Forming a subsea wellbore

Abstract

A riserless subsea drilling system includes a tubular drilling string including a drilling bit; a rotating control device including a seal sealed to the drilling string; and a drilling fluid circuit that circulates drilling fluid from a supply line, through an annulus exterior the drilling string and below the rotating control device, through the drilling bit at a downhole end of the drilling string, and through the drilling string toward an uphole end of the drilling string.

What is claimed is: 
     
         1 . A riserless subsea drilling system, comprising:
 a tubular drilling string comprising a drilling bit;   a rotating control device comprising a seal sealed to the drilling string; and   a drilling fluid circuit that circulates drilling fluid from a supply line, through an annulus exterior the drilling string and below the rotating control device, through the drilling bit at a downhole end of the drilling string, and through the drilling string toward an uphole end of the drilling string.   
     
     
         2 . The riserless subsea drilling system of  claim 1 , further comprising:
 a mud motor coupled to the drilling bit and having a center bypass bore; and   a flow control device in or upstream of the center bypass bore.   
     
     
         3 . The riserless subsea drilling system of  claim 1 , further comprising a flow control apparatus positioned in the drilling fluid circuit to control the circulation of drilling fluid. 
     
     
         4 . The riserless subsea drilling system of  claim 1 , further comprising a buffer chamber above the drilling bit. 
     
     
         5 . The riserless subsea drilling system of  claim 1 , further comprising pressure sensors positioned to measure a pressure differential across the seal of the rotating control device. 
     
     
         6 . The riserless subsea drilling system of  claim 1 , wherein the rotating control device further comprises a seawater inlet and a seawater outlet to circulate a flow of seawater across the seal. 
     
     
         7 . The riserless subsea drilling system of  claim 6 , wherein the rotating control device further comprises an additive to circulate with the flow seawater across the seal. 
     
     
         8 . The riserless subsea drilling system of  claim 7 , wherein the additive comprises detergent. 
     
     
         9 . The riserless subsea drilling system of  claim 1 , wherein the supply line extends from at or near a sea surface, through a subsea environment, to the annulus. 
     
     
         10 . The riserless subsea drilling system of  claim 9 , wherein the supply line fluidly connects to the annulus downhole of a blowout preventer. 
     
     
         11 . A method for forming a subsea wellbore, comprising:
 sealing, with a rotating control device, a portion of a tubular drilling string against a subsea environment, the tubular drilling string extending from at or near a subsea surface through the subsea environment; and   circulating, from at or near the sea surface, a drilling fluid in a downhole direction through an annulus between the drilling string and a wellbore, and then in an uphole direction through the drilling string towards the sea surface.   
     
     
         12 . The method of  claim 11 , further comprising circulating the drilling fluid through a supply line, connected to the annulus downhole of the rotating control device, that extends through the subsea environment to the annulus. 
     
     
         13 . The method of  claim 12 , further comprising:
 determining density and rheological properties of the drilling fluid entering the supply line;   determining density and rheological properties of the drilling fluid returned through the drilling string;   determining, based on the determined densities and rheological properties, a hydrostatic pressure difference between the drilling fluid entering the supply line and the drilling fluid returned through the drilling string; and   determining, based on the hydrostatic pressure difference, a desired setpoint drilling fluid pressure at a flow control apparatus through which the drilling fluid circulates that results in a predetermined drilling pressure at a predetermined downhole location.   
     
     
         14 . The method of  claim 13 , further comprising adjusting the flow control apparatus to maintain the desired setpoint drilling fluid pressure. 
     
     
         15 . The method of  claim 11 , further comprising circulating the drilling fluid through the annulus downhole to a drilling bit at an end of the drilling string, and from the drilling bit into the tubular drilling string. 
     
     
         16 . The method of  claim 15 , further comprising circulating the drilling fluid through a reverse circulating mud motor. 
     
     
         17 . A subsea drilling system, comprising:
 a tubular drilling string that comprises a drilling bit at a downhole end of the drilling string, the drilling string configured to extend through and in contact with a subsea environment;   a rotating control device positioned on the drilling string above a blowout preventer stack and configured to seal a portion of the drilling string against the subsea environment;   a first drilling fluid pathway comprising a first conduit that extends through the subsea environment and an annulus between the drilling string and a wellbore formed in a subterranean formation;   a second drilling fluid pathway in fluid communication with the first drilling fluid pathway through a bottom hole assembly, the second drilling fluid pathway comprising the drilling string; and   a circulation system configured to circulate a drilling fluid into and through the first drilling fluid pathway in a downhole direction and then through the second drilling fluid pathway in an uphole direction.   
     
     
         18 . The subsea drilling system of  claim 17 , further comprising a guide sleeve positioned around the drilling string above the rotating control device. 
     
     
         19 . The subsea drilling system of  claim 17 , wherein the second drilling fluid pathway further comprises a second conduit that extends from at or near the rotating control device through the subsea environment. 
     
     
         20 . The subsea drilling system of  claim 17 , further comprising a flow control system configured to control a pressure of the drilling fluid.