of the gripper
assembly of
Figure 18, shown in an actuated or gripping position.
Detailed Description of the Preferred Embodiment
[0046] Figure 1 is a schematic diagram illustrating a hydraulic tractor 100
during
use for moving equipment within a passage. The tractor is shown being used in
conjunction
with a coiled tubing drilling system 20 and adjoining downhole equipment 32.
The coiled
tubing drilling system 20 may include a power supply 22, tubing reel 24,
tubing guide 26,
tubing injector 28, and coiled tubing 30, all of which are well known in the
art. The tractor
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100 is configured to move within a borehole having an inner surface 42. An
annulus 40 is
provided in the space between the outer surface of the tractor 100 and the
inner surface 42 of
the borehole.
[0047] The downhole equipment 32 may include various types of equipment that
the tractor 100 is designed to move within the passage. For example, the
equipment 32 may
comprise a perforation gun assembly, an acidizing assembly, a sandwashing
assembly, a bore
plug setting assembly, an E-line, a logging assembly, a bore casing assembly,
a measurement
while drilling (MWD) assembly, or a fishing tool. Alternatively, the equipment
32 may
comprise a combination of these items. If the tractor 100 is used for
drilling, the equipment
32 will preferably include an MWD system 34, a downhole motor 36, and a drill
bit 38, all of
which are also known in the art. Of course, the downhole equipment 32 may
include many
other types of equipment for non-drilling applications, such as intervention
and completion
applications. While the equipment 32 is illustrated on the forward end of the
tractor, in
alternative configurations, the downhole equipment may be connected aft and/or
forward of ,
the tractor.
[0048] It will be appreciated by those skilled in the art that a hydraulic
tractor of
the type shown may be used to move a wide variety of tools and equipment
within a borehole
or other passage. For example, the tractor can be utilized for well completion
and production
work, pipeline installation and maintenance, laying and movement of
communication lines,
well logging activities, washing and acidizing of sands and solids, retrieval
of tools and
debris, and the like. Also, while preferred for intervention operations, the
tractor may also be
used for drilling applications, including petroleum drilling and mineral
deposit drilling. The
tractor can be used in conjunction with different types of drilling equipment,
including rotary
drilling equipment and coiled tubing equipment.
[0049] One of ordinary skill in the art will understand that oil and gas well
completion typically requires that the reservoir be logged using a variety of
sensors. These
sensors may operate using resistivity, radioactivity, acoustics, and the like.
Other logging
activities include measurement of formation dip and borehole geometry,
formation sampling,
and production logging. With the help of a tractor, these completion
activities can be
accomplished in a variety of inclined and horizontal boreholes. For instance,
the tractor can
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deliver these various types of logging sensors to regions of interest. The
tractor can either
place the sensors in the desired location, or it can idle in a stationary
position to allow the
measurements to be taken at the desired locations. The tractor can also be
used to retrieve the
sensors from the well.
[0050] Examples of production work that can be performed with a hydraulic
tractor include sands and solids washing and acidizing. It is known that wells
sometimes
become clogged with sand, hydrocarbon debris, and other solids that prevent
the free flow of
oil through the borehole. To remove this debris, specially designed washing
tools are
delivered to the region and fluid is injected to wash the region. The fluid
and debris then
return to the surface. Such tools include acid washing tools. These washing
tools can be
delivered to the region of interest for performance of washing activity and
then returned to
the ground surface by a preferred embodiment of the tractor of the invention.
[0051] In another example, a hydraulic tractor can be used to retrieve
objects,
such as, for example, damaged equipment and debris, from the borehole.
Equipment may
become separated from the drill string, or objects may fall into the borehole.
These objects
must be retrieved, or the borehole must be abandoned and plugged. Because
abandonment
and plugging of a borehole is very expensive, retrieval of the object is
usually preferred if
possible. A variety of retrieval tools known to the industry are available to
capture these lost
objects. In use, the tractor is used to transport retrieving tools to the
appropriate location,
retrieve the object, and then return the retrieved object to the surface.
[0052] In yet another example, a hydraulic tractor can be used for coiled
tubing
completions. As known in the art, continuous-completion drill string
deployment is
becoming increasingly important in areas where it is undesirable to damage
sensitive
formations in order to run production tubing. These operations require the
installation and
retrieval of fully assembled completion drill string in boreholes with surface
pressure. The
tractor can be used in conjunction with the deployment of conventional
velocity string and
simple primary production tubing installations. The tractor can also be used
with the
deployment of artificial lift devices such as gas lift and downhole flow
control devices.
[0053] In yet another example, a tractor can be used to service plugged
pipelines
or other similar passages. Frequently, pipelines are difficult to service due
to physical
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constraints such as location in deep water or proximity to metropolitan areas.
Various types
of cleaning devices are currently available for cleaning pipelines. These
various types of
cleaning tools can be attached to the tractor so that the cleaning tools can
be moved within
the pipeline.
[0054] In still another example, a tractor can be used to move communication
lines or equipment within a passage. Frequently, it is desirable to run or
move various types
of cables or communication lines through various types of conduits. The
tractor can move
these cables to the desired location within a passage.
Overview of Tractor Components
[0055] Figure 2 illustrates one preferred embodiment of the tractor 100, shown
with the aft end on the left and the forward end on the right. The tractor 100
generally
comprises a central control assembly 102, an aft gripper assembly 104, a
forward gripper
assembly 106, an aft propulsion cylinder 108, a forward propulsion cylinder
114, an aft shaft
assembly 118, a forward shaft assembly 124, tool joint assemblies 116 and 129,
and flex
joints or adapters 120 and 128. The tool joint assembly 116 is disposed along
the aft end of
the aft shaft assembly 118 for connecting the drill string (e.g., coiled
tubing) to the aft shaft
assembly 118. The aft gripper assembly 104, aft propulsion cylinder 108, and
flex joint 120
are assembled together end-to-end and are all axially slidably engaged with
the aft shaft
assembly 118. Similarly, the forward gripper assembly 106, forward propulsion
cylinders
114, and flex joint 128 are assembled together end-to-end and are axially
slidably engaged
with the forward shaft assembly 124. The tool joint assembly 129 is preferably
configured
for coupling the tractor 100 to downhole equipment 32, as shown in Figure 1.
The aft shaft
assembly 118, the control assembly 102 and the forward shaft assembly 124 are
axially fixed
with respect to one another and are generally referred to herein as the body
of the tractor.
Conventionally, the body of the tractor is axially fixed with respect to the
drill string and the
downhole tools.
[0056] The gripper assemblies 104, 106 and propulsion cylinders 108, 114 are
axially slidable along the body for providing the tractor 100 with the
capability of pulling
and/or pushing downhole equipment 32 of various weights through the borehole
(or passage).
In one embodiment, the tractor 100 is capable of pulling and/or pushing a
total weight of 100
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lbs, in addition to the weight of the tractor itself. In various other
embodiments, the tractor is
capable of pulling and/or pushing a total weight of 500, 3000, and 15,000 lbs.
[00571 In order to prevent damage to a surrounding formation or casing wall,
the
gripper assemblies 104, 106 are preferably constructed to limit the radial
gripping load (i.e.,
force) exerted on a surface. In one embodiment, the gripper assemblies 104,
106 exert no
more than 25 psi on a surface surrounding the tractor. This embodiment is
particularly useful
in softer formations, such as gumbo. In various other embodiments, the gripper
assemblies
104, 106 exert no more than 100, 3000, and 50,000 psi on a surface surrounding
the tractor.
At radial gripping loads of 50,000 psi or less, the tractor generally can be
used safely in steel
tube casing.
[0058] The tractor 100 preferably receives pressurized operating fluid from a
supply source at the surface. A supply line extends down from the surface and
passes
through an internal passage in the tractor for supplying operating fluid to
the downhole
equipment. As the operating fluid passes through the internal passage, a
portiori of the
operating fluid is diverted into the control assembly 102 for providing
hydraulic power to the
tractor. More particularly, the control assembly 102 houses a valve system
that distributes
operating fluid to and from the gripper assemblies 104, 106 and the propulsion
cylinders 108,
114 for controlling tractor movement. Preferred embodiments of the control
assembly and
the valve system are described in more detail below. Using the specification
and figures of
the present application along with the principles of design and space
management known to
those skilled in the art through Applicant's co-owned U.S. Patent No.
6,347,674 and U.S.
Patent No. 6,679,341, one of ordinary skill in the art will understand how to
build a tractor
having an improved valve system as described herein.
[00591 The tractor 100 can be any desirable length, but for oilfield
applications
the length is typically approximately 25 to 35 feet. The maximum diameter of
the tractor will
vary with the size of the hole, thrust requirements, and the restrictions that
the tractor must
pass through. The gripper assemblies 104, 106 can be designed to operate
within boreholes
of various sizes, but typically are configured to expand to a diameter of 3.75
to 7.0 inches.
[0060] The flex adapters 120 and 128 are preferably hollow structural members
that provide a region of reduced flexural rigidity (i.e., increased
flexibility). This region of
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reduced flexural rigidity facilitates the tractor's ability to negotiate sharp
turns. In one
preferred embodiment, the adapters are fowled of a relatively low modulus
material such as
Copper Beryllium (CuBe) and/or Titanium. Occasionally, there are applications
that require
the use of non-magnetic materials for the tractor. Otherwise, depending on the
required
turning capability of the tractor and resultant stresses, various stainless
steels may be used in
many areas of the tractor.
[0061] The tool joint assembly 116 preferably couples the aft end of the aft
shaft
assembly 118 to a coiled tubing drill string, preferably via a threaded
connection. As
discussed above, downhole equipment may also be placed at the aft end of the
tractor,
connected to the, tool joint assembly 116. However, in a typical operation,
the tool joint
assembly 129 will be coupled to downhole equipment. The interface threads of
the tool joint
assemblies are preferably API threads or proprietary threads (such as Hydril
casing threads).
The tool joint assemblies can be prepared with conventional equipment (tongs)
to a specified
torque (e.g., 1000-3000 ft-lbs). The tool joint assemblies can be formed from
a variety of
materials, including CuBe, steel, and other metals.
[0062] As discussed above, the aft and forward shaft assemblies 118 and 124,
along with the control assembly 102, form the body of the tractor 100. The aft
and forward
shaft assemblies 118 and 124 are each preferably formed with a segment having
an expanded
diameter that forms a piston. Preferably, the aft and forward pistons have
outer diameters
that are substantially similar to the inner diameters of the aft and forward
propulsion
cylinders 104, 108. The aft and forward pistons are slidably housed within the
aft and
forward propulsion cylinders 104, 108 and separate the interiors of each
cylinder into a power
chamber and a reset chamber. Accordingly, the aft and forward propulsion
cylinder 104, 108
form, at least in part, aft and forward propulsion assemblies that are
configured for advancing
the tractor body through the borehole relative to the aft and forward gripper
assemblies.
Although preferred embodiments of the tractor utilize aft and forward
propulsion cylinders, it
will be appreciated that a wide variety of aft and forward propulsion
assemblies may be used
for producing advancement of the tractor body.
[0063] As will be described in more detail below, pressurized fluid is
alternately
directed to the power chamber in the aft or forward propulsion cylinder for
propelling the
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body through the borehole when the aft or forward gripper assembly is anchored
to the inner
surface. Pressurized fluid is alternately directed to the reset chamber in the
aft or forward
propulsion cylinder for resetting the position of the aft or forward gripper
assembly relative to
the body (i.e., in preparation for another power stroke) while the aft or
forward gripper
assembly is disengaged. Accordingly, the tractor steps through the borehole by
thrusting
itself forward relative to the aft or forward gripper assembly.
[0064) The aft and forward shaft assemblies 118 and 124 may be constructed
from any suitable material. In one preferred embodiment, the shafts are formed
from a
flexible material, such as CuBe, in order to permit the tractor 100 to
negotiate sharper turns.
In other embodiments CuBe is not used, as it is relatively expensive. Other
acceptable
materials include Titanium and steel (when low flexibility is sufficient). In
a preferred
configuration, each shaft includes a central internal bore which together
form, in part, the
internal passage for the flow of pressurized operating fluid to the downhole
equipment and to
the control assembly 102. The bore in each shaft assembly preferably extends
the entire
length of the shaft. Each shaft may also include numerous other passages for
the flow of
fluid to the gripper assemblies and propulsion cylinders. These fluid passages
range in length
and are equal to or less than the overall length of the tractor. Multiple
fluid passages can be
drilled in the shaft for the same function, such as to feed a single
propulsion chamber.
Preferably, the bore and the other internal fluid passages are arranged so as
to minimize stress
and provide sufficient space and strength for other design features, such as
the pistons
slidably housed within the cylinders. Each shaft is preferably provided with
threads on one
end for connection to the tool joint assemblies 116 and 129, and with a flange
on the other
end to allow bolting to the control assembly 102.
[0065] It will be appreciated by those skilled in the art that the tractor 100
described herein is particularly well adapted for intervention applications.
While intervention
tractors can be made any size, they are typically operated within 5-inch or 7-
inch casing. The
inside diameter of a 5-inch casing can range from 4.5 to 4.8 inches. The
inside diameter of a
7-inch casing can range from 5.8 to 6.4 inches. The primary structural
components of the
tractor 100 are the shafts 118 and 124. In a preferred embodiment, the shafts
have an outside
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diameter of 1.75 inches and an inside bore diameter of 0.8 inches. The
remaining fluid
passages of the shafts are preferably smaller. The pistons can have varying
outside diameters.
[0066] For intervention applications, the tractor 100 described herein is very
reliable and efficient. Prior art intervention tools that utilize rotary drill
strings are as much
as 150% more expensive than the illustrated tractor 100 used with coiled
tubing equipment.
In addition, the tractor 100 is more time-conservative, as the longer rig-up
time associated
with rotary equipment is avoided. Furthermore, the use of coiled tubing is
particularly
advantageous when operating perforation guns.
[0067] The tractor 100 is at least in part hydraulically powered by the
operating
fluid pumped down the drill string, such as brine, sea water, drilling mud, or
other hydraulic
fluid. As discussed above, the same fluid supply line that operates the
downhole equipment
32 (see Figure 1) also preferably powers the tractor. This avoids the need to
provide
additional fluid channels in the tool. Preferably, liquid brine or sea water
is used in an open
system. Alternatively, fluid may be used in a closed system, if desired.
Referring again to
Figure 1, in operation, operating fluid flows from the drill string 30 through
the tractor 100
and down to the downhole equipment 32.
Preferred Configuration of Valve System
[0068] The control assembly 102 preferably houses a plurality of hydraulically
and/or electrically controlled valves configured for selectively controlling
the flow of
operating fluid to and from the gripper assemblies 104 and 106 and to and from
the
propulsion cylinders 108 and 114 for producing tractor movement. It will be
appreciated that
the term "valve" as used herein is a broad term that generally refers to any
device capable of
regulating or controlling the distribution of fluid. Preferably, the valves
contained within the
control assembly 102 are entirely hydraulically controlled. Hydraulically
controlled tractors
are generally more desirable than electrically controlled valves, particularly
for intervention
applications, because they are less expensive and are generally safer to use
in combination
with certain types of downhole equipment, such as perforation guns. In
addition,
hydraulically controlled valves eliminate the need for electronic components,
thereby saving
space, which allows for larger internal flow passages. As a result, tractors
using hydraulically
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controlled valves are generally faster and more powerful than tractors using
electrically
controlled valves.
[0069] Preferred embodiments of the present invention disclose an improved
valve system that provides a significant improvement over valve systems known
heretofore.
For example, embodiments of the improved valve system disclosed herein provide
much
greater control of tractor movement as compared with existing hydraulically
controlled
tractors. The improved valve system also provides improved regulation of fluid
pressure and
allows the tractor to operate effectively within a larger zone of parameters.
Furthermore, the
improved valve system is configured to improve the reliability and extend the
life of the
internal components, thereby saving time and reducing costs.
[0070] Referring now to Figure 3, for purposes of illustration, one preferred
embodiment of an improved valve system 300 is schematically shown. The portion
of the
valve system 300 housed within the control assembly 102 generally includes a
start/stop
valve 308, a propulsion control (or main sequence) valve 310, a gripper
control (or pilot)
valve 312, an aft sequence valve 314, a forward sequence valve 316, an aft
vent valve 318, a
forward vent valve 320 and a pressure reducing valve 326. In addition, a
pressure relief valve
306 is provided for regulating the supply pressure in the internal passage.
The pressure relief
valve 306 is preferably included in the control assembly; however, the
pressure relief valve
may be located elsewhere, such as on the surface.
[0071] To effectively control the sequence of valve operation, it is desirable
to
accurately detect when the tractor body has completed an advancement stroke
relative to the
anchored aft or forward gripper assembly. Due to pressure fluctuations in the
valve system,
the use of pressure-responsive valves is not always effective for detecting
and signaling the
end of an advancement stroke. Accordingly, one embodiment of an improved valve
system
for an intervention tractor incorporates at least one mechanically actuated
valve mechanism
into the propulsion control assembly for quickly and accurately detecting and
signaling the
completion of a piston stroke.
[0072] In one preferred embodiment, the mechanically actuated valve is a
poppet
valve that is integrated into the piston. As the piston completes its stroke,
the poppet valve
(or other mechanically actuated valve) is mechanically actuated to open a seal
and thereby
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allow fluid to pass through a passage. As a result, the outlet flow from the
poppet valve may
be used to actuate or pilot another valve. The use of a poppet valve to detect
the end of the
piston stroke, rather than a pressure-responsive valve, improves the
efficiency and reliability
of the hydraulic control assembly.
[0073] Figure 3 schematically illustrates an aft piston poppet valve 322 and a
forward piston poppet valve 324, each of which cooperates with the valves
housed within the
control assembly 102 to control tractor movement. As will be described in more
detail
below, the aft and forward piston poppet valves 322, 324 are preferably
integrated into the aft
and forward pistons on the aft and forward shaft assemblies. In preferred
embodiments, the
aft and forward piston poppet valves 322, 324 are preferably substantially
identical in
structure and operation.
Pressure Relief Valve
[00741 With continued reference to Figure 3, one embodiment of an improved
valve system is illustrated wherein the tractor receives pressurized fluid
from the surface
through a supply line 302. As the fluid enters the internal passage in the
tractor body, a
portion of the fluid from the supply line 302 is diverted to a pressure relief
valve 306 along
flow path 352. Also, a portion of the pressurized fluid from the supply line
302 is diverted to
the start-stop valve 308 along flow path 350. The remaining pressurized fluid
passes through
the internal passage to the downhole equipment along flow path 303.
[0075] In the illustrated embodiment, the pressure relief valve 306 regulates
the
fluid pressure in the supply line 302. As a result, the pressure relief valve
306 also regulates
the pressure of the "working" fluid that enters the start-stop valve 308 along
flow path 350.
The working fluid provides hydraulic power for producing movement of the
tractor.
Accordingly, it will be appreciated that the pressure relief valve regulates
the pressure of the
fluid entering the gripper assemblies 104, 106 and the propulsion cylinders
108, 114 (see
Figure 2). Still further, the pressure relief valve 306 regulates the pressure
of the fluid that is
supplied to the downhole equipment along flow path 303. Although the pressure
relief valve
is desirably housed within the control assembly (as shown in Figure 3), the
pressure relief
valve may also be provided in other locations, such as along other portions of
the tractor or
on the surface.
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[0076] In a preferred embodiment, the pressure relief valve 306 has a variable
orifice that opens as a function of the fluid pressure. If the pressure in the
supply line 302
increases rapidly, the variable orifice will open wider to vent more fluid. As
a result, the
pressure relief valve 306 responds quickly and fluid in the supply line 302
may be
advantageously maintained at a regulated pressure.
[0077] During use, when the differential pressure between the supply line 302
and
the annulus 40 increases above a pre-selected threshold pressure, the pressure
relief valve 306
opens to vent fluid to the annulus 40, thereby lowering the pressure in the
supply line. In
various embodiments, the pre-selected threshold pressure is desirably at least
600 psid, 800
psid, 900 psid, 1100 psid, 1200 psid, 1400 psid and 1600 psid. In a preferred
embodiment,
the pre-selected threshold pressure is 1400 psid. Other pre-selected threshold
pressures may
also be desirable in some circumstances. The pressure relief valve is
preferably sized for
diverting fluid to the annulus 40 at a maximum rate of up to 20 to 25 gallons
per minute. In
preferred embodiments, the pressure relief valve 306 may be selectively
rendered non-
operational (i.e., turned off) when it is desirable to supply high-pressure
fluid to the downhole
equipment for certain operations.
[0078] The pressure relief valve 306 is particularly advantageous for use with
valve systems that use a relatively large percentage of the flow through the
supply line 302
for powering the tractor. Valve systems that use a large percentage of the
system flow
typically produce large pressure fluctuations in the system pressure during
operation. For
example, when the tractor completes a power stroke, the shifting in valve
positions may
temporarily stop the flow of fluid through the valve system. Without the
pressure relief
valve, the reduction in flow could produce a large swing in system pressure
that could
produce surges in motion, valve instability or stalling of the tractor.
Accordingly, those
skilled in the art will appreciate that the embodiments of the pressure relief
valve 306
disclosed herein provide a significant advancement in the field of tractors.
[0079] With reference now to Figure 4, a cross-sectional view of the internal
components 400 of one preferred embodiment of a pressure relief valve is
shown. The
pressure-relief valve is preferably a pilot operated, spring return, two-
position valve that is
piloted by the pressure in the fluid path 354 from the start-stop valve 308
(as illustrated in
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Figure 3). The internal components 400 of the pressure relief valve generally
comprise a
body 402 formed with a hollow interior and a spool 404 slidably housed within
the hollow
interior. First and second inlet ports 430, 432 and first and second outlet
ports 434, 436 are
provided through the body 402 for providing fluid communication with the
hollow interior.
[0080] In the illustrated embodiment, a spring cartridge 414 is coupled to the
left
end of the spool 404 via a ball 412. The spring cartridge 414 and the spool
404 are axially
fixed with respect to each other. The right end of the cartridge 414 is
slidably maintained
within the body 404 by a retainer 410. A coiled spring 422 extends around a
middle portion
of the spring cartridge 414. As illustrated, the left end of the spring 422 is
in contact with a
fixed stop 426, which prevents movement of the spring 422 away from the body
402 (to the
left in Figure 4). The spring 422 is preferably compressed between the fixed
stop 426 and a
flange 428 on the cartridge 414. The spring 404 provides a biasing force that
urges the
cartridge 414 and the spool 404 away from the body 402 (to the right in Figure
4).
Preferably, the pressure relief valve is configured such that the biasing
force varies according
to the pressure in the annulus such that the pressure relief valve operates
off a differential
pressure between the supply line and the annulus. A stop 406 is provided
within the housing
402 for limiting the translation of the spool to the right. A pilot assembly
416 is attached to
the right end of the body 402 opposite the spring 404. A pilot stem 418 is
slidably housed
within the pilot assembly 416 such that the left end of the stem 418 is in
contact with the
right end of the spool 404.
[0081] Figure 4 shows the internal components 400 of the pressure relief valve
in
an open position such that pressurized fluid may pass therethrough. In
operation, pressurized
fluid enters the pilot assembly 416 through a pilot port 420. The fluid passes
into a chamber
424 wherein the fluid pressure acts on one end of the pilot stem 418. When the
spool is in
contact with the stop 406, the inlet ports 430, 432 are blocked such that no
fluid passes
through the pressure relief valve. However, when the fluid pressure is
sufficient to overcome
the biasing force of the spring, the stem 418 moves to the left, thereby
causing the spool 404
to translate to the left through the body 402. As the spool 404 moves to the
left, the spring
422 is compressed. As the spool 404 translates to the left relative to the
body 402, the inlet
ports 430, 432 open to allow fluid to enter into the interior chamber of the
body. The fluid
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passes around the spool and exits through the outlet ports 434, 436,
preferably to the annulus.
Due to the configuration of the spool and inlet ports, the first and second
inlet ports 430, 432
open further as the spool moves further to the left to allow more fluid to
pass therethrough.
In a preferred configuration, the first and second inlet ports 430, 432 are
staggered such that
the first inlet port 430 opens before the second inlet port 432. Accordingly,
the pressure
relief valve vents only a small amount of fluid when the fluid pressure is
only slightly above
the threshold. However, when the fluid pressure is significantly larger than
the threshold
pressure, both the first and second inlet ports 430, 432 are open for allowing
a large volume
of fluid to pass.
[0082] With reference again to Figure 3, the pressure relief valve 306
advantageously provides the ability to regulate the pressure of the fluid that
is supplied to
both the valve system (via flow path 350) and to the downhole equipment (via
flow path
303). In one advantage of this arrangement, the working fluid entering the
valve system is
regulated independently of the tractor load and speed. In another advantage,
the valve system
is protected from large pressure fluctuations that can damage the internal
hardware. In
another advantage, the tractor is prevented from surging or stalling due to
large pressure
fluctuations in the supply line. Still further, because the pressure in flow
path 303 is
regulated, the tractor has improved compatibility with downhole equipment.
Still further, the
regulated pressure allows preferred embodiments of the tractor to be used over
a substantially
greater range of flow rates. The increased range further enhances the
tractor's ability to be
used with a wide variety of downhole equipment in a various field
applications.
Start/Stop Valve
[0083] With reference again to Figure 3, a portion of the pressurized fluid is
preferably diverted from the supply line 302 (i.e., internal passage) into
flow path 350 for
providing hydraulic power to move the tractor through the borehole.
Preferably, a filter 304
is provided along flow path 350 for removing particles from the fluid. The
removal of large
particles from the fluid protects internal valve system components (e.g.,
valve spools) that are
used for controlling the operation of the tractor.
[0084] As illustrated in Figure 3, the pressurized fluid in flow path 350
enters the
start-stop valve 308. The start/stop valve 308 is preferably a pilot operated,
spring return,
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indexed, two position, two-way valve that is piloted by the pressure of the
fluid in flow path
350. When in a closed position, the start/stop valve 308 prevents fluid from
passing through
the valve system, thereby rendering the tractor non-operational. When in an
open position,
the start/stop valve 308 allows pressurized fluid to pass through to flow path
354. The
pressurized fluid in flow path 354 flows to the propulsion control valve 310
and the pressure
reducing valve 326, thereby allowing for tractor operation. The start-stop
valve 308 is
configured to move into the open position when the fluid pressure in flow path
350 (i.e., the
supply line) exceeds a pre-selected threshold pressure. However, the start-
stop valve 308 is
preferably indexed such that the valve may be selectively prevented from
opening when the
fluid pressure exceeds the pre-selected threshold.
[0085] With reference now to Figure 5, an exploded view of one preferred
embodiment of a start-stop valve 308 is shown. The primary components of the
start-stop
valve 308 generally comprise a body 502 formed with a hollow interior and a
spool 506
slidably housed within the hollow interior. The slidable spool 506 is
preferably coupled at a
first end to a spring cartridge 524 via a ball 522. In one embodiment, the
ball 522 is made of
stainless steel. The spool 506 is preferably coupled at a second end to an
index sleeve 510
with a spacer 512 located therebetween. An index guide 508 extends through a
center
portion of the index sleeve 510 and a washer 514 is provided therebetween. The
spool 506,
the index guide 508, and the index sleeve 510 are all slidably housed within
the body 502.
The spring cartridge 524 is preferably coupled to a first end of the body 502
by a slotted
retainer 504. The spring cartridge is configured to urge the spool 506 into
the closed
position. A pilot assembly 520 is preferably coupled to a second end of the
body 502 via a
retainer 518. Under sufficient fluid pressure, the pilot assembly 520
compresses the spring
on the spring cartridge 524 for changing the position of the index sleeve 510
and moving the
spool into the open position.
[0086] During use, as the pressure in the flow path 350 increases above a pre-
selected threshold (e.g., 900 psi), the fluid pressure acts on the pilot
assembly 520, which in
turn causes the index sleeve 510 to rotate about the index guide 508. The
rotational position
of the index sleeve 510 determines whether the start-stop valve 308 opens or
remains closed
as the pressure of the fluid increases above the pre-selected threshold.
Accordingly, the start-
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stop valve 308 provides a mechanism for turning the tractor on and off by
varying the supply
pressure. If the index sleeve 510 is in the off position, a pressure cycle
(e.g., dropping the
pressure to 0 psi and then back up to 900 psi) will change the index sleeve
510 into the on
position. When the index sleeve 510 is in the on position, the spool may slide
within the
hollow interior of the body 502 for opening a passage between the inlet and
outlet ports (not
shown) and thereby allowing fluid to pass through the start-stop valve 308.
More details on
valves having indexed drums can be found in Applicant's U.S. Patent No.
6,679,341.
[0087] With reference again to Figure 3, in preferred embodiments, the
fluid
pressure in the flow path 354 from the start-stop valve 308 is used to pilot
the pressure relief
valve 306. As a result, the pressure relief valve 306 is only operational when
the start-stop
. valve 308 is in the open position. Accordingly, the pressure relief valve
306 is effectively
"turned off' when the index sleeve is in the off position such that the start-
stop valve will not
open regardless of the fluid pressure in flow path 350. This is an important
feature because it
allows the fluid pressure in the internal passage 302, 303 to be increased
above the pressure
threshold of the pressure relief valve. This advantageously allows the
operator to provide
fluid at any pressure to a bottom hole assembly or other downhole equipment
when desired.
Propulsion Control Valve
[0088] As discussed above, when the start/stop valve 308 is open,
pressurized
operating fluid flows through the passage 354 to the propulsion control valve
310. In a
preferred embodiment, the propulsion control valve 310 is a two-position,
sliding-spool
directional flow valve. In a first position, as shown in Figure 3, the spool
of the valve 310
provides a flow path 360 for the flow of fluid to the power chamber of the aft
cylinder, and
also to the reset chamber of the forward cylinder. In the first position, the
valve 310 also
provides a flow path 362 for the flow of fluid from the power chamber of the
forward
cylinder to the annulus 40, and from the reset chamber of the aft cylinder to
the annulus 40.
[0089] The spool of the propulsion control valve 310 also has a second
position,
(e.g., which would be shifted to the left in Figure 3). When the spool of the
valve 310 is in
its second position, the valve 310 provides a flow path 362 for the flow of
fluid to the power
chamber of the forward cylinder, and also to the reset chamber of the aft
cylinder. In the
second position, the valve 310 also provides a flow path 360 for the flow of
fluid from the
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power chamber of the aft cylinder to the annulus 40, and also from the reset
chamber of the
forward cylinder to the annulus 40.
[0090] With continued reference to Figure 3, the spool of the propulsion
control
valve 310 has a first end surface 330 and a second end surface 332. The first
end surface 330
is in fluid communication with the aft gripper assembly along fluid path 364.
The second end
surface 332 is in fluid communication with the forward gripper assembly along
fluid path
366. The first and second end surfaces 330 and 332 of the propulsion control
valve 310 are
configured to receive respective fluid pressure forces that act on the valve
spool. The first
end surface 330 receives a pressure force from the fluid in the aft gripper
assembly that tends
to move the spool of the valve 310 toward its first position, (e.g., to the
right as shown in
Figure 3). The second end surface 332 receives a pressure force from the fluid
in the forward
gripper assembly that tends to move the spool toward its second position,
(e.g., which would
be shifted to the left in Figure 3).
Aft and Forward Sequence Valves
[0091] With continued reference to Figure 3, an aft sequence valve 314 is
preferably provided along the fluid path 364 extending from the aft gripper
assembly to the
first end surface 330. In addition, a forward sequence valve 316 is preferably
provided along
the fluid path 366 extending from the forward gripper assembly to the second
end surface
332.
[0092] Referring only to the aft sequence valve 314 for purposes of
illustration,
the aft sequence valve 314 opens when the fluid pressure in the flow path 364
exceeds a pre-
selected threshold (e.g., 900 psid). When the aft sequence valve 314 is open,
the fluid
pressure in flow path 364 acts on the first end surface 330 for urging the
propulsion control
valve to the right as shown in Figure 3. When the fluid pressure in the flow
path 364 is
below the pre-selected threshold, the aft sequence valve 314 is closed such
that the fluid
pressure in flow path 364 cannot act on the first end surface 330. In
addition, when the aft
sequence valve 314 is closed, and the fluid in the portion of the flow path
between the aft
sequence valve 314 and the propulsion control valve 310 is vented to the
annulus 40, thereby
removing any remaining force acting on the first end surface 330. It will be
understood that
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the forward sequence valve 316 preferably operates in the same manner as the
aft sequence
valve 314.
[0093] The aft and forward sequence valves 314, 316 used in combination with
the propulsion control valve 310 significantly improve the efficiency of the
tractor operation.
In particular, the aft and forward sequence valves 314, 316 provide a reliable
and constant
pressure threshold in the flow paths 364, 366 that must be overcome in order
to pilot the
propulsion control valve 310. Because the aft and forward sequence valves 314,
316 provide
a reliable pressure threshold, the fluid flow rates through the valve system
may be increased
substantially without having an adverse effect on the operation of the
tractor. As a result, the
gripper assemblies may be actuated more quickly, which in turn decreases the
dwell time
(i.e., the delay time between power strokes) and substantially increases the
overall tractor
speed through the borehole. Furthermore, due to the reliability of the
tractor, the educational
and skill requirements for service personnel are reduced, which thereby
reduces operational
costs.
[0094] With reference now to Figure 6, the primary components 600 of one
preferred embodiment of an aft sequence valve (see element 314 in Figure 3)
are shown in a
longitudinal sectional view. The components 600 of the aft sequence valve are
preferably
identical to the components of the forward sequence valve and therefore only
the components
of the aft sequence valve will be described. The illustrated components 600 of
the aft
sequence valve generally comprises a body 602 formed with a hollow interior
and a spool
610 slidably housed within the hollow interior. An inlet port 620, a working
port 622 and an
exhaust port 624 are provided through the body 602 for communication with the
hollow
interior. A bore 632 is foimed through the spool 610. The slidable spool 610
is preferably
coupled to a spring guide 614 via a ball 612. In one embodiment, the ball 612
is made of
silicon-nitride. A spring 616 extends around the guide 614 and contacts a stop
618 at one
end. A plug 604 at the other end of the body 602 provides a fluid tight seal.
The plug 604
and stop 618 are preferably coupled to the body 602 via a pin or dowel 608.
[0095] During use, pressurized fluid (e.g., from fluid passage 364 as shown in
Figure 3) enters the inlet port 620 of the aft sequence valve. The fluid
enters the annular
region 630 located between the spool 610, the body 602 and the plug 604. The
fluid pressure
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urges the spool 610 to move to the left. At the same time, the spring 616
provides a biasing
force that urges the spool to the right. When the fluid pressure in the
annular region 630
exceeds a pre-selected threshold (e.g., 900 psid), the spool 610 will move to
the left a
sufficient distance such that the bore 632 communicates with the working port
622. As a
result, fluid may pass from the inlet port through the bore 632 and out
through the working
port 622 (e.g. for piloting the propulsion control valve 310 in Figure 3).
When the pressure is
below the threshold, the spool 610 is located hardover to the right, as shown
in Figure 6. In
this position, fluid may travel back through the working port 622, into the
annular region 634
and out through the exhaust port 624 to the annulus. This feature allows fluid
to vent to the
annulus when the fluid in the flow path 364 or 366 (see Figure 3) is not
pressurized.
Pressure Reducing Valve
[0096] With reference again to Figure 3, in a preferred embodiment, the outlet
flow from the start/stop valve 308 along fluid path 354 passes through the
pressure reducing
valve 326 before entering the gripper control valve 312. The pressure reducing
valve 326 is
preferably a direct operating valve that limits the pressure of the operating
fluid in the aft and
forward gripper assemblies, and thus provide a means for preventing possible
damage to the
gripper assembly components.
[0097] When the pressure downstream of the pressure reducing valve 326
increases above a pre-selected threshold (e.g., 1400 psid), the pressure
reducing valve closes
to protect the gripper assemblies from becoming over-pressurized. Thus, the
pressure
reducing valve 326 imposes an upper limit on the pressure in the passage 356
and thereby
prevents over-pressurization of the gripper assemblies by bleeding excess
pressure to the
annulus 40.
Gripper Control Valve
[0098] With continued reference to Figure 3, the gripper control valve 312
directs
fluid to either the aft gripper assembly or the forward gripper assembly. In
the illustrated
embodiment, the gripper control valve 312 is preferably a two-position,
sliding-spool
directional valve that functions in essentially the same manner as the
propulsion control valve
310 described above. For additional details regarding preferred embodiments of
the valves
310 and 312, see Applicant's U.S. Patent No. 6,679,341.
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[0099] The spool of the gripper control valve 312 has a first position (as
shown in
Figure 3) in which the gripper control valve 312 provides a flow path 370 to
the aft gripper
assembly. When the spool of the valve 312 is in its first position, the valve
312 also provides
a flow path 372 for the flow of fluid from the forward gripper assembly to the
annulus 40.
The spool of the gripper control valve 312 also has a second position, not
shown in Figure 3.
In the second position, the gripper control valve 312 provides a flow path 372
to the forward
gripper assembly. When the spool of the valve 312 is in its second position,
the valve also
provides a flow path 370 for the flow of fluid from the aft gripper assembly
to the annulus 40.
[0100] The spool of the gripper control valve 312 has a first end surface 334
and a
second end surface 336. The first end surface 334 is in fluid communication
with the
forward piston poppet valve 324 along flow path 380. The second end surface
336 is in fluid
communication with the aft piston poppet valve 322 along flow path 382. The
first and
second end surfaces 334 and 336 are configured to receive respective fluid
pressures from
flow paths 380 and 382 that act on the spool of the valve. The first end
surface 334 receives
a pressure force from the outlet of the forward piston poppet valve 324 that
tends to move the
spool of the gripper control valve 312 toward its first position, as shown in
Figure 3. The
second end surface 336 receives a pressure force from the outlet of the aft
piston poppet
valve 322 that tends to move the spool toward its second position, which would
be shifted to
the left in Figure 3. The structure and function of preferred embodiments of
the aft and
forward poppet valves 322, 324 are described in more detail below.
Vent Valves
[0101] With continued reference to Figure 3, an aft vent valve 318 is
preferably
provided along the fluid path 382 extending from the aft piston poppet valve
322 to the first
end surface 336 of the gripper control valve 312. In addition, a forward vent
valve 320 is
preferably provided along the fluid path 380 extending from the forward piston
poppet valve
324 to the second end surface 334 of the gripper control valve 312. Similar to
the aft and
forward sequence valves 314, 316 described above, the aft and forward vent
valves 318, 320
each prevents fluid from passing through their respective fluid path unless
the pressure fluid
in the path exceeds a pre-selected threshold. As a result, the aft and forward
vent valves
provide for reliable shifting of the spool in the gripper control valve 312
and further improve
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the timing and efficiency of the valve system. When the pressure drops below
the pre-
selected threshold, the aft and forward vent valves 318, 320 allow the fluid
in the flow paths
between the vent valves and end surfaces to be vented to the annulus 40. In
preferred
embodiments, the structure of the aft and forward vent valves 318, 320 is
substantially
identical to the aft and forward sequence valves 314, 316 described above with
reference to
Figure 6.
Preferred Configurations of Shaft Assemblies / Piston Poppet Valves
[01021 With reference again to Figure 2, aft and forward shaft assemblies 118,
124 are coupled to the aft and forward ends of the control assembly 102. The
aft and foward
shaft assemblies 118, 124, along with the control assembly 102, form the body
of the tractor
100. The aft gripper assembly 104 and aft propulsion cylinder 108 are slidably
coupled to the
aft shaft assembly 118. The forward gripper assembly 106 and forward
propulsion cylinder
114 are slidably coupled to the forward shaft assembly 124.
[01031 With reference now to Figure 7A, for purposes of illustration, an
exploded
view of the aft shaft assembly 118 is shown in combination with the aft
cylinder 108 and aft
tool joint assembly 116. The aft shaft assembly 118 generally includes an
elongate shaft 150
formed with a substantially cylindrical shape. In a preferred embodiment, the
aft cylinder
108 is substantially tubular in shape and is slidably disposed over the shaft
150 such that an
annular region is formed therebetween. The aft cylinder 108 is sealed at the
aft end by the
flex joint 120. The aft cylinder 108 is sealed at the forward end by a gland
seal 704. The aft
cylinder 108 is thus sealed at both ends and slidably houses the aft piston
for providing the aft
propulsion assembly. When fully assembled, a gripper assembly (not shown) is
also slidably
disposed over the shaft 150 and is preferably coupled to the flex joint 120
along the aft end.
[01041 With reference now to Figure 7B, an enlarged view of the aft piston 700
is
shown for purposes of illustration. The aft piston 700 is rigidly connected to
the aft shaft 150
and includes the aft piston poppet valve (see element 322 of Figure 3). The
aft piston 700
slides within the aft cylinder 108 and separates the power chamber from the
retract chamber.
10105] Figure 8 is a longitudinal sectional view illustrating the aft piston
700,
which includes the aft piston poppet valve (see element 322 of Figure 3). With
reference
now to both Figure 7B and Figure 8, the aft piston 700 generally comprises a
flange 708 and
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a hub 710. The flange 708 and hub 710 separate the power and retract chambers
within the
aft cylinder 108. The flange 708 is surrounded by a wear guide 746 and houses
a seat 730.
The seat 730 is maintained in place by an internal retaining ring 748 at the
aft end. A spring
712 is adjacent the seat 730 and extends from the flange 708 into the hub 710.
A stem 714 is
coupled to the spring 712 and is slidably housed within the hub 710. A portion
of the stem
714 extends from an end surface of the hub for contacting the seal gland 704.
The protruding
end of the stem 714 is guided by a stem guide 742, which is supported by an o-
ring 740 and a
retaining ring 744.
[0106] The protruding end of the poppet valve stem 714 is located for
contacting
the seal gland 704, or other inner wall, as the piston reaches the end of the
power stroke. As
the valve stem 714 contacts the seal gland 704, the valve stem slides axially
with respect to
the hub 710. As the stem slides, a seal washer 728 and a valve cap 732 are
displaced from a
valve seat 750 of the piston hub 710. As a result, pressurized fluid from the
power chamber
of the cylinder flows through a gap 716 between the outer diameter of the
piston flange 708
and the inner diameter of the cylinder 108. The fluid continues to flow
through a gap 718
located between the flange 708 and the hub 710, around the valve stem 714, and
through the
piston hub 710. The fluid then flows in a radial direction through a port 722
and then into the
pilot passage 716. The fluid in the pilot passage 716 may then be ported to
the control
assembly for controlling the position of the gripper control valve, as
schematically illustrated
and described above with respect to Figure 3.
[0107] With continued reference to Figures 7B and 8, as the piston 700 moves
away from the seal gland 704, the valve spring 712 applies a biasing force
that reseats the
seal washer 728 onto the valve seat 750 of the piston hub 708. As a result,
the pilot passage
706 becomes sealed from the fluid pressure on both sides of the piston. In an
important
aspect of the above-described embodiment, the presence of pressurized fluid in
the pilot
passage 706 provides a means for accurately detecting and indicating the
completion of the
aft power stroke. This provides a significant advantage over pressure-
responsive valves that
may shift prematurely due to pressure fluctuations.
[0108] As illustrated, the mechanically actuated valve is desirably provided
as a
piston poppet valve. When used with preferred embodiments of the tractor,
piston poppet
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valves have certain advantages over other mechanically actuated valves, such
as, for example,
reliability, small size and reliability. However, in alternative embodiments,
other types of
mechanically actuated valves may also be used for detecting the completion of
a power
stroke. For example, a diaphragm valve may be used to signal the completion of
a power
stroke. The diaphragm valve is mechanically actuated in a manner similar to
that described
above for the poppet valve to detect the completion of a power stroke. In
another preferred
embodiment, a shear valve may be used to signal the completion of the piston
stroke. The
shear valve includes a floating seal that slides to open or close an orifice.
The shear valve
may be mechanically actuated in a manner similar to that described above for
the poppet
valve to detect the completion of a power stroke. In addition, it will be
appreciated that a
piston poppet valves (or other mechanically actuated valve) may be located in
a variety of
different locations while still providing the ability to detect the completion
of the piston
stroke. In one alternative configuration, the valve may be integrated into the
cylinder, rather
than into the piston. Still further, in embodiments of a tractor that is
reversible in direction,
piston poppet valves, or other mechanically actuated valves, may be provided
on both sides
of a piston for detecting the completion of a power stroke in either
direction.
Preferred Configuration of Control Assembly
[0109] With reference now to Figures 9 and 10, a preferred embodiment of the
control assembly (see element 102 of Figure 2) is shown partially
disassembled. Figure 9
illustrates a control housing 202, which forms the central portion of the
control assembly.
Figure 10 illustrates the aft transition housing 204, the filter housing 206
and the forward
transition housing 206. Connectors 220 are provided for coupling the aft
transition housing
204 to the aft shaft and connectors 222 are provided for coupling the forward
transition
housing 206 to the forward shaft. Connectors 226 couple the aft transition
housing 204 and
the filter housing 206 to the control assembly 202. Connectors 224 couple the
forward
transition housing 208 to the control assembly 202.
[0110] With reference again to Figure 9, one preferred embodiment of the
control
housing 202 houses the propulsion control valve 310, the gripper control valve
312, the
pressure relief valve 306, the pressure reducing valve 326, the start/stop
valve 308, the aft
sequence valve 314, the forward sequence valve 316, the aft vent valves 318,
and the forward
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vent valve 320. Each of the valves preferably comprises a spool housed within
an elongate
valve housing defining a spool passage. In one configuration, the valves are
positioned
within recesses along the outer surface of the control housing 202.
[0111] The propulsion control valve 310, gripper control valve 312, pressure
reducing valve 326, vent valves 318, 320 and sequence valves 314, 316 are
preferably all
configured in a similar manner for ease of manufacture. In particular, each of
the valves is
provided in an elongate housing that fits within a recess along the outer
surface of the control
assembly 202. The valve housings are each attached to the body of the control
assembly via
two bolts or other appropriate attachment means. The pressure relief valve 306
and the
start/stop valve 308 are preferably configured in a similar manner. In one
embodiment, the
pressure relief valve 306 and start/stop valve 308 are both attached to the
body of the control
assembly via four bolts or other appropriate means for attachment.
[0112] The central housing 202 includes numerous internal fluid passages for
the
controlled flow of operating fluid to the downhole equipment (see element 32
of Figure 1),
between the valves, to the gripper assemblies, and to the propulsion
cylinders. In one
preferred embodiment, the fluid passages are configured to create the valve
system shown
schematically in Figure 3. Some of the fluid passages extend to corresponding
fluid passages
in the end surfaces of the transition housings 204, 206 and 208. In a
preferred embodiment,
the primary internal passage is shifted off center to maximize available space
for the various
valves and internal fluid passages.
[0113] An internal passage 250 extends through the aft transition housing 204,
the
filter housing 206 and the forward transition housing 208. The internal
passage also extends
through the aft and forward shafts and the control housing 202 such that
pressurized fluid
from the supply line may pass through the tractor body to the down.hole
assembly. As shown
in Figure 10, the filter housing 206 houses the filter/diffuser 304. The
filter/diffuser 304 is
generally cylindrical and has a plurality of side holes 210 for allowing
filtered fluid to pass
from the internal passage to the start/stop valve 308 (as shown schematically
in Figure 3). In
one preferred embodiment, the side holes 210 are angled so that the fluid
passing forward
through the filter/diffuser 304 must turn somewhat aftward to pass through.
This prevents
larger particles within the operating fluid from entering the start-stop valve
308, as it is more
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difficult for the larger particles to overcome forward momentum and flow
through the side
holes. Those of ordinary skill in the art will understand that any of a
variety of different types
of filters can be used instead of the illustrated diffuser 304.
Tractor Operation
[0114] With reference again to Figure 3, pressurized fluid is provided to the
control assembly from a supply source (e.g., on the surface) via a supply line
302. The
supply line 302 preferably extends through an internal passage in the elongate
tractor body
for providing pressurized fluid to the downhole equipment. When the pressure
in the supply
line 302 increases above a pre-selected threshold (e.g., 900 psi), the start-
stop valve 308
opens if the index is in the on position. If the index is in the off position,
a pressure cycle
(e.g., dropping the pressure to 0 psi and then back up to 900 psi) will change
the drum index
to the on position. When the start/stop valve 308 is open, the supply flow
takes parallel paths
to the pressure relief valve 306, the propulsion control valve 310 and the
pressure reducing
valve 326.
[0115] As discussed above, it has been found that the pressure of the
operating
fluid in the supply line 302 can fluctuate significantly during movement of
the tractor and/or
operation of the downhole equipment. Under certain circumstances, the pressure
fluctuations
can be substantial and can damage internal components and render other
hydraulically
coupled tools inoperable or incompatible. Accordingly, the pressure relief
valve 306 is
provided for regulating the fluid pressure in the supply line 302 (i.e., in
the internal passage),
and thus in the valve system located within the control assembly. In an
important feature, the
pressure of the fluid flowing to both the control assembly and the downhole
equipment is
desirably regulated. This feature improves the efficiency of the bottom hole
assemblies and
extends the life of the hardware components. In addition, the pressure relief
valve 306 is off
when the start-stop valve 308 is closed. This feature advantageously allows
high-pressure
(i.e., non-regulated) fluid to be selectively directed to the downhole
equipment when desired.
[0116] After passing through the start-stop valve 308, the pressurized fluid
flows
along path 354 to the pressure reduction valve 326 and then on to the gripper
control valve
312. In the illustrated configuration, the gripper control valve 312 is
shifted to the right such
that the fluid in flow path 370 is pressurized and the fluid in flow path 372
is depressurized.
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As a result, the aft gripper assembly begins expanding in a radial direction
for engagement
with the inner surface of the borehole and the forward gripper assembly
contracts radially for
disengagement from the inner surface of the borehole. When the aft gripper
assembly
become fully actuated, the fluid flow through flow path 370 stops and, as a
result, the fluid
pressure increases substantially (i.e., to the system pressure) in flow paths
370 and 364.
During this time, the pressure reducing valve 326 protects the aft gripper
assembly from
damage due to over-pressurization.
[0117] When the aft gripper assembly has becomes sufficiently fully engaged,
the
pressure in the flow path 364 exceeds the preset threshold (e.g., 900 psid) of
the aft sequence
valve 314. As a result, fluid flows through the aft sequence valve 314 and
acts on the first
end surface 330 of the propulsion control valve 310, thereby causing the spool
to shift to the
right (as shown in Figure 3). Accordingly, the valve system is configured such
that the
gripper assembly becomes fully actuated before the propulsion control valve
initiates a power
stroke.
[0118] In this position, pressurized fluid passes through the propulsion
control
valve 310 to the power chamber of the aft cylinder and to the reset chamber of
the forward
cylinder. As fluid enters the power chamber of the aft cylinder, the
pressurized fluid pushes
on the aft piston and thereby causes the tractor body to advance forward
through the borehole
relative to the aft gripper assembly (which is anchored to the inner surface).
Movement of
this type is generally referred to herein as a power stroke. At the same time,
as fluid enters
the reset chamber of the forward cylinder, the pressurized fluid pushes the
forward cylinder
and forward gripper assembly forward relative to the tractor body. This
movement resets the
position of the forward gripper assembly prepares the forward cylinder for a
subsequent
power stroke. Movement of this type is generally referred to herein as a reset
stroke.
Because the resistance to a reset stroke is relatively small, the reset stroke
is typically
completed before the power stroke is completed.
[0119] As the tractor body reaches the end of the power stroke with respect to
the
aft cylinder, the aft piston poppet valve 322 is actuated. This occurs when a
stein on the aft
piston poppet valve comes into contact with a portion of the aft cylinder such
that the stem is
mechanically depressed. When the stem is depressed, pressurized fluid enters a
flow passage
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382. When the pressure in flow path 382 becomes sufficiently large, the aft
vent valve 318
opens to allow pressurized fluid to pass through to the second end surface 336
of the gripper
control valve 312. The fluid pressure causes the spool in the gripper control
valve 3,12 to
shift to the left (i.e., to the position not shown in Figure 3).
[0120] After the gripper control valve 312 switches its position, the fluid
within
the flow path 370 becomes depressurized and the fluid within the flow paths
366 and 372
becomes pressurized. When the pressure in flow path 366 becomes sufficiently
large, the
forward sequence valve 316 opens such that pressurized fluid acts on second
end surface 332
of the propulsion control valve 310 and causes the spool to shift to the left
(i.e., to the
position not shown in Figure 3). The pressure in flow path 366 becomes
sufficiently large to
open the forward sequence valve 316 after the forward gripper assembly comes
into contact
with the inner surface of the borehole and is therefore prevented from
expanding any further.
When the forward gripper assembly stops expanding, the flow to the forward
gripper
assembly through flow path 372 is stopped, thereby producing an increase in
fluid pressure.
[0121] Due to the shifting of the spool in the propulsion control valve 310,
pressurized fluid within the flow path 354 flows through the propulsion
control valve 310
and into the forward chamber of the forward cylinder and the aft chamber of
the aft cylinder.
Simultaneously, fluid within the aft chamber of the foward cylinder, as well
as fluid within
the forward chamber of the aft cylinder, flows back through the propulsion
control valve 310
into the annulus 40. This causes the forward piston, and thus the entire
tractor body, to be
thrust forward through the borehole with respect to the actuated forward
gripper assembly in
another power stroke. Simultaneously, the aft cylinder is thrust forward with
respect to the
piston and the tractor body in a reset stroke.
[0122] As the tractor body reaches the end of the power stroke with respect to
the
forward cylinder, the forward piston poppet valve 324 is actuated. This occurs
when a stem
on the forward piston poppet valve comes into contact with a portion of the
forward cylinder
such that the stem on the forward piston poppet valve is mechanically
depressed. When the
stem is depressed, pressurized fluid enters flow passage 380. When the
pressure in flow path
380 is sufficiently large to overcome the pre-selected threshold pressure, the
forward vent
valve 320 opens to allow pressurized fluid to pass through to the first end
surface 334 of the
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gripper control valve 312. The fluid pressure causes the spool in the gripper
control valve
312 to shift back to the right (i.e., to the position shown in Figure 3). At
this point, all of the
valves have returned back to their original positions (i.e., to the positions
generally shown in
Figure 3). Thus, the above describes a complete cycle of operation of the
valve system
during forward motion.
[0123] Note that during forward or aft (i.e., backward) motion, the gripper
assemblies preferably shuttle between two extreme positions. First, the
gripper assemblies
move as far apart as possible toward opposite ends of the tractor. Second, the
gripper
assemblies move as close together as possible (with the propulsion cylinders
and control
assembly between them). During most of the operation of the tractor, one
gripper assembly is
in a power stroke while the other is in a reset stroke. When they switch
directions they also
switch gripper action. Hence, the tractor continually moves in one
longitudinal direction.
[0124] A significant advantage of the preferred configuration of the valve
system
is that the tractor body is assured of completing its forward advancement
(i.e., power stroke)
before the gripper assemblies are switched between their actuated and
retracted positions. As
described above, the reliability and efficiency of the tractor movement may be
improved by
the incorporation of the mechanically-actuated valves (e.g., piston poppet
valve) into the
valve system. The piston poppet valves provide a mechanism to detect and
signal the
completion of a power stroke. In addition, in a preferred configuration, the
outlet from the
gripper control valve 312 is used to pilot the propulsion control valve 310.
As a result, the
system ensures that the gripper is fully actuated before a power stroke
commences.
[0125] In one preferred embodiment, the flow rate of operating fluid into the
valve system in the control assembly can be up to about 23 gallons per minute.
Typically,
large positive displacement pumps are utilized at the ground surface to pump
fluid down the
coiled tubing and through the internal passage of the tractor. Such pumps
usually supply a
system flow rate of up to about 120 gpm. In one typical mode of operation, the
valve system
receives approximately 20% of the fluid passing through the internal passage
of the tractor
body. In other modes of operation, the valve system receives approximately 5%,
10%, 15%
or 25% of the fluid passing through the internal passage.
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[0126] In a preferred embodiment of the tractor wherein the valve system is
all-
hydraulic, the tractor's maximum speed may be greater than that of an
electrically controlled
tractor. The valve system does not include electrical conductors and other
electrical
elements, which allows for larger internal fluid passages, greater flow rates,
and improved
power density. The faster maximum speed of the tractor results in lower
operational costs,
especially for intervention applications. In one preferred embodiment of the
invention, the
tractor is capable of moving at speeds greater than or equal to 1350 feet per
hour.
Reversible Tractor
[0127] In another preferred embodiment, the tractor may be capable of movement
through a passage in both forward and aft directions. With reference now to
Figure 11, one
embodiment of an improved valve system 800 is illustrated for use with a
reversible tractor.
Similar to the valve system described above with reference to Figure 3, the
improved valve
system 800 illustrated in Figure 11 receives pressurized fluid from a supply
line 302. The
pressurized fluid passes through a start-stop valve 308 for providing
hydraulic power to the
tractor control assembly 102. To provide the tractor operator with the ability
to selectively
reverse directions, the valve system 800 in the control assembly further
comprises a main
reverser valve 390, an aft reverser valve 392, a forward reverser valve 394,
and a gripper
reverser valve 396. The main reverser valve 390 is piloted by fluid pressure
in the supply
line 302. The main reverser valve 390, in turn, pilots the aft reverser valve
392, the forward
reverser valve 394 and the gripper reverser valve 396.
[0128] Similar to the embodiment described above with respect to Figure 3, the
improved valve system 800 for use with a reversible tractor preferably
comprises an aft
piston poppet valve 322, and a forward piston poppet valve 324. The aft and
forward piston
poppet valves 322, 324 are adapted for detecting the completion of the piston
stroke during
forward advancement through the passage. In addition, the improved valve
system shown in
Figure 11 comprises a forward reverser piston poppet valve 323, and an aft
reverser piston
poppet valve 325 for detecting completion of the piston stroke during aft
movement through
the passage. Therefore, as shown in Figure 11, the improved valve system 800
is provided
with two piston poppet valves on both the forward and aft pistons. As a
result, the tractor is
capable of providing accurate and efficient valve sequencing during movement
in either the
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forward or aft direction. Because each piston includes two piston poppet
valves, two
independent pilot passages are preferably provided in the wall of the shaft
for each piston.
[0129] During use, when the main reverser valve 390 is in the closed position
(as
shown in Figure 11), no fluid passes through the main reverser valve and the
valve system
800 operates in a manner similar to the manner described above with respect to
Figure 3.
However, when the pressure in the supply line 302 is increased above a pre-
selected
threshold (e.g., 2000 psi), the main reverser valve 390 is indexed to the open
position. As a
result, the pressurized fluid in the supply line 302 passes through the main
reverser valve 390
to the aft reverser valve 392, the forward reverser valve 394, and the gripper
reverser valve
396. The fluid pressure causes the aft reverser valve 392, the forward
reverser valve 394, and
the gripper reverser valve 396 to change positions, thereby altering the
sequencing of the
valve operation. In particular, the aft and forward reverser valves 392, 394
allow the forward
reverser piston poppet valve 323 and aft reverser piston poppet valve 325 to
pilot the aft and
forward vent valves during aft movement through the passage. Furthermore, the
gripper
reverser valve 396 changes the flow path from the gripper control valve 312
such that the
desired gripper assembly is actuated before initiation of a power stroke.
[0130] In preferred alternative configurations, the improved valve system
illustrated in Figure 11 may also include a pressure relief valve 306 and aft
and forward
sequence valves 314, 316, as generally described above with reference to
Figure 3.
Additional details of a tractor having the ability to reverse directions may
be found in
Applicant's U.S. Patent No. 6,679,341.
Gripper Assemblies
[0131] Preferred embodiments of the tractor described herein may be used with
a
wide variety of different gripper assemblies. However, in preferred
embodiments, the gripper
assemblies 104 and 106 are embodied as a plurality of toes that are radially
expandable for
engaging the inner surface of the borehole. Figures 12-19 illustrate various
preferred
configurations of preferred gripper assemblies adapted for use with a tractor.
Additional
details can be found in Applicant's copending U.S. Application No. 10/004,963,
entitled
"IMPROVED GRIPPER ASSEMBLY FOR DOWNHOLE TRACTORS," filed on
December 3, 2001. In a preferred embodiment, the gripper assemblies 104 and
106 are
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substantially identical. Thus, the gripper assembly configurations shown in
Figures 12-19
may be considered to describe both aft and forward gripper assemblies 104 and
106.
[0132] Figure 12 shows one preferred embodiment of a gripper assembly 1000.
The illustrated gripper assembly includes an elongated generally tubular
mandrel 1002
configured to slide longitudinally along a length of the tractor 50.
Preferably, the interior
surface of the mandrel 1002 has a splined interface (e.g., tongue and groove
configuration)
with the exterior surface of the shaft, so that the mandrel 1002 is free to
slide longitudinally
yet is prevented from rotating with respect to the shaft. In another
embodiment, splines are
not included. Fixed mandrel caps 1004 and 1010 are connected to the forward
and aft ends
of the mandrel 1002, respectively. On the forward end of the mandrel 1002,
near the mandrel
cap 1004, a sliding toe support 1006 is longitudinally slidably engaged on the
mandrel 1002.
Preferably, the sliding toe support 1006 is prevented from rotating with
respect to the
mandrel 1002, such as by a splined interaction therebetween. On the aft end of
the mandrel
1002, a cylinder 1008 is positioned next to the mandrel cap 1010 and
concentrically encloses
the mandrel so as to form an annular space therebetween. As shown in Figure
12, this
annular space contains a piston 1038, an aft portion of a piston rod 1024, a
spring 1044, and
fluid seals, for reasons that will become apparent.
[0133] The cylinder 1008 is fixed with respect to the mandrel 1002. A toe
support 1018 is fixed onto the forward end of the cylinder 1008. A plurality
of gripper
portions 1012 are secured onto the gripper assembly 1000. In the illustrated
embodiment the
gripper portions comprise flexible toes or beams 1012. The toes 1012 have ends
1014
pivotally or hingedly secured to the fixed toe support 1018 and ends 1016
pivotally or
hingedly secured to the sliding toe support 1006. As used herein, "pivotally"
or "hingedly"
describes a connection that permits rotation, such as by an axle, pin, or
hinge. The ends of
the toes 1012 are preferably engaged on axles, rods, or pins secured to the
toe supports.
[0134] Those of skill in the art will understand that any number of toes 1012
may
be provided. As more toes are provided, the maximum radial load that can be
transmitted to
the borehole surface is increased. This improves the gripping power of the
gripper assembly
1000, and therefore permits greater radial thrust and drilling power of the
tractor. However,
it is preferred to have three toes 1012 for more reliable gripping of the
gripper assembly 1000
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onto the inner surface of a borehole. For example, a four-toed embodiment
could result in
only two toes making contact with the borehole surface in oval-shaped holes.
Additionally,
as the number of toes increases, so does the potential for synchronization and
alignment
problems of the toes. In addition, at least three toes 1012 are preferred, to
substantially
prevent the potential for rotation of the tractor about a transverse axis,
i.e., one that is
generally perpendicular to the longitudinal axis of the tractor body. For
example, the three-
bar linkage gripper described above has only two linkages. Even when both
linkages are
actuated, the tractor body can rotate about the axis defined by the two
contact points of the
linkages with the borehole surface. A three-toe embodiment of the present
invention
substantially prevents such rotation. Further, gripper assemblies having at
least three toes
1012 are more capable of traversing underground voids in a borehole.
[0135] A driver or slider element 1022 is slidably engaged on the mandrel 1002
and is longitudinally positioned generally at about a longitudinal central
region of the toes
1012. The slider element 1022 is positioned radially inward of the toes 1012,
for reasons that
will become apparent. A tubular piston rod 1024 is slidably engaged on the
mandrel 1002
and connected to the aft end of the slider element 1022. The piston rod 1024
is partially
enclosed by the cylinder 1008. The slider element 1022 and the piston rod 1024
are
preferably prevented from rotating with respect to the mandrel 1002, such as
by a splined
interface between such elements and the mandrel.
[0136] Figure 13 shows a longitudinal cross-section of a gripper assembly
1000.
Figures 14 and 15 show a gripper assembly 1000 in a partial cut-away view. As
seen in the
figures, the slider element 1022 includes a multiplicity of wedges or ramps
1026. Each ramp
1026 slopes between an inner radial level 1028 and an outer radial level 1030,
the inner level
1028 being radially closer to the surface of the mandrel 1002 than the outer
level 1030.
Desirably, the slider element 1022 includes at least one ramp 1026 for each
toe 1012. Of
course, the slider element 1022 may include any number of ramps 1026 for each
toe 1012. In
the illustrated embodiments, the slider element 1022 includes two ramps 1026
for each toe
1012. As more ramps 1026 are provided for each toe, the amount of force that
each ramp
must transmit is reduced, producing a longer fatigue life of the ramps. Also,
the provision of
additional ramps results in more uniform radial displacement of the toes 1012,
as well as
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radial displacement of a relatively longer length of the toes 1012, both
resulting in better
overall gripping onto the borehole surface.
[0137] In a preferred embodiment, two ramps 1026 are spaced apart generally by
the length of the central region 1048 of each toe 1012. In this embodiment,
when the gripper
assembly is actuated to grip onto a borehole surface, the central regions 1048
of the toes 1012
have a greater tendency to remain generally linear. This results in a greater
surface area of
contact between the toes and the borehole surface, for better overall
gripping. Also, a more
unifoim load is distributed to the toes to facilitate better gripping. With
more than two
ramps, there is a greater proclivity for uneven load distribution as a result
of manufacturing
variations in the radial dimensions of the ramps 1026, which can result in
premature fatigue
failure.
[0138] Each toe 1012 is provided with a driver interaction element on the
central
region of the toe. The driver interaction element interacts with the driver or
slider element
1022 to vary the radial position of the central region 1048 of the toe 1012.
Preferably, the
driver and driver interaction element are configured to interact substantially
without
production of sliding friction therebetween. In the illustrated embodiments,
the driver
interaction element comprises one or more rollers 1032 that are rotatably
secured on the toes
1012 and configured to roll upon the inclined surfaces of the ramps 1026.
Preferably, there is
one roller 1032 for every ramp 1026 on the slider element 1022. In the
illustrated
embodiments, the rollers 1032 of each toe 1012 are positioned within a recess
1034 on the
radially interior surface of the toe, the recess 1034 extending longitudinally
and being sized
to receive the ramps 1026. The rollers 1032 rotate on axles 1036 that extend
transversely
within the recess 1034. The ends of the axles 1036 are secured within holes in
the sidewalls
1035 that define the recess 1034.
[0139] The piston rod 1024 connects the slider element 1022 to a piston 1038
enclosed within the cylinder 1008. The piston 1038 has a generally tubular
shape. The
piston 1038 has an aft or actuation side 1039 and a forward or retraction side
1041. The
piston rod 1024 and the piston 1038 are longitudinally slidably engaged on the
mandrel 1002.
The forward end of the piston rod 1024 is attached to the slider element 1022.
The aft end of
the piston rod 1024 is attached to the retraction side 1041 of the piston
1038. The piston
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1038 fluidly divides the annular space between the mandrel 1002 and the
cylinder 1008 into
an aft or actuation chamber 1040 and a forward or retraction chamber 1042. A
seal 1043,
such as a rubber 0-ring, is preferably provided between the outer surface of
the piston 1038
and the inner surface of the cylinder 1008. A return spring 1044 is engaged on
the piston rod
1024 and enclosed within the cylinder 1008. The spring 1044 has an aft end
attached to
and/or biased against the retraction side 1041 of the piston 1038. A forward
end of the spring
1044 is attached to and/or biased against the interior surface of the forward
end of the
cylinder 1008. The spring 1044 biases the piston 1038, piston rod 1024, and
slider element
1022 toward the aft end of the mandrel 1002. In the illustrated embodiment,
the spring 1044
comprises a coil spring. The number of coils and spring diameter is preferably
chosen based
on the required return loads and the space available. Those of ordinary skill
in the art will
understand that other types of springs or biasing means may be used.
[0140] Figures 16 and 17 show a gripper assembly 1055 according to an
alternative embodiment of the invention. In this embodiment, the rollers 1032
are located on
a driver or slider element 1062. The toes 1012 include a driver interaction
element that
interacts with the driver to vary the radial position of the central sections
1048 of the toes. In
the illustrated embodiment, the driver interaction element comprises one or
more ramps 1060
on the interior surfaces of the central sections 1048. Each ramp 1060 slopes
from a base
1064 to a tip 1063. The slider element 1062 includes external recesses sized
to receive the
tips 1063 of the ramps 1060. The roller axles 1036 extend transversely across
these recesses,
into holes in the sidewalls of the recesses. Preferably, the ends of the
roller axles 1036 reside
within one or more lubrication reservoirs in the slider element 1062. More
preferably, such
lubrication reservoirs are pressure-compensated by pressure compensation
pistons, as
described above in relation to the embodiments shown in Figures 12-15.
[0141] Although the gripper assembly 1055 shown in Figures 16 and 17 has four
toes 1012, those of ordinary skill in the art will understand that any number
of toes 1012 can
be included. However, it is preferred to include three toes 1012, for more
efficient and
reliable contact with the inner surface of a passage or borehole. As in the
previous
embodiments, each toe 1012 may include any number of ramps 1060, although two
are
preferred. Desirably, there is at least one ramp 1060 per roller 1032.
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[0142] The gripper assembly 1055 shown in Figures 16 and 17 operates similarly
to the gripper assembly 1000 shown in the Figures 12-14. The actuation and
retraction of the
gripper assembly is controlled by the position of the piston 1038 inside the
cylinder 1008.
The fluid pressure in the actuation chamber 1040 controls the position of the
piston 1038.
Forward motion of the piston 1038 causes the slider element 1062 and the
rollers 1032 to
move forward as well. The rollers roll against the inclined surfaces or slopes
of the ramps
1060, forcing the central regions 1048 of the toes 1012 radially outward.
[0143] Figures 18 and 19 show a gripper assembly 1070 having toggles 1076 for
radially displacing the toes 1012. A slider element 1072 has toggle recesses
1074 configured
to receive ends of the toggles 1076. Similarly, the toes 1012 include toggle
recesses 1075
also configured to receive ends of the toggles. Each toggle 1076 has a first
end 1078 received
within a recess 1074 and rotatably maintained on the slider element 1072. Each
toggle 1076
also has a second end 1080 received within a recess 1075 and rotatably
maintained on one of
the toes 1012. The ends 1078 and 1080 of the toggles 1076 can be pivotally
secured to the
slider element 1072 and the toes 1012, such as by dowel pins or hinges
connected to the
slider element 1062 and the toes 1012. Those of ordinary skill in the art will
understand that
the recesses 1074 and 1075 are not necessary. The purpose of the toggles 1076
is to rotate
and thereby radially displace the toes 1012. This may be accomplished without
recesses for
the toggle ends, such as by pivoted connections of the ends.
[0144] In the illustrated embodiment, there are two toggles 1076 for each toe
1012. Those of ordinary skill in the art will understand that any number of
toggles can be
provided for each toe 1012. However, it is preferred to have two toggles
having second ends
1080 generally at or near the ends of the central section 1048 of each toe
1012. This
configuration results in a more linear shape of the central section 1048 when
the gripper
assembly 1070 is actuated to grip against a borehole surface. This results in
more surface
area of contact between the toe 1012 and the borehole, for better gripping and
more efficient
transmission of loads onto the borehole surface.
[0145] The gripper assembly 1070 operates similarly to the gripper assemblies
1000 and 1055 described above. The gripper assembly 1070 has an actuated
position in
which the toes 1012 are flexed radially outward, and a retracted position in
which the toes
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1012 are relaxed. In the retracted position, the toggles 1076 are oriented
substantially parallel
to the mandrel 1002, so that the second ends 1080 are relatively near the
surface of the
mandrel. As the piston 1038, piston rod 1024, and slider element 1072 move
forward, the
first ends 1078 of the toggles 1076 move forward as well. However, the second
ends 1080 of
the toggles are prevented from moving forward by the recesses 1075 on the toes
1012. Thus,
as the slider element 1072 moves forward, the toggles 1076 rotate outward so
that they are
oriented diagonally or even nearly perpendicular to the mandrel 1002. As the
toggles 1076
rotate, the second ends 1080 move radially outward, which causes radial
displacement of the
central sections 1048 of the toes 1012. This corresponds to the actuated
position of the
gripper assembly 1070. If the piston 1038 moves back toward the aft end of the
mandrel
1002, the toggles 1076 rotate back to their original position, substantially
parallel to the
mandrel 1002.
[0146] Compared to the gripper assemblies 1000 and 1055 described above, the
gripper assembly 1070 does not transmit significant radial loads onto the
borehole surface
when the toes 1012 are only slightly radially displaced. However, the gripper
assembly 1070
comprises a significant improvement over the three-bar linkage gripper design
of the prior
art. The toes 1012 of the gripper assembly 1055 comprise continuous beams, as
opposed to
multi-bar linkages. Continuous beams have significantly greater torsional
rigidity than multi-
bar linkages, due to the absence of hinges, pin joints, or axles connecting
different sections of
the toe. Thus, the gripper assembly 1070 is much more resistant to undesired
rotation or
twisting when it is actuated and in contact with the borehole surface. Also,
continuous beams
involve few if any stress concentrations and thus tend to last longer than
linkages. Another
advantage of the gripper assembly 1070 over the multi-bar linkage design is
that the toggles
1076 provide radial force at the central sections 1048 of the toes 1012. In
contrast, the multi-
bar linkage design involves moving together opposite ends of the linkage to
force a central
link radially outward against the borehole surface. Thus, the gripper assembly
1070 involves
a more direct application of force at the central section 1048 of the toe
1012, which contacts
the borehole surface. Another advantage of the gripper assembly 1070 is that
it can be
actuated and retracted substantially without any sliding friction.
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[0147] Although this invention has been disclosed in the context of certain
preferred embodiments and examples, it will be understood by those skilled in
the art that the
present invention extends beyond the specifically disclosed embodiments to
other alternative
embodiments and/or uses of the invention and obvious modifications and
equivalents thereof.
Further, the various features of this invention can be used alone, or in
combination with other
features of this invention other than as expressly described above. Thus, it
is intended that
the scope of the present invention herein disclosed should not be limited by
the particular
disclosed embodiments described above, but should be determined only by a fair
reading of
the claims that follow.
