FAILSAFE VALVE SYSTEM

A technique facilitates failsafe closure of a valve used in, for example, a subsea test tree. The technique utilizes a valve combined with a cutter oriented to sever well equipment passing through an interior passage of the valve. The valve is operatively coupled with an actuation system having an actuator piston which controls cutting and valve closure. The failsafe valve and the cutter are shifted to an open position by applying pressure in a control fluid chamber to shift the actuator piston. However, the actuator piston, and thus the valve and cutter, are biased toward a closed position via pressure applied in a pressure chamber and a gas precharge chamber. The combined pressure ensures adequate force for shearing of the well equipment and closure of the valve when hydraulic control pressure is lost. In some applications, additional closing force may be selectively provided to the actuator piston.