System and Methods for Controlled Mud Cap Drilling

A method for drilling wells in a body of water from a Mobile Offshore Drilling Unit (MODU) on a surface of the body of water, the method comprising:

• operating a drilling apparatus comprising a marine drilling riser extending from the MODU to a blowout preventer (BOP) on the bottom of the body of water, with at least one fluid return outlet in fluid communication with an interior of the marine drilling riser coupled to an inlet of a subsea mud pump, an outlet of the subsea mud pump connected to a return line extending to the MODU, an interface of gas and liquid in the marine drilling riser disposed at an elevation below the surface of the body of water, a conduit extending from the MODU through the marine drilling riser and BOP into a wellbore extending below the water bottom;
• pumping a liquid into the marine drilling riser through an exterior line on the marine drilling riser and from the top of the marine drilling riser through at least two distinct lines each connected to an additional pump, where both additional pumps pump at a higher rate than a wellbore fluid loss rate into an annulus between the wellbore and the conduit, wherein excess liquid pumped into the marine drilling riser by the additional pumps as required to maintain the interface level is pumped back to the MODU by controlling the pumping rate of the subsea mud pump, thereby controlling fluid loss rate into the formation;
• and
• at least one of, lowering the pumping rate of the subsea mud pump when the interface level drops to a predetermined minimum level, while maintaining a pumping rate of the additional pumps and decreasing the pumping rate of the subsea mud pump when the interface level rises to a predetermined maximum level, while maintaining a pumping rate of the additional pumps.