46 CFR §153.935

1. (a)Except as provided in paragraph (b) of this section, the master shall ensure that all cargo tank hatches, ullage openings, and tank cleaning openings are tightly closed at all times.
2. (b)The master may not authorize the opening of a cargo tank, except:
   1. (1)To clean a tank;
   2. (2)To transfer a cargo that Table 1 allows in a containment system having an open gauging system;
   3. (3)To sample a cargo that Table 1 allows in a containment system having an open gauging system; or
   4. (4)To sample a cargo that Table 1 allows in a containment system having a restricted gauging system if:
      1. (i)The tank is not being filled during sampling;
      2. (ii)The vent system has relieved any pressure in the tank;
      3. (iii)The person sampling the cargo wears the protective clothing required during cargo transfer; and
      4. (iv)The tank is closed tightly following sampling.
3. (c)The master shall make sure that cargoes requiring closed gauging are sampled only through the controlled sampling arrangement required by § 153.404(d).