for a significantly reduced drill floor height and consequently a lower vertical center of gravity for the rig application_number US 201313870789 A description the invention may best be understood by reference to the exemplary embodiment s illustrated in the drawing figures to permit lowering the drill floor of a semi submersible offshore drilling vessel the present invention uses an alternative mud return system that does not require the use of a slip joint advantage is taken of technology that already exists in dual gradient drilling and land underbalanced drilling in place of the slip joint and diverter under the drill floor an industry proven drilling rotating control head is used on top of the blowout preventer an optional in line progressive cavity pump that moves mud returns from the well to the shakers mud pits may be in fluid communication with the mud return line coming off the rotating head a flexible hose designed to the specifications for a diverter flow line may attached to the pump and connected to fixed pipe under the drill floor to direct mud returns to the shaers mud pits or to diverter overboard lines separated by remotely operated valves significantly reduced drill floor height with resulting reduction of the rig 39 s vertical center of gravity more convenient construction and installation of the substructure and derrick and deletion of the operational requirement for removing the slip joint during emergency evacuation procedures referring now to fig1 a drilling rig 10 of the prior art is depicted in side view fig1 shows a portion of a floating offshore drilling vessel such as a semi submersible vessel drilling derrick 12 is mounted on drill floor 14 and drill floor skid base 16 which in turn is supported on main deck 20 by support.