PREVENTING FOULING ON MARINE STRUCTURES

Abstract: Fouling of a marine structure, such as an oil production platform or a drilling rig, by seaweed, barnacles, mussels and the like, is prevented by securing to the underwater surface of the marine structure a coated flexible sheet material, the outermost surface of which is a layer of silicone rubber. The preferred coated flexible sheet material, useful as an anti-fouling cladding, is a coated fabric having an outermost coated surface which is a layer of silicone rubber/silicone oil mixture produced by curing a room-temperature-vulcanisable silicone rubber having hydroxyl end groups in the presence of a silicone oil.

Description: Preventing fouling on marine structures This invention relates to the prevention of fouling on - marine structures, (namely man-made structures, frequently of metal but also- of other structural materials such as concrete, which are at least partially immersed in sea- water during normal use, either as static structures or as moving objects), for example oil production platforms and drilling rigs. Such structures are liable to heavy fouling from seaweed, barnacles, mussels and the like. The resist¬ ance of a thick layer of fouling to waves and currents can cause unpredictable and potentially dangerous stresses in the marine structure particularly when this is resting on the sea bed in deep water ' . The use of anti-fouling paints containing biocides which are gradually released from the paint does not pro- vide a permanent solution to fouling of static marine struc¬ tures. These anti-fouling paints have a limited active life and re-painting an oil platform in situ is impractical. Moreover, marine structures such as oil production platforms have to be inspected periodically for corrosion and for stress cracks in the structure. The structure needs to be free from fouling for proper inspection. A method according to the invention for preventing fouling of a marine structure comprises securing to an underwater surface of the marine structure ( " i.e. a surface which in normal use of the marine structure is at or below the water level) a coated flexible sheet material, the outer¬ most (i.e. water-contacting) surface of which is a layer of silicone rubber. The invention also includes a marine structure having . secured thereto at and/or belcw the water-line a coated flexible sheet material, the outermost surface of which is a layer of silicone rubber. Silicone rubbers have been proposed for use in anti- fouling paints or films in British Patent Specifications Nos. 1,307,001 ,and 1,470,465- Their anti-fouling action is unique in that they do not poison marine organisms but pre¬ vent then securing satisfactory adhesion to the surface, apparently by a physical effect. There are, however, pract- ical difficulties in the use of silicone rubber anti-foul paints or films. It is difficult to make them adhere wel to a marine surface and the applied paint films or coatin are mechanically rather " weak and liable to damage. The present invention employs a reinforcing sheet coated with silicone rubber layer and this gives mechanical support t the silicone rubber and the fabric can be securely attach to the marine structure. The sheet material used as substrate for the silicon rubber layer should be a reinforcing sheet material, i.e. sheet of it should have substantially greater tensile and tear strength than a silicone rubber sheet of equal weigh Preferably it is a fabric ' , for example a plain woven fabr of nylon or polyester yarn. Alternatively, however, it c be a tough plastics film such as an oriented polyester or polypropylene film. The silicone rubber can be a cured room-temperature- vulcanisable silicone rubber or a heat-cured (i.e. cured heat-curable) silicone rubber. We have found that the be anti-fouling performance is obtained using a cured room- ' te perature-vulcanisable silicone rubber but the heat-cur silicone rubbers are also effective and can be readily applied by coating techniques such as dip coating. When a fabric is used as the substrate, more than on coating layer is usually applied to the fabric to build u sufficient thickness of coating to obscure the weave of t fabric. Only the outermost coating need be of silicone rubber and from cost considerations it is generally prefe ed that at least one inner layer is an alternative elasto such as natural rubber, nitrile rubber, neoprene polychlo prene rubber or Hypalon rubber which is based on polyethy ene substituted by chlorine and sulphonyl chloride groups The first coat applied to the fabric may be a keying coat for example a rubber solution containing a polyfunctional isocyanate, and an adhesion promoter can also be applied, for example by dip coating with a solution containing a resorcinol resin and/or an epoxy resin, as is known in fa ric coating. Good adhesion of the silicone rubber to the inner elastomer coating(s) can be achieved by the use of or more intermediate layers based on a mixture of the sili¬ cone rubber and the elastomer used for the inner layer(s). The thickness of the silicone rubber coating should be sufficient to give a continuous surface of silicone rubber. The thickness is usually equivalent to a coating weight of at least 10 g/m 2 and preferably about 25 g/m2 and can be up to 1 mm or even more. The room-temperature-vulcanisable silicone rubber is preferably an oligomeric silicone rubber having hydroxyl end groups, for example those sold under the trade marks Silocoset 105, Dow Corning RTV 3110 and General Electric RTV 11. Such a silicone rubber generally has a molecular weight in the range 40,000-100,000 and a viscosity of 10-1,000 Stokes before curing and can be represented by the formula where n is an integer corresponding to the degree of poly- merisation and R 1 and R2 are the same or different alkyl, aryl or vinyl groups, the repeating units being identical or different. The silicone subber is generally cured using an aminoacetoxyoxime or alkoxysilane crosslinking agent and a curing catalyst, for example dibutyl tin αilaurate, stannous octoate or a platinum salt. The heat-curable silicone rubber is generally based on a long-chain siloxane, for example a polydimethyl silox- ane which " can be crosslinked by heat in the presence of a tin-containing catalyst or one which can be crosslinked using an oxidising agent, for example by the incorporation of a peroxide curing agent. The room-temperature-vulcanisable silicone rubber can be subjected to an accelerated cure at temperatures above ambient, for example in the range 50°-100°C. An inter¬ mediate layer of a heat-curable silicone rubber can be used to improve the adhesion of the room-temperature-vulcanisable silicone rubber, optionally with a further layer which is a Q PI_ WIPO - mixture of the heat-curable silicone rubber and the room temperature-vulcanisable silicone rubber. The preferred curing procedure for such a multi-coated sheet is to hea cure, usually at a temperature in the range 100-200 C, w all the layers containing heat-curable material have bee applied and to cure a subsequently applied layer or laye of room-temperature-vulcanisable silicone rubber at a temperature which is in any case substantially below tha used for heat-curing and may be as low as ambient temper ture. The room-temperature-vulcanisable silicone rubbers are slow in curing compared to many fabric coatings. It may be preferred to interleave the final coated sheet mat erial with a sheet having a release surface of, for examp polyethylene, so that the coated sheet can be rolled up before the silicone rubber is fully cured. The silicone rubber preferably contains a silicone o for example as described and claimed in our British Pate Specification No. 1,470,465- The silicone oil is general a polymer of molecular weight 2,000-30,000 and viscosity 20-1,000 centistokes and comprises repeating units where R 3 and R4 are the same or different alkyl or aryl groups, the repeating units being identical or diffe ent. Particularly preferred silicone oils are those wher ' " 4 R is an alkyl group and R is an aryl group in at least some of the repeating units, for example methylphenyl sil cone oils such as those sold under the trade marks Dow Corning DC 510, DC 550 and DC 710. Silicone oils contain ing fluorocarbon groups can also be used. The silicone o is generally used in a proportion of 1-50 per cent by wei preferably 5~30 per cent by weight, based on the silicone rubber. A preferred material according to the invention for as an anti-fouling cladding for a marine structure theref OM comprises a coated flexible substrate, preferably a coated fabric, optionally one or more coated inner layers of an elastomer, and an outermost coated layer of a cured sili¬ cone rubber, preferably a layer of a silicone rubber/ silicone oil mixture produced by curing a heat curable silicone rubber or more preferably a room-temperature- vulcanisable silicone rubber having hydroxyl end groups in the presence of 1-50 per cent by weight of the silicone oil. When a silicone oil is used it is generally mixed with the silicone rubber before curing, so that the silicone rubber is cured in the presence of the silicone oil to give good dispersion of the oil on the rubber. The coated flexible sheet material, e.g. fabric, is preferably secured to the marine structure by wrapping it around an underwater surface of the marine structure and securing it by clamping bands. This method of securing allows relatively easy removal of the cladding for inspec¬ tion of the structure. The coated flexible sheet material can be in the form of a band which is spirally wrapped around the structure, or for larger diameter structures a sheet of the coated flexible sheet material can be secured around the structure by clamping bands. This procedure, unlike painting, can be carried out under water. In an alternative method of securing the coated flex- ible sheet material to the marine structure, the reverse face of the flexible sheet material is coated with an adhes¬ ive capable of bonding it to an underwater surface of the marine structure. The particular adhesive depends on the conditions under which the anti-fouling cladding is to be bonded to the marine structure. Where the cladding is to be applied to the structure before it is immersed in the se any sea-water-resistant adhesive can be used, for example a nitrile or neoprene rubber or an epoxy adhesive. Where the cladding is to be applied to a structure in situ, for examle at and/or below the water line of an oil production platfor already in position on the sea-bed, the adhesive must be capable of forming a bond under water. One example of an underwater adhesive is an epoxy resin, for example a low . fRE OMPI_ ΛJ. " W W I 1 P P C O T * molecular weight condensate of bisphenol-A epichlorhydrin used in conjunction with a polya ine curing agent which i insoluble in and insensitive to water. When adhesives are not used, the reverse face of the flexible sheet material can be left uncoated, or a protec tive rubber coating can be applied. Although the invention is particularly applicable to the prevention of fouling of marine structures which are static for long periods such as oil production platforms, drilling rigs and fish farming tanks, the coated flexible sheet material can also be applied to ships' hulls, for example by adhering sheets of the coated flexible sheet material to the ship's hull. The coated flexible sheet m erial is preferably applied to the marine structure from t highest point the sea reaches on the structure to a depth up to 10 metres below the lowest water line. The invention is illustrated by the following example Example 1 A plain weave fabric of 165 cm width formed from 940 decitex low shrinkage nylon yarn was dipped in a solution rescrcinol/formaldehyde resin, coated with a key coat of rubber solution containing a polyfunctional isocyanate an then successive coats of neoprene rubber to a total coati weight of 250 grams per square metre. This obscured the fabric weave. The back face of the fabric was coated simi larly to a total coating weight of 50 grams per square met The fabric was coated with a 25 grams per square metre coa of a mixture of nitrile rubber and silicone rubber in a weight ratio of 2:1 followed by a coat of a mixture of the two rubbers in a weight ratio of 1:2. The silicone rubber used was in each case a heat-curable elastomer based on a polydi ethyl siloxane sold by Dow Corning as "FC.227". The product was then coated with a 50 grams per square metre coat of a composition of the same silicone rubber containi 5 per cent by weight of a fluorinated methyl phenyl silico oil. The coated fabric was finally cured at l6θ°C for 3 minutes. The silicone rubber/silicone oil surface of the coate ( OMPI fabric, when immersed in sea water off the south coast of England, remained free from fouling for a period of six months, and had a much reduced rate of fouling thereafter compared to an uncoated surface. It can be applied to a marine structure by cutting lengthways into two or more bands which can be spirally wrapped around the marine struc¬ ture and then secured by clamping bands or the coated fabric can be wrapped at full width around a large marine structure such as the leg of an oil production platform and secured by clamping bands. Example 2 A neoprene rubber-coated fabric as described in Example • 1 was coated with a 25 g/m coat of 1:1 by weight mixture of neoprene rubber and "FC227" heat-curable silicone rubber followed by a 25 g/m coat of silicone rubber alone, then a 25 g/m coat of 1:1 by weight mixture of the "FC227" silicone rubber with Dow Corning RTV 3110 room-temperature-vulcanis¬ able silicone rubber. The coated fabric was then heat cured for 5 minutes at l4θ°C. 2 The coated fabric was further coated with a 25 g/m coat of the RTV 3110 silicone rubber which was allowed to 2 cure at ambient temperature and finally with a 25 g/m coat of a composition of the RTV 3110 silicone rubber containing 20 per cent by weight of a methyl phenyl silicone oil sold by Dow Corning under the trade mark "DC550". This was allowed to cure at ambient temperature and the fabric was uhen attached to a raft and immersed in sea water off the south coast of England. It has remained free from fouling for a period of six months. Panels having a similar sur- face of a cured room-temperature-vulcanisable silicone rub¬ ber compound with silicone oil have resisted fouling for up to five years. The fabric can be applied and secured to a marine struc¬ ture as described in Example 1. OMPI

Claims: CLAIMS 1. A method of preventing fouling of a marine struc ture characterised by securing to the underwater surface the marine structure a coated flexible sheet material, th outermost surface of which is a layer of silicone rubber. 2. A method according to claim 1 characterised in t the silicone rubber is a cured room-temperature-vulcanisa silicone rubber. 3. A method according to claim 1 characterised in t the silicone rubber is a heat-cured silicone rubber. 4. A method according to any of claims 1 to 3 chara terised in that the silicone rubber is mixed with a silic oil. 5. A method according to any of claims 1 to 3 chara terised in that the coated sheet material is a fabric whi is coated with at least one inner layer of an elastomer (other than silicone rubber) selected from natural rubber neoprene rubber or chlorinated polyethylene rubber as wel as an outermost layer of silicone rubber. 6. A method according to any of claims 1 tc 3 chara terised in that the thickness of the silicone rubber laye is from a thickness equivalent to a coating weight of 10 grams per square metre up to a thickness of one millimetr 7. An anti-fouling cladding for a marine structure composed of a coated flexible sheet material comprising a flexible substrate, optionally one or more inner coated layers of an elastomer and an outermost coated layer of a cured silicone rubber. 8. An anti-fouling cladding for a marine structure prising a coated fabric, the outermost coated surface of which is a layer of a silicone rubber/silicone oil mixtur produced by curing a room-temperature-vulcanisable silico rubber having hydroxyl end groups in the presence of 1 tc per cent by weight of the silicone oil. 9. An anti-fouling cladding according to claim 8 ch acterised in that the silicone oil is a methyl phenyl sil cone oil. OMPI 10. A marine structure having a reduced liability to fouling, characterised in that it has secured thereto at and/or below the waterline a coated flexible sheet material the outermost surface of which is a layer of silicone rubber.

Inventors: NEILD J, SYKES B
Applicants: INT PAINT CO, NEILD J, SYKES B, INT PAINT CO LTD
Application year: 1979
Main classification: B63B59
IPC codes: B08B17, B63B59, C08L83, C09D5