A subsea system for processing a fluid emanating from one or more subsea wells, comprising a fluid processing circuit and a base module (3) provided with at least one receiver (40) for receiving an insert module (4-8) comprising an appliance that forms part of the fluid processing circuit. The receiver (40) comprises a cavity (30) for accommodating the insert module (4-8). The insert module (4-8) is provided with a flange (31), which is adapted to bear on a corresponding flange (32) of the receiver (40) when the insert module (4-8) is mounted therein, a watertight seal (33) being arranged between said flanges (31, 32) so as to seal the space between the receiver (40) and the part of the insert module (4-8) received therein from the surrounding sea water.