FIELD OF THE INVENTION The present invention relates to a method and system for protecting a structure in a marine/aquatic environment against abrasion.

BACKGROUND OF THE INVENTION

Concrete and steel structures located in maritime and aquatic environments are subjected to abrasion, as water containing abrasive particles of sand, ice etc. flow past the structure. There are several environments where such abrasion is a problem, for example in spillways, quay and bridge columns, and also on ice breakers, off shore light houses and other installations in areas with drifting ice. This would be relevant for drifting ice in both fresh water and in sea water.

The traditional approach to reduce abrasion and thereby to increase the service life of such structures has been to increase the quality (hardness) of the construction material or to protect the structure itself. Apart from the practical problems, added complexity and costs in incorporating different materials during the construction, maintenance has turned out to be both impractical and expensive, especially in the case of permanent structures. Another effect that increases the abrasion on concrete structures is the freeze/thaw cycles, i.e. the number of times the concrete is frozen and then thawed again per time unit. Somewhat simplified will concrete in an environment with a higher number of freeze/thaw cycles indicate that the concrete is more exposed for abrasion than concrete in an environment with a lower number of freeze/thaw cycles.

The object of the present invention is to provide a method and system for protecting a structure against abrasion, where the above disadvantages are avoided. It should be easy to maintain, install and operate.

SUMMARY OF THE INVENTION Dette er en formell sak, kopi av claims:

The present invention relates to a method for protecting a structure in a marine/ aquatic environment against abrasion, comprising the following steps: cooling at least a part of a surface area of the structure; exposing the surface area to a fluid having a freezing temperature higher than the surface temperature of the structure, thereby causing the fluid to freeze onto the surface area and form a protective ice coating.

In one aspect, the cooling is performed by circulating a cooling fluid within or near the structure.

In one aspect, the the cooling fluid is circulated within or near the surface area of the structure.

In one aspect, the fluid is a freely available fluid surrounding the structure, such as sea water, fresh water etc. In one aspect, the cooling is performed in a part of the surface area being in a splash zone of the fluid.

In one aspect, the cooling is performed by controlling the cooling fluid to flow only to parts of the structure near the splash zone.

The present invention also relates to a system for protecting a structure in a marine/ aquatic environment against abrasion, comprising: cooling means for cooling at least a part of a surface area of the structure; a protective ice coating provided on the surface area of the structure, where the ice coating comprises a fluid having a freezing temperature higher than the surface temperature of the structure. In one aspect, the cooling means comprises a piping means for circulation of a cooling fluid within or near the structure.

In one aspect, the piping means is provided within or near the surface are of the structure.

In one aspect, the cooling means is provided for cooling the surface area of the structure to a temperature below a freezing temperature of the fluid.

In one aspect, the fluid is a freely available fluid surrounding the structure, such as sea water, fresh water etc.

In one aspect, the cooling means is provided for cooling down a part of the surface area being in a splash zone of the fluid. In one aspect, the cooling means comprises control means to control the supply of cooling fluid to the part of the surface area being in a splash zone of the fluid.

In one aspect, the cooling means comprises a refrigeration plant connected to the piping means, where the refrigeration plant is cooling down the cooling fluid In one aspect, the system comprises fluid applying means, for applying fluid onto the surface area.

In one aspect, the fluid applied by the fluid applying means is supercooled/precooled to a temperature below its freezing temperature.

DETAILED DESCRIPTION

Embodiments of the present invention will now be described in detail with reference to the enclosed drawings, where: Fig. 1 illustrates a first embodiment where the system is incorporated in the structure;

Fig. 2 illustrates an enlarged detailed view of the structure with piping;

Fig. 3 illustrates an enlarged detailed view of a second embodiment of the structure and piping; and Fig. 4 illustrates an enlarged detailed view of a third embodiment of the structure and piping.

It is now referred to fig. 1. A system 10 is provided on a structure 1 for protecting the structure 1 against abrasion. In this embodiment, the structure 1 is a semi- submerged structure, for example a leg of an oil platform. The structure 1 can also be a bridge column or other types of semi-submerged structures as mentioned in the introduction above. The structure is hollow, i.e. there is a compartment 2 inside the structure 1. The compartment 2 will normally contain air or water.

The semi-submerged structure 1 provided in the splash zone of the sea, having an (average) sea level as indicated in fig. 1. The splash zone is here used to describe the zone between a lower level and a upper level, where the water level is variating because of waves, tides, regulations (for example regulations between lower and upper water level in dam installations of hydro-electric power stations) etc.

Some of the environmental parameters should be mentioned: the air temperature Tl is -20°C - the sea water temperature T2 is 2°C the air/water compartment temperature T3 is 2°C

The system 10 comprises cooling means 11 for cooling the surface area of the structure. In fig. 1, the cooling means 11 (indicated by dashed lines in fig. 1) comprises piping means 12 for circulation of a cooling fluid within or near a surface The cooling means 11 provides a cooling of the surface area of the structure 11 to a temperature below the freezing temperature of the sea water. Hence, since sea waves are washed onto the surface area 3 of the structure 1 , some of the water will be frozen onto the surface area 3 of the structure as a protective ice coating 13 (indicated by a dotted area).

The cooling means 11 further comprises a refrigeration plant 14 for cooling down the cooling fluid. The cooling means 11 further comprises pumps, valves, fittings, supports, thermostats etc (hot shown,) for circulating the cooling fluid through the piping means 12. The cooling fluid can be any suitable cooling fluid. It should be noted that the refrigeration plant 14 would normally be a suitable commercial available refrigeration plant. The refrigeration plant 14 is considered known for a skilled person and will thus not be described here in detail.

In the present embodiment, only parts of the surface area of the structure are cooled down, this would be the part from the lowest water level to the highest water level (normally corresponding to the splash zone).

In some embodiments, especially in areas where there are variations in the wave height or where there are variations in how deep the structure itself would be submerged, the cooling means may also comprise control means (not shown). The control means controls the flow of the cooling fluid only to some pipes of the piping means 12, so that only necessary parts of the surface area are cooled down. This would reduce the energy consumption of the cooling means.

The temperature T4 of the protective ice coating 13 can for example be approximately -25 °C near the surface of the structure. Consequently, the ice coating 13 will be colder than most drifting ice floating in the sea. In fig. 1, a block of sea ice 16 is drifting towards the ice coated structure. This sea ice will have a temperature in the area -1°C to -5°C. The interface between the sea water and the ice will have a temperature of -2 ⁰ C.

Since ice with low temperature exhibit higher mechanical strength than warmer ice, hence, the protecting coating 13 will also protect the structure when ice floating in the sea collides with the protective coating 13.

Normally, sea ice and fresh ice show an increase in both compressive and tensile strength with decreasing temperature, and will, when put under strain, normally yield in areas with lowest strength. The ice coating 13 as shown in fig. 1 will have the lowest temperature near the surface of the structure, and a progressive increasing temperature at increasing distance from the surface. In fig. 1, an ice fracture zone or outer part 15 of the ice coating 13 is provided, illustrating a zone where ice will build up and fracture as floating ice 16 arrives towards the structure. Hence, the outer part 15 of the ice coating will yield before the inner part of the ice coating, and the drifting ice will yield before the inner part of the ice coating.

Under normal conditions the ice coating 13 would be relatively homogenous, with a lower salinity and porosity than natural ice 16, since natural ice 16 often contain salt water pockets, crushed ice and snow and is thus relatively inhomogeneous comparted with the ice coating 13.

According to the invention, a sacrificial, self-repairing ice coating is provided, where the ice coating is replenishing itself as it gets eroded. For offshore structures in arctic environment, the drifting ice will be deflected and break up before it gets in physical contact with the structures.

In addition to the abrasion shielding effect, the system 10 can limit the number of freeze/thaw cycles to only one per season (i.e. freezing the structure down during winter and thawing the structure during summer), thereby significantly enhance the abrasion resistance of concrete exposed to the combined effect of sea water and freezing/thawing .

It is now referred to fig. 2, showing details of the structure 1. The structure 1 is build up of vertical reinforcement bars 20, horizontal reinforcement bars 21, horizontal transverse reinforcement bars 22 and concrete 23.

As seen in fig. 2. the pipes of the piping means 12 can be provided as a part of the construction of the structure 1, for example supported by the reinforcement bars 20, 21 near the surface area 3 of the structure 1.

In second embodiment shown in fig. 3, the piping of the piping means 12 is located inside the air/water compartment 2. This may be the case if the system 10 is to be mounted onto the structure 1 as a separate operation, for example after the fabrication of the structure 1. In fig. 3, the piping means 12 is located near the inner surface of the structure 1. Outside the piping means 12, a casing 24 can be provided to protect and insulate the piping means 12. Insulation 25 can also be provided around the piping means 12. In this embodiment, a larger part of the structure 1 will be cooled than in the above embodiments. In a third embodiment shown in fig. 4, the piping of the piping means is provided outside the surface area 3 of the structure 1. The piping means 12 is located inside a protective coating 30 of a heat conducting material, for example a stainless steel or another type of metal or similar material. Consequently, the protective coating 30 is providing an additional surface area 3 a of the structure 1. An insulation layer 31 can be provided between the piping means 12 and the structure 1. In this embodiment, the concrete part 23 of the structure will not be cooled as much as in the above embodiments. In the following, the steps of the method for protecting a structure in a marine/aquatic environment against abrasion according to an embodiment of the invention will be described:

In a first step, the method comprises cooling at least a part of the the surface area 3, 3 a of the structure. Then, the surface area is exposed to a fluid, having a freezing temperature higher than the surface temperature of the structure, thereby causing the fluid to freeze onto the surface area and form a protective ice coating.

The cooling is performed by circulating a cooling fluid within or near the structure, or more specif! cially within or near the surface area of the structure. In most applications, the fluid is an an freely available fluid surrounding the structure, such as sea water, fresh water or brackish water.

The cooling can be performed by circulating cooling fluid in the whole structure. However, the cooling would in most applications be performed only in a part of the surface area being in the splash zone of the fluid. In some applications, the circulation of the cooling fluid is controlled to flow only to parts of the structure near the wave zone.

Alternative embodiments

In the embodiment described above, the fluid used to form the protective ice coating 13 is a freely available fluid surrounding the structure; normally this would be sea water or fresh water. However, it would also be possible to use other suitable fluids.

Moreover, it would also be possible that the system 10 comprises a fluid applying means, for applying the fluid onto the surface area manually.

In some embodiments, the fluid applied by the fluid applying means is supercooled or precooled to a temperature below freezing temperature before it is applied to the surface area of the structure. In this way, the ice coating will build up faster.

The abovementioned detailed description is especially provided to illustrate and to describe preferred embodiments of the invention. However, the description is by no means limiting the invention to the specific embodiments.