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CROSS-REFERENCE TO RELATED APPLICATIONS 
   None 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to systems for actuating one or more tools adapted for use in a wellbore. 
   2. Description of the Related Art 
   Hydrocarbons such as oil and gas are recovered from a subterranean formation using a wellbore drilled into the formation. A number of tools are used throughout the process of drilling and completing the wellbore and also during the production life of the well. Many of these tools are energized using pressurized fluid that is self-contained in the tool, pumped downhole from the surface, or fluid that is produced from the well itself. These tools, which are sometimes referred to as hydraulically actuated tools, can be put to a number of uses. 
   One use for hydraulically actuated tools is to set a liner hanger. During drilling, the wellbore is lined with a string of casing that is cemented in place to provide hydraulic isolation and wellbore integrity. Commonly, multiple strings of casing are set in a well in a successive fashion. For example, a first string of casing is set in the wellbore after the well is drilled to a first depth and a second string of casing is run into the wellbore after the well is drilled to a second depth. The second string is set such that the upper portion of the second string of casing overlaps with the lower portion of the first string of casing. Any string of casing that does not extend back to the surface is generally referred to as a liner. The second string is then cemented into the wellbore as well. This process may be repeated as needed. 
   The liner hanger is used to hang or anchor a liner off of a string of other casing string. Several types of liner hangers are known in the art, which includes hydraulic liner hangers. In conventional hydraulic liner hangers, fluid is supplied under pressure into an annular space between a mandrel and a surrounding cylinder. The hydrostatic pressure of the fluid between the cylinder and the mandrel creates a force on the inner surface area of the cylinder that causes the cylinder to slide longitudinally. 
   Conventionally, the hydraulic liner hanger is set by applying a predetermined level of hydrostatic pressure to the liner hanger. That is, the liner hanger is run into the wellbore while in contact with a fluid having a first hydrostatic pressure and then actuated by increasing the pressure in the fluid. In an conventional arrangement, a ball is dropped into the wellbore and landed on a seat that is positioned generally downhole of the liner hanger. Fluid is then injected into the wellbore under pressure in order to actuate the hydraulic liner hanger. 
   Conventional hydraulic liner hangers can prematurely set if there is a pressure spike of sufficient magnitude in the drill string or if the pressure of the fluid external to the liner hanger unexpected drops. Conventional measures to prevent unintended setting of the liner hanger include the use of shear pins to mechanically restrain the cylinder while the liner assembly is run into the hole and closures or flow restriction devices that prevent fluid from entering the hydraulic liner hanger until the liner hanger is ready to be set. 
   These conventional measures have various drawbacks that include, but are not limited to, expense and tool complexity. These conventional measures may also impose undesirable constraints in deployment of the liner hanger such as permissible bounds for drilling fluid circulation pressures and flow rates. Moreover, the drawbacks of conventional hydraulic liner hanger are merely illustrative of the general drawbacks of wellbore tools in general that operate using hydrostatic pressure while in the wellbore. 
   The present invention addresses these and other drawbacks of the prior art. 
   SUMMARY OF THE INVENTION 
   In aspects, the present invention provides systems, devices, and methods for actuating a wellbore tool. An exemplary actuator made in accordance with the present invention is operatively coupled to the wellbore tool and conveyed into a wellbore via a work string. When the fluid in the work string reaches a predetermined applied pressure, the actuator undertakes a specified action such as longitudinal motion, rotation, expansion, etc that actuates or operates the wellbore tool. Premature actuation of the wellbore tool is prevented by applying to the actuator a resistive force that, alone or in cooperation with another mechanism, arrests or restrains movement of the actuator. This resistive force is generated by applied pressure of the fluid in the work string. 
   In one arrangement adapted for use on a drill string, the actuator includes an actuating member having a first and a second pressure chamber. The actuator also includes a pressure control device that can control the pressure in the two chambers. The two pressure chambers are independently hydraulically coupled to the fluid in the drill string and are arranged such that the pressures in the chambers generate opposing forces, a motive force and a resistive force, on the actuating member. In one embodiment, the actuating member includes a cylinder slidably disposed on a mandrel. The pressure chambers, which are formed between the cylinder and mandrel, communicate with the drill string fluid via ports formed in the mandrel. When needed, the pressure control device forms a hydraulic seal between the two chambers by using, for example, a sealing member and occlusion member. This hydraulic seal hydraulically couples the first pressure chamber to the fluid uphole of the hydraulic seal. The fluid downhole of the hydraulic seal and the second pressure chamber are largely isolated from pressure increases in the uphole fluid due to the hydraulic seal. 
   To activate the actuator and thereby actuate the wellbore tool, the pressure in the first chamber is increased relative to the pressure in second chamber. For example, after the hydraulic seal is formed by the pressure control device, a surface pump can be energized to increase the applied pressure in the fluid uphole of the hydraulic seal. When so energized by the pressurized fluid, the magnitude of the motive force generated by the first pressure chamber increases. When an adequate pressure differential exists, the motive force overcomes the resistive force and the actuating member is thereby displaced. The displacement of the actuating member in turn actuates the wellbore tool. 
   The actuator can be configured to operate liner hangers as well as other tools used in the wellbore. Moreover, in addition to drilling fluid, the pressurized fluid can be water, synthetic material, hydraulic oil, or formation fluids. 
   It should be understood that examples of the more important features of the invention have been summarized rather broadly in order that detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For detailed understanding of the present invention, references should be made to the following detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, in which like elements have been given like numerals and wherein: 
       FIG. 1  schematically illustrates one embodiment of an actuating tool made in accordance with the present invention; 
       FIGS. 2A and 2B  schematically illustrate sectional views of an embodiment of an actuating tool made in accordance with the present invention that is adapted for use in connection with a liner hanger; 
       FIGS. 3A and 3B  illustrate sectional views of embodiment of pressure chambers in accordance with the present invention; 
       FIG. 4  schematically illustrates one embodiment of a pressure control device made in accordance with the present invention that uses a closure; 
       FIG. 5  schematically illustrates one embodiment of a pressure control device made in accordance with the present invention that uses a flow restriction device; and 
       FIG. 6  schematically illustrates a sectional elevation view of a liner drilling system utilizing an actuating tool made in accordance with the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention relates to devices and methods for actuating wellbore tools. The present invention is susceptible to embodiments of different forms. There are shown in the drawings, and herein will be described in detail, specific embodiments of the present invention with the understanding that the present disclosure is to be considered an exemplification of the principles of the invention, and is not intended to limit the invention to that illustrated and described herein. 
   Referring initially to  FIG. 1 , there is schematically illustrated one embodiment of a tool actuator  100  made in accordance with the present invention for operating a tool  10  conveyed via a work string  12  into a wellbore. The tool actuator  100 , as will be described in further detail below, operates in response to the applied pressure of the fluid. Applied pressure is generally defined as the total pressure applied by the fluid. The total pressure can be the hydrostatic pressure or can be the sum of several components such as hydrostatic pressure, dynamic pressure losses, and a pressure differentials caused by a device such as a surface mud pump or downhole pump. The actuator  100  includes an actuating member  102  connected directly or indirectly to the tool  10 , a first pressure chamber  104 , a second pressure chamber  106 , and a pressure control device  108 . The pressure control device  108  controls the pressure in each chamber  104  and  106 . The pressures in chamber  104  and  106  each generate a force on the actuating member  102  that substantially oppose one another. When the two chambers  104  and  106  have generally equal pressures, the opposing forces balance and the actuating member  102  remains stationary. When desired, the pressure control device  108  can vary the pressure in one of the two chambers  104  and  106  to cause a net force that causes the actuating member  102  react in a preset manner such as sliding, rotating, extending, retracting, etc. The reaction of the actuating member  102  thereby actuates the tool  10 . 
   In one arrangement, the actuator  100  is energized using pressurized fluid in a bore  14  of the work string  12 . The first and second pressure chambers  104  and  106  hydraulically communicate with the bore  14  via ports  110  and  112 , respectively. The first pressure chamber  104  generates a motive force F 1  adapted to displace the actuating member  102  whereas the second pressure chamber  106  generates a resistive force F 2  that temporarily or selectively offsets the force F 1  created by the first pressure chamber  104 . In such an arrangement, as long as chambers  104  and  106  communicate with pressurized fluid having the same hydraulic pressure, then the pressure values in the chambers  104  and  106  and corresponding generated forces will be substantially equal and the actuating member  102  will remain stationary, i.e., motion will be substantially arrested. 
   It should be appreciated that the actuating member  102  will remain stationary even if the applied pressure of the fluid in the bore  14  significantly and unexpectedly increases while the tool  100  is being run into the wellbore or sometime thereafter. This is so because the increased applied pressure will be applied to both chambers  104  and  106 . Thus, while the magnitude of the motive force F 2  may increase due to a pressure spike, the magnitude of the resistive force F 2  will also increase since the applied pressure of the fluid that energizes the first pressure chamber  104  also energizes the second pressure chamber  106 . Thus, the resisting force F 2  will act to cancel the motive force F 1  and thereby minimize the risk that the actuating member  102  will move. In like manner, if the pressure of the fluid external to the tool  100  unexpectedly drops, this pressure drop will not cause movement of the actuating member  102  because the second pressure chamber  104 , and the resistive force it generates, arrests movement of the actuating member  102  using the applied pressure of the fluid internal to the tool  100 . 
   When desired, the pressure control device  108  can cause a pressure imbalance or differential by allowing fluids having different applied pressures to communicate with each chamber  104  and  106 . Upon the pressure differential reaching a preset or predetermined value, the net force generated by the first pressure chamber  104  overcomes the opposing force of the second pressure chamber  106  and displaces the actuating member  102 , which then actuates the tool  10 . 
   It should be understood that the pressure chamber  106  need not provide the exclusive resistive force or mechanism for offsetting the motive force F 1 . For instance, a biasing member or spring can be utilized to provide a preset amount of resistance against movement of the actuating member  102 . Moreover, a shear pin or other frangible member can be used to increase the resistance the motive force F 1  must overcome before displacing the actuating member  102 . It should also be understood that the resisting force F 2  does not necessarily cause motion of the actuating member  102 . That is, the force F 2  can act to maintain the actuating member  102  at a limit or end point of a stroke of the actuating member  102 . 
   As will become apparent, the teachings of the present invention can be utilized for a variety of well tools and in all phases of well construction and production. Accordingly, the embodiments discussed below are merely illustrative of the applications of the present invention. 
   Referring now to  FIGS. 2A and 2B , there is shown an embodiment of an actuator  120  adapted to actuate a liner hanger  50 . The liner hanger is conventionally arranged and includes devices such as slips  52 , a slip retainer  54 , and a shear pin  56 . A work string or other suitable conveyance device (not shown) can be used to convey this and other equipment into a wellbore. The actuator  120  is energized by the applied pressure of fluid in an inner bore  126  of the actuator  120 . The actuator  120  is coupled to the slip retainer  54  and is configured to move the slips  52  longitudinally when the applied pressure in the bore  126  reaches a predetermined value. During this longitudinal movement, the slips  52  extend radially outward and engage a casing wall. 
   In one embodiment, the actuator  120  includes an inner mandrel  128  concentrically disposed within a surrounding cylinder  130 . The cylinder  130  is adapted to slide longitudinally along the mandrel  128 . For ease of assembly, the cylinder  130  includes an upper cylinder section  132 , a spacer  134 , and a lower cylinder section  136 . The spacer  134  connects together the upper and lower cylinder sections  132  and  136  such that the cylinder  130  operates as one integral member. Other embodiments of the cylinder  130 , of course, could have greater or fewer constituent parts. The actuator  120  includes a first pressure cavity or chamber  140  formed in the upper cylinder section  132  and a second pressure cavity or chamber  142  formed in the second cylinder section  136 . Ports  144  and  146  formed in the inner mandrel  128  hydraulically couple the chambers  140  and  142  to the inner bore  126 . As will be described in further detail below, a pressure imbalance or differential between the two chambers  140  and  142  create a net force that causes longitudinal movement of the cylinder  130 . 
   Referring now to  FIG. 3A , there is shown an exemplary arrangement of the chamber  140  for generating the motive force F 1  for displacing the cylinder  130 . During use, fluid in the bore  126  flows through the port  144  and fills the chamber  140 . The hydraulic pressure of the fluid in the chamber  140  applies a force to the surfaces defining the chamber  140 . Upon a predetermined pressure differential being caused between the chamber  140  and the chamber  142 , the cylinder  130  moves longitudinally along the direction specified with arrow B. To prevent or minimize the fluid from leaking out of the chamber  140 , the chamber can include seals  152 A and  152 B. In one embodiment, the seal  152 A is a movable sealing element that moves generally with the cylinder  130  and the seal  152 B is a stationary sealing element that is fixed to the inner mandrel  134  with suitable devices such as snap rings  153 . It should be understood, however, that other embodiments having different sealing elements may be utilized and that in still other embodiments the sealing elements can be omitted entirely. 
   Referring now to  FIG. 3B , there is shown an exemplary arrangement of the chamber  142  for providing a resisting force F 2  that at least partially offsets the motive force F 1  to at least temporarily arrest of restrain motion of the cylinder  130 . During use, fluid in the bore  126  flows through the port  146  and fills the chamber  142 . The hydraulic pressure of the fluid applies a force to the surfaces defining the chamber  142 . This force urges the cylinder  130  in the direction specified with arrow C, which is substantially opposite of arrow B. Similar to the chamber  140 , the chamber  142  can include seals  162 A and  162 B. In one embodiment, the seal  162 A is a movable sealing element that moves generally with the cylinder  130  and the seal  162 B is a stationary sealing element that is fixed to the inner mandrel  128  with suitable devices such as snap rings  163 . 
   It will be understood that the magnitude of the pressure differential that initiates motion of the cylinder  130  will depend on factors such as frictional forces, the applied pressure external to the tool actuator  100 , the shear strength of any shear pins that may be used to secure the slip assembly, etc. 
   Referring now to FIGS.  1 , 2 A- 2 B, a pressure control device  170  selectively controls the pressures in the chambers  140  and  142 . The pressure control device  170  is positioned between the ports  144  and  146  to thereby selectively hydraulically isolate the chambers to which the ports  144  and  146  respectively connect. The pressure control device  170  can maintain substantially equal pressures in the chambers  140  and  142  and also vary the pressure in either of the two chambers  140  and  142  to cause a pressure imbalance therebetween. For example, the pressure control device  170  can for one period of time maintain substantially equal pressures in the chambers  140  and  142  and in a successive period of time selectively increase the pressure in the chamber  140  or decrease the pressure in chamber  142 . Numerous embodiments of the pressure control device  170  can be utilized, a few of which are discussed below. 
   Referring still to FIGS.  1 , 2 A- 2 B, in one embodiment, the pressure control device  170  includes a sealing member  172  and an occlusion member  174  that cooperate to at least temporarily occlude the bore  126  of actuator  120 . During run in of the wellbore tool  100  and before actuation is required, the sealing member  172  permits flow through the bore  126 . To initiation activation of the actuator  120 , the occlusion member  174  is introduced at the surface into the tubular connecting the actuator  120  to the surface (e.g., drill string, coiled tubing, production string, etc.). The occlusion member  174  travels down the tubular and mates with the sealing member  172 , which has an opening or passage equal to or less than the size of the occlusion member  174 . The occlusion member  174  can include a ball, a plug or other object configured to create a barrier across the sealing member  172 . 
   When the occlusion member  174  and the sealing member  172  mate, a hydraulic seal is formed between the port  144  and the port  146 . This seal, which does not need to be a “zero leakage” seal, enables a substantial pressure differential thereacross. Thus, the pressure chambers  140  and  142  are in communication with two hydraulically independent bodies of fluid. The two bodies of fluid need not be completely isolated from one another, e.g., there can be some fluid or hydraulic communication between the two fluid bodies. 
   Merely for convenience, the fluid in region  180 , which communicates with the chamber  140 , will be referred to as the uphole fluid and the fluid in region  182 , which communicates with the chamber  142 , will be referred to as the downhole fluid. The pressure of the uphole fluid can be controlled, e.g., increased, using a device such as a mud pump. Increasing the pressure of the uphole fluid will, of course, increase the pressure in the first chamber  140 . Because of the seal provided by the pressure control device  170 , the pressure of the downhole fluid and the fluid in the second chamber  142  remains mostly at hydrostatic pressure and are largely unaffected by the increased pressure in the uphole fluid. 
   Thus, initially, the motive force F 1  and resistive force F 2  will cancel due to the first and second chambers  140  and  142  receiving fluid having the same applied pressure. However, after occlusion of the bore  126 , the increase of applied pressure in the uphole fluid and in the first chamber  140  will cause a corresponding increase in the magnitude of the force F 1 . Because the pressure in the downhole fluid is mostly static, the resistive force F 2  does not change. At a predetermined pressure differential between the chambers  140  and  142 , the motive force F 1  overcomes the resistive force F 2  and longitudinally displaces the cylinder  130 . The cylinder  130  via its connection to the slip retainer  54  actuates or sets the slips  52 . 
   It should be appreciated that the temporary occlusion in the well provides a hydraulic path to the chamber inducing the motive force while isolating or uncoupling the chamber inducing the resistive force from that hydraulic path. In addition to a surface pump increasing hydraulic pressure, other devices such as a downhole pump or even pyrotechnics can be used to selectively increase hydraulic pressure in that hydraulic path. 
   In one arrangement, after the slips  52  are set, pressure of the uphole fluid is further increased until the sealing member  172  deforms and allows the occlusion member  174  to pass therethrough. After the occlusion member  174  unseats and passes through the sealing member  172 , hydraulic communication and fluid flow is reestablished along the bore  126 . In certain embodiments, the sealing member  172  and occlusion member  174  can be configured to permit multiple selectively blockages of the bore  126 . 
   Other selective bore restriction devices suitable for use in embodiments of the present invention are disclosed in U.S. Pat. No. 5,146,992 and U.S. patent application Ser. No. 10/602,578 filed Jun. 24, 2003, titled “Plug and Expel Flow Control Device,” both of which are commonly assigned and are hereby incorporated by reference for all purposes. 
   Referring now to  FIG. 4 , there is shown a pressure control device  200  including an operator  202  that selectively displaces a closure member  204 . The closure member  204  is adapted to partially or completely seal the port  146  leading to the second pressure chamber  142  to thereby effectively isolate the second pressure chamber  142 . The pressure control device  200  can be adapted for either “one time” usage or multiple sealing and unsealing of the port  146  and can include a mechanical device, electro-mechanical device, hydraulic motor or other suitable device. For example, the operator can include a biasing member that applies a spring force, a pressure chamber actuated by hydraulic fluid, an electric motor, frangible devices that restrain the closure member  204 , etc. 
   Referring now to  FIG. 5 , there is shown another embodiment of a pressure control device  210  that includes a flow restriction device  212  such as a valve that selectively controls flow across the port  146 . The flow rate of the flow restriction device  212  can be adjusted using a solenoid or other suitable device. In still other embodiments, the pressure control device can merely include ports of differing cross-section flow areas. Referring now to  FIGS. 3A-3B , for example, the port (or ports) for the chamber  140  can have a larger cross-sectional flow area than the port (or ports) for the chamber  142 . The cross-sectional area differential can be selected such that the increase in hydraulic pressure in the bore is communicated faster to the chamber  140  than to chamber  142  to thereby provide a desired pressure differential between the chambers  140  and  142 . 
   It should be appreciated that the pressure control device, whatever the particular configuration, can control the degree to which hydraulic pressure in the bore is communicated to the pressure chambers. Moreover, it should be appreciated that fluid communication between the bore and the chambers need not be completely blocked in order to cause a desired pressure differential. 
   Referring now to  FIG. 6 , there is shown a well construction facility  230  positioned over subterranean formation  232 . While the facility  230  is shown as land-based, it can also be located offshore. The facility  230  can include known equipment and structures such as a derrick  234  at the earth&#39;s surface  236 , a casing  238 , and mud pumps  240 . A work string  242  suspended within a well bore  244  is used to convey tooling and equipment into the wellbore  244 . The work string  242  can include jointed tubulars, drill pipe, coiled tubing, production tubing, liners, casing and can include telemetry lines or other signal/power transmission mediums that establish one-way or two-way data communication and power transfer from the surface to a tool connected to an end of the work string  242 . A suitable telemetry system (not shown) can be known types as mud pulse, electrical signals, acoustic, or other suitable systems. The tooling and equipment conveyed into the wellbore can include, but are not limited to, bottomhole assemblies, tractors, thrusters, steering units, drilling motors, downhole pumps, completion equipment, perforating guns, tools for fracturing the formation, tools for washing the wellbore, screens and other production equipment. 
   For illustrative purposes, the work string  242  is shown as including a drill string conveying a bottomhole assembly adapted for liner drilling (“liner drilling assembly”)  246  into the wellbore  244 . Exemplary liner drilling systems are discussed commonly assigned U.S. Pat. Nos. 5,845,722 and 6,196,336, which are hereby incorporated by reference for all purposes. The liner drilling assembly  246  includes a liner hanger  248  and an actuator  250 . 
   Referring now to  FIGS. 2-6 , in an exemplary deployment, the liner drilling assembly  246  drills the wellbore  244  while the mud pump  240  circulates drilling fluid down the drill string  242 . The drilling fluid and entrained drill cuttings return up an annulus  252  formed by the drill string  242  and the wellbore  244 . During drilling, both pressure chambers  140 ,  142  of the actuator  120  communicate with the drilling fluid in the drill string  244  and thus both pressure chambers  140 ,  142  have approximately the same applied pressure as the drilling fluid in the drill string  242 . Accordingly, the opposing forces created by the pressures in the first and second chambers  140 ,  142  are substantially equal and balance each other. Thus, advantageously, the actuator  120  remains substantially stationary regardless of the applied pressure value or pressure fluctuations inside the drill string  242 . 
   Once the liner drilling assembly  246  drills to a desired depth, the liner hanger  248  can be actuated in the following manner. In embodiments utilizing occlusion of the bore  126 , such as in  FIG. 2A and 2B , drilling is halted and the occlusion member  174  is “dropped” into the drill string  242 . The occlusion member  174  flows down through the drill string  242  until it mates with the sealing member  172  to form an occlusion in the drill string  242  that hydraulically separates the first pressure chamber  140  from the second pressure chamber  142 . Thereafter, the mud pump  240  is operated to increase the applied pressure of the drilling fluid in the drill string  242 . Because of the occlusion, the applied pressure will increase only in the drilling fluid column inside the drill string  242  and uphole of the occlusion. The drilling fluid column in the drill string  242  and below the occlusion will remain at a lower applied pressure. Because the first pressure chamber  140  communicates with the fluid uphole of the occlusion, the applied pressure in first pressure chamber  140  increases relative to the pressure in the second pressure chamber  142 , which is communication with the drilling fluid downhole of the occlusion. Once a sufficient pressure differential is created between the first and second pressure chambers  140 ,  142 , the net force applied by the first pressure chamber  140  urges the cylinder  130  longitudinally toward the slips  52 . Via the slip retainer  54 , the cylinder  130  drives the hanger slips  52  into engagement with the casing  238 . 
   In addition to being largely immune from pressure fluctuations during drilling, the actuator  120  also cannot be inadvertently actuated by pressure fluctuations when the liner drilling assembly  248  and drill string  244  are run into the hole (e.g., due to surge). 
   It should be appreciated that embodiments of the present invention provide numerous operational and situational advantages. For example, during drilling, formations having relatively a low fracture pressure could be encountered. In such a situation, increasing the pressure in the wellbore to set a liner hanger could expose the formation to excessive applied pressures. With embodiments of the present invention, it should be seen that the increased applied pressure used for actuating the tool actuator and thereby setting the liner hanger is confined mostly within the drill string. Thus, the formation is largely protected from damage that would otherwise occur if exposed to applied pressure in excess of the formation fracture pressure. 
   In another example, during drilling, the hydrostatic pressure external to the drill string could be significantly lower than the hydrostatic pressure within the drill string. Such a situation could arise, for instance, where drilling fluid lost to the formation reduces the hydrostatic pressure of the drilling fluid flowing up the wellbore annulus. Because operation of the tool actuator is initiated by actively controlling pressure within the drill string, the tool actuator is largely immune to the value the hydrostatic pressure of fluid external to the drill string or tool actuator. That is, even a dramatic drop in external pressure will not induce movement of the actuator since the resistive force opposing movement utilizes hydrostatic pressure within the actuator to prevent unintended activation of the actuator. 
   It should further appreciated that the teachings of the present invention can be readily applied to numerous tools outside the liner drilling context. For example, in certain applications, fluids such as water, acids, fracturing fluids, may be circulated in the wellbore. Also, formation fluids such as oil and water can be utilized in some circumstances to energize the actuator. 
   Some embodiments of the present invention can be adapted for use in situations where fluid pressure is not used to energize a tool or device. For example, some tools may be actuated or energized by vibrations, mud pulse, motion of the tool, frequency, electronic signals, etc. Aspects of the present invention, including, but not limited to the use of opposing forces, can be advantageously applied in such circumstances. 
   Further, it should be understood that while the embodiments described illustrate only two pressure chambers, additional pressure chambers can be added to further extend the utility of devices made in accordance with the present invention. In the same regard, while actuation of the a wellbore tool has been described, embodiments of the present invention can be readily adapted to return a wellbore tool to a condition prior to actuation (e.g., turn a tool on and off, set and set a tool, etc.) 
   Additionally, it should be understood that the terms such as “first” and “second” and “uphole” and “downhole” do not signify any specific priority, importance, or orientation but are merely used in better describe the relative relationships between the items to which they are applied. Also, the term longitudinal generally refers to a direction along the long axis of a wellbore or tool, but as noted above, the actuator is not limited to motion in any particular direction. 
   The foregoing description is directed to particular embodiments of the present invention for the purpose of illustration and explanation. It will be apparent, however, to one skilled in the art that many modifications and changes to the embodiment set forth above are possible without departing from the scope and the spirit of the invention. It is intended that the following claims be interpreted to embrace all such modifications and changes.

Summary:
An actuator operatively coupled to a wellbore tool is activated upon receiving fluid that a predetermined applied pressure. When the fluid string reaches the predetermined applied pressure, the actuator undertakes a specified action such as longitudinal movement, rotation, expansion, etc. that actuates or operates the wellbore tool. Premature actuation of the wellbore tool is prevented by applying a resistive force to the actuator that, alone or in cooperation with another mechanism, arrests movement of the actuator. This resistive force is generated by applied pressure of the fluid in the work string.