Downhole Tool with Multiple Pistons

Abstract

A downhole tool includes a tool body comprising an inner housing and an outer housing to form an annulus between the inner housing and the outer housing and a primary piston positioned within the annulus to form a primary cavity within the annulus. The downhole tool further includes a backup piston positioned within the annulus to form a second cavity within the annulus and a pressure operating device in fluid communication with the annulus.

What is claimed is: 
     
         1 . A downhole tool, comprising:
 a tool body comprising an inner housing and an outer housing to form an annulus between the inner housing and the outer housing;   a primary piston positioned within the annulus to form a primary cavity within the annulus;   a backup piston positioned within the annulus to form a second cavity within the annulus; and   a pressure operating device in fluid communication with the annulus and configured to operate in response to a pressure in the annulus.   
     
     
         2 . The downhole tool of  claim 1 , wherein the pressure operating device is configured to operate in response to the pressure in the annulus being above an operating pressure. 
     
     
         3 . The downhole tool of  claim 2 , wherein the operating pressure is at or above about 3,000 psi (20,700 kPa). 
     
     
         4 . The downhole tool of  claim 2 , wherein the backup piston is positioned between the primary cavity and the second cavity and comprises a pressure inhibiting device configured to prevent pressure below a predetermined amount from communicating across the backup piston. 
     
     
         5 . The downhole tool of  claim 4 , wherein the predetermined amount of pressure for the pressure inhibiting device is lower than the operating pressure for the pressure operating device. 
     
     
         6 . The downhole tool of  claim 4 , wherein the predetermined amount for the pressure inhibiting device is at or lower than about 2,000 psi (13,800 kPa). 
     
     
         7 . The downhole tool of  claim 4 , wherein the pressure inhibiting device comprises at least one of a frangible element and a relief valve, and wherein the frangible element comprises a burst disc. 
     
     
         8 . The downhole tool of  claim 4 , further comprising another backup piston positioned within the annulus to form another backup cavity within the annulus positioned between the primary cavity and the other backup cavity. 
     
     
         9 . The downhole tool of  claim 8 , wherein:
 the other backup piston comprises a backup pressure inhibiting device configured to prevent pressure below a second predetermined amount from communicating across the other backup piston; and   the second predetermined amount for the backup pressure inhibiting device is higher than the first predetermined amount for the primary pressure inhibiting device.   
     
     
         10 . The downhole tool of  claim 1 , wherein the pressure operating device is positioned within the annulus and is in fluid communication with the second cavity. 
     
     
         11 . The downhole tool of  claim 1 , wherein the pressure operating device comprises an indexing sleeve. 
     
     
         12 . The downhole tool of  claim 1 , wherein a port is formed within inner housing to enable fluid communication between a bore of the inner housing and the annulus. 
     
     
         13 . The downhole tool of  claim 1 , wherein at least one of the primary piston and the backup piston comprises a seal to sealingly engage the tool body. 
     
     
         14 . A method to operate a pressure operating device in a downhole tool, the method comprising:
 introducing pressure into a bore of a tool body of the downhole tool, the tool body comprising an inner housing and an outer housing to form an annulus between the inner housing and the outer housing;   communicating the pressure across a primary piston positioned within the annulus;   communicating the pressure across a backup piston positioned within the annulus; and   operating the pressure operating device in response to the pressure in the annulus.   
     
     
         15 . The method of  claim 14 , wherein the communicating the pressure across the backup piston comprises communicating the pressure through a pressure inhibiting device of the backup piston if a pressure differential develops across the backup piston above a predetermined amount. 
     
     
         16 . The method of  claim 15 , wherein the pressure operating device operates in response to a pressure above an operating pressure, wherein the predetermined amount of pressure for the pressure inhibiting device is lower than the operating pressure for the pressure operating device. 
     
     
         17 . The method of  claim 15 , wherein the pressure inhibiting device comprises at least one of a frangible element and a relief valve, and wherein the frangible element comprises a burst disc. 
     
     
         18 . The method of  claim 15 , further comprising communicating the pressure across another backup piston positioned within the annulus, wherein the communicating the pressure across the other backup piston comprises communicating the pressure through a back pressure inhibiting device of the other backup piston if a pressure differential develops across the other backup piston above a second predetermined amount, and wherein the second predetermined amount for the backup pressure inhibiting device is higher than the first predetermined amount for the primary pressure inhibiting device. 
     
     
         19 . The method of  claim 14 , wherein the pressure operating device comprises an indexing sleeve. 
     
     
         20 . A downhole tool, comprising:
 a tool body comprising an inner housing and an outer housing to form an annulus between the inner housing and the outer housing with the annulus in fluid communication with a bore of the inner housing;   a primary piston positioned within the annulus to form a primary cavity within the annulus;   a backup piston positioned within the annulus to form a second cavity within the annulus with the backup piston positioned between the primary cavity and the second cavity;   a pressure inhibiting device configured to prevent pressure below a predetermined amount from communicating across the backup piston;   a pressure operating device in fluid communication with the annulus and configured to operate in response to a pressure in the annulus above an operating pressure; and   wherein the predetermined amount of pressure for the pressure inhibiting device is lower than the operating pressure for the pressure operating device.