shaped geometry of progressively variable section is particularly advantageous such a shape is particularly suitable for a vertically arranged capacity it can be advantageous to install a bypass line 10 bypassing the separation capacity in case of sanding up or dismantling for repair or maintenance fig2 a these valves can be operated by a rov remotely operated subsea vehicle line 11 is connected to the production wellheads line 12 is connected to the base of the production riser line 13 corresponds to the line feeding the gas into the riser the capacity can be arranged near to the base of the riser on the baseplate of the riser or at a short distance therefrom on an independent baseplate in this case the junctions of the lines between the capacity and the riser consist of connecting devices known to specialists as jumpers according to the architecture diagrammatically illustrated in fig2 b it is also possible to arrange the capacity in such a way that it forms a vertical continuation of riser 14 and to directly connect the riser to the capacity in this case foundation 15 of the separation capacity also acts as an anchor for the riser control valve 3 is intended to control the gas flow rate in the riser the type of valve and its dimensions are determined according to the nominal gas flow rate and to the pressure drop required for this nominal flow rate since the fluid flowing therethrough is wet gas with high velocity liquid droplets it may undergo wear through erosion of the metal the layout of this valve is suited to allow easy servicing for replacement of its inner.