Patent Application
20050082055
The present invention relates generally to grippers for downhole tractors and, specifically, to improved gripper assemblies.
Tractors for moving within underground boreholes are used for a variety of purposes, such as oil drilling, mining, laying communication lines, and many other purposes. In the petroleum industry, for example, a typical oil well comprises a vertical borehole that is drilled by a rotary drill bit attached to the end of a drill string. The drill string may be constructed of a series of connected links of drill pipe that extend between ground surface equipment and the aft end of the tractor. Alternatively, the drill string may comprise flexible tubing or “coiled tubing” connected to the aft end of the tractor. A drilling fluid, such as drilling mud, is pumped from the ground surface equipment through an interior flow channel of the drill string and through the tractor to the drill bit. The drilling fluid is used to cool and lubricate the bit, and to remove debris and rock chips from the borehole, which are created by the drilling process. 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.
Tractors for moving within downhole passages are often required to operate in harsh environments and limited space. For example, tractors used for oil drilling may encounter hydrostatic pressures as high as 16,000 psi and temperatures as high as 300° F. Typical boreholes for oil drilling are 3.5-27.5 inches in diameter. Further, to permit turning, the tractor length should be limited. Also, tractors must often have the capability to generate and exert substantial force against a formation. For example, operations such as drilling require thrust forces as high as 30,000 pounds.
As a result of the harsh working environment, space constraints, and desired force generation requirements, downhole tractors are used only in very limited situations, such as within existing well bore casing. While a number of the inventors of this application have previously developed a significantly improved design for a downhole tractor, further improvements are desirable to achieve performance levels that would permit downhole tractors to achieve commercial success in other environments, such as open bore drilling.
In one known design, a tractor comprises an elongated body, a propulsion system for applying thrust to the body, and grippers for anchoring the tractor to the inner surface of a borehole or passage while such thrust is applied to the body. Each gripper has an actuated position in which the gripper substantially prevents relative movement between the gripper and the inner surface of the passage, and a retracted position in which the gripper permits substantially free relative movement between the gripper and the inner surface of the passage. Typically, each gripper is slidingly engaged with the tractor body so that the body can be thrust longitudinally while the gripper is actuated. The grippers preferably do not substantially impede “flow-by,” the flow of fluid returning from the drill bit up to the ground surface through the annulus between the tractor and the borehole surface.
Tractors may have at least two grippers that alternately actuate and reset to assist the motion of the tractor. In one cycle of operation, the body is thrust longitudinally along a first stroke length while a first gripper is actuated and a second gripper is retracted. During the first stroke length, the second gripper moves along the tractor body in a reset motion. Then, the second gripper is actuated and the first gripper is subsequently retracted. The body is thrust longitudinally along a second stroke length. During the second stroke length, the first gripper moves along the tractor body in a reset motion. The first gripper is then actuated and the second gripper subsequently retracted. The cycle then repeats. Alternatively, a tractor may be equipped with only a single gripper for specialized applications of well intervention, such as movement of sliding sleeves or perforation equipment.
Grippers are often designed to be powered by fluid, such as drilling mud in an open tractor system or hydraulic fluid in a closed tractor system. Typically, a gripper assembly has an actuation fluid chamber that receives pressurized fluid to cause the gripper to move to its actuated position. The gripper assembly may also have a retraction fluid chamber that receives pressurized fluid to cause the gripper to move to its retracted position. Alternatively, the gripper assembly may have a mechanical retraction element, such as a coil spring or leaf spring, which biases the gripper back to its retracted position when the pressurized fluid is discharged. Motor-operated or hydraulically controlled valves in the tractor body can control the delivery of fluid to the various chambers of the gripper assembly.
The prior art includes a variety of different types of grippers for tractors. One type of gripper comprises a plurality of frictional elements, such as metallic friction pads, blocks, or plates, which are disposed about the circumference of the tractor body. The frictional elements are forced radially outward against the inner surface of a borehole under the force of fluid pressure. However, these gripper designs are either too large to fit within the small dimensions of a borehole or have limited radial expansion capabilities. Also, the size of these grippers often cause a large pressure drop in the flow-by fluid, i.e., the fluid returning from the drill bit up through the annulus between the tractor and the borehole. The pressure drop makes it harder to force the returning fluid up to the surface. Also, the pressure drop may cause drill cuttings to drop out of the main fluid path and clog up the annulus.
Another type of gripper comprises a bladder that is inflated by fluid to bear against the borehole surface. While inflatable bladders provide good conformance to the possibly irregular dimensions of a borehole, they do not provide very good torsional resistance. In other words, bladders tend to permit a certain degree of undesirable twisting or rotation of the tractor body, which may confuse the tractor's position sensors. Also, some bladder configurations may substantially impede the flow-by of fluid and drill cuttings returning up through the annulus to the surface.
Yet another type of gripper comprises a combination of bladders and flexible beams oriented generally parallel to the tractor body on the radial exterior of the bladders. The ends of the beams are maintained at a constant radial position near the surface of the tractor body, and may be permitted to slide longitudinally. Inflation of the bladders causes the beams to flex outwardly and contact the borehole wall. This design effectively separates the loads associated with radial expansion and torque. The bladders provide the loads for radial expansion and gripping onto the borehole wall, and the beams resist twisting or rotation of the tractor body. While this design represents a significant advancement over previous designs, the bladders provide limited radial expansion loads. As a result, the design is less effective in certain environments. Also, this design impedes to some extent the flow of fluid and drill cuttings upward through the annulus.
Yet another type of gripper comprises a pair of three-bar linkages separated by 180° about the circumference of the tractor body.
One major disadvantage of the three-bar linkage gripper design is that it is difficult to generate significant radial expansion loads against the inner surface of the borehole until the second link 204 has been radially displaced a substantial degree. As noted above, the radial load applied to the borehole is generated by applying longitudinally directed fluid pressure forces onto the first and third links. These fluid pressure forces cause the first end 208 of the first link 202 and the second end 218 of the third link 206 to move together until the second link 204 makes contact with the borehole. Then, the fluid pressure forces are transmitted through the first and third links to the second link and onto the borehole wall. However, the radial component of the transmitted forces is proportional to the sine of the angle θ between the first or third link and the tractor body 201. In the retracted position of the gripper, all three of the links are oriented generally parallel to the tractor body 201, so that θ is zero or very small. Thus, when the gripper is in or is near the retracted position, the gripper is incapable of transmitting any significant radial load to the borehole wall. In small diameter boreholes, in which the second link 204 is displaced only slightly before coming into contact with the borehole surface, the gripper provides a very limited radial load. Thus, in small diameter environments, the gripper cannot reliably anchor the tractor. As a result, this three-bar linkage gripper is not useful in small diameter boreholes or in small diameter sections of generally larger boreholes. If the three-bar linkage was modified so that the angle θ is always large, the linkage would then be able to accommodate only very small variations in the diameter of the borehole.
Another disadvantage of the three-bar linkage gripper design is that it is not sufficiently resistant to torque in the tractor body. The links are connected by hinges or axles that permit a certain degree of twisting of the tractor body when the gripper is actuated. During drilling, the borehole formation exerts a reaction torque onto the tractor body, opposite to the direction of drill bit rotation. This torque is transmitted through the tractor body to an actuated gripper. However, since the gripper does not have sufficient torsional rigidity, it does not transmit all of the torque to the borehole. The three-bar linkage permits a certain degree of rotation. This leads to excessive twisting and untwisting of the tractor body, which can confuse the tractor's position sensors and/or require repeated recalibration of the sensors. Yet another disadvantage of the multi-bar linkage gripper design is that it involves stress concentrations at the hinges or joints between the links. Such stress concentrations introduce a high probability of premature failure.
Some types of grippers have gripping elements that are actuated or retracted by causing different surfaces of the gripper assembly to slide against each other. Moving the gripper between its actuated and retracted positions involves substantial sliding friction between these sliding surfaces. The sliding friction is proportional to the normal forces between the sliding surfaces. A major disadvantage of these grippers is that the sliding friction can significantly impede their operation, especially if the normal forces between the sliding surfaces are large. The sliding friction may limit the extent of radial displacement of the gripping elements as well as the amount of radial gripping force that is applied to the inner surface of a borehole. Thus, it may be difficult to transmit larger loads to the passage, as may be required for certain operations, such as drilling. Another disadvantage of these grippers is that drilling fluid, drill cuttings, and other particles can get caught between and damage the sliding surfaces as they slide against one another. Also, such intermediate particles can add to the sliding friction and further impede actuation and retraction of the gripper.
In at least one embodiment of the present invention, there is provided an improved gripper assembly that overcomes the above-mentioned problems of the prior art.
In one aspect, there is provided a gripper assembly for anchoring a tool within a passage and for assisting movement of the tool within the passage. The gripper assembly is movable along an elongated shaft of the tool. The gripper assembly has an actuated position in which the gripper assembly substantially prevents movement between the gripper assembly and an inner surface of the passage, and a retracted position in which the gripper assembly permits substantially free relative movement between the gripper assembly and the inner surface of the passage. The gripper assembly comprises an elongated mandrel, a first toe support longitudinally fixed with respect to the mandrel, a second toe support longitudinally slidable with respect to the mandrel, a flexible elongated toe, a driver, and a driver interaction element. The mandrel surrounds and is configured to be longitudinally slidable with respect to the shaft of the tractor. The toe has a first end pivotally secured with respect to the first toe support and a second end pivotally secured with respect to the second toe support so that the first and second ends of the toe have an at least substantially constant radial position with respect to a longitudinal axis of the mandrel. The toe comprises a single beam.
The driver is longitudinally slidable with respect to the mandrel, and is slidable between a retraction position and an actuation position. The driver interaction element is positioned on a central region of the toe and is configured to interact with the driver. Longitudinal movement of the driver causes interaction between the driver and the driver interaction element substantially without sliding friction therebetween. The interaction between the driver and the driver interaction element varies the radial position of the central region of the toe. When the driver is in the retraction position, the central region of the toe is at a first radial distance from the longitudinal axis of the mandrel and the gripper assembly is in the retracted position. When the driver is in the actuation position, the central region of the toe is at a second radial distance from the longitudinal axis and the gripper assembly is in the actuated position. The second radial distance is greater than the first radial distance.
In another aspect, the present invention provides a gripper assembly for use with a tractor for moving within a passage. The gripper assembly is longitudinally slidable along an elongated shaft of the tractor. The gripper assembly has actuated and retracted positions as described above. The gripper assembly comprises an elongated mandrel, a first toe support longitudinally fixed with respect to the mandrel, a second toe support longitudinally slidable with respect to the mandrel, a flexible elongated toe, a ramp, and a roller. The mandrel is configured to be longitudinally slidable with respect to the shaft of the tractor. The toe has a first end pivotally secured with respect to the first toe support and a second end pivotally secured with respect to the second toe support. The ramp has an inclined surface that extends between an inner radial level and an outer radial level, the inner radial level being radially closer to the surface of the mandrel than the outer radial level. The ramp is longitudinally slidable with respect to the mandrel. The roller is rotatably secured to a center region of the toe and is configured to roll against the ramp. In a preferred embodiment, the toe preferably comprises a single beam.
Longitudinal movement of the ramp causes the roller to roll against the ramp between the inner and outer levels to vary the radial position of the center region of the toe between a radially inner position corresponding to the retracted position of the gripper assembly and a radially outer position corresponding to the actuated position of the gripper assembly. Preferably, the ramp is movable between first and second longitudinal positions relative to the mandrel. When the ramp is in the first position, the roller is at the inner radial level and the gripper assembly is in the retracted position. When the ramp is in the second position, the roller is at the outer radial level and the gripper assembly is in the actuated position.
In yet another aspect, the present invention provides a gripper assembly for use with a tractor for moving within a passage, the tractor having an elongated shaft. The gripper assembly has actuated and retracted positions as described above. The gripper assembly comprises an elongated mandrel, a first toe support longitudinally fixed with respect to the mandrel, a second beam support longitudinally slidable with respect to the mandrel, a flexible toe, a piston longitudinally slidable with respect to the mandrel, a ramp, a slider element, and a roller. The mandrel is configured to be longitudinally slidable with respect to the shaft of the tractor. The toe has a first end pivotally secured with respect to the first toe support and a second end pivotally secured with respect to the second toe support. The ramp is positioned on an inner surface of the toe. The ramp slopes from a first end to a second end, the second end being radially closer to the surface of the mandrel than the first end. The slider element is longitudinally slidable with respect to the mandrel and longitudinally fixed with respect to the piston. The roller is rotatably fixed with respect to the slider element and configured to roll against the ramp.
The ramp is oriented such that longitudinal movement of the slider element causes the roller to roll against the ramp to vary the radial position of the center region of the toe between a radially inner position corresponding to the retracted position of the gripper assembly and a radially outer position corresponding to the actuated position of the gripper assembly. The piston and the slider element are movable between first and second longitudinal positions relative to the mandrel. When the piston and the slider element are in the first position, the first end of the ramp bears against the roller and the gripper assembly is in the retracted position. When the piston and the slider element are in the second position, the second end of the ramp bears against the roller and the gripper assembly is in the actuated position.
In yet another aspect, the present invention provides a gripper assembly for use with a tractor for moving within a passage, the tractor having an elongated shaft. The gripper assembly has actuated and retracted positions as described above. The gripper assembly comprises an elongated mandrel, a first toe support longitudinally fixed with respect to the mandrel, a second toe support longitudinally slidable with respect to the mandrel, a flexible elongated toe, a slider element, and one or more elongated toggles. The mandrel is configured to be longitudinally slidable with respect to the shaft of the tractor. The toe has a first end pivotally secured with respect to the first toe support and a second end pivotally secured with respect to the second toe support. The slider element is longitudinally slidable with respect to the mandrel, and is slidable between first and second positions. The toggles have first ends rotatably maintained on the slider element and second ends rotatably maintained on a center region of the toe. The toe preferably comprises a single beam.
The toggles are adapted to rotate between a retracted position in which the second ends of the toggles and the center region of the toe are at a radially inner level that defines the retracted position of the gripper assembly, and an actuated position in which the second ends of the toggles and the center region of the toe are at a radially outer level that defines the actuated position of the gripper assembly. Longitudinal movement of the slider element causes longitudinal movement of the first ends of the toggles, to thereby rotate the toggles. When the slider element is in the first position the toggles are in the retracted position. When the slider element is in the second position the toggles are in the actuated position.
For purposes of summarizing the invention and the advantages achieved over the prior art, certain objects and advantages of the invention have been described above and as further described below. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.
All of these embodiments are intended to be within the scope of the invention herein disclosed. These and other embodiments of the present invention will 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.
Coiled Tubing Tractor Systems
Various embodiments of the gripper assemblies 100 are described herein. It should be noted that the gripper assemblies 100 may be used with a variety of different tractor designs, including, for example, (1) the “PULLER-THRUSTER DOWNHOLE TOOL,” shown and described in U.S. Pat. No. 6,003,606 to Moore et al.; (2) the “ELECTRICALLY SEQUENCED TRACTOR,” shown and described in U.S. Pat. No. 6,347,674; and (3) the “ELECTRO-HYDRAULICALLY CONTROLLED TRACTOR,” shown and described in U.S. Pat. No. 6,241,031, all of which are hereby incorporated herein by reference, in their entirety.
As used herein, “aft” refers to the uphole direction or portion of an element in a passage, and “forward” refers to the downhole direction or portion of an element. When an element is removed from a downhole passage, the aft end of the element emerges from the hole before the forward end.
Gripper Assembly with Rollers on Toes
The cylinder 108 is fixed with respect to the mandrel 102. A toe support 118 is fixed onto the forward end of the cylinder 108. A plurality of gripper portions 112 are secured onto the gripper assembly 100. In the illustrated embodiment the gripper portions comprise flexible toes or beams 112. The toes 112 have ends 114 pivotally or hingedly secured to the fixed toe support 118 and ends 116 pivotally or hingedly secured to the sliding toe support 106. As used herein, “pivotally” or “hingedly” describes a connection that permits rotation, such as by a pin or hinge. The ends of the toes 112 are engaged on rods or pins secured to the toe supports.
Those of skill in the art will understand that any number of toes 112 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 100, and therefore permits greater radial thrust and drilling power of the tractor. However, it is preferred to have three toes 112 for more reliable gripping of the gripper assembly 100 onto the inner surface of a borehole, such as the surface 42 in
A driver or slider element 122 is slidably engaged on the mandrel 102 and is longitudinally positioned generally at about a longitudinal central region of the toes 112. The slider element 122 is positioned radially inward of the toes 112, for reasons that will become apparent. A tubular piston rod 124 is slidably engaged on the mandrel 102 and connected to the aft end of the slider element 122. The piston rod 124 is partially enclosed by the cylinder 108. The slider element 122 and the piston rod 124 are preferably prevented from rotating with respect to the mandrel 102, such as by a splined interface between such elements and the mandrel.
In a preferred embodiment, two ramps 126 are spaced apart generally by the length of the central region 148 (
Each toe 112 is provided with a driver interaction element on the central region 148 (
The piston rod 124 connects the slider element 122 to a piston 138 enclosed within the cylinder 108. The piston 138 has a generally tubular shape. The piston 138 has an aft or actuation side 139 and a forward or retraction side 141. The piston rod 124 and the piston 138 are longitudinally slidably engaged on the mandrel 102. The forward end of the piston rod 124 is attached to the slider element 122. The aft end of the piston rod 124 is attached to the retraction side 141 of the piston 138. The piston 138 fluidly divides the annular space between the mandrel 102 and the cylinder 108 into an aft or actuation chamber 140 and a forward or retraction chamber 142. A seal 143, such as a rubber O-ring, is preferably provided between the outer surface of the piston 138 and the inner surface of the cylinder 108. A return spring 144 is engaged on the piston rod 124 and enclosed within the cylinder 108. The spring 144 has an aft end attached to and/or biased against the retraction side 141 of the piston 138. A forward end of the spring 144 is attached to and/or biased against the interior surface of the forward end of the cylinder 108. The spring 144 biases the piston 138, piston rod 124, and slider element 122 toward the aft end of the mandrel 102. In the illustrated embodiment, the spring 144 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.
The central section 148 of the toe 112 houses the rollers 132 and a pressure compensated lubrication system for the rollers. In the preferred embodiment, the lubrication system comprises two elongated lubrication reservoirs 152 (one in each sidewall 135), each housing a pressure compensation piston 154. The reservoirs 152 preferably contain a lubricant, such as oil or hydraulic fluid, which surrounds the ends of the roller axles 136. In the illustrated embodiment, each side wall 135 includes one reservoir 152 that lubricates the ends of the two axles 136 for the two rollers 132 contained within the toe 112. It will be understood by those of skill in the art that each toe 112 may instead include a single contiguous lubrication reservoir having sections in each of the side walls 135. Preferably, seals 158, such as O-ring or Teflon lip seals, are provided between the ends of the rollers 132 and the interior of the side walls 135 to prevent “flow-by” drilling fluid in the recess 134 from contacting the axles 136. As noted above, the axles 136 can be maintained in recesses in the inner surfaces of the sidewalls 135. Alternatively, the axles 136 can be maintained in holes that extend through the sidewalls 135, wherein the holes are sealed on the outer surfaces of the sidewalls 135 by plugs.
The pressure compensation pistons 154 maintain the lubricant pressure at about the pressure of the fluid in the annulus 40 (
The pressure compensation system provides better lubrication to the axles 136 and promotes longer life of the seals 158. As seen in
The gripper assembly 100 has an actuated position (as shown in
The positioning of the piston 138 controls the position of the gripper assembly 100 (i.e., actuated or retracted). Preferably, the position of the piston 138 is controlled by supplying pressurized drilling fluid to the actuation chamber 140. The drilling fluid exerts a pressure force onto the aft or actuation side 139 of the piston 138, which tends to move the piston toward the forward end of the mandrel 102 (i.e., toward the mandrel cap 104). The force of the spring 144 acting on the forward or retraction side 141 of the piston 138 opposes this pressure force. It should be noted that the opposing spring force increases as the piston 138 moves forward to compress the spring 144. Thus, the pressure of drilling fluid in the actuation chamber 140 controls the position of the piston 138. The piston diameter is sized to receive force to move the slider element 122 and piston rod 124. The surface area of contact of the piston 138 and the fluid is preferably within the range of 1.0-10.0 in2.
Forward motion of the piston 138 causes the piston rod 124 and the slider element 122 to move forward as well. As the slider element 122 moves forward to an actuation position, the ramps 126 move forward, causing the rollers 132 to roll up the inclined surfaces of the ramps. Thus, the forward motion of the slider element 122 and of the ramps 126 radially displaces the rollers 132 and the central sections 148 of the toes 112 outward. The toe support 106 slides in the aft direction to accommodate the outward flexure of the toes 112. The provision of a sliding toe support minimizes stress concentrations in the toes 112 and thus increases downhole life. In addition, the open end of the toe support 106 allows the portion of a failed toe to fall off of the gripper assembly, thus increasing the probability of retrieval of the tractor. The ends 114 and 116 of the toes 112 are pivotally secured to the toe supports 118 and 106, respectively, and thus maintain a constant radial position at all times.
Thus, the gripper assembly 100 is actuated by increasing the pressure in the actuation chamber 140 to a level such that the pressure force on the actuation side 139 of the piston 138 overcomes the force of the return spring 144 acting on the retraction side 141 of the piston. The gripper assembly 100 is retracted by decreasing the pressure in the actuation chamber 140 to a level such that the pressure force on the piston 138 is overcome by the force of the spring 144. The spring 144 then forces the piston 138, and thus the slider element 122, in the aft direction. This allows the rollers 136 to roll down the ramps 126 so that the toes 112 relax. When the slider element 122 slides back to a retraction position, the toes 112 are completely retracted and generally parallel to the mandrel 102. In addition, the toes 112 are somewhat self-retracting. The toes 112 comprise flexible beams that tend to straighten out independently. Thus, in certain embodiments of the present invention, the return spring 144 may be omitted. This is one of many significant advantages of the gripper assembly of the present invention over prior art grippers, such as the above-mentioned three-bar linkage design.
Another major advantage of the gripper assembly 100 over the prior art is that it can be actuated and retracted without substantial production of sliding friction. The rollers 132 roll along the ramps 126. The interaction of the rollers 132 and the ramps 126 provides relatively little impedance to the actuation and retraction of the gripper assembly. Though there is some rolling friction between the rollers 132 and the ramps 126, the impedance to actuation and retraction of the gripper assembly provided by rolling friction is much less than that caused by the sliding friction inherent in some prior art grippers.
In operation, the gripper assembly 100 slides along the body of the tractor, so that the tractor body can move longitudinally when the gripper assembly grips onto the inner surface of a borehole. In particular, the mandrel 102 slides along a shaft of the tractor body, such as the shafts 64 or 66 of
Advantageously, the toe support 106 on the forward end of the gripper assembly 100 permits the toes 112 to relax as the assembly is pulled out of a borehole from its aft end. While the gripper assembly is pulled out, the toe support 106 may be biased forward relative to the remainder of the assembly by the borehole formation, drilling fluids, rock cuttings, etc., so that it slides forward. This causes the toes 112 to retract from the borehole surface and facilitates removal of the assembly.
The gripper assembly 100 has seen substantial experimental verification of operation and fatigue life. An experimental version of the gripper assembly 100 has been operated and tested within steel pipe. These tests have demonstrated a fully functional operation with very little indication of wear after 32,000 cycles when the experimental gripper assembly was actuated with 1500 psi to produce 5000 lbs thrust and withstand 500-ft-lbs of torque. In addition, the experimental gripper assembly has “walked” down hole for 34,600 feet, drilled over 360 feet, operated for over 96 hours, and gripped formations of various compressive strengths ranging from 250-4000 psi. Under normal drilling conditions, the experimental gripper assembly has demonstrated resistance to contamination by rock cuttings. Under typical flow and pressure conditions, the experimental gripper assembly 100 has been shown to induce a flow-by pressure drop of less than 0.25 psi.
Gripper Assembly with Rollers on Slider Element
Although the gripper assembly 155 shown in
The gripper assembly 155 shown in
Radial Loads Transmitted to Borehole
The gripper assemblies 100 and 155 described above and shown in
As noted above, the ramps 126, 160 can be shaped to have a varying or non-varying angle of inclination with respect to the mandrel 102.
In addition to the embodiments shown in
In addition to the embodiments shown in
Gripper Assembly with Toggles
In the illustrated embodiment, there are two toggles 176 for each toe 112. Those of ordinary skill in the art will understand that any number of toggles can be provided for each toe 112. However, it is preferred to have two toggles having second ends 180 generally at or near the ends of the central section 148 of each toe 112. This configuration results in a more linear shape of the central section 148 when the gripper assembly 170 is actuated to grip against a borehole surface. This results in more surface area of contact between the toe 112 and the borehole, for better gripping and more efficient transmission of loads onto the borehole surface.
The gripper assembly 170 operates similarly to the gripper assemblies 100 and 155 described above. The gripper assembly 170 has an actuated position in which the toes 112 are flexed radially outward, and a retracted position in which the toes 112 are relaxed. In the retracted position, the toggles 176 are oriented substantially parallel to the mandrel 102, so that the second ends 180 are relatively near the surface of the mandrel. As the piston 138, piston rod 124, and slider element 172 move forward, the first ends 178 of the toggles 176 move forward as well. However, the second ends 180 of the toggles are prevented from moving forward by the recesses 175 on the toes 112. Thus, as the slider element 172 moves forward, the toggles 176 rotate outward so that they are oriented diagonally or even nearly perpendicular to the mandrel 102. As the toggles 176 rotate, the second ends 180 move radially outward, which causes radial displacement of the central sections 148 of the toes 112. This corresponds to the actuated position of the gripper assembly 170. If the piston 138 moves back toward the aft end of the mandrel 102, the toggles 176 rotate back to their original position, substantially parallel to the mandrel 102.
Compared to the gripper assemblies 100 and 155 described above, the gripper assembly 170 does not transmit significant radial loads onto the borehole surface when the toes 112 are only slightly radially displaced. However, the gripper assembly 170 comprises a significant improvement over the three-bar linkage gripper design of the prior art. The toes 112 of the gripper assembly 155 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 170 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 170 over the multi-bar linkage design is that the toggles 176 provide radial force at the central sections 148 of the toes 112. 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 170 involves a more direct application of force at the central section 148 of the toe 112, which contacts the borehole surface. Another advantage of the gripper assembly 170 is that it can be actuated and retracted substantially without any sliding friction.
Double-Acting Piston
With regard to all of the above-described gripper assemblies 100, 155, and 170, the return spring 144 may be eliminated. Instead, the piston 138 can be actuated on both sides by fluid pressure.
To actuate the gripper assembly 190, fluid is discharged from the retraction chamber 142 and delivered to the actuation chamber 140. To retract the gripper assembly 190, fluid is discharged from the actuation chamber 140 and delivered to the retraction chamber 142. In one embodiment, the surface area of the retraction side 141 of the piston 138 is greater than the surface area of the actuation side 139, so that the gripper assembly has a tendency to retract faster than it actuates. In this embodiment, the retraction force to release the gripper assembly from the borehole surface will be greater than the actuation force that was used to actuate it. This provides additional safety to assure release of the gripper assembly from the hole wall. Preferably, the ratio of the surface area of the retraction side 141 to the surface area of the actuation side 139 is between 1:1 to 6:1, with a preferred ratio being 2:1.
Failsafe Operation
In a preferred embodiment, the tractor 50 (
The failsafe assembly 230 comprises failsafe valves 232A and 232F. The valve 232A controls the fluid input and output of the gripper assembly 10A, while the valve 232F controls the fluid input and output of the gripper assembly 100F. Preferably, the tractor includes one failsafe valve 232 for each gripper assembly 100. In one embodiment, the failsafe valves 232A/F are two-position, two-way spool valves. These valves are preferably formed of materials that resist wear and erosion caused by exposure to drilling fluids, such as tungsten carbide.
In a preferred embodiment, the failsafe valves 232A/F are maintained in first positions (shown in
One advantage of restraints V comprising dents or protrusions without a spring return function on the failsafe valves 238A/F is that once the valves shift to their second positions, they will not return to their first positions while the tool is downhole. Advantageously, the gripper assemblies will remain retracted to facilitate removal of the tool from the hole.
The failsafe valve 232A is fluidly connected to the actuation and retraction chambers 140A and 142A. In its first position (shown in
The illustrated configuration also includes a motorized packerfoot valve 234, preferably a six-way spool valve. The packerfoot valve 234 controls the actuation and retraction of the gripper assemblies 100A/F by supplying fluid alternately thereto. The position of the packerfoot valve 234 is controlled by a motor 245. The packerfoot valve 234 fluidly communicates with a source of high pressure input fluid, typically drilling fluid pumped from the surface down to the tractor through the drill string. The packerfoot valve 234 also fluidly communicates with the annulus 40 (
In the position shown in
In the position shown in
Also, in the position shown in
Thus, in the illustrated position of the valves the aft gripper assembly 100A is retracted and the forward gripper assembly 100F is actuated. Those of ordinary skill in the art will understand that if the packerfoot value 234 is shifted to the right in
The same is true when the packerfoot valve 234 shifts so that the aft gripper assembly 100A is actuated and the forward gripper assembly 100F is retracted. In that case, loss of electrical control of the tractor will result in pressure buildup in the high pressure fluid chamber 238A. This will cause the failsafe valve 232A to switch positions so that high pressure fluid flows into the retraction chamber 142A of the gripper assembly 100A. The threshold pressure at which the failsafe valves switch their positions can be controlled by careful selection of the physical properties (geometry, materials, etc.) of the restraints V.
Materials for the Gripper Assemblies
The above-described gripper assemblies may utilize several different materials. Certain tractors may use magnetic sensors, such as magnetometers for measuring displacement. In such tractors, it is preferred to use non-magnetic materials to minimize any interference with the operation of the sensors. In other tractors, it may be preferred to use magnetic materials. In the gripper assemblies described above, the toes 112 are preferably made of a flexible high strength, fracture resistant, long fatigue life material. Non-magnetic candidate materials for the toes 112 include copper-beryllium, Inconel, and suitable titanium or titanium alloy. Other possible materials include nickel alloys and high strength steels. The exterior of the toes 112 may be coated with abrasion resistant materials, such as various plasma spray coatings of tungsten carbide, titanium carbide, and similar materials.
The mandrel 102, mandrel caps 104 and 110, piston rod 124, and cylinder 108 are preferably made of high strength magnetic metals such as steel or stainless steel, or non-magnetic materials such as copper-beryllium or titanium. The return spring 144 is preferably made of stainless steel that may be cold set to achieve proper spring characteristics. The rollers 132 are preferably made of copper-beryllium. The axles 136 of the rollers 132 are preferably made of a high strength material such as MP-35N alloy. The seal 143 for the piston 138 can be formed from various types of materials, but is preferably compatible with the drilling fluids. Examples of acceptable seal materials that are compatible with some drilling muds include HNBR, Viton, and Aflas, among others. The piston 138 is preferably compatible with drilling fluids. Candidate materials for the piston 138 include high strength, long life, and corrosion-resistant materials such as copper beryllium alloys, nickel alloys, nickel-cobalt-chromium alloys, and others. In addition, the piston 138 may be formed of steel, stainless steel, copper-beryllium, titanium, Teflon-like material, and other materials. Portions of the gripper assembly may be coated. For example the piston rods 124 and the mandrel 102 may be coated with chrome, nickel, multiple coatings of nickel and chrome, or other suitable abrasion resistant materials.
The ramps 126 (
The toggles 176 of the gripper assembly 170 can be made of various materials compatible with the toes 112. The toggles are preferably made of materials that are not chemically reactive in the presence of water, diesel oil, or other downhole fluids. Also, the materials are preferably abrasion and fretting resistant and have high compressive strength (80-200 ksi). Candidate materials include steel, tungsten carbide infiltrates, nickel steels, Inconel alloys, and others. The toggles may be coated with materials to prevent wear and decrease fretting or galling. Such coatings can be sprayed or otherwise applied (e.g., EB welded or diffusion bonded) to the toggles.
Performance
Many of the performance capabilities of the above-described gripper assemblies will depend on their physical and geometric characteristics. With specific regard to the gripper assemblies 100 and 155, the assembly can be adjusted to meet the requirements of gripping force and torque resistance. In one embodiment, the gripper assembly has a diameter of 4.40 inches in the retracted position and is approximately 42 inches long. This embodiment can be operated with fluid pressurized up to 2000 psi, can provide up to 6000 pounds of gripping force, and can resist up to 1000 foot-pounds of torque without slippage between the toes 112 and the borehole surface. In this embodiment, the toes 112 are designed to withstand approximately 50,000 cycles without failure.
The gripper assemblies of the present invention can be configured to operate over a range of diameters. In the above-mentioned embodiment of the gripper assemblies 100 and 155 having a collapsed diameter of 4.40 inches, the toes 112 can expand radially so that the assembly has a diameter of 5.9 inches. Other configurations of the design can have expansion up to 6.0 inches. It is expected that by varying the size of the toe 112 and the toe supports 106 and 118, a practical range for the gripper is 3.0 to 13.375 inches.
The size of the central sections 148 of the toes 112 can be varied to suit the compressive strength of the earth formation through which the tractor moves. For example, wider toes 112 may be desired in softer formations, such as “gumbo” shale of the Gulf of Mexico. The number of toes 112 can also be altered to meet specific requirement for “flow-by” of the returning drilling fluid. In a preferred embodiment, three toes 112 are provided, which assures that the loads will be distributed to three contact points on the borehole surface. In comparison, a four-toed configuration could result in only two points of contact in oval-shaped passages. Testing has demonstrated that the preferred configuration can safely operate in shales with compressive strengths as low as 250 psi. Alternative configurations can operate in shale with compressive strength as low as 150 psi.
The pressure compensation and lubrication system shown in
The above-described gripper assemblies are capable of surviving free expansion in open holes. The assemblies are designed to reach a maximum size and then cease expansion. This is because the ramps 126, 160 and the toggles 176 are of limited size and cannot radially displace the toes 112 beyond a certain extent. Moreover, the size of the ramps and toggles can be controlled to ensure that the toes 112 will not be radially displaced beyond a point at which damage may occur. Thus, potential damage due to free expansion is prevented.
The metallic toes 112 formed of copper-beryllium have a very long fatigue life compared to prior art gripper assemblies. The fatigue life of the toes 112 is greater than 50,000 cycles, producing greater downhole operational life of the gripper assembly. Further, the shape of the toes 112 provides very little resistance to flow-by, i.e., drilling fluid returning from the drill bit up through the annulus 40 (
Another advantage of the gripper assemblies of the present invention is that they provide relatively uniform borehole wall gripping. The gripping force is proportional to the actuation fluid pressure. Thus, at higher operating pressures, the gripper assemblies will grip the borehole wall more tightly.
Another advantage is that a certain degree of plastic deformation of the toes 112 does not substantially affect performance. It has been determined that when the gripper assembly is halfway in a passage or borehole, the portion of the toes 112 that are outside of the passage and are permitted to freely expand may experience a slight amount of plastic deformation. In particular, each toe 112 may plastically deform (i.e. bend) slightly in the sections 150 (
In summary, the gripper assemblies of various embodiments of the present invention provide significant utility and advantage. They are relatively easy to manufacture and install onto a variety of different types of tractors. They are capable of a wide range of expansion from their retracted to their actuated positions. They can be actuated with little or no production of sliding friction, and thus are capable of transmitting larger radial loads onto a borehole surface. They permit rapid actuation and retraction, and can safely and reliably disengage from the inner surface of a passage without getting stuck. They effectively resist contamination from drilling fluids and other sources. They are not damaged by unconstrained expansion, as may be experienced in washouts downhole. They are able to operate in harsh downhole conditions, including pressures as high as 16,000 psi and temperatures as high as 300° F. They are able to simultaneously resist thrusting or drag forces as well as torque from drilling, and have a long fatigue life under combined loads. They are equipped with a failsafe operation that assures disengagement from the borehole wall under drilling conditions. They have a very cost-effective life, estimated to be at least 100-150 hours of downhole operation. They can be immediately installed onto existing tractors without retrofitting.
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.
This application claims the benefit under 35 U.S.C. § 120 of U.S. patent application Ser. No. 10/268,604, filed Oct. 9, 2002, which claims the benefit under 35 U.S.C. § 120 of U.S. patent application Ser. No. 09/777,421, filed Feb. 6, 2001, now U.S. Pat. No. 6,464,003, which claims the benefit under 35 U.S.C. § 119 of U.S. Provisional Patent Application Ser. No. 60/182,995, entitled “FLUID POWERED FLEXTOE PACKERFOOT,” filed on Feb. 16, 2000; U.S. Provisional Patent Application Ser. No. 60/205,937, entitled “PACKERFOOT IMPROVEMENTS,” filed on May 18, 2000; and U.S. Provisional Patent Application Ser. No. 60/228,918, entitled “ROLLER TOE GRIPPER,” filed on Aug. 29, 2000. Each of the above-identified applications is hereby incorporated by reference in its entirety.
