Abstract: The transportation of oil and gas under high pressure in tanks on board a ship is carried out by loading and unloading the oil/gas utilizing a suitable pressurized liquid, for example water, in the individual tanks, whereby during loading, a tank or a group of tanks containing pressurized liquid are filled with oil and gas while the pressurized liquid simultaneously is displaced into the next tank or group of tanks which are to be filled, after which the said next tank/group of tanks are filled and the pressurized liquid displaced into a third tank/group of tanks, etc., and that during unloading, the cargo in one tank or one group of tanks is removed by introducing a pressurized liquid into the tank/group of tanks, and unloading of the cargo in the next tank/group of tanks occurs by transferring the pressurized liquid from the said first tank/group of tanks to the next.