DYNAMIC POSITIONING CONTROL

Abstract

A drive-off prevention controller for a dynamic positioning system, the drive off prevention controller being configured to communicate with the dynamic positioning system, wherein the dynamic positioning system is configured to determine a position of a vessel relative to a target position and to control a 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, based on one or more indicative conditions, selectively adjust the control of the propulsion system determined by the dynamic positioning system or that would otherwise have been determined by the dynamic positioning system; and at least one of the indicative conditions comprises the demand, or rate of increase in demand, of the propulsion system being greater than a first conditional threshold whilst weather or environmental data received by the drive-off prevention controller indicates that there has been no corresponding change in weather or environmental conditions.

Claims

1.-24. (canceled)

25. An offshore vessel comprising a drive-off prevention controller for a dynamic positioning system, a communication system, and a propulsion system of the offshore vessel, wherein the drive-off prevention controller is configured to: determine a position of a vessel relative to a target position and to control a propulsion system of the vessel based on the determined position of the vessel relative to the target position; communicate via the communication system with at least one remote server or weather service to determine one or more indicative conditions that depend (based on page 21, line 4) on weather or environmental conditions; and selectively adjust the control of the propulsion system determined by the dynamic positioning system, based on the one or more indicative conditions; and wherein at least one of the indicative conditions comprises a demand of the propulsion system being greater than a first conditional threshold while the weather or environmental conditions received by the drive-off prevention controller indicates that there has been no corresponding change in weather or environmental conditions.

26. The offshore vessel according to claim 25, wherein the weather or environmental conditions further comprise one or more of: wave conditions, wind conditions, current at the vessel, wave height, instantaneous wave speed, average wave speed, wave force, wind strength, wind direction, wind force, current strength, current speed, force due to currents, and current direction at the vessel.

27. The offshore vessel according to claim 25, wherein the weather or environmental conditions comprise horizontal wave drift force on the vessel, a speed, direction and/or strength of surface currents and/or a viscous effect due to or properties derived therefrom.

28. The offshore vessel according to claim 25, wherein the drive-off prevention controller is configured to receive or derive at least part of the weather or environmental data from one or more weather or environmental data servers.

29. The offshore vessel according to claim 25, wherein the drive-off prevention controller is configured to determine that there has been no corresponding change in weather or environmental conditions if one or more or all of the weather conditions change by less than a threshold amount, in a determined or predetermined or set time window or based on a determined or predetermined algorithm, machine learning algorithm or set of conditions or parameters.

30. The offshore vessel according to claim 25, wherein the drive-off prevention controller is configured to access vessel information data including vessel geometry and vessel behavioral properties determining if there has been a corresponding change in weather or environmental conditions dependent at least in part on the vessel information data.

31. The offshore vessel according to claim 25, wherein the drive-off prevention controller is configured to adjust control of the propulsion system by additionally limiting a power characteristic of the propulsion system, wherein the power characteristic that is additionally limited is a power characteristic determined based on the position of the vessel relative to the target position.

32. The offshore vessel according to claim 31, wherein the drive-off prevention controller is configured such that the control of the propulsion system is adjusted by selectively limiting the power characteristic of the propulsion system if the power demanded by the propulsion system is above a power threshold.

33. The offshore vessel according to claim 31, wherein the power characteristic comprises a power demanded or a rate of increase in power demanded by the propulsion system.

34. The offshore vessel according to claim 25, wherein the controller is configured such that an operation to adjust the control of the propulsion system can be manually overridden, switched off or cancelled.

35. The offshore vessel according to claim 25, wherein the drive-off prevention controller being configured to override or modify the control of a dynamic positioning system controller of the dynamic positioning system.

36. A drive-off prevention controller controlling a dynamic positioning system of a vessel, the drive-off prevention controller being configured to: determine a current position of the vessel; determine a difference between the determined current position and a target position; communicate with at least one remote database, at least one remote server, or a weather service to predict weather conditions; estimate power demand for a propulsion system of the vessel based on the determined difference between the determined current position and a target position; estimate a time window when a rise in power demand occur; operate the propulsion system: in a normal mode to minimize the difference between the determined current position and a target position, or in a limiting conditions procedure mode, wherein the limiting conditions procedure mode is applied when the estimated power demand for the propulsion system exceeds a power threshold, and the predicted weather conditions do not show a significant change during the time window when a rise in power demand occur.

37. The drive-off prevention controller according to claim 36 and being configured to adjust control of the propulsion system by selectively limiting a power characteristic of the propulsion system if the power consumed by the propulsion system is above a power threshold.

38. The drive-off prevention controller according to claim 36 and being configured to implement a time dependent limiting scheme during the limiting conditions procedure mode, wherein a limit on rate of increase in power is ended or increased after a certain period of time.

39. A method for dynamically positioning a vessel, the method comprising: determining a current position of the vessel; determining a difference between the determined current position and a target position; communicating with at least one remote database, at least one remote server, or a weather service to predict weather or environmental conditions; estimating power demand for the propulsion system of the vessel based on the determined difference between the determined current position and a target position; estimating a time window when a rise in power demand occur; operating the propulsion system: in a normal mode to minimize the difference between the determined current position and a target position, or in a limiting conditions procedure mode when both a estimated power demand for the propulsion system exceeds a power threshold, and a predicted weather conditions do not show a significant change during the time window when a rise in power demand occur.

40. The method according to claim 39, and further comprising receiving or deriving at least part of the weather or environmental conditions from one or more weather or environmental data servers.

41. The method according to claim 39, and further comprising determining that there has been no corresponding change in weather or environmental conditions if one or more or all of the weather conditions change by less than a threshold amount, in a determined or predetermined or set time window or based on a determined or predetermined algorithm, machine learning algorithm or set of conditions or parameters.

42. The method according to claim 39, and further comprising accessing vessel information data including vessel geometry and vessel behavioral properties determining if there has been a corresponding change in weather or environmental conditions dependent at least in part on the vessel information data.

43. The method according to claim 39, and further comprising adjusting control of the propulsion system by additionally limiting a power characteristic of the propulsion system, wherein the power characteristic that is additionally limited is a power characteristic determined based on the position of the vessel relative to the target position.

44. The method according to claim 39, and further comprising adjusting the propulsion system by selectively limiting a power characteristic of the propulsion system if the power demanded by the propulsion system is above a power threshold.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0095] These and other aspects of the present disclosure will now be described, by way of example only, with reference to the accompanying Figures, in which:

[0096] FIG. 1 is a schematic of a drilling arrangement;

[0097] FIG. 2 is a schematic of an example of a drilling arrangement involving a drilling vessel;

[0098] FIG. 3 is a schematic of a control system for a dynamic positioning system for the drilling vessel shown in FIG. 2;

[0099] FIG. 4 is a comparison of power consumption in a dynamic positioning control scheme implemented by the control system of FIG. 3 relative to that of a conventional control scheme; and

[0100] FIG. 5 is a flowchart illustrating a method of performing dynamic positioning.

DETAILED DESCRIPTION OF THE DRAWINGS

[0101] Various aspects and examples of the present disclosure relate to methods, systems and apparatus for the dynamic position control of offshore vessels. Any vessel may be considered, but for the purposes of the exemplary description provided below, a semi-submersible drilling vessel or “rig” is presented.

[0102] FIG. 1 shows a semi-submersible 5 that is connected to and supports one end of a riser 10, which may be a drilling riser. The riser 10 extends from the vessel 5, underwater to connect to a structure such as a blow-out preventer (BOP) 15 on the sea bed 20. The riser is provided with a pair of flex joints 25, one at the surface end and one at the subsea end. The vessel 5 can be moved, e.g. due to currents, waves and wind, and is maintained in a target position using a dynamic positioning system, as is well known in the art. The vessel comprises a propulsion system that comprises a plurality of propulsion devices such as thrusters 30. The thrusters 30 are configured to propel the vessel 5 in different directions and are selectively controllable to adjust the position of the vessel 5.

[0103] The vessel shown in FIG. 1 is a drilling rig, but it will be appreciated that use of dynamic positioning is not limited to this and can equally be used for ships, remotely operated vehicles or indeed any other suitable vessel or vehicle that uses a dynamic positioning system. Furthermore, although the riser 10 is connected at the subsea end to a BOP 15, it will be appreciated that the riser could be connected to any other suitable wellhead apparatus, such as a Xmas tree.

[0104] The dynamic positioning system of the vessel 5 is configured to determine the position of the vessel 5, compare the determined position of the vessel 5 with a target position and determine corrective control actions for specific thrusters 30 to, as far as possible, maintain the semi-submersible 5 substantially in the target position.

[0105] The dynamic positioning system can determine the position of the vessel using a range of sensors, such as satellite positioning sensors, beam based triangulation or relative positioning sensors, wire tension monitoring systems, sea monitoring sensors (e.g., acoustics), and/or the like. Although modern sensors used for position determination are generally reliable and failures can be mitigated by providing multiple sensors and different types of sensor, any failures or errors in the sensors used for position determination can cause the dynamic positioning system to believe that the vessel 5 has deviated further from the target position than it actually has. This in turn may cause the dynamic positioning system to request a sudden significant ramp up in the power of one or more of the thrusters 30 of the vessel 5 to counteract the false determination from the dynamic positioning system. This ramp up in thruster 30 operation resulting from the erroneous readings from the sensor used for position determination may have the effect of the vessel “driving off” from the target position.

[0106] Since the riser 10 is suspended between the vessel 5 and the BOP 15 or other well head component, drive-off of the vessel results in different parts of the riser being subjected to different degrees of bend, tilt or re-orientation. Generally the ends of the riser 10 that are connected to the wellhead and supported by the vessel 5 are the worst affected and most likely to suffer failure as a result of the drive-off. The flex joints 25 are designed to preferentially flex or bend and can provide a degree of protection against such failure, but improved mechanisms for preventing drive off would be advantageous.

[0107] FIG. 2 shows a vessel 5′ provided with a dynamic positioning system that comprises a controller 35 that is in communication with a plurality of sensors 40A-40G and a plurality of riser sensors 45A, 45B. At least one of the sensors 40A-40G may be sensors used by the dynamic positioning system to determine the position of the vessel. The controller 35 is also configured to communicate with and control a propulsion system that comprises a plurality of propulsion devices of the vessel 5′ in the form of thrusters 30. The propulsion system is a system specifically for maneuvering and repositioning the vessel.

[0108] As in the example of FIG. 1, the vessel 5′ supports an end of a riser 10′ that extends between the vessel 5′ and a wellhead 50 on the seabed 20. Each end of the riser 5′ is provided with a flex joint 25′ and each flex joint is provided with at least three of the riser sensors 45A, 45B that monitor at least one property of the flex joint 25′ that changes when the vessel 5′ is moved. Suitable properties of the flex joint include tilt, inclination, angle, orientation, flex, bend, stress or strain, or other suitable property of the respective flex joint 25′ or other part of the riser 10′. The controller 35 is configured to determine a value for the at least one property of each flex joint 25′ based on the readings from the plurality of riser sensors 45A, 45B for that flex joint 25′, e.g. by a majority decision, averaging or other statistical procedure. However, the riser sensors need not be located only at the flex joint 25′ and one or more riser sensors may be provided on or along the riser 10′, in addition to or as an alternative to the riser sensors 45A, 45B at the flex joints 25′.

[0109] The sensors 40A-40G include a variety of different sensor types. In this particular example, the sensors 40A-40G include at least two satellite navigation sensors 40A, 40B provided on the vessel 5. In the present example, each of the satellite navigation sensors 40A, 40B is a different type of satellite navigation sensor, e.g. one of the satellite navigation sensors 40A is a DGPS 700 sensor, whilst the other is a DGPS 5D sensor. However, it will be appreciated that the satellite navigation sensors may be the same type of sensor or may comprise other types of satellite navigation sensors, such as GLONASS, Galileo or other sensors.

[0110] The sensors 40A-40G in this example also include a wire sensor 40C on the vessel 5 that determines a tension or force on a taut wire that is suspended between the vessel 5 and a fixed external point, e.g. on the sea bed. As the vessel 5 moves relative to the fixed external point, then the tension or force on the wire changes, which is measured by the wire sensor 40C and can be used in the estimation of position of the vessel 5.

[0111] The sensors 40A-40G in this example also include a plurality of motion sensors 40D, in this example in the form of four gyroscopic sensors, which are configured to measure motion of the vessel. However, other numbers or types of motion sensors 40D, such as accelerometers, could be used.

[0112] The sensors 40A-40G in this example further include a plurality of beam position sensors 40E, 40F, including a surface based beam sensor 40E that sends a beam to and/or receives a beam from a reference point, e.g. on land or on another vessel or other sea structure. The surface based beam sensor 40E can be used to determine changes in position of the vessel 5 relative to the reference point. Similarly, the vessel 5 comprises a plurality of underwater beam sensors 40F, such as sonar or other sonic sensors, that are each in communication with a plurality of reference points located on the sea bed using sonic or sonar signals and can be used to determine changes in the location of the vessel 5 using the relative timing and/or strength of the sonic or sonar signal received from each reference point. A plurality of reference points for each sensor 40F allows direction of motion of the vessel to be determined, e.g. using triangulation or other suitable techniques.

[0113] The sensors 40A-40G in this example further include condition sensors 40G, which include weather sensors such as wind sensors, and also include water current sensors to determine the speed, direction and/or strength of surface currents, e.g. currents at a depth of less than a depth threshold. The weather sensors are configured to determine parameters indicative of, or that are usable to derive, weather or environmental conditions experienced by the vessel. The weather or environmental conditions could, for example, comprise one or more of: wave conditions, wind conditions, current, wave height, instantaneous wave speed, average wave speed, wave force, wind strength, wind direction, wind force, current strength, current speed, force due to currents, and/or current direction at the vessel. For example, the condition sensors 40G may comprise at least one of: a water speed sensor, a tide direction sensor, a tide strength sensor, a current sensor such as a surface current sensor, a wave height sensor and/or the like.

[0114] The controller 35 is not limited to weather or environmental conditions directly derived from the condition sensor(s) 40G but instead may obtain at least some of the weather or environmental conditions from a database, remote server or weather service provider or derive the weather or environmental conditions from the data from the condition sensors 40G and/or the database, server or weather service data. In one example, the weather or environmental conditions comprise horizontal wave drift force on the vessel, which can be determined by the controller 35 using 3-D radiation-diffraction theory, under potential theory, empirically using wave drift force coefficients and/or the like. The weather or environmental conditions may comprise a viscous effect of the vessel or properties derived therefrom, which the controller may be configured to determine, e.g. from the data from the condition sensors 40G and/or the database, server or weather service data, such as by using the methods outlined in Dev, Arun. (2011). Environmental Forces for Dynamic Positioning: Ships vs. Semi-Submersibles. 64-78, Conference: 4th International Conference on Technology and Operation of Offshore Support Vessels, August 2011, 10.3850/978-981-08-9731-4_05V2011-08, or in Dev, A.K.: “Viscous Effects in Drift Forces on Semi-submersibles”, PhD Thesis, Ship Hydromechanics Laboratory, Delft University of Technology, Delft, The Netherlands, 1996, the contents of each of which are incorporated by reference as if set out in full herein.

[0115] The controller 35 is in communication with the sensors 40A-40G and uses the data collected by at least some of the sensors 40A-40G to determine a position of the vessel 5, e.g. according to a predetermined algorithm or other suitable relation.

[0116] A detailed schematic of the controller 35 is shown in FIG. 3. The controller 35 comprises a processor 55, a communications system 60 and a data store in the form of a memory 65. The communications system 60 is configured to communicate via wires 70 or wirelessly with the plurality of sensors 40A-40G used for position determination and the plurality of control sensors 45A, 45B to receive data signals therefrom. The communications system 60 is also configured to communicate via wires 70 or wirelessly with the vessel control system 75 that controls the thrusters 30 of the vessel 5 so as to be able to send control signals to the vessel control system 75 to control the thrusters 30. The wireless communications could comprise acoustic communications, e.g. along the riser 10, 10′.

[0117] Although an example of a controller 35 is shown in FIG. 3, it will be appreciated that other controller configurations could be used. For example, the controller 35 may comprise a plurality of processors 55 and/or a plurality of data stores 60, which may be contained in a single unit or distributed, e.g. over several systems, some of which may be remote.

[0118] Furthermore, in an example, the controller could be functionally, logically or physically separated into a drive-off prevention controller and a dynamic position system controller. The dynamic positioning system controller determines the position of the vessel relative to the target position and controls the propulsion system of the vessel based on the determined position of the vessel relative to the target position. The drive-off prevention controller is configured to override or modify the control of the dynamic positioning system by the dynamic positioning system controller. In particular, the drive off prevention controller is configured to monitor one or more indicative conditions that may be indicative of the vessel being at the target position or not. The drive off prevention controller is then configured to selectively adjust the operation of the dynamic positioning system to control of the thrusters 30 when the indicative conditions are met. In this way, the drive-off control functionality may be retro-fitted to an existing dynamic positioning system by retro-fitting the drive-off controller, which communicates with (or is provided by a common control or computer system with) the dynamic positioning system controller of the existing dynamic positioning system.

[0119] In this way, the controller 35 can receive data from the plurality of sensors 40A-40G, which is processed by the controller 35 to determine the position of the vessel 5. The controller 35 can also use the data from the sensors 40A-40G, from a database or remote server and/or form riser sensors 45A, 45B, which is processed by the controller 35 to determine one or more indicative conditions that may be indicative of the vessel being at the target position or not. Examples of suitable indicative conditions include changes in the weather or environmental conditions, at least one property, e.g. inclination, of at least part of the riser 10, such as the inclination or orientation of the flex joints 25, amongst other possibilities.

[0120] For example, many conventional dynamic positioning adjustments may be due to weather or environmental conditions. If the dynamic positioning system determines that a significant positional adjustment is required such that a sudden increase in power demand is determined without a corresponding change in weather or environmental conditions, then this may be indicative that the determined control of the propulsion unit based on the determined position of the vessel is incorrect.

[0121] The actions performed by the controller 35 are described in relation to FIGS. 4 and 5.

[0122] In 105 of FIG. 5, the controller 35 determines a position of the vessel 5 from data from the plurality of sensors 40A-40G. In 110 of FIG. 5, the controller 35 determines the difference between the determined position and a target position, stored in the data store 65 and determines a control operation of the thrusters that is dependent on the determined difference, e.g. based on a look-up table, algorithm or some other predetermined relationship. The control operation is communicated from the controller 35 to the vessel control system 75 which then controls the thrusters 30 accordingly.

[0123] The power consumption of the thrusters 30 is monitored and communicated to the controller 35.

[0124] In steps 115 and 120 of FIG. 5, the controller determines if the indicative condition is met. In this example, the indicative condition is the demand, or rate of increase in demand, of the propulsion system being greater than a first conditional threshold whilst weather or environmental data received by the drive-off prevention controller indicates that there has been no corresponding change in weather or environmental conditions (e.g. a change in weather conditions of greater than a threshold amount within a threshold period of time of the demand or change in demand).

[0125] In 115 of FIG. 5, the controller 35 receives and/or determines the weather or environmental conditions as outlined above (e.g. one or more of: wave conditions, wind conditions, current, wave height, instantaneous wave speed, average wave speed, wave force, wind strength, wind direction, wind force, current strength, current speed, force due to currents, current direction, water speed, tide direction sensor, tide strength, surface currents, wave height and/or the like) from the condition sensor(s) 40G, from a database, remote server or weather service. As other examples, the weather or environmental conditions may comprise horizontal wave drift force on the vessel, and/or a viscous effect arising from the movement of water around the vessel or properties derived therefrom. For example, these may be determined as outlined above, e.g. using 3-D radiation-diffraction theory, under potential theory, empirically using wave drift force coefficients and/or the like or any of the other methods outlined in Dev, Arun. (2011). Environmental Forces for Dynamic Positioning: Ships vs. Semi-Submersibles. 64-78, Conference: 4th International Conference on Technology and Operation of Offshore Support Vessels, August 2011, 10.3850/978-981-08-9731-4_OSV2011-08 or in Dev, A.K.: “Viscous Effects in Drift Forces on Semi-submersibles”, PhD Thesis, Ship Hydromechanics Laboratory, Delft University of Technology, Delft, The Netherlands, 1996, the contents of each of which are incorporated by reference as if set out in full herein.

[0126] In 120 of FIG. 5, the controller 35 determines any change in the determined weather or environmental conditions within a set or preset timeframe of the rise in power demand and compares the determined change in the determined weather or environmental conditions with a respective threshold or operational range. The threshold or operational range may be pre-programmed and stored in the memory 65, calculated or manually input.

[0127] If the controller 35 determines that the value of the change in the determined weather or environmental conditions is above the threshold or outwith the operational range, then this is indicative of the riser 10 being in a condition in which the vessel 5 may have moved significantly away from the target position and the controller 35 is configured to operate the thrusters 30 in accordance with the control operation determined from the difference between the determined position of the vessel 5 and the target position (125 of FIG. 5), i.e. the controller 35 performs conventional dynamic positioning.

[0128] However, if the controller 35 determines that the change in weather or environmental condition is below the threshold or within the operational range, then this is indicative of the vessel 5 being close to the target position (i.e. there is no change in weather or environmental condition to give rise to the sudden change in power demand). In this case, the controller 35 determines if the power currently being consumed by the thrusters 30 is above a power threshold 80 (130 of FIG. 5) and, if so, implements a limiting procedure in which the control operation of the thrusters 30 is adjusted such that the rate of increase in power supplied to the thrusters above a threshold power 80 is limited (135 of FIG. 5). If not, the controller continues to operate the thrusters 30 in accordance with the control operation determined from the difference between the determined position of the vessel 5 and the target position (125 of FIG. 5).

[0129] In this way, as can be seen in region A of FIG. 4, the controller 35 can control the thrusters 30 with small variations in power to make small adjustments to the position of the vessel 5, as would be expected to respond to tides, current, waves and wind, for example. However, if the operational control determined by the controller 35 results in the power supplied to the thrusters 30 being above the power threshold 80, then the controller 35 adjusts control of the thrusters such that the rate of increase in power consumed by the thrusters is limited, as can be seen in region B of FIG. 4. This prevents a large, sudden increase in power drawn by the thrusters 30, which would otherwise be the case (see region C in FIG. 4) and instead results in a much slower ramp up of thruster 30 operation.

[0130] In this way, whilst the indicative condition is met, then it is likely that the vessel is close to the target position such that any large, sudden increases in thruster operation 35 are more likely to be the result of erroneous reading from one of more of the sensors 40A-40G used for position determination and are as such delayed by limiting the rate of increase of thruster power 35 beyond a power threshold. This allows an operator much more time to assess if the situation requiring the ramp up in thruster 30 power is genuine and to make a manual intervention if it is not. However, it does not prevent low power operations of the thrusters 30 that result in a power draw that is below the power threshold that are much more likely to be genuinely needed. Beneficially, the limiting of the rate of increase of power in the manner described also does not prevent increases in thruster 30 operation, it only delays them and only in cases where the control sensors indicate that the riser is in a configuration associated with the vessel being close to the target range. This ensures that any genuine requirement for increased thruster 30 operation above the power threshold will get implemented, albeit delayed. Furthermore, the controller 35 may optionally be configured to implement a time dependent limiting scheme, wherein the limit on rate of increase in power is ended or increased after a certain period of time.

[0131] In view of the above, examples of the controller 35 provide improved prevention of drive-off due to errors in the determination of the position of the vessel 5, e.g. due to faulty sensors and the like.

[0132] Although, specific examples are described above in relation to the Figures, it will be appreciated that variations on the above examples are possible.

[0133] Although examples described above use power consumed or rate of increase in power consumed, as a metric or threshold, it will be appreciated that other metrics such as power demand, rate of increase in power demand, power output or rate of increase in power output may be used instead.

[0134] In addition, in the example given above, the controller is configured to selectively apply the adjustment of the control of the propulsion system based on one or more conditions in order to protect against drive-off. One of the conditions may be that the demand, or rate of increase in demand, of the propulsion system is greater than a first conditional threshold whilst weather or environmental data received by the controller indicates that there has been no corresponding change in weather or environmental conditions, as outlined above.

[0135] However, the controller may be configured to use other conditions in addition to or instead of any of the conditions above. For example, one of those conditions may be a property of the riser (e.g. bend, tile, inclination and/or the like) determined by the controller is below the associated threshold or within the operational range. Another condition may be that the amount of power demanded or consumed by the propulsion system is above the power threshold. One of the conditions may be that the demand, or rate of increase in demand, of the propulsion system is greater than a second conditional threshold resulting from a change in position that is greater than a distance threshold, e.g. greater than 5 m or 10 m, over a predetermined or determined period of time.

[0136] If one or more or all of the conditions are met, then the controller selectively applies the adjustment of the control of the propulsion system, e.g. by limiting the power characteristic of the propulsion system, otherwise the system simply controls the propulsion system based on the determined position of the vessel relative to the target position without the adjustment of the control of the propulsion system.

[0137] Furthermore, the vessel 5 need not be a drilling rig and other vessels such as ships or ROVs, drill ships, semi-submersibles, FPSOs, or any other type of vessel or vehicle that uses or could use a dynamic positioning system could be used instead.

[0138] In examples given above, a change (i.e. a corresponding change) in the weather or environmental conditions is determined to have taken place if there is a change in the weather or environmental conditions determined by the controller 35 that is greater than a threshold and occurs within a set or preset time period of the increase in power or the power being determined to be above the first conditional threshold. However, the present disclosure is not limited to this. For example, a change (i.e. a corresponding change) in the weather or environmental conditions could be determined to have taken place if there is a change in the weather or environmental conditions determined by the controller 35 implementing an algorithm such as a machine learning algorithm (e.g. a neural network or deep neural network) that maps weather or environmental conditions as inputs to determinations of whether there has been a corresponding change in weather or environmental conditions as outputs. Other possibilities for determining a change in weather conditions that excludes insignificant variation or noise in weather or environmental data may be used.

[0139] Although one possible example of controller 35 is descried in relation to FIG. 3, the controller 35 configuration is not limited to this. Method steps of the invention can be performed by one or more programmable processors executing a computer program to perform functions of the invention by operating on input data and generating output. Method steps can also be performed by special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application-specific integrated circuit) or other customised circuitry. Processors suitable for the execution of a computer program include CPUs and microprocessors, and any one or more processors. Generally, a processor will receive instructions and data from a read-only memory or a random access memory or both. The essential elements of a computer are a processor for executing instructions and one or more memory devices for storing instructions and data. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto-optical disks, or optical disks. Information carriers suitable for embodying computer program instructions and data include all forms of non-volatile memory, including by way of example semiconductor memory devices, e.g. EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto-optical disks; and CD-ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in special purpose logic circuitry.

[0140] As such, the present invention is not limited by the examples shown in the drawings but only by the claims.