Isolation valve with debris control and flow tube protection

1. An isolation valve for use as part of a casing string, the isolation valve comprising:

a housing having a bore and a valve cavity;

a valve member movable between a first position in which the valve member obstructs the bore of the housing and a second position in which the valve member is disposed in the valve cavity, the valve member having a locked position in which the valve member is locked in the valve cavity;

a flow tube configured to allow movement of the valve member between the first and second positions;

a piston coupled to the flow tube;

an engagement assembly attached to the housing by a shearable member, the piston configured to move the flow tube into engagement with the engagement assembly to substantially prevent debris from entering the valve cavity when the valve member is in the second position, and configured to move the flow tube out of engagement with the engagement assembly when the valve member is in the first position, wherein the engagement assembly is movable relative to the housing upon release of the shearable member when the valve member is in the locked position; and

a compression spring disposed between the piston and the flow tube and configured to bias the flow tube into contact with the engagement assembly when the valve member is in the second position.

2. The isolation valve of claim 1 , wherein an end of the flow tube includes a shaped surface that is configured to contact a surface on the engagement assembly.

3. The isolation valve of claim 1 , wherein the engagement assembly includes a guide member and a sleeve.

4. The isolation valve of claim 3 , wherein the guide member includes a tapered portion that is configured to centralize wellbore tools moving through the bore of the housing.

5. The isolation valve of claim 1 , wherein the piston is connected to the flow tube via a releasable connection.

6. The isolation valve of claim 5 , wherein the piston is configured to move the flow tube within the housing when fluid pressure acts on the piston.

7. The isolation valve of claim 5 , wherein the valve member is configured to be locked in the second position when the connection between the piston and the flow tube is released and a connection is formed between the flow tube and the housing.

8. The isolation valve of claim 1 , further comprising a hinge arrangement having a first portion connected to the valve member and a second portion operatively connected to the housing.

9. The isolation valve of claim 8 , wherein the second portion includes a hinge mating surface in contact with the housing.

10. The isolation valve of claim 9 , wherein the hinge arrangement includes a locking connection member that is configured to prevent axial direction movement along a plane of the hinge mating surface during operation of the valve member.

11. The isolation valve of claim 1 , wherein the engagement assembly is biased into contact with the flow tube.

12. The isolation valve of claim 1 , wherein the shearable member is a shear pin.

13. A method of operating an isolation valve in a wellbore, the method comprising:

placing the isolation valve in the wellbore, the isolation valve including a housing, a valve member, a flow tube, a piston coupled to the flow tube, a compression spring disposed between the piston and the flow tube, and an engagement assembly, the engagement assembly attached to the housing by a shearable member;

moving the valve member from an opened position to a closed position by moving the flow tube via the piston in a first direction through the housing, the flow tube being disengaged from the engagement assembly;

moving the valve member from the closed position to the opened position by moving the flow tube via the piston in a second direction, wherein the compression spring biases an end of the flow tube into contact with an end of the engagement assembly; and

moving the valve member to a locked position by moving the flow tube further in the second direction which releases the shearable member and causes the engagement assembly to move in the second direction relative to the housing.

14. The method of claim 13 , further comprising biasing the flow tube against the engagement assembly using the biasing member.

15. The method of claim 13 , further comprising releasing a connection between the flow tube and the piston, which allows the flow tube to move independently from the piston.

16. The method of claim 15 , further comprising forming a connection between the flow tube and the housing to maintain a flow path through the isolation valve.

17. The method of claim 13 , wherein the shearable member is a shear pin.

18. An isolation valve comprising:

a housing having a bore;

a flapper pivotally movable between a closed position in which the bore is blocked and an open position in which the bore is open to fluid flow;

a flow tube for shifting the flapper between the open position and the closed position;

a piston coupled to the flow tube;

an engagement assembly attached to the housing by a releasable resilient connection member, the engagement assembly having an end configured to abut an end of the flow tube after the flow tube moves the flapper to the open position; and

a compression spring disposed between the piston and the flow tube and configured to bias the flow tube into contact with the engagement assembly when the flapper is in the open position wherein the flow tube is movable to a further location in the housing thereby displacing the engagement assembly and locking the flapper in the open position.

19. The isolation valve of claim 18 , wherein the releasable resilient connection member is configured to bias the engagement assembly into contact with the flow tube while the flapper is in the open position.

20. The isolation valve of claim 18 , wherein the releasable resilient connection member is configured to be released when the valve member is moved to a locked position.