Abstract: An offshore oil storage facility on a sea bed which can withstand the effects of waves, tides and oil transfer even though it is of lightweight steel construction. The oil in the tank floats on a layer of water and standpipes extend up from the water layer to a location under the sea surface. Oil containment casings in the form of tubular shells surround the upper ends of the standpipes and extend above the sea surface. Differential pressure sensors arranged to sense differential pressure across the tank walls are used to control the pumping of fluids into and out from the tank to minimize such pressure differentials.