Dynamic positioning control

1. A drive-off prevention controller for controlling a propulsion system of a vessel with a plurality of thrusters, a riser extending from the vessel, and a plurality of control sensors configured for monitoring riser properties, wherein the controller is configured for:

determining a difference between a determined position for the vessel and a target position,

determining a control operation for thrusters, the control operation having a power characteristic dependent on the determined difference,

determining an adjusted control operation for the thrusters, wherein the adjusted control operation implements a limiting procedure of the power characteristic to below a threshold less than the power characteristic of the control operation,

determining one or more properties of the riser from data supplied by the plurality of control sensors, and

determining whether the one or more properties of the riser is within an operational range, the operational range comprising a maximum value of the one or more determined properties at which riser disconnection can occur;

when the one or more properties of the riser are out of the operational range, operating the thruster by applying the control operation, and

when the one or more properties of the riser are within the operational range, operating the thruster by applying the adjusted control operation.

2. The controller of claim 1 , further comprising operating the thruster by applying the adjusted control operation when the property of at least part of the riser is within an associated threshold or operational range.

3. The controller according to claim 2 , wherein the controller is configured to increase the associated threshold or operational range when the vessel current increases or is above a current threshold.

4. The controller of claim 1 further configured for determining the power demanded by the propulsion system for operating the thruster by applying the control operation, and for determining the adjusted control operation for the propulsion system by selectively limiting the operational power characteristic of the propulsion system if the power demanded by the propulsion system is above a power threshold.

5. The controller according to claim 1 , further comprising applying the adjusted control operation for the propulsion system characteristic of the propulsion system determined based on the position of the vessel relative to the target position when the property of the riser determined by the controller is within an associated threshold or operational limit and when the power demanded by the propulsion system is above a power threshold.

6. The controller according to claim 1 , wherein monitoring the property of at least part of the riser extending from the vessel comprises monitoring one or more of: an angle, orientation, inclination, bending, flex, stress or strain of at least part or all of the riser.

7. The controller according to claim 1 , wherein the property of the riser is measured by a plurality of control sensors comprising at least two control sensors that are duplicates of each other and the controller is configured to select the control operation or the adjusted control operation for the thrusters on the basis of signals from the at least two control sensors.

8. The controller according to claim 1 , configured such that, when the property of the riser is out with a threshold or operational range, then the controller controls the propulsion system according to the control operation for the thrusters without the adjustment or limiting of the propulsion system.

9. The controller according to claim 1 , wherein the adjusted control operation of the propulsion system is time dependent.

10. The controller according to claim 1 , wherein the power characteristic comprises a power demanded or a rate of increase in power demanded by the propulsion system.

11. The controller according to claim 1 configured to automatically disconnect the riser, and/or provide an alert to an operator to disconnect the riser, if the monitored one or more properties of the riser exceeds a disconnection threshold.

12. The controller according to claim 1 , wherein the position of the vessel is at least partially determined using variations in tension in a taut line fixed at one end to a stationary point.

13. The controller according to claim 1 , configured such that operation of the thruster by applying the adjusted control operation can be manually overridden, switched off or cancelled.

14. The controller according to claim 1 , being configured to override or modify control of a dynamic positioning system by a dynamic positioning system controller.

15. The controller according to claim 14 , wherein the dynamic positioning system controller is configured to determine the position of the vessel relative to the target position and to determine a control the propulsion system of the vessel based on the determined position of the vessel relative to the target position and the drive-off prevention controller is configured to monitor the property of at least the part of the riser, and to apply the adjusted control operation for the propulsion system determined by the dynamic positioning system controller according to the monitored property of the riser.

16. The controller of claim 1 and being configured to determine at least two properties of the riser from data supplied from at least two control sensors, wherein the controller is configured for operating the thruster by applying the adjusted control operation with the limitation of the power characteristic when one of the at least two properties indicates operation without the operational range.

17. The controller of claim 1 , wherein the threshold less than the power characteristic of the control operation comprises at least one of at least one of: a power demand or a rate of increase in power demand of the propulsion system; power consumed by the propulsion system; a rate of increase in the power consumed by the system, power output by the propulsion system, or a rate of increase in power output by the propulsion system.

18. A method for dynamically positioning a vessel having a riser extending towards seabed and a plurality of control sensors configured for monitoring riser properties, the method comprising:

determining a position of the vessel,

determining a difference between the determined position and a target position,

determining a control operation for thrusters, the control operation having a power characteristic being dependent on the determined difference,

determining an adjusted control operation for the thrusters, wherein the adjusted control operation implements a limiting procedure of the power characteristic to below a threshold less than the power characteristic of the control operation,

determining one or more properties of the riser from data supplied by the plurality of control sensors,

determining whether the one or more determined properties of the riser is within an operational range, the operational range comprising a maximum value of the one or more determined properties at which riser disconnection can occur,

operating the thruster by applying the control operation when the one or more determined properties of the riser are out of the operational range, and

operating the thruster by applying the adjusted control operation when the one or more determined properties of the riser are within the operational range.

19. The method of claim 18 , wherein the threshold less than the power characteristic of the control operation comprises at least one of at least one of: a power demand or a rate of increase in power demand of the propulsion system; power consumed by the propulsion system; a rate of increase in the power consumed by the system, power output by the propulsion system, or a rate of increase in power output by the propulsion system.

20. A floating drilling vessel comprising:

a riser extending towards seabed,

at least one control sensor for monitoring at least one property of the riser,

a dynamic positioning system configured to determine a position of a vessel relative to a target position and to

determine a control operation having a power characteristic being dependent on the difference between the determined position of the vessel relative to the target position, and

control a propulsion system of the vessel based on the control operation,

a drive-off prevention controller controlling the dynamic positioning system, and configured for:

determining the control operation for the thrusters, and

determining an adjusted control operation for the thrusters, wherein the adjusted control operation implements a limiting procedure of the power characteristic to below a threshold less than the power characteristic of the control operation,

determining one or more properties of the riser from data supplied by the at least one control sensor, and

determining whether the one or more properties of the riser is within an operational range, the operational range comprising a maximum value of the one or more properties at which riser disconnection can occur,

operating the thruster by applying the control operation, when the one or more properties of the riser are out of the operational range, and

operating the thruster by applying the adjusted control operation when the one or more properties of the riser are within the operational range.

21. The floating drilling vessel of claim 20 , wherein the threshold less than the power characteristic of the control operation comprises at least one of at least one of: a power demand or a rate of increase in power demand of the propulsion system; power consumed by the propulsion system; a rate of increase in the power consumed by the system, power output by the propulsion system, or a rate of increase in power output by the propulsion system.