An apparatus comprising: a tube having at least one flow outlet in communication with an interior of the tube, wherein the tube forms a segment of a riser extending between a wellbore and a drilling unit on the surface of a body of water; a pump having an inlet connected to a first pump conduit and an outlet connected to a second pump conduit; a mud return line extending from a manifold to the drilling unit, the pump configured to lift mud to the drilling unit through the mud return line; a first plurality of valves (18, 19, 26) for selectively diverting flow within the tube by connecting the flow outlet to an inlet of the pump via the first pump conduit, wherein when the flow outlet is connected to the inlet of the pump, the pump operates to lift the mud to the drilling unit so as to maintain a selected pressure in the wellbore; a second plurality of valves (18, 19, 27, 40) for selectively diverting flow within the tube by connecting the flow outlet to the mud return line and for closing the pump outlet, wherein the flow outlet is connected to a first flow line via a flow block, the flow line is connected to the manifold and to the mud return line and the flow outlet is connected to an outlet of the pump via the second pump conduit, wherein when the flow outlet is connected to the mud return line and the pump outlet is closed, flow in the riser above the tube is stopped; wherein the flow outlet of the tube comprises a second manifold having two separate flow outlets, each in fluid communication with the interior of the tube and wherein each of the two separate flow outlets comprises a valve (18, 19) of the first plurality of valves arranged to close fluid communication between the respective flow outlet and the interior of the tube; and wherein the apparatus further comprises a flow tee connected to each valve (18, 19) of the first plurality of valves arranged to close fluid communication between the respective flow outlet and the interior of the tube, a first outlet of one of the flow tees being connected to the first pump conduit and a second outlet being connected to a second flowline and a first outlet of the other of the flow tees being connected to the second pump conduit and a second outlet being connected to the first flow line. a tube having at least one flow outlet in communication with an interior of the tube, wherein the tube forms a segment of a riser extending between a wellbore and a drilling unit on the surface of a body of water; a pump having an inlet connected to a first pump conduit and an outlet connected to a second pump conduit; a mud return line extending from a manifold to the drilling unit, the pump configured to lift mud to the drilling unit through the mud return line; a first plurality of valves (18, 19, 26) for selectively diverting flow within the tube by connecting the flow outlet to an inlet of the pump via the first pump conduit, wherein when the flow outlet is connected to the inlet of the pump, the pump operates to lift the mud to the drilling unit so as to maintain a selected pressure in the wellbore; a second plurality of valves (18, 19, 27, 40) for selectively diverting flow within the tube by connecting the flow outlet to the mud return line and for closing the pump outlet, wherein the flow outlet is connected to a first flow line via a flow block, the flow line is connected to the manifold and to the mud return line and the flow outlet is connected to an outlet of the pump via the second pump conduit, wherein when the flow outlet is connected to the mud return line and the pump outlet is closed, flow in the riser above the tube is stopped; wherein the flow outlet of the tube comprises a second manifold having two separate flow outlets, each in fluid communication with the interior of the tube and wherein each of the two separate flow outlets comprises a valve (18, 19) of the first plurality of valves arranged to close fluid communication between the respective flow outlet and the interior of the tube; and wherein the apparatus further comprises a flow tee connected to each valve (18, 19) of the first plurality of valves arranged to close fluid communication between the respective flow outlet and the interior of the tube, a first outlet of one of the flow tees being connected to the first pump conduit and a second outlet being connected to a second flowline and a first outlet of the other of the flow tees being connected to the second pump conduit and a second outlet being connected to the first flow line.