A coupling system between a riser and an underwater supporting structure, comprising: - a coupling seat and a coupling head, one connectable to the underwater supporting structure and the other to the riser, wherein the coupling head and the coupling seat are shaped for a mutual insertion thereof along an insertion direction from a completely detached position to an inserted position, - a pull connector adapted to constrain the coupling head to the coupling seat, in a coupling region, so as to prevent the extraction of the coupling head towards the detached position, but so as to allow rotations of the coupling head with respect to the coupling seat at least about axes transversal to the insertion direction, - a clamping connector adapted to constrain the coupling head to the coupling seat, in at least one locking region spaced apart from the coupling region, so as to prevent translations transversal to the insertion direction and rotations between the head and the coupling seat, wherein the pull connector and the clamping connector are adapted to form a complete clamping of the coupling head in the coupling seat with prevention of relative rotations therebetween, wherein the pull connector can be actuated alone and independently from the clamping connector to make a provisional, pull-only connection between the head and the coupling seat, wherein the clamping connector can be actuated independently from the pull connector to allow postponing the complete clamping with respect to the provisional connection, characterized in that the clamping connector comprises one or more expansion members positioned in a locking portion of the coupling head and displaceable between a retracted position, in which they allow the head to move in the seat, and a protracted position, in which they expand the radial dimension of the head in pressing contact against a locking surface of the coupling seat. - a coupling seat and a coupling head, one connectable to the underwater supporting structure and the other to the riser, wherein the coupling head and the coupling seat are shaped for a mutual insertion thereof along an insertion direction from a completely detached position to an inserted position, - a pull connector adapted to constrain the coupling head to the coupling seat, in a coupling region, so as to prevent the extraction of the coupling head towards the detached position, but so as to allow rotations of the coupling head with respect to the coupling seat at least about axes transversal to the insertion direction, - a clamping connector adapted to constrain the coupling head to the coupling seat, in at least one locking region spaced apart from the coupling region, so as to prevent translations transversal to the insertion direction and rotations between the head and the coupling seat, - a pull connector adapted to constrain the coupling head to the coupling seat, in a coupling region, so as to prevent the extraction of the coupling head towards the detached position, but so as to allow rotations of the coupling head with respect to the coupling seat at least about axes transversal to the insertion direction, - a clamping connector adapted to constrain the coupling head to the coupling seat, in at least one locking region spaced apart from the coupling region, so as to prevent translations transversal to the insertion direction and rotations between the head and the coupling seat, wherein the pull connector and the clamping connector are adapted to form a complete clamping of the coupling head in the coupling seat with prevention of relative rotations therebetween, wherein the pull connector can be actuated alone and independently from the clamping connector to make a provisional, pull-only connection between the head and the coupling seat, wherein the clamping connector can be actuated independently from the pull connector to allow postponing the complete clamping with respect to the provisional connection, characterized in that the clamping connector comprises one or more expansion members positioned in a locking portion of the coupling head and displaceable between a retracted position, in which they allow the head to move in the seat, and a protracted position, in which they expand the radial dimension of the head in pressing contact against a locking surface of the coupling seat.