Inner drilling riser tie-back internal connector

1. A system for coupling a platform to a subsea wellhead comprising:

a riser extending between the platform and the subsea wellhead, wherein the riser comprises an inner riser and an outer riser; and

an inner drilling riser tie-back connector (ITBC) coupled to the inner riser, wherein the ITBC comprises:

an outer body;

an inner body at least partially disposed within the outer body, wherein the inner body is translatable with respect to the outer body along a longitudinal axis of the ITBC between a first unlocked position and a second locked position; and

a locking mechanism comprising a first threaded ring disposed about an outer circumference of the inner body and a second threaded ring disposed along an inner circumference of the outer body, wherein the second threaded ring comprises a series of teeth extending along an outer circumference of the second threaded ring and the outer body comprises a series of corresponding and complementary alternating teeth extending along an inner circumference of the outer body.

2. The system of claim 1 , wherein in the first unlocked position the first threaded ring and second threaded ring are not engaged.

3. The system of claim 1 , wherein in the second locked position the first threaded ring and the second threaded ring are engaged and the series of teeth extending along the outer circumference of the second threaded ring and the corresponding and complementary teeth extending along the inner circumference of the outer body are engaged.

4. The system of claim 1 , wherein the first threaded ring comprises a ring that is separate from the inner body, wherein the first threaded ring comprises a series of teeth extending along an inner circumference thereof, and wherein the inner body comprises a series of corresponding and complementary teeth extending along an outer circumference thereof.

5. The system of claim 4 , wherein the first threaded ring further comprises one or more longitudinal protrusions extending in an axial direction from a ring-shaped body of the first threaded ring, the one or more longitudinal protrusions being received into corresponding slots formed into the outer circumference of the inner body.

6. The system of claim 1 , wherein the first threaded ring is integrally formed with the inner body.

7. The system of claim 1 , wherein an outer circumference of the first threaded ring has a frustoconical shape that slopes in a radially inward direction from an upper end to a lower end of the first threaded ring.

8. The system of claim 1 , wherein the second threaded ring further comprises a fixed pivot at a lower end thereof and one or more grooves extending from an upper end thereof opposite the lower end.

9. The system of claim 1 , further comprising:

a lower sub coupled to a lower end of the outer body; and

a seal element coupled to the inner body for establishing at least a partially fluid tight seal between the inner body and the lower sub.

10. A method of coupling a subsea wellhead to a platform, comprising:

coupling a first terminal end of a riser to the platform and a second terminal end of the riser to the subsea wellhead, the riser comprising an inner riser and an outer riser;

coupling the second terminal end of the inner riser to an inner drilling riser tie-back connector (ITBC) having an outer body and an inner body at least partially disposed within the outer body;

landing and locking the outer body of the ITBC within the subsea wellhead; and

applying a downward weight to the inner body to at least partially actuate a locking mechanism of the ITBC, wherein applying the downward weight to the inner body translates the inner body with respect to the outer body along a longitudinal axis of the ITBC from a first unlocked position to a second locked position.

11. The method of claim 10 , wherein the locking mechanism comprises a first threaded ring disposed about an outer circumference of the inner body and a second threaded ring disposed along an inner circumference of the outer body, wherein the second threaded ring comprises a series of teeth extending along an outer circumference of the second threaded ring and the outer body comprises a series of corresponding and complementary teeth extending along an inner circumference of the outer body.

12. The method of claim 11 , wherein translating the first threaded ring from a first unlocked position to a second locked position ratchets the first threaded ring over the second threaded ring and flexes the second threaded ring in a radially outward direction so the teeth on the second threaded ring engage with the corresponding and complementary teeth on the inner circumference of the outer body.

13. The method of claim 12 , further comprising maintaining a lower end of the second threaded ring in a fixed position against the outer body of the ITBC.

14. The method of claim 11 , further comprising:

after applying the downward weight to the inner body, rotating the inner body relative to the outer body to fully lock the ITBC to the wellhead via the locking mechanism.

15. The method of claim 14 , wherein rotating the inner body relative to the outer body causes the first threaded ring to move farther down along the second threaded ring via engagement of threads on the first and second threaded rings.

16. The method of claim 14 , wherein rotating the inner body relative to the outer body activates a seal between the inner body and a lower sub coupled to a lower end of the outer body.

17. The method of claim 11 , wherein the first threaded ring is integrally formed with the inner body.

18. The method of claim 11 , wherein the first threaded ring comprises a ring that is separate from the inner body and attached to the outer circumference of the inner body via engagement of teeth.

19. The method of claim 18 , further comprising maintaining the first threaded ring in a consistent orientation with respect to the inner body via one or more longitudinal protrusions extending in an axial direction from the first threaded ring that are received into corresponding slots formed into the outer circumference of the inner body.

20. The method of claim 10 , further comprising disengaging the ITBC from the subsea wellhead, wherein disengaging the ITBC from the subsea well head comprises rotating the inner riser to disengage the locking mechanism.