Wellhead seal assembly lockdown system

1. A seal assembly for sealing a casing annulus in a subsea wellhead having a radially internal recess therein and a casing hanger positioned within the wellhead including a radially external recess therein, the seal assembly comprising:

a seal body supporting one or more radially internal seals and one or more radially external seals;

an actuation sleeve axially movable with respect to the subsea wellhead in response to a running tool;

a first locking member radially movable by the actuation sleeve to move into the internal recess in the wellhead to axially connect the seal body and the wellhead;

a second locking member radially movable by the actuation sleeve to move into the external recess in the casing hanger to axially connect the seal body and the casing hanger;

upper and lower stops supported on the seal body for engaging each of the first locking member and the second locking member to limit radial movement of the first and second locking members; and

wherein the first locking member is a C-shaped locking ring supported on the seal body and radially movable outward to fit within the internal recess in the wellhead.

2. The seal assembly as defined in claim 1 , wherein the second locking member is a C-shaped locking ring supported on the seal body and radially movable inward to fit within the external recess in the casing hanger.

3. The seal assembly as defined in claim 1 , further comprising:

the upper stop and the lower stop engage respective upper and lower protrusions on the first locking member and the second locking member to limit radial movement of the first and second locking members.

4. The seal assembly as defined in claim 1 , wherein the seal body comprises:

a radially inner seal body forming the upper first and second stops each for engaging a respective one of the first and second locking members; and

a radially outer seal body supported on the inner seal body and forming the lower first and second stops each for engaging a respective one of the first and second locking members.

5. The seal assembly as defined in claim 4 , wherein the radially inner seal body is threadably connected to the radially outer seal body.

6. The seal assembly as defined in claim 1 , wherein the upper and lower stops of the seal body are each substantially ring-shaped stops secured to the seal body.

7. The seal assembly as defined in claim 6 , wherein each upper and lower stop engages a substantially C-shaped protrusion on a respective one of the first and second locking members.

8. A seal assembly run in a well on a running tool for sealing a casing annulus in a subsea wellhead with a casing hanger positioned within the wellhead and having an external recess therein, the wellhead having a radially internal recess therein, the seal assembly comprising:

a seal body supporting one or more radially internal seals and one or more radially external seals;

an actuation sleeve axially movable with respect to the subsea wellhead in response to the running tool;

a first locking ring supported on the seal body and radially movable outward in response to movement of the actuation sleeve to move at least a portion of the first locking ring into the internal recess in the wellhead to axially connect the seal body and the wellhead;

a second locking ring supported on the seal body and radially movable inward and in response to movement of the actuation sleeve to move at least a portion of the second locking ring into the external recess in the casing hanger to axially connect the seal body and the casing hanger;

an upper stop and a lower stop supported on the seal body for engaging each of the first and second locking rings to limit radial movement of each of the first and second locking rings, and

wherein the first locking ring is a C-shaped locking ring supported on the seal body and radially movable outward to fit within the internal recess in the wellhead.

9. The seal assembly and running tool as defined in claim 8 , wherein the upper and lower stops on the seal body engage upper and lower protrusions on each of the first and second locking rings.

10. The seal assembly and running tool as defined in claim 8 , wherein the running tool moves the actuation sleeve which activates both the first locking ring and the second locking ring.

11. The seal assembly and running tool as defined in claim 10 , wherein the running tool includes a piston movable in response to fluid pressure in the running tool.

12. The seal assembly and running tool as defined in claim 8 , wherein the running tool is released from the seal assembly by rotation of the running tool and shearing of one or more shear members.

13. The seal assembly and running tool as defined in claim 8 , wherein the seal body comprises:

a radially inner seal body forming the upper first and second stops each for engaging a respective one of the first and second locking members; and

a radially outer seal body supported on the inner seal body and forming the lower first and second stops each for engaging a respective one of the first and second locking members.

14. The seal assembly and running tool as defined in claim 13 , wherein the radially inner seal body is threadably connected to the radially outer seal body.

15. The seal assembly and running tool as defined in claim 8 , wherein the upper and lower stops of the seal body are each substantially ring-shaped stops secured to the seal body.

16. The seal assembly and running tool as defined in claim 15 , wherein each upper and lower stop engages a substantially C-shaped protrusion on a respective one of the first and second locking members.

17. A method of sealing a casing hanger having a radially external recess to a wellhead having a radially internal recess, comprising:

providing a seal body having one or more seals;

radially moving a first locking member to move into the internal recess in the wellhead to axially connect the seal body and the wellhead;

radially moving a second locking member to move into the external recess in the casing hanger to connect the seal body and the casing hanger, such that the seal body seals the casing annulus while being interconnected with the wellhead and the casing hanger;

limiting radial movement of each of the first and second locking members by upper and lower stops supported on the seal body; and

wherein the first locking member is a C-shaped locking ring supported on the seal body and radially moveable outward to fit within the internal recess in the wellhead.

18. The method as defined in claim 17 , wherein each of the first and second locking members is activated during downward movement of an actuation sleeve.

19. The method as defined in claim 17 , wherein each of the upper and lower stops is formed to have a substantially ring-shaped configuration.