Buoyant system for installing a casing string

1. A sealing device, comprising:

a tubular body having a bore;

a collet seat having a plurality of collets releasably coupled to the tubular body;

a frangible sealing element disposed in the collet seat and blocking fluid communication through the bore, the collet seat longitudinally supporting the frangible sealing element in the tubular body; and

a releasable sleeve releasably attached to the tubular body and retaining the collet seat against the tubular body, wherein the collet seat is longitudinally movable relative to the tubular body upon release from the tubular body, wherein the collet seat is in a first longitudinal position when retained by the sleeve, and wherein the collet seat, when released from the sleeve, is movable to a second longitudinal position while supporting the sealing element.

2. The device of claim 1 , wherein the frangible sealing element breaks when the collet seat reaches the second position.

3. The device of claim 2 , wherein the collet seat contacts a shoulder in the tubular body when the collet seat reaches the second position.

4. The device of claim 1 , further comprising an annular chamber formed between the sleeve and the tubular body.

5. The device of claim 4 , further comprising a port for fluid communication between the annular chamber and an exterior of the tubular body.

6. The device of claim 1 , wherein the frangible sealing element comprises a frangible material selected from the group consisting of ceramics, metals, glass, porcelains, carbides, and combinations thereof.

7. The device of claim 1 , wherein the frangible sealing element comprises a dome.

8. The device of claim 1 , wherein each collet includes a collet head engaged with a groove formed in the tubular body.

9. The device of claim 8 , wherein the releasable sleeve, in a first position, prevents the collet head from disengaging from the groove, and, in a second position, allows the collet head to disengage from the groove.

10. The device of claim 9 , wherein the second position of the sleeve is located higher than the first position of the sleeve relative to the groove.

11. The sealing device of claim 1 , wherein the frangible sealing element is longitudinally movable with the collet seat.

12. The sealing device of claim 1 , wherein the releasable sleeve is configured to move in an opposite longitudinal direction as the collet seat to release the collet seat.

13. The sealing device of claim 1 , wherein the collet seat includes a collet head engaged with a groove formed in the tubular body, and wherein the releasable sleeve prevents the collet head from disengaging from the groove.

14. A tubular assembly disposed in a wellbore, comprising:

a tubular string having a bore;

a sealing device of claim 1 disposed in the tubular string and blocking fluid flow through the bore of the sealing device;

a valve assembly disposed in the tubular string and downstream from the sealing device, the valve assembly blocking fluid flow through the bore of the tubular string;

a buoyant chamber formed between the sealing device and the valve assembly, the buoyant chamber including a fluid having a lower specific gravity than a fluid in the wellbore.

15. The tubular assembly of claim 14 , wherein the frangible sealing element of claim 1 comprises a dome, and a convex surface of the dome is oriented upward, and the concave surface of the dome is oriented downward toward the valve assembly.

16. The tubular assembly of clam 14 , further comprising a circulation tool having an injection tube with a tapered bore.

17. A tubular assembly disposed in a wellbore, comprising:

a tubular string having a bore;

a lower sealing device disposed in the tubular string and blocking fluid flow through the bore of the tubular string;

an upper sealing device disposed in the tubular string and located upstream from the lower sealing device, the upper sealing device blocking fluid flow through the bore of the tubular string, wherein the upper sealing device includes:

a tubular body having a bore;

a frangible sealing element disposed in the tubular body and blocking fluid communication through the bore of the tubular body, the frangible sealing element includes a dome having a concave surface oriented toward the lower sealing device; and

a collet seat longitudinally movable relative to the tubular body, wherein the frangible sealing element is longitudinally supported by the collet seat in the tubular body and is longitudinally movable with the collet seat;

a releasable sleeve releasably attached to the tubular body and retaining the collet seat against the tubular body, wherein the collet seat is longitudinally movable relative to the tubular body upon release from the tubular body, wherein the collet seat is in a first longitudinal position when retained by the sleeve, and wherein the collet seat, when released from the sleeve, is movable to a second longitudinal position while supporting the sealing element; and

a buoyant chamber formed between the lower sealing device and a upper sealing device, the buoyant chamber including a fluid having a lower specific gravity than a fluid in the wellbore.

18. The tubular assembly of claim 17 , wherein the collet seat is movable into contact with a portion of the tubular body to cause the frangible sealing element to shatter.

19. The tubular assembly of claim 17 , wherein the lower sealing device includes:

a tubular body having a bore;

a plug housing coupled to the tubular body;

a plug releasably attached to the plug housing and blocking fluid flow through the plug housing.

20. The tubular assembly of claim 17 , wherein the lower sealing device includes:

a tubular body having a bore; and

a frangible sealing element disposed in the tubular body and blocking fluid communication through the bore of the tubular body, the frangible sealing element includes a dome having a concave surface oriented toward the upper sealing device.

21. The tubular assembly of claim 17 , further comprising a valve assembly disposed between the upper sealing device and the lower sealing device.