You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
FIELD OF THE INVENTION 
       [0001]    The field of the invention is locks that keep mechanically set packers from setting prematurely and more particularly techniques for providing housings in very deep applications that have sufficient pressure rating while functioning as a releasable lock assembly. 
       BACKGROUND OF THE INVENTION 
       [0002]    Mechanically set packers typically involve a set of drag block so that one component of two that are relatively moveable with respect to the other can be held stationary while a mandrel is moved generally axially for setting the packer. Typically a j-slot assembly connects the stationary and the movable components so that reciprocation of the string in combination with the j-slot mechanism induces a rotational movement as a pin follows a slot so that the packer is not only set but the set condition is locked. Reversal of the movement with have the pin follow the slot in reverse to release the mechanically set packer as the slips have the cones pulled out from under them that in turn allows the compressed sealing element to reduce in diameter and increase in length so that the assembly can be removed from the wellbore using the string that supports the packer mandrel. 
         [0003]    Getting the packer to the desired location especially in a deviated borehole involves string manipulation in the axial and rotational directions where the packer body can scrape the casing or tubing. If the setting mechanism was not locked during run in it is possible that the packer manipulations to get it to the desired location could inadvertently set it. This would be disadvantageous especially if the packer was not of a retrievable design. In those cases it would require a release from the packer and another trip into the wellbore to mill it out with a mill. Even if the packer is resettable, it can be damaged by being forced to the desired location if it is in the deployed position. 
         [0004]    The lock to prevent packer setting has to be a compact design so that it will not increase the drift dimension of the packer assembly during run in. The design also has to operate reliably and to be cost effective. One way to keep the cost down for such a lock assembly is to have it release in response to annulus pressure above a predetermined value. One such design that has been developed is shown in U.S. Pat. No. 5,320,183 where access to annulus pressure is made available upon a pressure buildup to a predetermined value in the annulus that opens a barrier to let that pressure into a chamber  30   c   3   a  that in turn pushes on an annular piston  30   b   3   a  shown in  FIG. 2  so that the sleeve  30   c   2 ,  30   c   1  moves down and the dogs  30   a  move out of groove  10   a . The mandrel  10  can then be manipulated. An alternative embodiment removes support for a dog extending into the mandrel to fixate it and in the process pulls a sleeve into the groove that the dog has vacated as a result of initial mandrel movement. The actuation of the sleeve to remove support for the dogs does not remove the dogs from their groove until after sleeve movement and mandrel manipulation. The subsequent covering of the groove formerly occupied by the dogs is taken to insure that the dogs cannot get back into the groove and prevent relative movement with respect to the packer that is now in the process of being set. 
         [0005]    As packer setting locations get deeper the pressure rating of housings to withstand the higher hydrostatic forces from such depths has to be much higher than previously needed. The use of an annular piston as described above forces a reduction in wall thickness for the housing as there has to be a large annular volume to accommodate the piston and its travel distance. This forces a thinner housing wall in a location with a given drift diameter. Below certain depths such a design is not serviceable as the pressure rating on the housing cannot meet system requirements. 
         [0006]    The present invention addresses the need for a higher housing pressure rating with a thicker wall made possible by drilling a piston bore into the housing wall in which is located a rod shaped piston. At a predetermined pressure the annulus pressure is allowed to communicate to one side of the piston that had preferably been at atmospheric pressure. The opposite side of the piston is preferably at atmospheric pressure so that the resulting piston movement liberates a c-ring that had previously extended into a mandrel groove to prevent relative movement between the housing and the mandrel. The c-ring is manufactured so that it springs away from the groove when the piston is stroked so that the c-ring will not re-engage the groove once the lock assembly is unlocked. These details and others about the present invention will be more readily apparent to those skilled in the art from a review of the details of the preferred embodiment and other embodiments described below while realizing that the full scope of the invention is to be determined by the appended claims. 
       SUMMARY OF THE INVENTION 
       [0007]    A lock assembly for a mechanically set packer for deep deployments is described. The lock housing wall has at least one bore for a rod piston that is selectively actuated when wellbore hydrostatic is allowed to reach one side of the piston. Movement of the piston, after breaking a shear pin, allows a c-ring to spring outwardly and out of a locking groove in the mandrel so that the mandrel can be string manipulated with respect to the housing to set the packer. Once unlocked the lock assembly remains defeated. The piston can be optionally exposed to hydrostatic and will unlock at a given depth without manipulation of the wellbore annulus pressure. Other mechanisms to admit wellbore hydrostatic to move the piston or to move the piston in general by other techniques are described. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a section view of the lock assembly in the run in position; 
           [0009]      FIG. 2  is a close-up view of  FIG. 1  focusing on the c-ring design; 
           [0010]      FIG. 3  is a view along section lines  3 - 3  of  FIG. 1 ; 
           [0011]      FIG. 4  is a view along line  4 - 4  of  FIG. 1 ; 
           [0012]      FIG. 5  is a released view of the lock assembly shown in  FIG. 1 ; 
           [0013]      FIG. 6  shows the use of a solenoid valve to admit annulus pressure to defeat the lock with the solenoid in the closed position and the lock in the locked position; 
           [0014]      FIG. 7  is the view of  FIG. 6  with the lock defeated by operation of the solenoid; 
           [0015]      FIG. 8  is a view of the lock in the locked position using a plug to initially isolate annulus pressure; and 
           [0016]      FIG. 9  is the view of  FIG. 8  with the plug out of the way allowing the piston to release the lock. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0017]    The mechanically set packer is very well known to those skilled in the art so it is not shown in detail in the drawings. It suffices to state that the packer can be selectively set by manipulation of the mandrel  10  with respect to the packer body, a portion of which  12  is shown in  FIG. 1 . Generally drag blocks that are not shown are used to hold the packer body in position as the mandrel  10  is manipulated with respect to the body  12  generally using a j-slot or equivalent to get the sealing element compressed and the cones under slips to lock in the set position of the sealing element. As stated before, the packer cannot set until there is relative movement between the mandrel  10  and the packer body  12 . 
         [0018]    What prevents such relative movement is the lock housing  14  that has a body  16  and a bottom sub  18  connected at thread  20  with a fluid displacement port  22  in the bottom sub  18 . A retainer  24  that is also shown in the section view of  FIG. 4  is generally rounded in shape and preferably has a flat  28  that faces the c-ring  30  preferably at 180 degrees from the split  32 . Shear pin  26  initially fixates the retainer  24  in such a manner as to have the c-ring  30  supported in groove  34  and as shown in  FIG. 2  the groove  34  is made up of a series of spaced grooves that match a profile  36  on the face of the c-ring  30  that faces the mandrel  10 . End  38  abuts housing  16  that is part of the mandrel body  12  to prevent uphole movement in the direction of arrow  42  until after the c-ring  30  is released by movement of the retainer  24 . Bore  44  in  FIG. 2  accepts the shear pin or pins  26 . 
         [0019]    Going back to  FIG. 1  the housing  16  has at least one bore  40  in which piston  46  resides for slidable movement. Seals  48  are a smaller diameter than seals  50  which reside in a larger diameter portion  52  of the bore  40 . In between seals  48  and  50  is a chamber  54  that is preferably at atmospheric pressure but can be higher. Above the piston  46  is another chamber  56  also seen in  FIG. 3 . This chamber is defined by a rupture disc retainer  58  held by a lock nut  60  threaded in bore  62 . Bore  62  is preferably at 90 degrees to the piston bore  40  and communicates to the annular space  64  surrounding the housing assembly  14 . 
         [0020]    When the mechanically set packer is at the desired location and it is time to set it, the pressure in the annulus  64  is raised or reaches a predetermined level and the rupture disc  59  breaks to communicate pressure in the annulus  64  into chamber  56  to start driving the piston  46 . Initially the shear pin  26  breaks after which further piston  46  movement pushes the retainer  24  to a location offset from the c-ring  30  which allows the stored potential energy in c-ring  30  to be released as the c-ring  30  springs to a larger diameter taking the profile  36  out of the facing groove  34  that has a similar profile. The c-ring  30  is then no longer in the profile  34  of the mandrel  10  and the mandrel  10  may now be manipulated axially and/or rotationally to set the packer using relative movement of the mandrel  10  to the packer housing  12  that is in most cases supported in the wellbore with drag blocks (not shown). 
         [0021]    The advantage of the present design can be readily seen from the drawings and the above description. The housing  16  can have a massive wall thickness as shown in  FIG. 3  that is interrupted by the bore  40  for each respective piston  46 . In well depths of 20,000 feet or more the hydrostatic pressures can be so high that a very thick wall for housing  16  is necessary to get the required pressure rating which can range in the order of 20,000 PSI or more depending on the depth and fluid density in the annular space. Using a smaller bore for a piston  46  allows the ability to withstand such high differential pressures with minimal or no bore distortion. As shown in  FIG. 5 , once the c-ring  30  is sprung, its built in potential energy keeps it in the  FIG. 5  position and out of the profile  34  so that once there is an unlocking of the mandrel  10  to move with respect to the packer body portion  12  there is no risk of a re-latching that could otherwise prevent the packer from fully setting. 
         [0022]    There are options that can be employed. For example, the c-ring  30  that is shown in  FIG. 2  as retained against movement in the direction of arrow  42  can also be retained against axial movement in opposed directions. The use of the rupture disc  59  is optional and it can be omitted. This would let hydrostatic pressure in the annulus  64  act on the piston  46  as the packer body  12  is lowered. At some predetermined depth that results in the breakage of the shear pin  26  the packer will be unlocked to set and prevented from relocking as previously described. 
         [0023]    Rather than using a rupture disc to communicate pressure in annulus  64  to the piston  46  some other type of device can be used. The valve can be a smart valve with an associated operator and a power supply and processor to receive signals from the surface to trigger the valve to open when desired. For some examples the sensor associated with the valve can be responsive to a predetermined movement in opposed directions of the packer body, an acoustic signal through the supporting tubular string or the annulus fluid or an applied magnetic field such as with the delivery of a sonde through the supporting string. 
         [0024]      FIGS. 6 and 7  illustrate an example of this in the form of a solenoid valve  80  that initially blocks a port  82  to the annulus  84  using seals  86  and  88  on a valve member  90 . When a signal makes the valve member  90  move uphole, the seal  88  is pulled past the port  82  allowing annulus pressure into chamber  92  to push the piston  94  against the retainer  96  to break the shear pin  98  so that the lock ring  100  is no longer supported and can spring away from the mandrel  102  to which it was previously locked.  FIG. 7  shows the serrations on ring  100  having parted from the mating serrations on mandrel  102  so that relative movement is now possible for setting the packer using the drag blocks and string manipulation in the manner previously described. 
         [0025]      FIG. 8  shows a port  200  that is blocked by a plug  202  that is a shape memory plug. On exposure to a predetermined temperature for a predetermined time the plug  202  reverts to its original smaller shape and moves away from the port  200  to allow pressure in the annulus  204  to reach the piston  206  through chamber  208 . The piston moves the retainer  210  away from over the lock ring  212  and the lock ring  212  springs out and away from the mandrel  214 . The packer can now be set using the drag blocks and string manipulation as previously described.  FIG. 9  shows the released position with the shear pin  216  having been broken by the initial movement of the retainer  210 . 
         [0026]    Other options involve a preload force on the piston that is retained against stroking by a locking member that is defeated by well fluids such as by dissolving or other chemical attack from well fluid exposure. Another option is a piston that is made of a shape memory material that responds to the temperature of well fluids to revert to an original shape that results in liberating the c-ring to release the mandrel for movement. Instead of a c-ring, ring segments can be used that simply fall away from a profile in the mandrel when the piston shifts to remove support for the segments. More than a single piston can be used such as a symmetrical or asymmetrical array of pistons in a circumferential orientation that collectively push on a release for the mandrel lock to free the mandrel for movement such as when the rupture disc  59  breaks annulus  64  pressure can act on multiple pistons in tandem or in sequence. One or more pistons can have a plurality of axially spaced piston areas so that breaking of the rupture disc for example feeds hydrostatic pressure to the multiple piston areas to enhance the force acting on the piston. A sleeve can be used that is pushed by one or multiple pistons using wellbore annulus pressure such that the sleeve shifts enough to let the c-ring spring out or to otherwise release a locking member from a mandrel groove. 
         [0027]    The lock can be defeated with application and removal of pressure in the annulus or in the tubing although a design without wall penetrations in the tubing is preferable tipping the balance in favor of actuation through the annulus. The annulus can be pressured against the formation for unlocking the lock to then allow setting the packer. 
         [0028]    If the packer is releasable with string manipulation, the lock can be configured to re-latch after release to again hold the packer in the released position for removal from the wellbore. The retainer could be manipulated back toward the sprung c-ring to compress it as it is pushed back into alignment with the groove in the mandrel and the retainer can then jump over the c-ring to relock it in as the assembly is then pulled out of the hole locked in the retracted position of the packer. 
         [0029]    The above description is illustrative of the preferred embodiment and many modifications may be made by those skilled in the art without departing from the invention whose scope is to be determined from the literal and equivalent scope of the claims below.

Summary:
A lock assembly for a mechanically set packer for deep deployments is described. The lock housing wall has at least one bore for a rod piston that is selectively actuated when wellbore hydrostatic is allowed to reach on side of the piston. Movement of the piston, after breaking a shear pin, allows a c-ring to spring outwardly and out of a locking groove in the mandrel so that the mandrel can be string manipulated with respect to the housing to set the packer. Once unlocked the lock assembly remains defeated. The piston can be optionally exposed to hydrostatic and will unlock at a given depth without manipulation of the wellbore annulus pressure. Other mechanisms to admit wellbore hydrostatic to move the piston or to move the piston in general by other techniques are described.