Abstract: A system, which is preferably located on board a ship having at least one tank for oil or another liquid cargo and is provided for carrying out operations in connection with the unloading and the draining of cargo and/or wash liquid and the cleaning of the cargo tank as well as the handling and refining of polluted wash liquid, comprises a main pump, which is connected to the cargo tank through one main suction pipe and is adapted to substantially empty the cargo tank, means connecting the inlet of the main pump to a wash liquid source, spray means which is provided in the cargo tank for the cleaning thereof and is connected to a pressure fluid source, an air and gas suction off system, which is preferably connected to the main suction line through a gas separation tank, and a drainage pump. This drainage pump, which is preferably of the centrifugal type, is provided for wash-draining and/or after-draining and is driven by a liquid turbine.