46 CFR §108.171

1. (a)An enclosed space that contains any part of the mud circulating system that has an opening into the space and is between the well and final degassing discharge.
2. (b)An enclosed or semi-enclosed location that is below the drill floor, and contains a possible source of gas release.
3. (c)An enclosed space that is on the drill floor, and is not separated by a solid, gas-light floor from the spaces specified in paragraph (b) of this section.
4. (d)A space that would normally be considered a Division 2 location under § 108.173 but where combustible or flammable gases might accumulate.
5. (e)A location in the weather, or a semi-enclosed location, except as provided in paragraph (b) of this section that is within 1.5 m (5 ft.) of the boundary of any—
   1. (1)Equipment or opening specified in paragraph (a) of this section;
   2. (2)Ventilation outlet, access, or other opening to a Class I, Division 1 space; or
   3. (3)Gas vent outlet.
6. (f)Except as provided in § 108.175, an enclosed space that has an opening into a Class I, Division 1 location.