Abstract: An offshore riserless drilling system drills a subsea borehole from a platform and through a cased borehole. The system comprises a lightweight drill string suspending a bottomhole assembly and extending from the platform downwardly through a depth of water into the cased borehole; a first limiter limiting the range of motion of the drill string adjacent the platform; and a second limiter limiting the range of motion of the drill string adjacent the cased borehole. The preferred methods include lowering a bottomhole assembly suspended on a lightweight drill string from a platform through a depth of water; limiting the bend radius of the drill string adjacent the platform; guiding the bottomhole assembly into a cased borehole; limiting the bend radius of the drill string adjacent the cased borehole; maintaining the bottomhole assembly in the cased borehole, and drilling the subsea borehole.

Abstract: Methods and apparatus for drilling subsea wells in deep water using dual-gradient drilling techniques. One preferred embodiment includes a high pressure riser running from a drilling platform at the surface to the seafloor. The base of the riser is connected to a wellhead that is anchored to the seafloor. Pressure control equipment is disposed on the upper end of the riser at the drilling platform. A coiled tubing drill string is run through the riser and into the subsea formation. The drill string preferably includes a pressure sensing device that can be used in transmitting real-time downhole pressure data to the surface. A riser injection system is provided to inject a lower density fluid into the riser annulus in order to reduce the density of the fluids in the riser annulus and therefore reduce the hydrostatic pressure within the wellbore.

Abstract: An apparatus and method for offshore riserless drilling are disclosed. The preferred embodiments comprise a system for riserless drilling of a subsea borehole from a platform and through a cased borehole, the system comprising a lightweight drill string suspending a bottomhole assembly and extending from the platform downwardly through a depth of water into the cased borehole; a first limiter limiting the range of motion of the drill string adjacent the platform; and a second limiter limiting the range of motion of the drill string adjacent the cased borehole. The preferred methods include lowering a bottomhole assembly suspended on a lightweight drill string from a platform through a depth of water; limiting the bend radius of the drill string adjacent the platform; guiding the bottomhole assembly into a cased borehole; limiting the bend radius of the drill string adjacent the cased borehole; maintaining the bottomhole assembly in the cased borehole, and drilling the subsea borehole.