A system is described for use at offshore locations of large depth, for mooring a production vessel or floating unit (14) at a location over a hydrocarbon reservoir (26) and for connecting risers (101) that can be carrying hydrocarbons up from the sea floor to a production vessel that stores the hydrocarbons, flowlines for water injection, gas lift, gas export, umbilicals and mooring lines that moor the vessel. Both the mooring lines and the risers are disconnectably connected to the vessel though a connection buoy, or connector (16). The invention concerns a system that allows a connector (16) to be used that is of minimum mass and volume, to ease its handling especially during its connection and disconnection to and from a vessel.