Offshore drilling system

A system for drilling a subsea well from a rig through a subsea wellhead below the rig, comprising:

• a wellhead stack mounted on the subsea wellhead, the wellhead stack comprising at least a subsea blowout preventer stack and a subsea diverter;
• a drill string extending from the rig through the wellhead stack into the well, the drill string for conducting drilling fluid from the rig to a drill bit in the well;
• a well annulus extending from the bottom of the well to the subsea diverter, the well annulus for conducting fluid away from the drill bit;
• and
• a positive-displacement pump having a suction side in communication with the well annulus and a discharge side in communication with the rig, the pump being operable such that a selected pressure gradient is maintained in the well annulus, the pump comprising a first chamber in communication with the well annulus, the first chamber being provided to selectively receive fluid from and dispense fluid to the well annulus wherein the first chamber is defined in a vessel having a second chamber defined therein and a movable member disposed between the first and second chambers, the movable member being arranged to move within the vessel in response to pressure differential between the first and second chambers.

A system for drilling a subsea well from a rig through a subsea wellhead below the rig, comprising:

• a subsea blowout preventer stack having a first end coupled to the subsea wellhead;
• a drill string extending from the rig through the subsea blowout preventer stack and wellhead into the well, the drill string for conducting drilling fluid from the rig to a drill bit in the well;
• a rotating subsea diverter coupled to a second end of the subsea blowout preventer stack and adapted to slidingly receive and sealingly engage the drill string;
• a well annulus extending from the bottom of the well to the rotating subsea diverter, the well annulus for conducting fluid away from the drill bit;
• and
• a pump having a suction side in communication with the well annulus and a discharge side in communication with the rig, the pump being operable such that a selected pressure gradient is maintained in the well annulus, the pump comprising a device coupled to an inlet side thereof to control a size of solid particles entering the pump.