Subsea wellbore drilling system for reducing bottom hole pressure

A wellbore system for performing subsea downhole wellbore operations at an offshore location and for controlling pressure at the bottom of the wellbore, comprising:

• (a) a tubing receiving fluid under pressure adjacent an upper end thereof;
• (b) a bottomhole assembly adjacent a lower end of the tubing;
• (c) a subsea wellhead assembly at top of the wellbore receiving the tubing and the bottomhole assembly, said wellhead assembly adapted to receive said fluid after it has passed down through said tubing and back up through an annulus between the tubing and the wellbore;
• (d) a fluid return line extending up from the wellhead assembly to the sea level for conveying return fluid from the wellhead to the sea level, with the tubing, annulus, wellhead and return line constituting a subsea fluid circulation system;
• (e) a pump in the return line for controlling the pressure at the bottom of the wellbore at predetermined values during downhole operations and to move the return fluid to the surface;
• and
• (f) a control circuit for controlling the pump to control the pressure at the bottom of the subsea wellbore at the predetermined values during downhole operations.

A drilling system for drilling a wellbore at an offshore location comprising:

• (a) tubing receiving drilling fluid under pressure adjacent the upper end thereof;
• (b) a bottomhole assembly adjacent the lower end of the tubing;
• (c) a subsea wellhead assembly at the top of the wellbore receiving the tubing and the bottomhole assembly, said wellhead assembly adapted to receive said fluid after it has passed through said tubing and through the annulus between the tubing and the wellbore;
• (d) a fluid return line separate and spaced apart from the tubing extending up from the wellhead assembly to the sea level for conveying said fluid from the wellhead to the sea level, with the tubing, annulus, wellhead and return line constituting a fluid circulation system;
• (e) a source of flowable material having a density lower than the density of the return fluid;
• (f) an injector in fluid communication with the fluid circulation system for injecting said flowable material into the return fluid to maintain the bottomhole pressure at predetermined values during downhole operations in the wellbore to overcome at least a portion of the hydrostatic pressure and friction loss pressures in the return fluid;
• and
• (g) at least two flow control devices in the fluid circulation system, one device in the tubing and the other in fluid communication with the return fluid to block flow of fluid when the injector is not in operation.

A wellbore system for performing downhole subsea operations in a wellbore at an offshore location, comprising:

• (a) tubing receiving fluid under pressure adjacent the upper end thereof;
• (b) a bottom hole assembly adjacent the lower end of the tubing;
• (c) a subsea wellhead assembly at the top of the wellbore receiving the tubing and the bottom hole assembly, said wellhead assembly adapted to receive said fluid after it has passed down through said tubing and back up through the annulus between the tubing and the wellbore;
• (d) a fluid return line separate and spaced apart from the tubing extending up from the wellhead assembly to the sea level for conveying return fluid from the wellhead to the sea level, with the tubing, annulus, wellhead and return line constituting a subsea fluid circulation system;
• (e) an adjustable fluid lift in fluid communication with the subsea fluid circulation system for regulating the fluid pressure at predetermined values during downhole operations in the wellbore by overcoming at least a portion of the hydrostatic pressure and friction loss pressures of the return fluid;
• and
• (f) a fluid surge vessel extending up from adjacent the wellhead to the surface and in fluid communication with return fluid from the annulus, said vessel holding a lower column of return fluid and an upper column of water with the height of the column of return fluid indicative of the differential pressure of the return fluid and the sea water adjacent the wellhead.

A method of controlling the pressure in a subsea wellbore during drilling of the wellbore by a drilling system having a tubing, a bottomhole assembly carried by the tubing adjacent a lower end thereof, a subsea wellhead assembly at the top of the wellbore receiving the tubing and the bottomhole assembly, and a fluid return line extending from the wellhead assembly to the surface, wherein during drilling of the wellbore the bottomhole assembly is positioned in the wellbore below the wellhead assembly and a drilling fluid is supplied under pressure to the bottomhole assembly through the tubing, which drilling fluid returns to the wellhead assembly via an annulus between the tubing and the wellbore and then to the surface through the fluid return line, the tubing, annulus, wellhead assembly and the fluid return line constituting a closed-loop fluid circulation system during drilling of the wellbore, wherein the improvement comprising:

• (a) providing a pump in the return line for pumping drilling fluid to the surface and for controlling pressure at the bottom of the wellbore at a desired pressure during drilling of said wellbore;
• (b) determining bottomhole pressure during drilling of the wellbore;
• and
• (c) providing a control circuit that controls the speed of the pump in response to the determined bottomhole pressure to control the bottomhole pressure at the desired pressure.

A method of controlling bottomhole pressure during drilling of a subsea wellbore wherein a drilling fluid is returned to the surface via a separate return line, said method comprising:

• (a) selecting a desired bottomhole pressure;
• (b) determining the bottomhole pressure during drilling of the subsea wellbore;
• and
• (c) controlling a pump in the return line in response to the determined pressure to control the bottomhole pressure at the desired pressure by changing the speed of the pump.