Variable diameter piston assembly for safety valve

Abstract

Disclosed are subsurface safety valves having a telescoping piston assembly. One disclosed safety valve includes a housing defining a piston bore that provides an upper bore section having a first bore diameter and a lower bore section having a second bore diameter smaller than the first bore diameter, and a piston assembly movably arranged within the piston bore and comprising a piston rod that includes an upper portion, a lower portion, and a radial shoulder defined between the upper and lower portions, the piston assembly further comprising a piston movably arranged on the upper portion, wherein the piston dynamically seals an inner wall of the upper bore section when the piston assembly moves within the piston bore and dynamically seals an outer surface of the upper portion of the piston rod when the piston engages the bore shoulder and the piston rod continues moving.

What is claimed is: 
     
         1 . A safety valve, comprising:
 a housing defining a piston bore configured to receive control pressure, the piston bore providing an upper bore section having a first bore diameter and a lower bore section having a second bore diameter smaller than the first bore diameter, the piston bore further defining a bore shoulder; and   a piston assembly movably arranged within the piston bore and comprising a piston rod that extends longitudinally within the piston bore and includes an upper portion, a lower portion, and a radial shoulder defined between the upper and lower portions, the piston assembly further comprising a piston movably arranged on the upper portion of the piston rod,   wherein the piston is configured to dynamically seal an inner wall of the upper bore section when the piston assembly moves within the piston bore and dynamically seal an outer surface of the upper portion of the piston rod when the piston engages the bore shoulder and the piston rod continues moving within the piston bore.   
     
     
         2 . The safety valve of  claim 1 , wherein the upper portion of the piston rod exhibits a first diameter and the lower portion of the piston rod exhibits a second diameter greater than the first diameter. 
     
     
         3 . The safety valve of  claim 1 , wherein the piston is cylindrical and the piston rod extends through the piston. 
     
     
         4 . The safety valve of  claim 1 , wherein the control pressure acts on the piston and the piston rod to move the piston assembly. 
     
     
         5 . The safety valve of  claim 4 , wherein the piston axially biases the radial shoulder and hydraulic force derived from the control pressure acting on the piston is transferred to the piston rod via the radial shoulder. 
     
     
         6 . The safety valve of  claim 1 , wherein the piston is a first piston and the bore shoulder is a first bore shoulder, the safety valve further comprising:
 an intermediate bore section defined within the piston bore and interposing the upper and lower bore sections, the intermediate bore section exhibiting a third bore diameter smaller than the first bore diameter but greater than the second bore diameter;   a second bore shoulder defined in the piston bore; and   a second piston movably arranged on the upper portion of the piston rod, wherein the second piston is configured to dynamically seal an inner wall of the intermediate bore section when the piston assembly moves within the piston bore and dynamically seal the outer surface of the upper portion of the piston rod when the second piston engages the second bore shoulder and the piston rod continues moving within the piston bore.   
     
     
         7 . The safety valve of  claim 6 , wherein the first bore shoulder provides a transition from the upper bore section to the intermediate bore section and the second bore shoulder provides a transition from the intermediate bore section to the lower bore section. 
     
     
         8 . The safety valve of  claim 6 , wherein the piston rod further defines a piston rod neck that extends axially from the upper portion and an annular groove defined between the upper portion and the piston rod neck, and wherein the piston rod neck exhibits a third diameter smaller than the first diameter such that a gap is formed between the first piston and the piston rod neck when the annular groove axially surpasses the first piston. 
     
     
         9 . The safety valve of  claim 6 , wherein the piston rod and the first piston cooperatively exhibit a first piston area, and the piston rod and the second piston cooperatively exhibit a second piston area smaller than the first piston area. 
     
     
         10 . The safety valve of  claim 1 , wherein the piston includes one or more dynamic seals configured to sealingly engage the inner wall of the upper bore section and the outer surface of the upper portion of the piston rod. 
     
     
         11 . The safety valve of  claim 1 , further comprising:
 a flow tube operably coupled to the piston rod and movably arranged within a flow passage defined in the safety valve in response to the movement of the piston assembly;   a valve closure device movable between an open position and a closed position and adapted to restrict fluid flow through the flow passage when in the closed position, wherein the flow tube is adapted to shift the valve closure device between open and closed positions; and   a power spring arranged within a lower chamber defined within the housing and configured to bias the piston assembly upwardly within the piston bore.   
     
     
         12 . A method of actuating a safety valve, comprising:
 conveying control pressure to a piston bore that provides an upper bore section having a first bore diameter and a lower bore section having a second bore diameter smaller than the first bore diameter, the piston bore further defining a bore shoulder;   axially displacing a piston assembly arranged within the piston bore with the control pressure, the piston assembly comprising a piston rod that extends longitudinally within the piston bore and includes an upper portion, a lower portion, and a radial shoulder defined between the upper and lower portions, the piston assembly further comprising a piston movably arranged on the upper portion of the piston rod;   dynamically sealing an inner wall of the upper bore section with the piston when the piston assembly moves within the piston bore; and   dynamically sealing an outer surface of the upper portion of the piston rod with the piston when the piston engages the bore shoulder and the piston rod continues moving within the piston bore.   
     
     
         13 . The method of  claim 12 , further comprising:
 moving the piston assembly within the piston bore as the control pressure acts on the piston and the piston rod; and   transferring hydraulic force derived from the control pressure to the piston rod when the piston engages the radial shoulder.   
     
     
         14 . The method of  claim 12 , wherein the piston rod is operably coupled to a flow tube movably arranged within a flow passage defined in the safety valve, the method further comprising:
 axially displacing the flow tube as the piston assembly moves within the piston bore;   compressing a power spring as the piston assembly is axially displaced by the hydraulic fluid pressure; and   moving a valve closure device with the flow tube from a closed position, which restricts fluid flow through the flow passage to an open position.   
     
     
         15 . The method of  claim 14 , further comprising:
 reducing the control pressure within the piston bore;   biasing the piston assembly upwardly within the piston bore with the power spring;   engaging the piston assembly on an up stop defined in the piston bore; and   generating a mechanical seal between the up stop and the piston assembly.   
     
     
         16 . The method of  claim 12 , wherein the piston is a first piston, the bore shoulder is a first bore shoulder, and wherein the piston bore further defines a second bore shoulder and an intermediate bore section that interposes the upper and lower bore sections, and wherein the piston assembly further includes a second piston movably arranged on the upper portion of the piston rod, the method further comprising:
 dynamically sealing an inner wall of the intermediate bore section with the second piston as the piston assembly moves within the piston bore, the intermediate bore section exhibiting a third bore diameter smaller than the first bore diameter but greater than the second bore diameter; and   dynamically sealing the outer surface of the upper portion of the piston rod with the second piston when the second piston engages the second bore shoulder and the piston rod continues moving within the piston bore.   
     
     
         17 . The method of  claim 16 , wherein the piston rod and the first piston cooperatively exhibit a first piston area, and the piston rod and the second piston cooperatively exhibit a second piston area smaller than the first piston area. 
     
     
         18 . The method of  claim 16 , wherein the piston rod further defines a piston rod neck that extends axially from the upper portion and an annular groove defined between the upper portion and the piston rod neck, the piston rod neck exhibiting a third diameter smaller than the first diameter, the method further comprising:
 advancing the piston rod in the piston bore until the annular groove axially surpasses the first piston;   allowing the control pressure to migrate past the first piston via a gap defined between the first piston and the piston rod neck; and   axially displacing the piston assembly further within the piston bore as the control pressure acts on the second piston and the piston rod, wherein hydraulic force from the control pressure is transferred from the second piston to the piston rod when the second piston axially biases the radial shoulder.   
     
     
         19 . The method of  claim 18 , further comprising separating the radial shoulder from the second piston as the control pressure acts on the piston rod and advances the piston rod further within the piston bore. 
     
     
         20 . The method of  claim 12 , wherein conveying the control pressure to the piston bore comprises conveying hydraulic fluid to the piston bore via a control line.