Patent ID: 7108006

Claim:
A method of operating a subsea valve, comprising: connecting to the valve an actuator having a piston with first and second sides and a spring acting against the second side; exposing one side of a fluid accumulator to sea water hydrostatic pressure and communicating an opposite side of the fluid accumulator to a lower pressure side of a pressure intensifier; communicating a higher pressure side of the pressure intensifier with a second side of the piston, thereby applying a hydraulic fluid pressure to the second side of the piston that is greater than the sea water hydrostatic pressure; moving the valve to a first position by applying hydraulic fluid pressure to the first side of the piston at a level sufficient to overcome forces created by the fluid pressure on the second side of the piston and the spring; and in the event the hydraulic fluid pressure on the first side of the piston to drops to the sea water hydrostatic pressure, moving the valve to a fail safe position with the forces created by the fluid pressure on the second side of the piston and the spring.