The present invention relates to a buoyancy device for connection to a vessel, and to a vessel incorporating a buoyancy device. It is known for sea vessels to be provided with flotation or buoyancy devices to allow the deck of the vessel to sit well above sea-level and particularly above the likely height of any waves. A common example of a vessel of this sort is a catamaran, which is a twin hull vessel that is designed to sit largely with its deck and superstructure well above the sea level, and with its twin hulls partially submerged providing buoyancy. A similar style of vessel is a "small waterplane area twin hull" (SWATH) design, wherein the cross sectional area of the hull of the boat is minimized at sea level, such that the parts of the hull providing the majority of the buoyancy (i.e. the ballast chambers) are situated below the level of the waves, and the body of the vessel is situated above sea level. This design improves the stability of the vessel over that of a catamaran, since the area of the vessel exposed to wave action is reduced.

A drawback of SWATH vessels is that they tend to create a large water resistance, due to a large volume of the vessel located below sea level. This causes the vessels to be slower compared to an equivalently powered catamaran, or to require greater power in order to achieve the same speed, as the catamaran sits at the surface of the water thereby meeting less water resistance when in motion. Furthermore, SWATH vessels are designed to have increased stability over a catamaran when moving, but when stationary, the vessels are typically still affected to a large degree by the effects of large waves. Semi-submersible vessels are designed to be capable of taking water ballast into ballast chambers within their hulls, such that the vessels sink lower in the water. This is generally accomplished by incorporating pontoon hulls that are attached to the upper part of the vessel structure by fixed structural members. The addition of ballast water causes the pontoon to sink below the surface so that the vessel as a whole is more stable and less susceptible to the effects of waves and swell. However, semi-submersible vessels are typically very slow in transit mode. It is desirable to create a more stable vessel, such that the deck and superstructure of the vessel are less prone to the pitch and roll caused by large waves. In particular, if a vessel is used to access a structure at sea such as a wind turbine, for example, personnel may be required to embark or disembark from the vessel. Disembarking from a vessel while it is subjected to large degrees of pitch and roll by large waves acting on the body/ballast chamber of the vessel may be very dangerous. In order to limit the risk of injury to personnel working on a vessel in such conditions, it is preferable for the vessel to have the ability to be stabilised whilst maintaining the ability to travel at high speed.

According to a first aspect of the invention, we provide a buoyancy device for connection to a vessel for use at sea, the device including a ballast chamber for holding gas and liquid, means for adjusting the ratio of gas and liquid in the chamber, a connection means for connection to a vessel, wherein the connection means is moveable so as to adjust a distance between the ballast chamber and the vessel, and an actuation means operable to cause movement of the ballast chamber relative to the vessel.

According to a second aspect of the invention, we provide a vessel including a buoyancy device, the buoyancy device extending downwardly away from a hull of the vessel and including a ballast chamber for holding gas and liquid, means for adjusting the ratio of gas and liquid in the chamber, a connection means connecting the buoyancy device and the hull of the vessel, wherein the ballast chamber is moveable relative to the hull of the vessel for adjusting a distance therebetween, and an actuation means operable to cause movement of the ballast chamber relative to the hull of the vessel.

According to a third aspect of the invention, we provide a method of operating a vessel including a buoyancy device, the buoyancy device including a ballast chamber for holding gas and liquid, means for adjusting the ratio of gas and liquid in the chamber, a connection means connecting the buoyancy device to the hull of the vessel, and an actuation means operable to cause movement of the ballast chamber relative to the hull of the vessel, the method comprising the steps of

operating the actuation means so as to move the ballast chamber relative to a hull of the vessel so as to adjust the distance therebetween, and using the means for adjusting the ratio of gas and liquid in the chamber to decrease or increase the buoyancy of the ballast chamber.

Further features of the first, second and third aspects of the invention are set forth in the dependent claims appended hereto.

Embodiments of the invention will be described by way of example only, with reference to the accompanying drawings, of which:- Figure 1 is a side view of a vessel including a buoyancy device according to the invention;

Figure 2 is a front view of the vessel of Figure 1 ;

Figure 3 is a side view of the vessel of Figure 1 , wherein the submersion elements of the buoyancy devices are extended away from the hull of the vessel; Figure 4 is a cross-sectional view through the width of a lower-portion of a vessel including buoyancy devices, wherein the submersion elements of the buoyancy devices are extended away from the hull of the vessel;

Figure 5 is a cross-sectional view of the vessel of Figure 4, wherein the submersion elements are in a raised position;

Figure 6 is a side view of another vessel according to the invention, the vessel being an aircraft carrier;

Figure 7 is a side view of the vessel of Figure 6, wherein the submersion elements are extended away from the hull of the vessel;

Figure 8 is a cross-sectional view through the width of a lower-portion of a vessel including buoyancy devices of an embodiment of the invention;

Figure 9 is a cross-sectional view of the vessel and buoyancy devices of Figure 8, in a rolled orientation;

Figure 10 is a cross-sectional view of another embodiment of a vessel and buoyancy devices; and

Figure 1 1 is a cross-sectional view of the vessel and buoyancy devices of Figure 10, wherein the submersion elements of the buoyancy devices are extended away from the hull of the vessel. Referring to the figures, there is shown a vessel 10 that has a body 12 which supports the superstructure 14 of the vessel, and a pair of buoyancy devices 20, disposed below the body 12. The body 12 provides a deck, upon which cabins and other structural features including control rooms may be located. The buoyancy devices 20 comprise submersion elements 22 that are connected to the body 12 of the vessel by a connection means 26, each submersion element 22 comprising a ballast chamber for holding a volume of gas and/or liquid. In this example the connection element 26 comprises a plurality of upright supports 18 and a plurality of cross-support members 28, 30, 38, 40. The connection means 26 is moveable to cause relative movement of the submersion elements 22 and the body 12, such that the submersion elements 22 may be moved between a raised position (as shown in Figures 1 and 2) and a lowered position (as shown in Figure 3).

A substantial proportion of the outer surfaces of the buoyancy devices 20 may be constructed from steel or steel alloy. Alternatively, or in addition to steel, a composite material may be used.

In this example, the buoyancy devices 20 form a lower part of the vessel 10, such that when the submersion elements 22 are in the raised position relative to the body 12 of the vessel, the buoyancy devices 20 effectively form twin hulls of the vessel. In other words, the buoyancy devices 20 are positioned relative to the sea level such that the submersion elements 22 are either partly submerged, or submerged in their entirety, with the body 12 of the vessel supported on them, so as to form a lower part of the vessel itself. It should, however, be understood that the buoyancy devices 20 may not form an integral part of the vessel 10 itself, and may instead be supplied as a separate entity for connection to the hull of the vessel. In other words, it is possible that the buoyancy devices 20 are not vital to the ability of the vessel to float and/or to otherwise function at sea. Although the body 12 of the vessel may be buoyant in its own right, and may be fully functional as a sea-worthy vessel without the buoyancy devices 20, it is not necessary that the body be buoyant in its own right. The vessel may be fully functional where the lower part of the vessel, comprising at least one buoyancy device 20, is capable of supporting the vessel in the water.

Therefore, while buoyancy devices 20 comprising submersion elements 22 as shown in Figure 1 , for instance, are typically referred to as "hulls" (in the context of SWATH and semi-submersible vessels, for instance), in the context of this description the term 'hull' refers to the lower part of the body of the vessel 10, which in this example is the lower part or underside of body 12. When in their raised configuration, the submersion elements 22 are designed to form a standard "wave piercer" formation, so as to glide through, rather than over, the waves. As shown in Figures 1 and 2, the vessel 10 includes two buoyancy devices 20, aligned side-by-side across its width. Figure 2 shows the vessel 10 from the front, having a respective buoyancy device 20 on either side of its width, each being connected to the body 12 of the vessel by a connection element 26. The connection element 26 comprises pairs of cross-support members 28, 30 and 38, 40, a first end 28b, 30b of each member being moveable lengthwise of the vessel, through a sliding connection, for example, and a second, opposite, end 28a, 30a being pivotally connected to the submersion elements 22 or to the hull, respectively, such that the cross-support members 'expand' and 'collapse' in a scissor action to move the submersion elements 22 away from or towards the hull of the vessel. Of course, cross-support members may or may not be required, depending on the design and structural requirements of the particular vessel.

The buoyancy device 20, or the vessel 10, may comprise one or more actuation means operable to cause movement of the submersion elements 22, including the ballast chambers, relative to the hull of the vessel 10. The actuation means is operable to move the cross-support members between their expanded and collapsed configurations, so as to move the submersion elements 22 towards or away from the hull of the vessel. The actuation means (not shown) may comprise one or more hydraulic systems, such as hydraulic rams disposed horizontally so as to cause the first end 28b, 30b of at least one support member of each pair of cross-support members to move lengthwise of the vessel along a track. Alternatively, the actuation means may comprise a mechanical system such as scissor jacks, gears, lifting wires, or any other suitable system. The track may be located either on the hull of the vessel or on the submersion element 22, and may be recessed into the hull or submersion element 26. The first end 28b, 30b is moved from a central region of the vessel towards an end of the vessel, so as to cause the support member to pivot about its second end 28a, 30a (located on the other of the hull of the vessel or the submersion element 22). This causes the cross-support member to move from a generally horizontal to a generally upright orientation, and in so doing, moves the submersion element 22 downwardly away from the hull of the vessel. The pairs of cross-support members may be pivotally connected to one another about a mid-point along their lengths, so that movement of one support member also causes similar movement of the other, to provide a scissor action.

It will of course be understood that while in this example the first, sliding, ends of each support member are moveable by the actuation means, alternatively the second, pivoted, end of each member may be moveable by the actuation means. As a further alternative, both ends of each member may be moveable by the actuation means. As another alternative, the cross-support members may not be acted on directly by the actuation means, and a further moveable part such as the upright support members 18 may be moveable by the actuation means.

By incorporating actuation means that are mechanically or hydraulically operated, it is possible to move the submersion elements 22 away from the body 12 of the vessel, without necessarily requiring the submersion elements 22 to be ballasted with water (i.e. to cause them to sink). In this way, the centre of gravity of the vessel may be maintained, substantially, whilst the body 12 rises and the submersion elements 22 move downwardly.

The upright supports 18 are positioned such that two are located towards the stern of the vessel and two towards the bow, with one of each pair located near to the starboard side and one near to the port side. The upright supports are connected to the submersion elements 22, and move relative to the body of the vessel, such that when the submersion elements 22 are in the raised configuration the upright support members 18 project upwardly from the deck of the vessel (as shown in Figure 1 ), and when the submersion elements 22 are in the lowered configuration, the upright support members 18 project downwardly from the hull of the vessel. The upright support members 18 provide a strengthening formation between the body 12 of the vessel and the submersion elements 22. The upright support members 18 may also provide a secure and stable passage for power and communications wiring and/or personnel access between the body 12 of the vessel and the submersion elements 22. It will be understood that the number of upright supports 18 included in the vessel depends on the size of the vessel. Larger vessels may incorporate a greater number of upright supports.

To move the submersion elements 22 back towards the hull of the vessel, an opposite force may be applied to the first ends 28b, 30b of the support members, so as to move them back along the slide track in an opposite direction to that described above (i.e. towards the central region lengthwise of the vessel). As the submersion elements 22 are lowered relative to the body 12 (i.e. during the lowering process), water may be taken into the ballast chambers. Valves 24 are provided in the walls of the submersion elements 22, which are operable to allow water into the ballast chamber under its natural water pressure. The valves 24 may be regulated. Alternatively, a pump may be provided to pump seawater into the chamber and to control the flow rate thereof.

Alternatively, water may be taken into the ballast chambers before, or after, the lowering operation. In taking water into the ballast chamber, the ratio of liquid to gas in the ballast chamber is increased, and the weight of he ballast chamber increases. As a result, the centre of gravity of the buoyancy device 20 and vessel 10 is lowered. Lowering the centre of gravity of the vessel 10 causes the stability of the vessel to increase.

As the separation of the body 12 from the submersion elements 22 increases, the underwater volume of the vessel increases slightly as the cross-support members 28, 30, 38, 40 and upright supports 18 extend into the water. As the submersion elements 22 are lowered, the increased underwater volume causes an increase in buoyancy, to some degree. .

When the submersion elements 22 are to be raised, water within the ballast chambers is expelled, so as to lower the ratio of liquid to gas in the chambers, and thus reduce the weight of the submersion elements 22. Expulsion of ballast may be achieved using compressed air supplied under high pressure from one or more compressed air cylinders. The cylinders may be provided within the submersion elements 22 or may be disposed within or on the body 12 of the vessel with pipes for transport of gas to the ballast chamber. Alternatively, or in addition to the use of compressed air, pumps may be used to expel water from the chamber.

As liquid is expelled from the ballast chambers and replaced by air, the weight of the submersion elements 22 is reduced, and therefore the overall buoyancy effect (i.e. the weight of the buoyancy device offset against the buoyancy produced in terms of the volume of water displaced) is increased. This causes the vessel to rise. Preferably, the increased buoyancy is such that the body 12 rises from the water, to a position in which the underside of the body 12 is clear of the water (as shown in Figure 3). In embodiments, the submersion elements 22 and body 12 of the vessel 10 are moved away from each other by operation of the actuation means. This active separation of the submersion elements 22 from the body 12 (as opposed to 'passive' ballasting of the submersion elements 22, to cause them to move downwards relative to the body 12) means that the body 12 may move upwardly as the submersion elements 22 drop. In this way, the body 12 of the vessel may rise above the wave energy zone, close to the surface of the water, in which the waves have greatest effect on the vessel. By moving the body 12 out of this zone, the effect of rough seas and high wave-power acting on the vessel is decreased.

The vessel 10 includes at least one propulsion device, for moving the vessel through the water. The propulsion device may be a Hamilton water jet propulsion system or may alternatively include one or more propellers. Alternatively, or in addition, a propulsion device may be included within each buoyancy device, forming a part of the submersion element 22. In addition, each submersion element 22 may include retractable azimuth thrusters, which may extend from the submersion elements 22 when they are in their lowered configuration. The thrusters provide increased manoeuvrability for fine directional control of the vessel 10.

Generators for providing power to the engines and other electronic components may be provided in the superstructure of the vessel, and the power transmitted to the engines via suitable cables provided through the connection means 26. The cables may be housed within the upright supports 18 or within one or more the cross-support members 28, 30, 38, 40. Alternatively, the generators may be housed within the submersion elements 22. An engine control system may be provided on the body 12 of the vessel, or alternatively may be provided within one or more of the submersion elements 22. If the engine control system is provided in the superstructure, and the engines are provided within the submersion elements 22, communication between the two may be provided through cabling housed within the connection element 26. The connection element 26 may also provide personnel access between the submersion elements 22 and the body 12 of the vessel.

Support braces 32 (see figures 4 and 5) may be disposed between the submersion elements 22, and in particular between the upright supports 18, to hold the submersion elements 22relative to one another in order to counteract any effects of transverse loading, e.g. from waves.

The principles of design discussed herein may be applied to a vessel of any dimension, whether a mono-hull or multi-hull vessel. The upper part of the vessel (i.e. the body, not including the buoyancy device) may be interchanged - for instance, the body may form part of a passenger ferry, a vehicle carrier, a bulk freight carrier, or any other form of vessel. In this way the superstructure of the vessel may be changed, and a new superstructure connected to the buoyancy device forming the lower portion of the vessel.

A vessel suitable for performing marine wind farm maintenance may have a width of approximately 20 metres and a length of approximately 40 metres. A vessel of these dimensions has a base sufficient to straddle a base of a wind turbine base, for example. A vessel of these dimensions is likely to achieve approximately 12 metres of separation between the submersion elements 22 and the hull of vessel 10, in which case the deck of the vessel is likely to be in the region of 5 to 6 metres above sea level, and the submersion element 6 to 7 metres below sea level. Such a configuration is suitable for use in seas with waves of 4 to 5 metres, without being unduly affected by the waves causing any significant degree of instability on deck. In addition to providing cabins, facilities and control equipment for personnel in the superstructure of the vessel, walkway 16 is provided for transfer of personnel. The walkway 16 provides a set of steps to allow personnel to safely embark or disembark the vessel 10. The walkway 16 may be located on an upper level of the deck of the vessel, and is configured to move between a first position in which the steps are largely over the body 12 of the vessel, and a second position in which the steps extend outwardly and upwardly from the body 12 of the vessel so as to overhang a side of the vessel. For example, to disembark personnel from the vessel to a wind turbine tower, as shown in Figure 3, the vessel is manoeuvred so that it is positioned with its stern to the structure, so that the vessel is downwind from the structure (so that in a power cut, or other failure situation, the vessel is blown away from the structure and not towards it). Once near to the structure, the valves 24 of the ballast chambers are operated to regulate the ratio of gas and water in the chambers, and the hydraulic rams are actuated to move the submersion elements 22 away from the hull of the vessel (either at the same time as, before, or after operating the valves 24). The body 12 of the vessel rises as the submersion elements 22 are lowered and as the air replaces ballast in the ballast chambers, as the decrease in their weight causes the overall upward buoyancy effect of the buoyancy device to increase. Once in their lowered configuration, the stability of the vessel increases due to the lower centre of gravity of the vessel. The arm 16 is then extended upwardly and outwardly from the body 12 of the vessel, so that personnel may climb up from the vessel and disembark onto the structure.

Figures 6 and 7 show another embodiment of the invention, wherein the vessel is a monohull aircraft carrier that includes a buoyancy device according to the invention. Figure 6 shows the vessel in a first configuration with the submersion element 22 raised, having the appearance and performance of a standard aircraft carrier. The underwater volume of the vessel, including the buoyancy device, increases to some extent as the submersion element 22 is lowered. The overall resultant upward force created by the buoyancy device increases as the ratio of gas to fluid is increased in the ballast chamber, lessening the weight of the device. Figure 7 shows the vessel in a second configuration, with the submersion element 22 lowered relative to the body of the vessel. In its second configuration, the body 12 of the vessel is raised relative to its position in the first configuration.

Figures 8 and 9 show another embodiment of the buoyancy device of the present invention, wherein the connection elements 42 are shaped so as to provide greater stability to the vessel. Each connection element 42 has an upper end 42a connected to the body 12, and a lower end 42b connected to a submersion element 22. Each connection element 42 increases in cross- sectional area, and is configured so that the portion of greater cross-sectional area 42a lies at or just above the water level when the submersion elements 22 (including the ballast chambers) are at a position spaced from the hull of the vessel 10.

If the wave motion of the sea causes the vessel 10 to roll, the buoyancy and self-righting effect of the submerged volume of the vessel and buoyancy devices 20 is raised by the increased volume of the upper portion of the connection element 42. In a rolled position, as shown in Figure 9, the vessel is unstable, and forces acting on the vessel cause it to return to a righted position in which it is stable. To increase this effect, the connection element 42 of the part of the vessel that has rolled into the water will be submersed so that the portion of increased area 42a enters the water. This creates a greater buoyancy effect at that point, due to the relative increase of the volume of water displaced, and so that portion of the vessel gains greater buoyancy, relatively, than the portions that have not been submersed. Therefore, the vessel will right itself more rapidly, due to the increase in buoyancy at that position. Of course, whilst the portion of increased area 42a has been described as fornning part of the connection element 42, such portions of increased area 42a may form any part of the connection means 26. The portions of increased area 42a may be tapered, and may be substantially conical.

Alternatively, a further mechanical means may be provided in the connection elements 18, to act outwardly on a portion of the connection element 18 so as to increase the cross-sectional area of that portion. Alternatively, as shown in Figures 10 and 1 1 , additional flotation devices 44 may be provided at positions disposed about portions of the connection elements 18, which may enter the water in the event that the vessel rolls or pitches, to provide the same effect as that outlined above. The flotation devices 44 may be lowered into the water automatically, as part of a stabilising system, when it is determined that it is necessary to do so. In embodiments, and as shown in Figures 10 and 1 1 , the vessel comprises a separate flotation device 46 to provide additional buoyancy volume, to be submersed in the water in the event that the vessel pitches or rolls. This separate flotation device may be provided on a lower part of the body 12 of the vessel, at a position just above the water level when the body 12 is in a raised orientation (see Figure 1 1 ). The flotation devices 44, 46 may be moveable between a position in which they are stowed inside, or adjacent, the body 12 of the vessel, and a position in which they are disposed below, and either adjacent or spaced from, the body 12 of the vessel.

In embodiments, buoyancy devices 20 may be provided with combined actuation means for controlling the movement of the submersion elements 22 relative to the body 12. In this way, the submersion elements 22 may be moved towards and away from the body 12 at the same time as one another. Alternatively, the actuation means for controlling each submersion element 22 may be separate. In this way, submersion elements 22 (and buoyancy devices 20) may be operated in isolation from one another. A stabilising system may be provided on a vessel 10, having means for detecting the orientation and positioning of the deck, or any other part of the vessel, so as to allow automatic operation of the actuation means so as to perform stabilisation of the body 12. In this way, submersion elements 22 may be moved automatically, to adjust their positioning relative to one another, so as to stabilize the vessel 10.

When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.