Underwater safety valve.
Source: API SPEC 6A, Specification for Wellhead and Christmas Tree Equipment, Twentieth Edition, October 2010 (Addendum November 2012). Global Standards
Source: API SPEC 17F, Specification for Subsea Production Control Systems, Second Edition, December 2006 (Reaffirmed April 2011). Global Standards
Underwater safety valve (see ISO 10423).
Source: API SPEC 17D, Design and Operation of Subsea Production Systems—Subsea Wellhead and Tree Equipment, Upstream Segment, Second Edition May 2011 (Errata September 2011). Global Standards