Offshore well drilling system with nested drilling risers

1. An offshore well system for supporting a tubular string positioned within a riser system including an internal riser and an external riser at a subsea well, the system comprising:

an internal riser tension device configured to apply tension to the internal riser; and

an external riser tension device configured to apply tension to the external riser such that the internal riser is able to extend from an upper end of the external riser when installed, wherein the external riser tension device is configured to be supported independent of the internal riser tension device and independent of a platform of the offshore well system.

2. The system of claim 1 , further comprising:

the platform;

the external riser extending from the subsea well;

the internal riser nested within the external riser and extending from the subsea well to the platform at a sea surface; and

the external riser tension device configured to apply tension to the external riser such that the external riser is a freestanding riser supported independent of the platform.

3. The system of claim 2 , further comprising well pressure control equipment located on the platform and connected with the internal riser, the well pressure control equipment being the only well pressure control equipment for the well.

4. The system of claim 2 , wherein the internal riser is free to move within the external riser.

5. The system of claim 1 , wherein the internal riser tension device is configured to apply tension to the internal riser such that the internal riser is supported at least partially independent of the external riser tension device.

6. The system of claim 2 , wherein the internal riser is configured to connect with the external riser such that the external riser is capable of applying tension to the internal riser.

7. The system of claim 2 , wherein the internal riser tension device is configured to place the internal riser in tension dynamically.

8. The system of claim 2 , wherein the external riser tension device comprises a buoyancy device.

9. The system of claim 8 , wherein the buoyancy device comprises at least one of an air can, balloon, and foam.

10. A method for drilling a subsea well from a platform with a riser system including an internal riser and an external riser, the method comprising:

tensioning the internal riser with an internal riser tension device;

tensioning the external riser with an external riser tension device supported independent of the internal riser tension device and independent of the platform, such that only a portion of the internal riser is nested within the external riser; and

drilling the subsea well with a tubular string positioned within the internal riser and the external riser under tension.

11. The method of claim 10 , wherein the tensioning the external riser comprises providing a buoyancy device on the external riser.

12. The method of claim 11 , wherein the buoyancy device comprises at least one of an air can, balloon, and foam.

13. The method of claim 10 , wherein the tensioning the internal riser with the internal riser tension device is at least partially independent of the external riser tension device.

14. The method of claim 10 , wherein the tensioning the internal riser comprises dynamically tensioning the internal riser with the internal riser tension device.

15. The method of claim 10 further comprising connecting the external riser to the internal riser such that the external riser applies tension to the internal riser.

16. The method of claim 10 , further comprising connecting well pressure control equipment located on the platform with the internal riser, the well pressure control equipment being the only well pressure control equipment for the well.

17. The method of claim 10 , further comprising moving the internal riser within the external riser.

18. An offshore well system for supporting a tubular string positioned within a riser system including an internal riser and an external riser at a subsea well, the system comprising:

an internal riser tension device configured to apply tension to the internal riser and;

an external riser tension device configured to apply tension to the external riser such that only a portion of the internal riser is nested within the external riser when installed, wherein the external riser tension device is configured to be supported independent of the internal riser tension device and independent of a platform of the offshore well system.

19. The system of claim 18 , further comprising the external riser and the internal riser nested within the external riser, wherein the external riser tension device is configured to apply tension to the external riser such that the external riser is a freestanding riser supported independent of the platform.