Subsea wellhead providing controlled access to a casing annulus

1. A subsea wellhead assembly including:

a wellhead housing having a generally cylindrical wall configured to support an outer casing string, and further defining an axial bore, the wellhead housing being disposed to support within said bore a casing hanger for the support of an inner casing string within said outer casing string whereby to form a monitoring annulus between the inner and outer casing strings, the wellhead housing including within said wall thereof a communication passageway which is in communication with said monitoring annulus near a lower end of the wellhead housing and which extends upwardly in said wall of the wellhead housing towards an upper end of the wellhead housing; and

a valve which is accommodated near the upper end of said wellhead housing and disposed for control from a production tree to control fluid flow passage between (a) the monitoring annulus through said communication passageway within the wellhead housing wall and (b) the production tree while the production tree is in place.

2. The subsea wellhead assembly of claim 1 in which said valve is controllable from the production tree by way of an isolation sleeve.

3. The subsea wellhead assembly of claim 2 , in which said valve is moveable between open and closed positions in response to fluid pressure applicable to the valve by way of said isolation sleeve.

4. The subsea wellhead assembly of claim 3 and including passages for the application of said fluid pressure, extending within a body of said isolation sleeve.

5. The subsea wellhead assembly of claim 2 , in which said valve controls a port in communication with a passage in said isolation sleeve.

6. The subsea wellhead assembly of claim 5 , in which said valve is moveable between an open position connecting said passageway and said port and a closed position blocking communication between said passageway and said port.

7. The subsea wellhead assembly of claim 6 , and including at least one biasing spring for urging said valve to said closed position.

8. The subsea wellhead assembly of claim 5 , in which said passage in said isolation sleeve communicates with a gallery in said production tree.

9. The subsea wellhead assembly of claim 5 , in which said valve has a gallery providing communication between said communication passageway and said port.

10. The subsea wellhead assembly of claim 1 , in which said valve is a slide valve and the assembly includes a valve sleeve fitting within said bore and defining therewith a chamber in which said valve is disposed.

11. A subsea wellhead assembly including:

a wellhead housing having a generally cylindrical wall configured to support an outer casing string and further disposed to support a casing hanger for the support of an inner casing string within said outer casing string to form a monitoring annulus between the inner and outer casing strings;

said wellhead housing being disposed to support an isolation sleeve for a production tree and including a passageway within said wall of the wellhead housing in communication near a lower end of the wellhead housing with said monitoring annulus between casing strings and extending upwardly within said wall of the wellhead housing to an upper end of the wellhead housing to provide communication between the isolation sleeve and said monitoring annulus between inner and outer casing strings; and

a valve which controls communication through said passageway and is operable between open and closed positions by means of fluid pressure applied to the valve by way of said isolation sleeve.

12. The subsea wellhead assembly of claim 11 , in which said isolation sleeve has passages for the application of said fluid pressure, these passages extending within a body wall of the isolation sleeve.

13. The subsea wellhead assembly of claim 11 wherein said valve is accommodated in said wellhead housing below said isolation sleeve and provides controlled communication between said passageway and a passage in said isolation sleeve.

14. A subsea wellhead assembly including:

a wellhead housing having a generally cylindrical wall configured to support an outer casing string, and further defining an axial bore, the wellhead housing being disposed to support within the bore a casing hanger for the support of an inner casing string within said outer casing string to form a monitoring annulus between the inner and outer casing strings, wherein:

said wellhead housing includes a communication passageway within said wall of the wellhead housing which is in communication with said monitoring annulus near a lower end of the wellhead housing and extends upwardly and entirely within said wall of the wellhead housing to an upper end of the wellhead housing;

said wellhead housing includes a port in said bore; and

a valve is accommodated near the upper end of said wellhead housing and is disposed to control fluid communication between said passageway and said port.

15. The subsea wellhead assembly of claim 14 in which said valve comprises a slide valve which is moveable between an open position connecting said passageway and said isolation sleeve and a closed position blocking communication between said passageway and said isolation sleeve.

16. The subsea wellhead assembly of claim 15 and including a spring bias for urging said valve to said closed position.

17. The subsea wellhead assembly of claim 14 , and further comprising an isolation sleeve for a production tree, said isolation sleeve being disposed in said bore at the upper end thereof, wherein said isolation sleeve includes a passageway for fluid communication between said port and said production tree.

18. The subsea wellhead assembly of claim 17 , in which said isolation sleeve is disposed to communicate fluid pressure from said tree to operate said valve.