An adjustable buoyancy device, use of the device, and methods for mounting and demounting ship thrusters

Abstract

The adjustable buoyancy device (100, 200) for mounting and / or demounting a thruster (301) of a ship comprises a main frame (110, 210) comprising a first structure (111, 211) forming a first side, a second structure (112, 212) forming a second side, a third structure (113, 213) forming a third side connecting the first structure to the second structure, a free space formed between the first, second and third structures to allow at least the portion of the thruster to pass from at least partially above to at least partially below the main frame, and one or more buoyancy tanks (121, 122, 220-224) formed at least partially by or within the first, second and / or third structures, means (130) for controlling the amount of water in one or more of the buoyancy tanks, and supporting means (140, 240) attached to the main frame and configured to support at least the portion of the thruster.

Description

[0001] An adjustable buoyancy device, use of the device, and methods for mounting and demounting ship thrusters

[0002] Technical field of the invention

[0003] The present invention relates to the field of mounting and demounting, especially underwater, thrusters of ships. More specifically, the invention concerns an adjustable buoyancy device for mounting and demounting thrusters. The invention further concerns use of said adjustable buoyancy device for mounting and demounting thrusters as well as a method for mounting and a method for demounting thrusters using said adjustable buoyancy device.

[0004] Background of the invention

[0005] Thrusters are an essential part of a ship and its propulsion system. Thrusters are typically mounted and demounted onshore or in dry docks, however, such methods are expensive and time consuming as it requires the ship to be routed to a harbor or a large dry dock needs to be brought to ship’s location to replace or repair a thruster.

[0006] Underwater replacement of thrusters enables mounting and demounting to be carried out in sea. However, such replacement of thrusters usually requires strand jacks, large team of workers, deep water and good weather conditions. In some conventional underwater methods, demounting of thrusters is carried out by first lowering the thruster from its mounting point with strand jacks or other means attached to the ship and hooking, usually by a team of divers, the thruster up to a main lifting cable and a side pull wire. The thruster is then lowered further while pulling on the side pull wire until there is sufficient tension in the main lifting cable, at which point the cables attaching the thruster to the ship can be released and the main lifting cable is used to lift the thruster up to the surface. Such lifting method, however, requires a large depth of water under the ship to maneuver the thruster with safety margin. Larger depths further increase safety risks and make the replacement method dependent on location and weather conditions, which may result in delays and hence higher costs. Such method also requires a large team of workers.Summary of the invention

[0007] An object of the invention is to provide an adjustable buoyancy device that solves at least some of the problems of prior art. Another object of the invention is to provide a use of said device for mounting and demounting ship thrusters or at least a portion of a ship thruster. Additional object of the invention is to provide a method for mounting and demounting ship thrusters or their portions. The invention is defined in the independent claims.

[0008] Adjustable buoyancy device and device will be used interchangeably throughout this application. Terms such as top and bottom, up and down, above and below are defined in this application with respect to the seabed, i.e. bottom of the sea where bottom surface of an object faces the bottom of the sea, whereas top surface faces away from the bottom of the sea, i.e. towards water surface or sky. Similarly, up and down refers to directions towards and away from the seabed, whereas above and below refers to positions further away and closer to the seabed respectively. In this application, the term seabed refers to the bed or bottom of any water body, e.g. a sea, canal, river or lake without considering any local terrain variations. Similarly, the water surface is assumed to be flat, not considering the unevenness due to waves. The adjustable buoyancy device may float or be fully submerged at a required depth depending on the buoyancy force provided.

[0009] Instead of always referring to at least a portion of a thruster, term thruster refers in this application to either whole or part of the thruster that is to be mounted or demounted, i.e. the removable portion of the thruster, which could be the lower gearbox, the lower gearbox together with the steering section, the whole thruster, or other combination. The at least a portion of a thruster to be mounted or demounted preferably comprises at least the gear housing, propeller, and nozzle. It may comprise also the steering section.

[0010] The adjustable buoyancy device according to the invention comprises a main frame comprising a first structure forming a first side, a second structure forming a second side, a third structure forming a third side connecting the first structure to the second structure, a free space formed between the first, second and third structures to allow at least the portion of the thruster to pass from at least partially above to at least partially below the main frame, and one or more buoyancy tanks formed at least partially by or within the first, secondand / or third structures. The adjustable buoyancy device further comprises means for controlling the amount of water in one or more of the one or more buoyancy tanks, and supporting means attached to the main frame and configured to support at least the portion of the thruster. The means for controlling the amount of water may be more generally referred to as means for adjusting buoyancy of the device.

[0011] The adjustable buoyancy device according to the invention enables mounting and demounting of thrusters in shallower and possibly sheltered waters as the required depth when using device of the invention is reduced to about the height of the thruster, with an added safety margin. The shallower and / or sheltered waters further reduces the risks due to weather conditions thus providing safer operation and less delays. The design of the main frame comprising the free space, as defined in the independent claim, further provides a highly versatile device that can be used with thrusters of various dimensions as well as made by various manufacturers. The device according to the invention thus provides means for mounting and demounting thrusters that may be simpler, cheaper, safer, require less manual labor, less dependent on the weather conditions, and more versatile and compatible with various thrusters.

[0012] The means for controlling the amount of water are configured to allow controlling the amount of water at least in one tank formed by or within the first structure and at least in one tank formed by or within the second structure. This allows keeping the device stabile.

[0013] The main frame may be U-shaped. The main frame may further comprise a fourth side parallel or at least opposite to the third side and connecting the first side with the second side. The U-shaped main frame may thus be defined as a main frame where said fourth side is open. In other words, the free space of the main frame may further be configured to allow at least the portion of the thruster to pass from a side outside the main frame other than above or below the main frame to the free space surrounded by the first, second and third structures. U-shape is preferably configured to be used in substantially horizontal orientation, i.e. with all three structures being substantially parallel to the water surface. The term substantially horizontal or parallel takes into account deviation from the horizontal plane due to imperfect balancing of buoyancy of the device. A structure is considered substantially parallel to the water surface if the structure’s longest edge is substantially parallel to the watersurface. The main frame can, however, have a different shape, such as rectangular or square. For example, the main frame may comprise a fourth structure forming a fourth side. The fourth side may be parallel or at least opposite to the third side and connect the first side with the second side. The main frame may comprise more than three or four structures. The U-shape allows the device to be moved in around the thruster without needing to go fully under the bottom of the thruster, thus reducing the depth of water required under the hull of the ship for mounting or demounting of the thruster.

[0014] The device may be configured to provide a buoyancy force sufficient to lift thrusters weighing 25-80 metric tons.

[0015] The dimensions of each of the first structure and the second structure may substantially correspond to the dimensions of a 20-foot shipping container or each of the structures may comprise a module having dimensions that substantially correspond to the dimensions of a 20-foot shipping container. This allows easy transportability of the device.

[0016] The first structure and the second structure may each be in the form of or formed by a shipping container. Each structure may be in the form or formed by a plurality of shipping containers. Preferably the first and second structures are each made of or in the form of the same number of containers. The shipping container may be a 20-foot shipping container, such as defined by the ISO standard 668:2020 thus allowing easy transportability, low cost and availability. Instead of a 20-foot container, the container could, be, for instance, a 40-foot high-cube container. Any or each of the main frame structures may be in the form of or made of one or more of such containers, where a main frame structure made of more than one container provides a higher buoyancy force. The at least one of the one or more buoyancy tanks may comprise a bottom opening to allow surrounding water to flow into the buoyancy tank. The bottom opening may enable a simpler and cheaper construction of the adjustable buoyancy device compared to a pressurized vessel. The bottom opening is preferably located at the bottom side of the buoyancy tank; however, it may also be located at the lower half portion of any of the sides adjoined to the bottom side. The bottom opening may refer to fully open bottom side, i.e. , the buoyancy tank may have an open bottom.Instead of using buoyancy tanks with bottom openings, closed buoyancy tanks could be used. At least one of the tanks can be closed or all the tanks can be closed. A benefit of closed buoyancy tanks is that they provide better stability. If an open buoyancy tank is tilted, part of the air within the tank may escape. Also, the displaced water volume of a closed buoyancy tank is constant and also the buoyancy is thus constant. In buoyancy tanks with bottom openings the air volume depends on the depth and the buoyancy thus decreases as a function of the submersion depth of the tank.

[0017] Closed buoyancy tanks can have, for instance, a cylindrical or spherical shape to withstand the pressure difference between the interior and the exterior of the tank.

[0018] At least one of the one or more buoyancy tanks may comprise at least two compartments. The device may be configured to allow separate buoyancy control of different parts of said buoyancy tank. By dividing the buoyancy tanks into two or more compartments, stability of the tanks is increased. Separate compartments within a buoyancy tank may allow adjusting the buoyancy force in the tank and its compartments separately, thus enabling multi-directional tilt control and enhancing the device’s stability and maneuverability while maintaining the main frame substantially parallel to the water surface when in use. Multiple compartments further provide an additional redundancy, allowing to limit air loss in case of an accident or failure.

[0019] In case of closed buoyance tanks, each tank may be divided into two or more fluid-tight compartments.

[0020] The main frame may comprise at least two of the one or more buoyancy tanks, a first buoyancy tank arranged in or as the first structure and a second buoyancy tank arranged in or as the second structure. Similarly to the multiple compartments discussed above, the main frame comprising at least two buoyancy tanks enables tilt and device stability control when in use.

[0021] The means for controlling the amount of water in at least one of the one or more buoyancy tanks may comprise means for supplying air into and means for removing air from said buoyancy tank, the means for supplying air comprising a supply inlet. The supply inlet may be arranged on said buoyancy tank and configured to be connected to an air supply.The means for supplying air may comprise a supply pipe for directing air from the supply inlet to a supply outlet at a predefined part of said buoyancy tank. The supply pipe may provide easier access to supply inlets, such as by enabling their arrangement in the vicinity of each other. A single supply pipe may be sufficient to provide air from a single supply inlet to a plurality of supply outlets, such as in a plurality of buoyancy tanks or plurality of buoyancy tank compartments.

[0022] The means for removing air may comprise a removal outlet with a valve thus providing a simple and cheap means for removing air. The means for removing air may further comprise a removal pipe for directing air from a removal inlet to the removal outlet.

[0023] The means for controlling the amount of water may comprise a control unit attached to the adjustable buoyancy device and accessible by an operator in the vicinity of the adjustable buoyancy device. This may provide simple and cheap means for controlling the amount of water. The means for controlling the amount of water may alternatively or in addition comprise a remote-control unit connected to at least a part of the means for controlling the amount of water via a wireless and / or wired connection and accessible by an operator at a distance from the adjustable buoyancy device. The means for controlling the amount of water may alternatively or in addition comprise an automated control system controlling the amount of water in at least one of the one or more buoyancy tanks based on sensor data and / or other known parameters, such as parameters of the ship, thruster, and transportation route. The control unit in the vicinity of the device, the remote-control unit and / or the automated control system may be provided individually or in combination with others for increased redundancy.

[0024] The adjustable buoyancy device may further comprise a bumper on the top side of the main frame to reduce damage in case of collision with the hull of the ship or other objects.

[0025] The supporting means may comprise hanging means configured to hold at least the portion of the thruster suspended from the main frame. Suspending the thruster from hanging means may allow the thruster to maintain its natural orientation and / or enable orienting the thruster more freely. Moreover,suspending the thruster may increase versatility and compatibility of the device with various thruster heights.

[0026] In addition, or alternatively, the supporting means may comprise a support frame configured to hold at least the portion of the thruster resting on the support frame. The support frame may provide added stability and enable use of the device with thrusters to which hanging means may not be easily attached. The support frame may be arranged at least partly under the main frame or under the free space of the main frame.

[0027] The support frame may be configured to lift and lower at least the portion of the thruster along an axis perpendicular to the water surface. This may contribute to easier mounting and demounting of thrusters, especially non-under-water demountable trusters or thrusters that do not have attachment points for attaching supporting means such as hanging means described above. The liftable support frame may be moved closer to the thruster to reduce the distance between the bottom of the thruster and the support frame during thruster detachment or move the thruster closer to the mounting point to reduce the distance the thruster needs to be lifted towards the mounting point for mounting. The liftable support frame may further provide protection for the thruster top from collisions by lowering the thruster below the top of the main frame.

[0028] The adjustable buoyancy device may further comprise a work platform attached to the main frame. The work platform may facilitate thruster attachment and detachment work, providing a platform for workers to stand and work on, which may be particularly beneficial for non-underwater demountable thrusters that may require manual work to detach the thruster from its mounting point. According to an aspect of the invention, the adjustable buoyancy device as described above is used for mounting and / or demounting at least a portion of a thruster of a ship. The device may be used fully submerged underwater. Preferably, the mounting and / or demounting of at least the portion of the thruster is performed under the water surface so that at least the device is fully submerged underwater at some time during use.

[0029] The method for mounting or demounting at least a portion of a thruster to or from a mounting point on a ship according to the invention comprises providing an adjustable buoyancy device as described above around the portion of thethruster that is to be mounted or demounted. In case of demounting of the thruster from the ship, the method further comprises detaching the thruster from the mounting point and lowering the thruster downwards, i.e. in a direction from the hull of the ship towards the seabed. The method further comprises supporting the thruster by the supporting means of the adjustable buoyancy device, moving the adjustable buoyancy structure with the thruster into a desired area, and in case of mounting of the thruster to the ship, raising the thruster in a direction from the seabed towards the mounting point. The order of the method steps may, but does not have to, be as described above. For example, in case of demounting of the thruster, the device may be provided around the thruster before, after or even at the same time as the thruster is being lowered. Preferably, the device is provided around the thruster or around the downward projection of the thruster before the lowering of the thruster. The method for mounting at least a portion of a thruster to a mounting point on a ship comprises providing an adjustable buoyancy device as described above at least around the portion of the thruster, supporting the portion of the thruster by the supporting means of the adjustable buoyancy device, moving the adjustable buoyancy device with at least the portion of the thruster to under the hull of the ship and towards the mounting point, raising at least the portion of the thruster in a direction from the seabed towards the mounting point, and attaching at least the portion of the thruster to the mounting point.

[0030] The method for demounting at least a portion of a thruster from a ship, wherein the portion of the thruster is attached to the ship at a mounting point, comprises providing an adjustable buoyancy device as described above under the mounting point and at least around the portion of the thruster, detaching at least the portion of the thruster from the mounting point, lowering at least the portion of the thruster in a direction from the mounting point towards the seabed, supporting at least the portion of the thruster by the supporting means of the adjustable buoyancy device, and moving the adjustable buoyancy device with at least the portion of the thruster into a desired area.

[0031] The adjustable buoyancy device provided in the method for mounting as well as the method for demounting may be fully submergible underwater. The method for mounting as well as the method for demounting may comprise fully submerging the device underwater at least during some of the method steps.The methods for mounting as well as for demounting may further comprise sealing off the top of the portion of the thruster and the hull at the mounting point to prevent water from entering the portion of the thruster and the hull. In cases where the mounting point is at a recessed portion of the hull of the ship, the methods for mounting as well for demounting may further comprise supplying air into the recessed portion to surround by air the mounting point and the top of the portion of the thruster before the portion of the thruster is attached to or detached from the mounting point. Supplying air into the recessed portion provides a dry working space, such as for detaching or attaching non-underwater demountable thrusters, without needing to dry dock the whole ship.

[0032] Brief description of the drawings

[0033] Embodiments of the invention are described below in more detail with reference to the accompanying drawings, in which

[0034] Fig. 1 shows different views of an adjustable buoyancy device according to a first embodiment of the invention,

[0035] Fig. 2 shows different views of an adjustable buoyancy device according to a second embodiment of the invention,

[0036] Fig. 3 shows a sideview of a ship’s hull and a truster detached from the ship and supported by an adjustable buoyancy device according to the second embodiment of the invention,

[0037] Fig. 4 shows an adjustable buoyancy device according to a third embodiment of the invention,

[0038] Fig. 5 shows a flowchart of the method according to the invention for mounting a thruster, and

[0039] Fig. 6 shows a flowchart of the method according to the invention for demounting a thruster.

[0040] Detailed description of embodiment of the inventionDifferent embodiments of the adjustable buoyancy device 100, 200 are shown in various views of Figs. 1 , 2 and 4. It should be noted, however, that these embodiments are mere examples and include features that are not essential for the adjustable buoyancy device according to the invention. Moreover, features of the different embodiments are inter-compatible and may be combined or interchanged to make new embodiments within the scope of the current invention.

[0041] Fig. 1 shows an adjustable buoyancy device 100 according to a first embodiment of the invention. For clarity, different views of the device are shown in Figs. 1 A-G with a thruster 301 attached to the device shown in Figs. 1 A-D. Fig.

[0042] 1A shows a three-dimensional view of the device 100 from the top, Fig. 1B a three-dimensional view from the bottom, Fig. 1C shows a two-dimensional top view and Fig. 1 D shows a two-dimensional cross-section side view showing the thruster and a portion of the device from a side. Figs. 1 E-G focus on details of means 130 for controlling the amount of water in the buoyancy tanks.

[0043] The adjustable buoyancy device 100 of Fig. 1 comprises a main frame 110 comprising three structures - a first structure 111 forming a first side of the main frame, a second structure 112 forming a second side, a third structure 113 forming a third side connecting the first structure to the second structure and a free space formed between the first, second and third structures to allow the thruster to pass from at least partially above to at least partially below the main frame. The free space may extend substantially all the way to the first, second and third structures as shown in Fig. 1 , however, it is sufficient that the free space is at least large enough to allow the thruster to pass from at least partially above the main frame through the free space to at least partially below the main frame as can be seen also in Fig. 2. The space between the first, second and third structures in the first embodiment may comprise a structure, such as the work platform 260 described with the second embodiment, configured to allow the thruster to pass through the free space.

[0044] The free space may be defined to have a top side and a bottom side arranged on the opposite sides of the main frame, where the top side is above the main frame and extends to the uppermost point of the main frame and the bottom side is below the main frame and extends to the lowermost point of the main frame. Said top and bottom sides may be fully open as shown in Fig. 1, however, it is sufficient that they are at least partially open to allow the thruster topass through the top and bottom sides. The top and / or bottom sides may comprise a structure, such as the work platform 260 described below, configured to allow the thruster to pass through the top and bottom sides.

[0045] The main frame 110 of Fig. 1 further comprises two buoyancy tanks, a first buoyancy tank 121 arranged as the first structure 111 and a second buoyancy tank 122 arranged as the second structure 112. The third structure 113 is shown as a connection portion made of beams or pipes connecting the first structure with the second structure and thus providing structural stability. The third structure could have different construction and additional functionality. For example, the third structure could be arranged as a third buoyancy tank. The main frame could comprise one or more buoyancy tanks, e.g., each of the first, second and / or third structures could be a separate buoyancy tank, or the three structures or any two of them could form one, or at least a portion of one buoyancy tank.

[0046] The main frame of Fig. 1 is shown to be U-shaped. The U-shape is preferably configured to be used in substantially horizontal orientation, i.e. with all three structures being substantially parallel to the water surface. The main frame may have a different shape. For example, the main frame may comprise a fourth structure 214 forming a fourth side. The fourth side may be parallel or at least opposite to the third side and connect the first side with the second side as shown in Fig. 2, in which case the main frame may be rectangular or square shaped. The main frame may comprise more than three or four structures. In Fig. 1 , the first and the second structures are each in the form of a shipping container, more specifically, in the form of a 40-foot high-cube shipping container, as defined by the ISO standard 668:2020. Any or each of the main frame structures may be in the form of or made of one or more of such containers. The buoyancy force of a device comprising two 40-foot high-cube shipping container sized buoyancy tanks is estimated to be sufficient to lift thrusters weighing 25-80 metric tons. Preferably, the first and the second structures are each made of or in the form of the same number of containers. Instead of using the 40-foot containers mentioned above, 20-foot containers could be used. If the buoyancy of a single container is not sufficient, each of the first and the second structures could be formed by two containers. The use of 20-foot containers allows easier transport of the buoyance device.As seen in Figs. 1B and 1 G, the first and second buoyancy tanks 121, 122 have three bottom openings 123 at the bottom sides of the buoyancy tanks, i.e. facing the seabed when in use to allow water to flow into the first and second buoyancy tanks and maintain same pressure inside the buoyancy tanks as outside. A buoyancy tank can comprise one or more of such bottom openings, or the whole bottom side could be open. One or more of the buoyancy tanks could alternatively be closed, i.e. such tank can be referred to as a pressurized vessel.

[0047] The two buoyancy tanks as in Fig. 1 allows to adjust buoyancy forces between the two buoyancy tanks and thus to adjust device tilt at least around one axis. Fig. 1 G shows an example of the inside of the first structure 111 being the first buoyancy tank 121. A three-dimensional view with one of the sidewalls of the buoyancy tank removed is shown in Fig. 1 G. As seen in Fig. 1 G, the buoyancy tank may comprise two compartments 121a, 121b to allow separate buoyancy control of different parts of said buoyancy tank. Instead of dividing a single buoyancy tank into two or more compartments, the first structure may comprise more than one buoyancy tanks. Additional buoyancy tanks and / or separate compartments within each buoyancy tank allows to adjust the buoyancy force in each tank and its compartments separately, enabling adjustment of device tilt around two perpendicular axes. Such multi-directional tilt control enhances the device’s stability and maneuverability and allows to keep the main frame substantially parallel to the water surface when in use. Even if the buoyancy tanks are divided into two or more compartments, it is not necessary to have independent control of the buoyancy force in the compartments.

[0048] As shown in Fig. 1A, and C-G, the device may further comprise bumpers 150 on the top side of the main frame. Although four bumpers are illustrated in the figures, the adjustable buoyancy device may comprise any number, such as one or more of such bumpers.

[0049] The adjustable buoyancy device 100 of Fig. 1 further comprises means 130 for controlling the amount of water in the first and second buoyancy tanks. Figs. 1 C, E-G shows the means for controlling the amount of water comprise means 130a for supplying air into and means 130b for removing air from said buoyancy tanks. The means for supplying air may comprise a supply inlet 131 a for each buoyancy tank. The supply inlets for the first and the second buoyancytanks in Fig. 1C are seen to be arranged within the third structure, however, they could be arranged on or in close proximity of the buoyancy tanks. The supply inlet could be an opening in the buoyancy tank provided with a valve. The supply inlet may be configured to be connected to an air supply.

[0050] The means for supplying air may further comprise a hose (not shown) comprising a first end connected to the supply inlet and a second end extending or connecting to the air supply. The air supply may be configured to contain pressurized or non-pressurized air. In case of non-pressurized air, further means, such as a pump, may be provided to pressurize the air before supplying it into the buoyancy tank. The air supply may comprise a tank filled with pressurized or non-pressurized air. The air supply tank may be arranged within or in near proximity of the adjustable buoyancy device, such as attached to the buoyancy device or configured to be carried by a diver or a remote or manually operated vehicle. The air supply tank may be connected to the supply inlet via the hose. Alternatively, the air supply may be the ambient air above the water surface or air above the hull of the ship, in which case the second end of the hose may extend to above the water surface or above the hull of the ship.

[0051] Fig. 1G shows one example of means for supplying air to the first buoyancy tank 121 comprising two compartments 121a, 121b. In this example, the means for supplying air into the first buoyancy tank comprises two supply pipes 132a for directing air from two supply inlets 131 a to two supply outlets 133a at predefined parts of the first buoyancy tank. Although the air supply pipes and supply inlets are arranged at the upper portion of the buoyancy tank in Fig. 1 G, they could be arranged at a different part, such as at the lower portion of the buoyancy tank. The specific number of supply pipes, inlets and outlets may be different and depend on the number of compartments or other requirements. The means for supplying air may thus comprise a supply pipe 132a or a plurality of supply pipes for directing air from one or more supply inlets 131a to one or more supply outlets 133a at one or more predefined parts of the one or more buoyancy tanks.

[0052] Fig. 1G further shows an example of means for removing air from the first buoyancy tank 121 comprising two compartments 121a, 121b. In this example, the means for removing air from the first buoyancy tank comprises six removal pipes 132b for directing air from one of the compartments of the buoyancy tank to the respective removal outlets 133b. The specific number of removal pipes,inlets and outlets for air removal from the buoyancy tank, however, may depend on the number of compartments or other requirements. Preferably, one removal inlet, pipe and outlet are sufficient per compartment of the buoyancy tank. The means for removing air from a buoyancy tank may also be provided without any removal pipes, instead comprising a removal opening with a valve at a side of the buoyancy tank. Preferably, the removal opening with the valve is located at the top side or an upper portion of a side adjoint to the top side of the buoyancy tank allowing air to come out once the valve is open. Removal of air from the buoyancy tank leads to water flowing into the buoyancy tank via the bottom openings 123. Although the removal pipes and removal inlets are shown arranged at different heights of the buoyancy tank, a different arrangement, such as one or more of the removal pipes and inlets being arranged at the upper portion, is possible.

[0053] The means for controlling the amount of water may comprise a control unit attached to the adjustable buoyancy device, such as the main frame of it, and accessible by an operator in the vicinity of the adjustable buoyancy device. Fig. 1 shows such control unit as valves accessible by an operator. Alternatively, or in addition, the means for controlling the amount of water may comprise a remote-control unit connected to at least a part of the means for controlling the amount of water via a wireless and / or wired connection and accessible by an operator at a distance from the adjustable buoyancy device. Alternatively, or in addition, the means for controlling the amount of water may comprise an automated control system controlling the amount of water in at least one of the one or more buoyancy tanks based on sensor date and / or other known parameters, such as parameters of the ship, thruster, and transportation route.

[0054] The device 100 of Fig. 1 further comprises supporting means 140 attached to the main frame and configured to support the thruster. The supporting means seen in Figs. 1A-D comprises hanging means 141 configured to hold the thruster suspended from the main frame. The hanging means may comprise chains 142 and hooks 143 as shown in Fig. 1. Figs. 1A-D show one end of chains attached to one of the first, second or third structures and the other end of said chains to the thruster 301 as the supporting means 140. In Figs. 1A-D eight chains are shown, three attached to the first structure, three to the second structure and two to the third structure, each chain being attached to thethruster. In this example, the chains are attached to three different points on the thruster. Other hanging means than chains could be used, such as heavy-duty straps or cables. Different number of hanging means can be used.

[0055] As mentioned earlier, Figs. 2A-D (and Fig. 3) illustrate an adjustable buoyancy device 200 according to a second embodiment of the invention. In this embodiment, the device comprises a rectangularly shaped main frame 210 comprising four structures 211, 212, 213, 214 forming four sides of the main frame. As in the first embodiment, the main frame further comprises a free space, in this case formed between the first, second, third and fourth structures to allow the thruster to pass from at least partially above to at least partially below the main frame. The device of the second embodiment preferably comprises four buoyancy tanks (221, 222, 223, 224), each formed by the first, second, third and fourth structures respectively. Alternatively, the device of the second embodiment could comprise a different number of buoyancy tanks, such as one buoyancy tank (220) formed by all the four said structures or two buoyancy tanks (e.g. 221 and 222 or 223 and 224 shown in Fig. 2A) formed by any of the two opposing structures of the said four. Each buoyancy tank may be as described above, e.g., be in the form of a shipping container, such as a 20-foot container or a 40-foot high-cube shipping container, comprise a bottom opening 123 and / or comprise one, two or more compartments 121a, 121 b. One or more of the buoyancy tanks could also be a closed tank.

[0056] The main frame of the second embodiment may comprise only three structures as in the first embodiment. As described for the first embodiment, the main frame may be U-shaped. Although not shown, the second embodiment may comprise bumpers as described above. Although not shown in the figures, the second embodiment of Fig. 2 may comprise the same means as described above for controlling the amount of water in the one or more buoyancy tanks. Instead of the supporting means 140 of the first embodiment shown in Fig. 1, the supporting means 240 of the second embodiment comprises a support frame 241 configured to hold the thruster resting on the support frame as shown in Figs. 2A-D and 3. As seen in Fig. 2, the support frame may be arranged at least partly under the main frame or more precisely at least partly under the free space of the main frame. Figs. 2A-C show that the supporting frame can be fully under the free space to hold the thruster so that its top is fully under the top of the main frame. Figs. 2D and 3 show that the supportframe may be arranged to be partially above the bottom of the free space, in which case the thruster may be held at least partially above the top of the main frame depending on the thruster and the device dimensions.

[0057] The support frame is attached to the main frame with attachment means 242. In Figs. 2A-D, the attachment means is shown to be a hydraulic adjustable length attachment means 242 to lift the support frame along a direction perpendicular to water surface or substantially perpendicular to the main frame. Other attachment means can be used, such as adjustable or fixed length, comprising elements such as chains, cables, straps, hoists and / or winches.

[0058] As mentioned above, the support frame of the second embodiment is liftable, i.e. configured to lift and lower the thruster along an axis perpendicular to the water surface. Figs. 2A-C show the adjustable buoyancy device 200 with the support frame in a lowered position, and Figs. 2D and 3 show the support frame in a raised position. The liftable support frame enables versatility of the device for use with different size thrusters and may allow to raise the liftable support frame to receive a detached thruster with reduced drop distance from thruster’s mounting point, and to lower the liftable support frame to lower the thruster below the top of the main frame thus providing clearance and protection of the thruster in case of collision of the device top with other objects, such as the hull of the ship.

[0059] The support means 240 is further shown in Fig. 2 comprising a cradle 243 attached to the support frame 241 for supporting the thruster in its vertical orientation, i.e., substantially as the orientation when the thruster is in use. The cradle may comprise one or more parts, such as two as shown in Fig. 2. The parts of the cradle may be curved following the curvature of thruster parts to be supported.

[0060] The supporting means of the first and the second embodiments may comprise either or both of the supporting means 140, 240 described above, i.e., the supporting means may comprise the hanging means 141 and / or the support frame 241. It is worth noting that not all thrusters comprise attachment points for attaching to hanging means as described in connection with Fig. 1. In such a case, the device according to the second embodiment or at least comprising a support frame as described above should be used.The device of Fig. 2 further comprises a work platform 260 attached to the main frame and configured to allow the thruster to pass from at least partially above the main frame through the free space to at least partially below the main frame. The work platform may be arranged surrounded by the four structures of the main frame as can be seen in Fig. 2B. Alternatively, the work platform may be arranged above the main frame, such as resting on top of at least one of the four structures. The work platform may be configured to provide space and support for workers to stand on when detaching or attaching a thruster to its mounting point on the ship.

[0061] The device of Fig. 2 further comprises a set of suction cups 250 attached to the main frame and configured to be attachable to the hull of the ship as shown in Fig. 3. Suction cups may be considered as a form of a bumper. Fig. 2B shows the device comprising four suction cups facing up to be attached to the horizontal portion of the hull and four suction cups facing a direction substantially perpendicular to the other four suction cups to be attached to a vertical portion of the hull, such as walls of a recess 305 in the hull 303 of the ship as shown in Fig. 3. Other attachment means than suction cups could be used as known by a skilled person. Any number of attachment means could be used as long as they securely attach the device to the ship. Attachment means are not necessary. However, they may provide added stability to the device and thus safety especially in cases where workers are present on the work platform 260.

[0062] Figure 4 shows a buoyancy device 100 according to a third embodiment of the invention. The third embodiment is similar to the first embodiment, but the buoyancy tanks of the device 100 are closed tanks.

[0063] In the embodiment of figure 4, each of the first structure 111 and the second structure 112 is formed by a frame that accommodates several closed buoyancy tanks. The buoyancy tanks are cylindrically shaped to withstand the pressure difference between the interior and the exterior of the tanks. In case of having several buoyancy tanks, the buoyancy of all the tanks may not need to be controllable, but it may be sufficient to control the amount of water in only some of the tanks to control the buoyancy of the device 100.

[0064] With closed buoyancy tanks, the device can be lowered by introducing water into one or more of the buoyancy tanks. For raising the device, water can beremoved from one or more of the buoyancy tanks by introducing pressurized air into the tanks. One of the benefits of closed tanks is that inclination of the device does not cause escaping of air from the buoyancy tanks. Also, the buoy-ance is not affected by the depth. The closed tanks also improve the stability of the device.

[0065] In the embodiment of figure 4, the frame of each of the first structure 111 and the second structure 112 is divided into a lower portion and an upper portion. The dimensions of each of the first structure 111 and the second structure 112 as whole, or the dimensions of the lower portion and / or the upper portion may substantially correspond to the dimensions of a shipping container, such as a 20-foot container. This makes the parts of the device 100 easy to transport. The method for mounting 400 a thruster to a mounting point 302 on a ship according to an embodiment of the invention is shown in the flowchart of Fig.

[0066] 5. The method 400 comprises providing 401 an adjustable buoyancy device 100, 200 as described above around the thruster 301, supporting 402 the thruster by the supporting means 140, 240 of the device, moving 403 the device with the thruster to under the hull 303 of the ship and towards the mounting point, raising 404 the thruster in a direction from the seabed towards the mounting point, and attaching 405 the thruster to the mounting point.

[0067] The method for demounting 500 from a ship a thruster 301 attached to the ship at a mounting point 302 according to an embodiment of the invention is shown in the flowchart of Fig. 6. The method for demounting is essentially the method for mounting as described above performed in reverse order. The method for demounting 500 comprises providing 501 an adjustable buoyancy device 100, 200 as described earlier under the mounting point and around the thruster, detaching 502 the thruster from the mounting point, lowering 503 the thruster in a direction from the mounting point towards the seabed, supporting 504 the thruster by the supporting means of the device, and moving 505 the device with the thruster into a desired area, such as another ship or harbor.

[0068] The raising 404 and / or lowering 503 may be performed by strand-jack lines 304 (shown in Figs. 1A, D) extending through the interior of the ship and attached to the thruster. Alternatively, other means could be used, such as hydraulic system, retractable steering section 306, ropes or chains, winch withcable, or the liftable support frame 241 to minimize the distance the thruster needs to be lowered or dropped or raised.

[0069] The moving 403, 505 could be performed by assistance of divers, remote operated vehicle, crane with cable, or propulsion means provided to the device. The method for mounting and / or demounting may comprise fully submerging the device underwater at least during some of the method steps, such as during the steps of moving and / or raising and / or lowering respectively.

[0070] The method for mounting and / or demounting may further comprise sealing off the top of the removable part of the thruster and the hull at the mounting point to prevent water from entering the removable thruster part and the hull.

[0071] In cases where at least a portion of the thruster 301 is located within a recessed portion 305 of the hull 303 of the ship as shown in Fig. 3, either of the above methods may further comprise supplying air into the recessed portion before attaching or detaching the thruster to or from the ship. The point at which the thruster is detached from or attached to the ship may be referred to as a mounting point 302 and if the mounting point is arranged within the recessed portion of the hull, the method may comprise supplying air into the recessed portion to surround by air the mounting point and the top of the removable thruster portion during some time before the thruster is attached to or detached from the mounting point thus creating dry dock conditions and preventing water from entering the thruster and the hull at the moment of detachment or attachment.

[0072] Both devices of the first and second embodiment as well as various modifications of the provided examples may be used for mounting and / or demounting a thruster. Both devices can be fully submerged and thus can be used to mount and / or demount a thruster underwater so that at least the adjustable device is fully submerged underwater at some time during use.

[0073] The thrusters may be divided into two types - underwater demountable thrusters (as shown in Fig. 1) and non-underwater demountable thrusters (Figs. 2 and 3). The non-underwater demountable thrusters are not designed to be detached from the ship under water and usually do not have attachment points for attachment or hanging means such as chains or cables. The underwater demountable thrusters are thrusters that have inbuilt features allowing easierattachment and detachment of thrusters underwater, such as comprising caps to seal off the top of the removable thruster part as well as the hull from water. While both the first and the second embodiment devices can be used for mounting and / or demounting underwater demountable thrusters, the first embodiment device would require at least the support frame as described above for mounting and / or demounting non-underwater demountable thrusters if such thrusters do not have attachment means.

[0074] The device 200 of the second embodiment is configured for and particularly suitable for mounting and demounting non-underwater demountable thrusters. As such thrusters usually do not comprise attachment points for attaching to hanging means 141, a device comprising a support frame 241 allows to support a thruster during transportation. The liftable support frame further allows to lift the support frame closer towards the thruster bottom to reduce the distance the thruster needs to be dropped or be moved down after the thruster is detached from at the mounting point 302 at the ship or more specifically at the steering section 306 of the thruster. The work platform 260 provides a platform for workers to stand on while detaching or attaching the removable part of the thruster, such as its lower gearbox, to or from the mounting point of the removable part of the thruster, such as at the steering section. The device is configured to enable demounting and mounting of a thruster under the sea level, i.e. with the ship being in its operational position in water, facilitated by arranging a thruster into a recess 305 in the hull 303 of the ship. In such a case the recess may be supplied with air, thus surrounding the mounting point with air. The device of the first embodiment is particularly suitable for mounting and demounting of underwater demountable thrusters. As such thrusters are ready made for underwater detachment, a work platform is not needed for detaching or attaching the thruster and the thruster may be suspended from the main frame via attachment means attached to attachment points on the thruster without a support platform. The first structure may provide a simpler, cheaper and more robust structure that is suitable for a wide range of thruster dimensions, as the thruster can be suspended by hanging means with the bottom of the thruster sticking out below the main frame as much as needed.

Claims

Claims:

1. An adjustable buoyancy device (100, 200) for mounting and / or demounting at least a portion of a thruster (301 ) of a ship, the adjustable buoyancy device comprising:- a main frame (110, 210) comprising- a first structure (111, 211) forming a first side,- a second structure (112, 212) forming a second side, - a third structure (113, 213) forming a third side connecting the first structure to the second structure,- a free space formed between the first, second and third structures to allow at least the portion of the thruster to pass from at least partially above to at least partially below the main frame, and- one or more buoyancy tanks (121 , 122, 220-224) formed at least partially by or within the first, second and / or third structures, - means (130) for controlling the amount of water in one or more of the one or more buoyancy tanks, and- supporting means (140, 240) attached to the main frame and configured to support at least the portion of the thruster.

2. The adjustable buoyancy device (100) according to claim 1 , wherein the means (130) for controlling the amount of water are configured to allow controlling the amount of water at least in one tank formed by or within the first structure and at least in one tank formed by or within the second structure.

3. The adjustable buoyancy device according to claim 1 or 2, wherein the main frame (110) is U-shaped.

4. The adjustable buoyancy device according to any of the preceding claims, wherein the dimensions of each of the first structure and the second structure substantially correspond to the dimensions of a 20-foot shipping container or each of the first structure and the second structure comprises a module having dimensions that substantially correspond to the dimensions of a 20-foot shipping container.

5. The adjustable buoyancy device according to any of the preceding claims, wherein the first structure and the second structure are each in the form of a shipping container.

6. The adjustable buoyancy device according to claim 5, wherein the shipping container is a 20-foot shipping container.

7. The adjustable buoyancy device according to any of the preceding claims, wherein at least one of the one or more buoyancy tanks comprises a bottom opening (123) to allow surrounding water to flow into the buoyancy tank.

8. The adjustable buoyancy device according to any of claims 1 to 7, wherein at least one of the buoyancy tanks is a closed tank.

9. The adjustable buoyancy device according to claim 8, wherein all the buoyancy tanks are closed tanks.

10. The adjustable buoyancy device according to any of the preceding claims, wherein at least one of the one or more buoyancy tanks comprises at least two compartments (121a, 121b).

11. The adjustable buoyancy device according to any of the preceding claims, wherein the main frame comprises at least two of the one or more buoyancy tanks (121, 122, 221-224), a first buoyancy tank (121, 221, 223) arranged in or as the first structure and a second buoyancy tank (122, 222, 224) arranged in or as the second structure.

12. The adjustable buoyancy device according to any of the preceding claims, wherein the means for controlling the amount of water in at least one of the one or more buoyancy tanks comprises means (130a) for supplying air into and means (130b) for removing air from said buoyancy tank, the means for supplying air comprising a supply inlet (131a).

13. The adjustable buoyancy device according to claim 12, wherein the supply inlet (131a) is arranged on said buoyancy tank and configured to be connected to an air supply.

14. The adjustable buoyancy device according to claim 12 or 13, wherein the means for supplying air comprises a supply pipe (132a) for directing airfrom the supply inlet (131a) to a supply outlet (133a) at a predefined part of said buoyancy tank.

15. The adjustable buoyancy device according to any of claims 12-14, wherein the means for removing air comprises a removal outlet (133b) with a valve.

16. The adjustable buoyancy device according to claim 15, wherein the means for removing air further comprises a removal pipe (132b) for directing air from a removal inlet (131b) to the removal outlet (133b).

17. The adjustable buoyancy device according to any of the preceding claims, wherein the means for controlling the amount of water comprises a control unit attached to the adjustable buoyancy device and accessible by an operator in the vicinity of the adjustable buoyancy device.

18. The adjustable buoyancy device according to any of the preceding claims, wherein the means for controlling the amount of water comprises a remote-control unit connected to at least a part of the means for controlling the amount of water via a wireless and / or wired connection and accessible by an operator at a distance from the adjustable buoyancy device.

19. The adjustable buoyancy device according to any of the preceding claims, wherein the means for controlling the amount of water comprises an automated control system controlling the amount of water in at least one of the one or more buoyancy tanks based on sensor data and / or other known parameters, such as parameters of the ship, thruster, and transportation route.

20. The adjustable buoyancy device according to any of the preceding claims, further comprising a bumper (150, 250) on the top side of the main frame.

21. The adjustable buoyancy device according to any of the preceding claims, wherein the supporting means comprises hanging means (141) configured to hold at least the portion of the thruster suspended from the main frame.

22. The adjustable buoyancy device according to any of the preceding claims, wherein the supporting means (240) comprises a support frame (241) configured to hold at least the portion of the thruster resting on the support frame.

23. The adjustable buoyancy device according to claim 22, wherein the support frame is arranged at least partly under the main frame.

24. The adjustable buoyancy device according to claim 22 or 23, wherein the support frame is configured to lift and lower at least the portion of the thruster along an axis perpendicular to the water surface.

25. The adjustable buoyancy device according to any of the preceding claims, further comprising a work platform (260) attached to the main frame.

26. Use of the adjustable buoyancy device according to any of the preceding claims for mounting and / or demounting of at least a portion of a thruster.

27. Use of the adjustable buoyancy device according to claim 26, wherein the mounting and / or demounting of at least the portion of the thruster is performed underwater so that at least the adjustable buoyancy device is fully submerged underwater at some time during use.

28. A method for mounting (400) at least a portion of a thruster (301) to a mounting point (302) on a ship, the method comprising:- providing (401 ) an adjustable buoyancy device according to any of claims 1-25 at least around the portion of the thruster, - supporting (402) the portion of the thruster by the supporting means of the adjustable buoyancy device,- moving (403) the adjustable buoyancy device with at least the portion of the thruster to under the hull (303) of the ship and towards the mounting point,- raising (404) at least the portion of the thruster in a direction from the seabed towards the mounting point, and- attaching (405) at least the portion of the thruster to the mounting point.

29. A method for demounting (500) at least a portion of a thruster from a ship, said portion being attached to the ship at a mounting point, the method comprising:- providing (501 ) an adjustable buoyancy device according to any of claims 1-25 under the mounting point and at least around the portion of the thruster,- detaching (502) at least the portion of the thruster from the mounting point,- lowering (503) at least the portion of the thruster in a direction from the mounting point towards the seabed,- supporting (504) at least the portion of the thruster by the supporting means of the adjustable buoyancy device, and- moving (505) the adjustable buoyancy device with at least the portion of the thruster into a desired area.

30. The method (400, 500) according to claim 28 or 29, comprising fully submerging the adjustable buoyancy device underwater at least during some of the method steps.

31. The method according to any of claims 28-30, further comprising sealing off the top of the portion of the thruster and the hull at the mounting point to prevent water from entering the portion of the thruster and the hull.

32. The method according to any of claims 28-31 , wherein the mounting point is at a recessed portion of the hull of the ship, the method further comprising supplying air into the recessed portion to surround by air the mounting point and the top of the portion of the thruster during some time before the portion of the thruster is attached to or detached from the mounting point.

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