Active barriers are typically barriers that are designed to be routinely actuated either manually (e.g. by a diver or ROV) or by some form of remote control (e.g. via the production control system) or by reverse flow (e.g. check valves), such as the following: downhole SCSSVs and SSCSVs; subsea tree valves (including valves in the production and annulus flow paths, as well as valves in hydraulic and chemical injection lines); manifold valves (including hydraulically actuated and ROV-operated valves); flowline isolation valves (including those on a manifold, as well as at the top of a riser); check valves (including those in downhole gaslift valves and in chemical injection lines). Barriers such as downhole sliding sleeves can be classified as either passive or active, depending on the activation method and the anticipated activation frequency.
Source: API RP 17A, Design and Operation of Subsea Production Systems—General Requirements and Recommendations, Fourth Edition, Reaffirmed 2011. Global Standards