AIR LUBRICATION SYSTEM FOR REDUCING MARINE GROWTH ON A VESSEL

Abstract

A method for reducing marine growth on a vessel, includes providing an air lubrication system and covering at least a part of the hull with air bubbles. Also described is a vessel having an air lubricating system with releasable connection of the deflectors across the cavity, a closeable outlet valve in the air outlet duct, connection of a compressor to each cavity or pair of cavities and an air inlet opening in the top of the cavity.

Claims

1. A method for reducing marine growth on a vessel, comprising: providing an air lubrication system, the air lubrication system comprising an air cavity system having sidewalls (18,18) and a top wall (19) defining a cavity (33) with an opening (20) situated in an interface plane (30) that is transverse to the sidewalls, substantially at a level of a hull of the vessel, or at a level of a substantially flat bottom, the opening having a front end (22) and a rear end (21) when seen in a length direction of the cavity, and a compressor being connected to the cavity, the compressor being connected to the cavity for supplying air into the cavity via an air outlet duct (14); and covering at least a part of the hull with air bubbles.

2. The method according to claim 1, further comprising: providing air bubbles on the hull when the vessel is stationary or in a moored configuration.

3. The method according to claim 1, further comprising: providing an air system on the sidewalls of the vessel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Some embodiments of an air lubrication system according to the invention and a vessel comprising such a system will, by way of non-limiting example, be described in detail with reference to the accompanying drawings. In the drawings:

[0057] FIG. 1 shows a schematic side view of a vessel comprising an air lubrication system according to the invention,

[0058] FIG. 2 shows a perspective view of an air lubrication system according to the invention,

[0059] FIG. 3a shows a cross-sectional view of the system of FIG. 2,

[0060] FIGS. 3b and 3c show an enlarged detail of different embodiments of a releaseable fastening member for the deflectos,

[0061] FIG. 4 shows a schematic side view of a cavity with an elongate deflector according to the invention,

[0062] FIGS. 5a-5c show different embodiments of a deflector according to the invention,

[0063] FIG. 6 shows a partly cut-away view of a vessel comprising for each cavity a respective compressor situated on a support deck near the bow,

[0064] FIG. 7 shows a number of cavities near the bow in a V-shaped configuration, and

[0065] FIG. 8 shows an embodiment of a bullet-shaped cavity with a rounded front part.

DETAILED DESCRIPTION OF THE INVENTION

[0066] FIG. 1 shows a vessel 1 having a length Lv of between 20 m and 500 m, and a width between 5 m and 75 m. The vessel 1 may have a water displacement of at least 10000 ton, preferably at least 50000 ton and is an ocean going vessel. The vessel 1 has a hull 4 with a bow 2, a stern 3, sides 5 a substantially flat bottom 6 and a propeller 10. Air lubricating cavities 7,8 that are open in the plane of the bottom 6, are distributed along the bottom 6 to generate a layer of bubbles 9 travelling towards the stern 3, along the flat bottom 6. Compressors 11,12 are connected to each cavity 7,8 for supplying air at the hydrostatic pressure inside each cavity at the prevailing draught level of the vessel.

[0067] The compressors 11,12 are with an air outlet duct 14 connected to the cavities 7,8 and have an air inlet duct 13 for taking in ambient air. The compressors 11,12 are controlled by a controller 15, for regulating the air supply in dependence of the sailing speed, sea state and during starting and stopping. The air outlet ducts 14 are connected to the cavities 7,8 via a valve 82 which may be closed in case no air is supplied to the cavities and the air lubrication system is in its inoperative state.

[0068] FIG. 1. Shows for each cavity 7,8 a flushing fluid duct 87 connected to a flushing fluid source 89 via a valve 88. This valve may comprise for instance a butterfly valve. When sailing in shallow waters, the cavities 7,8 and air outlet ducts 14 may become filled with silt or other material originating from the sea bed. When the valve 88 is opened, the air outlet duct 14 and the cavities 7,8 are flushed by high-pressure flushing fluid from the fluid source 89. The valves 82 are opened in order to allow fluid to exit the cavities 7,8. The fluid source 89 may be comprised of the standard fire extinguishing system present on the vessel 1, which is able to supply water to the air outlet ducts 14 at pressures of for instance up to ten bar.

[0069] FIG. 2 shows an air lubrication system 16 that is constructed as an integral module forming a cavity 33 that can be fitted into the bottom 6 of the hull 4 of a vessel 1. The system 16 comprises sidewalls 18, 18 and a top wall 19. The sidewalls 18,18 are supported on a flange 17 that can be welded into the flat bottom 6 of the vessel 1. The sidewalls 18,18 delimit an opening 20 that is substantially level with the flat bottom surface of the vessel, the opening 20 forming a smooth air-water interface plane in which air is mixed into the water due to the Kelvin Helmholtz mixing effect. Air bubbles that are mixed with the water at the interface plane leave the cavity along rear edge 21 to pass in a smooth transition from the cavity onto the bottom and to travel unrestricted along the flat bottom 6 in the direction of the stern 3. A concavely curved, downwardly sloping wall part 27 connects the top wall 19 with the rear edge 21 to guide the air and water inside the cavity in a smooth flow pattern to the exit point situated along lower rear edge 21.

[0070] The front end 22 of the cavity 33 is dagger-shaped and an air inlet 23 is situated in the top wall 19. The air inlet 19 can be connected to one of the air outlet ducts 14 of the compressors 11,12.

[0071] Inside the cavity 33, a number of curved wave deflectors 24,25, 26 extends across the width W of the cavity and are connected to the sidewalls 18, 18. The length Lc of the cavity 33 may be about 4 m, the width W being about 75 cm, and the height Hc being about 45 cm. The sidewalls 18,18 may have a thickness of 16 mm, whereas the flange 17 and top wall 19 may have a thickness of 20 mm.

[0072] The inventors have discovered that the following key principles apply for proper air lubrication system design:

[0073] The wave deflectors inside the cavity stabilize the water flow inside the cavity. This is important for two reasons: Firstly the deflectors enable filling the cavity with air during speed of the vessel. Secondly, the deflectors minimize resistance of the cavity while the system is off (without air input).

[0074] The wave deflectors are to be positioned above the interface plane of the cavity for obtaining an undisturbed flow of water pass the cavity during speed of the vessel. When the cavity is full of air, the deflectors are free of the water surface. They also help maintaining the surface of the water stable during roll motions of the vessel.

[0075] The slope at the rear wall of the cavity helps smooth release of the air bubbles into the boundary layer of the vessel and is designed to help to inject the bubbles that are formed by Kelvin Helmholtz mixing into the immediate vessel surface boundary layer, minimizing vertical dispersion and optimizing drag reduction.

[0076] The shape of the front of the cavity, i.e. wedge-shaped or bullet-shaped, controls water flow and minimizes wave instability at the air/water interface and improves consistent air mixing into the boundary layer by the Kelvin Helmholtz effect.

[0077] The length of the cavity is to be chosen sufficient to create a stable Kelvin Helmholtz air mixing effect for constant air bubble generation and flow of air bubbles into the boundary layer.

[0078] The relative positioning of the cavities under the hull is important to maximize the air-lubricated surface area of the hull.

[0079] The size of the cavity determines both the volume of air required for stable air bubble generation and required for recovery of the cavity after air pocket collapse. Optimizing the size of the cavity determines the overall lubrication effectiveness and the efficiency of the total air generation.

[0080] As is clear from FIG. 3a, the wave deflectors 24,24;-26,26 each have a horizontal part 29 extending at a distance h1 of about 5 cm from the open interface plane 30 in which the boundary layer between air inside the cavity 16 and the water flowing along the flat bottom 6 is situated. The wave deflector horizontal part 29 has a length Lwh of about 20 cm, and the wave deflector curved part 31 having a length Lwc of about 20 cm. The distance hu of the horizontal wave deflector parts 29 from the top wall 19 is about 30 cm. The horizontal parts 29 of all wave deflectors lie at substantially the same height in a deflector plane 32. The height he of the curved deflector part is about 11 cm. The distance g1 between adjacent wave deflectors 24, 24 is about 5 cm. The projected surface area of the wave deflectors 24-26; on the interface plane 30 covers at least 25%, preferably at least 50%, most preferably at least 75% of the surface area of the interface plane.

[0081] The deflectors 24-26 are connected to the cavity walls or to the hull via bolts 80 which are accessible via the open interface plane 30 when the vessel 1 is in dry dock for maintenance. As shown in the enlarged details of FIG. 3b and FIG. 3c, the bolt 80 may provided with slots or may comprise hexagon-heads or recessed heads. By easy removal of the deflectors 24-26, the interior of the cavity 16 can be easily accessed for maintenance and/or inspection.

[0082] The air inlet 23 is provided with a relatively wide section 34 connecting to a smaller diameter compressor outlet duct 35 which wide section reduces the air speed and provides a gradual inflow of air into the cavity 3.

[0083] FIG. 4 shows a schematic rendering of an air lubrication system 16 comprising a number of substantially horizontal deflector members 34, 34. The deflector members 34,34 may be separate strips supported across the width of the cavity, or may be part of a unitary deflector 28 of a type as schematically shown in FIGS. 5a-5c.

[0084] In the embodiment of FIG. 5a, the deflector 28 comprises a plate-shaped body with a number of slits 36,36. The elongate deflector parts 34, 34 are part of a unitary plate-shaped deflector 28.

[0085] In the embodiment of FIG. 5b, the deflector 28 is in the form of a perforated plate. The holes 37,37 define elongate deflector parts 34, 34.

[0086] In the embodiment of FIG. 5c, the deflector 28 is lattice or framework shaped, wherein the elongate deflector members 34, 34 are interconnected by transverse girders 35,35.

[0087] As can be seen in FIG. 6, a number of compressors 11 is supported on a compressor supporting deck 40 near the bow 2 of the vessel 1. Other compressors 12 are situated near the bow 2 at the level of upper deck 41. One compressor 11,12 is provided for each cavity 7,8.

[0088] The flushing fluid source 82 is connected to air supply duct 90 of the cavities 7,8 via a valve 85. When valve 86 is closed, the air supply duct 90 downstream of the valve 86 may be flushed by high pressure water upon opening of valve 85. The flushing fluid duct 84 may be fixed, but may also be laid out for temporary use and may for instance be formed by a regular fire hose.

[0089] In FIG. 7 it is shown that a number of cavities 54,54-59, 59 is distributed along lines running from the center line 50 to the sides 51, 52 when going in a rearward direction. Two central cavities 53,53 are provided in proximity to the center line 50. The center line of the cavities 54-59 is at a slight angle with respect to the centre line 50. For cavities 54,55,56 and 57 and 54, 55, 56 and 57 the front part 70 is located closer to the bow 2 than the rear part 71 of the cavity ahead. This overlap provides an even distribution of air bubbles across the flat bottom 6.

[0090] As can be seen in FIG. 8, the cavity 33 has at its front end 22 a rounded head, such as to be bullet-shaped. It was found that both the rounded bullet-shaped front end 22 as well as the dagger-shaped front end result in the formation of a stable air-water interface inside the cavity 33 without wave formation along the interface plane.