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Title:
U-TANK ACTIVE ROLL DAMPENING SYSTEM FOR AND METHOD FOR ACTIVE ROLL DAMPENING OF A VESSEL
Document Type and Number:
WIPO Patent Application WO/2019/143257
Kind Code:
A1
Abstract:
U-tank active roll dampening system and method for active roll dampening of a vessel provided with a U-tank enabling controlling of fluid level in side tanks in anti-phase in front of vessel roll motion period. At least one vacuum and air pressure manipulation unit is arranged to the upper part of an associated side tank and at least one aeration device against open air is connected to the upper part of the other side tank, wherein the at least one vacuum and air pressure manipulation unit is arranged to supply air pressure or vacuum on the fluid surface in the associated tank to control the fluid level in both tanks.

Inventors:
SPORSHEIM, Ove Sigbjörn (Torleifv 11B, 6416 MOLDE, 6416, NO)
Application Number:
NO2019/050011
Publication Date:
July 25, 2019
Filing Date:
January 18, 2019
Export Citation:
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Assignee:
MRPC AS (Fannestrandvegen 85, 6416 MOLDE, 6416, NO)
International Classes:
B63B39/03
Attorney, Agent or Firm:
CURO AS (Vestre Rosten 81, 7075 Tiller, 7075, NO)
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Claims:
Claims

1. U-tank active roll dampening system for vessel (100), wherein the U-tank (10) is formed by side tanks (lla-b), one arranged at port side of the vessel (100) and one arranged at starboard side of the vessel (100), the side tanks (lla-b) at upper end sealed and at lower end connected by a crossover duct (12), wherein the U-tank (10) is partly filled with a fluid, characterized in that at least one vacuum and air pressure manipulation unit (20) is arranged to upper part of an associated side tank (lla-b) and at least one aeration device (30) against open air is connected to upper part of the other side tank (lla-b), wherein the at least one vacuum and air pressure manipulation unit (20) is arranged to supply air pressure or vacuum on fluid surface in the associated side tank (11a) to control fluid level in both side tanks (lla-b) in anti-phase in front/advance of vessel roll motion period.

2. U-tank active roll dampening system for vessel (100) according to claim 1, characterized in that the vacuum and air pressure manipulation unit (20) is provided with a valve (23) for opening or closing the connection to the associated side tank (lla-b) and the aeration device (30) is provided with a valve (31) for opening or closing the aeration to open air.

3. U-tank active roll dampening system for vessel (100) according to claim 1, characterized in that the vacuum and air pressure manipulation unit (20) is arranged for aeration against open air of the associated side tank (lla-b).

4. U-tank active roll dampening system for vessel (100) according to claim 1, characterized in that the system comprises at least one fluid level sensors (40) arranged in each of the side tanks (lla-b) for measuring fluid level in the respective side tank (lla-b).

5. U-tank active roll dampening system for vessel (100) according to any one of the preceding claims, characterized in that the at least one vacuum and air pressure manipulation unit (20) is arranged to supply pressure on fluid surface of the other side tank (lla-b) or that at least one separate air pressure unit is arranged to supply pressure on fluid surface of the other side tank (lla-b), and that aeration devices (30) provided with controllable valves (31) for opening or closing the aeration to open air are arranged to both side tanks (lla-b).

6. U-tank active roll dampening system for vessel according to claim 5, characterized in that the at least one vacuum and air pressure manipulation unit (20) is arranged to the other side tank (lla-b) via a controllable valve (24) and a pipe (25) extending from pressure side of the vacuum and air pressure manipulation unit (20) to upper part of the other side tank (lla-b).

7. U-tank active roll dampening system for vessel (100) according to any one of the preceding claims, characterized in that the system comprises a dedicated motion reference unit (50) for measuring accelerations of the vessel (100) in a roll motion.

8. U-tank active roll dampening system for vessel (100) according to any one of the preceding claims, characterized in that the system comprises a control unit (60) arranged for controlling the at least one vacuum and air pressure manipulation unit (20), aeration device(s) (30) and valves (23, 31, 24), and further provided with means and/or software for controlling the fluid level in the side tanks (lla-b) based on measurements by the dedicated motion reference unit (50) and measurements from the fluid level sensors (40) so as to control the fluid level in the respective side tanks (lla-b) in anti-phase in front/advance of the vessel roll motion period.

9. U-tank active roll dampening system for vessel (100) according to any one of the preceding claims, characterized in that the vacuum and air pressure manipulation unit (20) comprises at least one vacuum and air pressure generation unit (21) connected to a demand controlled ventilation unit (22).

10. Method for active roll dampening of a vessel (100) provided with a the U-tank (10) formed side tanks (lla-b), one arranged at port side of the vessel (100) and one arranged at starboard side of the vessel (100), the side tanks (lla-b) at upper end sealed and at lower end connected by a crossover duct (12), wherein the U-tank (10) is partly filled with a fluid, characterized by controlling fluid level in both the side tanks (lla-b) by supplying air pressure or vacuum by means of at least one vacuum and air pressure manipulation unit (20) on fluid surface in an associated side tank (11a- b) and aeration against open air of the other side tank (lla-b) by means of an aeration device (30), and controlling the fluid level in both side tanks (lla-b) in anti-phase in front/advance of vessel roll motion period.

11. Method according to claim 10, characterized by controlling the fluid level in the side tanks (11a- b) based on measurements by a dedicated motion reference unit (50) and measurements from fluid level sensors (40) associated with the side tanks (lla-b) so as to control the fluid level in the respective side tank (lla-b) in anti-phase in front/advance of vessel roll motion period, approximately 90 degrees in front/advance of the vessel roll motion period.

12. Method according to claims 10-11, characterized by closing the aeration of the other side tank (lla-b) and opening for aeration of the side tank (lla-b) associated with the at least one vacuum and air pressure manipulation unit (20) by means of at least one aeration device (30) arranged thereto, and supplying pressure utilizing the at least one vacuum and air pressure manipulation unit (20) or at least one separate air pressure unit on fluid surface of the other side tank (lla-b) when additional increasing of the fluid level in the side tank (lla-b) associated with the vacuum and air pressure manipulation unit (20) is required.

Description:
U-tank active roll dampening system for and method for active roll dampening of a vessel

The present invention is related to a U-tank active roll dampening system for vessel, according to the preamble of claim 1.

The present invention is also related to a method for active roll dampening of a vessel provided with a U-tank, according to the preamble of claim 10.

Background

The use of U-tube tanks was pioneered by Frahm in Germany at the start of the 20th century and they are often referred to as Frahm tanks. These partially filled tanks consists of two side tanks connected at the bottom by a substantial crossover duct. The air columns above the liquid in the two tanks are also connected by a duct. As in the free surface tanks, as the ship begins to roll the fluid flows from side tank to side tank causing a time varying roll moment to the ship and with careful design this roll moment is of correct phasing to reduce the roll motion of the ship. They do not restrict fore and aft passage as space above and below the crossover duct is available for other purposes.

JP7251793A, KR20160104884A, JP7251793A, KR101235329 B1 and KR20150045323A are examples of such a system where air supply means are arranged to both the side tanks, at upper end thereof.

There is also known to use a pump in the crossover duet to control the fluid volume in the side tanks.

These pumps have the problem to active transfer enough volume capacity for roll damping, but sufficient in "Anti-Fleeling" system.

GB 1213853 A also describes a system as mentioned above requiring an upper connection of the side tanks to ensure that the air spaces above the liquid in both of the tanks are at the same pressure by a vent against atmospheric pressure. In GB 1213853 A a booster arrangement is arranged supplying air to both the side tanks, one at the time when controlled in relation to roll measurements. I.e. in GB 1213853 A the system works such that when supplying air to first side tank, the fluid level in the second tank rises, and when the fluid level in the second tank is to be lowered, air is supplied to the second tank, lowering the fluid level in the second tank and raising the fluid level in the first tank. Excess air is flowing back to reservoirs via the interconnecting duct at upper part of the two tanks. GB 1213853 A thus describes a closed system wherein each tank is connected to a booster arrangement via piping at upper part thereof. It is clearly stated in GB 1213853 A that the system cannot be classified as an active stabilizer, as the booster arrangement is incapable of providing a sufficient force to produce the necessary flow of liquid between the tanks in order to stabilize roll motion of the ship.

GB 2087818 A discloses a system similar to the system in GB 1213853 A working in the similar manner as GB 1213853 A and suffer from the same disadvantages as GB 1213853 A.

The mentioned U-tank systems are mainly designed for Anti-heeling when utilizing blowers at top of the tanks, but in addition these systems can provide passive roll dampening by means of free flow of fluid between the tanks as a result of the vessel roll motions, with full opening of tank airings between the top of the tanks, wherein the fluid flow is designed to the shortest roll period of the vessel. If the vessel should have a longer roll period the tank airings at top of the tanks are reduced for reducing the velocity of the fluid flow between the tanks. In such a passive roll dampening mode the system attempts to achieve the flow of fluid as close to 90 degrees after the vessel roll movement period.

Accordingly, prior art solutions fail to provide an active roll dampening and there is a need for a U- tank active roll dampening system for and method for active roll dampening of a vessel provided with a U-tank.

The prior art solutions further suffer from that they are not capable of providing the volume capacity or response time required to counteract the roll affection from waves before they have started to affect the vessel, only after, and accordingly will not be able to provide an active roll dampening of the vessel. Accordingly, prior art solution are not arranged or capable of moving sufficient fluid volume in advance, approximately 90 degrees in front of a roll motion.

Object

The main object of the present invention is to provide a U-tank active roll dampening system for and a method for active roll dampening of a vessel that partly or entirely solve the drawbacks of prior art.

It is further an object of the present invention to provide a U-tank active roll dampening system for and method for active roll dampening of a vessel enabling counteraction of roll motions on the vessel before the waves have started to affect the vessel. An object of the present invention is to provide a U-tank active roll dampening system for and method for active roll dampening of a vessel enabling sufficient volume capacity capable of moving sufficient fluid volume between side tanks thereof in anti-phase in front of vessel roll motion period.

It is an object of the present invention to provide a U-tank active roll dampening system for and method for active roll dampening of a vessel wherein controlling fluid level in one of the side tanks thereof also controls the fluid level in the other side tank.

It is an object of the present invention to provide a U-tank active roll dampening system for and method for active roll dampening of a vessel that can make use of present U-tank system installed on vessel and used for anti-heeling. Further objects of the present invention will appear from the following description, claims and attached drawings.

The invention

A U-tank active roll dampening system for vessel according to the present invention is disclosed in claim 1. Preferable features of the system are disclosed in claims 2-9.

A method for active roll dampening of a vessel provided with a U-tank is disclosed in claim 10. Preferable features of the method are disclosed in claims 11-12.

The present invention discloses an active roll dampening system for and method for active roll dampening of a vessel based on a U-tank. The U-tank is formed by a port side tank and a starboard side tank, the side tanks at upper end sealed and at lower end connected by a crossover duct, wherein the U-tank is partly filled with a fluid, which typically is water or seawater. The actual fluid volume inside the U-tank must be defined by the crew based on the actual loading of the vessel. The U-tank active roll dampening system according to the present invention further comprises at least one vacuum and air pressure manipulation unit arranged to upper part of an associated side tank and at least one aeration device against open air connected to upper part of the other side tank. According to the present invention the at least one vacuum and air pressure manipulation unit is arranged to supply air pressure or vacuum on fluid surface in the associated side tank with the other side tank open for aeration to open air to control fluid level in both side tanks. According to a second embodiment of the U-tank active roll dampening system according to the present invention the at least one vacuum and air pressure manipulation unit is arranged to supply air pressure on fluid surface of the other side tank via a controllable valve. Accordingly, the other tank is connected to the pressure side of the vacuum and air pressure manipulation unit via the controllable valve. In this embodiment there are further arranged aeration devices to both the side tanks. This embodiment will be especially suitable for solutions where the fluid level difference between the two side tanks have to be larger than 5 meters, i.e. the fluid level in the side tank associated with the at least one vacuum and air pressure manipulation unit is to be higher than 5 meters above the fluid level in the other side tank. By this embodiment, after a fluid level difference of 5 meters is achieved, the aeration device of the side tank associated with the vacuum and air pressure manipulation unit is set open to air, while the aeration device of the other side tank is closed and wherein the vacuum and air pressure manipulation unit is set to supply air pressure on the fluid surface of other side tank, whereupon the fluid level will further be lowered in the other side tank and correspondingly increased in the side tank associated with the vacuum and air pressure manipulation unit. In an alternative embodiment at least one separate air pressure unit is connected to the other tank instead of the vacuum and air pressure manipulation unit for supplying pressure on the fluid surface in the other side tank.

In one embodiment of the U-tank active roll dampening system according to the present invention the vacuum and air pressure manipulation unit comprises at least one vacuum and air pressure generation unit connected to a demand controlled ventilation unit enabling controlled switching between supply of vacuum or pressure. In an alternative embodiment two or more vacuum and air pressure generation units are connected to a common demand controlled ventilation unit. According to a further embodiment the U-tank active roll dampening system according to the present invention it comprises two or more of the vacuum and air pressure manipulation units arranged to the same side tank.

This will enable different configurations as well as redundancy.

According to a further embodiment of the system according to the present invention the vacuum and air pressure manipulation unit is provided with a valve for opening or closing the connection to the associated side tank and the aeration device(s) is/are provided with a valve for opening or closing the aeration to open air.

In a further embodiment of the U-tank active roll dampening system for vessel according to the present invention, the vacuum and air pressure manipulation unit is arranged for aeration against open air of the associated side tank. By this the system according to the present invention can operate in passive mode. Alternatively a separate aeration device can be arranged for this.

By means of the mentioned valves of the vacuum and air pressure manipulation unit and aeration device(s), respectively, the system according to the present invention can operate in a stabilized mode locking the fluid level in the side tanks at a desired level by closing the mentioned valves.

According to the present invention the U-tank active roll dampening system comprises at least one fluid level sensors arranged in each of the side tanks for measuring fluid level in the associated tank.

The U-tank active roll dampening system for vessel according to the present invention further comprises a dedicated motion reference unit (M RU) for measuring accelerations of the vessel in a roll motion.

According to the present invention the U-tank active roll dampening system further comprises a control unit arranged for controlling the vacuum and air pressure manipulation unit and aeration device(s), as well as associated valves and further is provided with means and/or software for controlling the fluid level in the side tanks based on measurements by the dedicated motion reference unit and measurements from the fluid level sensors so as to control the fluid level in the respective side tanks in anti-phase in front of the vessel roll motion period, approximately 90 degrees in front of a roll motion period.

Accordingly, the present invention provides a U-tank active roll dampening system capable of controlling fluid volume in the side tanks to move in anti-phase in front of the vessel roll motion period. Accordingly, the U-tank active roll dampening system according to the present invention is capable of moving sufficient fluid volume from one of the side tanks to the other side tank approximately 90 degrees in front of a roll motion period, and by this counteracting the affection of a passing wave before the wave starts to affect the vessel.

There is no description or mentioning in prior art of this way to active roll dampening to counteract roll motion of a vessel.

The controlling of the system according to the present invention is plain, as the fluid level/volume in only one of the side tanks requires to be controlled, which directly affects the fluid level/volume in the other side tank. This reduces the space required in the vessel for arrangement of the system, as well reduces the number of components of the system, resulting in low maintenance and installation costs. A method for active roll dampening of a vessel provided with a the U-tank according to the present invention comprises controlling fluid level in both the side tanks by supplying air pressure or vacuum by means of at least one vacuum and air pressure manipulation unit on fluid surface in an associated side tank and aeration against open air of the other side tank by means of an aeration device, and controlling the fluid level in both side tanks in anti-phase in front of vessel roll motion period.

According to a further embodiment of the method according to the present invention it comprises controlling the fluid level in the side tanks based on measurements by a dedicated motion reference unit and measurements from fluid level sensors associated with the side tanks so as to control the fluid level in the respective side tank in anti-phase in front of vessel roll motion period, approximately 90 degrees in front of the vessel roll motion period.

According to a further embodiment of the method according to the present invention the method comprises closing the aeration of the other side tank and opening for aeration of the side tank associated with the at least one vacuum and air pressure manipulation unit by means of at least one aeration device arranged thereto, and supplying pressure utilizing the at least one vacuum and air pressure manipulation unit or at least one separate air pressure unit on fluid surface of the other side tank when additional increasing of the fluid level in the side tank associated with the vacuum and air pressure manipulation unit is required.

In practice the fluid level in both the side tanks will attempt to maintain in horizontal even when the vessel is tilting. By adding sufficient amount of vacuum or over-pressure in one of the side tanks, i.e. the side tank associated with the vacuum and air pressure manipulation unit, one with the present invention is able to move the fluid rapidly enough that one achieve controlled roll dampening. The earlier, i.e. in front/advance, the movement of fluid between the two side tanks before the vessel movement has developed, the less fluid (amount/weight) is required for dampening the roll motion/angle.

Accordingly, the method according to the present invention and the control unit of the system will be arranged to use latest historical information from the dedicated motion reference unit regarding roll motion of the vessel, and based on this predict probable future next roll motion/sequence and based on this control the fluid level in the side tanks in front/advance of the affection of the waves on the vessel.

In addition to the above stated, the system can operate in passive roll dampening mode with aeration of both the side tanks such that fluid passively flows freely between the two side tanks. This will match the vessels minimum roll period. If this period is to be extended this will be done by controlling the opening of the valves of the vacuum and air pressure manipulation unit and aeration device.

The system can further be set in a stabilizing mode, wherein the present fluid level in the side tanks are locked by closing the valves of the vacuum and air pressure manipulation unit and aeration device(s).

The present invention can further make use of tanks already present in the vessel, such as Anti heeling system tanks, and make use of the same tank volumes.

Further preferable features and advantageous details of the present invention will appear from the following example description, claims and attached drawings.

Example

The present invention will below be described in further detail with references to the attached drawings, where:

Figure 1 is a principle drawing of a U-tank active roll dampening system for a vessel according to a first embodiment of the present invention,

Figure 2 is a principle drawing of a U-tank active roll dampening system for a vessel according to a second embodiment of the present invention,

Figure 3 is a block diagram of the system according to the present invention, and

Figures 4a-c are principle drawings of different states of the fluid level in the side tanks of the U- tank.

Reference is now made to Figure 1 which is a principle drawing of a U-tank active roll dampening system for a vessel 100 according to the present invention. The U-tank 10 is formed by a pair of side tanks lla-b, one arranged on the starboard side of the vessel 100 and one arranged on port side of the vessel 100. The side tanks lla-b are sealed at upper end and are at lower end connected by a crossover duct 12, thus forming a U-tank 10 inside the vessel 100. The U-tank 10 will initially be filled with a fluid volume that is defined based on actual loading of the vessel 100. This configuration of a U-tank 10 is well known for a skilled person. The crossover duct 12 will be designed to allow sufficient flow of fluid between the side tanks lla-b to counteract a part of the roll movement within natural roll period of the vessel 100.

The mentioned side tanks lla-b and crossover duct 12 thus form a closed fluid system in the vessel 100. In the present invention the only connection between the two side tanks lla-b is the crossover duct 12.

The present invention is related how to use this known U-tank for active roll dampening of the vessel 100.

According to the present invention the system further comprises at least one vacuum and air pressure manipulation unit 20 connected to upper part of one of an associated side tank lla-b, and an aeration device 30 connected to upper part of the other side tank lla-b. In the shown example the vacuum and air pressure manipulation unit 20 is connected to the port side tank 11a and the aeration device 30 is connected to the starboard side tank lib, but the vacuum and air pressure manipulation unit 20 can be connected to the starboard side tank lib and the aeration device 30 to the port side tank 11a.

The vacuum and air pressure manipulation unit 20 comprises at least one vacuum and air pressure generation unit 21 connected to a demand controlled ventilation unit 22 connected to upper part of the interior of associated side tank 11a via a controllable valve 23. The vacuum and air pressure manipulation unit 20 is arranged to supply air pressure (overpressure) or vacuum (underpressure) on the fluid surface in the associated side tank 11a.

The aeration device 30 further comprises a controllable valve 31 for controlling the aeration of the mentioned side tank lib.

Reference is now made to Figure 2 which is a principle drawing of a second embodiment of the U- tank active roll dampening system according to the present invention. In the second embodiment the at least one vacuum and air pressure manipulation unit 20 is in addition arranged to supply air pressure on fluid surface of the other side tank lib via a controllable valve 24 and a pipe 25 extending from the pressure side of the vacuum and air pressure manipulation unit 20 to upper part of the other side tank lib. In this embodiment there are further arranged aeration devices 30 to both the side tanks lla-b. This embodiment will be especially suitable for solutions where the fluid level difference between the two side tanks lla-b have to be larger than 5 meters, i.e. the fluid level in the associated side tank 11a is to be higher than 5 meters above the fluid level in the other side tank lib. In an alternative embodiment at least one separate air pressure unit is arranged to upper part of the side tank lib to supply pressure to the fluid surface therein in the same manner as the vacuum and air pressure manipulation unit 20 as described above.

Accordingly, the system according to the present invention provides a U-tank active roll dampening system that can operate both in a passive mode and in an active mode for performing roll damping. By the system according to the present invention is achieved a system that provides full control of fluid moving between the two side tanks lla-b.

The system according to the present invention further comprises fluid level sensors 40 arranged in the side tanks lla-b for reading/measuring current fluid level in the side tanks lla-b.

Reference is now made to Figure 3 which is block diagram of a system according to the present invention. The system according to the present invention further comprises a dedicated motion reference unit 50 for measuring accelerations of the vessel 100 in a roll motion, hereunder providing roll angle for the vessel which can be used for controlling of the fluid level/volume in the side tanks lla-b.

The system according to the present invention further comprises a control unit 60 arranged for controlling the at least one air pressure manipulation unit 20, aeration device(s) 30 and valves 23, 31, 24. The control unit 60 is provided with means and/or software for controlling the vacuum and air pressure manipulation unit 20, aeration device(s) 30, as well as valves 23, 31, 24, based on operation mode, hereunder passive or active roll dampening, or stabilization mode.

In active mode the control unit 60 is arranged to set the aeration device 30 of the starboard side tank lib open towards open air and the vacuum and air pressure manipulation unit 20 is set to supply air pressure or vacuum in the port side tank 11a based on measurement from the motion reference unit 50 and measurement of current fluid level in the side tanks lla-b from the fluid level sensors 40. Accordingly, in active mode, instead of the fluid moving passively in relation to the vessel 100 motions and thus moving approximately 90 degrees behind the vessel roll motion, active control of the fluid movement is achieved by means of the system according to the present invention, and able to move the fluid in the respective side tanks lla-b in anti-phase in front/advance of the vessel roll motion period. By means the vacuum and air pressure manipulation unit 20 capability of supplying air pressure or vacuum on the fluid surface in the port side tank 11a one can control the fluid to move between the mentioned side tanks lla-b. With the aeration device 30 of the starboard side tank lib open against air and by supplying air pressure in the port side tank 11a the fluid level in port side tank 11a will be reduced by that the air pressure will force the fluid to move from the port side tank 11a via the crossover duct 12 to the starboard side tank lib, thus correspondingly elevating the fluid level in the starboard side tank lib. Similarly, with the aeration device 30 of the starboard side tank lib open against air and by supplying vacuum to the fluid surface in the port side tank 11a, this will result in pressure difference between the mentioned side tanks lla-b that will draw/force the fluid to move from the starboard side tank lib to the port side tank 11a via the crossover duct 12, elevating the fluid level in port side tank 11a and reducing the fluid level in the starboard side tank lib. Accordingly, by controlling the pressure on the fluid surface in the port side tank 11a, full control of the fluid level in both side tanks lla-b, i.e. the entire U-tank is achieved in an active manner.

If a fluid difference of more than 5 meter is required, i.e. the fluid level in the port side tank 11a will have to be larger than 5 meters above the fluid level in the starboard side tank lib, the above described second embodiment, as shown in Figure 2, can be used in active mode. The second embodiment works as the first embodiment as described above, but when the fluid level difference of 5 meters is achieved, the aeration device 30 of the port side tank 11a is set open to air, while the aeration device 30 of the starboard side tank lib is closed and wherein the vacuum and air pressure manipulation unit 20 is set to supply air pressure on the fluid surface of the starboard side tank lib, whereupon the fluid level will further be lowered in the starboard side tank lib and correspondingly increased in the port side tank 11a. For most vessels a fluid level difference of 5 meters between the side tanks lla-b will provide sufficient roll dampening, but for larger vessel there might be a need for larger fluid level difference which can be achieved by the second embodiment.

The second embodiment will further result in that one more rapid can achieve the required pressure difference in the two side tanks lla-b in a more rapid manner.

Accordingly, by measuring the accelerations of the vessel 100 in a roll motion, this information can be used to move the fluid between the side tanks lla-b, i.e. the fluid level in the respective side tanks lla-b, in anti-phase in front/advance of the vessel roll motion period. Based on the measurements of the motion reference unit 50 the control unit 60 can predict and calculate the fluid level in the respective side tanks lla-b such that the fluid level/volume can be controlled to be in anti-phase in front/advance of the vessel roll motion period. Accordingly, instead of the waves executing a force to the vessel weight (tonnage) in a certain roll motion, the system according to the present invention supplies a sufficient amount of fluid in the respective side tanks lla-b in correct anti-phase point in time in front of the roll motion period (frequency) of the vessel 100, i.e. optimally moving the fluid level in the respective side tanks lla-b approximately 90 degrees in front/advance of the vessel roll period. Accordingly, by the controlled fluid flow between the side tanks lla-b the affection from the waves on the vessel 100 can effectively be counteracted in an active manner by that the fluid is forced to flow from side tank lla-b to side tank lla-b counteracting the vessels roll motion, which will reduce the vessels roll considerably.

The control unit 60 is for this provided with means and/or software for software for controlling the fluid level in the side tanks lla-b based on measurements by the dedicated motion reference unit 50 and measurements from the fluid level sensors 40 by calculating and provide optimal fluid level in the side tanks lla-b.

By means of readings/measurements from the motion reference unit 50 of the vessel accelerations, the control unit 60 is arranged to calculate and provide optimal fluid level in the side tanks lla-b based on input from a dedicated motion reference unit 50 measuring the vessel motion accelerations and in this way capable of setting the fluid level in anti-phase in front of the vessel roll period.

Accordingly, the control unit 60 will be provided with means and/or software for using latest historical information from the dedicated motion reference unit 50 regarding roll motion of the vessel, and based on this predict probable future next roll motion/sequence and based on this control the fluid level in the side tanks lla-b such that the fluid level is controlled in front/advance of the affection of the waves on the vessel.

The initial fluid volume inside the U-tank 10 must be defined by the crew based on the actual loading of the vessel 100.

The present invention can utilize the same tank volumes as Anti-heeling systems already present on a vessel or the vessel can be provided with a U-tank according to the present invention.

In addition to the above described active mode, the system according to the present invention can also be set in a passive mode. In passive mode the control unit 60 will set the vacuum and air pressure manipulation unit 20 in aeration mode, where the port side tank 11a is set open towards air, and the aeration device 30 of the starboard side tank lib is set open towards air. Accordingly, in passive mode the fluid can flow freely (passively) between the side tanks lla-b via the crossover duct 12 according to the vessels minimum roll period, 90 degrees behind the roll motion period. If this period is to be extended, one can achieve this by controlling the valves 23 and 31 via the control unit 60. For the second embodiment also the aeration device 30 in the port side tank 11a can be used for this purpose. The system according to the present invention can further be provided with a stabilization mode, wherein the control unit 60 is arranged to close the valve 23 of the air pressure manipulation unit 20 and valve 31 of the aeration unit 30 in starboard side tank lib locking the current fluid level in the side tanks lla-b. In the second embodiment also the aeration device 30 of the port side tank 11a have to be closed.

The U-tank 10 of the present invention can further be arranged to a fluid supply and removal system for supply and removal of fluid in the U-tank 10, for changing the total fluid amount in the U-tank, which is well known in prior art and requires no further description herein.

The system according to the present invention can further be provided with two or more vacuum and air pressure manipulation units 20 and aeration devices 30, respectively, connected to the associated side tanks lla-b, respectively, for redundancy. The vacuum and air pressure manipulation unit 20 can further be provided with two or more vacuum and air pressure generation units 21 connected to the same demand controlled ventilation unit 22 dimensioned for this. In an alternative embodiment the system can further comprise two or more vacuum and air pressure manipulation units 20 separately arranged to the same side tank 11a via separate valves 23 or via the same valve 23.

Reference is now made to Figure 4a which show an initial state of fluid level in the side tanks lla-b where the fluid level of in the port side tank 11a and the starboard side tank lib are levelled, for the first embodiment. Figure 4a could also represent a stabilized mode, where the valves 23 and 31 are closed. In connection with stabilized mode, the valves 23 and 31 can also be closed with different fluid levels in the port side tank 11a and starboard side tank lib if desired.

Reference is now made to Figure 4b which show a state, for the first embodiment of the system, where the fluid level in port side tank 11a is elevated in relation to the fluid level in starboard side tank lib, by that the vacuum and air pressure manipulation unit 20 supplies a vacuum on the fluid surface of the fluid in the port side tank 11a, resulting in a pressure difference between the side tanks lla-b resulting in that fluid flows from the starboard tank lib and into the port side tank 11a, and thus compensating for roll movements caused by a wave hitting on port side or a wave passing from starboard side.

Reference is now made to Figure 4c which show a state, for the first embodiment of the system, where the fluid level in starboard side tank lib is elevated in relation to the fluid level in port side tank 11a, by that the vacuum and air pressure manipulation unit 20 supplies pressure on the fluid surface of the port side tank 11a forcing the fluid to flow to the starboard side tank lib, reducing the fluid level in the port side tank 11a and elevating the fluid level in the starboard side tank lib, and thus compensating for roll movement caused by a wave hitting on starboard side or a wave passing from port side.

Accordingly, by the present invention is achieved a system where the fluid level in the respective side tanks lla-b are set to be in anti-phase in front/advance of the vessel roll motion period, and optimally approximately 90 degrees in front/advance of roll motion period. The fluid level difference between the side tanks lla-b will return to levelled with almost none use of energy due to the pressure difference between the side tanks lla-b. Accordingly, the vacuum and air pressure manipulation unit 20 will only require power consumption for half of the required pressure difference, as the first half of the pressure difference can be achieved with the vacuum and air pressure manipulation unit 20 in idle running.

Further, by using air pressure and vacuum in only one of the side tanks for controlling the fluid level in both of the side tanks lla-b this will results in only approx. 50 % power requirement compared to prior art systems, as the present invention will utilize the fluid height difference and will only have to perform half the work for moving the fluid volume from one side tank lla-b to the other side tank lla-b due to the use of the combination of pressure and vacuum.

The shape and volume of the side tanks lla-b will have to be adapted the actual properties of vessel. Further, it will an advantage, that the tanks have large volume and that the system can operate with low pressure differences. The above described embodiments can be combined to form alternative embodiments within the scope of the attached claims.