46 CFR §108.173

1. (a)An enclosed space that has any open portion of the mud circulating system from the final degassing discharge to the mud suction connection at the mud pit.
2. (b)A location in the weather that is—
   1. (1)Within the boundaries of the drilling derrick up to a height of 3 m (10 ft.) above the drill floor;
   2. (2)Below the drill floor and within a radius of 3 m (10 ft.) of a possible source of gas release; or
   3. (3)Within 1.5 m (5 ft.) of the boundaries of any ventilation outlet, access, or other opening to a Class I, Division 2 space.
3. (c)A location that is—
   1. (1)Within 1.5 m (5 ft.) of a semi-enclosed Class I, Division 1 location indicated in § 108.171(b); or
   2. (2)Within 1.5 m (5 ft.) of a Class I, Division 1 space indicated in § 108.171(e).
4. (d)A semi-enclosed area that is below and contiguous with the drill floor to the boundaries of the derrick or to the extent of any enclosure which is liable to trap gasses.
5. (e)A semi-enclosed derrick to the extent of its enclosure above the drill floor or to a height of 3 m (10 ft.) above the drill floor, whichever is greater.
6. (f)Except as provided in § 108.175 an enclosed space that has an opening into a Class I, Division 2 location.