OF A VESSEL

The invention relates to a method for the dynamic positioning of a vessel, in which the water-jet propulsion system includes two control blocks each comprising at least one water-jet propulsion device together with an engine, and a control unit for controlling the control block, in which each water-jet propulsion device includes a nozzle and a reversing deflector, in which method the vessel is steered automatically in the following stages,

- the desired position and heading are selected for the vessel,

- the vessel's momentary position and heading is de termined,

- the desired position and the momentary position are compared to determine the position deviation,

- the desired heading and momentary heading are compared to determine the heading deviation,

- with the aid of the control unit, a thrust vector, which is needed to steer the vessel to the desired position and heading, is defined on the basis of the position deviation and the heading deviation,

- the thrust vector is divided into water-jet propulsion device-dependent thrust vector components,

- on the basis of the thrust vector components, control block control commands are formed for each control block for each of the water-jet propulsion devices, and

- the vessel is steered on the basis of the control commands of each water-jet propulsion device's nozzles and reversing deflectors.

The invention also relates to a corresponding water-jet pro pulsion system. Publication EP 2024226 Bl, which discloses a system and method intended to steer a vessel equipped with two or more water-jet propulsion devices, is known from the prior art. The steering of the vessel of the publication takes place only and solely using water-jet propulsion devices located on the stern, without bow thrusters or anything similar. The vessel can be steered au tomatically on the basis of the desired position, in such a way that the vessel remains in place at the desired position against external forces. In this context, the term external forces refers, for example, to waves, currents, and winds, which seek to move the vessel away from the desired position. The external forces can also be, for example, a water cannon used in a fire boat or the movement of a cargo, liquids, or bilge water inside the vessel. Automatic steering to hold a vessel in place is generally known by the term "dynamic positioning", i.e. DP positioning.

In the aforementioned publication, the dynamic positioning uses two control blocks, in each of which blocks there is one or more water-jet propulsion devices on each side of the vessel's centre-line. In terms of the dynamic positioning, the only independently adjustable variable is the attitude of the wa ter-jet propulsion device's reversing deflector. In addition to this, collectively block-dependently adjustable variables are the attitudes of the nozzles guiding the heading of both control blocks water-jet propulsion devices' water jets, which are adjusted in a synchronized manner, i.e. jointly in both control blocks. A further adjustable variable is also the rpm of the engines of both control blocks' water-jet propulsion device, but the engines' rpm are partly bound to each other for reasons that will be explained later. The vessel's movement forwards or backwards can be easily implemented using the reversing de flector, but the vessel's lateral movement without rotating the vessel demands the combined use of the water-jet propulsion devices, in such a way that the desired heading of the pro pulsion-force vector is obtained as the vector computation's sum.

Figures la and lb show a situation, in which it is wished to move a vessel 100 only to port without rotation, using the control system and method of the prior art according to publication EP 2024226 B1. The lateral thrust vector of the water-jet propulsion devices 12 (shown in Figure la as a simplified ball at the vessel's stern) is created by rotating the nozzles in such a way that water- et propulsion devices' 12 water jets 40 point laterally in the heading of the desired thrust vector F and through the vessel's 100 centre of gravity 42. With the aid of the thrust vector F acting through the centre of gravity the movement of the vessel only in the desired heading is achieved, without creating a moment that rotates the vessel. So that the forwards-backwards directed components of the thrust vector are cancelled to hold the vessel 100 in place in a forward-backward heading, the control-blocks' water-jet propulsion device or device's 12.1 on the vessel's port side are used with the aid of the reversing deflector to direct power backwards and the control block's water-jet propulsion device or devices 12.2 on the vessel's starboard side are in turn used to direct power towards the vessel's bows. Because the effective operating efficiency of the water-jet propulsion device when using the reversing deflector is considerably lower than when running without the deflector, the poorer efficiency of the water-jet propulsion device when using the reversing deflector must be compensated by using higher engine rpm, so that the force vectors directed both forwards and backwards cancel each other and movement takes place only in the vessel's lateral heading.

The rpm ranges in the different headings and magnitudes of the thrust vectors of the different control blocks of the steering methods of the prior art are shown by the rpm curves 44 and 46 of Figure lb.The inner, smaller rpm curve 46 depicts the rpm range of the starboard-side water-jet propulsion device 12.2 of Figure la, whereas the outer, larger rpm curve 44 depicts the rpm range of the port-side water- et propulsion device 12.1 using the reversing deflector. The rpm ranges of the engines of the wa ter-jet propulsion devices of the different control blocks are bound to each other, so that the components of the for- wards-backwards directed thrust vector are cancelled, thus achieving, however, the desired lateral thrust vector to port.

However, problems are associated with such a control imple mentation. In a significant seaway, holding the vessel stationary requires rapid control commands and controlling changes to the water-jet propulsion device. The control system based on con trolling the engine's rpm is then imprecise, as the increase in the engine's rpm has its own slowness. For example, when using diesel engines equipped with a turbocharger, the engine can be outside the effective rpm range of the turbocharger, in which case when raising the fuel feed the engine reacts slowly, the tur bocharger's boost pressures being low, until the increase in the turbocharger's rpm lifts the boost pressures. In other words, the increase in rpm is slowed by the so-called turbo lag.

Another problem related to the control is its dependence on the engine's rpm. There can be various accessories in vessels that take their operational power from the same engine as the water-jet propulsion device. An example of such is a fire-fighting vessel, the water cannon of which are operated with the aid of the power provided by the water-jet propulsion device's engines during the operation of the dynamic positioning. The operation of the water cannon can demand an increase in the engine's rpm, for sufficient power for both the water cannon and the dynamic positioning, when the increase in rpm in turn interferes with the vessel's steering by increasing the thrust vector. The invention is intended to create a method and system for the dynamic positioning of a vessel, i.e. to hold it automatically at a desired position and heading with the aid of a water-jet propulsion, independently of the rpm of the water-jet propulsion system's engines. The characteristic features of the method according to the present invention are stated in the accompanying Claim 1 and the characteristic features of the water-jet pro pulsion system are stated in the accompanying Claim 8.

This intention can be achieved by means of the method for the dynamic positioning of a vessel, in which the water-jet propulsion system includes a control block comprising at least one water-jet propulsion system together with its engine and a control unit controlling the control blocks, in which each water-jet pro pulsion device includes a nozzle and a reversing deflector. In the method, the vessel is controlled automatically in the following stages, in which the desired position and heading are selected for the vessel, the vessel's momentary position and heading are determined, the desired position and momentary position being compared to determine the position deviation and the desired heading and momentary heading being compared to determine the heading deviation. In addition, in the method on the basis of the position deviation and the heading deviation the thrust vector, which is needed to steer the vessel to the desired position and heading, is determined with the aid of the control unit, the thrust vector is divided into water-jet propulsion device dependent thrust vector components, to form on the basis of the thrust vector components separate control commands of the control blocks for each control block's water-jet propulsion device's nozzle and reversing deflector, in which the different control blocks' control commands at least for part of the time deviate from each other in the case of the control of the nozzles. Further, in the method the nozzles and reversing deflectors of each of the vessel's water-jet propulsion device are controlled on the basis of separate control commands, the engine's rpm remaining constant in terms of steering.

By controlling each control block's nozzle independently in addition to the reversing deflector the desired controlling thrust vector can be formed with the aid of the independent control of each control block's nozzle and reversing deflector without a need to alter the engine's rpm and, on the other hand, without a need for bow thrusters or similar added to the thrust devices to implement control. In other words, the controlling thrust vector can be achieved irrespective of the engine's rpm necessary to achieve the required thrust, when the rpm can be set to the desired level. The rpm can be raised, for example, according to the external power required, such as, for example, when operating a fire-fighting water cannon with the aid of the water-jet propulsion system's engine. In other words, a deviation is, if necessary, permitted between the different attitudes of the control blocks' nozzle, so that the required thrust vector can be formed, without actively changing the rpm of the water-jet propulsion device's engine.

In other words, in the method a separate control command is formed for each control block on the basis of the required controlling thrust vector, without altering the engine's rpm. The nozzle of each control block can then be controlled independently of the other control block.

In this context, the term thrust vector refers to the force created together by the water-jet propulsion devices, which has magnitude and heading. The thrust vector is the resultant vector of the thrust vector components, which acts on the vessel's centre of gravity, the thrust-vector components formed by the water-jet propulsion devices acting at the vessel's stern at a distance from its centre of gravity.

The thrust vector required to steer the vessel is preferably formed only with the aid of the water-jet propulsion devices attached to the vessel's stern.

According to one embodiment, the engine's rpm are kept constant while forming the control commands. A change in the rpm need not then be taken into account when computing the control commands.

Preferably the vessel is steered using only the nozzle and reversing deflector of the water-jet propulsion device of each control block.

Each control command can be computed in the following stages, in which the heading and magnitude of the controlling thrust vector required are determined on the basis of the position deviation and the heading deviation, the momentary rpm of each engine are determined, and the attitudes for the nozzle and reversing deflector of each control block's water-jet propulsion device are determined, in which the controlling thrust vector desired with the aid of the water-jet propulsion devices of both control blocks is achieved in the case of both heading and magnitude, computed according to the momentary rpm of the engines.

Pre-computed and recorded thrust functions for each of the engine's rpm are preferably used in the method, which thrust function comprises all the attitudes of the nozzle and reversing deflector as a result of the thrust vectors achieved as a result of the magnitudes of the thrust vectors achieved at the relevant rpm. This permits rapid computation of the control, as the amount of computation is reduced. The attitudes of the nozzle and reversing deflector of the water-jet propulsion device of each control block are preferably computed with the aid of the thrust function, on the basis of the thrust vector.Thus, the control unit can rapidly form the control command comprising the attitudes of the nozzle and reversing deflector implementing the desired thrust vector, using a simple thrust function without a need for real-time complex computation based on flow calculation.

Alternatively, functions modelling real flow dynamics can also be used in the method, which can be used in computation even in real time, if the computation capacity permits this.

In the method, the changes in attitude of the nozzle and reversing deflector are preferably implemented at the same speed of change. The vessel's movements and the accelerations acting on it during dynamic positioning are gentle and even and not pulse-like. More specifically, the changes in the operating points can optimally be made in such a way that the transition stages take place in a co-ordinated manner along a set of curves formed by optimal operating points. By this means more stable operation can be achieved with minimal wander and flutter, even though rpm may be used more fully.

In other words, in the method according to the invention the vessel is steered with the aid of the water-jet propulsion system without changing the engine's rpm. The engine's rpm can then be selected to be optimal in terms of the total power requirement.The engine's revolutions need not be actively adjusted to achieve operation of the method according to the invention. The levels of the rpm at any time are determined by the operating range and upper limit of performance. The system can be used, however at constant rpm, which can be the same or different in both control blocks. The aim is, however, in a normal case to use the lowest possible rpm optimally in terms of energy saving.

According to one embodiment, the control unit includes block controls for each control block, and in the method data on the engine's revolutions and the nozzle's and reversing deflector's attitudes of each control block's water-jet propulsion device is also transferred to the block control of the other control block, to duplicate control in case of a breakdown. Such a procedure improves the redundancy of the water-get propulsion system's control, i.e. the system's control still operates, even though an error occurs in the control of one block.

According to one embodiment, in the method, in both water-jet propulsion devices (12) nozzles (20) are used, the adjustment of which from one extreme position to the other takes 0.3 - 2.0 s, preferably 0.5 - 0.8s, and the control pressure of the water-jet propulsion system's nozzles (20) and reversing deflector is 20 - 200 bar.Thus the method reacts sufficiently quickly in steering and achieves high precision in dynamic positioning.

The intention of the water-jet propulsion system according to the invention can be achieved by a water-jet propulsion system for the dynamic positioning of a vessel, in which the water-jet propulsion system includes two water-jet propulsion devices and their engines, as well as a control unit controlling the water-jet propulsion devices, in which each water-jet propulsion device includes a nozzle and a reversing deflector and their operating devices. The control unit is arranged to perform the following operations to steer the vessel automatically on the basis of the desired position and heading, i.e. to determine the vessel's momentary position, to compare the desired position and the momentary position to determine the position deviation, to compare the desired heading and the momentary heading to determine the heading deviation, and on the basis of the position deviation and the heading deviation to determine the thrust vector, which is needed to steer the vessel to the desired position and heading. In addition, the control unit is arranged to divide the thrust vector into water-jet propulsion device-dependent thrust-vector components and on the basis of the thrust-vector components to form separate control-block control commands for each control block's water- et propulsion devices, in which the control commands of the different control blocks (16) differ from each other for at least part of the time in the case of the nozzle's (20) control. Further, the control unit is arranged to control the nozzles and reversing deflectors of each of the vessel's water-jet propulsion devices on the basis of separate control commands, the engine's rpm being constant in terms of control.

Such a water-jet propulsion system is very precise and especially advantageous for use in vessels in which there are external devices using the power of the water-jet propulsion devices, for which the engines' rpm must be raised. In the control of the nozzles and reversing deflectors there is no corresponding lag to the raising of the engine's rpm, so that control can be faster.

In other words, the control unit is arranged to control the water-jet propulsion devices without changing the engine's rpm. Such optimal revolutions can be selected according to the external effective factors, for example, according to the power re quirement of additional devices operated with the aid of the water-jet propulsion device and its engine.

The water-jet propulsion system is preferably arranged to form a thrust vector only with the aid of a water-jet propulsion device fitted to the stern of the vessel. According to one embodiment, the control unit includes separate block controllers arranged to receive the control commands of manual controllers belonging to the vessel and to translate the requests into control commands for the nozzles and reversing deflectors of each water-jet propulsion device, and an oper ating-device controller arranged to control the nozzle and reversing deflector of each water-jet propulsion device sepa rately on the basis of the block controller's commands, to steer the vessel to the desired position and heading. Using such an implementation the block control and operating-device controller can be different manufacturers'components, allowing more freedom in the implementation of the water-jet propulsion system.

The control unit preferably includes a separate system controller arranged to determine the position deviation and heading de viation and to receive the commands of the vessel's manual controller. With the aid of the separate system controller the calculation load of the block controllers can be reduced and data on the momentary position and heading collected in a single unit, which divides the data with the block controller of each control block.

According to an alternative embodiment, the block controller is arranged to determine the position deviation and the heading deviation and to receive the commands of the vessel's manual controller. The control unit can then be implemented without a separate system controller.

For each control block there is preferably its own block con troller and operating-device controller. The redundancy of the water-jet propulsion system can then be improved considerably, as the block controllers and operating-device controllers are doubled, when the vessel will be steerable even if a fault occurs in one of the control block's block controllers or operat ing-device controllers.

The control unit preferably includes a memory, in which there are predefined and recorded thrust functions for each of the engine's rpm, which thrust function comprises the magnitudes of the thrust vectors achieved at the relevant rpm as a result of all of the attitudes of the nozzle and reversing deflector, and the control unit is arranged to compute the attitudes of the nozzle and reversing deflector of each water-jet propulsion device of each control block on the basis of the necessary thrust vector with the aid of the thrust function corresponding to the momentary rpm of the water- et propulsion device's engine. Thus, only the attitudes of the nozzle and reversing deflector are computed in real time for each control block with the aid of a simplified thrust function, nor does the heavy and complex computation of the flow dynamics of the attitudes need to be performed in real time. The computation of the attitudes of the nozzle and reversing deflector is based on flow computation and demands a large amount of computing power, for the computation to be performed suf ficiently quickly for precise dynamic positioning.

According to one embodiment, an intermediate form of a model utilizing a thrust function and precise computation of the flow dynamics can be computation based on simpler averaged and more approximate mathematical functions, which are lighter and faster to calculate.

The operating devices are preferably hydraulic cylinder devices, the accuracy of which in the cylinder's stroke is 0.1 - 1.0 mm, if the stroke is 70 - 150 mm. Then by control of the nozzles and reversing deflectors of each control block sufficiently precise and rapid control can be achieved to hold the vessel in place automatically without control of the rpm of the water-jet propulsion system's engines.

The speed of adjusting a nozzle from one extreme position to the other can be 0.3 - 2.0 s, preferably 0.5 - 0.8 s, and the water-jet propulsion system can includes a hydraulic pump, the maxi mum-pressure level of which can be 20 - 200 bar, preferably 80 - 120 bar, and its normal operating range 5-30 bar. The control unit's control commands can then be implemented rapidly with the aid of the water-jet propulsion device, thus reducing the error in the operating response of the water-jet propulsion system due to the operating devices' delay.

The control unit preferably includes a PID-controller to form control commands, both in the x and y headings of the positions and in the vessel's z heading. The advantages of a PID controller are its rapid operation and low noise. In this context, the term the vessel's direction refers to the English-language term "heading".

Alternatively, the PID-controller can also use a Sliding Mode controller or a combination of a PID-controller and some other controller.

The control unit's internal calculation frequency in the case of the operating-device controller can be 0.5 Hz - 20 Hz, preferably 1 - 10 Hz. The control will then be sufficiently rapid to hold the vessel in place. At high computation frequencies it is particularly advantageous to use thrust functions, when the determination of the control commands of the attitudes of each water-jet propulsion device's nozzle and reversing deflector at the prevailing revolutions of the water-jet propulsion device's engine can be quickly implemented. The use of the method and water-jet propulsion system according to the invention achieves the vessel's controlled steerability in all situations, without actively changing the engine's rpm, so that the engine's rpm can be raised to a high level, thus permitting take-off of the engine's power for other purposes too than only the water-jet propulsion device, without interfering with the vessel's automatic dynamic positioning.

In the following, the invention is described in detail with reference to the accompanying drawings showing some embodiments of the invention, in which

Figures la and lb show the principle of the method according to the prior art,

Figure lc shows the principle of the method according to the invention, as shown in Figure lb,

Figure 2 shows schematically an embodiment of the wa ter-jet propulsion system according to the in vention,

Figure 3 shows a block diagram of the method according to the invention,

Figure 4a shows the vessel's deviation from the desired position and heading of the method according to the invention,

Figure 4b shows a block diagram of the principle of the dynamic positioning in greater detail,

Figure 4c shows the thrust vector achieved by both water-jet propulsion devices of the method according to the invention, and its division into con trol-block-dependent components,

Figure 4d shows the operation of the vessel's wa ter-jet-propulsion devices of the method ac cording to the invention, to achieve a desired thrust vector. With reference to Figure la and lc, the water-jet propulsion system according to the invention is intended to steer a vessel 100 automatically, to hold the vessel 100 in place at a desired position pi and heading zl, using the vessel's 100 water-jet propulsion device 12. The term vessel refers to a boat, ship, or other vessel moving on the surface of the water using water-jet propulsion devices, which is steered with the aid of a water jet created by the water-jet propulsion devices. According to Figure 2, the water-jet propulsion system 10 according to the invention comprises, in all embodiments, two control blocks 16, each comprising at least one water-jet propulsion device 12 with an engine 14, and a control unit 18 controlling the control blocks 16. Each water-jet propulsion device 12 includes a nozzle 20 and a reversing deflector 22 and their operating devices 28. In the water-jet propulsion system according to the invention, the vessel 100 is preferably steered only with the aid of water-jet propulsion devices 12 situated at the stern 48 of the vessel 100 according to Figure la, when the resultant thrust vector F steering the vessel is formed as the sum of only two thrust-vector components Fi and F2 formed by the water-jet propulsion devices 12 of the two control blocks.

In this context it should be understood that in each control block there can be one or more water-jet propulsion device, and the water-jet propulsion devices in each block are controlled block-dependently identically, but the control blocks are controlled independently. According to Figure 2, the water-jet propulsion devices 12 in one control block 16 are preferably equipped with their own engines 14 or in some cases a single engine can operate several water-jet propulsion devices in the same control block. Such an implementation is particularly possible when using electric motors. In some cases there can be even three control blocks, but then the third control block situated between the two control blocks acts only to boost the vessel's longi tudinal movement as a so-called "booster", without power com ponents in the vessel's lateral heading.

In this context, the term control unit 18 preferably refers to the totality of system controllers 25, block controllers 24, and operating-device controllers 26 shown in Figure 2, preferably included in the water-jet propulsion system. Alternatively, the control unit can also be a single unit, which performs both computation and the creation of control commands to the operating devices of the water-jet propulsion devices nozzles and con trollers.

In a preferred embodiment of the water-jet propulsion system 10 according to the invention, shown in Figure 2, the control unit 18 belonging to the system controller's 25 upper level control device is not essential from the point of view of the operability of the water-jet propulsion system according to the invention. Hereinafter, the name CDU (computing display unit) will be used for the system controller. The CDU is a monitoring tool, with the aid of which the operator sees the state of the water-jet propulsion system and from that the state of the individual water-jet propulsion devices in greater detail.At the same time, the CDU is advantageously a computing unit, which comprises a dynamic-positioning controller and receives the manual con trollers' 30 control requests, as well as position and heading data from the positioning device. The dynamic positioning performed by the CDU 25 can be bypassed using the manual con trollers 30, i.e. the steering wheel and the throttle lever.

The CDU receives an external GPS signal and calculates on that basis the x and y displacements relative to the desired position and heading, so that error can be minimized. In this context the positioning device is preferably a so-called Heading Receiver antenna, which has in it two GPS antennae for heading data. The antenna can be, for example, a Hemisphere V104 or its successor, in which acceleration and gyro sensors are used to stabilize hunting, when good performance will be obtained relative to the antenna's size. An alternative to the positioning device is an IMU unit using a magnetic field (flux compass) as a reference. The term the vessel's momentary position data p2 arriving from an external source can refer, for example, to a GPS signal or other similar positioning signal, on the basis of which the vessel's momentary position can be determined. POSE attitude data can possibly also be determined with the aid of the positioning device to improve precision.The determining of heading can also be based on a GPS signal or a separate compass. Other alternatives are, for example, the use of a high-performance inertia navigation system or headingal position data sent by external buoys or 5G base stations. Further, determination of heading can also be implemented, for example, by SLAM (Simultaneous Localization and Mapping), using for example a LIDAR laser scanner, marine radar, or an oblique echo-sounder.A second vessel or a shore base station may also be able to transmit the vessel's position data from outside it.

The method and water-jet propulsion system according to the invention aim at an accuracy, which in good weather in the order of 1 m on a vessel 10 - 15-m long, and 2 - 3 m in small waves. In other words, the automatic dynamic positioning is able to hold the vessel at the desired position and heading with the aforementioned accuracy.

The block controller 24 of Figure 2 is arranged to receive the control requests of the manual controllers 30 belonging to the vessel and to translate the requests into control commands to the operating-device controller 26. Hereinafter the name HCU (Helm Control Unit) is used for the block controller 24. If the control unit is implemented without a separate system controller, the block controller also takes care of the aforementioned tasks of the system controller. The operating-device controller 26 is in turn arranged to control the nozzle 20 and reversing deflector 22 of each water-jet propulsion device 12 on the basis of the HMU's 24 control commands separately to steer the vessel 100 to the desired position pi and heading zl of Figure la. Hereinafter the name JCU (Jet Control Unit) is used for the operating-device controller 26. The JCU's task is also to receive the sensor data obtained from the water-jet propulsion device, for example, on the nozzle's and reversing deflector's attitude and to forward it to the HCU. Because the HCU 24 controls the attitudes of the nozzle 20 and reversing deflector 22 of the water-jet propulsion devices 12 of each control block 16, the HCU 24 performs the control without affecting the rpm of each control block's 16 engines 14.

Each control block 16 has preferably its own HCU 24 and JCU 26 according to Figure 2, of which the JCU 26 preferably controls the operating devices of the same control block's water-jet propulsion devices in a centralized manner, if there is more than one water-jet propulsion device in the control block. In other words, the control unit can be partly doubled, to increase redundancy. This means that if one of the control block's HCUs fails, the vessel will still retain steerability, as the vessel can be steered on the basis of the other HCU. In other words, each HCU preferably computes, as a mirror image, the other control block's control commands in addition to its own control block's control commands, in case the other HCU fails.

If, however, the control unit is implemented using only a single HCU that computes the control commands for the JCUs of both control blocks, the HCU must have separate computation for the attitudes of the nozzle and reversing deflector for each control block, in order to implement the method according to the invention. Then, however, the failure of the HCU can cause failure of dynamic positioning and control must be transferred to manual con trollers.

Data transfer between the control unit 18 the water-jet propulsion device 12 takes place using a separate field bus 32. In the embodiment of Figure 2, the CDU 25 is connected over a field bus 32 to each control block's 16 HCU 24, which in turn is further connected over the field bus 32 to the same control block's 16 JCU 26. The field buses 32 are preferably independent in that each control block 16 has its own field bus to improve redundancy. The field bus is preferably a CAN bus, but can also be some other similar bus suitable for the purpose. The reference number 33 shows an auxiliary bus, which is an "intermediate bus" joining the independent HCU-JCU power lines, which permits, among other things the co-ordination of manual steering and DP steering.Both data transfer and current transfer from the batteries 34 through the switches 36 to the control unit 18 take place over the field bus 32 and the auxiliary bus 33. Preferably each control block 16 has its own battery 34 and switches 36 to increase redundancy. Each water-jet propulsion device 12 can be an independent bus unit on the field bus 32. In addition to an engine the water-jet propulsion device preferably includes, according to the prior art, one or two hydraulic cylinders and valve manifolds for controlling the hydraulic flows adjusted by them, which are not shown in this connection.

Next is the principle of the method according to the invention and referred to in Figures 3 and 4a - 4d, which consists of two parts. The first part is needed to determine the magnitude and heading of the necessary thrust vector and is generally known dynamic positioning, the stages of which are in the dynamic positioning control block 214. The core of the invention rep- resents independent control-block-dependent steering, in each of which control blocks the reversing deflector and nozzle of the water-jet propulsion device of each control block is controlled independently. The principle of dynamic positioning is presented, for example, in the article published in 1996 "Control and Stabilization of an Underactuated Surface Vessel" (Mahmut Reyhanoglu; Proceedings of the 35th Conference of Decision and Control; Kobe, Japan, December 1996).

Apart from the control of the water-jet propulsion system, the computation of dynamic positioning is computation known from the prior art, in which the departure point is the desired position pi and heading zl of the block 206 given in stage 202 using the operator's manual controllers, by which the vessel is sought to be held in place. As a second departure point, are the vessel's momentary position p2 and heading z2, shown in block 208, which are obtained, for example from GPS positioning, in section 204. The vessel's desired position pi and heading zl, as well as its momentary position p2 and heading z2 are also shown in Figure 4a. By comparing in stage 210 the vessel's desired position pi and its momentary position p2 the position deviation Dr is formed, and by comparing the vessel's desired heading zl and its momentary heading z2, its heading deviation Dz is formed. The position deviation and the heading deviation are the basis of computation of the water-jet propulsion system's control.

In Figure 4a, the difference of the position data's x and y co-ordinates' desired and momentary positions are shown by the markings e_x and e_y. In addition, other factors such as wind can also be taken into account in the computation, and can be compensated with the aid of filters. In stage 212, the control unit computes of the basis of these data the magnitude of such a thrust vector F, that will hold the vessel in place at the desired position and heading, or move the vessel back to an initial position and heading according to Figure lc. The same thrust vector F is preferably selected for both control blocks 16 for the control of the water-jet propulsion devices.

To improve the precision of control and reduce noise, the control unit's computation preferably include a PID controller for each channel, i.e. heading, which acts as an error-correcting control algorithm in relation to the vessel's transverse x-heading, the longitudinal y-heading and the vessel's rotational angle, i.e. z-heading. The controller is preferably a PID controller.

Next the principles of the method and water-jet propulsion system according to the invention is explained with reference to Figures 4a - 4d. Naturally moving a vessel using water-jet propulsion devices to move straight forwards or backwards is easily im plemented by using the water-jet propulsion devices, the nozzles of the water-jet propulsion devices being completely parallel to the vessel and using or not using the reversing deflector de pending on the heading of travel. The method according to the invention is particularly advantageous in a situation in which the dynamic positioning detects the vessel 100 as having moved laterally, i.e. in a transverse heading to the longitudinally heading between the vessel's bow and stern, from the desired position pi. If the necessary thrust vector F comprises the vessel's 100 transverse force component, the transverse force component must be achieved mainly by the vessels' 100 longi tudinally water-spraying water-jet propulsion devices 12. When it is also taken into account that the desired thrust vector must be of the correct size in the vessel's longitudinal heading, the control block's water-jet propulsion devices must be used to direct opposite forces in the vessel's longitudinal heading to achieve a transverse force of the correct magnitude. The dynamic positioning's detection is based on the position deviation Dr and the heading deviation Dz, which are preferably sent from the CDU to the HCU, which computes the necessary thrust vector F, by which the vessel is brought to the desired position pi and heading zl. If there is no need to change the vessel's heading, the thrust vector should run through the water-jet propulsion device's power point and the vessel's centre of gravity, so that a lateral force is created in the vessel, which does not, however, turn the vessel's heading, but only moves the vessel laterally. If it is desired to change the vessel's heading, the thrust vector is then aimed past the vessel's centre of gravity to the desired rotation heading. The theory of steering taking place with the aid of the power vector through the centre of gravity works especially when the speed relative to the water is nearly zero. On a viscose base the vessel then naturally tends to rotate around its total centre of gravity. If the difference in movement of the vessel and the water increases, the vessel's bottom's hydrodynamic properties may complicate the matter. Such a situation can be, for example, when seeking to remain stationary in a flowing river. The aim is to be stationary relative to the shore, though the vessel may be moving relative to the water. It may be necessary to compensate such situations.

When computing the thrust vector, the desired thrust vector is divided as a vector equation into its vessel-heading and transverse-heading thrust-vector components in the case of each control block. This is shown in stages 216 and 222 of Figure 4b. When using the reversing deflector, the efficiency of the wa ter-jet propulsion device is much poorer than when running without the reversing deflector. For this reason, the magnitude of the control block's lateral vector of the control block operating the reversing deflector must first be determined at the water-jet propulsion device's momentary rpm, and then compensate the final heading and magnitude of the thrust vector by setting principally the attitude of the nozzles and reversing deflector of the control block operating without a reversing deflector.This also achieves a better immediate nozzle response, because the nozzle's sen sitivity to lateral forces is greater, in other words a change in the heading of the nozzle changes in practice the heading of the steering vector to a higher degree.

In practice the reversing deflector is preferably always op erating to some extent, because the reversing deflector is always then "in the operating range" without a discontinuity. This is more advantageous than using the deflector at full for a small moment of time.Here a special situation is thrust full ahead using two jets towards the target point, but even then the adjustment is preferably made, so that the reversing deflectors slightly "brake".

The reversing deflector can also be used to deviate part of the flow of the water-jet propulsion device in the opposite heading, when using most of the flow with the reversing deflector partly in the reversing attitude. In this connection, the reference to reversing means creating such a force that will create a force pushing the vessel in the reversing heading, even if the vessel does not move in the reversing heading. On the basis of the momentary rpm of the water-jet propulsion device's engine the force in the vessel's heading created by each control block and laterally with the different attitudes of the nozzle and reversing deflector can be computed.

A role in dynamic positioning depending on the installation position is preferably defined for the water-jet propulsion device or water-jet propulsion devices of both control blocks, which is in practice thrusting in the reversing heading in the case of one control block, and thrusting in the heading of the bows in the case of the water-jet propulsion devices'of the other control block. Computation is started from the control block operating the reversing deflector and then repeated for the other control block, so that with the aid of two independent control blocks the necessary thrust vectors is obtained, without altering the engine's rpm of water-jet propulsion devices'engine of either control block for steering.

More specifically, for computation the thrust-force functions, for example, of the block control 24 of Figure 2 can be pre-defined in the memory 38, which present the heading and magnitude of the thrust vector for all the attitudes of the nozzle and reversing deflector, each thrust-force function for the engine's different rpm ranges. The thrust functions 232 or tables in question are shown in stages 228 and 230. In other words, each table contains the magnitude of the force created at each rpm at all the different attitudes of the nozzle and reversing deflector and their combinations. Computation of the forces'magnitudes can be based, for example, on flow calculation, which can be quite difficult to perform in real time. It is therefore preferable that for computation of the magnitudes of the forces a table of simplified mathematically discrete matrix functions, i.e. thrust function, is defined already through complex flow calculations, when the computation power of the HCU can be considerably smaller than if the calculation was to be performed in real time based on flow calculation. Alternatively, the computation can also be performed in real time based on flow calculation, if the HCU's computation power and other properties permit this.

Preferably on the basis of the needed steering thrust vector, using the thrust function the control commands for the attitude of the nozzle are computed on the basis of the momentary rpm of the engine of the water-jet propulsion device of each control block 16, according to stages 218 and 224 of Figure 3. The aforementioned computation takes place preferably in the control unit's HCU, i.e. each control block's HCU performs the computation separately. Thus, the necessary control commands are computed for each control block's JCU. The magnitude of the thrust vector is affected by the attitude of the reversing deflector and nozzle, so that the greater the attitude used, the greater the force created. In other words, the more the reversing deflector is lowered while the nozzle is in the centre, the greater the thrust vector's component in the vessels' longitudinal heading is created. I.e., if the reversing deflector is fully raised, a maximum thrust forwards is created, the reversing deflector being in the middle the force is zero, and being fully lowered maximum pull is created, i.e. a maximum force in the reverse heading. If the nozzle is deviated by, for example, 25 degrees (maximum deviation) and the reversing deflector is raised, a maximum thrust in the heading 25 degrees is implemented. If the nozzle is still deviated by 25 degrees and the reversing deflector is begun to be lowered, the heading of the thrust vector will exceed the 25 degrees, until the deflector is in the middle attitude, when the thrust vector will be 90 degrees to the side and the efficiency considerably reduced.

Because the water-jet propulsion device's rpm can be selected freely in terms of the vessel's steerability between the pre vailing minimum rpm and the engine-dependent maximum rpm, if necessary each engine's rpm can be selected independently to be quite high, so that power will suffice for the vessel's ac cessories, in addition to the dynamic positioning. Then a small thrust vector is then needed, and the attitudes of the reversing deflector and the nozzle should the limited to be quite small. The power vector pushing forwards of the water-jet propulsion device must preferably be limited by limiting the angle of the nozzle and the reversing deflector, so that the power vector will not be too great. Once the necessary attitudes of the reversing deflector and nozzle to create the necessary steering thrust vector have been defined in stages 228 and 230 for the water-jet propulsion devices of both control blocks, control commands are sent to the JCUs of both control blocks.

The water-jet propulsion system according to the invention can be implemented in connection with existing vessels by making the necessary changes to the existing equipment.Most preferably, the invention is implemented in connection with new vessels.

If the water-jet propulsion devices'engine's rpm are unnecessary high in terms of the dynamic positioning's request, the power request can nevertheless be met by suitably limiting the angles of the nozzle and the reversing deflector. The power output laterally is about 1/10 of that in the forward-backward heading and for reasons of efficiency the maximal backwards directed power only 40 % of the forwards directed.

According to one embodiment, the water-jet propulsion system according to the invention can be implemented using, for example, Bosch Rextroth's valve manifolds and the HCU and JCU can be implemented using, for example, some of the following components on the market: CrossControl CrossFire SX, CrossControl CrossCore XM, BOSCH RCIO-IO Series 31, Bosch BODAS RC4-5/30, Bosch BODAS RC40, Epee 5050 Control Unit, or Exertus HCM2010S. The CDU can be, for example, a Bosch BODAS DI 4 display, an Epee 6107 display unit, an Epee 6112 display unit, CrossControl VC, CrossControl CCpilot x900, or an Exertus MID2170S Multi Information Display. In principle, the JCU or HCU controller can be any programmable device controlling proportional valves, which has sufficient computation power and the necessary I/O-interfaces for sensors and field buses. Such devices can be, for example, the appropriate products of the aforementioned suppliers.

The CDU's computation platform can be compared to any platform whatever that runs, for example, Codesys-, C++-, or C-code, (generally algorithms), which communicates with the field buses. The logic can be designed on a model base using the (MDB)principle in a Matlab/Simulink environment, from which the algorithm itself is generated to form code. This code can be included in the controllers, in which the necessary physical outputs and inputs are formed.