Patent ID: 8387705

Claim:
A system comprising, in combination: a riser conduit for containing a mixed-density drilling mud above the mud line, the mixed-density mud formed by mixing a portion of a relatively high-density mud with a portion of a relatively low-density mud, the riser conduit fluidly connecting a floating or semi-submersed platform to a subsea wellhead located substantially at the mud line, the wellhead fluidly connecting the riser conduit and a subsea well accessing a subsea formation of interest; a plurality of casing members comprising a first casing member and a last casing member run into the well for casing the subsea well in the presence of a relatively high-density drilling mud in the well; a drill pipe landing string comprising a surface-controlled valve at its distal end, and a ported circulation sub located just above the surface-controlled valve, wherein the distal end of the first casing member comprises an auto-fill casing float collar, wherein the distal end of the drill pipe landing string fluidly connects to a proximal end of the last casing member run in the well, and wherein the surface-controlled valve and the ported circulation sub are positioned to substantially maintain the dual gradient mud system in the riser conduit and well during running in of casing into the well.