Abstract: Embodiments of the present invention relate to an underwater gas/liquid-liquid modular separation system which enables the disposal of produced water in undersea environment. The underwater gas/liquid-liquid modular system separation consists of a gravitational separator and a water polishing system consisting of deoiling membranes capable of withdrawing the residual oil contained in water. Furthermore, the underwater gas/liquid-liquid modular separation system also features a demulsifier injection which promotes the breakdown of the water-oil emulsion. Embodiments further relate to an underwater gas/liquid-liquid separation method and to the use of deoiling membranes in gas/liquid-underwater liquid separation systems.