Full opening safety valve (short for FOSV) is ball type safety valve used to stop flow through the drill string when the drilling is being withdrawn from the well.
FOSV is dual body full-opening safety valve, so it does not interfere with the running of tools such as core barrels or survey instruments. It is designed to be stabled into the top joint of drill pipe or tubing string at the rig floor and closed quickly in case a well kicks.
The ball-type design permits the valve to be compact, easy to handle, and yet have great strength, standard test pressure is 10000PSI, but higher pressure ratings are available.
FOSV and operating wrench should be maintained on the rig floor at all times.