(n.) Well Completions. The process of disassembling well-control or pressure-control equipment on the wellhead. Depending on the configuration of the wellhead and casing strings, it may be necessary to nipple-down and nipple-up the blowout preventer (BOP) system as each casing string is run. Ant: nipple up, nipple-up. Alt. form: nipple downSee: Blowout Preventer, Casing String