WO2013070083A1

WO2013070083A1 - Method and device to dampening of roll movements of a ship

Info

Publication number: WO2013070083A1
Application number: PCT/NO2012/000062
Authority: WO
Inventors: Eirik Hellesvik
Original assignee: Eirik Hellesvik
Priority date: 2011-10-26
Filing date: 2012-10-26
Publication date: 2013-05-16
Also published as: BR112014010167B1; NO20111444A1; BR112014010167A2; WO2013070083A9; NO336785B1

Abstract

A method is described for the dampening of the roll movements of a ship at sea, said ship comprises one or more, in the main, transversely placed, partially filled with liquid, roll dampening tanks, where the liquid in the tank is set up to swing in antiphase with the roll movements of the ship to hinder that the ship rolls, where a measuring and a control system is used to register antiphase and possible deviations from said antiphase and the amount of liquid in the tank is regulated to achieve said antiphase. The method is characterised in that the height of the liquid up along the wall surfaces of the tank, in particular along the end wall surfaces, is measured and registered at any time with the help of one or more sensor elements, said height measurement data from the one or more sensor elements is registered in a monitoring system that determines said deviation from the ideal antiphase, and on the basis of said measurement data the system carries out a regulation of the amount of fluid in the tank to achieve said antiphase. Also described is a device for carrying out the method.

Description

METHOD AND DEVICE FOR DAMPENING OF ROLL MOVEMENTS OF A SHIP The present invention relates to a method and a device for a roll dampening tank in a ship where the tank is set up to contain an amount of water which, during swinging movements in the tank and in antiphase with the movements of the ship, can prevent the ship from rolling in the sea and where the system is set up to fill the tank with water when the wave comes late and to let out liquid from the tank when the wave comes early. With the present invention one aims to provide that the time difference between the swing movements of the water and the rolling movements of the ship is equal to zero.

Many solutions to these principles are known, with reference being made to the applicant's own Norwegian application 20092940 and International Patent application PCT/NO10/000297 that are considered to be closest to the present invention as it will be described in the following.

Patent documents that describe the relationship between the wave pattern in the tank and the rolling movements of the ship to achieve optimal antiphase between these are previously known. Reference is made to US-3,915,109, WO201 1/016730, JP1 1 -51745, DE-30 02 1 18, CN-101514915, US-3,968,353, US-3,521 ,593, US-3,580,205 and DE-19 16 587 and the publication Ship Safety Bulletin 01/2005, TP 3231 E. Transport Canada, 2005.05.18, JP- 20062191 14 and also Palazzl, L. de Kat, Model Experiments and simulation on a damaged ship with air flow taken into account.

With the exception of the applicant's own two mentioned patent publications, there are no other known solutions that use this knowledge to regulate the amount of water in the tank all the time.

The previous PCT application of the applicant relates to registering the amplitude in the wave pattern along the length of the tank, and thereby computer model when it is probably in perfect antiphase, and then so that water shall be filled into the tank when the wave arrives late and be let out from the tank when the wave arrives early in relation to this antiphase. To achieve this, an appliance to take pictures, such as a camera inside the tank, is used to register how this antiphase/amplitude is set, i.e. too early or too late. Also described is that the tank shall be painted internally in bright colours and be lit up so that the camera can "see" and register the wave pattern, i.e. the amplitude along the whole length (transverse) of the tank.

About roll dampening tanks in general

Roll dampening tanks are described as a passive system for the dampening of the rolling movements mainly about the longitudinal axis of the ship. It has been shown that this is a dynamic system which must be adjusted to what exists of stability/trim in the ship, in addition to the wave frequency. Today, this is impossible, in particular on large vessels with several roll dampening tanks. To be on board in such large vessels and to be able to feel on the ship as to when the time for these tanks is correctly adjusted and "synchronised" is not only difficult, but nearly impossible. In the stability papers of ships with roll dampening tanks 1 or 2 conditions with a given amount for filling of these tanks are given. These can only be considered to be examples and it may be that these examples will work in practice in said conditions but in all other conditions they will at best work poorly and in the worst case, work against their intention. Consequently, there are no systems for continuous monitoring of such tanks to use the features of the roll dampening tanks optimally. This means that to a large extent the incorrect amount of liquid is used in the tanks. However, this constitutes a safety risk for the crew as the ship will roll more. This constitutes a risk for damage to the ship and a risk for damage to the cargo and crew.

The relationship between the wave movements of a body of water - impacts up along the walls of the tank - and volume relative to the rolling of the ship, is that the more water there is in the tank, the quicker the water wave moves along towards the end walls, relatively speaking. If water is taken out of the tank the water wave moves relatively slower towards the end walls of the tank. By obtaining a monitoring system the damage to the cargo and stress to the ship are reduced and, in addition, it will be much more pleasant to be on board the ship. Today, there appears to be no good monitoring system for roll dampening tanks, which should be obligatory and a legal requirement, other than in the applicant's previously mentioned applications, where one has come somewhat closer to an ideal solution for the registering of the ideal antiphase by careful monitoring and registering of the wave pattern inside the tank.

It is an aim of the invention to provide an even simpler solution than the above mentioned, and which in some versions of the embodiment can use camera technology as mentioned above. It is an aim to provide a solution where one reduces the rolling movement of the ship about the longitudinal axis of the ship.

The method (claim 1 ) according to the invention is characterised in that the height of the liquid up along the wall surfaces of the tank, in particular, along the surfaces of the end walls, is measured and registered at any time with the help of one or more sensor elements, said height measuring data from the one or more of the sensor elements are registered in a monitoring system that calculates said deviation from the ideal antiphase, and on the basis of said measurement data the system carries out a regulation of the amount of water in the tank to achieve said antiphase.

The preferred embodiments of the method appear in the dependent claims 2- 12. The device (claim 13) according to the invention is characterised in that the surfaces of the tank walls comprise one or more sensor elements to measure how high the water wave reaches along the wall at any time and the one or more sensors are connected to the monitoring system via a signal which in turn is connected via signals to means that carry out said regulation to antiphase.

The preferred embodiments of the device appear in the claims 14-22. According to the present invention ideal antiphase is registered in relation to when the wave inside the tank reaches its highest point up on/along the transverse wall alternately on each side of the tank (in correlation to the maximum rolling movements of the ship). In the case where the height up along the wall is registered by a camera as described in the previous applications, the inner wall of the tank ought to be bright and furthermore lit by a lamp.

However, the best way to solve this is to use a sensor fitted into the transverse wall of the tank, such as a rod sensor or the like, or a radar apparatus inside the tank that can measure when the water is at its highest up along the wall and correlate this to the maximum rolling movement of the ship.

According to the invention the sensor is connected to the computer system that registers the time when the liquid/water reaches highest up on the wall and, as in the previous solutions, so that water shall be filled in the tank when the wave comes too late up along the wall and is let out from the tank when the wave arrives early in relation to this antiphase.

Lit up roll dampening tanks with camera monitoring.

By said camera monitoring (as described previously) of the roll dampening tanks one will see how the "wave" moves up the surface of the transverse wall in the tank and one can easily adjust the level of the antiphase. If the wave comes too high up on the wall it is an indication that there is not enough liquid in the tank and it must be filled. If the wave arrives early this indicates that there is too much liquid in the tank and some is it let out. Where one has several roll dampening tanks it will be very easy to "synchronise" the tanks so that one uses the moment from the power in the free liquid surface to the maximum. Such a system will also increase the understanding of the function of the roll dampening tanks. And also help the ship constructors in further development of such tanks.

The invention shall be explained in more detail with reference to the figures, in which:

Figure 1A shows a side section of a ship 100 in which two transverse roll dampening tank arrangements 200 are implemented in the whole breadth of the hull. Figure 1 B shows a plane outline of the foredeck of the ship and the transverse tank 200.

Figure 2 shows how the regulating system for the operation of each roll dampening tank arrangement can be set up schematically for efficient dampening of the rolling movements of the ship at sea.

Figure 3 shows an enlarged outline of the one end wall of a such tank to illustrate the central feature (as an example) for the registering of the liquid level/height up along one end wall of the tank.

Figures 4,5 and 6 show a schematic cross section of the roll dampening tank in the ship and show how the liquid/water in the tank behaves when the tank/the ship rolls from side to side in swells.

With reference to figure 1 a ship 100 is shown, the hull of which comprises one or more roll dampening arrangements 200 with an associated tank 20 that can be filled with a liquid and which is arranged approximately horizontally and across the whole breadth of the hull. The liquid which is filled into the tank is preferably water, but can be oil, for example, a low viscosity oil to which corrosion preventing materials can be added to prevent corrosion of the tanks. Shown in figure 1 are forward and aft roll dampening arrangements 200 in the hull with their respective transverse tanks 20. Figure 2 shows schematically a set-up composed so that the numbers represent the details shown in the figures, in particular figure 2:

1 1 . 1 piece PLS control box.

12. 1 piece MRU (Motion Registration Unit).

13. 2 pieces rod sensor/tank radar for measuring the height of the liquid.

14. 1 piece computer.

15. 2 pieces pumps.

16. 4 pieces valves. A such roll dampening arrangement 200 is shown in figure 2 in the form of an extended, rectangular shaped tank 20 in an open section with a bottom, sidewalls, end walls and top part. The two end walls are shown by 1 10a and 1 10b. The tank 20 is set up to be filled with liquid/water 30 shown in figures 4-6 such that there are is air 40 (figure 5) above the liquid surface 50 in the tank. The principle for the invention is that the volume of the water in the tank is arranged to be such that the wave pattern (the body of water swings to and fro) in the tank is in ideal antiphase with the rolling movement of the ship. When the ship in its rolling motion to both sides, lies "rolled" about its longitudinal axis furthest down on one side and the movement starts up again, most of the body of water or the wave shall be on the opposite side inside the tank; or as high up as possible on the side wall of the tank which is the measuring parameter according to the present invention. Then the moment of the free liquid surface has the greatest effect.

The extent of filling for such roll dampening tanks will vary with the stability of the ship and the shape of the tank, with a degree of filling between 20 and 80% being normally used. It is in this range that the free liquid surface moment is considered to have considerable effect on the dampening of the rolling.

To carry out the dampening each end wall 1 10a,b is fitted with its own one or more measuring instruments 13 that continuously measure the water level/wave level up along the walls 1 10a,b. This can be a rod sensor or a tank radar to measure the height of the liquid, as will appear later. Consequently, it relates to a non-horizontal at each end of the tank. With the expression end surface is meant until about 10 cm from said end surface and most preferred within 5 cm from the end plate. Most preferred is sensors integrated into the surface of the end wall itself.

The tank 20 is extended and comprises the end walls 1 10a, 1 10b which function as wave reflectors and these are implemented as flat wall surfaces or have a special shape to maintain the wave energy when the wave hits against the end walls 1 10. Furthermore, the apparatus comprises a control unit 1 1 connected to a roll angle sensor MRU (Motion registration unit) 12. The control unit 1 1 is connected to a computer 14 to coordinate the operation of the apparatus.

The roll angle sensor 12 MRU can decide the roll angle a in true time and provides a roll angle indicating signal to the control unit 1 1 with which it is connected via a signal. Furthermore, the apparatus comprises a sensor arrangement in all the wall surfaces to monitor how high up along the walls 1 10a and 1 10b the wave gets in the tank 20; with the sensor arrangement preferably being implemented as a rod sensor 13 that can be formed by using several technologies for measuring water heights up along the wall as will become clear in the following.

The sensors 13a and 13b in the two facing wall surfaces are, via signal leads 131 a and 131 b, respectively, connected via a signal to the control unit and further with the computer 14 which calculates any deviation from ideal roll antiphase.

With the sensor arrangement one can measure the amplitude of the water wave 30 in the form of height up on the wall. Furthermore, the apparatus 200 comprises an arrangement in the tank 20 to supply and remove water. On the one side is a pump 15a on the pipeline In set up to pump water into the tank from a water source that can be a tank filled with water in the ship or seawater from outside. The line is marked In in figure 2 and is connected to a valve 16 on the pipeline connected by a pipeline connection to the control unit 1 1 and the computer 14 and which can decide when water shall be pumped in, dependent on the antiphase conditions.

At the opposite end of the tank a line marked OUT leads correspondingly into the tank 2. A pump 151 is connected and is associated with a valve 161 on the pipeline, where the valve is connected via a line to the control unit 1 1 and the computer 1 1 corresponding to the opposite side of the tank. The control unit decides when water is to be let out (pumped out) dependent on the antiphase conditions between the body of water in the tank and the roll movements of the ship.

The opening and closing, respectively, of the valves 16,161 and the operation of the pumps 15,151 are controlled by the control unit 1 1 and the computer 14. By coordinating the use of the pumps 15,151 and the valves 16,161 , the control unit 1 1 can dynamically regulate the total volume/amount of water (or the total depth of water) d (illustrated in figure 5) of the liquid 30 in the tank 20 and also stimulate the formation of waves or the displacement of the mass of water - it swings to and fro - by removing water from the one end and adding water from the other end. Both these systems can be used to supply or remove liquid from the tanks.

The comparison between the wave pattern and the roll of the ship is carried out by the computer 14 and control signals can be sent from the unit 1 1 to the pumps to either supply or remove water from the tank when it is not in ideal antiphase. Even if the filling or removing of water can take place continuously, it is preferred that this supplying/removing takes place on the basis of a registering of how many single wave impacts Ab of a total number of wave impacts Tb up on the wall come too early, or come too late in relation to the ideal roll movement. This means that the regulation can be set such that it is first when, for example, Ab=50 wave impacts out of 200 wave impacts Tb comes too early, that the system in a controlled way removes water from the tanks. And conversely when a corresponding part of wave impacts come too late, the system will pump water into the tank, i.e. when the relationship between Ab and Tb, in the form of the ratio F=Ab/Tb, exceeds a given limiting value F=g, given in the example mentioned by a ratio f=50/200=g=0.25.

In the case where cameras 18a, b are also used, one on either side of the tank, then these are pointed directly towards the wall surface to register when the water wave is at its highest directly onto the walls 1 10, respectively. It is preferably a standard CCD (charge-coupled device) or CMOS (complementary metal-oxide-semiconductor) optical camera that is operated to provide a so- called pixels stream data to the control unit 1 1 . The control unit 1 1 can, in combination with the computer 14, form a picture of how high up along the wall

1 10 the wave gets and can process this as a function of time to determine the amplitude and also the propagation speed of the wave, as a wave position as a function of time. The control unit 1 1 generates data to present a series of pictures of the surface that can be shown on the screen 17. The camera 18 is connected to the control unit via cables 181 a, 181 b. The picture material from the cameras can be used in the computer 14 together with the height measurements from the sensor 13 to carry out the regulating.

Figures 4, 5 and 6 show the roll movements of the ship about its longitudinal, imaginary axis A via the setting of the tank 20 for when the ship rolls to and fro. The sensor 13, the liquid height H the longitudinal axis A of the ship and the depth D of water in the tank are all shown in their normal position. Figure 4 shows when the ship rolls to the left and tilts at an angle a and the mass of water 30 "hits" up to the right at a height equal to H.

Figure 5 shows the situation when the ship swings back and is in the mid position and lies completely straight in the sea. Here, the water depth in the tank is shown by the symbol D.

In figure 6 the ship tilts over to the other side at a corresponding angle a.

As mentioned, many types of rod sensors can be used to register the water level up along the walls 1 10.

According to one embodiment a rod sensor can comprise one or more float elements that are fitted into a longitudinal groove cut out in the wall surface 13, and how high up the float rises in the groove is registered by the computer 1 1 , and this together with the roll movements of the ship allows the system to fill or remove water from the tank 20.

According to an alternative solution, the side walls 1 10c and 1 10d (marked in figures 1 , 2 and 3) can also comprise a number of rod sensors which measure the height of the liquid and the wave pattern along the long walls.

Measurement data from these long wall sensors can be combined with measurement data taken from the transverse walls 1 10 such that the system can carry out the regulation and setting to the ideal antiphase between the two swing movements even more accurately.

According to another preferred solution the rod sensor 13 comprises a number of electrically conducting electrodes, the conductivity of which is influenced by the liquid in the tank, and the electrodes can thereby "read" the height of the liquid up along the wall and this is registered in the computer 1 1 for further automatic regulation of the amount of water.

Alternative technologies built into the instrument relate to one or more ultrasonic elements set up to acoustically register/read the fluid when the fluid wave at least partially covers the element, alternatively the element is an electromagnetic resonance element or the like, that sends different signals to the system depending on whether it is covered by the water/liquid (up along the wall) or not (is in contact with the air only). According to yet another embodiment the instrument 13 is composed by a combination of said sensor elements, arranged mutually spaced apart vertically in the wall surface to provide accurate measurements of the liquid height at the end wall surfaces.

As a variant of the above mentioned float construction that can be raised or lowered depending on the water level up along the wall, the instrument is organised in a vertical arrangement for pivoting mini floats and such that when these are contacted by water up along the wall they will flip over and can activate micro-switches and/or magnetic switches. Such mini floats (of a size around 2 cm) are fastened to mountings that can swing up, and activate the micro-switches, so-called magnetic reed switches or the like when they are flooded by water in that they flip over their mainly horizontal axis of mounting and make a circuit. As they swing and activate the switches, the signal is sent. As one knows exactly how the individual float switches are placed along the wall one knows also how high up on the transverse wall the water wave hits.

For the embodiment where several sensor instruments are used in the same wall surface, the sensors are arranged in a zigzag displaced pattern to give increased measurement resolution. This can solve the challenges met when the wave or the water moves unevenly up along the wall. In this context a number of instruments are arranged at both the end walls of the tank, as there are preferably several sensors arranged so that an average liquid height can be generated for the case where the liquid wave is displaced as it propagates to and fro in the tank.

These rod sensor types can be fitted into all the walls of the roll dampening tank, i.e. both in the short walls and in the long walls, and are used to give the best possible picture of the wave pattern inside the tank.

With reference to figure 5a a system description of "Max control with roll dampening tanks" is given.

Function:

The registering unit MRU (Motion registration unit) will show a true tilt angle at any time. Here expressed as (+starboard) and (-port side). The rod sensor or radar 13, fitted on either side 1 10 of the tank 20 is here expressed as (+Starboard) and (-port side), measures the liquid level (the height H) at any time on both sides of the tank. These data are compiled in the control box 1 1 which senses whether the maximum liquid level H up on the wall comes before or after the maximum tilt angle. If the maximum liquid level H up along the wall comes after the maximum tilt angle a there is too little liquid in the tank and the converse if the maximum liquid level comes before the maximum tilt angle there will be too much liquid in the tank. One measures the time difference between these two swing movements and the nearer 0 one comes with respect to the time difference the better it is.

According to the invention there shall be a flattening out possibility so that one inputs that the control box 1 1 senses for an optional time how much time should pass before the system reacts to deviations that occur and starts to fill or empty the tank 20 of fluid. This is necessary to avoid the pumps and valves running continuously. Example:

When the MRU shows the maximum impact to (+) the rod sensor/radar (-) shall show the maximum liquid level and the rod sensor/radar (+) show the minimum liquid level. If the rod sensor (-) here shows maximum liquid level after the MRU has shown the max impact to (+) there is too little liquid in the tank and conversely, if the rod sensor/radar (-)shows the max liquid height before MRU shows the max impact to (+) there is too much liquid in the tank and the control box 1 1 sends a message to the valves 16 and pumps 15 to fill or reduce the amount of liquid so that the max impact comes at the same time. In this way a perfect antiphase is achieved.

Reserve/Backup

The use of camera monitoring connected to the system is meant as a reserve solution or backup solution to the automatic system, but the camera function can also be used at the same time as a supplement. For this solution it is considered to use lit up roll dampening tanks with cameras directed towards the end wall surfaces as given in the previous application. By having camera monitoring of the roll dampening tanks one will see how the "wave" travels in the tank and one can easily adjust the level to antiphase. If the wave arrives late this indicates that there is not enough liquid in the tank and one supplies more. If the wave comes early this indicates that there is too much liquid in the tank and one removes some of it.

Additionally, if one has several roll dampening tanks, it is very simple to synchronise the tanks so that one uses the moment from the power in the free liquid surface maximally. It is well known that one alters the speed in liquid according to the depth. The stability of a ship leads to a roll frequency/roll speed. With accurate measurements of the liquid in correlation with the rolling of the ship one can adjust the speed of the liquid to result in antiphase and stay there until one alters the stability (roll frequency). Then one alters the depth again.

Several types of rod sensors can be fitted at the same time, something which gives more reliable measurement results that are sent to the control system. The height at which the wave impacts up along the wall can be measured with an accuracy of a few centimetres and often down to an order of magnitude of millimetres.

With the invention very favourable dampening of the roll movements can be achieved. The present invention is particularly suited to all types of ocean going liners, oil tankers, ships and other installations that are used for drilling and production of oil or other purposes.

Claims

C L A I M S

1 . Method for the dampening of the rolling movements of a ship at sea, said ship comprises one or more, in the main, set up crosswise, roll dampening tanks partially filled with a liquid, where the liquid in the tank is set up to swing in the antiphase of the rolling movements of the ship to counteract the rolling movements of the ship, where a measuring system and a control system are used to register an antiphase and any deviation from said antiphase and the amount of liquid in the tank is regulated to achieve said antiphase, characterised in that

the height of liquid up along the wall surfaces of the tank, in particular along the end wall surfaces, is measured and registered at any time with the help of one or more sensor elements, said height measurement data from the one or more of the sensor elements are registered in a monitoring system that decides on said deviation from the ideal antiphase, and on the basis of said measurement data the system carries out the regulation of the amount of liquid in the tank to arrive at said antiphase.

2. Method according to claim 1 , characterised in that the sensor element used is one or more rod sensors comprising float elements arranged in a vertical groove in the wall surface (13) and the position of the float according to the water level in the groove is registered in the monitoring system (1 1 ).

3. Method according to claim 1 , characterised in that an ultrasonic element is used set up to acoustically register the liquid when the liquid at least partially covers the element, alternatively the element is an electromagnetic resonance element or the like.

4. Method according to one of the preceding claims, characterised in that a vertical arrangement of pivoting mini floats is used which, when they are contacted by water up on the wall, flip over and activate micro switches and/or magnetic switches that make a circuit and signals are sent about the height of the liquid to the monitoring system (1 1 ).

5. Method according to claim 1 , characterised in that a number of electrically conducting electrodes are used, the conductivity of which are influenced by the liquid, whereby the electrodes "read" the height of liquid on the wall and this is registered in the monitoring system (1 1 ) for further automatic regulation of the amount of liquid.

6. Method according to one of the preceding claims, characterised in that a combination is used of the same or different types of said sensor elements, arranged mutually spaced apart in the wall(s) for an accurate measurement of the liquid height at the end wall surfaces.

7. Method according to claims 1 -6, characterised in that the rod sensors are placed in a zigzag displaced pattern to provide increased measurement resolution.

8. Method according to one of the preceding claims, characterised in that the liquid height is measured by using the rod sensors both in the short walls and in the long walls of the roll dampening tank to map the wave motion inside the tank.

9. Method according to one of the preceding claims, characterised in that instruments are placed at both of the end walls of the tank, as several sensors are preferably placed so that an average liquid height up on the walls can be registered/measured in the case where the liquid wave is displaced lengthwise (in relation to the longitudinal axis of the ship) as it propagates to and fro across the tank.

10. Method according to one of the preceding claims, characterised in that water must be added (pumped in) with the help of a pump (15) in the tank when the wave arrives late and liquid is let out of the tank when the wave arrives early.

1 1 . Method according to one of the preceding claims, characterised in that the filling/emptying of water takes place continuously or is initiated on the basis of a registration of how many single wave impacts Ab of a total number of wave impacts Tb up on the wall arrive too early or arrive too late in relation to the ideal rolling movement, i.e. when the relationship between Ab and Tb, in the form of the ratio = Ab/Tb exceeds a given limiting value.

12. Method according to claim 1 1 , characterised in that first when Ab=50 wave impacts of a total of 200 wave impacts Tb comes too early, the system provides that water is emptied from the tank in a controlled way, and when a corresponding number of "wave impacts" are too late, the system provides that water is pumped into the tank, i.e. f=50/200=0.25.

13. Device for a roll dampening tank in a ship where the tank is set up, in the main, across the longitudinal direction of the ship and arranged to contain an amount of liquid which, by swinging movements in the tank and in antiphase to the movement of the ship, is brought to counteract that the ship rolls in the sea as the device comprises a monitoring system to register the phase relationship between the swing movement of the liquid and the movement of the ship, and applications to regulate the amount of liquid in the tank to achieve said antiphase, characterised in that the wall surfaces (1 10) of the tank comprise one or more sensor elements to measure how high the water wave is at any time up on the wall, and the one or more rod sensors are connected via signals to the monitoring system (1 1 ) which in turn is connected via signals to means that carry out said regulation of the antiphase.

14. Device according to claim 13, characterised in that the sensor elements are arranged in the end wall surfaces (1 10a,1 10b) of the tank.

15. Device according to claim 13, characterised in that the one or more sensor elements are rod sensors comprising float elements arranged in the wall surface (13) preferably in vertical cut grooves in the wall.

16. Device according to claim 13, characterised in that each sensor element is set up to acoustically register the liquid when the liquid at least partially covers the element, alternatively the element is an electromagnetic resonance element or the like.

17. Device according to any of the preceding claims 13-16, characterised in that the sensor element is an arrangement of mini floats that can swing (pivot) which, when contacted by water up on the wall, flip over and activate micro switches and/or magnetic switches that make a circuit and signals about the height of the liquid are sent to the monitoring system (1 1 ).

18. Device according to any of the preceding claims 13-17, characterised in that the sensor element is a number of electrically conducting electrodes, the conductivity of which is influenced by the liquid whereby the electrodes "read" the height of the liquid up on the wall and this is registered in the monitoring system (1 1 ) for further automatic regulation of the amount of liquid.

19. Device according to any of the preceding claims 13-18, characterised in that the sensor elements constitute a combination of the same or different types of said sensor elements, arranged mutually spaced apart in the wall(s) for accurate measurement of the liquid height at the end wall surfaces.

20. Device according to claims 13-19, characterised in that the one or more sensors are arranged in a zigzag displaced pattern to provide increased measurement resolution.

21 . Device according to any of the preceding claims 13-20, characterised in that both the short walls and the long walls of the roll dampening tank comprise sensor elements to be able to map the wave pattern inside the tank.

22. Device according to any of the preceding claims 13-21 , characterised in that water is filled into the tank with help of a pump (15) when the wave arrives late and liquid is removed/pumped out of the tank when the wave arrives early.