Method and apparatus for subsea well plug and abandonment operations

1. A riser-less method of performing a plug and abandonment operation on a subsea hydrocarbon well from a vessel, the method comprising:

providing an apparatus, the apparatus comprising:

a wellhead interface module located on a wellhead, the wellhead interface module comprising a body defining a chamber which accommodates a volume of wellbore fluid in fluid communication with the wellbore, an upper end of the chamber being open to the subsea environment;

a subsea flow control package;

a fluid conduit extending between the subsea flow control package and surface;

and

a system control module;

wherein the subsea flow control package defines a first flow path between the wellhead interface module and the fluid conduit via a pump and defines a second flow path between the wellhead interface module and the fluid conduit via a flow control valve;

removing a length of tubing or casing from the wellbore;

controlling, using the subsea flow control package, the flow of a wellbore fluid from the fluid conduit to re-fill the chamber of the wellhead interface module;

enabling wellbore fluid to flow from the chamber into the wellbore;

measuring, using a level sensor of the wellhead interface module, a level of the wellbore fluid in the chamber and outputting a measurement signal to the system control module in dependence on the level of wellbore fluid;

deriving volume data relating to a change in volume of wellbore fluid in the chamber, said change in volume being derived from the change in the level of the wellbore fluid in the chamber; and

comparing the derived volume data with a volume change expected due to the removal of tubing or casing from the wellbore.

2. The method according to claim 1 comprising analysing the measurement signal to identify a condition of the wellbore, and characterising the condition as one or more of the conditions in the group consisting of: a steady state; a fluid influx state; a fluid loss state; a tubing run-in state; or a tubing pull-out state.

3. The method according to claim 1 comprising:

removing a second length of tubing or casing from the wellbore;

monitoring the level of the wellbore fluid in the chamber; and

outputting a measurement signal to the system control module in dependence on the level of wellbore fluid.

4. The method according to claim 1 comprising providing wellbore fluid from a wellbore fluid source to the apparatus in response to the derived volume data.

5. The method according to claim 4 comprising repeating the steps of removing a tubing or casing and providing wellbore fluid from the wellbore fluid source to the apparatus in response to the measurement signal.

6. The method according to claim 4 comprising providing additional wellbore fluid from the wellbore fluid source to the chamber of the wellhead interface module in response to the derived volume data, wherein the wellbore fluid source is at the surface.

7. The method according to claim 4 comprising providing wellbore fluid from the wellbore fluid source to the apparatus while the tubing or casing is stationary.

8. The method according to claim 4 comprising providing wellbore fluid from the wellbore fluid source to the apparatus while the tubing or casing is being removed from the wellbore.

9. The method according to claim 1 comprising cutting a length of tubing or casing during the provision of wellbore fluid from a wellbore fluid source to the wellbore or apparatus.

10. The method according to claim 4 comprising pumping wellbore fluid from the wellbore fluid source.

11. The method according to claim 1 comprising controlling the flow of wellbore fluid to the chamber using the flow control valve.

12. The method according to claim 11 comprising choking the flow of wellbore fluid to the chamber using the flow control valve.

13. The method according to claim 1 comprising pumping wellbore fluid from the chamber to the surface.

14. The method according to claim 1 comprising providing a vessel without a marine riser system.

15. An apparatus for monitoring and/or controlling a volume of a fluid in a subsea wellbore system during a riser-less plug and abandonment operation, the apparatus comprising:

a wellhead interface module configured to be disposed on a wellhead, the wellhead interface module comprising a body defining a chamber which accommodates a volume of wellbore fluid in fluid communication with the wellbore, an upper end of the chamber being open to the subsea environment;

a subsea flow control package;

a fluid conduit extending between the subsea flow control package and surface;

and a system control module;

wherein the subsea flow control package defines a first flow path between the wellhead interface module and the fluid conduit via a pump, and defines a second flow path between the wellhead interface module and the fluid conduit via a flow control valve;

wherein the wellhead interface module comprises a sensor that measures the level of fluid in the chamber and outputs a measurement signal to the system control module;

wherein the system control module is configured to derive volume data relating to a change in volume of wellbore fluid in the chamber and compare the derived volume data with a volume change expected due to the removal of tubing or casing from the wellbore, said change in volume being derived from a change in the level of the wellbore fluid in the chamber;

and wherein the subsea flow control package is configured to control the flow of fluid from the fluid conduit to the wellhead interface module.

16. The apparatus according to claim 15 wherein the system control module is configured to characterise a change in wellbore conditions according to the group comprising: a steady state; a fluid influx state; a fluid loss state; a tubing run-in state; or a tubing pull-out state.

17. The apparatus according to claim 15 further comprising a surface flow control package, wherein the fluid conduit connects the subsea flow control package with the surface flow control package.

18. The apparatus according to claim 15 further comprising a wellbore fluid source connected to the wellhead interface module via the fluid conduit.

19. The apparatus according to claim 15 wherein the system control module is configured to direct the flow from the wellhead interface module to the fluid conduit via the first flow path.

20. The apparatus according to claim 15 wherein the system control module is configured to direct the flow from the fluid conduit to the wellhead interface module via the second flow path.

21. The apparatus according to claim 15 wherein the pump is a variable speed pump.

22. The apparatus according to claim 15 wherein the flow control valve is configured to choke the flow from the fluid conduit to the wellhead interface module.

23. A system comprising the apparatus according to claim 15 , a vessel, and a wellbore on which the wellhead interface module is disposed, wherein the vessel is without a marine riser system.

24. The method according to claim 23 comprising removing a length of tubing or casing from the wellbore using a lifting cable or drill string deployed from the vessel.

25. The method according to claim 23 comprising deploying the apparatus from the vessel.