Abstract: A new tank ship, or an older ship which might not otherwise be economically retrofitted to comply with current regulations, is provided with a double hull requiring essentially no internal steel structural support by distributing and compacting sand in the bottom of the tank or cargo hold, applying a layer of reinforced concrete to the sand, laying steel plates on the surface of the cured concrete and welding the abutting edges of the plates to the hull, interior bulkheads, and to each other, to form a second fluid-tight bottom in the tank. The construction method can be used to retrofit older tankers to comply with Regulation 13G of MARPOL.