Patent ID: 8517111

Claim:
A method of drilling a subsea well bore using a drill pipe, a drilling riser package comprising one or more drilling riser conduits fluidly connecting a drilling platform to a subsea wellhead located substantially at the mud line, the wellhead fluidly connecting the riser conduits and a subsea well accessing a subsea formation of interest, and a dual gradient mud system including a first mud having a first density and a second mud having a second density greater than the first density, the method comprising: a) drilling the subsea well bore while employing a subsea pumping system, a subsea choke manifold, and one or more mud return risers to implement the dual gradient mud system; b) detecting a well bore influx and shutting in the well bore; c) determining i) a size of the influx; and ii) a reduction in the weight of the mud system to match a hydrostatic head of the dual gradient mud system before the influx reaches a take point of the subsea pump; d) circulating a single gradient kill weight fluid having a third density less than the first density down the drill pipe using a surface pumping system and into an annulus between the drill pipe and the drilling riser, maintaining a constant bottom hole pressure, and using the subsea choke manifold to control flow to the subsea pump and maintain the constant bottom hole pressure; e) pumping a sufficient amount of the single gradient kill weight fluid into the annulus using the surface pumping system and a surface choke manifold until fluid in the annulus has a density sufficient to control the influx and has a density equivalent to the dual gradient mud system; and f) isolating the subsea pumping system, the subsea choke manifold, and the one or more mud return risers while circulating the influx up the annulus or one or more other fluid passages in the drilling riser package using the surface pumping system, through the wellhead, and out the surface choke manifold.