Subsea control system

Abstract

A technique enables protection of subsea wells. The technique employs a subsea test tree and associated control system to ensure control over the well in a variety of situations. The subsea test tree may be formed with an upper portion releasably coupled to a lower portion. The upper portion employs at least one upper shut-off valve, and the lower portion employs at least one lower shut-off valve to protect against unwanted release of fluids from either above or below the subsea test tree. The subsea test tree also is coupled with the control system in a manner which allows control to be exercised over the at least one upper shut-often valve and the at least one lower shut-off valve.

1 . A subsea wellhead control system, comprising:
 a subsea test tree having an upper part and a lower part connecting at a disconnect point;
 the upper part comprising a retainer valve; 
 the lower part comprising a ball valve and a flapper valve; 
   a control system for controlling the actuation of the subsea test tree including the retainer valve, the ball valve and the flapper valve; and   a pressure balanced accumulator associated with the control system and in hydraulic communication with the subsea test tree.   
     
     
         2 . The subsea wellhead control system as recited in  claim 1 , wherein the pressure balanced accumulator comprises a housing having a generally tubular-shaped member having first and second ends. 
     
     
         3 . The subsea wellhead control system as recited in  claim 2 , wherein the pressure balanced accumulator further comprises an accumulator mechanism located within the housing proximate the first end of the housing wherein the accumulator mechanism comprises a first chamber for receiving a pressurized gas at a first pressure and a second chamber for receiving a first pressurized fluid at a second pressure and where the first and second chambers are hermetically sealed from one another. 
     
     
         4 . The subsea wellhead control system as recited in  claim 3 , wherein the pressure balanced accumulator further comprises a third chamber in the housing which abuts one end of the accumulator mechanism, where the third chamber contains an oil fluid. 
     
     
         5 . The subsea wellhead control system as recited in  claim 4 , wherein the pressure balanced accumulator further comprises a movable piston which is located within the housing proximate the second end of the housing, the movable piston having first and second ends with first and second cross-sectional areas, respectively, where the piston is movable between a first position and a second position, wherein the second end of the housing includes a port to permit ambient subsea pressure to impinge on the first end of the piston, where the second end of the piston contacts the third chamber, and where the cross-sectional areas of the first and second ends of the piston are selected so as to optimize the pressure in the second chamber at which the piston begins to expel fluid from the second chamber of the accumulator. 
     
     
         6 . The subsea wellhead control system as recited in  claim 1 , wherein the control system comprises surface components and subsea components. 
     
     
         7 . The subsea wellhead control system as recited in  claim 1 , further comprising a blowout preventer stack independently controlled with respect to the subsea test tree. 
     
     
         8 . The subsea wellhead control system as recited in  claim 1 , further comprising at least one pipe ram. 
     
     
         9 . The subsea wellhead control system as recited in  claim 8 , further comprising at least one shear ram. 
     
     
         10 . The subsea wellhead control system as recited in  claim 1 , wherein the upper part and the lower part operate within a riser. 
     
     
         11 . The subsea wellhead control system as recited in  claim 6 , wherein the control system comprises a programmable logic controller. 
     
     
         12 . A subsea control system, comprising:
 a subsea installation, comprising:
 a blowout preventer; and 
 a subsea test tree controlled independently of the blowout preventer, the subsea test tree comprising:
 a retainer valve located in an upper portion; 
 at least one valve located in a lower portion; and 
 a latch mechanism releasably coupling the upper portion to the lower portion. 
 
   
     
     
         13 . The subsea control system as recited in  claim 12 , wherein the at least one valve located in the lower portion comprises a ball valve. 
     
     
         14 . The subsea control system as recited in  claim 13 , wherein the at least one valve located in the lower portion comprises a flapper valve separated from the ball valve. 
     
     
         15 . The subsea control system as recited in  claim 14 , further comprising a control system coupled to the subsea test tree to control actuation of the retainer valve, the ball valve and the flapper valve. 
     
     
         16 . The subsea control system as recited in  claim 12 , further comprising a control system coupled to the subsea test tree and a pressure balanced accumulator connected in cooperation with the control system. 
     
     
         17 . The subsea control system as recited in  claim 16 , wherein the pressure balanced accumulator is operable to control the retainer valve, located in the upper portion, and the at least one valve, located in the lower portion. 
     
     
         18 . The subsea control system as recited in  claim 17 , wherein the pressure balanced accumulator comprises a movable piston to exert a desired hydraulic supply pressure. 
     
     
         19 . A method for controlling a subsea wellhead, comprising:
 forming a subsea test tree with an upper portion, having at least one upper shut-off valve, coupled to a lower portion, having at least one lower shut-off valve;   positioning the subsea test tree in a subsea installation having separate emergency control features;   coupling the subsea test tree with a control system to control the at least one upper shut-off valve and the at least one lower shut-often valve; and   utilizing a pressure balanced accumulator in cooperation with the control system to supply pressurized fluid when needed.   
     
     
         20 . The method as recited in  claim 19 , wherein forming comprises forming the upper portion with a retainer valve. 
     
     
         21 . The method as recited in  claim 20 , wherein forming comprises forming the lower portion with a ball valve. 
     
     
         22 . The method as recited in  claim 20 , wherein forming comprises forming the lower portion with a ball valve and a flapper valve. 
     
     
         23 . The method as recited in  claim 19 , wherein utilizing comprises using a plurality of pressure chambers in cooperation with a movable piston.