FI61538C - MARIN CONSTRUCTION ATT ANBRING PAO OEPPET HAVSOMRAODE - Google Patents

Description

μ ^ Γτΐ Γ, ANNOUNCEMENT 61538 mA (11) UTLÄGGNINGSSKRIFT D 1000 C Patent aySncUy '0 03 172Ί (51) Kv.ik? /In..c.3 E 02 B 17/00 // B 63 B 35/12 ENGLISH I —FI N LAN D pi) hwittllnkMiui —hmnwilml »* 752092 (22) Htkwniifilvi - Anattknlnpd ·! 21.07 * 73 (23) AlkupMv · —GlMghttadag 21.07 * 75 (41) Tulkit JulklMkil - Bllvlt offwitllg 30.01 * 76

National Board of Patents and Registration (44) NihavUuiptnon | · kuL | uikti * un pvm. -

Patent and Registration Office of the United States of America 30 April 1982 (32) (33) (31) Fyydetty * tuolk «i» —Begird prtoritet 29 * 07. jh USA (US) 1 * 9017 ^ (71) Chevron Research Company , 200 Bush Street, San Francisco, California 9I + IOI *, USA (72) Thomas Allan Hudson, San Francisco, California, Gordon Edward Strickland,

Jr., Yorba Linda, California, USA (US) (7 * 0 Oy Kolster Ab (5I *) Marine structure to be located in the offshore area - Marin konstruktion att anbringas p & Popp havsomrade i

This invention relates to a marine structure to be located and held relatively fixedly in its location in an offshore area where the surface of the seawater freezes due to the surrounding natural conditions, which structure comprises a wall portion that is railed.

Teva inward and upward in the area where it first comes into contact with the ice floe floating in the sea, so that the part of the ice floe moving in contact with the structure rises upwards and deviates from its normal position on the surface of the water. In the northern and southern polar waters, winter ice can normally reach a thickness of 1.8 to 3.0 meters and more, and ice rafts, stagnant ice, and other ice deposits can cause ice thickness in places to be many times greater than the original ice thickness. Ice layers are wide-ranging, and although they can normally move slowly with wind and water currents, moving ice can cause quite large forces on its pathway structure. The compressive strength of such ice can be between about 45.7 and 70.3 kg / cm 2, and a structure strong enough to withstand the crushing force of ice would thus have to be quite massive and correspondingly expensive to build.

It has previously been proposed that instead of constructing a structure that is strong enough to withstand the total crushing force of ice, i. strong enough for the ice to crush against the structure and thus allow the ice layer to flow around the structure, the structure would be provided with ramp-like surfaces forcing the edge of the moving ice upwards from its normal position on the water when the ice came into test with the structure, thus bending the ice layer. Since the bending strength of ice is only about 6 kg / cm, a correspondingly lower force would be applied to the structure, because when the ice collides with it, the tensile stress would break.

Several embodiments with a sloping circumferential wall for installation in waters where they are exposed to the forces of moving ice are disclosed in J.V. Danys' presentation entitled "Effect of Cone Shaped Structures on Impact Forces of Ice Floes", presented at the first International Conference on Port and Ocean Construction Technology in Arctic Conditions, held at the Norwegian University of Technology, Trondheim, Norway 13 ... August 30, 1971.

Another publication of interest in this regard is a presentation by Ben C. Gerwick Jr. and Ronald R. Lloyd entitled "Design and Construction Procedures for Proposed Arctic Offshore Structures," presented at a meeting of the Offshore Technology Conference in Houston, Texas, in April 1970. .

When examining scale models of structures intended for the distance from the shore in the laboratory cold room in accordance with the above-mentioned structural principle, the ramp-type surface, moving in contact with the solid ice, caused a significantly lower force on the raft structure perpendicular to the ice, such as in a situation where a relatively large-diameter statue or kiln comes into contact with a passing layer of ice. However, it was found that this is true only for 3 3 53 538 as long as the ice layer may move relative to the ferry, and that, as will be explained below, this marine structure is usually subjected to much greater forces before the bond between it and ice is broken so that this relative movement goes head on.

The practical installation of a marine structure in Arctic waters is intended to build and assemble the structure at the shipyard and tow the assembled structure to its destination at a distance from the shore, where it will be attached during the season when the waters are open and relatively ice-free. During this time, the structure is brought into contact with the underwater ground and piles can be penetrated into the ground to hold the structure in place against horizontal forces acting on it. Piles can also be used to help support vertical loads on the structure.

In more distant Arctic waters, such as the waters off the northern slope of Alaska, the open water season is relatively short, approximately six weeks, after which ice begins to form in the open waters. Ice forms around and on top of a marine structure attached to an offshore destination. This situation has been simulated in the laboratory to determine the effect of the newly formed ice sheet on a scale model of a ramp-side, offshore structure as described above, and in particular to determine what forces would be applied to it.

As the thickness of the ice layer on the surface of the water surrounding the model increased, it also froze on the surface of the structure in contact with the water. When the ice layer reached the required thickness for the test, it was found that much more force was required to initiate relative motion between the model and the ice layer adhering to it than to maintain relative motion after the adhesive bond had broken. Under the experimental conditions, the ice layer applied about 5-10 times as much force to the miniature model, depending on the conditions, before the bond broke as it was subjected to the start of the relative movement.

The force initially applied to the structure of the ice layer, of course, depends on the shape, dimensions and properties of the structure and the dimensions and properties of the ice. But in all cases, insofar as the 4 61538 problem is now understood, the structure is subjected to a much greater force initially before the adhesive bond between the structure and the ice is broken than after this bond is broken. Normally, under these conditions, it would be necessary to make the structure so strong that it can withstand the forces initially applied to it by the ice layer, despite the fact that the forces applied to it during most of its service life would not require such a supportive structure. A structure made strong enough to withstand the initial forces of ice would be correspondingly more expensive to build and more difficult to install than a structure designed to withstand only the load of the ice layer moving relative to it. It is an object of the present invention to alleviate this situation of high initial load on an offshore structure with the device described below.

According to the present invention, there is provided a marine structure according to the preamble of claim 1, characterized in that the surface of the wall part facing the ice floe, at least to the extent of first contact between the ice floe and the wall part, is made of a material with low ice adhesion. to facilitate the movement of the part which comes into contact with the structure over the surface of the wall part, so that the force on the structure is reduced, since the adhesion coefficient between ice and the surface should not exceed 7 kg / cm.

The invention will now be described as being applied in particular to a nearshore structure used primarily for drilling oil wells or as an auxiliary device for the production of oil from underwater oil fields in areas of the earth where open waters freeze to considerably thick ice. For the sake of simplicity, such a structure is hereinafter referred to as an offshore rig, although it should be noted that the principles of this invention can be applied to other types of offshore structures such as offshore production rafts, offshore oil tanker loading and unloading stations, lighthouses, in place and are exposed to the forces of ice layers moving on the surface of the water.

5,61538

The present invention also encompasses both an outer wall coated with a substance with little or no adhesion to ice and a peripheral wall made entirely of such a substance.

Some embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic view, partially cut away and with some members rearranged for clarity, of an offshore rig according to the present invention; Fig. 2 is a schematic view of a sectional view taken along line 2-2 of Fig. 1, Fig. 3 is a schematic elevational view, partly in section, of another embodiment of the device of the present invention and illustrating the use of heat transfer panels for heating above the melting point of the natural ice surrounding the structure, Fig. 4 is a schematic plan view in section of line 4-4 of Fig. 3; Fig. 5 is a schematic view of a marine raft with an outer shell made of little or no ice-adhesive material, Fig. 6 is a schematic plan view, partly in section along the line 6-6 of Fig. 5, with parts broken away from the assembly; for disclosure, Fig. 7 is a schematic view, partly in section, of an offshore drilling rig with a coated outer surface. There is no external surface heating; however, internal heating for ferry personnel 161538 is shown, Fig. 8 is a schematic plan view, partly in section along line 8-8 of Fig. 7, with parts broken away to reveal details of the combination.

Figure 1 shows an offshore drilling rig 10 mounted on a body of water 12 in contact with an underwater base 14 to which it is temporarily attached by piles 16. The rig is specially designed for installation in arctic waters with thick ice layers 10 depending on the seasons. which extends into the water and forms a base which supports the lid part 22 above the water surface in a vertical position. The lower part of the raft is exposed to the forces of water and ice in its vicinity, and it is the part of the raft which is mainly concerned with the present invention. The top of the raft may include multiple deck layers and may be enclosed and heated to provide a reasonably comfortable working environment and protection for men and equipment from winter weather, during which the temperature may drop to approximately ~51 ° C. Without proper heating equipment in work areas, the use of this Arctic rig would become virtually impossible for most of the year.

Because it is both expensive and difficult to build and install a rig in Arctic waters, it is advisable to make a rig that can drill multiple wells in the same location. For example, the drilling rig of Figure 1 may be constructed for drilling 10 or more wells from the same location on the rig to a depth of approximately 4900 m, so that it is made large enough to accommodate the machinery and equipment required for this purpose. By way of illustration only, such a raft with a production capacity for installation in 12.2 m deep water may have a bottom diameter of approximately 55 m and a waterline diameter of approximately 36.6 m. The base section may be 26 m high and may support a deck above it. and other relevant equipment, including a drilling rig extending approximately 48.8 m above sea level.

A drilling rig of the above dimensions weighs several thousand tons before any machinery and equipment necessary for drilling is installed. The weight of a raft increases proportionally, insofar as it is built to withstand the greater forces of nature; 'and totfska φ; 1' 7 61538 the weight of the structure affects its price, the price increases proportionally as the weight increases. The present invention relates to a method for reducing the forces exerted by natural ice formations on the base of a raft, as a result of which less structural materials are required for the raft and its mass is correspondingly reduced and the price is reduced.

A drilling rig of the above-mentioned size and drilling capacity requires energy generators capable of generating approximately 2461 kW for the use of equipment and auxiliary equipment required for drilling a rotary table, traction units, sludge pumps, etc. According to an embodiment of the invention, the waste heat of the energy source is used to heat the coated shell of the raft above the melting point of the surrounding natural ice in the manner and for the purpose described in more detail below. For example, if a turbine is selected as the energy source and three 820 kW turbines are used to generate the energy required to operate the rig, more than 33,761,800 kilojoules per hour will be available to heat the shell of the support structure. This amount of heat is sufficient to maintain the above-mentioned shell continuously at a temperature above the melting point of the natural ice formed in the surrounding water.

The structure shown in Figure 1 represents a rig that is towed to the drilling site fully assembled and equipped and that requires no additional construction work at its location except to bring it into contact with the base and, if necessary, to secure it with piles. Ballast tanks 24 (Figures 1 and 2) are constructed in the base part 20 as an integral part thereof for loading the raft with ballast as it is towed and immersed in water, in contact with the seabed. The ballast tanks are each provided with appropriate bottom taps 26 which can be remotely controlled from the deck of the raft by devices not shown but known in the art. Connected to each ballast tank is a tank-specific blow-down pipe 28 which receives compressed air from the compressor 30. The bottom taps can be opened to allow water to enter the ballast tanks or alternatively compressed air can be led to the tanks to push water out of the tanks through the bottom taps.

When the raft is being towed, enough water is let into the cargo tanks to a depth of 2.4 to 3 m, and the volume of the ballast tanks is so large that there is sufficient lifting space above the ballast water to provide stability to the raft when towed. Bottom cargo tanks can, of course, be trimmed as needed to compensate for any unevenness in the weight distribution of the raft.

The drilling rig of Figure 1 is constructed so that it can be quickly made fully operational at the selected drilling site and can be moved from one location to another without delay. In order to promote this mobility, it is desirable that the ferry be constructed in such a way that it can be stabilized at its offshore location so that, in order to secure it in place to withstand the forces to which it is exposed, as little additional construction measures as piling are required.

The raft is provided with a plurality of leg members 31 »mounted so that they can be moved vertically relative to the raft body with appropriate force-fitting arrangements 33. The legs have expanded foot portions 35 and include internal guides 32 (Figure 2) for the piles 16. When the ferry arrives at the drilling site and while it is still floating, the hind limbs are lowered into contact with the seabed. When the lever levers are each in contact with their feet, the bottom taps 26 are opened to allow additional water to enter the ballast tanks 24, which increases the weight of the raft sufficiently to give it a negative lifting force. The feet 35 are designed to control the penetration of the feet into the waterbed as the weight of the raft increases.

The lever levers 33 are now actuated to lower the raft in a controlled horizontal position up to the feet until the bottom 37 of the raft comes into contact with the seabed. Even more water can now be poured into the ballast tanks so that the weight of the raft increases so much that the forces of nature cannot move the raft.

In locations where the raft is exposed to difficult conditions such as pushing the layer ice, the piles 16 can be penetrated into the underwater ground to keep the raft firmly in place against horizontal loads on it. It is an object of the invention to provide a device for considerably reducing the force which would otherwise be applied to a stationary ferry by a layer of ice moving against it, and thus makes it possible to assemble a structure which is more suitable for the operating conditions described above.

9 61538

When the raft is to be moved to another location, the piles 16, if used, are cut across the underside 35 or otherwise separated from the raft. Compressors 30 are used to push water out of the ballast tanks until the carrying capacity of the combination is reached so that it can be lifted up from the bottom up to the feet 31 under controlled conditions. During this operation, the amount of water in the ballast tanks is preferably kept such that the raft has a slightly negative lifting force, and the raft is lifted all the way to the legs using and guiding the lever levers 33. The ferry is raised to its floating position, the ballast is set, and the combination is towed to its new location, where it is lowered to the bottom as described above.

Figure 1 shows a rig mounted at a drilling site and equipped for drilling delivery. The drilling rig 39 is encased to protect it from the weather and extends inside the body of the structure to a deck 41 with a rotating table bracket 43 · The drilling rig has a tower beam 45 which can be moved alternately with a plurality of stations 47 on the base plate 49 for vertical extension. . A rotating table, not shown, is likewise constructed so that it can optionally be moved into extension with each station 47. When the well is drilled, a sleeve 51 is fitted to the well bore and sealed with a waterproof joint 53 relative to the plate 49. If the raft needs to be moved to another location, the sleeve is removed from the raft and a waterproof cover is attached over the relevant opening.

As previously mentioned, gas turbines can be used as the primary source of energy for the ferry. The two energy-generating gas turbines 34 and 36 are schematically shown in Figure 1 in terms of their shape and position. The exhaust gases from each turbine are led down the turbine-specific lines 38 and 40 to heat exchangers represented by coils 42 and 44. The invention also includes a group of heat exchangers are connected to all turbine exhaust lines that the fact that a separate heat exchanger is fitted to each turbine. It is important, however, that there is some extra space in this part of the appliance to provide sufficient functional heat exchanger capacity if any part of the energy generation or heat exchanger equipment needs to be shut down for maintenance or repair work. In addition, the circumferential wall has a coating of H .ε. * * 10 61538 made of a material which reduces the adhesive power of the ice and enables a corresponding reduction in heating capacity as the adhesive force between the coating and the ice is lower. Coating 131 has another advantage that is directly due to its poor adhesion, namely its minimal resistance to ice movement over it. The coating is either mechanically or chemically bonded to the shell.

In this embodiment illustrating the invention, after the rig is operational, the ballast tanks 24 are substantially completely filled with heat transfer fluid. At the top of the tanks, air spaces 48 are left to act as a sink chamber and to allow space for the liquid to expand. On the other hand, the ballast tanks may be combined with auxiliary sinks not shown for this purpose.

The heat transfer medium may be seawater to which a suitable anti-corrosion agent has been added to protect the steel surfaces in contact with it. An antifreeze should be added to the water to prevent it from freezing solid in the ballast tanks and to keep it pumpable unless the water is heated during a time when the temperature of the support structure has dropped below the freezing point. If sufficient fresh water is available, the ballast tanks can be flushed clean of any brine they may contain and filled with fresh water to which an anti-corrosion agent, antifreeze agent, and an algaecide agent have been added to form a heat transfer fluid mixture.

Antifreeze agents available for this purpose include, for example, soluble salts such as sodium chloride and calcium chloride, an alcohol such as methanol or a glycol such as ethylene glycol, or a glycerol, or any of a number of other known antifreeze agents, any of which may be added to the ballast water tank. from freezing to pumping to an impossible condition in a predetermined temperature range, at a time when no heat is added to the water. The corrosion inhibitor is selected to be compatible and interact with the antifreeze agent selected for use.

The heat exchangers 42 and 44 are connected by means of appropriate pumps 50 and 52, respectively, to a common manifold 54, from which the respective lines 56 and 58 are connected to each individual ffcnt 'ϊί.

11,61538 with the top of the tank 24 below the surface 59 of the water it contains. The lower part of each tank communicates with the common lower part 60 along the respective Alisia lines 61 and 62. The heat exchangers 42 and 44 are connected to the manifold 60 by respective lines 63 and 64, and the pumps operate by sucking water from the top of the tanks and pumping it through the heat exchangers and from there to the bottom manifold 60 from where the water is led to the bottom of the tanks 24. Although only a single pump can be used to recycle the heat transfer medium through the tanks 24, it is advisable to keep at least one other pump connected to the system as either a functional unit or a backup unit to ensure system continuity if any of the units fail. Appropriate valves located in the upper and lower lines, such as valve 65 in line 56 and valve 66 in line 61, respectively, provide a means to control the flow of heat transfer fluid through each tank independently of adjacent flow, and also provide a means of isolating a private tank from a heat transfer fluid circulation system. for maintenance work.

Ballast tanks for rafts of the dimensions described above are designed to draw a total of more than 3,180,000 liters of water. The heat transfer medium recycling system is designed to recycle the medium through these tanks at a rate of approximately 3,000 liters per minute when the ferry is in normal operation, making 33,761,800 kilojoules per hour available from energy generating turbines to heat this medium. When the medium in ballast tanks is heated to such a temperature that it can maintain the outer surface of the shell of the support structure at approximately 0.6 ° C, the heat in the ballast tanks stores enough heat to keep the temperature above the freezing point of the shell for 24 hours, which is a safe time for repairs or for closing wells and abandoning the ferry if the energy generation system fails under conditions that endanger the safety of the ferry.

The ferry according to Figures 1 and 2 is shown by way of example with six ballast tanks 24. However, this is not a critical number, and a smaller and larger number of tanks may be suitable for private ferries. The tanks according to the drawing are separated by radially watertight walls ί · * <.2 it 12 615 3 8, i.e. by bulkheads 67, and closed radially by their inner sides by cylindrical walls, i.e. by bulkheads 68.

The ballast tanks are also shown in Figure 1 extending from the waterproof base 37 of the raft to the bottom cover 74 of the top 22, so that the heat transfer medium in the ballast tanks is in direct heat transfer contact with the inner surface 76 of the outer wall 70 over substantially this area. However, on some ferries, it is sufficient to reserve tanks for the heat transfer medium which, although of sufficient capacity, are smaller in volume than those shown in the drawing. Such smaller tanks are distributed on the inner surface 76 of the wall 10 and constructed so as to bring this inner surface into contact with the heat transfer medium throughout the area where natural ice can freeze in the shell, extending above and below the surface of the water surrounding the raft to remain in this zone. above the melting point of natural ice. Thanks to this structure, a heavy dry ballast can be used, which requires less space than a water ballast and thus provides more dry working space on the raft.

In the embodiment according to the drawing, the cylindrical bulkhead 68 delimits in the heart of the raft a working space in which appropriate decks 41, 78 and 80 are arranged to support the crew and machinery. Although this space is maintained heated to a comfortable working temperature, which may normally be higher than the temperature of the medium in the tanks 24, an insulating layer 84 is nevertheless provided radially against the inner surface 86 of the bulkhead 68 to reduce operating losses from these tanks in case the core area 28 The temperature would be lower than the tanks.

Attached to the outer surface of the shell is preferably a wear plate 90 of a substance which reduces the adhesion of the ice to the zone of the raft in contact with the ice to strengthen this area and to receive the impact and abrasion effect of the ice layer protruding against the support structure. Due to the non-existent or low tendency of the wear plate 90 to adhere to the ice, the adhesion of the ice to the shell in this area is minimal.

Figures 3 and 4 represent an alternative arrangement of the device according to the invention and also show a modified ferry shape to which the device is adapted. The same reference numerals as used above are also used, where applicable, in connection with Figures 3 and b, to denote the corresponding bodies.

In this modification, the lower support portion 20 and the upper cover portion 22 may be constructed as separate units that are assembled together at an offshore location. The support part has pile guides 32 built-in along its circumference and through its central part, and a corresponding number of piles 16 are arranged in them.

The raft support section is towed to a selected offshore location and immersed in contact with the seabed by increasing the weight of the ballast. The piles 16 are then inserted through the pile guides 32 and penetrated into the underwater ground. The support section is placed horizontally and the piles are cemented to the pile guides to hold the raft firmly in place against horizontal and vertical loads on it.

Also in this embodiment, as in the raft shown in Figure 1, a waterproof bulkhead 68 surrounds the central area 88 of the raft and forms an inner wall for compartments 100 and 102 that can be used as ballast tanks for trimming the raft when towing and sinking it to a drilling site as described above. However, instead of filling the compartments with heat transfer medium to maintain the support shell at a temperature above the freezing point of the surrounding water, tubular coils * are attached to the inner surface of the shell. in the same way as explained above. In this embodiment of the invention, the water can be displaced from the private compartments after the rig is attached to the underwater bottom. The compartments are then loaded with a sufficient amount of dry ballast material to compensate for any remaining load on the assembled raft. This procedure alleviates the problem of corrosion of the internal surfaces of the compartments caused by the water in the tanks and also provides additional dry working or storage space inside the raft.

As further shown in Fig. 3, the heating panels 10¾ are positioned against the inner surface 76 of the shell 70 so that its coating 131 is Tmi, c'i »1.1 61538 with them in the heat transfer station over the entire area to which in contact with the ice layer 18 formed on the surface of the surrounding water, and preferably extends some distance above and below the thickness of the ice layer so that this area of the coated shell is sure to be heated to a temperature above the melting point of the surrounding ice ^ A wear plate 90 preferably made of adhesive attached to the outer surface of the shell in this area for the purpose explained above. The heating coil panel is covered on its inner surface with a layer of insulator 106, for example foamed urethane, to limit the heat given by the panels to the hull of the raft in this area. The insulator, in turn, is preferably covered by a lid 107 that is watertight attached to the surface 76 to prevent water in the ballast tank from coming into contact with the heating panels and the insulator.

During operation, a heat transfer medium of the type described above flows from drain tanks such as 108 and 110 to a manifold 54 from which it is sucked by pumps 50 and 52. The pumps press the medium into heat exchangers 42 and 44 which receive heat from the exhaust gases of the raft energy generating machines 34 and 36 via lines 38 and 40. The medium flows from the heat exchangers to the manifold 112 and from the manifold the respective lines 114 lead to the medium heat transfer panels 104. The medium is pumped through the panel piping 116 and then flows through the respective lines 118 to a manifold 120 from where it leads through tubes 122 to relevant drain tanks 110 such as 108.

Appropriate valves are provided in the system to control the circulation of the medium to each panel part or each sink tank and to allow these parts of the equipment to be separated from the system for maintenance and repair work. Thus, the respective valves 124 are arranged in the lines 114 leading from the manifold 112 to the respective parts of the heat transfer panels 104 and the respective valves 126 are arranged in the lines 118 leading from the heat transfer panels to the manifold 120. Similarly, each part of the drain tank can be lead from the manifold 120 to the respective valves 128 located in the expansion chamber and a corresponding valve 65 arranged from the different sink tanks such as 108 and 110 to the manifold 54 to the manifolds 54 and 58.

is 615 3 8

Since in the embodiment of Figure 3 the heat for the raft support shell 70 is concentrated in the zone where ice forms in the surrounding water, a lower total amount of heat is required to keep this portion of the coated shell above the melting point of natural ice than in the embodiment of the invention described in Figure 1. .

It is also within the scope of the invention that, under certain prevailing weather and raft shape conditions, the raft shell can be heated to a temperature above the melting point of the raft by directing gaseous removal media from energy generating machines through appropriate conduits to heat transfer to the shell. to act as a heat transfer medium. It is also within the scope of the invention that sufficient energy generating devices are provided on the board for generating energy for panels consisting of electric heating elements arranged in the same way as the heat transfer panels described in connection with Figures 3 and 4.

The schematic representations of Figures 5 and 6 illustrate a raft whose shell is made entirely of material 137 »which minimizes ice adhesion. As such, halogenated carbon resins such as tetrafluoroethylene-hexafluoropropylene copolymers, tetrafluoroethylene polymers, chlorotrifluoroethylene polymers or nylons such as polyamide polymers or copolymers and polylactams can be used.

"The raft may also have a heated outer surface, although it is not necessary to heat the surface. If heating is used, either the heating apparatus of Figures 5 and 6 or a heating system similar to that shown in Figure 3 may be used.

Figures 7 and 8 show an embodiment of the raft without heating of the outer wall surface of the raft. In this embodiment, the ballast tanks 24 are either filled with a mixture of seawater and antifreeze or are minimally heated to prevent freezing of the ballast water therein. Heat exchangers 42 and 44, connected via appropriate pumps 50 and 52 and lines 63 and 64, distribute heat to the surrounding shell 70. 2 .ί »16 61538 to the area used by the ferry staff. The advantage of this arrangement is the cost savings in terms of piping, fuel and heating equipment that would be required to heat the outer surface of the shell.

If it is desired to keep the ferry in place after the drilling is completed, in which case it is no longer necessary to develop the amount of energy required for drilling, auxiliary energy sources can be used immediately to supply the heat needed to prevent ice from sticking to the ferry. Thus, a steam boiler may be used which is primarily intended to supply heat transfer medium to the ballast tanks 24 or heating panels 104, or heat may be supplied from an external energy source to the raft, for example by connecting panels 104 of electric heating elements to an electrical energy source separate from the raft.

The idea of the invention is directed to a method and a suitable device for reducing the forces exerted by natural ice on an offshore ferry. In the above description, for ferries of the size in question, for example, the total force exerted by the 2.4-meter-thick layer of ice frozen on the raft and adhering to its shell by the movement of the raft is approximately 4,536,000 to 9,072,000 kp. When the shell is coated and heated above the melting point of the ice and the adhesion is broken, the force exerted by the ice layer on the raft is 5-10 times less, so that the total force is approximately 907,200 kp.

Preferred embodiments of the present invention and modifications thereof have been described above. However, it is clear that other modifications can be made to the application examples of the device according to the invention described above without departing from the scope of the invention, and it is intended that the invention comprise all modifications and equivalents within the scope of the appended claims.

K- '‘·»,

Claims (3)

17 61 5 38 A marine structure to be positioned and held relatively fixedly in its location in an offshore area (12) where the seawater surface freezes due to ambient natural conditions, the structure (10) comprising a wall portion (70) inclined inward and upward in the area of first contact with an ice float (18) floating in the sea so that the part of the ice floe moving in contact with the structure (10) rises and deviates from its normal position on the water surface, characterized in that the surface of the wall part (70) facing the ice floe (18) is at least in so far as the first contact between the ice board and the wall part is made of a material (131; 137) with low ice adhesion, to facilitate the movement of the part of the ice board that comes into contact with the structure (10) over the surface of the wall part (70) (10) the force applied is reduced because the coefficient of adhesion between the ice and the surface should not exceed 2 7kg / cm. Marine structure according to claim 1, characterized in that said material is arranged as a coating (131) on the wall part. Marine structure according to claim 1, characterized in that the wall part is made of said low ice-adhesive material (137). Tl. V '