CONTROL SYSTEM AND METHOD FOR CONTROLLING A TOWED MARINE OBJECT - Patent application

Abstract

A control system for a towed object (1, 6, 20, 30, 50) in or on a body of water comprises one or more towing members (3) extending between the object (1) and a towing vessel (2). One or more ballast members (4) are movably connected to respective ones of the one or more towing members (3) and are controllably movable at least a distance along their respective towing members. The towed object (1) is controllable by moving the one or more ballast members (4) on the towing members (3) to control a catenary (c) of the towing members (3). Each ballast member (4) may be individually manipulated to correct, preserve, or otherwise change the geometry of the trawl.

Description

[Technical field]

[0001] The present invention relates to a system and method that can be used in controlling one or more towed marine objects, as set out by the preambles of claims 1 and 11. The marine objects may be submerged objects (e.g. a trawl net or a seismic streamer cable) or objects that float on the surface (e.g. a barge, a floating dock or a ship). [Background technology]

[0002] Trawl nets are commonly used to capture various types of marine life and also to collect other objects on or near the seabed. In general, and with reference to FIG. 2, one type of trawl net comprises a funnel-shaped bag 1 of flexible material with a net or mesh structure that allows water to pass through. The trawl mouth (front end) is open and the narrower rear end is closed. The mesh size is determined according to the intended catch. The trawl net is usually towed through the water by one or more vessels 2 (trawlers) and maintained at a desired depth by suitable ballast elements. In one configuration, bottom trawl fishing, the trawl net is towed along or near the seabed. In another configuration, mid-water trawl (also called pelagic trawl), the trawl net is towed through the free water above the seabed. Mid-water trawls catch various types of zooplankton (such as krill), pelagic fish such as anchovies, shrimp, tuna, and mackerel, while bottom trawls target both demersal and semi-pelagic fish such as cod, halibut, and rockfish, as well as other objects on the sea floor.

[0003] The vertical extent of the trawl bag (net) is usually provided by one or more buoyancy elements 10 in the upper region of the trawl mouth (front end) and one or more ballast elements (weights) 7 at or near the bottom of the trawl mouth. The horizontal extent of the trawl bag is usually provided by two trawl doors 6, one on each side of the trawl mouth. Trawl doors come in various sizes and shapes and are designed to maintain contact with the seabed (in bottom trawl fishing) or to maintain lift at a desired depth. Trawl doors essentially act as wings with a shape and / or slope (angle of attack) to provide hydrodynamic forces and maintain horizontal extent. The towing vessel must maintain a certain speed for the trawl doors, and therefore the trawl itself, to remain functional. Thus, there is usually a minimum speed at which the trawl net can be towed and still maintain its integrity and fulfill its intended purpose. This minimum trawl speed also affects the turning radius of the towing vessel and can cause the trawl to "stall" and break apart if the turn is too sharp.

[0004] During trawling, the catch can be collected by reeling the trawl net up to the surface or to a vessel, or by pumping the catch up to the surface via a hose connected to the rear end of the trawl bag. An example is described in International Application No. WO2018 / 174723, which is incorporated herein by reference. The latter method is used, for example, to collect zooplankton.

[0005] Another example can be found in Norwegian Application No. NO20181676, which describes a trawl arrangement comprising a trawl vessel, a trawl bag assembly coupled to the trawl vessel by several separately controllable trawl connections, such as suitable wires, chains or ropes, and at least two outer trawl net doors at least indirectly connected to (or near) the side extremities of the trawl bag assembly. Trawls are well known in the art and need not be described in more detail here.

[0006] The prior art also includes International Application No. WO2014 / 122494A1, which describes a surveying apparatus having at least one survey cable, each having a proximal end attached to a mother ship, a distal end connected to at least one underwater towing vessel, and at least one surveying device connected to the survey cable between the proximal and distal ends. The survey cables extend in a direction perpendicular to the longitudinal axis of the mother ship during surveying. The survey cables extend a distance laterally from the mother ship, for example under an ice cap, for example a tightly packed ice or ice floe.

[0007] Figures 1 and 2 show a trawl net 1 with two trawl doors 6, towed by a trawl vessel 2 in a body of water W, at a distance l1 behind the trawl vessel and below the water surface S. The trawl doors 6 are similar in practice, but are arranged to deflect the trawl mouth in opposite directions. In this specification, the trawl net and the trawl doors can be considered as one unit, collectively called the trawl net. The trawl vessel moves with a speed v relative to the water and is connected to the trawl net (through the trawl doors) by two trawl wires 3. Those skilled in the art will understand that a greater or lesser number of trawl wires can be used. For example, a third intermediate trawl wire can be added to provide support for a hose that runs from the rear end of the trawl bag to the vessel.

[0008] The trawl 2, the trawl net 1 and the trawl wires 3a, b together form a dynamic system in which the trawl wires in effect act as springs by their ability to form a catenary. The trawl wires take up the slack that would otherwise form due to the relative movement between the trawl and the net. In figure 2 the catenary is indicated as c and is the deflection of the net relative to an imaginary line L between the trawl and the net (here the trawl door).

[0009] During trawling, and especially during mesopelagic (pelagic) trawling, it may be necessary or desirable to raise the trawl net in waters of greater or lesser depth. In prior art methods, this is done by reeling in the trawl wire, which reduces the length of the deployed trawl wire and the distance between the trawl vessel and the trawl net. This condition allows for a very short catenary, or in the extreme case no catenary at all, in which practically all elasticity (and therefore braking) in the system is greatly reduced. This leads to significant dynamic loads on the trawl vessel, the trawl net and the wire, which may terminate the trawl operation. The reeling operation also involves the risk of the trawl wire getting entangled in the trawl vessel's propeller or rudder. The further reduction in distance leads to a less controlled spread of the trawl vessel and the trawl net, especially during turns. Finally, especially when trawling for krill or other types of zooplankton, the reduction in the distance between the trawl vessel and the trawl net makes it necessary to reposition the aforementioned hose between the rear end of the trawl bag and the trawl vessel.

[0010] A problem associated with bottom trawl fishing is that the trawl net doors, and sometimes parts of the net, are dragged along the seabed, causing considerable damage to the marine flora and fauna on the seabed.

[0011] The prior art includes British Application No. GB811853A (published in 1959) which discloses a variable buoyancy floating structure for towing trawl nets, adapted to be towed at variable depths. The floating structure comprises at least one tank filled with compressed air, at least one chamber having a port through which seawater can be fed or discharged, means for controlling the inflow and outflow of compressed air in response to variations in the amount of seawater in the chamber and therefore for regulating the buoyancy of the float, and a compressed air container carried by the float and providing the inflow and outflow means of air. The air container comprises an air carrying valve and an air releasing valve resiliently mounted towards a closed position, the two valves being connected for alternate opening and closing by moving means consisting of a member movable by a bulkhead, which receives on one side the pressure in the chamber and on the other side air at a pressure adapted to vary according to the desired diving depth. The amount of seawater in the chamber is adjusted so that the float is in equilibrium and by blowing seawater out or allowing seawater to flow in through the port, the float is raised or lowered to a new desired depth. By blowing out the water that was allowed in, or vice versa, equilibrium is restored again at the new depth.

[0012] Another trawl type is the beam trawl, in which the horizontal extent of the trawl bag is provided by a beam that is positioned generally horizontally in the water. Beam trawls do not utilise trawl doors, but some of the issues discussed above also apply to this trawl type. Summary of the Invention

[0013] Therefore, there is a need for a system that can control the vertical position of an object being towed through a body of water without the drawbacks associated with the prior art.

[0014] The present invention is set forth and characterized in the independent claims, while the dependent claims describe further characteristics of the invention.

[0015] Thus, there is provided a control system for an object being towed in or on a body of water, with one or more towing members extending between the object and the towing vessel, characterised by one or more ballast members movably connected to respective ones of the one or more towing members and controllably movable at least a distance along their respective towing members.

[0016] At least one of the ballast members and / or the towed object may comprise sensing and detection means for determining the distance to the seabed. In one embodiment, the control system comprises a control device that can be used in controlling the position of the ballast member on the towed member. A weight member is connectable to the ballast member and extends a distance from the ballast member towards the seabed.

[0017] The towed object may be a trawl net, with elongated objects such as trawl boards, cables or risers, etc., and may comprise buoyancy elements that may make the towed object neutrally buoyant. The towed object may be a vessel that is towed across the water surface.

[0018] The towed object and / or the at least one ballast member in one embodiment comprises sensing and detection means configured to determine the distance between the means and the seabed, the sensing and detection means configured to communicate with a transmitter / receiver at a distal location, the transmitter / receiver configured to communicate with a control unit configured to communicate with a device for controlling the position of the at least one ballast member. The control unit may comprise a programmable control unit.

[0019] There is also provided a method of controlling a towed object in or on a body of water, characterized by controllably moving one or more ballast members movably connected to a respective one of the towing members extending between the towed object and the towing vessel at least a distance along its respective towing member to control the catenary of the towing members. The individual ballast members may be individually manipulated to correct, preserve or otherwise change the geometry of the towed object, the towed object being a trawl net. In one embodiment, the movement of the one or more ballast members is controlled based on information provided by sensing and detection means configured to determine the distance between the means and the seabed. The distance may be predetermined and input into the control unit.

[0020] These and other characteristics of the invention will become apparent from the following description of one embodiment, given as a non-limiting example, with reference to the attached schematic drawings. [Brief description of the drawings]

[0021] [Figure 1] 1 is a sketch in a perspective view showing, in principle, a towed object (trawl net) towed at a distance behind a towing vessel (trawler) in a body of water according to the prior art; [Diagram 2] 1 is a sketch in a side view according to the prior art, showing in principle a trawl net being towed at a distance behind a trawl vessel in a body of water, and also showing how the trawl wires form a catenary in the water. [Diagram 3] FIG. 1 is a sketch in a side view showing, in principle, a trawl net towed at a distance behind a trawl vessel in a body of water and showing how the trawl wire catenary can be controlled by means of ballast members. [Figure 4] 4 is a sketch in a perspective view corresponding to FIG. 3. [Diagram 5] FIG. 4 is an enlarged view of area "A" in FIG. [Figure 6] FIG. 1 is a schematic side view showing an embodiment of the present invention. [Figure 7] FIG. 1 is a schematic side view showing an embodiment of the present invention. [Figure 8] FIG. 1 is a schematic side view illustrating an embodiment of the present invention in which the object being towed is a seismic streamer cable. [Figure 9] FIG. 1 is a schematic side view illustrating an embodiment of the present invention in which the object being towed is a seismic streamer cable. [Figure 10] FIG. 1 is a schematic side view of one embodiment of the present invention in which the object being towed is an object being dragged or moved along the ocean floor. [Figure 11] FIG. 1 is a schematic top view showing a misaligned trawl net. [Figure 12] FIG. 1 is a schematic top view showing a turning trawl and trawl net. [Figure 13] FIG. 1 is a schematic side view of a surface vessel being towed and how the present invention is used to control a towing member catenary; [Figure 14] FIG. 1 is a schematic side view of a surface vessel being towed and how the present invention is used to control a towing member catenary; [Figure 15] FIG. 1 is a schematic side view showing one embodiment of the present invention. [Figure 16] FIG. 1 is a schematic side view of an embodiment of the present invention, particularly showing distance measurement equipment and configuration; [Figure 17] FIG. 2 is a block diagram showing how a towed object can be controlled. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] The following description may use terms such as "horizontal", "vertical", "side", "front-rear", "top-bottom", "upper", "lower", "inner", "outer", "front", "rear", etc. These terms generally refer to views and orientations as shown in the figures and associated with normal use of the invention. The terms are used for the convenience of the reader only and should not be considered limiting.

[0023] A first embodiment of the invention will now be described with reference to figures 3 to 5. A trawl net 1 with two trawl doors 6 is towed by a trawl vessel 2 in a body of water W, at a distance behind the trawl vessel and below the water surface S. In the embodiment shown, the two trawl doors 6 are substantially similar, but are arranged to deflect the trawl mouth in opposite directions. The notations a, b, c etc. after the reference numbers will be used below to distinguish the items where necessary.

[0024] The trawl net itself is generally described above with reference to FIG. 2 and is further known in the art. Herein, the trawl net and the trawl doors can be considered as one unit, collectively referred to as the trawl net or towed object. The trawl vessel moves with a speed v relative to the water and is connected to the trawl net (here via the trawl doors) by towing devices 3, here in the form of two trawl wires 3, one for each trawl door. Those skilled in the art will appreciate that a greater or lesser number of towing devices can be used. For example, a third intermediate trawl wire can be added to provide support for the hoses that run from the rear end of the trawl bag to the vessel. The invention is not limited by the number of towing devices, trawl doors, trawl types, or number of trawls.

[0025] As mentioned before, the trawl 2, the trawl net 1, the trawl door 6 and the trawl wire 3 together form a dynamic system in which the trawl wires in effect act as springs due to their ability to form a catenary and the friction between the wires and the water. The trawl wires (for example one wire in the case of a beam trawl, or more wires) take up the slack that would otherwise form due to the relative movement between the trawl and the towed object (here the trawl net). However, to control and increase this catenary, the invented control system comprises a ballast member 4 movably arranged on each towing member (here the trawl wire) 3. The double arrow M in FIG. 5 indicates such a movement. The ballast member 4 can be a clamp weight or any other solid object with negative buoyancy suitable for the intended purpose. The ballast member can have a hydrodynamically favorable shape in order to minimize the hydrodynamic drag. The ballast member can also comprise means for controlling its weight. The ballast member may be connected to the towing member by means of a housing with wheels, rollers (not shown) or the like. Reference number 5 denotes a control cable which can be used in adjusting the position of the ballast member 4 on the trawl wire 3. The cable 5 may be connected to a winch means (not shown in Figures 3 to 5) on the vessel (trawler) 2. Pulling the control cable 5 raises the ballast member 4 in the water, thus decreasing the catenary, and paying out the control cable causes the ballast member 4 to sink deeper in the water (due to its negative buoyancy), thus increasing the catenary.

[0026] Thus, by lowering the ballast members 4 as shown in Figure 5, the catenary c is significantly increased compared to the catenary in the prior art configuration (Figure 2), and the part of the towing member 3 extending between the ballast members 4 and the trawl net, designated 3' in Figures 3 to 5, is substantially horizontal. Thus, there is practically no upward pull on the trawl net (trawl door and trawl net) and the trawl depth can be controlled by manipulating the ballast members 4.

[0027] The invented control system allows for a significant reduction in trawl speed without compromising the integrity of the trawl net, which is also desirable when trawling for krill and other zooplankton.

[0028] It will be understood that the invented control system is also applicable to beam trawls, where the buoyancy elements 10 and the trawl door 6 in the upper region of the trawl mouth (front end) are replaced by a buoyancy beam (not shown) of, for example (but not limited to) 1 meter diameter and 30 meters long (i.e. generally horizontal and perpendicular to the trawl bag mouth).

[0029] A second embodiment of the invention will now be described with reference to figures 6 and 16. Unless otherwise stated, the features and aspects described with reference to figures 3 to 5 also apply to this second embodiment and the embodiments can be combined. Here, the trawl net 1 is provided with a buoyancy element 22 and the trawl door 6 comprises a buoyancy chamber 21. The buoyancy element 22 and the buoyancy chamber 21 are adjustable and can be controlled so that the trawl net and the trawl door have neutral buoyancy. The buoyancy can also be adjusted to be slightly positive, so that the trawl net can float to the water surface when the ballast member 4 is lifted towards the surface. This can be done without reeling in the trawl wire.

[0030] The ballast member 4 or the housing connecting it to the trawl wire 3 is fitted with a distance measuring device 73 for emitting and receiving signals 11 for determining the distance to the seabed B. Distance measuring devices (e.g. or echo sounders or sonars) are well known in the art and need not be described further. The signals 11 can be communicated to a control system (not shown in Figures 6 and 16) on the vessel 2 and the position (i.e. depth) of the ballast member 4 can be controlled based on the sensed distance to the seabed. As the trawl net and trawl doors are neutrally buoyant, they are balanced in the water body and the distance h to the seabed can be controlled by manipulation of the ballast member 4. Thus, contact forces between the towed object and the seabed can be minimized or eliminated.

[0031] 16 and 17, in addition to or as an alternative to the distance measuring device 73 on the ballast member 4 described above, a distance measuring device 80, 80' may be attached to the trawl 1 and / or the trawl door 6 and configured to emit and receive signals 11 for determining the distance between the towed object (the trawl and / or the trawl door) and the seabed B. Signals containing the distance information (indicated by arrows 12 in FIG. 6) may be communicated to a control system 71 on the vessel 2. The control system is configured to control the operation of the ballast member control cables 5, whereby the distance between the towed object and the seabed may be controlled and adjusted. This is shown diagrammatically by the block diagram of FIG. 17, where reference numeral 73 denotes the distance measuring device on the ballast member 4 and reference numerals 80, 80' denote the distance measuring devices on the trawl door and on the trawl. It will be appreciated that the trawl and trawl board may be considered as one object, and as previously mentioned the ends may be replaced by any other towed object (eg a beam trawl).

[0032] The determined distance between the towed object and the seabed is communicated via signal 11 to a transmitter / receiver (e.g. a transceiver or any other transmitter / receiver device known in the art) 70 on the vessel 2. The transceiver 70 communicates the distance information to a control unit 71 which uses this information to send control signals to the trawl wire winch 61 and / or a winch or other device 72 which controls the position of the ballast member 4. The trawl wire 3 and / or the control cable 5 are thus operable based on input from the distance measuring device 80, 80'. The control unit 71 may also be connected to the vessel's engine and / or rudder control 75, so that the speed and / or direction of the vessel may be controlled based on data sensed from the towed object.

[0033] The control unit 71 may comprise a programmable control unit (PLC) whereby the operator can input the desired distance (between the object to be towed and the seabed) into the PLC which will then control the device 72 controlling the ballast members and / or the object to be towed such that this pre-determined desired distance is automatically maintained based on the information provided by the signal 11.

[0034] The towed object may also be controlled based on sensed depth measurements provided by distance measuring equipment (echo sounders, sonar, etc.) 74 on the vessel.

[0035] Referring again to FIG. 16, the distance l1 between the vessel and the object being towed may be a significant distance, for example on the order of 1000 meters. The distance l between the ballast member 4 and the object being towed may be a significant distance, for example on the order of 1000 meters. bmay be a significant distance, for example approximately 300-500 meters. Thus, in one mode of operation, the distance to the seabed sensed by the distance measuring equipment 74 on the vessel may be used to control the depth of the towed object by manipulating the ballast members and / or the towed object's wires and / or the vessel's engines. The control unit 71 may also include a proportional-integral-derivative controller (PID) or similar device, whereby input data from any one of the distance measuring equipment units 80, 80', 73, 74 may be used to control the towed object as desired. It is noted that other sensors (not shown) (e.g. temperature, current, speed) may be provided to the control unit and used as parameters for controlling the towed object.

[0036] Communications between the devices identified in FIG. 17 can be by any means known in the art, hardwired or wireless.

[0037] The setup shown in Figures 16 and 17 can be used as a control system for a balanced trawl or other object. Device 74 includes instrumentation built into the vessel, typically as an echo sounder, sonar, or other type of instrumentation that can survey the bottom and position of the vessel, fish or krill, or objects in the water. Device 73 includes instrumentation built into the ballast member that is used to manipulate the towed object (e.g., the trawl). This provides feedback on the response to operate the ballast member or the main trawl winch. Devices 80 and / or 80' include instrumentation that provides feedback on how the towed object reacts based on the position of the ballast member or the operation of the trawl winch.

[0038] The control unit 71 comprises a computer program that receives inputs from the sensors 80, 80', 73, 74 and works with the data from these sensors. This program makes it possible to operate the trawl in a very controlled manner. It makes it possible to stay very close to the seabed and to maintain a constant distance during any towing condition. This reduces any physical contact with the seabed and therefore any damage to it. Damage to the trawl net can also be avoided.

[0039] The system can be set in a "hunting" mode where a particular depth region is defined within which the trawl should operate, for example a depth between 400 and 800 meters. Within this window, the winch computer control system should also be able to determine the true sonar or echo sounder to position the trawl in such a way as to optimize catchability (fish, krill, etc.). The system can also be operated manually.

[0040] The control system can follow any given parameters regarding water depth or distance to the seabed and actively make its own decisions to position the trawl. The control system has feedback from any and all of the sensors and devices and actively operates both the trawl winches and the winches for the ballast members to balance the trawl.

[0041] A third embodiment of the invention will now be described with reference to Figures 7 and 15. Unless otherwise stated, the features and aspects described above with reference to Figures 3 to 6 also apply to this third embodiment, and the embodiments can be combined. Here, the ballast member 4 or the housing connecting the ballast member to the trawl wire 3 is equipped with a clamping weight object 24 (or other solid mass) attached to a connecting member 23 (e.g. a chain, a tether, a rope, or an object with a similar function). The clamping weight object 24 can be equipped with wheels or rollers to minimize friction against the seabed. The length e of the connecting member 23 can be dimensioned so that the towed object balanced in the water maintains a minimum distance h above the seabed B. The object 24 can be shaped as a skid and can also be arranged with respect to the trawl door and the connecting member under the trawl, as shown in Figure 15. The trawl can also comprise sections (not shown) and the object 24, and the connecting member 23 can be arranged under each such section.

[0042] A fourth embodiment of the invention will now be described with reference to Figures 8 and 9. Unless otherwise stated, the features and aspects described above with reference to Figures 3 to 7 also apply to this fourth embodiment, and the embodiments can be combined. Here, the aforementioned trawl net and trawl door are replaced by a seismic streamer cable 30 with a buoyancy element 22. Although Figure 8 shows only one streamer, it should be understood that several streamers 30 can be towed by a corresponding number of towing devices (wires) 3, the catenary of which can be controlled by the operation of individual ballast members 4 as described above. A deflector (not shown) similar to the aforementioned trawl door can also be used to form the spread of the seismic streamer cable, as is known in the art. Figure 8 also shows a variation of the ballast member 4' shaped to exhibit favorable hydrodynamic properties. This variation can also be used in other embodiments of the invention. The invented control system thus facilitates the towing of a streamer spread at a desired (and constant) depth well below the water surface S. In this way, exposure to wind, waves and other surface conditions is mitigated or avoided. The present invention is not limited to seismic streamer cables but is also applicable to other towed objects. Examples of such towed objects are cables, pipelines, hoses, umbilicals, rises, etc. used in the oil industry.

[0043] A fifth embodiment of the invention will now be described with reference to FIG. 10. Unless otherwise stated, the features and aspects described above with reference to FIGS. 3 to 9 also apply to this fifth embodiment, and the embodiments can be combined. The towed object here is an object 50 that is dragged along the seabed. This dragged object 50 can be, for example, a plough, a scraper, or a collector container used to collect objects from the seabed. One possible use for such a dragged object is mineral mining. By adjusting the length e and / or manipulating the ballast member 4 on the towing device (wire, rope, etc.) 3, the inclination angle α of the part 3″ of the towing device that extends between the ballast member 4 and the towed (dragged) object can be changed, thereby increasing or decreasing the downward force exerted by the towed object 50 on the seabed. Although FIG. 10 shows only one towed object 50, it should be understood that several objects 50 may be towed by several towing devices (wires) 3, as previously described.

[0044] Figures 11 and 12 show two examples of how the invented control system can be used to control a trawl or similar towed object. In Figure 11 the entrance of the trawl is misaligned. This misalignment can be corrected by moving one ballast member 4a towards the trawl and / or the other ballast member 4b towards the towing vessel 2, as indicated by the arrows. Thus, in general the trawl geometry can be controlled by the manipulation of the ballast members 4. It will be appreciated that the same principle applies to the spreading of seismic streamer cables.

[0045] FIG. 12 shows how the invented control system can be used to control a trawl or similar towed object during a turn. When the towing vessel 2 deviates from a straight course (dotted line) and enters a turn, the inner ballast member 4a (with respect to the turn) is lowered in the water (moved away from the vessel) and the outer ballast member 4b is raised (moved towards the vessel). The ballast members can be used to trim the trawl before and during the turn, thus preventing it from collapsing or becoming misaligned. With this control system, the vessel speed can be reduced and the turn can be sharper compared to prior art systems. It is understood that the same method also applies to the spreading of seismic streamer cables.

[0046] Although the invented control system is described above in relation to the control of a submerged towed object, it is also applicable to control towed objects floating on the water surface, such as barges, pontoons, ships, and other floating vessels. An example of such an application is shown in Fig. 13 and Fig. 14, where a towed surface object 20 is towed by a towing vessel 2 via a towing device 3. It will be understood that multiple towing devices can be used. The towing device 3 can be a steel wire, a chain, a synthetic rope, etc. An adjustable (movable) ballast member 4 is used to control the towing device catenary. The ballast member 4 can have a shape with favorable hydrodynamic damping properties. It will be understood that a deep catenary (as shown in Fig. 14) prevents cavitation.

Claims

1. A control system for a towed object (1, 6, 20, 30, 50) in or on a body of water, comprising: In said control system, one or more towing members (3) extend between said object (1) and a towing vessel (2), The control system includes: one or more ballast members (4) movably connected to respective ones of the one or more towing members (3) and controllably movable at least a distance along their respective towing members; a control device (5) for controlling the ballast members on the towing member (3); A control system comprising:

2. 2. A control system according to claim 1, wherein at least one ballast member (4) and / or the towed object is equipped with a distance measuring device for determining the distance to the seabed (B).

3. 3. A control system as claimed in claim 1 or 2, wherein the control device (5) comprises a control cable connected to a winch means on the towing vessel (2).

4. A control system as claimed in any one of claims 1 to 3, further comprising a weighted object (24) connected to the one or more ballast members (4) and extending a distance (e) from the one or more ballast members (4) towards the seabed.

5. A control system according to any one of claims 1 to 4, wherein the towed object is a trawl net (1) with a trawl board (6).

6. A control system according to any one of claims 1 to 4, wherein the object to be towed is an elongated object.

7. The control system of claim 6, wherein the elongated object is a seismic cable (30), or a cable, pipeline, hose, umbilical used in the oil industry.

8. A control system as claimed in any one of claims 1 to 4 or claim 7, wherein the towed object comprises a buoyancy element (22) such that the towed object is neutrally buoyant.

9. A control system as claimed in any one of claims 1 to 4, wherein the object to be towed is a vessel (20) towed over a water surface (S).

10. At least one of the towed object and / or the ballast member comprises a distance measuring device configured to determine a distance to the seabed (B), The distance measuring device is configured to communicate with a transmitter / receiver (70); The transmitter / receiver (70) is configured to communicate with a control unit (71); A control system according to any one of the preceding claims, wherein the control unit (71) is configured to communicate with a device (72) for controlling the position of at least one ballast member (4).

11. The control system of claim 10, wherein the control unit (71) comprises a programmable control unit (PLC).

12. A method for controlling a towed object (1) in or on a body of water, comprising the steps of: controllably moving one or more ballast members (4) movably connected to respective ones of the towing members (3) at least a distance along their respective towing members (3) to control the catenary (c) of said towing members (3); The towing member (3) extends between the towed object (1) and a towing vessel (2).

13. each ballast member (4) is individually manipulated to correct or preserve the geometry of the towed object; The method of claim 12 , wherein the towed object is a trawl net.

14. 14. The method according to claim 12 or 13, wherein the movement of the one or more ballast members (4) is controlled based on information provided by a distance measuring device arranged to determine a distance to the seabed (B).

15. The method of claim 14 , wherein the distance is predetermined.

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