Emergency disconnect system

1. A control system for controlling disconnection of a riser system that extends between a vessel and a subsea location that comprises a wellbore, the riser system accommodating a string,

wherein the control system is configured to:

determine a critical condition of at least one of (a) the riser system and (b) the vessel;

automatically disconnect the riser system based on the critical condition according to a sequence of operations, the sequence of operations comprising disconnecting the riser system prior to or simultaneous with cutting the string and/or sealing the well bore;

determine a position of the vessel and a control enabling event, the control enabling event including a blackout or other power failure event of the vessel;

determine if a disconnect condition has been met based on the determined position of the vessel and the determination of the control enabling event, the meeting of the disconnect condition indicating that the riser system should be disconnected; and

determine a disconnection threshold past which a critical condition of the riser system will be reached before the riser system would be disconnected if the vessel continues to move or drift at its current or a predicted rate, rate of change and/or direction of movement or drift.

2. The control system according to claim 1 wherein the sequence of operation comprises successively disconnecting the riser system and cutting the string then sealing the well bore.

3. The control system according to claim 1 wherein the control system is configured to directly control, or indirectly control via a further controller, a latching mechanism comprised in a lower marine riser package (LMRP) or completion riser, and the control system is configured to disconnect the riser system by switching the latching mechanism into an unlatched configuration from a latched configuration.

4. The control system according to claim 1 wherein the cutting of the string comprises cutting the string using at least one casing shear ram (CSR) of a blowout preventer (BOP) stack.

5. The control system according to claim 4 wherein the sealing of the wellbore comprises sealing the wellbore using blind shear rams of the blowout preventer (BOP) stack.

6. The control system according to claim 1 , wherein the control system is configured to determine the disconnection threshold at least partly based on a time to disconnect the riser system with a current emergency disconnection scheme.

7. The control system according to claim 1 , wherein the control system is configured to determine the disconnect condition by determining the deviation of the vessel or the deviation of the vessel from the reference position is within an operational margin of, meets or exceeds the disconnection threshold.

8. A control system for controlling disconnection of a riser system that extends between a vessel and a subsea location that comprises a wellbore, the riser system accommodating a string,

wherein the control system is configured to:

determine a critical condition of at least one of (a) the riser system and (b) the vessel;

automatically disconnect the riser system based on the critical condition according to a sequence of operations, the sequence of operations comprising disconnecting the riser system prior to or simultaneous with cutting the string and/or sealing the well bore;

determine a position of the vessel and a control enabling event, the control enabling event including a blackout or other power failure event of the vessel; determine if a disconnect condition has been met based on the determined position of the vessel and the determination of the control enabling event, the meeting of the disconnect condition indicating that the riser system should be disconnected; and

disable, abort or render dormant the control of the disconnection of the riser system if it determines that the control enabling event no longer applies, including if the power is reinstated to the vessel.

9. A method of controlling disconnection of a riser system that extends between a vessel and a subsea location that comprises the steps of:

determining a critical condition of at least one of (a) the riser system and (b) the vessel; and

automatically disconnecting the riser system based on the critical condition according to a sequence of operations, the sequence of operations comprising disconnecting the riser system prior to or simultaneous with cutting the string and/or sealing the well bore,

determining a position of the vessel and a control enabling event, the control enabling event including a blackout or other power failure event of the vessel, and to determining if a disconnect condition has been met based on the determined position of the vessel and the determination of the control enabling event, the meeting of the disconnect condition indicating that the riser system should be disconnected,

determining a disconnection threshold past which a critical condition of the riser system will be reached before the riser system would be disconnected if the vessel continues to move or drift at its current or a predicted rate, rate of change and/or direction of movement or drift.

10. A system comprising a lower blow out preventer (BOP) stack and a control system for controlling disconnection of a riser system that extends between a vessel and a subsea location that comprises a wellbore, the riser system accommodating a string,

wherein the control system is configured to: implement the method according to claim 9 .

11. The method according to claim 9 comprising operating a lower blow out preventer (BOP) stack that is configured to selectively couple with a lower marine riser package (LMRP), and the lower blow out preventer (BOP) stack comprising at least one casing shear ram (CSR), and triggering, the triggering including automatically triggering, the operation of the at least one casing shear ram (CSR) to cut a string in the event of the lower marine riser package (LMRP) disconnecting from the lower blow out preventer (BOP) stack.

12. A computer program product configured such that, when implemented on a control system or processing device causes the control system or processing device to implement claim 11 .

13. A computer program product configured such that, when implemented on a control system or processing device causes the control system or processing device to implement claim 9 .