A drilling system comprising: a wellhead on a seafloor of a body of water, the wellhead defining a passage; a rotating control device having a housing carried atop the wellhead, the housing defining a passage in fluid communication with the passage of the wellhead; an offshore platform disposed above a surface of the body of water; a concentric dual drill pipe string carried by the platform and extending through the passage of the rotating control device into the passage of the wellhead, the wellhead and the string defining an annulus therebetween, the rotating control device including a sealing element that dynamically seals against an outer wall of the string so as to fluidly isolate the annulus from the body of water, the outer wall of the string above the rotating control device being in contact with the body of water; a hydraulic power unit near the seafloor and coupled to the rotating control device so as to supply a lubricant to the sealing element; a source of pressurized fluid selectively fluidly coupled to the annulus; a bottom hole assembly carried at a distal end of the string; a blowout preventer carried atop the wellhead at a position below the rotating control device, the blowout preventer having a passage formed therethrough that is in fluid communication with the passages (30, 42) of the wellhead and the rotating control device, the blowout preventer including a closure device (22, 24, 26) arranged so as to selectively isolate the passage of the wellhead from the passage of the rotating control device; a tubular spacer carried atop the blowout preventer at a position below the rotating control device, the spacer having an axial length great enough so that the bottom hole assembly can be positioned between the closure device (22, 24, 26) of the blowout preventer and the sealing element of the rotating control device; and at least one communication link operable between a location at the surface of the body of water and at least one of the hydraulic power unit and the source of pressurized fluid; characterized in that the concentric dual drill pipe string further comprises: an inner pipe disposed within an outer pipe; a first valve disposed along the string selectively fluidly coupling an interior of the inner pipe with the annulus, wherein the first valve is a distal crossover port; a second valve disposed along the string selectively fluidly coupling an interior of the inner pipe with the annulus, wherein the second valve is a medial crossover port; wherein the first and second valves can be independently and remotely actuated. a wellhead on a seafloor of a body of water, the wellhead defining a passage; a rotating control device having a housing carried atop the wellhead, the housing defining a passage in fluid communication with the passage of the wellhead; an offshore platform disposed above a surface of the body of water; a concentric dual drill pipe string carried by the platform and extending through the passage of the rotating control device into the passage of the wellhead, the wellhead and the string defining an annulus therebetween, the rotating control device including a sealing element that dynamically seals against an outer wall of the string so as to fluidly isolate the annulus from the body of water, the outer wall of the string above the rotating control device being in contact with the body of water; a hydraulic power unit near the seafloor and coupled to the rotating control device so as to supply a lubricant to the sealing element; a source of pressurized fluid selectively fluidly coupled to the annulus; a bottom hole assembly carried at a distal end of the string; a blowout preventer carried atop the wellhead at a position below the rotating control device, the blowout preventer having a passage formed therethrough that is in fluid communication with the passages (30, 42) of the wellhead and the rotating control device, the blowout preventer including a closure device (22, 24, 26) arranged so as to selectively isolate the passage of the wellhead from the passage of the rotating control device; a tubular spacer carried atop the blowout preventer at a position below the rotating control device, the spacer having an axial length great enough so that the bottom hole assembly can be positioned between the closure device (22, 24, 26) of the blowout preventer and the sealing element of the rotating control device; and at least one communication link operable between a location at the surface of the body of water and at least one of the hydraulic power unit and the source of pressurized fluid; characterized in that the concentric dual drill pipe string further comprises: an inner pipe disposed within an outer pipe; a first valve disposed along the string selectively fluidly coupling an interior of the inner pipe with the annulus, wherein the first valve is a distal crossover port; a second valve disposed along the string selectively fluidly coupling an interior of the inner pipe with the annulus, wherein the second valve is a medial crossover port; wherein the first and second valves can be independently and remotely actuated. an inner pipe disposed within an outer pipe; a first valve disposed along the string selectively fluidly coupling an interior of the inner pipe with the annulus, wherein the first valve is a distal crossover port; a second valve disposed along the string selectively fluidly coupling an interior of the inner pipe with the annulus, wherein the second valve is a medial crossover port; wherein the first and second valves can be independently and remotely actuated.