A method for handling a marine vessel actuator

Abstract

Embodiments of the invention provide a method for handing a hydraulic actuator (12, 13) of a marine surface vessel (MV), which actuator (12, 13) is arranged to adjust a position of a blade (3) of a propeller of the vessel, wherein a supply of hydraulic fluid to the actuator (12, 13) is controlled by an actuator control device (31), the method comprising - while controlling the actuator control device (31) so as to keep the vessel control element (3) in a desired position, determining a setting of the actuator control device (31), and determining values of a pitch setting of the blade and the rotational speed of the propeller, and - storing the determined actuator control device setting so as to be correlated with the determined pitch setting and rotational speed, and - using the stored actuator control device setting in a subsequent operational event of or for the actuator (12, 13).

Description

[0001] A METHOD FOR HANDLING A MARINE VESSEL ACTUATOR

[0002] TECHNICAL FIELD

[0003] The invention relates to a method for handling a hydraulic actuator of a marine vessel, for example a marine surface vessel, which actuator is arranged to adjust a position of a vessel control element for controlling movements of the vessel.

[0004] BACKGROUND

[0005] In a marine surface vessel, such as a passenger ship, a cargo ship, or a war ship, a hydraulic actuator may be arranged to adjust a position of a vessel control element for controlling movements of the vessel. For example, the vessel control element may be a blade of a propeller of the vessel, and the actuator may be a hydraulic pitch change servomotor coupled to the blade.

[0006] With time and use, the actuator may start leaking hydraulic fluid used in its operation. US3545881A describes supplying operating pressures for a controllable pitch propeller, wherein fluid is constantly supplied to compensate for leakage and to hold the pitch during steady state operation of the propeller.

[0007] There is nevertheless a desire to improve the handling of such hydraulic actuators in view of wear and leakage.

[0008] SUMMARY

[0009] An object of the invention is to improve the handling of a hydraulic actuator of a marine surface vessel, which actuator is arranged to adjust a position of a vessel control element for controlling movements of the vessel, in view of wear and leakage of the actuator.

[0010] This object is achieved with a method according to claim 1. Thus, the invention provides a method for handling a hydraulic actuator of a marine vessel, which actuator is arranged to adjust a position of a vessel control element for controlling movements of the vessel, wherein a supply of hydraulic fluid to the actuator is controlled by an actuator control device, the method comprising controlling the actuator control device so as to keep the vessel control element in a desired position, determining one or more values of one or more respective operational parameters which influence a force or a moment to which the vessel control element is subjected during the step of controlling the actuator control device so as to keep the vessel control element in the desired position, determining a setting of the actuator control device while the actuator control device is controlled to keep the vessel control element in the desired position, storing the determined actuator control device setting so as to be correlated with the determined operational parameter values, and

[0011] - using the stored actuator control device setting in a subsequent operational event of or for the actuator.

[0012] Thus, the method may comprise storing for the desired position of the vessel control element the determined actuator control device setting so as to be correlated with the determined operational parameter values. The stored actuator control device setting of or for the actuator may be used in a subsequent operational event for the same desired position of the vessel control element.

[0013] By storing the determined actuator control device setting so as to be correlated with the determined operational parameter values, an accurate way of determining a degree of wear of a hydraulic actuator of a marine surface vessel is provided. The invention allows for an adaptive compensation for leakage caused by wear.

[0014] The vessel control element may be provided for controlling the propulsion and / or the movement direction of the vessel. The determined operational parameter values are preferably present during the step of controlling the control device so as to keep the actuator in the desired position. The operational parameters may influence a moment in the form of a torque to which the vessel control element is subjected. As exemplified below, the determined control device setting may be a valve position or a pump rotational speed. The control device may be controlled in a closed loop comprising determining the desired position of the actuator, determining an actual position of the actuator, determining an error of the actual position of the actuator, and adjusting, based on the error, the control device so as to reduce the error. The determined control device setting may be stored so as to be mapped to the determined operational parameter values.

[0015] In some embodiments, the vessel control element is a blade of a propeller of the vessel, and the actuator is a hydraulic pitch change servomotor coupled to the blade. Thereby, the one or more operational parameters may comprise a pitch setting of the blade. Further, the one or more operational parameters may comprise a rotational speed of the propeller or a rotational speed of a part of a drivetrain for the propeller. Thereby, the dependence on the pitch setting and the propeller rotational speed, e.g. for the hydrodynamic torque on the blades, may be taken into account for accurately determining the leakage of hydraulic fluid. The pressure needed to keep the pitch setting in a desired position may be proportional to the square of the propeller speed. The pressure needed to keep the pitch setting in a desired position may be non-linearly dependent on the desired pitch position.

[0016] It should be noted however that in some embodiments, where the vessel control element is a steering device, such as a rudder, a waterjet nozzle steering actuator, or an azimuthal thruster turning device, the operational parameters may comprise a degree of steering device actuation, and the vessel speed. In some embodiments, the vessel control element may be a waterjet reversing scoop actuator.

[0017] Preferably, the method comprises providing, for each of the one or more operational parameters, a plurality of predetermined intervals of values of the respective operational parameter, and selecting, for each of the one or more operational parameters, a respective one of the predetermined intervals, which selected predetermined interval includes the determined value of the respective operational parameter, wherein storing the determined actuator control device setting so as to be correlated with the determined operational parameter values is done by storing the determined actuator control device setting so as to be correlated with the one or more selected predetermined intervals of the operational parameter values.

[0018] Thereby, intervals of different parameters may be organized in groups which are handled by respective control device setting determinators, as exemplified below. Thereby, the method may provide a control device setting for each group of intervals of different parameters. Suitably, there could be 5-15, e.g. 10 groups of intervals of different parameters. Each group may include a combination of intervals which is different from the interval combinations of all other groups.

[0019] In some embodiments, using the stored actuator control device setting comprises, in a subsequent operational condition of the vessel, determining one or more further values of the respective one or more operational parameters, and, if the operational parameter values are within the respective selected predetermined intervals, using the determined actuator control device setting for a further control of the actuator control device so as to keep the vessel control element in a desired position. For example, the determined actuator control device setting may be feed forwarded for a future control of the actuator control device so as to keep the vessel control element in the desired position. Thereby, when the vessel control element is controlled to be moved from a first desired position to a second desired position, overshooting or undershooting the actual vessel control element position in relation to the second desired position may be considerably reduced.

[0020] In some embodiments, the method comprises determining, for one or more of the predetermined intervals of values of the respective operational parameter, a respective reference actuator control device setting as a setting of the actuator control device when the actuator control device is controlled so as to keep the vessel control element in the desired position, and when there is no leakage of the hydraulic fluid supplied to the actuator, comparing the stored determined actuator control device setting with the reference actuator control device setting, and determining in dependence on the comparison an amount of leakage of the hydraulic fluid supplied to the actuator. Thereby, an absolute value of a leakage of hydraulic fluid supplied to the actuator can be determined.

[0021] In some embodiments, the method comprises, subsequently to storing the determined actuator control device setting, providing a further control of the actuator control device so as to keep the vessel control element in a desired position, determining one or more further values of one or more respective operational parameters which influence a force or a moment to which the vessel control element is subjected during the step of controlling the actuator control device so as to keep the vessel control element in the desired position, determining a further setting of the actuator control device during the further control of the actuator control device so as to keep the vessel control element in the desired position, if the further operational parameter values are within the respective selected predetermined intervals of the operational parameter values, storing the further determined actuator control device setting so as to be correlated with the one or more selected predetermined intervals, determining in dependence on the stored actuator control device settings, a trend of the stored actuator control device settings in a time domain.

[0022] The trend of the stored actuator control device settings in a time domain may be a time derivative of the stored actuator control device settings. Determining the trend of the stored actuator control device settings may comprise determining one or more differences between two or more of the stored actuator control device settings. Determining the trend of the stored actuator control device settings may comprise determining one or more differences in time between points in time of determinations of actuator control device settings. The time domain may be uninterrupted, or it may be restricted to the time during which the hydraulic actuator is in use.

[0023] By determining, in dependence on the stored actuator control device settings, the trend of the stored actuator control device settings in a time domain, a wear condition of the hydraulic actuator of may be provided. For example, the actuator control device settings may be determined at substantially the same values of the operational parameter. The settings may be compared and from a difference between them it can be concluded that the setting was changed to compensate for an increased hydraulic leakage due to wear of the actuator.

[0024] Thereby, using the stored actuator control device setting may comprise, based on the amount of leakage and / or the determined trend, deciding on a vessel maintenance service operation involving the actuator. Alternatively, or in addition, using the stored actuator control device setting may comprise, based on the amount of leakage and / or the determined trend, providing an alert signal, e.g. a sound an alarm, for an operator of the vessel. Alternatively, or in addition, where the supply of hydraulic fluid to the actuator is powered by a pump, using the stored actuator control device setting may comprise, based on the amount of leakage and / or the determined trend, activating an additional pump for the supply of hydraulic fluid to the actuator.

[0025] The object is also reached with a computer program according to claim 12, a carrier according to claim 13, or a control module according to claim 14.

[0026] DESCRIPTION OF THE DRAWINGS

[0027] Below embodiments of the invention will be described with reference to the drawings in which, fig. 1 is a partially sectioned side view of a marine vessel, in the form of a cargo ship, fig. 2 is a schematic axial sectional view of a propeller of the vessel in fig. 1, fig. 3 is a simplified illustration of a hydraulic circuit for a system for adjusting the pitch of the blades of the propeller in fig. 2, fig. 4 is a flow diagram depicting steps in an embodiment of a method according to the invention, fig. 5 is a diagram of components in a regulator used for the method in fig. 4, fig. 6 is a schematic representation of a control module for carrying out the method in fig 4, fig. 7 is an illustration similar to the one in fig. 3, of a hydraulic circuit for an alternative system for adjusting the pitch of the blades of the propeller in fig. 2, and fig. 8 is an illustration similar to the one in fig. 3, of a hydraulic circuit for a system for controlling a rudder of a marine vessel.

[0028] DETAILED DESCRIPTION

[0029] Fig. 1 shows a marine vessel MV comprising a hull VI. The vessel presents a bow V2, and a stern V3. A propeller shaft 1, made in a metal material and presenting a rotational axis R, carries a propeller V4. The propeller shaft 1 extends from a power providing device in the form of an engine or motor V5, through a structure of the hull VI, to the propeller V4. The vessel is provided with a rudder V6. The vessel also comprises a superstructure V7. Fig. 2 illustrates the propeller V4 and a part of the propeller shaft 1. The propeller in a variable pitch propeller which is well-known per se. A propeller hub 2 is attached to the propeller shaft 1. Propeller blades 3, herein more generally referred to as vessel control elements, are mounted in the propeller hub 2 for rotation about their mounting axes. For the purpose of changing the pitch of the propeller blades 3 there is provided a piston rod 6 which is connected in a known manner to the propeller blades 3, such that axial displacement of the piston rod 6 results in a commensurate change in the pitch angle of the propeller blades.

[0030] The piston rod 6 is connected to a control rod 10 which extends through the shaft 1.

[0031] An actuating system for the propeller blades 3 comprises a control module Cl described closer below. A pitch sensor SEI is arranged to provide to the control module Cl data corresponding to the pitch setting of the blades 3. An RPM sensor SE2 is arranged to provide to the control module Cl data corresponding to the rotational speed of the propeller shaft.

[0032] Fig. 3 illustrates a simplified hydraulic circuit for the actuating system.

[0033] An actuator in the form of a pitch change servomotor 12, 13 is provided for actuating the control rod 10. Displacement of the piston rod 6 is effected with the aid of the servomotor 12, 13.

[0034] As illustrated by way of example in Fig. 3, the auxiliary servomotor 12, 13 can be actuated by means of an actuator control device in the form of a control valve 31, which in the illustrated embodiment is electrically operated by means of the control module Cl. The control module is arranged to receive control commands from a maneuvering device 32, e.g. a lever provided in a control console, arranged on a bridge of the vessel.

[0035] The hydraulic system also incorporates two alternative pressure medium sources, in the form of hydraulic pumps, generally designated 33 and 34, respectively. Thus, the system includes an ordinary pressure-medium source 33, and also a pressure-medium source 34 which can be used as a reserve source should the ordinary pressure-medium source 33 become faulty. Alternatively, the pumps 33, 34 may work alternatingly so as to expose them to the same rate of wear over time. In some embodiments, one of the pumps may serve as a backup power reserve. Thereby, when the system needs a pressure that is above the capacity of one of the pumps, the pumps 33, 34 may be in operation simultaneously.

[0036] It should be noted that the invention is applicable to a variety of hydraulic propeller pitch adjustment systems. For example, instead of being mechanically linked to the blades and thereby arranged to adjust the blades directly, as in the example in fig. 2 and fig. 3, the actuator 12, 13 may be arranged to adjust the propeller blades via a hub mounted pitch change servomotor, as is known per se e.g. through W08700815A1.

[0037] With reference to fig. 4, a method for handling the actuator 12, 13 will be described.

[0038] Reference is made also to fig. 5. The method comprises controlling SI the actuator control device 31, in this example the control valve 31, so as to keep the blades 3 in a desired pitch position. In this example, the control valve 31 is proportional valve.

[0039] Based on a command by the operator for the speed of the vessel, the control module determines a desired position of the blades 3. The control module comprises a regulator. In a feedback loop an error of the blade pitch position PD-ERROR is determined as a difference between the desired position and the actual position, and the control valve 31 is adjusted to as to reduce the error. For this, the regulator comprises a pitch integrator, herein referred to as pitch integrator 1, which integrates time-sequential values of the blade pitch position error PD-ERROR, to produce an integrated blade pitch position error, herein referred to as a first integrated blade pitch position error PD-INT1. The integrated blade pitch position error PD-INT1 is added to the blade pitch position error PD-ERROR to produce a setting command SET for the control valve 31.

[0040] The method further comprises determining S2 a value of a first operational parameter in the form of a pitch setting of the blades 3, and determining S3 a value of a second operational parameter in the form of a rotational speed of the propeller shaft 1. The control module stores, for each of the shaft rotational speed and the pitch setting, a plurality of predetermined intervals of values. Each pair of intervals of operational parameter values, in this example shaft rotational speed and pitch setting values, can be said to represent an operational condition. For example, the control module may store ten such pairs of intervals. Each interval pair may be correlated with a control device setting determinator. In fig. 5 some control device setting determinators are represented as INTI, INT2, etc. Thus, in this example, based on the determined shaft rotational speed and pitch setting values one of the control device setting determinators INTI is selected S4. The selected control device setting determinator INTI is correlated with intervals including the determined values of the shaft rotational speed and the pitch setting.

[0041] The selected control device setting determinator INTI comprises a subtractor and a further pitch integrator, herein referred to as pitch integrator 2 which are arranged to produce based on the first integrated blade pitch position error PD-INT1 a second integrated blade pitch position error PD-INT2. More specifically, the produced second integrated blade pitch position error PD-INT2 is fed back to be subtracted from the first integrated blade pitch position error PD-INT1 to produce a position error difference PD-DIFF, and the pitch integrator 2 integrates time-sequential values of the position error difference PD-DIFF, to produce the second integrated blade pitch position error PD-INT2. Thereby, the control device setting determinator INTI forms a filter producing a mean value of the first integrated blade pitch position error PD-INT1.

[0042] The second integrated blade pitch position error PD-INT2 is multiplied by a gain factor PGAIN to produce a storage value SET-STORE of the setting of the actuator control device, in this example the control valve 31. Thereby, an actuator control device setting is determined S5 and stored S6 so as to be correlated with the determined values of the shaft rotational speed and the pitch setting.

[0043] It should be noted that in some embodiments, a filter including the pitch integrator 2 can be omitted, and the setting command SET for the actuator control device 31 may be stored so as to be correlated with the determined values of the shaft rotational speed and the pitch setting.

[0044] Storage values SET-STORE of the actuator control device setting, produced in this manner are stored at regular time intervals, e.g. once an hour. For this the steps S1-S6 described above are repeated. Thereby, as the control of the actuator control device 31, so as to keep the vessel control element 3 in a desired position, continues further settings of the actuator control device 31 are determined. If further values the shaft rotational speed and the pitch setting are within the parameter intervals of the previously selected control device setting determinator INTI, the further settings of the actuator control device 31 are stored correlated with these intervals.

[0045] The stored actuator control device settings could indicate an increased leakage of hydraulic fluid due to wear, e.g. of the actuator 12, 13. Thus, the stored settings of the actuator control device 31 may be used to detect an increase in the flow of hydraulic fluid required to maintain the vessel control element 3 in a desired position. Detecting this change in the flow can be used to schedule a maintenance service or sound an alarm when the flow exceeds a threshold value, as exemplified below. For this, the most recently stored actuator control device setting, and or changes in the stored actuator control device settings may be used.

[0046] In the embodiment depicted in fig. 4, a trend of the stored actuator control device settings in a time domain is determined S7, in dependence on the stored actuator control device settings, all correlated with the same intervals of values of the shaft rotational speed and the pitch setting.

[0047] Further, using reference actuator control device settings, for each control device setting determinator INTI, INT2 an amount of leakage of the hydraulic fluid supplied to the actuator 12, 13 may be determined S8. More specifically, for each control device setting determinator INTI, INT2 a reference actuator control device setting is determined as a setting of the actuator control device 31 when the actuator control device is controlled so as to keep the blades 3 in the desired pitch setting, and when there is no leakage of the hydraulic fluid supplied to the actuator 12, 13. One or more of the determined actuator control device settings which are stored in correlation with the respective control device setting determinator INTI, INT2, are compared with the respective reference actuator control device setting. In dependence on the comparison, the amount of leakage of the hydraulic fluid supplied to the actuator 12, 13 is determined.

[0048] The amount of leakage can be expressed as a value of an absolute parameter, e.g. liters per hour, or a relative parameter, e.g. a percentage of the maximum flow capacity of the system.

[0049] The determined trend and / or the amount of leakage may be used for a variety of purposes. For example, based on the trend and / or the amount of leakage, a decision may be made to perform a vessel maintenance service operation involving the actuator 12, 13. For example, if the time derivative of the stored actuator control device settings, or the amount of leakage, exceeds respective first predetermined threshold values S9, S10, it may be decided to replace SI 1 the actuator 12, 13.

[0050] As a further example, if the time derivative of the stored actuator control device settings, or the amount of leakage, exceeds a predetermined threshold value, an alert signal for an operator of the vessel may be provided. In another example, if the time derivative of the stored actuator control device settings, or the amount of leakage, exceeds respective second predetermined threshold value SI 2, SI 3, which may be lower than the respective first threshold values, the additional pump 34 (fig. 3) for the supply of hydraulic fluid to the actuator 12, 13 may be activated S14.

[0051] The comparisons of the time derivative of the stored actuator control device settings or the amount of leakage with respective threshold values is preferably repeated periodically.

[0052] In addition, a stored actuator control device setting may be used to improve the actuator control. More specifically, the vessel may be maneuvered so that the shaft rotational speed and the pitch setting changes. Thereby, a stored actuator control device setting correlated with intervals including the changed the shaft rotational speed and the pitch setting, may be used, e.g. feed forwarded, for the control of the actuator control device 31 in the new operational condition of the vessel.

[0053] With reference to Fig. 6, a schematic block diagram of embodiments of the control module Cl of fig. 4 is shown. The control module Cl, such as a computer, a processing device, an automation control unit etc., may be comprised in the vessel MV, the hull structure, or the like.

[0054] The control module Cl may comprise a processing module 401, such as a means for performing the methods described herein. The means may be embodied in the form of one or more hardware modules and / or one or more software modules. The term “module” may thus refer to a circuit, a software block or the like according to various embodiments as described below.

[0055] The control module Cl may further comprise a memory 402. The memory may comprise, such as contain or store, instructions, e.g. in the form of a computer program 403, which may comprise computer readable code units.

[0056] According to some embodiments herein, the control module Cl, e.g. the processing module 401 thereof, comprises a processing circuit 404 as an exemplifying hardware module.

[0057] Accordingly, the processing module 401 may be embodied in the form of, or ‘realized by’, the processing circuit 404. The instructions may be executable by the processing circuit 404, whereby the control module Cl is operative to perform a method of an embodiment of the invention. As another example, the instructions, when executed by the control module Cl, e.g. the processing module 401 thereof, may cause the control module Cl to perform a method of an embodiment of the invention.

[0058] Fig. 6 further illustrates a carrier 405, or program carrier, which provides, such as comprises, mediates, supplies and the like, the computer program 403 as described above. The carrier 405 may be one of, or a combination of, an electronic signal, an optical signal, a radio signal and a computer readable medium.

[0059] In further embodiments, the control module Cl, e.g. the processing module 401 thereof, may comprise one or more of a directing module 410 and a running module 420 as exemplifying hardware modules. The term “module” may refer to a circuit when the term “module” refers to a hardware module. In other examples, one or more of the aforementioned exemplifying hardware modules may be implemented as one or more software modules.

[0060] Moreover, the control module Cl, e.g. the processing module 401 thereof, may comprise an Input / Output module 406, which may be exemplified by a receiving module and / or a sending module when applicable. The receiving module may receive commands and / or information from various devices, and the sending module may send commands and / or information to various devices. Fig. 7 shows a system in an alternative embodiment, which system is similar to the one described with reference to fig. 1 - fig. 6, but with the following difference: The actuator control device is an axial piston pump 31. The output pressure of the pump is proportional to the rotational speed of the pump. The pump can operate at low speeds all the way to standstill. By changing rotational direction of the pump, the output to the actuator 12, 13 can be reversed.

[0061] In embodiments of the method where such an axial piston pump provides the function of the actuator control device 31, determining a setting of the actuator control device 31 while the actuator control device 31 is controlled to keep the vessel control element 3 (fig. 2) in the desired position, may comprise determining the rotational direction and the rotational speed of the pump 31. Thereby, the rotational direction and the rotational speed of the pump 31 may be stored so as to be correlated with the determined operational parameter values, e.g. the pitch position and the propeller rotational speed.

[0062] Fig. 8 shows a system in a further embodiment, which system is similar to the one described with reference to fig. 1 - fig. 6, but with the following differences: The vessel control element, the position of which the actuator 12, 13 is arranged to adjust, is a hydraulically controlled rudder V6 of a marine vessel. The maneuvering device 32 from which the control module is arranged to receive control commands, may be a steering wheel arranged on a bridge of the vessel.

[0063] Thereby, the operational parameters, values of which are determined in embodiments of the invention, may be the vessel speed and the rudder position or deflection. Thus, embodiments of the method with such a system may comprise determining an actuator control device setting while the actuator control device 31 is controlled to keep the rudder V6 in a desired position, and storing the determined actuator control device setting so as to be correlated with values of the vessel speed and the rudder position also determined while the actuator control device 31 is controlled to keep the rudder V6 in the desired position.

Claims

CLAIMS1. A method for handling a hydraulic actuator (12, 13) of a marine vessel (MV), which actuator (12, 13) is arranged to adjust a position of a vessel control element (3) for controlling movements of the vessel, wherein a supply of hydraulic fluid to the actuator (12, 13) is controlled by an actuator control device (31), the method comprising controlling the actuator control device (31) so as to keep the vessel control element (3) in a desired position, determining one or more values of one or more respective operational parameters which influence a force or a moment to which the vessel control element (3) is subjected during the step of controlling the actuator control device (31) so as to keep the vessel control element (3) in the desired position, determining a setting of the actuator control device (31) while the actuator control device (31) is controlled to keep the vessel control element (3) in the desired position, storing the determined actuator control device setting so as to be correlated with the determined operational parameter values, and- using the stored actuator control device setting in a subsequent operational event of or for the actuator (12, 13).

2. A method according to claim 1, wherein the vessel control element (3) is a blade of a propeller of the vessel, and the actuator (12, 13) is a hydraulic pitch change servomotor coupled to the blade.

3. A method according to claim 2, wherein the one or more operational parameters comprise a pitch setting of the blade.

4. A method according to any one of claims 2-3, wherein the one or more operational parameters comprise a rotational speed of the propeller or a rotational speed of a part of a drivetrain for the propeller.

5. A method according to any one of the preceding claims, comprising providing, for each of the one or more operational parameters, a plurality of predetermined intervalsof values of the respective operational parameter, and selecting, for each of the one or more operational parameters, a respective one of the predetermined intervals, which selected predetermined interval includes the determined value of the respective operational parameter, wherein storing the determined actuator control device setting so as to be correlated with the determined operational parameter values is done by storing the determined actuator control device setting so as to be correlated with the one or more selected predetermined intervals of the operational parameter values.

6. A method according to claim 5, wherein using the stored actuator control device setting comprises, in a subsequent operational condition of the vessel, determining one or more further values of the respective one or more operational parameters, and, if the operational parameter values are within the respective selected predetermined intervals, using the determined actuator control device setting for a further control of the actuator control device (31) so as to keep the vessel control element (3) in a desired position.

7. A method according to any one of claims 5-6, comprising determining, for one or more of the predetermined intervals of values of the respective operational parameter, a respective reference actuator control device setting as a setting of the actuator control device (31) when the actuator control device (31) is controlled so as to keep the vessel control element (3) in the desired position, and when there is no leakage of the hydraulic fluid supplied to the actuator (12, 13), comparing the stored determined actuator control device setting with the reference actuator control device setting, and determining in dependence on the comparison an amount of leakage of the hydraulic fluid supplied to the actuator (12, 13).

8. A method according to any one of claims 5-7, wherein the method comprises, subsequently to storing the determined actuator control device setting, providing a further control of the actuator control device (31) so as to keep the vessel control element (3) in a desired position, determining one or more further values of one or more respective operational parameters which influence a force or a moment to which the vessel control element (3) is subjected during the step of controlling the actuator control device (31) so as to keep the vessel control element (3) in the desired position,determining a further setting of the actuator control device (31) during the further control of the actuator control device (31) so as to keep the vessel control element (3) in the desired position, if the further operational parameter values are within the respective selected predetermined intervals of the operational parameter values, storing the further determined actuator control device setting so as to be correlated with the one or more selected predetermined intervals, determining in dependence on the stored actuator control device settings, a trend of the stored actuator control device settings in a time domain.

9. A method according to any one of claims 7-8, wherein using the stored actuator control device setting comprises, based on the amount of leakage and / or the determined trend, deciding on a vessel maintenance service operation involving the actuator (12, 13).

10. A method according to any one of claims 7-9, wherein using the stored actuator control device setting comprises, based on the amount of leakage and / or the determined trend, providing an alert signal for an operator of the vessel.

11. A method according to any one of claims 7-10, wherein the supply of hydraulic fluid to the actuator (12, 13) is powered by a pump, wherein using the stored actuator control device setting comprises, based on the amount of leakage and / or the determined trend, activating an additional pump for the supply of hydraulic fluid to the actuator (12, 13).

12. A computer program, comprising computer readable code units which when executed on a control module causes the control module to perform the method according to any one of the preceding claims.

13. A carrier comprising the computer program according to the preceding claim, wherein the carrier is at least one of an electronic signal, an optical signal, a radio signal, and a computer readable medium.

14. A control module configured to perform the method according to any one of claims 1-11.

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