Method of and apparatus for drilling a subterranean wellbore

1. A method of drilling a subterranean well bore comprising, monitoring the well bore for influx of formation fluid into the well bore, and, on detection of an influx:

a) stopping any pump pumping fluid into the well bore;

b) operating a first blow out preventer so that it closes within a first period of time;

c) operating a second blow out preventer so that it closes within a second period of time, the second blow out preventer being located below the first blow out preventer, the second period of time being longer than the first period of time; and

d) circulating the influx out of the well bore via a flow line extending from below the second blow out preventer.

2. The method according to claim 1 wherein the second blow out preventer is capable of containing fluid at a higher pressure than the first blow out preventer.

3. The method according to claim 1 wherein there is a flow spool provided between the first blow out preventer and second blow out preventer with a fluid flow line hereinafter referred to as a pressure relief line extending from the flow spool, the pressure relief line containing a pressure relief valve which is normally closed so as to substantially prevent flow of fluid along the pressure relief line, and the method further includes the step of, after the closing of the first blow out preventer, opening the pressure relief valve to allow flow of fluid along the pressure relief line if the fluid pressure in the flow spool exceeds a predetermined level.

4. The method according to claim 3 wherein the flow spool is provided with a second fluid flow line hereinafter referred to as a second pressure relief line extending from the flow spool, the second pressure relief line containing a second pressure relief valve which is normally closed so as to substantially prevent flow of fluid along the second pressure relief line, and the method further includes the step of, after the closing of the first blow out preventer, opening the second pressure relief valve to allow flow of fluid along the second pressure relief line if the fluid pressure in the flow spool exceeds a second predetermined level.

5. The method according to claim 3 wherein the pressure relief line extends from the flow spool to a mud gas separator.

6. The method according to claim 4 wherein the second pressure relief line extends from the flow spool to an overboard diversion point.

7. An apparatus for use in drilling a subterranean well bore comprising:

a first blow out preventer;

a second blow out preventer, the second blow out preventer being located below the first blow out preventer and secured to a well head, wherein the first blow out preventer is operable to close within a first period of time, and the second blow out preventer is operable to close within a second period of time, the second period of time being longer than the first period of time;

a flow spool provided between the first blow out preventer and second blow out preventer with a fluid flow line hereinafter referred to as a pressure relief line extending from the flow spool, the pressure relief line containing a pressure relief valve which is normally closed so as to substantially prevent flow of fluid along the pressure relief line and which opens to allow flow of fluid along the pressure relief line if the fluid pressure in the flow spool exceeds a predetermined level; and

a flow line extending from below the second blow out preventer, the flow line being configured to circulate the influx out of the well bore.

8. The apparatus according to claim 7 wherein the second blow out preventer is capable of containing fluid at a higher pressure than the first blow out preventer.

9. The apparatus according to claim 7 wherein the flow spool is provided with a second fluid flow line hereinafter referred to as a second pressure relief line extending from the flow spool, the second pressure relief line containing a second pressure relief valve which is normally closed so as to substantially prevent flow of fluid along the second pressure relief line and which opens to allow flow of fluid along the second pressure relief line if the fluid pressure in the flow spool exceeds a second predetermined level.

10. The apparatus according to claim 7 wherein the pressure relief line extends from the flow spool to a mud gas separator.

11. The apparatus according to claim 9 wherein the second pressure relief line extends from the flow spool to an overboard diversion point.

12. The apparatus according to claim 7 wherein first blow out preventer is mounted on a riser which extends upwardly from the second blow out preventer.

13. The apparatus according to claim 12 wherein the flow spool is mounted on an upper end of the riser between the blowout preventer and the riser.

14. The apparatus according to claim 12 wherein the riser includes a slip joint, and the first blowout preventer is mounted in the riser below the slip joint.

15. The apparatus according to claim 7 wherein the second blow out preventer is mounted on a riser which extends upwardly from the well head.

16. A method of drilling a subterranean well bore comprising, monitoring the well bore for influx of formation fluid into the well bore, and, on detection of an influx:

a) stopping any pump pumping fluid into the well bore;

b) operating a first blow out preventer so that it closes within a first period of time;

c) opening a pressure relief valve to allow a flow of fluid along the pressure relief line if a fluid pressure in a flow spool exceeds a predetermined level, the flow spool being provided between the first blow out preventer and second blow out preventer with the pressure relief line extending from the flow spool, the pressure relief line containing the pressure relief valve normally being closed so as to substantially prevent the flow of fluid along the pressure relief line;

d) operating a second blow out preventer so that it closes within a second period of time, the second blow out preventer being located below the first blow out preventer and being secured to a well head, the second period of time being longer than the first period of time; and

e) circulating the influx out of the well bore via a flow line extending from below the second blow out preventer.