document: FIELD OF THE INVENTION 
     The field of this invention is generally downhole vibratory tools and more specifically those tools that selectively allow passage therethrough for other tools. 
     BACKGROUND OF THE INVENTION 
     Vibratory tools are used to dislodge a stuck object known as a fish from a downhole location. They have other applications such as allowing a pulling force to be transmitted from the surface to a fish stuck in a deviated wellbore. In that application the vibratory devices can be placed in the deviation such that their presence helps transmit forces to the fish that would have otherwise been resisted by the deviated wellbore through which the string extended to reach the fish. An illustration of such as application is U.S. Pat. No. 6,502,638. 
     Vibratory tools known in the art have operated on a similar principle. An overpull is applied to the string supporting the tool and pressure is applied within the string. A piston then travels against the bias of a spring, in effect stretching the string while compressing the spring. At some point of travel, the force applied by the spring that acts on a valve member becomes higher than the pressure applied from above to that valve member. When this happens, there is relative movement that takes the valve member off a seat. The pressure that had been keeping the valve member on the seat up to that point is suddenly relieved as the valve member is biased off the seat by the rising spring force due to compression of the spring. Once the valve member is off the seat, the pressure acting on the piston that drove the mandrel down against the spring in the first place is suddenly relieved. Flow through the tool causes a sudden drop in the applied pressure causing the piston to snap back under the spring force and re-close the valve. At that point the cycle repeats. There are variations on this basic concept. Some designs employ a piston or opposed pistons that drive the mandrel in opposed directions. 
     There are other common features of known designs that limit their utility. Most earlier designs did not have a capability to have a central passageway clear so that a wireline could be run through the tool to determine conditions in the vicinity of the fish. Using those designs, the vibratory tool had to be removed to run a wireline or other tools down to the fish. Most all of these designs had the dump valve that relieved pressure located in the center of the tool preventing a clear run through the tool for a wireline or other tools. A few examples of such designs are U.S. Pat. Nos. 6,062,324 and 6,206,101. 
     More recently a drop in dart that incorporates the working components of the vibratory tool has been developed as shown in U.S. Pat. No. 6,866,104. This patent offered a solution to the need to have wireline access through the tool body and the dart could be retrieved after the vibration operation that commenced with the landing of the dart and application of pressure. While this design allowed for wireline access through the tool it also included additional compromises unique to the design of a dart that landed and sealed around a seat downhole. The main area of compromise was that the components of the vibratory tool had to be made to fit in the dart and the dart was limited in outside diameter so that it could fit into the receptacle in the tool body. Doing this required miniaturization of the vibratory tool key components which limited the power delivery of the generated vibrations from the tool. The use of smaller components also increased the effects of fatigue on the moving parts of the vibratory tool and there were also many components to the dart assembly making it fairly costly to build and maintain. 
     Other issues that affected reliable operation in the previous designs included a dump valve assembly that was pounded against a seat with each cycle resulting in rapid wear and potential loss of sealing contact. Another problem in the past had been the limited power delivery from the driving piston since its area was limited by the maximum available inside diameter in the tool housing. Many applications simply needed a higher power delivery to get the fish released. 
     A few other examples of known designs for vibratory tools are U.S. Pat. Nos. 6,474,421; 6,182,775; 6,164,393; 5,875,842 and 5,375,671. 
     What is needed and is addressed by the tool described below is a collection of features that solve the issues with prior design and lead to a more economical and reliable design. The dump valve is reconfigured into an annular shape to keep the middle of the tool free and clear. This allows a central passage to exist to permit a wireline operation through the tool when the tool is not set up to be in vibratory mode. The tool can be simply put in vibratory mode by dropping a removable plug onto a seat. The dump valve opens and closes without getting slammed against a seat. The mandrel is powered by stacked pistons in the tool body to magnify the delivered power from the vibratory tool. Since the essential parts of the vibratory tool are in the housing and only the delivery of the plug is required to initiate operations, the remaining components can be designed to be more beefy so as to run longer and more reliably as compared to the prior design where the key movable components were delivered into the tool housing on a dart. The tool can be configured so that the pistons can travel their entire stroke without being banged against travel stops. The tool has the capability to tolerate continued downward mandrel movement to dissipate its momentum even after the dump valve opens. The components are then configured to apply power to the mandrel for a down cycle when the dump valve closes close to the point where the pistons reach their upward travel limit. In this way a longer power stroke is achieved in an effort to free the fish. The tool can be run to apply up oscillating forces with or without impacts depending on how the tool is operated by rig personnel. These and other advantages of the present invention and its scope will become more apparent to those skilled in the art from a review of the description of the preferred embodiment, the drawings and the claims appended below. 
     SUMMARY OF THE INVENTION 
     A vibratory tool for downhole use is capable of letting a wireline or other tools pass a passage therethrough that can be subsequently closed by landing a plug on a seat. The dump valve is disposed annularly about the central passage so that cycling the tool does not cause it to be slammed against a seat. Since only a plug is delivered to a passage the functioning parts already in the housing are made stronger to improve reliability. Additional power is delivered per stroke from modular stacked piston units. The tool can be run in a manner where the high amplitude low frequency oscillating forces are delivered to the stuck fish without impacts of the pistons on the housing. 
  
  
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a section view of the tool with the plug in place and ready to vibrate; 
         FIG. 2  is a section view of the modular piston stack that can be used in the tool; 
         FIG. 3  is a section view of the lower end of the tool without the plug in position; 
         FIG. 4  is the view of  FIG. 3  with the plug seated and pressure being applied; 
         FIG. 5  is the view of  FIG. 4  just before the dump valve opens; 
         FIG. 6  is the view of  FIG. 5  as the dump valve trips open; 
         FIG. 7  is the view of  FIG. 6  after sufficient uphole movement of the mandrel to close the dump valve again and repeat the cycle. 
     
  
  
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The tool has an outer body  20  with a lower end  22  that is attached to a stuck object or fish  24 . The outer body  20  has an upper end  26 . Within the outer body  20  and extending uphole from upper end  26  is the mandrel  28 . Mandrel  28  is connected to the surface through a string  30 . Mandrel  28  has a passage  32  that is in fluid communication with the passage in the string  30  so that pressure can be delivered from the surface to lateral ports such as  34  or  36 . Ports  34  and  36  are at different elevations. Although only two rows of such ports are illustrated in the preferred embodiment, the construction of the tool is preferably modular so that different numbers of rows of ports can be used. A row of ports such as  34  lead to an annular space  38  with which there is communication to a piston  40  that is attached to the mandrel  28 . Pressure in space  38  pushed down piston  40  and with it mandrel  28  at the same time displacing fluid from chamber  42  through opening  44 .  FIG. 2  shows that this type of piston arrangement is modular allowing as many or as few pistons such as  40  to be stacked. More pistons such as  40  connected to the mandrel  28  mean more force imparted in a downward direction on the string  30  while at the same time creating an opposite reaction force on the outer body  20  that is attached to the fish  24 . It should be noted that space  38  and chamber  42  are created between mandrel  28  and outer body  20 . Chamber  42  sees downhole pressures through opening  44 . Stacking pistons  40  in effect increases the area of total pistons exposed to the applied pressure thus increasing the delivered power of the tool to considerably more by orders of magnitude than had been available in prior art tools. 
     Referring now to  FIG. 4 , the mandrel  28  has a lower end  9  that marks the end of passage  32  and a nearby shoulder  46 . A drop in plug  16  is shown landed on shoulder  46  to close off passage  32 . Those skilled in the art will appreciate that before plug  16  is dropped the passage  32  is open, as shown in  FIG. 3 , so that a wireline or other tool can be run through passage  32  and into the stuck fish or further down to collect any required data that may be helpful in determining the progress of the operation trying to get the fish unstuck or for any other reasons. The plug  16  is preferably retrievable and for that purpose has a fishing neck  48  so that it can be captured and returned to the surface with known tools. Plug  16  also has a seal  50  to help close off passage  32  and build pressure in it. Lower end  9  features openings  52  that lead into chamber  54 . Dump valve  12  is shown closing off chamber  54  so that application of pressure to passage  32  will build pressure on piston(s)  40  to move the mandrel  28  downwardly. It should be noted that valve  12  is cylindrically shaped with a seal ring  11  initially riding on surface  56  to hold pressure in chamber  54  as the movement of the mandrel  28  stretches out string  30  that is connected to it. At some point the ring seal  11  moves off of surface  56  to surface  58  that represents an increase in inside diameter and as a result a loss of sealing contact that had previously closed off passage  32 . For a time the pressure in passage  32  drives the valve  12  in tandem with the mandrel  28  due to applied pressure in chamber  54  from ports  52 . The movement of valve  12  is against the bias of spring  14  bearing on spring stop  13 . At some point of pressure buildup in passage  32  and tandem movement of mandrel  28  and valve  12  the force of spring  14  on stop  13  exceeds the downward force on valve  12  from pressure in passage  32 . This results in the valve  12  being moved uphole with respect to the mandrel  28  to relieve the pressure built up in the passage  32 . This happens due to ring seal  11  now being placed in juxtaposition with surface  58  of valve  12 , breaking the seal, as shown in  FIG. 5 . The mandrel  28  continues to move downhole due to momentum from the extension force applied from the pressure with the passage  32  closed off at the bottom and piston(s)  40  forcing the mandrel  28  down. However, the valve  12  in the open position and the pressure in passage  32  dissipated the momentum of mandrel  28  carrying it further downhole quickly dissipates as it reaches its lowest position shown in  FIG. 6 . 
     With the pressure dissipated in passage  32  the stretching of the string  30  that accompanied the downhole movement of the mandrel  28  now reverses as the string  30 , now no longer exposed to a stretching force goes into a contraction cycle. With the fish  24  still stuck and holding the outer housing  20  in position, the mandrel  28  and the piston(s)  40  attached to it move up relative to the housing  20 . At some point preferably before the piston(s)  40  slams into a radial surface in chamber  38  the seal ring  11  gets back into sealing contact with surface  56  of valve  12  closing off passage  32  again to allow pressure buildup and to reverse the direction of movement of mandrel  28  to allow the next cycle to begin, as shown in  FIG. 7 . It should be noted that the tool can be operated so that there are jarring blows delivered in every cycle or by avoiding such jarring blows. The factor that controls this is the amount of surface overpull applied to string  30  before and during when passage  32  is pressurized. 
     The basic operation of the tool having been reviewed, the features of the tool of the present invention can now be explored in greater detail. One such feature is the ability to stack pistons  40  to increase the available piston area in a confined downhole space so as to increase the power of the pressure spike that is applied to the fish  24 . The impacting of pistons  40  on the housing  20  is optional and depends of the applied overpull to string  30 . The cycling continues until applied pressure is turned off, the overpull force is removed from the surface or by the fish  24  becoming unstuck. It should be noted that without plug  16  in position, the tool can&#39;t cycle but wireline and other operations are possible through passage  32 . The tool is activated by dropping a simple and cheap plug  16  into passage  32  to seal its lower end. The design of the valve  12  as an annular ring gets it out of the center of the tool to allow the wireline access feature through passage  32  before the plug  16  is dropped. It further allows the opening and closing of the valve  12  to occur without slamming any part of the valve against a seat, as in some prior designs. Instead, the ring seal  11  simply slides between surfaces  56  and  58  respectively to close and open the valve. The configuration of the valve  12  and the spring  14  about the central bore of the tool allows those components to be designed to better perform in a cyclical loading environment without fatigue or failure. It also takes away the need, as in the prior art to put all the workings of the tool in a dart that is seated in the tool body after a wireline operation below the tool body. Instead, the components of the tool are delivered within the body and still are configured to leave a passage open for wireline or other activity through the passage  32  before the plug  16  is dropped into position. This means that the components delivered with the tool initially can be bigger than they could have been as part of a dart and will give longer trouble free service. It also means that the plug  16  is simple and cheap because it has no moving parts. Additionally, the tool can be made to operate with fewer moving parts than the previous design that involved dropping the critical tool components as part of the dart assembly. 
     The design of valve  12  eliminates significant cyclical impacts on opening and closing due to the cylindrical shape and the seal ring  11  simply moving into alignment and misalignment with the surface that surrounds it. The use of a cylindrically shaped valve  12  allows for the spring  14  to be more beefy thus reducing the stresses on it and extending its life. 
     The modular design that allows selection of the number of pistons allows for a tool design to be matched to the power required for the particular work string, or the surface equipment available or the anticipated downhole conditions with the stuck fish. Presenting the valve  12  outside the mandrel  28  and the piston(s)  40  opens the center and allows the use of the simple plug  16 . Wear on the valve is eliminated by avoiding banging valve components on a valve seat. Special materials can also be used for seal ring  11  to increase resistance to wear. The layout of the components allows the mandrel  28  to continue moving downhole after valve  12  opens. The result is that forces created in the modular piston  40  assembly stay in phase with the oscillating string  30  or the fish  24 . This is accomplished by engaging the power stroke near the upper end of piston movement, after valve  12  opens, and before valve  12  is allowed to close again. In that manner if the momentum from the string  30  allows for a longer stroke the tool can accommodate that by not engaging the power stroke until the pistons are at or near their maximum uphole travel. On the other hand the tool can also be operated to have impacts on each cycle with the pistons  40  against the housing  20 . These impacts can be on the up or down stroke and can be induced during operation by varying the overpull amount. The tool can operate without impact of the pistons  40  and can still be effective in releasing a fish  24 . The tool may also be used in stimulation or fishing operations. It provides large amplitude vibrations in a tubing string. It relies on a single valve for operation. 
     The tool can also have a rotational lock between the mandrel  28  and the housing  20  for the purposes of torque transmission. 
     It is to be understood that this disclosure is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended other than as described in the appended claims.
summary: A vibratory tool for downhole use is capable of letting a wireline or other tools pass a passage therethrough that can be subsequently closed by landing a plug on a seat. The dump valve is disposed annularly about the central passage so that cycling the tool does not cause it to be slammed against a seat. Since only a plug is delivered to a passage the functioning parts already in the housing are made stronger to improve reliability. Additional power is delivered per stroke from modular stacked piston units. The tool can be run in a manner where the high amplitude low frequency oscillating forces are delivered to the stuck fish without impacts of the pistons on the housing.