GB2040232A - Submersible or semi-submersible structures - Google Patents

Abstract

A submersible or semi- submersible structure, such as a boat, oil ring or pipeline, has an anti-fouling covering secured thereto. The covering comprises a mesh (2) of copper or copper alloy embedded in a carrier (1), preferably of plastics material. The mesh (2) has knuckles (3) which on one surface (4) are close to or protrude from the carrier (1) so as to be accessible to sea water. The knuckles are spaced from the other surface (5) so as to prevent contact between the structure and the mesh (2). A sea water permeable coating may be provided on the outer surface (4). <IMAGE>

Description

SPECIFICATION Submersible or semi-submersible structures This invention relates generally to the retardation of marine growth on submersible or semi-submersible structures.

Marine growth, for example slimes, barnacles, tubeworms, molluscs and certain animal organisms such as sponges and anemones, adversely affect such structures. Such growth may accelerate corrosion and, particularly on fixed off-shore structures, for example pipelines or oil and gas platforms, increases the hydrodynamic loading and seriously impedes inspection and maintenance of the structure. On movable structures, for example ships' hulls, certain molluscs such as mussels are unable to adhere to the surface of the structures at normal operational speeds. However, fouling by other growth requires the ship to be put into dry dock at regular intervals for cleaning and re-painting or at least to be put into a calm site for in-water cleaning of the hull.

It has long been known to use copper to retard or prevent marine growth on structures subjected to sea water. Copper sheets were commonly fixed to the hulls of wooden ships to prevent such growth.

However the use of copper sheets is impractical on structures of ferrous metal, for example ships' hulls or fixed off-shore structures, because of the expense of cladding with copper sheets and also because contact between the copper and the structure results in extreme corrosion of the ferrous metal and rapid deterioration of the structure. More recently it has been proposed to coat the structure with a paint containing copper salts from which the copper ions are released to poison the marine growth. A disadvantage of such a method is that the life of the coating is relatively short and repainting has to be carried out at frequent regular intervals.

In accordance with with one aspect of the present invention there is provided a submersible or semisubmersible structure having secured thereto an anti-fouling covering, said covering comprising a mesh including a copper or copper alloy embedded in a carrier material, the copper being accessible to sea water at the outer surface of the covring.

In accordance with another feature of the invention, there is provided a covering for such a submersible or semi-submersible structure, comprising a copper or copper alloy mesh in a carrier material The carrier material is preferably a plastics material, for example a thermoplastic polymer, such as polypropylene, or a thermosetting polymer, such as a polyester. Such a polymer may be used with a glass fibre reinforcement either as a mat or in the form of random choppings or rovings.

Pure copper or many copper alloys could be used as the material for the mesh, preferably with a copper content of at least 60%. It is envisaged for example that for use in static conditions, such as on fixed offshore structures, or for low-speed applications, bronze could be used containing, for example, from 95% copper. For higher speed applications, for example on ships' hulls, cupro-nickels could be used, for example approximately a 90/10 alloy with small amounts of iron and/or other elements, the alloy constituents being changed to improve errosion/corrosion properties for increases speed of operation. Other copper alloys could also be used, for example aluminium bronze containing a portion of copper greater than 99%. It has been found that, generally, the greater the copper content of an alloy, the greater its effectiveness in retarding marine growth.

Conveniently, the mesh size is in the range of 3 - 60 mesh/inches (1.2 -24 mesh/cm.), the wire diameter being of from 0.05 to 1.5 mm. We have discovered that for the reduction of certain marine growth it is desirable to use a mesh size of at least 12 mesh/inch (4.7 mesh/cm.) but considering other factors, such as ease of production and costs, the optimum mesh size is probably in the region of 20 mesh/inch (8.0 mesh/cm.).

Preferably, the mesh is exposed to the sea water at the outer surface, the knuckles preferably protruding through or being level with the outer surface of the carrier material. The knuckles are preferably all in the same plane, so that for a given mesh size and wire gauge more metal is available at the outer surface which is in use subjected to sea water, and are regularly spaced apart. For mesh of pure copper our investigations indicate that the area of exposure of the metal is desirably greater that about 13% of the area of the covering.

The covering may be formed as panels and bonded to the structure using suitable adhesive, for example and air-curing or underwater-curing twopart epoxy cement, and are preferably flexible to permit attachment to the hull of a ship or to an off-shore structure. The panels may, however, be secured to the structure by mechanical means, for example by tying. The panels may be formed by any suitable means, for example casting, spreading, hot pressing, or calendering. Preferably, the thickness of each panel is between 0.05 mm and 2.0P mm.

Optionally, the carrier material could be applied directly to the structure, for example by spraying the carrier material on to the structure and embedding the mesh in the carrier on the structure.

In order that the mesh may project from the outer surface of the covering and in use be in direct contact with the sea water the panels may be formed with the mesh exposed at the outer surface, or the outer surface may be adbraded to expose the mesh.

However, particularly for movable structures the mesh need not be exposed and in direct contact with the sea water but may be coated with a layer of coating material which is permeable to water, preferably a thin coating of plastics material through which the copper can diffuse. The surface coating may take the form of a polyester or epoxy film which may be filled, either with an insoluble material which enhances the water permeability of the layer, such as cotton fibre, wood fibre or wood flour, or with a water soluble material such as copper oxide which would be leached by the sea water to give a porous structure. Optionally, the coating film may be of the same material as the carrier and integral with the carrier.

A boat having a covering in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure lisa perspective view of the boat having panels secured to the hull thereof.

Figure 2 is an enlarged detail view of area A of the boat of Figure 1 illustrating four juxtaposed panels, Figure 3 is a perspective view of one form of panel partly cut away to illustrate a mesh configuration, and Figure 4 is an enlarged detail section of another form of panel attached to the hull of the boat.

Each panel P comprises a layer 1 of carrier material in the form of a glass-reinforced polyester and a mesh 2 of copper alloy embedded therein. The alloy has a copper content of at least 70% and preferably greater than 90%.

The mesh has knuckles 3 which, in the form of panel illustrated in Figure 3, protrude through, or are level with, the front surface of the panel so as to be in direct contact with the sea water. However, the knuckles are not level with the rear surface 5 of the panel.

In an alternative form of panel illustrated in Figure 4 the panel is secured to the hull 6 of the boat by an adhesive layer 7 and is covered by a thin layer 8 of resin material, filled with cotton fibre or the like, through which sea water can permeate. Optionally, the mesh may be totally embedded in the carrier material 1, a thin layer of the carrier material coating the mesh and being permeable by sea water.

In each form of panel the knuckles are regularly spaced apart, the distance between adjacent knuckles being from 0.25 to 7 mm.

In use a plurality of such panels are secured to the surface of a structure, for example a ship's hull, with the rear surface 5 being bonded by suitable cement to the structure. Because the copper alloy mesh is insulated from the rear surface 5, there can be no electro-chemical interaction between the mesh and the structure. Furthermore the carrier material and cement provide a barrier which effectively reduces corrosion of the structure by sea water.

The mesh may be woven or knitted, or may be an expanded metal mesh. In the case of woven or knitted meshes, the copper strands may be interwoven with strands or other material, for example plastics. As an example, the copper strands may all extend in one direction and the plastic strands in the other direction.

In standard woven meshes, half the number of knuckles on one side of the mesh are generally disposed in one plane and the other half are disposed in a different plane, the planes being slightly spaced apart. It is preferred in the present embodiment that the knuckles are disposed in a single plane so that all of them project from or are level with the front carrier surface 4. The formation of knuckles in a single plane may be achieved for example by rolling standard meshes or by a suitable weaving process.

During the life of the panel, copper from the mesh dissolves and in the panel of Figure 1 diffuses through the coating layer 8. However, the mesh presents a continuous run of copper alloy which is gradually leached away by the sea water. Thus, the life of the covering may be controlled by suitable choice of the alloy, the mesh size and the diameter of mesh wire, to suit a particular application.

The rate of dispersion of the copper may be further controlled by the application of an electric potential to the mesh, which controls the quantity of copper liberated. Optionally the mesh may include strands of metal other than copper or copper alloy to control the rate of dispersion. For example, nickel may be interwoven to enhance the toxicity of the covering or zinc may be used to inhibit the toxicity.

The above-described covering has the advantages over copper or copper alloy sheets that it is relatively simple to attach to the structure, the electrochemical reaction between the structure and the copper is avoided, that emission of the copper can be controlled, and that there is a cost advantage for selected mesh sizes.

Claims (37)

1. A submersible or semi-submersible structure having secured thereto an anti-fouling covering, said covering comprising a mesh including a copper or copper alloy embedded in a carrier material, the copper being accessible to sea water at the outer surface of the covering.

2. A structure according to claim 1, wherein the mesh is exposed at the outer surface of the covering.

3. A structure according to claim 2, wherein the mesh has knuckles which are exposed at the outer surface of the covering.

4. A structure according to claim 2 or 3, wherein the area of exposure of the mesh is greater than about 13% of the total area of the covering.

5. A structure according to claim 1, wherein the mesh is coated with a layer of coating material which is permeable by sea water and which provides said outer surface.

6. A structure according to claim 5, wherein the coating material is the same as the carrier material and is integral therewith.

7. A structure according to claim 5, wherein said coating material is a resin filled with a material which is insoluble in water and which enhances the permeability of said coating layer.

8. A structure according to claim 7, wherein said filler material is cotton fibre, wood fibre or wood.

9. A structure to claim 5, wherein said coating material is a resin filled with a material which is soluble in water and which can be leached out in water to provide a porous coating layer.

10. A structure according to claim 9, wherein said soluble material is copper oxide.

11. A structure according to any of the preceding claims, wherein the mesh is a woven mesh.

12. A structure according to any of the preceding claims, wherein the mesh size is in the range of 3 to 60 mesh/inch (1.2-24 mesh/cm).

13. A structure according to claim 11, wherein the mesh size is greater than about 12 mesh/inch (4.7 mesh cm.)

14. A structure according to claim 12, wherein the mesh size is about 20 mesh/inch (8.0 mesh/cm.).

15. A structure according to any of the preceding claims, wherein the mesh is formed from wire having a diameter of from 0.05 mm to 1.5 mm.

16. A structure according to any of the preceding claims, wherein the mesh includes a plurality of strands of copper or copper alloy extending in one direction and strands of different material extending in the other direction.

17. A structure according to claim 16, wherein said strands of different material are of plastics material.

18. A structure according to claim 17, wherein said strands of different material are of metal other than copper or copper alloy.

19. A structure according to any of the preceding claims, wherein the mesh includes a copper alloy containing at least 60% copper.

20. A structure according to claim 19, wherein the copper alloy contains at least 70% copper.

21. A structure according to claim 19, wherein the copper alloy contains at least 90% copper.

22. A structure according to claim 21, whrein the alloy contains at least 99% copper.

23. A structure according to any of claims 19 to 22, wherein the copper alloy is bronze.

24. A structure according to any of claim 19 to 22, wherein the copper alloy is cupro-nickel.

25. A structure according to claim 24, wherein said alloy includes small amounts of iron or other elements.

26. A structure according to any of the preceding claims, wherein the carrier material is a plastics material.

27. A structure according to claim 26, wherein the carrier material is polypropylene.

28. A structure according to claim 26, wherein the carrier material is polyester.

29. A structure according to any of claims 26 to 28, wherein the carrier material is reinforced with glass fibre.

30. A structure according to any of the preceding claims, wherein the mesh has knuckles, all of said knuckles being in the same plane in said panel.

31. A structure according to any of the preceding claims, wherein the mesh has knuckles which are spaced apart in a regular pattern.

32. A structure according to any of the preceding claims, wherein the covering comprises a plurality of juxtaposed panels.

33. A submersible or semi-submersible structure, substantially as herein described with reference to the accompanying drawings.

34. An anti-fouling covering for a submersible or semi-submersible structure according to any of the preceding claims.

35. A method of covering a submersible or semi-submersible structure according to any of claims 1 to 33, comprising the step of coating the carrier material on the structure and embedding the mesh therein.

36. A method of covering a submersible or semi-submersible structure according to claim 32, comprising the step of securing the panels to the structure.

37. A method according to claim 36, wherein the panels are secured to the structure by an adhesive.