Arrangement and method for controlling and regulating bottom hole pressure when drilling deepwater offshore wells

1. A method for circulating out a formation influx from a subterranean formation below a subsea blowout preventer, comprising:

letting gas associated with a formation influx into a well escape from below a subsea blowout preventer to a riser connected to the subsea blowout preventer; adjusting the level of a liquid/gas interface between a liquid in a lower part of the riser and gas at atmospheric pressure in an upper part of the riser to maintain a bottom hole pressure between formation fracture pressure and pore pressure;

allowing the gas associated with the influx to collect in the upper part of the riser above an outlet in the riser, said outlet being substantially below seawater level and above seabed;

pumping liquid out the outlet by way of a subsea pump connected by a flow return line to the outlet; and

venting the gas collected in the upper part of the riser at atmospheric pressure at the upper end of the riser.

2. The method according to claim 1 , further comprising:

a seabed BOP at the lower end of the drilling riser, the subsea BOP having at least one drill string shear ram and one pipe ram and containing at least one by-pass line with the ability to bypass at least said shear and pipe rams in the subsea BOP when said rams are closed, the by-pass line containing at least one bypass shutoff valve; and

if a variation in bottom hole pressure in the well created by the gas associated with a formation influx into the well exceeds the available equivalent adjustment of the level of the liquid/gas interface in the riser above the riser outlet; then

closing at least one said ram in the subsea BOP and adjusting the level of the liquid/gas interface in the riser whereby pressure above said closed ram is equalized to the pressure below said ram; and

providing fluid communication between the riser and the well below said closed ram by opening the bypass shutoff valve in the bypass line.

3. The method according to claim 1 , wherein the flow return line between the outlet and the subsea pump is adapted to prevent free gas from entering the subsea pump by having a U-shaped loop acting as a gas-lock.

4. The method of claim 1 , further comprising:

filling the drilling riser with a gas or liquid through a filling line coupled to the drilling riser substantially below sea level and above the riser outlet.

5. The method according to claim 2 , wherein a gas escape line is connected to the riser below a near surface BOP at the top of the riser.

6. The method of claim 2 , further comprising:

pumping and circulating drilling fluid down through a drill pipe and drill bit extending through the riser into the well and up an annulus around the drill pipe.

7. The method of claim 3 , further comprising:

filling the drilling riser with a gas or liquid through a filling line coupled to the drilling riser substantially below sea level and above the riser outlet.

8. The method of claim 3 , further comprising:

altering the density of the drilling fluid thereby altering the pressure gradient.

9. A method for circulating out a formation influx from a subterranean formation below a subsea blowout preventer, comprising:

letting a gas associated with a formation influx escape from below a subsea blowout preventer to a riser connected to the subsea blowout preventer;

adjusting the level of a liquid/gas interface between liquid in the lower part of the riser and gas in the upper part of the riser so as to maintain a bottom hole pressure between fracture and pore pressure of the formation;

sucking out gas in the upper part of the riser to create a pressure below atmospheric pressure;

allowing the gas associated with the influx to collect in the upper part of the riser above an outlet in the riser, said outlet being substantially below seawater level and above seabed;

pumping liquid out the outlet by way of a subsea pump connected by flow return line to the outlet; and

sucking out the gas collected in the upper part of the riser at a pressure below atmospheric pressure through a gas escape line at the upper end of the riser.

10. The method of claim 9 , further comprising:

filling the drilling riser with a gas or liquid through a filling line coupled to the drilling riser substantially below sea level and above the riser outlet.

11. A method for circulating out a formation influx from a subterranean formation into a wellbore, comprising:

adjusting the level of a liquid/gas interface between a liquid in the lower part of the riser and gas at atmospheric pressure in the upper part of the riser so as to maintain a bottom hole pressure close to the formation pressure;

allowing gas associated with a formation influx to collect in the upper part of the riser above an outlet in the riser, said outlet being substantially below seawater level and above seabed;

pumping liquid out the outlet by way of a subsea pump connected by a flow return line to the outlet and back to surface; and

venting the gas collected in the upper part of the riser to the surrounding at atmospheric pressure.

12. The method of claim 11 , further comprising:

filling the drilling riser with a gas or liquid through a filling line coupled to the drilling riser substantially below sea level and above the riser outlet.

13. The method according to claim 11 , wherein the flow return line between the outlet and the subsea pump is adapted to prevent free gas from entering the subsea pump by having a U-shaped loop acting as a gas-lock.

14. The method according to claim 11 , further comprising:

a seabed BOP at the lower end of the drilling riser, the subsea BOP having at least one drill string shear ram and one pipe ram and containing at least one by-pass line with the ability to bypass at least said shear and pipe rams in the subsea BOP when said rams are closed, the by-pass line containing at least one bypass shutoff valve; and

if a variation in bottom hole pressure in the well created by the gas associated with a formation influx into the well exceeds the available equivalent adjustment of fluid level in the riser above the riser outlet; then

closing at least one said ram in the subsea BOP and adjusting the drilling fluid level in the riser whereby pressure above said closed ram is equalized to the pressure below said ram; and

providing fluid communication between the riser and the well below said closed ram by opening the bypass shutoff valve in the bypass line.

15. The method according to claim 14 , wherein a gas escape line is connected to the riser below a near surface BOP at the top of the riser.

16. A method for circulating out a formation influx entering an annulus bore in a well during subsea drilling resulting from drilling activities, comprising:

maintaining pressure at the top of a drilling riser extending from a subsea wellhead on a subsea well to the surface, at or below atmospheric pressure

operating a drilling fluid return pump and flow return line connected to the drilling riser by a riser outlet located above seafloor level so as to maintain a drilling fluid level in the drilling riser between the riser outlet and the surface and a bottom hole pressure between formation fracture pressure and pore pressure;

adjusting the level of a liquid/gas interface inside the riser in response to variation of bottom hole pressure in the well created by gas associated with a formation influx and thereby maintaining a constant bottom hole pressure;

allowing the gas from the influx to collect in the riser; and

removing the gas from the riser by other than the drilling fluid return pump.

17. The method of claim 16 , further comprising:

pumping and circulating drilling fluid down through a drill pipe and drill bit extending into the well and up an annulus around the drill pipe.

18. The method of claim 16 , comprising:

the flow return line between the riser outlet and the drilling fluid return pump having a U-shaped loop acting as a gas-lock thereby preventing free gas from entering the return pump.

19. The method of claim 16 , comprising:

said removing the gas from the riser comprising using a gas escape line connected to the drilling riser below a near surface BOP at the top of the drilling riser.

20. The method of claim 16 , said allowing the gas from the influx to collect in the riser comprising letting the gas escape from below a subsea blowout preventer into the drilling a riser.

21. The method of claim 16 , said maintaining pressure at the top of a drilling riser at or below atmosphere pressure comprising the top of the riser being vented or left open to ambient atmospheric pressure.

22. The method of claim 16 , further comprising:

preventing free gas from entering the fluid return pump by means of a U-shaped loop in the fluid return line acting as a gas-lock.

23. The method of claim 16 , further comprising:

the drilling riser connecting a floating drilling unit to a subsea wellhead and containing a drillstring, a near surface BOP at the upper end of the drilling riser, the near surface BOP having a gas bleeding outlet from below the BOP connected to a pressure regulating valve manifold, and a seabed BOP at the lower end of the drilling riser, the subsea BOP having at least one drill string shear ram and one pipe ram and containing at least one by-pass line with the ability to bypass at least said shear and pipe rams in the subsea BOP when said rams are closed, the by-pass line containing at least one bypass shutoff valve, the drilling fluid return pump and flow return line connecting the riser outlet to the floating drilling unit; and

if a variation in bottom hole pressure in the well created by the gas associated with a formation influx into the well exceeds the available equivalent adjustment of fluid level in the riser above the riser outlet; then

closing at least one said ram in the subsea BOP and adjusting the drilling fluid level in the riser whereby pressure above said closed ram is equalized to the pressure below said ram; and

providing fluid communication between the riser and the well below said closed ram by opening the bypass shutoff valve in the bypass line.

24. The method of claim 16 , further comprising a valve in the fluid return line, and a particle collection box in the fluid return line, the valve being adapted for opening and closing the communication between the particle collection box and the fluid return line.

25. The method of claim 17 , further comprising:

altering the density of the drilling fluid thereby altering the pressure gradient.

26. The method of claim 23 , further comprising:

pumping and circulating drilling fluid down through a drill pipe and drill bit extending into the well and up an annulus around the drill pipe.

27. The method of claim 23 , further comprising:

said removing the gas from the riser comprising closing the near surface BOP and passing the gas through the gas bleeding outlet, said gas bleeding outlet being connected to a choke line in communication with a high pressure choke and stand pipe manifold on the floating drilling unit.

28. The method of claim 23 , further comprising:

filling the drilling riser with a gas or liquid through a filling line coupled to the drilling riser substantially below sea level and above the riser outlet.

29. The method of claim 24 ,wherein the particle collection box is hanging underneath the drilling fluid return pump and the particle collection box has a re-circulation and jetting means for breaking down particle size to prevent particle build up.

30. The method of claim 28 , wherein the gas or liquid is an inert gas for displacement of the air above the drilling fluid.