Patent ID: 7677329

Claim:
A method of controlling drilling fluid pressure during drilling offshore, comprising: flowing drilling fluid down into a borehole in a sea bed beneath a body of water; flowing drilling fluid back out of the borehole and into a conduit, wherein the conduit also contains a volume of riser fluid, wherein the riser fluid has a different density than the drilling fluid, and wherein the volume of the riser fluid is located above the drilling fluid starting at a demarcation zone between the two fluids in the conduit; regulating a distance between a first level and the demarcation zone while flowing drilling fluid into the borehole and out of the borehole and into the conduit; and removing drilling fluid from the conduit utilizing a pump with an inlet in fluid communication with the conduit; and regulating a pressure of the drilling fluid at the inlet to the pump to regulate the distance between the first level and the demarcation zone.