Patent application title: VESSEL
Richard Parkinson (Falmouth, GB)
IPC8 Class: AB63H2542FI
Class name: Marine propulsion means to control the supply of energy responsive to a sensed condition
Publication date: 2014-12-25
Patent application number: 20140378012
A vessel comprising a plurality of hulls, a propulsion system including
propulsion devices at respective end regions of the plurality of hulls
and a control system connected to the propulsion system to control
operation of the propulsion devices so as to position the hulls, wherein
the propulsion devices are angled relative to the vertical longitudinally
of each hull. Such vessel can perform installation functions of
sub-surface assets such as tidal turbines, wave energy devices, cable
laying and the like, or to facilitate foundation installations and also
the function of inspecting sub-surface areas.
1. A vessel comprising a plurality of hulls, a propulsion system
including propulsion devices at respective end regions of said plurality
of hulls and a control system connected to said propulsion system to
control operation of said propulsion devices so as to position the hulls,
wherein said propulsion devices are angled relative to the vertical
longitudinally of each hull.
2. A vessel according to claim 1, wherein said plurality of hulls is two.
3. A vessel according to claim 1, wherein said propulsion system includes four propulsion devices located at both end regions of each hull.
4. A vessel according to claim 1, wherein the propulsion devices are located in corner regions of the vessel taken as a whole.
5. A vessel according to claim 1, and further comprising skegs and/or rudders provided on each hull.
6. A vessel according to claim 5, wherein said skegs and/or rudders are retractable.
8. A vessel according to claim 1, wherein said control system is a computer controlled system which automatically maintains the vessel's position and heading by using the propulsion devices and wherein position reference sensors, wind sensors and motion sensors together with gyro compasses provide information to the computer pertaining to the vessel's position and the magnitude and direction of environmental forces affecting its position.
13. A vessel according to claim 1, the arrangement being such that each hull is symmetrical about a substantially vertical plane transverse to a longitudinal axis of each hull.
14. A vessel according to claim 1, and further comprising an inspecting arrangement mounted to at least two of the plurality of hulls to inspect an area outwardly away from said vessel.
15. A method comprising positioning at a location a vessel having a plurality of hulls and utilising a control system to operate a propulsion system of said vessel in order to substantially maintain the position of the hulls at the location.
17. A method according to claim 15, wherein said control system is a computer controlled system which automatically maintains the vessel's position and heading by using propulsion devices of the propulsion system and wherein position reference sensors, wind sensors and motion sensors together with gyro compasses provide information to the computer pertaining to the vessel's position and the magnitude and direction of environmental forces affecting its position.
18. A method according to claim 15, and further comprising inspecting an area outwardly away from said vessel.
19. A method according to claim 15, wherein said vessel is according to claim 1.
20. A vessel comprising a plurality of hulls substantially parallel to each other, the arrangement being such that each hull is symmetrical about a substantially vertical plane transverse to a longitudinal axis of each hull.
21. A vessel according to claim 20, wherein each hull is symmetrical from front-to-back about a substantially vertical central transverse plane across each hull.
22. A vessel according to claim 20, wherein the shape of a bow and a stern of said vessel are substantially identical.
23. A vessel according to claim 20, and further comprising a moon pool in a deck structure supported by the hulls.
24. A vessel according to claim 20, wherein the hulls have an elliptical form geometry.
 This invention relates to a specialised vessel for use in strong
current environments to perform installation functions of sub-surface
assets such as tidal turbines, wave energy devices, cable laying and the
like, or to facilitate foundation installations and also the function of
inspecting sub-surface areas.
 There are numerous requirements for the installation of assets such as tidal turbines, foundations, cables, etc., in locations with strong currents. Tidal energy, for example, presents a reliable source of energy in certain areas of the globe. The main impediment to the development of the industry of utilising that energy at present is the extremely high cost of installation. This installation cost is very high due to the developing industry lacking suitable ships/vessels which are custom-built for the particular environment. Current practice is to utilise vessels developed for the oil and gas sector. These vessels are not developed to operate in the fast currents of a tidal environment and are therefore not suitably effective for tidal device installation (including associated device-to-shore connections), maintenance and decommissioning. Furthermore, the charter day rates of these vessels are very high and variable. Seasonal price fluctuations in orders of magnitude are not uncommon.
 To a lesser extent, the offshore wind and the wave energy industry face similar problems because by choosing sites where there is a lot of energy, be it current, wave or wind, installation vessels are needed which can maintain station (position) accurately in a very unsteady and dynamic environment. Surges in the current occur where the tide will change speed and/or direction quickly and gusts of wind and/or complicated wave climates need to be dealt with. Traditional offshore construction vessels have limited ability in working under certain wind and current conditions, primarily on the beam, i.e. the side of the vessel. Such current and/or wind surges can lead to the vessel losing station with no warning and having to abort an operation, and can cause serious damage to equipment and potential loss of life.
 To date, existing offshore construction vessels (currently utilised for marine renewable energy system installation and servicing) have been unable to demonstrate reliable station-keeping in extreme wind and water flow environments.
 Mine-hunting is the process of locating sea-mines, but not detonating them. Once located they can be defused, recovered, or destroyed in a controlled detonation Thus the use of sonar equipment is a fundamental technology employed in mine-hunting. Conventional mine-hunting sonar involves the use of a single high-definition, short range hull mounted sonar which is used to view an object from different aspects in order to classify it. To date navies employ such technology on monohull vessels, it is therefore analogous to `single eye vision`, and to identify and classify an object as a mine, the vessel must be moved relative to the mine.
 Three-dimensional sonar imaging technology currently involves using Side Scan Sonar (SSS) which involves the moving of the sonar device over a target and taking a series of "snap-shots" which are then combined together along the direction of motion to form an image of the sea bottom within the swath (coverage width) of the sonar beam. It is profoundly dangerous to pass over the top of a sub-marine mine, simply because of the risk of detonation of the mine by the triggering of one or more of an array of sensors incorporated in the mine itself.
 The same limitation applies to the more sophisticated Synthetic Aperture Sonar (SAS) which combines a number of acoustic pings to form an image with much higher resolution than conventional sonars. The principle of SAS is to move a sonar device along a path and `illuminate` the same spot on the sea floor with several active pings. By coherent post-processing of the sonar data from all the pings, an image is produced with improved along-path resolution.
 Naval organisations, especially military ones, are increasingly financially constricted and the availability of relatively low-cost, high-capability vessels would be advantageous.
 Remaining on-site at a fixed position above a submerged asset in fast water flow speeds, both in rivers and sites of high tidal current speed, can only be performed by a very small number of relatively large, powerful and expensive ships. These ships are still only able to operate within tight windows of opportunity and have limited capability owing to their relatively high wetted surface, i.e. the surface of a ship's hull in contact with the water under specified conditions. Their large size and resulting high inertia also does not facilitate high levels of manoeuvrability and rapid response to demands for station-keeping. In addition, moorings are required for traditional vessels to hold station. Owing to the size of the vessels the moorings are complex to install and of relatively very high cost. If such moorings drag they pose a hazard to other sub-sea equipment (cables, devices etc.) and if break, they present a potentially dangerous situation.
 A key problem is that traditional vessels are optimised for travel in one direction (ahead) whilst having to hold station above a subsea asset for any length of time, even if the tide will change direction. Since these vessels are not optimised to work in this type of environment, they usually need to re-orientate themselves by turning through 180 degrees to remain pointing into the tide. This re-orientation needs to occur at slack water so that the vessel can remain on-station. However, this period is also the working window for performing the necessary sub-sea operations (for example using Remotely Operated Vehicles (ROVs)), therefore such manoeuvres dramatically reduce the already limited time to perform such tasks.
 According to one aspect of the present invention, there is provided a vessel comprising a plurality of hulls, a propulsion system including propulsion devices at respective end regions of said plurality of hulls and a control system connected to said propulsion system to control operation of said propulsion devices so as to position the hulls, wherein said propulsion devices are angled relative to the vertical longitudinally of each hull.
 According to a second aspect of the present invention, there is provided a method comprising positioning at a location a vessel having a plurality of hulls and utilising a control system to operate a propulsion system of said vessel in order to substantially maintain the position of the hulls at the location.
 Owing to these aspects, it is possible to provide a vessel that substantially retains its position with respect to a fixed point on a seabed or an offshore structure, such as offshore oil and gas, tidal energy devices, or wind structures.
 The vessel is of particular use in areas of fast flow including tidal streams, ocean currents and rivers regardless of changes in the current velocity, the wave direction, or the wind direction.
 Advantageously, there are two hulls and the propulsion system includes four propulsion devices located at both end regions of each hull. The propulsion devices are preferably able to vector-thrust the vessel, i.e. the ability of the vessel to manipulate the direction of the thrust from its propulsion devices in order to control its attitude.
 The propulsion devices are advantageously located in the corner regions of the vessel taken as a whole and can provide thrust in any direction. Such devices can be, for example, vertical axis Voith-Schneider thrusters (also known as a cycloidal drive), but any propulsion device capable of vectored thrust can be utilised (e.g. vectored propellers, water jets or azimuth thrusters). In this way, the bias of the propulsive thrust is to be provided by the two upstream propulsion devices or thrusters (`pulling` the vessel) in order that the vessel can naturally/passively weathervane into the tide; this being the equivalent of front wheel drive in a road vehicle. The vessel can remain on-station without constant thrust variations and rudder movements as is the case with traditional vessels which are driven (`pushed`) by the stern-mounted propulsion unit, and must thus dynamically stem the tide through the use of a rudder and the bow thruster. To use another road vehicle analogy, the set up with the present vessel results in under-steer (stable in the direction of the current) rather than over-steer of the traditional vessels which needs continuous correction to prevent loss of directional stability.
 Furthermore, it is preferable that respective skegs and/or rudders are provided on each hull and may be retractable so as to be deployed at the "stern" under a given operative condition and lifted at the "bow". Such skegs and/or rudders give greater stability in an aquatic environment than would be possible if relying solely on the propulsion devices.
 According to a third aspect of the present invention, there is provided a vessel comprising a plurality of hulls substantially parallel to each other, the arrangement being such that each hull is symmetrical about a substantially vertical plane transverse to a longitudinal axis of each hull.
 Owing to this aspect, it is possible to provide a vessel which can operate in a turning tide condition.
 The vessel is preferably a twin-hulled catamaran-type of vessel, each hull being symmetrical from front-to-back about a substantially vertical central transverse plane across each hull, with the bow and stern shape being substantially identical.
 This front-to-back symmetry allows operation of the vessel in turning tides without the requirement to re-position the vessel, and also allows back-and-forth motion with equal capability. This symmetry allows safe, economical operation in a wide range of conditions.
 The catamaran-type of vessel allows a relatively large deck space bridging the plurality of hulls. Preferably, a central "moon pool" is included between two hulls to facilitate underwater operations such as lifting or lowering from a substantially central stable position.
 Advantageously, a Dynamic Positioning (DP) control system is provided whereby a computer controlled system automatically maintains the vessel's position and heading by using the propulsion devices. Sensors, such as position reference sensors, wind sensors and motion sensors together with gyro compasses provide information to the computer pertaining to the vessel's position and the magnitude and direction of environmental forces affecting its position. The DP system also operates during propulsive redundancy, i.e. the failure of one of the propulsion devices, so that the failure of one does not result in a loss of position.
 The hulls preferably have an elliptical form geometry, optimised for drag reduction for a range of vessel heading angles as opposed to conventional vessels which are normally optimised for low-drag in the forward direction only.
 According to a fourth aspect of the present invention, there is provided a vessel comprising a plurality of hulls, a propulsion system including propulsion devices at respective end regions of said plurality of hulls, a control system connected to said propulsion system to control operation of said propulsion devices so as to position the hulls and an inspecting arrangement mounted to at least two of the plurality of hulls to inspect an area outwardly away from said vessel.
 According to a fifth aspect of the present invention, there is provided a method comprising positioning at a location a vessel having a plurality of hulls, utilising a control system to operate a propulsion system of said vessel in order to substantially maintain the position of the hulls at the location and inspecting an area outwardly away from said vessel.
 Owing to these aspects, it is possible to provide a vessel that inspects a sub-surface area in order to find objects without having to pass over the objects.
 Each inspecting arrangement comprises an emitting device serving to emit an inspection medium, a receiving device arranged to receive any reflected inspection medium from objects in the inspected area. A data processing device connected to the inspection arrangements serves to generate first and second inspection data sets, one from each inspection arrangement.
 The inspection arrangements mounted at the front end regions, or the bow regions, of at least two hulls are preferably forward-looking sonar devices and preferably high-definition forward-looking sonar devices. The equivalent of `stereoscopic` vision is thus afforded to the data processing device.
 Furthermore, the hulls are preferably made from relatively light-weight materials such as aluminium and/or composites (not limited to lass/fibre-reinforced plastics (G/FRP or similar). The vessel is preferably a twin-hulled catamaran-type of vessel, each hull being symmetrical from front-to-back about a substantially vertical central transverse plane across each hull, with the bow and stern shape being substantially identical.
 The vessel is of particular use in aquatic minefields where there may be fast flow including tidal streams, ocean currents and rivers regardless of changes in the current velocity, the wave direction, or the wind direction.
 In order that the present invention be clearly and completely disclosed, reference will now be made, by way of example, to the accompanying drawings, in which:
 FIG. 1 schematically illustrates the layout of a vessel in plan view comprising two symmetrical hulls with propulsion devices at each end region of the hulls,
 FIG. 2 is similar to FIG. 1, but showing how the vessel is "pulled" into a current direction,
 FIG. 3 is similar to FIG. 2, but showing how, when the current flow reverses, the the operation of the propulsion devices can be changed,
 FIG. 4 shows how lateral disturbances from wind or tide can be countered,
 FIG. 5a shows a side view of a vessel having constant cross-sectional area hulls,
 FIG. 5b shows and end view of the vessel of FIG. 5a,
 FIG. 6a shows a side view of an alternative embodiment of the vessel having constant cross-sectional area hulls,
 FIG. 6b shows and end view of the vessel of FIG. 6a,
 FIGS. 7a-d are views of a further embodiment of the vessel,
 FIGS. 8a-d show respective perspective, side, end and plan views of the hull form of the vessel of FIGS. 7a-d,
 FIG. 9 is a view similar to FIGS. 1 to 4, but showing how the propulsion devices may be used to rotate the vessel and to work against lateral currents, winds or waves, and
 FIG. 10 shows a schematic perspective view from below of a vessel comprising two symmetrical hulls having bow-mounted sonar devices, and
 FIG. 11 is a schematic perpective view from above of the vessel of FIG. 10 inspecting an area in front of the vessel.
 Referring to FIGS. 1 to 4, a vessel 2 comprises two hulls 4 arranged substantially parallely to each other and propulsion devices 6 located at respective opposite end regions of each of the hulls 4, such that there is one propulsion device 6 in each corner region of the vessel 2. Referring specifically to FIG. 2, the current direction is shown by the arrow 8 and the direction of thrust provided by the pair of upstream propulsion devices 6 is shown by the arrows 10 which direction of thrust is substantially parallel to the direction of current. Thus, the drag of the hulls 4 causes them to align with the current direction and maintain a stable orientation in the current in relation to a sub-surface target zone 12.
 As shown in FIG. 3, in a situation like a tide race, when the current flow 8a reverses, operation 10a of the propulsion devices 6 can be changed to the opposite pair of propulsion devices without repositioning or re-orientating the vessel to maintain station in relation of the zone 12. FIG. 4 shows how lateral disturbances L from factors such as wind or tide can be countered by arranging for the propulsion devices 6 to apply a sideways thrust either using all four thrusters 6 or just those on the windward or lee side of the vessel 2.
 The propulsion devices 6 on each hull 4 are arranged so that in cross-section across the vessel 2 the propulsion devices are mounted substantially vertically. If, however, a longitudinal section of each hull 4 is taken, the propulsion devices 6 at each end of the hull 4 are angled relative to the vertical (as seen in FIG. 7b). This arrangement of the propulsion devices 6 achieves relatively good propulsive efficiency in extreme wind and water flow environments, such as fast tidal flows. As water moves around the hulls 4, the propulsion devices 6 need to be substantially perpendicular to this flow to operate efficiently and since the shape of each hull 4 alters the water flow, the propulsion devices 6 need to be offset at an angle longitudinally to achieve the most efficient substantially perpendicular inflow velocity.
 Referring to FIGS. 5a and 5b, the vessel 2 has hulls 4 with a substantially constant cross-sectional area (for relatively low cost) where the propulsion devices 6 are deployed fore and aft of each hull 4 in a central vertical plane of each hull (the upstream propulsion device shown as being in a retracted state contained within the volume of the hull 4). Skegs or rudders 14 may also be provided for and aft of each hull 4 also in the vertical plane and which are retractable, only to be lowered and deployed when needed (the downstream skeg/rudder being shown in a retracted state above the water level WL). The propulsion devices in FIGS. 5a and 5b are preferably cycloidal drive, Voith-Schneider, type propulsors, but they could equally use vectored azimuth thrust propellers or water jets.
 In normal operation the upstream propulsion devices 6 will be lowered and activated to pull the vessel 2 against the current and the skeg/rudder 14 at the downstream or aft end will be lowered to help maintain directional stability.
 The layout of the propulsion devices 6 means that the vessel 2 can maintain position and directional stability in a variety of flow directions and speeds by utilising the thrust from the upstream propulsion devices 6, so that the rest of the vessel passively "weathercocks" and remains in-line with the flow in a balanced and stable manner.
 The geometry of the hulls 4 are optimised to incorporate the propulsion devices allowing for a substantially equal drag on the vessel 2 in both the forward and aft direction. Each hull 4, at least the part of the hull under the water, is symmetrical about a single central substantially vertical plane at amidships giving front-to-back symmetry to each hull 4. This enables the vessel 2 to maintain station when the tide turns and flows from the opposite direction. Thus the vessel 2 does not itself need to rotate to face the current and thus operational time is maximised. This also ensures that the DP system is more easily optimised to the vessel as its characteristics are the same in both directions of flow.
 The skegs or rudders 14 may also be deployed fore and aft of a centre line of the vessel 2 as shown in FIGS. 6a and 6b. In these circumstances, in normal operation, the rudder or skeg 14 at the downstream or aft end of the vessel 2 will be deployed into the water and the one at the upstream or forward end will be raised.
 The skegs/rudders 14 provide passive or active keel areas to improve directional stability and help the vessel 2 to weathercock into the flow direction. In short, the skegs/rudders 14 are needed to provide the following:
 Directional stability when transitting or sitting in a fast tidal flow. Thus reducing the need to use the DP system to maintain a heading.
 Provide steering on passage.
 Substantially horizontal pitchable foils or hydrovanes (not shown) can optionally be deployed fore and aft on each hull 4 either between the hulls, or outboard of the hulls (or both inboard and outboard) so as to develop substantial up-thrust or down-thrust in order to level the hulls 4 when they are subjected to a vertical load, such as when lifting using an on-board crane or winch over the end of the vessel 2 or when in transit to assist in levelling the hulls against an asymmetric load such as when cable laying. Such foils or hydrovanes may also in some cases be dynamically positionable in pitch using suitable sensors and software so as to act as stabilisers in order to reduce wave-induced hull motion in pitch and roll directions.
 The hulls 4 may also be optimised geometrically in the transverse direction to reduce lateral drag (i.e. port-to-starboard), which enables the vessel 2 to be very manoeuvrable. Even during slack water, as the tide changes direction, the tide may try to force the vessel 2 sideways. With the optimised hull geometries the vessel 2 is able to remain on station above the subsea asset.
 The unique hull shape has the following advantageous characteristics:
 Symmetrical elliptical hulls 4 that are optimised for low drag in both forward and reverse directions.
 The elliptical hulls 4 also give improved wave riding capability in higher sea states.
 The high freeboard, i.e. the distance from the waterline to an upper deck level 16, allows high lift capability for a low-drag hull-form
 The high freeboard keeps the working deck 16 dry in higher sea states allowing improved operating windows.
 The use of the elliptical hull form in a Catamaran layout removes the need for bilge keels which are normally required on mono-hull vessels.
 To provide a space at each end of the hulls 4 to permit the propulsion devices 6 to be protected from grounding in shallow water or if the vessel 2 needs to settle on the seabed at low tide.
 The DP control system enables the vessel 2 to maintain position in aggressive and difficult sea-states such as tide races, and/or if the vessel is to be moving at speed such as when laying a sub-sea cable. This also gives the vessel 2 the ability to manoeuvre in any direction and maintain its heading/orientation. The hulls 4 are optimised to reduce the drag both in-line with the water flow (as per normal ships) but also when the flow is at an oblique angle to the longitudinal hull axis. The combination of the vectoring thrust from each corner region of the vessel 2 and the refined hull form result in a vessel that is extremely capable in such an environment, and fundamentally efficient owing to the fact that the DP system can seek an optimal orientation in order to minimise drag. This ensures that the vessel 2 is particularly fuel economical.
 The propulsion devices are under direct control by the DP software. This software is used to co-ordinate the thrust to maintain the position vessel 2 and its orientation at a target location and thus to maximise efficiency. The DP system therefore has to be specifically tailored to the hull configuration and propulsion devices.
 FIGS. 6a and 6b also show the possibility of mounting the propulsion devices externally of the hulls 4 instead of using retractable propulsion devices 6 as in FIGS. 5a and 5b. These externally mounted propulsion devices 6 can be raised, for example, by swivelling or rotating them about a pivot axis 18. The propulsion devices 6 may also be mounted externally of the hulls 4 so as to be simply raised from an extension to the deck 16. If the propulsion devices 6 are so mounted as shown in FIGS. 6a and 6b, then a flat plate (not shown) immediately above them may be provided in order to prevent air from being sucked into them and thereby reducing their efficiency.
 FIGS. 7a-d show the vessel 2 and how the hulls 4 are arranged to support the deck 16 with a moon pool 20, i.e. an opening in the deck 16 to gain access to the surface of the water, a bridge 22 and a lifting device 24 which may be in the form of a pair of shearlegs 26 to which lifting tackle may be suspended to permit lifting either over the end of the vessel 2 or through the moon pool 20. The vessel 2 is designed to provide the maximum deck space by utilising the plurality of hulls 4.
 FIGS. 8a-d illustrates more clearly the symmetrical eliptical hull form to minimise drag, a substantially vertical transverse plane 32 at the amidships position of each hull 4 showing the front-to-back symmetry (shown in FIG. 8d). It can be seen that the central section 28 of the underside of the hulls 4 is deeper in the water than the end sections 30, thereby facilitating the use of non-retractable propulsion devices 6 in shallow water operations.
 Catamaran-types of vessel have a relatively low wetted surface and in combination with the elliptical geometry of the hulls 4, the vessel 2 provides a wave-resistant water-contact area.
 FIG. 9 shows how the propulsion devices 6 may be used to rotate the vessel 2 and to work against lateral currents, winds or waves L. In some respects the vessel 2 can be manoeuvred like a tracked vehicle on land which can be turned on the spot by running its tracks in opposite directions. By operating the upstream propulsion devices 6 in the same general direction as the current and the downstream propulsion devices 6 in a direction substantially orthogonally to the direction of the upstream propulsion devices 6 and with the respective pairs of propulsion devices being operated with differential thrust power (controlled by the DP system), the vessel 2 may be kept on-station over the sub-sea asset target zone 12 and orientate itself with the direction L (shown by the chain line hulls 4 in the Figure).
 The vessel 2 is extremely manoeuvrable, capable of stable positioning in strong currents and other disruptive sea conditions without the need for costly/risky moorings.
 The vessel 2 does not need to re-orientate owing to the fact that it is optimised to operate in bi-directional flow regimes, being symmetrical in the direction of travel and similar in performance moving forwards or in reverse, therefore maximising the time available for performing sub-sea operations.
 In the case of tidal energy related installations in particular, the vessel 2 offers the means to dramatically reduce costs and risks associated with installing which are currently the main hurdles to overcome in order to advance the industry. The vessel 2 would provide the technical means (at an acceptable cost) to complete construction and possibly maintenance tasks throughout a project life cycle.
 The vessel 2 would further provide safe and reliable means to complete tasks such as (but not limited to); site investigation, foundation installation, subsea drilling support, cable installation, cable repair, cable protection, turbine installation/removal, site decommissioning and submerged sub-station maintenance.
 Referring to FIGS. 10 and 11, the vessel 2 comprises two hulls 4 arranged substantially parallely to each other in a catamaran style, in a similar manner to that of the vessel as described above. The propulsion devices (not shown in FIGS. 10 and 11) are, likewise, preferably located at respective opposite end regions of each of the hulls 4, such that there is one propulsion device in each corner region of the vessel 2. In operation, the arrangement of propulsion devices is such that the vessel 2 maintains a stable orientation in the current in relation to a sub-surface object, such as a mine 34 (shown in FIG. 11).
 The retractable skegs or rudders (described hereinabove) may also be provided fore and aft of each hull 4 to be lowered and deployed when needed.
 As already mentioned, the layout of the propulsion devices means that the vessel 2 can maintain position and directional stability in a variety of flow directions and speeds to remain in-line with the flow in a balanced and stable manner. This is advantageous when mine-hunting since the location of mines can be accurately mapped.
 The hulls 4 are to be made, as mentioned before, of relatively light-weight materials, such as aluminium and/or G/FRP. If G/FRP is used, the resulting structure is strong in tension and would be largely free of corrosion. Aluminium results in a lighter and stronger hull form than if the vessel 2 were built from G/FRP. Marine-grade aluminium has such high impact resistance, that the vessel 2 could withstand a collision that would seriously damage a G/FRP hull, and thus may be much more preferable for a mine hunting vessel. Marine-grade aluminium also has excellent corrosion resistance, and in most cases aluminium boats will last up to 50 years in a harsh saltwater environment. Aluminium is furthermore by far and away the lowest maintenance material that could be used for boat manufacturing.
 Again, substantially horizontal pitchable foils or hydrovanes (not shown) can optionally be deployed fore and aft on each hull 4 either between the hulls, or outboard of the hulls (or both inboard and outboard) so as to develop substantial up-thrust or down-thrust in order to level the hulls 4 when they are subjected to a vertical load or when in transit to assist in levelling the hulls against an asymmetric load. Again, such foils or hydrovanes may also in some cases be dynamically positionable in pitch using suitable sensors and software so as to act as stabilisers in order to reduce wave-induced hull motion in pitch and roll directions.
 A DP control system enables the vessel 2 to maintain position in aggressive and difficult sea-states such as tide races, and/or if the vessel is to be moving at speed.
 Located at the underside of one of the end sections 30, the bow end section 30, is an inspection arrangement 36 which comprises an emitting means for emitting an inspection medium in an inspection area and a receiving device for receiving any emitted inspection medium which is reflected back from any object present in the inspection region. In aquatic environments it is known that sonar devices are reliable for the inspection of underwater environments using an emitting device to emit sound energy and a receiving device to receive any of that sound energy emitted which has been reflected back towards the sonar device by an object in the area being inspected. In mine-hunting, it is advantageous not to have to pass over an explosive mine before it is locatable, as is the case with current SSS and SAS mine-hunting vessels. Each inspection arrangement 36 serves to emit sound energy in a direction projecting outwardly away from the vessel, and preferably forwardly and downwardly into the water with respect to the heading of the vessel 2, i.e. a forward-looking sonar arrangement. In this way the inspection area, or field of view, for each sonar arrangement 12 overlap. Each sonar arrangement 36 is connected to a data processing device in order to generate respective first and second inspection data of the inspection area. A substantially identical image of the inspection area can thus be produced by the data processing device for each sonar arrangement 36. The two images from the sonar arrangements located at different points in space can then be electronically, stereoscopically combined by the or another data processing device with suitable software to produce a three-dimensional image of the inspection area covered by both sonar arrangements 36. Preferably, the sonar arrangements enable high-definition images to be produced. Any object 34, such as an explosive mine, can be found without the vessel 2 having to pass over it. The position of the mine 34 can thus be recorded with significant accuracy when the inspection data is connected to the DP system.
 Referring to FIG. 11, the sonar arrangements 36, inspecting a region in front of the vessel 2 (shown by the arrows 38) can view the mine 34 under the water surface which mine may be provided with an array of sensors, one or more of which could trigger the mine 34 to explode if the vessel 2 passed over the mine 34. The high degree of manoeuvrability of the vessel 2 thus makes the mine 34 easily avoidable.
 The vessel 2 could be provided with mine-sweeping equipment to clear the inspection area of mines or the location data could be transmitted to another dedicated mine-sweeping vessel.
 As a matter of safety, degaussing coils are fitted to the hulls 4 just in case the vessel passes over a mine 34 in order to decrease or eliminate an unwanted magnetic field produced by the vessel 2 which can trigger magnetic sensors in some mines. As a further safety feature, the propulsion devices to be used are relatively very quiet compared to other propulsion systems in order to prevent triggering sensors in mines which trigger an explosion by the detection of noise above certain threshold levels or specific vessel noises.
 The vessel 2 would also be useful for sub-marine bottom surface mapping and surveying, especially in marine environments such as coral reefs owing to the fact that the propulsion system is such that there is no need to drop an anchor into ecologically fragile areas which may or may not be protected by a rule of law.
 Moreover, the vessel 2 would serve an advantageous purpose in submarine rescue. In instances where a submarine becomes disabled and an urgent evacuation of submariners by way of submarine escape immersion equipment suits is not required, then a rescue vehicle can be employed. A rescue vehicle is preferred as it allows submariners to survive with essentially no injuries since they are protected from the great amount of pressure at ocean depths and the exposure to cold water is avoided. The vessel 2 could reliably locate a disabled submarine with its forward-looking sonar inspection arrangements and initiate a rescue operation whilst maintaining station on the water surface by way of the propulsion system. Furthermore, other types of salvage work could be carried out or contributed to by the vessel 2.
Patent applications in class MEANS TO CONTROL THE SUPPLY OF ENERGY RESPONSIVE TO A SENSED CONDITION
Patent applications in all subclasses MEANS TO CONTROL THE SUPPLY OF ENERGY RESPONSIVE TO A SENSED CONDITION