46 CFR §151.13-5

1. (a)The configurations listed in this paragraph refer to the separation of the cargo from its surroundings and list the various degrees of segregation required. Paragraphs and (2) of this section explain the symbols used in lines 1 and 2, in order, under the tank segregation column of Table 151.05.
   1. (1)Segregation of cargo from surrounding waters (Line 1 of Table 151.05).
   2. (2)Segregation of cargo space from machinery spaces and other spaces which have or could have a source of ignition (Line 2 of Table 151.05).
2. (b)[Reserved]
3. (c)If a cofferdam is required for segregation purposes and a secondary barrier is required for low temperature protection by § 151.15-3(d)(4), the void space between the primary and secondary barriers shall not be acceptable in lieu of the required cofferdam.