Definition
Full-Opening Safety Valve — A full-opening safety valve is a manually operated, full-bore ball valve stabbed into the top of the drill string to shut in the pipe bore during a kick, while its full-diameter opening allows tools to pass and circulation to resume once closed and reopened.
Also known as: FOSV, full opening safety valve, kelly cock, TIW valve, lower kelly valve
What a full-opening safety valve does
A full-opening safety valve (FOSV), sometimes called a TIW or lower kelly valve, is a manually actuated ball valve used to close the bore of the drill string in a well-control situation. When the well flows up through the pipe, the crew stabs the FOSV onto the open string and closes it with a wrench to establish a barrier in the string bore. Its full-opening ball leaves an unrestricted bore when open, so wireline tools and darts can pass and full-rate circulation is possible.
Unlike a check-type inside BOP, the FOSV can be opened and closed on command, which lets the crew hold pressure, then reopen to pump down or drop a dart valve as needed.
Role in the drill-string barrier
The FOSV, an inside BOP and the surface BOP stack together form the well-control barrier envelope. The FOSV is kept on the rig floor ready to stab, and its connection and pressure rating must match the drill pipe and expected shut-in pressures.
Why it matters for rentals
An FOSV is a pressure-containing barrier that must be certified and pressure tested. Rental valves should match your string connection and working pressure and carry current documentation. See our pressure control equipment and our quality program, or contact us.