Definition(s)
Lower Stack
Lower section of a subsea BOP stack that contains BOP spools, valves, hydraulic connectors, and nipples that typically connect to the subsea wellhead, subsea tree, or tubing head spool (below) and LMRP (above).
Source: API Specification 16Q, Design, Selection, Operation, and Maintenance of Marine Drilling Riser Systems, Second Edition, April 2017. Global Standards