46 CFR § 153.256 - Trunks, domes, and openings of cargo tanks.
(a) The hatch of a cargo tank must:
(1) Be at the highest point of the tank; and
(2) Open on or above the weatherdeck.
(b) To be endorsed to carry a cargo requiring an independent cargo tank, a tank must have:
(1) A trunk or dome at the uppermost part of the tank, extending above the weatherdeck;
(2) Its hatch at the top of the trunk or dome; and
(3) No openings below the weatherdeck.