A separator tank (1) for separating oil and gas from water, and comprising separator tank units (2, 2′, 2″) arranged on top of one another within an annular enclosure (4). An inlet pipe (14) is connected with the inlet for fluid in a first of the at least two separator tank units. A second outlet (9) in the first separator tank unit (2) is connected with the inlet (7) for fluid in a second of the at least two separator tank units (2′). A pressure control device controls the pressure downstream of the first outlets for oil and gas (8) so that the pressure at the first outlets (8) is lower than the pressure at the water outlet (26) on the separator tank.