With the ongoing phase out of single hull oil tankers under the IMO directive, Owners are giving consideration to converting these vessels into double hull tankers by means of adding an internal or external double hull. SeaTec have completed numerous studies to for both internal and external Owners into the feasibility of this conversion. Several of these have resulted in actual ship conversions, and we have experience of both internal and external double hulling. For internal hulling, new side longitudinal bulkheads are added to create wing ballast tanks, and a new tank top is added to create a double bottom ballast space. For external hulling a new outer skin is added over the original hull of the cargo tank region, at the requisite depth to satisfy the MARPOL minimum side tank requirements, and the new void spaces are dedicated to ballast tanks. Internal hulling generally involves less steelwork and does not affect ship performance, however cargo capacity can be reduced by up to 10%. Whereas the external skin option will retain the full cargo capacity of the vessel but affect ships speed, fuel consumption and propeller immersion. In both conversion cases the cargo and ballast tanks will require significant modification of the current pumping systems.