46 U.S.C. § 7310

For service on a vessel of less than 500 gross tons as measured under section 14502 of this title , or an alternate tonnage measured under section 14302 of this title as prescribed by the Secretary under section 14104 of this title engaged in support of exploration, exploitation, or production of offshore mineral or energy resources, an individual may be rated as able seaman—offshore supply vessels if the individual has at least 6 months’ service on deck on board vessels operating on the oceans or the navigable waters of the United States (including the Great Lakes).