Abstract: A hydraulic actuator for a valve that fails as-is in the event of control line failure. The actuator has a housing carrying an actuator piston for actuating the valve open or closed. An elongate gripper shaft extends upward from the actuator piston. A collet member is carried within the housing and adapted to engage the gripper shaft when moved upward relative to the housing. The collet member is biased upward by a spring. A release piston abutting the dog members prevents their upward movement when actuated to a lower position. The release piston is supplied with the same hydraulic fluid pressure as the actuator piston. If pressure is removed, the spring moves the collet member into locking engagement with the shaft.