The present invention provides drilling systems for drilling
subsea wellbores. The
drilling system includes a tubing that passes through a
sea bottom wellhead and carries a
drill bit. A
drilling fluid system continuously supplies
drilling fluid into the tubing, which discharges at the
drill bit bottom and returns to the
wellhead through an annulus between the tubing and the
wellbore carrying the
drill cuttings. A fluid return line extending from the
wellhead equipment to the drilling vessel transports the returning fluid to the surface. In a riserless arrangement, the return fluid line is separate and spaced apart from the tubing. In a
system using a riser, the return fluid line may be the riser or a separate line carried by the riser. The tubing may be
coiled tubing with a drilling motor in the
bottom hole assembly driving the
drill bit. A suction pump coupled to the annulus is used to control the
bottom hole pressure during drilling operations, making it possible to use heavier drilling muds and drill to greater depths than would be possible without the suction pump. An optional
delivery system continuously injects a flowable material, whose
fluid density is less than the density of the
drilling fluid, into the returning fluid at one or more suitable locations the rate of such lighter material can be controlled to provide supplementary regulation of the pressure. Various pressure, temperature, flow rate and kick sensors included in the
drilling system provide signals to a controller that controls the suction pump, the surface
mud pump, a number of flow control devices, and the optional
delivery system.