The Anglo-Saxon Petroleum Company, led by John Lamb, offered the British Admiralty to convert tankers into escort carriers. Initially the Admiralty felt that there would be a larger fire risk and turned down the offer, however in Spetember 1942 they changed their minds.
The tankers had their superstructures reduced and the funnel replaced by a vent that went out the sides.
Since no hangar was installed all the Swordfish had to be carried on deck.
The Rapana class ships carried only "black oil" and would sometimes refuel convoy escorts.