This invention relates to apparatus for removing surface material from an underwater surface. While the invention is primarily directed to the cleaning of subsea structures, it is equally applicable in any underwater situation where surface erosion is required, whether this be for cleaning purposes or for any other surface erosion process.

Man-made subsea structures, such as the subsea parts of oil platforms, tend to become encrusted with marine growth, rust and other unwanted coatings. These must be removed to facilitate inspection, to maintain the mass and stability properties of the structure, to avoid premature fatigue and stress-related cracking, to prevent important outlet apertures from becoming blocked, and to avoid accelerated corrosion.

The use of divers to carry out cleaning involves some personal danger, can only be performed in relatively calm weather and is expensive, particularly for depths in excess of 30 metres. There is a trend for remotel -operated vehicles (ROVs) to replace divers in the cleaning task.

The grit (slurry) jet system is the leading cleaning system on the market at present. In this system, grit particles are carried in a

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hi gh speed jet of sea water towards the su rface to be cl eaned. The impact of the grit and sea water removes unwanted coatings. The grit cl eaning system, when used in conjuncti on with an ROV, has di sadvantages in that it i nvol ves the carrying of a bul ky grit hose from above the sea surface duri ng the operation. This can cause entanglement problems under water. More importantly perhaps , some of the grit tends to get into the moving parts of the manipulator (i .e. the steerable arm that al l ows the jet to be di rected as requi red) leading to abrasi on damage to seals and oi l ways. Costly downtime and vehicle maintenance requi rements result from this.

One object of the present inventi on is to avoi d any abrasion damage to the slurry dispensi ng equipment, in parti cular the manipulator.

According to the invention there is provided apparatus for removing surface materi al from an underwater surface by erosi on , which comprises mi xing means for forming a slurry of pieces of ice in a water medium and jet means for projecti ng the slurry in a jet di rected at the surface.

The apparatus may compri se a vessel adapted to be located at or above the water surface, an underwater vehicl e which carries the jet means , hose means l inking the vessel with the vehicl e, and pump means adapted to pump, via the hose means , the slurry or water for use in forming the sl urry from the surface vessel to the underwater vehicle.

In one embodiment of the invention the mixing means is carried by the surface vessel , the pump means being adapted to pump the slurry to the underwater vehicle.

In another embodiment the mixing means i s carried by the underwater vehicle, the slurry bei ng formed underwater from cooled water suppl ied via the hose means , the surface vessel carrying means for producing the cooled water, and the underwater vehi cle carrying refri geration means for forming ice pieces from the cooled water.

In a further embodiment the mixing means and the jet means are carried by an underwater vehicle. In this case the mixing means may be adapted to use ambient water in the formation of the slurry. The vehicle may carry a container in which ice pieces , made previously above the water

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surface, can be stored prior to mixing with the water medium.

In all embodiments the jet means may comprises a nozzle from which the jet is issued, mounted on a manipulator arm adapted to control the direction of the jet, the manipulator arm incorporating heating means to prevent damage from the impact of stray ice fragments.

The underwater vehicle is preferably remotely operated. The surface vessel may be adapted to be secured to the seabed. For example, the vessel may be an oil rig.

Various forms of apparatus for cleaning a subsea structure according to the invention will now be described, by way of example only. One -such apparatus is illustrated in the accompanying drawing.

In the first embodiment of the invention a remotely-operated vehicle is controlled to survey the beams or other members of a subsea structure, for example an oil rig, that are liable to be encrusted with extraneous matter.

The ROV is fitted with a nozzle of perhaps 10 cm diameter which terminates a hose extending down from a vessel above on the surface. In the case of an oil rig the vessel would generally be the rig itself. The hose is supplied with a slurry of ice blocks and water. The slurry, formed by a mixer carried by the surface vessel, is pumped down the hose to the ROV nozzle from which it emerges in a high pressure jet. A suitable size for the pieces of ice, i.e. the blocks, may be say, 3 cm 'diameter', although they do not have to be spherical, or indeed of any particular shape. The smaller the pieces the greater has to be the pressure at which the slurry is pumped to give them sufficient energy. In the above case of ice blocks several centimetres across, a pressure of between one thousand and several thousand millimetres of mercury above the ambient water pressure at the nozzle is adequate.

In operation a conventional manipulator is steered to direct the nozzle at the surface in question and the slurry pump on the vessel above is energised. On hitting the surface to be cleaned, the ice may shatter into even smaller blocks which will melt quickly. If any of these fragments (or even the unfragmented ice blocks) should survive long enougto

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to get into the moving parts of the manipulator they will cause very little abrasion damage because they will melt quickly therein. To further ensure the prevention of damage by stray ice fragments heating coils or other heating arrangements can be incorporated in the manipulator at strategic positions. The melting of the ice would be yet further accelerated by the conversion, on impact, of some of its kinetic energy into heat, and perhaps by a depression of its melting point due to it acquiring impurities through contact with the surface. Small fragments and blocks will generally drift through the water slowly, a factor which limits their chance of ever getting back to the manipulator.

In this particular embodiment the ice is supplied ^' directly from above the sea surface via a hose analogous to the grit hose used in the grit jet system, the ice blocks being preferably within several degrees of their melting point.

In a second embodiment, schematically illustrated in the accompanying drawing, a supply of water, preferably at a temperature just above (i.e. within several degrees of) the freezing point, is sent through a hose from above the sea surface and final chilling and conversion to ice blocks is performed by a refrigerator unit carried by the ROV. The drawing (which is not to scale) shows the apparatus in operation cleaning one support pillar 1 of an oil rig platform 2. Water for forming the jet slurry is fed to an underwater ROV 3 on the seabed 13 via a hose 4. The ROV 3 would generally be connected by cable (not shown) to a control vessel at the water surface. The water, obtained from a supply pipe 5 on the platform, is cooled in a cooling plant 6 before being pumped by a pump 7 to the ROV 3. The underwater end of the hose 4 is coupled to equipment carried by the ROV which comprises a refrigeration plant 8 and a mixer 9. The plant 8 produces ice blocks of suitable size for subsequent combination in the mixer 9 with untreated water from the cooling plant 6 to form the required slurry. The slurry output of the mixer 9 is fed to a nozzle 10 to generate the cleaning jet 11. The nozzle 10 is mounted on a manipulator arm 12 which enables the jet to be controllably directed at and scanned over the pillar surface to be treated. The manipulator arm 12

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also supports the slurry feed hose to the nozzle 10. This embodiment has an advantage over the direct supply of ice blocks which may have a low packing density in the hose and may give rise to occasional blockages of the hose.

A further alternative allows the hose from above the surface to be dispensed with entirely, thereby avoiding entanglement risks. The ice blocks are formed from the ambient sea water by a refrigerator unit in the ROV. The sea water would probably be filtered before freezing to prevent large bodies such as fish, and unwanted impurities, from entering and reducing the effectiveness of the process by causing blockages or otherwise.

A fourth alternative, and one which again allows the hose from above _, the surface to be dispensed with, involves storing the ice pieces made previously above the surface in an insulated container on the .ROV. The ice pieces are dispensed as and when required for mixing with water to form the slurry. Unlike grit, the ice is nearly neutrally buoyant and can be taken underwater in bulk without need to make major adjustments to the overall density and centre of gravity of the ROV or other carrier.

A final advantage of the use of ice instead of grit is that possible damage to the environment is avoided.

Thus, in each of the alternatives described, a system for cleaning subsea structures is provided in which small blocks of ice just below (i.e. within several degrees of) the melting point are carried in a high speed jet of sea water to strike the surface to be cleaned and thereby dislodge extraneous materials from that surface by erosion. The ice fragments will melt rapidly after the impact so they can do little or no damage to the cleaning equipment.

While the primary application for the apparatus claimed is the cleaning of subsea structures, it will be clear that there exist applications in the 'working' of underwater surfaces by erosion where in fact no extraneous matter may be involved.

It will be appreciated that in all the described embodiments, the underwater vehicle is not essentially remotely-operated. In certain, critical operations it may be desirable to use a manned vehicle.

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