A vessel (14) that stores crude oil in tanks (12A-12H) in the vessel hull, maintains oxygen-free gas in the tank spaces (32) that lie above the crude oil. This is achieved by flowing in hydrocarbon-inert gas (gas that does not react with hydrocarbons), such as flue gas, when crude oil is removed so the crude oil level drops, and by flowing out vent gas that includes the inert gas and gaseous hydrocarbons that come from the crude oil, when crude oil flows into the tank and the oil level rises. An inert gas pipe (50) is used to flow inert flue gas into the tanks, and a separate vent gas pipe (52) is used to flow out the vent gas. A vacuum is applied to the vent pipe by connecting the vent pipe to an eductor (102) through which pressured gas, such as steam, is flowed. The vent gas is usually released into the atmosphere, but is burned when there is almost no wind, to prevent a buildup of hydrocarbon gas on the vessel deck.