Accumulator having operating fluid volume independent of external hydrostatic pressure

Abstract

A method and device for maintaining pressure in an accumulator for subsea wellbore operations is disclosed.

1 . An accumulator for subsea wellbore operations having hydraulic charge pressure maintained at a substantially constant pressure above hydrostatic pressure at any depth in a body of water, the accumulator comprising:
 a generally cylindrical housing having a first longitudinal end and a second longitudinal end, each longitudinal and having a port therein, the housing divided into two sections by a bulkhead;   a first piston disposed in the housing on one side of the bulkhead;   a second piston disposed in the housing on the other side of the bulkhead;   a connecting rod disposed between the first and second pistons, the connecting rod having an atmospheric pressure or vacuum chamber defined in a longitudinal end thereof in contact with the second piston; and   wherein the first piston defines an hydraulic fluid chamber between the first piston and the first longitudinal end of the housing and a gas charge pressure chamber disposed between the bulkhead and the first piston, and wherein the second piston defines a hydrostatic pressure chamber between the second piston and the second longitudinal end of the housing, the first and second pistons having substantially equal cross sectional areas on both sides thereof such that a pressure of fluid in the hydraulic fluid pressure chamber is substantially equal to a pressure of gas in the gas charge pressure chamber and a hydrostatic pressure applied to the hydrostatic pressure chamber.   
     
     
         2 . The accumulator of  claim 1  further comprising annular supports disposed in the gas charge pressure chamber, the annular supports having openings generally in a center thereof to enable passage of the connecting rod and openings to enable flow of gas in the gas charge pressure chamber. 
     
     
         3 . The accumulator of  claim 1  wherein the accumulator is disposed in a riser coupled to a subsea test tree, and the port in the first longitudinal end of the housing is coupled to the subsea test tree to provide hydraulic pressure to operate components thereof. 
     
     
         4 . The accumulator of  claim 3  wherein the port in the second longitudinal end of the housing is exposed to fluid in the riser. 
     
     
         5 . The accumulator of  claim 1  wherein a ratio of cross sectional area of the first piston exposed to the hydraulic fluid chamber and exposed to the gas charge pressure chamber is selected such that a selected gas charge pressure results in a selected hydraulic fluid pressure. 
     
     
         6 . The accumulator of  claim 1  wherein an initial pressure of the gas charge is about 5,000 pounds per square inch. 
     
     
         7 . The accumulator of  claim 6  wherein an initial pressure of the hydraulic fluid is about 3,000 pounds per square inch plus the hydrostatic pressure. 
     
     
         8 . A method for maintaining pressure in an accumulator for subsea wellbore operations having hydraulic fluid pressure maintained at a substantially constant pressure above hydrostatic pressure at any depth in a body of water, the method comprising:
 communicating pressure of a pressurized gas in a container to a body of hydraulic fluid and to a chamber having at least one of atmospheric pressure and vacuum therein;   communicating hydrostatic pressure to the body of hydraulic fluid such that a pressure of the hydraulic fluid is substantially always equal to a sum of the pressure in the container and the hydrostatic pressure.   
     
     
         9 . The method of  claim 8  further comprising coupling the body of hydraulic fluid to a control in a subsea test tree. 
     
     
         10 . The method of  claim 8  further comprising discharging the hydraulic fluid to operate the control. 
     
     
         11 . The method of  claim 8  wherein the communicating hydrostatic pressure is performed by a first and a second piston each disposed at an opposed longitudinal end of a connecting rod, the connecting rod passing through the container, wherein the hydrostatic pressure exerted on the first piston is communicated through the connecting rod to the second piston in contact with the hydraulic fluid. 
     
     
         12 . The method of  claim 8  wherein the first piston separates compartments in an accumulator housing having the hydrostatic pressure in a first compartment and the at least one of atmospheric pressure and vacuum in a second compartment. 
     
     
         13 . The method of  claim 8  wherein an initial pressure of the pressurized gas is about 5,000 pounds per square inch. 
     
     
         14 . The method of  claim 8  wherein an initial pressure of the hydraulic fluid is about 3,000 pounds per square inch plus the hydrostatic pressure. 
     
     
         15 . An accumulator for subsea wellbore operations having hydraulic charge pressure maintained at a substantially constant pressure above hydrostatic pressure at any depth in a body of water, the accumulator comprising:
 a generally cylindrical housing having a first longitudinal end and a second longitudinal end, each longitudinal and having a port therein, the housing divided into two sections by a bulkhead;   a first piston disposed in the housing on one side of the bulkhead;   a second piston disposed in the housing on the other side of the bulkhead;   a connecting rod disposed between the first and second pistons, the connecting rod having an atmospheric pressure or vacuum chamber defined in a longitudinal end thereof in contact with the second piston;   wherein the first piston defines an hydraulic fluid chamber between the first piston and the first longitudinal end of the housing and a gas charge pressure chamber disposed between the bulkhead and the first piston, and wherein the second piston defines a hydrostatic pressure chamber between the second piston and the second longitudinal end of the housing, the first and second pistons having substantially equal cross sectional areas on both sides thereof such that a pressure of fluid in the hydraulic fluid pressure chamber is substantially equal to a pressure of gas in the gas charge pressure chamber and a hydrostatic pressure applied to the hydrostatic pressure chamber; and   wherein the accumulator is disposed in a riser coupled to a subsea test tree, and the port in the first longitudinal end of the housing is coupled to the subsea test tree to provide hydraulic pressure to operate components thereof.   
     
     
         16 . The accumulator of  claim 15  further comprising annular supports disposed in the gas charge pressure chamber, the annular supports having openings generally in a center thereof to enable passage of the connecting rod and openings to enable flow of gas in the gas charge pressure chamber. 
     
     
         17 . The accumulator of  claim 15  wherein the port in the second longitudinal end of the housing is exposed to fluid in the riser. 
     
     
         18 . The accumulator of  claim 15  wherein a ratio of cross sectional area of the first piston exposed to the hydraulic fluid chamber and exposed to the gas charge pressure chamber is selected such that a selected gas charge pressure results in a selected hydraulic fluid pressure. 
     
     
         19 . The accumulator of  claim 15  wherein an initial pressure of the gas charge is about 5,000 pounds per square inch. 
     
     
         20 . The accumulator of  claim 15  wherein an initial pressure of the hydraulic fluid is about 3,000 pounds per square inch plus the hydrostatic pressure.