GB2485861A

GB2485861A - Wave piercing offshore supply, maintenance or patrol vessel

Info

Publication number: GB2485861A
Application number: GB1110853.7A
Authority: GB
Inventors: Stephen John Beadsmoore
Original assignee: Individual
Current assignee: Individual
Priority date: 2010-11-26
Filing date: 2011-06-27
Publication date: 2012-05-30
Anticipated expiration: 2031-06-27
Also published as: GB2485861B; GB201020053D0; GB201110853D0

Abstract

A long, narrow, wave piercing maintenance and supply vessel 1 is taught having powerful engines (11, Fig 3) to be able to travel quickly in heavy seas. Trim planes (21, Fig 4) on either side of the bow may be provided to either operate together to promote wave piercing, or operate independently to reduce rolling and/or yawing, or in a combination of both modes. Control of the trim planes would be via a gyro-stabiliser(s) (15, Fig 2) which also stabilises the vessel itself. The vessel transfers personnel and equipment between a base and a wind turbine, by mooring stern-on to the mast of the wind turbine in the lee of the mast, to transfer personnel and equipment to the access point of the mast. The transfer may be assisted by an elevatable transfer platform (30, Fig 70 provided at the stern of the vessel. This platform may also be gyro-stabilised.

Description

OFFSHORE SUPPLY, MAINTENANCE OR PATROL VESSEL This specification particularly relates to a vessel able to ferry equipment and personnel to S and from offshore installations, e.g. windfarms, where speed of transit and ability to carry items of equipment and maintenance personnel are key requirements. However, the vessel will be well suited to other offshore duties, e.g. in connection with oil and gas rigs, tidal turbines, patrols and border control, smuggling interdiction, etc. To explain the invention fully, this specification is written with respect to the supply and maintenance of the equipment on, or associated with, an offshore windfarm but the skilled person will understand its many other parallel applications in the offshore environment, all falling within the scope of the invention.

The apparent changes in the climate of the planet, believed to be largely related to the increase in carbon dioxide in the atmosphere, have increased the demand for renewable, so-called, green' energy. One of the main sources of this energy is the wind and, because of environmental objections, many so-called windfarms' are located offshore in the shallower parts of the continental shelf Such windfarms can consist of over one hundred separate turbines covering a large area of sea. All of these turbines require regular maintenance, which can be planned, and breakdown maintenance, which cannot. Thus, a vessel(s) is(are) required to ferry equipment and personnel between the individual turbines on their support columns and a base, which may be an adjacent port or an offshore mother ship.

Windfarms are best sited where strong winds regularly blow and the most favoured locations are often where the wind has blown over a long fetch of open sea or ocean.

Strong winds, blowing over a long fetch of water, usually mean large waves and I or a significant swell. As most of the locations close to shore have been used, sites further from shore are now being developed. While regular maintenance can be left for calmer weather, breakdowns have to be attended to as and when they occur and as quickly as possible, so that the windfarm can niaximise its power production, as downtime is very costly.

This means getting personnel and replacement equipment to the failed turbine as quickly as possible, i.e. the need for a fast vessel. Breakdowns often occur in extreme conditions, i.e. in stormy weather, so that the vessel must not only be fast in calm water but also in heavy seas. This poses a further problem as trying to sail fast through rough seas can be a violent passage; it is no point getting there fast if the maintenance personnel on board are too seasick on arrival to undertake their work! To maximise wind turbine availability, the maintenance vessel must be able to travel S quickly but also be able to carry, often heavy, replacement items of equipment.

Traditionally, powerful monohull boats have been used but these have tended to roll excessively in heavy seas and the problem gets worse with more distant sites. Monohulls with a broader beam tend to roll less but they are slower and / or use more fuel. The current standard maintenance vessels are catamaran and larger versions are being built to give greater stability but the capital and running costs are greater.

Thus, there is a need for a fast windfarm maintenance and supply vessel, which can carry equipment and personnel quickly and comfortably to and from the individual wind turbines and disembark it and them safely at the wind turbine.

According to the invention, there is provided an offshore supply, maintenance or patrol vessel comprising:- (i) a long, narrow hull having a length to beam ratio of 5.4: 1 or more; (ii) a hull which in plan view has an essentially triangular form, having a wave piercing bow and a stein; (ii) a gyro-stabiliser arranged to act to maintain the vessel in a normal upright attitude; (iii) heavy equipment, including the propulsion machinery, located as low as possible in and below the hull to maximise stability; (iv) accommodation for passengers / crew and their equipment; (v) means to load equipment onto the vessel, store it securely during transit and unload both equipment and passengers and transfer it and them to the access point of an offshore installation; and (vi) a helmsman's position offering good forward and rearward visibility; characterised in that the bow is designed to pierce waves rather than ride up and over them so that, together with the gyro-stabiliser, pitching, rolling and yawing are minimised allowing higher transit speeds with minimal discomfort to the passengers I crew and further characterised in that, when supplying or maintaining an offshore installation and on arriving at that installation, the vessel is sailed astern up to an appropriate access point of the installation and the stern is secured thereto so that the vessel is essentially at a radius from the access point of the installation which minimises the motion of the vessel thus facilitating transfer of personnel and equipment to and from the access point of the installation.

According to a first variation of the apparatus of the invention, the hull is made of glass reinforced plastic (GRP) or metal alloy.

According to a second variation of the apparatus of the invention, the means of propulsion is via a propeller(s).

According to a third variation of the apparatus of the invention, the means of propulsion is via a water jet(s).

According to a fourth variation of the apparatus of the invention, the means of propulsion is provided with a degree of azimuth rotational capability.

According to a fifth variation of the apparatus of the invention, a gyro-stabiliser is located on I adjacent to the fore-aft axis of the hull.

S

According to a sixth variation of the apparatus of the invention, all the vessel's permanent.

heavy equipment is located as low in the hull as possible.

According to a seventh variation of the apparatus of the invention, hydrodynamic trim planes are provided near the bow and above the waterline to cut through large waves and minimise the tendency of the buoyant bow to rise on encountering waves but minimise drag when sailing on calm water.

According to an eighth variation of the apparatus of the invention, the angle of the hydrodynamic trim planes is adjustable to control the degree of wave piercing of the bow.

According to a ninth variation of the apparatus of the invention, the superstructure surrounding the helmsman's seat is shaped to deflect water breaking over the bow.

According to a tenth variation of the apparatus of the invention, the helmsman's seat can rotate through 1800 so that the helmsman can steer the vessel both when sailing ahead and astern.

According to an eleventh variation of the apparatus of the invention, windows are provided to maximise the helmsman's forward and rearward visibility.

According to a twelfth variation of the apparatus of the invention, a loading platform is provided atl near the stern of the vessel.

According to a thirteenth variation of the apparatus of the invention, means is provided to secure equipment to the loading platform to prevent it moving during transits to / from the access point of an offshore installation.

According to a fourteenth variation of the apparatus of the invention, a transfer platform is provided as an integral part of the loading platform.

According to a fifteenth variation of the apparatus of the inventi?n, the transfer platform is elevatable to facilitate embarkation I disembarkation of people and loading / unloading equipment onto / off the vessel.

According to a sixteenth variation of the apparatus of the invention, the height of the elevatable transfer platform is continuously adjustable to absorb the rise and fall of the vessel as waves pass under the vessel to facilitate the transfer of people at sea and I or equipment from the transfer platform to the access point of an offshore installation.

According to a seventeenth variation of the apparatus of the invention, the transfer platform is gyro-stabilised with the rest of the vessel.

According to an eighteenth variation of the apparatus of the invention, means is provided on the transfer platform, or elsewhere, to secure equipment or personnel being transferred from the vessel to the access point of an offshore structure.

According to a nineteenth variation of the apparatus of the invention, the form of the hull is such that its forward motion through the water entraps air below the hull so reducing drag.

According to a twentieth variation of the apparatus of the invention, the engine exhaust is discharged through the hull below the waterline, so reducing drag.

In a preferred application of the invention, a long, narrow, wave piercing maintenance, supply or patrol vessel is provided with powerful engines to be able to travel quickly in heavy seas. Wave piercing vessels, moving quickly in rough seas, follow an undulating course as opposed to the violent up-and-down motion of a small vessel riding the waves; this gives a much more comfortable voyage to the passengers, who will thus be better able to undertake maintenance activities on arrival at the wind turbine than would have been the case had they been violently sea sick in transit.

To reduce the drag of the water on the hull and improve passenger comfort, the hull has a form designed so that its forward motion entraps air underneath, thus gaining some benefits of Surface Effect Ships, e.g. hovercraft. This effect is further enhanced by ducting the diesel engine exhaust to exit points below the hull.

To improve further the quality of the transit, gyro-stabilisation is provided, ideally operating in conjunction with trim planes at I near the bow of the vessel. Trim planes on either side of the bow may either operate together, e.g. to promote wave piercing, or operate independently, e.g. to reduce rolling and I or yawing when the swell is coming at the vessel from an angle to the course being sailed or operate in a combination of both modes.

Control of the trim planes would be via the gyro-stabiliser, in accordance with an overall setting determined by the helmsman, taking into account the prevailing weather and sea conditions.

Comfortable accommodation is provided for the passengers and crew, together with appropriate storage for their equipment. On arrival at the wind turbine requiring maintenance, the vessel would approach stern-first and moor to the mast of the wind turbine, preferably, in the lee of the mast so that personnel and equipment could be transferred to the access point of the mast as safely and conveniently as possible.

Preferably, an elevatable transfer platform is provided at the stern of the vessel to facilitate the transfer of personnel and equipment and ideally this platform is gyro-stabilised to make such transfers as safe as possible.

For a clearer understanding of the invention and to show how it may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings in which:-Figure 1 is a side elevation of a vessel according to the invention; Figure 2 is a cutaway side elevation of the vessel of Fig. 1; Figure 3 is a plan view of the vessel in Fig. 1; Figure 4 is a side elevation of the bow of the vessel showing the trim planes and operating mechanism; Figure 5 is a plan view of the bow of the vessel in Fig. 4 showing the location of the trim planes and operating mechanism; Figure 6 is a plan view of the stern of the vessel moored to the mast of a wind S turbine; and Figure 7 is a side elevation of the stern of the vessel in Fig. 6 moored to the mast of a wind turbine showing the juxtaposition of the transfer platform in its elevated position to the access deck on the mast of the wind turbine.

In the following description, the same reference numeral is used for identical components in different Figures or for different components fulfilling an identical function.

Referring to Fig. 1, vessel I has a wave-piercing bow 2, an enclosed forward section 3 and an enclosed bridge 4, provided with large windows 5. Mast 6 carries the radar, communications equipment and riding lights. Aft of bridge 4 is loading platform 7, extending to the stern 8. A fender 9 is provided above waterline 10. As shown in Fig. 2, there is very little draft at the bow but slightly more at the stern. This attitude, combined with the high speed of vessel 1 will promote wave piercing to give a smoother passage over (and through) the water. Bridge 4 has a steeply raked front 4A to deflect waves running over forward section 3 and away from loading platform 7.

A series of adjustable trim planes 21 (Fig. 4) are provided near bow 2 and above waterline 10. When bow 2 enters a wave (hot shown), planes 2lwill counteract the buoyancy and cause bow 2 and forward section 3 to pierce' the wave thus minimising pitching and giving a more even sailing motion. This will allow vessel 1 to travel much faster while giving the passengers (not shown) a more pleasant voyage. The angle of the planes 21 is controllable so that it may be adjusted according to the sea state. This adjustment may be equal on either side of bow 2, e.g. to control pitching, or asymmetric, e.g. to control roll or yaw. This adjustment may be dynamic, i.e. changed as the vessel passes through each wave, or static, i.e. preset for a particular sea state. When travelling on calm water, the planes 21 would be mostly out of the water thus reducing drag.

The operation of trim planes 21 is shown in Figs. 4 and 5. Each of planes 21 is mounted onan axle 22 which is rotatably fast with hull 13. A number of planes 21 are provided on both sides of bow 2, as shown. In the example shown, five planes 21 are provided on each side. The advantage of having a series of planes 21 is that, as the sea gets rougher and the buoyancy force on the bow increases, more planes 21 will be submerged thus spreading the greater downward reaction force more evenly over the planes 21 and bow 2.

The angle of planes 21 is adjustable via common actuating rail 27 which is rotatably fast s 27A with each plane 21. A double-acting piston (not shown), movable in cylinder 25, drives rod 24 to operate rail 27 via L-shaped connecting member 23. Rotatable mounting 26 locates cylinder 25. As shown in Fig. 5, separate means are provided to actuate the planes 21 on each side of bow 2 so that they may be operated independently, as and when required. Actuating means 24, 25 may be powered by hydraulic or electrical means.

Gyro-stabiliser 15 would operate the plane actuating means 24, 25 in response from the input of accelerometers (not shown). Helmsman 17 would set the sailing parameters on gyro-stabiliser 15 according to the prevailing weather and sea conditions.

A shield 28 (Fig. 4) protects actuators 23, 24 and 25 from wave damage as the water would break violently over bow 2 when piercing waves at high speed. Similarly, shield 29 (Fig. 5) protects rail 27, connections 27A and axles 22 from wave impact damage. (Shields 28 and 29 are not shown in Figs. 5 and 4 respectively to avoid confusing detail.) Trim planes 21 are constructed and mounted 22 robustly in view of the wave impacts to which they will be constantly subjected. The actual profiles of planes 21 will be designed to accommodate this arduous duty and one possible design shape is shown in Fig. 5.

Fig. 2 shows engines 11 located low down in hull 13 and driving, for example, a pair of contra-rotating propellers 12 mounted below hull 13 so that their weight and location adds to the stability of vessel 1. Propellers 12 are mounted in an azimuth housing allowing a degree of angular movement. Water jets (not shown), also with azimuth movement capability are equally possible. Fuel is stored in tank 20, below waterline 10 to improve stability. If propulsion was by electric motors 11, the batteries could be located in compartment 20.

Gyro-stabiliser(s) 15 (Figs. 2 and 3) are located on, or adjacent to fore I aft midline 1 5A.

Traditionally, the rolling of a vessel may be reduced by increasing the beam and adding bilge keels but both will reduce the speed for a given engine power. However, another method is to use gyro-stabilisers 15; these act more efficiently on a narrow hull 13 as the effect of the turning moment generated is greater than would be for a broad-beamed hull.

A narrow hull 13 has less water resistance and so is faster for the same given engine power than a broad one. Thus, the long, narrow hull 13, with gyro-stabilising 15, a wave piercing bow 2 and bow trim planes 21 offer a new concept in naval architecture for smooth, fast passages for small vessels 1, which will be much appreciated by the passengers in seats 14. This concept is a development of the hydrofoil principle, i.e. smoothing out the wave profiles.

The fast, gyro-stabilised, wave piercing concept places additional loadings on hull 13 but a strong light structure 13, e.g. made of aluminium alloy, will be more than adequate for the required duty. Hull 13 is based on the VSV Monohull principle and, in a preferred design, is 24Mm long with a maximum beam of 4Am. This gives a length to beam ratio of 5.853:1.

The concept can be scaled in order to build larger or smaller vessels. Traditionally, ocean going ships have a length to beam ratio of about 7: 1 and for most windfarm supply vessels, it is much smaller than this, particularly for catamarans. However, the effect of gyro-stabilisation 15 is much greater on a narrow hull than on one with a broad beam or a catamaran.

The design of hull 13 is such that forward motion of vessel 1 causes air to become entrapped underneath the hull so that bubbles, or even a film of air, between the hull and the water effectively reduces the drag allowing higher speeds for a given engine power or reduced fuel consumption for a given speed. This effect can be increased if all, or part, of the diesel engine 11 exhaust is discharged below the waterline. Depending on the actual layout, all / part of this exhaust may be directed forward and discharged adjacent to the keel to gain the maximum effect. As all the heavy propulsion equipment is located well aft, vessel I will ride in a slightly bow-up attitude at speed so that the exhaust would not have to be discharged very far forward to gain a significant benefit. This slightly bow-up attitude would not affect the wave-piercing aspects of the vessel.

Loading platform 7 is located aft of bridge 4 and extends forward from stern 8. The area of platform 7 is such as to be able to carry replacement parts, as and when required. Built into platform 7 is a transfer platform 30, which may be elevated by an electro-hydraulic or electro-mechanical power unit (not shown) to an appropriate height above loading platform 7. Figs. 6 and 7 show the operation of transfer platform 30. Vessel I is manoeuvred up to the mast 35 of a wind turbine and held (not shown) thereto, i.e. effectively at a radius to mast 35.

Surrounding mast 35 is annular access deck 34 [which may be accessed by a ladder (not shown)]. By reversing vessel 1, preferably into the lee side of mast 35, i.e. as shown by the direction of arrow 36, h&msman 16 will minimise the effects of wind, rolling, pitching and yawing of vessel 1. Arrow 36 is double headed to indicate both wind and swell direction; if these are the same, this is ideal; if not, helmsman 16 will chose the best compromise to minimise the motion of vessel 1 relative to mast 35.

Transfer platform 30 is stored in housing 33 and is elevatable therefrom by means (not shown) in location 33. Two telescopic sections 31, 32 are shown as one form of elevatable structure suitable for this application.

When not in use, deck 7A of platform 30 is flush with after deck 7. When moored at a wind turbine mast 35, deck 7A may be elevated so that it essentially aligns with access deck 34, as shown (Fig. 7) or an access ladder (not shown). Deck 7A will be moved automatically up and down and from side to side in opposition to the fall and rise and roll of vessel 1, i.e. keeping deck 7A essentially still with respect to mast 35. Thus, when deck 7A is flush with deck 7, personnel may step from deck 7 onto deck 7A, be lifted (Fig. 7) and step across to mast deck 34 or access ladder (not shown). Platform 30 and deck 7A are stabilised by gyro-stabiliser 15; if conditions are not too rough and helmsman 16 has chosen the right mooring point on mast 35, deck 7A will be relatively stable next to deck 34 (or access ladder), allowing personnel to step safely from one to the other. (Hand rails have not been shown to avoid confusing detail but may be provided, if required.) The gyro-stabilisation 15 of transfer platform 30 may be up and down (to allow for vessel I rising and falling as waves pass underneath) and angled from the base so that deck 7A moves from side-to-side and / or fore-aft (in opposition to the movement of vessel I to accommodate rolling, pitching and yawing) so that deck 7A remains effectively stationary relative to deck 34 or the access ladder.

Similarly, equipment may (not shown) be placed on deck 7A, raised and transferred to deck 34. When sea conditions are exceptionally rough, the equipment may be secured to attachments (not shown) on deck 7A for the actual elevation; additionally, a rope may be tied to the equipment, with the other end secured to deck 34 so that anything falling off will not be lost.

Transfer platform 30 may also be used to take away damaged equipment for repair and for loading / unloading equipment in port.

Persons skilled in Naval Architecture will note the location of all the heavy equipment, i.e. the propulsion machinery, close to the stern of vessel 1, thus giving a centre of gravity well aft and close to the line of action of elevatable transfer platform 30. This aspect of the design reduces the amount of rolling, pitching and yawing experienced at the deck 7A of transfer platform 30 and hence the duty on gyrostabiliser 15 so making the operation of transfer platform 30 safer, easier and quicker. A further benefit of having the centre of gravity well aft is that the bow tends to ride up slightly giving a better sailing attitude and greater fuel efficiency, combined with the wave-piercing capabilities.

The operating procedure for vessel I would be to load equipment and personnel at its base and sail at best possible speed, depending on the sea conditions, to the broken wind turbine. Locations (not shown) on deck 7 allow heavy items of equipment to be secured during transits to and from wind turbines. On arrival, helmsman 16 would turn his seat 17 through 180° to face astern 19 and manoeuvre backwards up to mast 35 of the failed wind turbine at the optimum angle 36 and moor, stern first, thereto. As shown in Fig. 2, forward 18 and rearward 19 visibility are excellent so that helmsman 16 will have no difficulty manoeuvring vessel 1, e.g. with the two contra-rotating propellers 12 in their azimuth mountings.

Where the access is only at one or two points around mast 35, helmsman 16 will choose the one giving the calmer(calmest) position to moor vessel 1. Similarly, if wind and swell 36 are from different directions, he / she would choose the optimum position for unloading / loading personnel and equipment via transfer platform 30.

For very large windfarms, e.g. as proposed for the Irish Sea, a mother ship may be the best maintenance solution. Because vessel 1 is 24.Om long with beam of only 4.1 m, a dock could be provided in the stern of the mother ship, which would ease loading and transfer problems and permit routine maintenance on vessel 1. The dock could be sized to accommodate two vessels 1 side-by-side.

Claims

Claims:- 1. An offshore supply, maintenance or patrol vessel comprising:- (i) a long, narrow hull having a length to beam ratio of 5.4: 1 or more; S (ii) a hull which in plan view has an essentially triangular form, having a wave piercing bow and a stern; (ii) a gyro-stabiliser arranged to act to maintain the vessel in a normal upright attitude; (iii) heavy equipment, including the propulsion machinery, located as low as possible in and below the hull to maximise stability; (iv) accommodation for passengers / crew and their equipment; (v) means to load equipment onto the vessel, store it securely during transit and unload both equipment and passengers and transfer it and them to the access point of an offshore installation; and (vi) a helmsman's position offering good forward and rearward visibility; characterised in that the bow is designed to pierce waves rather than ride up and over them so that, together with the gyro-stabiliser, pitching, rolling and yawing are minimised allowing higher transit speeds with minimal discomfort to the passengers / crew and further characterised in that, when supplying or maintaining an offshore installation and on arriving at that installation, the vessel is sailed astern up to an appropriate access point of the installation and the stern is secured thereto so that the vessel is essentially at a radius from the access point of the installation which minimises the motion of the vessel thus facilitating transfer of personnel and equipment to and from the access point of the installation.
2. An offshore supply, maintenance or patrol vessel, as claimed in claim 1, wherein the hull is made of glass reinforced plastic (GRP) or metal alloy.
3. An offshore supply, maintenance or patrol vessel, as claimed in claim 1, wherein the means of propulsion is via a propeller(s).
4. An offshore supply, maintenance or patrol vessel, as claimed in claim 1, wherein the means of propulsion is via a water jet(s).
5. An offshore supply, maintenance or patrol vessel, as claimed in claims 3 and 4, wherein the means of propulsion is provided with a degree of azimuth rotational capability.
6. An offshore supply, maintenance or patrol vessel, as claimed in claim 1, wherein a gyro-stabiliser is located on / adjacent to the fore-aft axis of the hull.
7. An offshore supply, maintenance or patrol vessel, as claimed in claim 1, wherein all the vessel's permanent, heavy equipment is located as low in the hull as possible.
8. An offshore supply, maintenance or patrol vessel, as claimed in claim 1, wherein s hydrodynamic trim planes are provided near the bow and above the waterline to cut through large waves and minimise the tendency of the buoyant bow to rise on encountering waves but minimise drag when sailing on calm water.
9. An offshore supply, maintenance or patrol vessel, as claimed in claim 8, wherein the angle of the hydrodynamic trim planes is adjustable to control the degree of wave piercing of the bow.
10. An offshore supply, maintenance or patrol vessel, as claimed in claim 1, wherein the superstructure surrounding the helmsman's seat is shaped to deflect water breaking over the bow.
11. An offshore supply, maintenance or patrol vessel, as claimed in claim 1, wherein the helmsman's seat can rotate through 1800 so that the helmsman can steer the vessel both when sailing ahead and astern.
12. An offshore supply, maintenance or patrol vessel, as claimed in claims 10 and 11, wherein windows are provided to maximise the helmsman's forward and rearward visibility.
13. An offshore supply, maintenance or patrol vessel, as claimed in claim 1, wherein a loading platform is provided at / near the stern of the vessel.
14. An offshore supply, maintenance or patrol vessel, as claimed in claim 13, wherein means is provided to secure equipment to the loading platform to prevent it moving during transits to / from the access point of an offshore installation.
15. An offshore supply, maintenance or patrol vessel, as claimed in claim 14, wherein a transfer platform is provided as an integral part of the loading platform.
16. An offshore supply, maintenance or patrol vessel, as claimed in claim 15, wherein the transfer platform is elevatable to facilitate embarkation / disembarkation of people and loading / unloading equipment onto I off the vessel.
17. An offshore supply, maintenance or patrol vessel, as claimed in claim 16, wherein the height of the elevatable transfer platform is continuously adjustable to absorb the rise and fall of the vessel as waves pass under the vessel to facilitate the transfer of people and / or transfer of equipment from the transfer platform to the access point of an offshore installation.
18. An offshore supply maintenance or patrol vessel, as claimed in claims 15 to 17, wherein the transfer platform is gyro-stabilised with the rest of the vessel.
19. An offshore supply, maintenance or patrol vessel, as claimed in claim 1, wherein means is provided on the transfer platform, or elsewhere, to secure equipment or personnel being transferred from the vessel to the access point of an offshore structure.
20. An offshore supply, maintenance or patrol vessel, as claimed in claim 1, wherein the form of the hull is such that its forward motion through the water entraps air below the hull so reducing drag.
21 An offshore supply, maintenance or patrol vessel, as claimed in claim 1, wherein the engine exhaust is discharged through the hull below the waterline, so reducing drag.
22. An offshore supply, maintenance or patrol vessel, as described in and by the above statement with reference to the accompanying drawings.