46 CFR §153.256

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