Method and apparatus for well tubular flotation

1. A wellbore tubular flotation device, comprising:

a housing having a locking element disposed thereon, the housing shaped to move through an interior of a wellbore tubular segment, the locking element shaped to engage the interior of the wellbore tubular segment, the locking element comprising a locking mechanism configured to urge the locking element into contact with the interior of the wellbore tubular by moving the locking element radially outwardly;

a burst disk engaged with the housing and shaped to close the tubular segment to fluid flow; and

a release mechanism configured to reverse the urging of the locking mechanism by releasing the locking element from contact with the wellbore tubular segment by radially retracting when a release tool is moved through the housing.

2. The device of claim 1 wherein the locking mechanism comprises at least one piston, the release mechanism comprising a sleeve slidably engaged with an interior of the housing, the housing having hydraulic pressure passages therein connected to the piston, a bore of the sleeve defining a volume exposed to the hydraulic pressure passages when the sleeve is moved by the release tool.

3. The device of claim 2 wherein the release tool comprises a pumpable plug having a smaller diameter nose than a body of the plug, and wherein the sleeve comprises an internal ring having a diameter larger than a diameter of the nose, the diameter of the internal ring smaller than a diameter of the pumpable plug body.

4. The device of claim 3 wherein the plug comprises a cement wiper plug.

5. The device of claim 1 wherein the housing comprises wiper seals engageable with the interior of the tubular element.

6. The device of claim 1 wherein the locking mechanism comprises at least one cam pivotally coupled to the housing so as to urge the locking element into contact with the interior of the wellbore tubular segment when the at least one cam is rotated in a first direction and to reverse movement of the locking element from the interior of the wellbore tubular segment when rotated opposite to the first direction.

7. The device of claim 6 wherein the release tool comprises a pumpable plug having a smaller diameter nose than a body of the plug, and wherein the sleeve comprises an internal ring having a diameter larger than a diameter of the nose, the diameter of the internal ring smaller than a diameter of the pumpable plug body.

8. The device of claim 7 wherein the plug comprises a cement wiper plug.

9. A method for moving a tubular string into a wellbore comprising:

locking a flotation device comprising a housing and a burst disk in the tubular string, wherein the locking the flotation device comprises applying hydraulic pressure to a piston engaged with a locking element;

moving the tubular string to a selected depth in the wellbore;

pumping on the tubular string to rupture the burst disk; and

releasing the flotation device from the tubular string by pumping a release tool into the flotation device.

10. The method of claim 9 wherein the releasing comprises moving a release sleeve in the flotation device to expose the hydraulic pressure to a volume to relieve the hydraulic pressure.

11. The method of claim 10 wherein the moving the pumping the release tool comprises pumping a plug into a ring in the flotation device, the ring having a diameter larger than a nose of the plug and smaller than a body of the plug, whereby pumping the plug moves the ring and consequently the release sleeve.