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FIELD OF THE INVENTION 
     The field of this invention relates to running tools and, more particularly, wireline-supported tools which are automatically resettable and which will not prematurely release the downhole tool being run until a predetermined hydraulic force is applied after the tool is landed on location. 
     BACKGROUND OF THE INVENTION 
     In some facilities, the appropriate rig is not available and tools cannot be run-in on rigid or coiled tubing. In those instances, the downhole tools are connected to a running tool which is, in turn, supported by one type or another of a line. One common form is a wireline; however, other types of line supports are intended to be encompassed in the term “line” or “wireline” as used in this application. One of the problems in the past with running in tools on wireline has been that if an obstruction of sorts is encountered prior to reaching the desired depth, the running tools of the prior art would release. In some designs, if the downhole tool becomes supported, allowing the wireline to go slack and the wireline is subsequently tensioned, the running tool releases from the downhole tool. One variation in a wireline-supported running tool, that has been developed by Halliburton in its Modular Gun System, involves up and down movement on the wireline to set a gun hanger, followed by a decrease in wireline weight at the surface to verify that such a hanger had been set. When thereafter additional weight was slacked off, oil metered through an orifice flowed in the hydraulic running tool. After delay of some 5 minutes, the tool automatically released from the gun hanger. While this design allowed surface personnel to react to avoid an inadvertent release due to the time delay provided by metering the oil flow through a restriction orifice, a better design was needed to ensure that the tool being conveyed will not release from the running tool until it is properly positioned at the appropriate depth. Another requirement was to allow the running tool to automatically reset so that it could be reused for multiple-trip operations without having to be disassembled and redressed. This type of an issue is common in designs that break shear pins to allow a release mechanism to operate. 
     Some systems have been tried which incorporated a rupture disk which, in order to release, involved an increase in wellbore pressure to break the rupture disk. This, in turn, created an unbalanced force which broke a shear pin on a release piston, which in turn pulled locking collets off of their support. These designs were good for a single use and had to be disassembled to be redressed to replace the shear pins. An example of this design is the model GRD Running Tool, product No. 493-46 made by Baker Oil Tools. 
     Various tubing-conveyed fishing tools have been used which apply a force generated by fluid flow through an orifice for release. These tools would automatically reset after the hydraulic pressure was removed from the tubing. Typical examples of such tools are U.S. Pat. Nos. 5,242,201 and 5,581,014. However, these tools were not configured to operate on wireline. Yet other tools using wireline worked on the jarring concept. A Model W Running Tool from Baker Oil Tools required upward jarring to release the downhole tool. The Model M Running and Pulling Tool made by Baker Oil Tools required jarring down to shear a shear pin to remove support for dogs which held the downhole tool so that a release could occur. The soft release running tool, product No. 811-40 by Baker Oil Tools, released by an upward pull followed by a slacking off. Also of general interest in this area are U.S. Pat. Nos. 4,361,188 and 5,180,015. 
     The shortcoming of the prior art tools was that for a wireline application, they would not give assurance of premature release should the downhole tool become supported in a location above the desired depth. Additionally, these tools did not facilitate many trips in succession because they had to be redressed after each release due to their use of a shear pin or pins in the release mechanisms. Yet other designs in the prior art which provided the automatic resetting feature and released with hydraulic pressure required the running tool or fishing tool to be run-in the wellbore on rigid or coiled tubing. Accordingly, one of the objectives of the present invention is, in applications where equipment is not available to run rigid or coiled tubing, to have a running tool supported on a wireline which can give assurance that it will not prematurely drop the downhole tool, while at the same time providing features of automatic resetting, coupled with simple and safe operation. These objectives will be more readily understood by those skilled in the art from a review of the preferred embodiment described below. 
     SUMMARY OF THE INVENTION 
     A running tool is disclosed to deliver tools downhole, preferably supported on a wireline. The running tool will not release the downhole tool before the desired depth is reached, even if an obstruction is encountered. The tool has the ability to release upon application of pressure in the wellbore with the tool supported in the wellbore. The tool features a floating piston with a pre-charged chamber on one side. Hydrostatic pressure acts on the opposite side of the floating piston as the running tool descends. When the downhole tool reaches its desired depth and becomes supported, slacking on the wireline traps the hydrostatic on one side of the floating piston. Applied wellbore pressure, acting on a release piston exposed to the trapped hydrostatic on its opposite side, shifts the release piston and releases the running tool from the downhole tool. On the way uphole, the trapped hydrostatic pressure is released. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS.  1   a  and  b  show in sectional elevation the downhole tool being inserted into the running tool prior to lowering into the well. 
     FIGS.  2   a  and  b  are a sectional elevational view of the running tool supporting the downhole tool on the trip downhole. 
     FIGS.  3   a  and  b  are the view of FIGS.  2   a  and  b,  shown after the downhole tool is firmly supported and the wireline is slacked off. 
     FIGS.  4   a  and  b  show the tool of FIGS.  3   a  and  b,  with the release piston shifted due to application of pressure in the wellbore. 
     FIGS.  5   a  and  b  show the release piston further shifted and the downhole tool fully released. 
     FIGS.  6   a  and  b  show the running tool being pulled out of the wellbore, with the trapped hydrostatic pressure vented off as the running tool rises out of the wellbore. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS.  1   a  and  b,  the apparatus A has a connection  10  on adapter  12  which can be used as an attachment point for a line or wireline, shown schematically as  14 . Connected to adapter  12  is top sub  16 , which has a fill port  18 . Top sub  16  is connected to mandrel  20  at thread  22 ′. Fill port  18  communicates with passage  24 . Passage  24  is isolated from passage  26  by plug  28 . 
     Outer sleeve  30  is in sealing engagement with top sub  16  due to seal  32 . Sleeve  30  defines an annular cavity  34  around the mandrel  20 . Passages  36  and  38  provide fluid communication from passage  26  into annular cavity  34 . Passages  36  and  38  are in the mandrel  20 . Mandrel  20  is connected to top sub  16  at thread  22 . At the lower end of annular cavity  34  is floating piston  42 . Piston  42  has seals  44  and  46 , thus sealingly isolating the annular cavity  34  at its lower end. 
     Surrounding the outer sleeve  30  is a multi-component outer body  48  which begins with sleeve  50  at its top end and terminates at centralizer  52  at its lower end. Supported between the mandrel  20  and the outer body  48  is a gripping ring  54 , which is biased by spring  56  in a downward direction toward shoulder  58  on outer body  48 . The gripping ring  54  has an outer surface  60  of a series of fingers which have an inwardly oriented shoulder  62 . Also between the gripping ring  54  and the mandrel  20  is a release piston  64 . Release piston  64  extends between outer sleeve  30  and mandrel  20  and is sealed respectively by seals  66  and  68 . A passage  70  in sleeve  30  leads to annular passage  72 . Annular passage  72  communicates with passages  74  and  76  to poppet  78  which is biased by spring  80 . Poppet  78  seals against a shoulder  82  which surrounds passage  76  such that when the pressure in passage  76  is higher than the hydrostatic pressure in the wellbore, the spring  80  is compressed, venting any pressure in passage  76  through passage  84 . 
     The outer body  48  is supported off of outer sleeve  30  by virtue of spring  86 . In the run-in position shown in FIG.  1   b,  outer body  48  obstructs passage  70 . However, when the downhole tool  88  is suspended on outer body  48 , the spring  86  is compressed, bringing recessed surface  90  opposite passage  70 , as shown in FIG.  2   b , so as to expose annular passage  72  to hydrostatic wellbore pressure. The critical components of the preferred embodiment now having been described, its operation will be reviewed in greater detail. 
     Referring to FIG.  1   b,  the downhole tool  88  has a recess  92  and an upper end  94 . When upper end  94  is pushed against gripping ring  54 , it displaces the gripping ring upwardly, away from shoulder  58  and outwardly on tapered surface  96 . This allows the upper end  94  to advance beyond shoulder  62 , whereupon the spring  56  pushes the gripping ring  54  back down against tapered surface  96  such that shoulder  62  now finds itself within recess  92 , as shown in FIG.  1   b.  When the assembly is picked up for lowering into the wellbore, the view of FIG. 2 is achieved where the only difference between FIGS. 1 and 2 is that in FIG. 2, the shoulder  62  has caught the shoulder  98  at the upper end of recess  92 . This is the position of the apparatus A with the downhole tool  88  as the assembly is lowered in the wellbore. As the apparatus A is being lowered in the wellbore, the suspension of the weight of the downhole tool  88  results in compression of spring  86  and presentation of recessed surface  90  opposite passage  70 . Thus, as the apparatus A descends, the pressure in annular passage  72  reflects the surrounding hydrostatic pressure in the wellbore. The annular cavity  34  has been precharged with preferably nitrogen gas or some other compressible fluid to a pressure slightly below the anticipated hydrostatic in the wellbore at the desired depth for the downhole tool  88 . This pressurization of the annular cavity  34  occurs by hooking up a source of nitrogen to filler port  18  while backing off the plug  28 , thus providing fluid communication from passage  24  through passages  26 ,  36  and  38  into annular cavity  34 . When the desired pressure is reached, the plug  28  is again rotated to seal off passage  26  from passage  24 , thus trapping in the precharged pressure in annular cavity  34 . As the apparatus A descends with hydrostatic pressure building in annular passage  72 , the floating piston  42  stays in its lowermost position until such time as the hydrostatic pressure in annular passage  72  is greater than the precharged pressure in annular cavity  34 . 
     Looking at FIG. 3, the downhole tool  88  has either reached its desired depth and become supported or has hit an obstruction along the way. Because the downhole tool  88  is supported and the wire  14  is allowed to go slack, the result is that the gripping ring  54  travels to the lower end of the recess  92  but is still firmly engaged into recess  92  due to the support that it receives from the outer body  48 . Accordingly, even if an obstruction is encountered, there will be no release as the gripping ring  54  will continue to retain the downhole tool  88  due to the fact that it is firmly supported in the recess  92  by outer body  48 . However, when the ultimate depth required is, in fact, reached, the same movement shown in FIG. 3 will occur as the gripping ring  54  moves downwardly in recess  92 , all the while retaining the connection to the downhole tool  88 . A release can occur only when the downhole tool  88  is supported downhole and pressure is applied to port  100 . 
     At this time, pressure is applied through port  100 , as shown in FIG.  4 . It should be noted that when the downhole tool is supported and the wire  14  is slacked off, the port  70  becomes sealingly obstructed due to seals  102  and  104 , as shown in FIG.  3   b . As shown in FIG.  4   b , application of pressure at port  100  results in an upward force on end  106  of release piston  64 . End  108  of piston  64  is exposed to the trapped pressure in annular passage  72 . Eventually the pressure on end  106 , through a build-up of pressure in the wellbore communicated through port  100 , results in an unbalanced force on release piston  64 . Release piston  64  has a shoulder  110  which engages a shoulder  112  on gripping ring  54 . When these two shoulders connect, further upward movement of the release piston  64  brings up with it the gripping ring  54  and pulls the gripping ring  54  away from shoulder  58 , as can be seen by comparing FIGS.  4   b  and  5   b.  The gripping ring  54  has tapered surfaces  113  which ultimately engage a taper  114  on the mandrel  20 . Thus, upward movement of the release piston  64  cams the fingers which comprise the lower end of the gripping ring  54  radially outwardly, as shown in FIG.  5   b , to bring shoulder  62  out of recess  92  to effect a complete release of the downhole tool  88  when an upward force is applied at the same time as the application of wellbore pressure. 
     Those skilled in the art can see that the precharging of annular cavity  34 , which acts on piston  42 , allows a reference hydrostatic pressure to be trapped in annular passage  72  against the compressible fluid trapped in passage  34  when the downhole tool  88  is supported downhole. This occurs because passage  70  is sealingly closed, as illustrated by comparing FIGS.  2   b  and  3   b , as the recess surface  90  moves away from passage  70  and seals  102  and  104  effectively straddle passage  70 , which is now fully covered by the outer body  48 . With that reference pressure trapped, which is generally a pressure close to the wellbore hydrostatic at the desired location for release from the downhole tool  88 , applied pressure on the wellbore on the release piston  64 , one end of which  108  is exposed to the trapped hydrostatic pressure in the annular passage  72 , results in the release sequence just described. It also moves the floating piston  42  and compresses the fluid in chamber  34 . 
     FIGS.  6   a  and  b  illustrate that on the way up the hole, annular passage  72  is still isolated from wellbore hydrostatic as passage  70  continues to be sealed off due to the upward force applied by spring  86 , which keeps the outer body  48  over the passage  70 , with seals  102  and  104  acting to prevent pressure loss out of annular passage  72 . However, the hydrostatic pressure is decreasing as the apparatus A is elevated, and such reduced pressure is sensed at passage  84 . Thus, as the apparatus A is raised, lowering the pressure in passage  84 , the poppet  78  eventually sees a sufficient unbalanced force to overcome the spring  80 , thus moving the poppet  78  off of the sealing surface or shoulder  82  so that the pressure in annular passage  72  can dissipate by flow through passage  116  and poppet  78 , which becomes exposed when it is moved to the position shown in FIG.  6   b . As the pressure in annular passage  72  decreases, the pressure in annular cavity  34  correspondingly decreases such that by the time the apparatus A is withdrawn from the wellbore, the originally charged pressure into annular cavity  34  is once again present. 
     The pressure in annular cavity  34  can be manually bled off by hooking up the requisite valving and piping to the fill port  18  and backing off plug  28 . 
     Those skilled in the art will now appreciate that what has been shown is a running tool which can be run on a wireline  14  or, for that matter, on rigid or coiled tubing as an alternative. There will be no release of the downhole tool  88 , even if the downhole tool  88  becomes supported in the wellbore at a depth higher than its ultimate destination. The apparatus A is released by application of pressure in the wellbore to a release piston, the other side of which sees a trapped hydrostatic pressure. The floating piston  42 , acting on a compressible fluid, such as nitrogen, in annular cavity  34 , provides the capability of compressing the compressible fluid to enable movement of the release piston  64 . An upward pull on line  14  with applied wellbore pressure through port  100  will release the downhole tool  88 . Withdrawal of the applied pressure through port  100  will simply allow the spring  56  to push down the gripping ring  54  into the position shown in FIG.  6   b  so that it is now ready to accept, when removed from the wellbore, another tool which can be run and engaged to the tool  88  which is already in the wellbore. Accordingly, the apparatus A does not need to be redressed whenever it is brought out of the well. There are no shear pins involved in the design which must be removed and replaced after an individual use. The apparatus A is designed to bleed off the trapped hydrostatic pressure in annular passage  72  so that when it is withdrawn from the well, the only internal pressures are the initial charge pressure to annular cavity  34 . That pressure in cavity  34  can be safely bled off using the fill port  18  and plug  28 , with appropriate piping. The apparatus A is simple and reliable. It is preferred to charge the annular cavity  34  with a pressure slightly below the anticipated hydrostatic at the depth to which the downhole tool  88  can be delivered. Any type of downhole tools can be conveyed with the apparatus A, including perforating guns and packers or bridge plugs, as an example. The tool can also be used as a fishing tool to grab any downhole tool which has a fishing neck defined by a recess, such as  92 . Those skilled in the art will appreciate that the parts of the apparatus can be reconfigured so that when used in a fishing application, it can either act as an overshot, as disclosed in these figures, or as a spear to go inside of a stuck tool that happens to have an internal recess for fishing purposes. Although the apparatus A has been shown as ideal for use with a line  14 , rigid or coiled tubing can also be connected to connection  10  without departing from the spirit of the invention. 
     The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the size, shape and materials, as well as in the details of the illustrated construction, may be made without departing from the spirit of the invention.

Summary:
A running tool is disclosed to deliver tools downhole, preferably supported on a wireline. The running tool will not release the downhole tool before the desired depth is reached, even if an obstruction is encountered. The tool has the ability to release upon application of pressure in the wellbore. The tool features a floating piston with a pre-charged chamber on one side. Hydrostatic pressure acts on the opposite side of the floating piston as the running tool descends. When the downhole tool reaches its desired depth and becomes supported, slacking on the wireline traps the hydrostatic on one side of the floating piston. Applied wellbore pressure, acting on a release piston exposed to the trapped hydrostatic on its opposite side, shifts the release piston and releases the running tool from the downhole tool. On the way uphole, the trapped hydrostatic pressure is released.