Technical field

The present invention relates to an offshore structure comprising a lower portion, an intermediate portion and at least one upper portion, which lower portion forms a foundation to be located on a sea floor, which intermediate portion forms a base column fixed to the foundation and arranged to support the at least one upper portion, according to the preamble of claim 1 . The present in- vention also relates to a method for manufacturing an offshore structure and a method for deploying an offshore structure on a sea floor.

Background art

There are various known approaches for installing offshore structures on the sea floor. A common feature with offshore structures is that they are substantially large and heavy, which poses difficulties in transporting the structures to intended sites. Another common feature is that they are very difficult to install, whereby they require very large and costly specialized vessels. A known way to transport such offshore structures to a site is by towing. This is realized by providing the foundation of the structure with ballasting compartments, that are empty during towing and that are filled with ballast, e.g. sea water or other ballast medium, when the foundation is sunk and anchored to the sea floor. Towing large and heavy floating structures is extremely weather dependent and slow, which causes additional difficulties and expenses. Examples of such structures may be found e.g. in EP 2 479 103 A1 , EP 1 876 093 A1 , GB 2 017 794 A, WO 2010/143967 A2, and US 5,383,748. The known structures are unnecessarily massive and complex, partly due to the required ballasting measures.

An alternative approach is to form the foundation as a single shell structure that is either made to sink into the sea floor or made to rest on the sea floor. US 6,371 ,695 B1 , WO 2005/038146 and US 720,997 describe solutions in which a central shell is sunk into the seafloor by establishing a suction effect inside the central shell. GB 2460551 , WO 2010/138978, US 201 1/0158750 and WO 2012/019050 describe solutions with flat and fixed base plates, which are intended to be set on sea floors. Additional ballasting is necessary for keeping the structures in place. This sets limitations on the type of sea floors the known structures can be installed on. Further, the known shell structures have to be very large throughout their height in order to provide sufficient stability. Such a voluminous structure is difficult to produce and to transport. Fur- thermore, it causes undesired obstructions at the installation site.

Summary of invention

An object of the present invention is to overcome the drawbacks of background art and to achieve an offshore structure that has a lightweight and sim- pie design and that can easily be transported to an offshore site in a variety of weather conditions and deployed at the offshore site regardless of site depth and sea floor soil character. This object can be attained by means of an offshore structure according to claim 1 . The offshore structure according to the invention is intended as a support for a superstructure, such as a windmill or a thereto related substation, or for other offshore operation units, including drilling platforms, lighthouses, or other offshore platforms, as well as for bridges. In the case of a multi-legged or multi- pillar construction, the offshore structure according to the invention is intended as a support for each individual leg or pillar of the construction, which together support a superstructure as mentioned above.

The offshore structure of the present invention has a gravity based foundation which keeps the supported arrangements in an upright position.

The basic idea of the invention is to provide a hollow and modular assembly including standardized components that readily can be assembled into an offshore structure of given dimensions. This is realized by configuring the foundation of the offshore structure as a unitary hollow frame structure with an open bottom surface and with an open top surface arranged around the base column. The unitary hollow frame structure is composed of bulkhead elements attached to each other in order to form adjacent open cells with respective open bottom surface sections and open top surface sections within the unitary hollow frame structure. In other words the open bottom surface sections and the open top surface sections of the open cells together form the open bottom surface and the open top surface of the unitary hollow frame structure. The unitary hollow frame structure composed of bulkhead elements provides for standardization. Furthermore, the unitary hollow frame structure facilitates lowering the offshore structure on to the sea floor as practically no resistance is encountered since water freely can flow through the open cells when the offshore structure is lowered onto the sea floor. When the foundation is set on the sea floor, a concrete layer can be cast within the open cells and subsequently the open cells can be filled with ballasting material in order to secure the frame structure, and consequently the offshore structure to the sea floor.

Terms like bottom, downwards, and lower indicate an orientation towards the sea floor, and terms like top, upwards, and upper indicate an orientation towards the water level. An offshore structure is generally vertically positioned vis-a-vis the sea floor.

The bulkhead elements are advantageously arranged radially and circumferen- tially with respect to the base column. In this way the open cells can be evenly distributed around the central column in a balanced configuration.

Alternatively, the base column is advantageously arranged eccentrically with respect to the unitary hollow frame structure of the foundation. Such an eccen- trie configuration is advantageous e.g. when the offshore structure is arranged as a support for one leg or for one pillar of a multi-legged or multi-pillar construction.

Preferably, the base column is supported by first bracket elements. The first bracket elements are preferably fastened to the upper side of radially arranged bulkhead elements adjacent the base column and to the outside of the base column. This gives the necessary stability to the base column with respect to its height. The size, form and possible reinforcement of the first bracket elements can be adapted to the prevailing conditions, such as the height of the offshore structure or a given normal water level.

The at least one upper portion of the offshore structure advantageously includes a second column that is fastened to and supported by the base column. In addition it is advantageous, that the cross-section of the base column and the cross-section of the second column are equal. This provides for a streamlined configuration that can be manufactured from standardized parts. The base column and the second column may thus advantageously be in the form of tubes of equal diameter.

Particularly for arctic and semi-arctic conditions, the second column is advan- tageously provided with an ice cone. In order to provide the ice cone with sufficient strength, the ice cone is formed by second bracket elements which are to be arranged circumferentially around the second column. The second bracket elements are preferably covered by sheet elements in order to provide a surface structure for the ice cone.

The ice cone advantageously has a horizontally oriented top surface and a vertically oriented side surface, whereby the vertically oriented side surface includes a surface portion arranged to converge downwards towards the second column. Such a configuration is beneficial in order to avoid undesired loads caused by surrounding ice formations. This configuration is advantageous for deep water. In addition the side surface provides a practical quay side and the top surface a suitable access deck for an offshore supply vessel or other craft. Alternatively, the ice cone has a horizontally oriented bottom surface and a vertically oriented side surface, whereby the vertically oriented side surface in- eludes a surface portion arranged to converge upwards towards the second column. Such a configuration is beneficial in order to avoid undesired loads caused by surrounding ice formations. This configuration is advantageous for shallow water, where the ice formations thus may assist in pressing the ice cone and consequently also the offshore structure towards the sea floor.

The at least one upper portion of the offshore structure advantageously includes additionally an intermediate column that can be inserted between the base column and the second column. The intermediate column can be used for adjusting the height of the offshore structure.

Further, it is advantageous, that the cross-section of the base column, the cross-section of the intermediate column and the cross-section of the second column are equal. This provides for a streamlined configuration that can be manufactured from standardized parts. The base column, the intermediate column and the second column may thus advantageously be in the form of tubes of equal diameter. Advantageously at least a number of the open cells, particularly those filled with ballast material, are provided with cover elements. The cover elements prevent the erosion of ballast material contained by the cells when the offshore structure is deployed on the sea floor. The cover elements are preferably of concrete in order to enhance erosion resistance. The cover elements also add to the weight of the foundation when anchored to the sea floor.

The bulkhead elements of the foundation are made of corrugated steel plates or planar flat steel plates or curved steel plates. This is advantageous in view of manufacturing and allows for standardized pre-fabrication according to measure. Alternatively, other materials facilitating manufacture could be used as well, e.g. concrete, fiberglass, etc. depending on required resistance at the intended deployment site of the offshore structure. At least the foundation, the base column, the first bracket elements, the second column, the second bracket elements, the intermediate column, and the sheet elements are advantageously made of steel. This is advantageous in view of manufacturing and allows for standardized pre-fabrication according to measure thus providing for an easily modularized offshore structure. Alterna- tively, other materials facilitating manufacture could be used as well, e.g. concrete, fiberglass, etc. depending on required resistance at the intended deployment site of the offshore structure.

The offshore structure provides a support for a superstructure, whereby the superstructure is supported on the upper portion of the offshore structure. The superstructure may e.g. include a windmill or a thereto related substation, or other offshore operation units as mentioned above. In case of a multi-legged or multi-pillar construction, the superstructure would thus be supported on each of the uppermost columns of each of the offshore structures.

The advantageous features of the offshore structure are given in claims 2-20. The method for manufacturing the offshore structure is defined in claims 21 -29. The method for deploying the offshore structure on the sea floor is defined in claims 30-39. Brief description of drawings

In the following the invention will be described, by way of example only, with reference to the accompanying schematic drawings, in which Figures 1 -10 illustrates an example of the components of the offshore structure and how they can be assembled into an offshore structure according to the present invention,

Figure 1 1 illustrates a first embodiment of the offshore structure installed on the sea floor,

Figure 12 illustrates a second embodiment of the offshore structure installed on the sea floor, Figure 13 illustrates a third embodiment of the offshore structure installed on the sea floor,

Figure 14 illustrates a detail of a fourth embodiment of the offshore structure, Figure 15 illustrates a detail of a fifth embodiment of the offshore structure,

Figure 16 illustrates a first embodiment of the foundation of the offshore structure, Figure 17 illustrates a second embodiment of the foundation of the offshore structure,

Figure 18 illustrates a third embodiment of the foundation of the offshore structure,

Figure 19 illustrates a detail of a sixth embodiment of the offshore structure, and

Figure 20 illustrates an example of deploying the offshore structure according to Figure 19. Detailed description

Figures 1 -10 illustrate an example of an offshore structure 1 according to the invention from component level to an assembled state. Figure 10 shows the offshore structure 1 comprising a lower portion, an intermediate portion and at least one upper portion in an assembled state. The lower portion forms a foundation 2 in the form of a unitary hollow frame structure 27 composed of bulkhead elements 21 , 22, which form adjacent open cells 28 of the unitary hollow frame structure 27. The intermediate portion forms a base column 3 fixed to the foundation 2. The at least one upper portion forms a second column 41 . The second column 41 is fixed on top of the base column 3. The second column 41 is provided with an ice cone 5. The lower portion, the intermediate portion, and the upper portion will be discussed below in more detail. The at least one upper portion could also include an addi- tional intermediate column as discussed in more detail in connection with Figures 1 1 -13.

Figure 1 shows two different bulkhead elements 21 , 22. The bulkhead element 21 is made of a corrugated steel plate and the bulkhead element 22 is made of a planar flat steel plate. The flat steel plate can be provided with stiffeners 23 for reinforcement. The bulkhead element combination as indicated by reference numeral 25 is composed of a corrugated steel plate and a flat steel plate joined together by a flange 24 fastened to opposite sides of the joined bulkhead elements.

The bulkhead elements of the unitary hollow frame structure 27 forming the foundation 2 can be of the same type or of different type. As an alternative, the bulkhead element 26 can also be made of a planar curved steel plate as shown in Figures 17 and 18. The choice of the type of bulkhead elements can be made based on the size and the expected load of the offshore structure, or for example on the basis of available production facilities.

Figure 2 shows the base column 3. This is advantageously made of steel tube, which is easy to manufacture and to standardize. In this embodiment, the base column 3 is provided with a bottom flange 31 for closing the bottom of the base column 3. The base column 3 does not have to be in the form of a tube, it can also have a different cross-section, e.g. a polygon. Figure 3 shows one half of the foundation 2 in an assembled state, Figure 4 shows the base column 3 attached to the first half of the foundation 2 and Figure 5 shows the foundation 2 and the base column 3 assembled as one unit. The foundation 2 is formed of a unitary hollow frame structure 27 with an open bottom surface and an open top surface. The bulkhead elements 21 , 22 included in the unitary hollow frame structure 27 are attached to each other so that they are arranged radially and circumferentially with respect to the base column 3, which is thus located in the center of the foundation 2. The unitary hollow frame structure 27 is thus composed of the bulkhead elements 21 , 22 attached to each other in order to form adjacent open cells 28 with respective open bottom surface sections and respective open top surface sections of the unitary hollow frame structure 27. In this embodiment the open cells 28 are distributed in two rings, an inner ring around the base column 3 and an outer ring around the inner ring. If desirable, e.g. for stability reason, further outer rings, and thus more cells, can be added to the foundation. Also the height of the bulkhead elements can be varied in order to influence the size of the open cells 28 and the amount of ballast material they can receive (discussed below in connection with Figures 1 1 -15).

Figure 6 shows first bracket elements 32, which are intended to be fastened to the foundation 2 on one hand and to the base column 3 on the other hand as shown in Figure 7. The first bracket elements 32 shown in this embodiment are triangular plates made of steel and provided with stiffeners 33 and flanges 34. Their purpose is to strengthen and support the base column 3 with respect to the foundation 2. In order to enhance this purpose, the first bracket elements 32 are provided with flanges 34. The first bracket elements 32 are fastened to the upper side of the radially arranged bulkhead elements 22, in this embodiment to the flanges 24, in the above mentioned inner ring around the base col- umn 3.

Wires, poles, rails, or other corresponding parts could be used as an alternative for the first bracket elements depending on the prevailing support demands.

Figures 8 and 9 show an upper portion of the offshore structure, i.e. in this embodiment a second column 41 , which advantageously is provided with an ice cone 5. The second column 41 is advantageously made of a steel tube, which is easy to manufacture and to standardize. As discussed above with regard to the base column 3, also the cross-section of the second column 41 can be different, e.g. a polygon. In this connection, it is preferable that the cross- section of the base column and the cross-section of the second column are equal. In this embodiment, the tubes would thus have an equal diameter. In this way the base column 3 and the second column 41 can easily be attached to each other forming a streamlined body. The second column 41 is attached on top of the base column 3.

The ice cone 5 is particularly advantageous, if the offshore structure is intended for arctic or semi-arctic conditions. Figure 8 shows the ice cone 5 as intended to be arranged around the second column 41 . The ice cone 5 has a configuration constituted partly of a cylinder (upper part) and partly of a trun- cated cone (lower part). The body of the ice cone 5 is formed of second bracket elements 51 arranged radially so as to be arranged around the circumference of the second column 41 when fastened thereto. The second bracket elements 51 are covered by sheet elements 52 in order to provide the ice cone 5 with a horizontally oriented top surface 53, a vertically oriented side surface 54, and a surface portion 55 arranged to converge downwards towards the second column 41 . The configuration of the ice cone 5 is achieved by the shape of the second bracket elements 51 .

Clearly there are other ways in which to achieve the configuration of the ice cone, e.g. by a frame structure and supporting webs.

An ice cone according to this embodiment has a configuration advantageous for relatively deep waters since the form is suitable for breaking ice formations that engage the ice cone.

The ice cone can also be inverted with respect to the ice cone of the above discussed embodiment. An inverted ice cone is discussed in connection with Figure 14. In conclusion, Figure 10 shows the offshore structure in an assembled state with the components fastened to each other comprising the foundation 2, the base column 3, and the second column 41 provided with the ice cone 5. The offshore structure according to the present invention discussed above can be a so-called stand-alone structure for independently supporting a superstructure, such as a windmill or a thereto related substation, or for supporting other offshore operation units, including drilling platforms, lighthouses, or other off- shore platforms. The superstructure would thus be supported on the uppermost column of the offshore structure.

However, the offshore structure of the present invention can also be designed as a support for one leg of e.g. a jacket-structure having for example three legs, which are joined together for supporting a common superstructure, e.g. a windmill or other offshore operation units, or as a support for the legs of a multi-legged offshore platform or the like. In this case each of the legs would have an independent support provided by an offshore structure according to the invention. The superstructure would thus be supported on the respective upper- most columns of the respective offshore structures.

Figures 1 1 , 12 and 13 show three embodiments of the offshore structure 1 according to the invention located on a sea floor S. A given normal water level is indicated by WL. The main difference between the embodiments is mainly de- pendent on the prevailing water depth at the site where the offshore structure is intended to be deployed. The height of the offshore structure can easily be adjusted by means of the height of the base column 3. This provides for a modularized and standardized component. Additionally or alternatively, the height can be adjusted by an optional intermediate column 42 of varying height. Clearly, the height can also be adjusted by varying the height of the second column 41 .

In this way a variety of modules can be produced in order to adapt the offshore structure for any intended site, particularly with regard to varying water depths and potential ice conditions.

In all three embodiments the foundation 2, the base column 3, the second column 41 and the ice cone 5 are provided with a general configuration as dis- cussed above in connection with Figures 1 -10. The base column 3 is presented in three modularized versions of different heights in Figures 1 1 -13.

In Figure 1 1 the base column 3 has a given first height hi with the support of first bracket elements 32 including stiffeners 33 and flanges 34 and is arranged as discussed above in connection with Figures 2 and 6. An optional intermediate column is indicated by reference numeral 42.

In Figure 12 the base column 3 is shorter than in Figure 1 1 with a given se- cond height h2 and with a correspondingly dimensioned support. An optional intermediate column is indicated by reference numeral 42.

As discussed above with regard to the base column 3 and the second column 41 , it is advantageous the cross-section of the intermediate column 42, the base column 3 and the second column 41 are equal, e.g. tubes with equal diameter. Clearly, the cross-sections can be different, e.g. a polygon.

Figure 13 shows an offshore structure for very shallow water, in which a limited third height h3 of the base column 3 is only supported by relatively small first brackets 32. An additional advantage of such small first brackets 32 in very shallow waters is discussed below in connection with Figure 15.

In the three shown embodiments the foundation 2 with the base column 3 and the second column 41 with the ice cone 5 are formed of principally the same, easily standardized components. Clearly, the height of the offshore structure may as well be achieved by varying the height of the second column 41 or by using an optional intermediate column 42 in between the base column 3 and the second column 41 as noted above. Support for an elevated height of the base column 3 can easily be achieved also by designing the foundation 2 so that the open cells 28 of the unitary hollow frame structure 27 are deeper and can receive more ballast material (discussed more in detail below). Basically this means that the bulkhead elements are dimensioned accordingly.

Figure 14 shows a detail of a further embodiment of the offshore structure 1 according to the invention with an inverted ice cone 5. In this embodiment, the second brackets 51 are in an inverted position with respect to the position of the ice cone discussed above. The inverted ice cone 5 has a configuration constituted partly of a cylinder (lower part) and partly of a truncated cone (upper part). Consequently, this ice cone 5 is provided with a horizontally oriented bottom surface 56 and a vertically oriented side surface 57 with a surface portion 58 arranged to converge upwards towards the second column 41 . The configuration of the ice cone 5 is achieved by the shape of the second bracket elements 51 . The inverted ice cone 5 is preferably covered with corresponding sheet elements 52 as discussed above in connection with Figure 8. A given normal water level in this case is indicated by WL.

Clearly there are other ways in which to achieve the configuration of the ice cone, e.g. by a frame structure and supporting webs. Such an ice cone is particularly advantageous at shallow waters in arctic or semi-arctic conditions. Due to the upwards turned truncated-cone configuration the ice formations thus may assist in pressing the ice cone and consequently also the offshore structure towards the sea floor. This prevents the ice formations from lifting the offshore structure from the sea floor.

Figure 15 shows yet another embodiment of the offshore structure 1 according to the invention, which allows for handling ice formations in very shallow waters. In this embodiment the ice formations can be broken up by means of the smaller first brackets 32 arranged around the base column 3 as discussed above in connection with Figure 13. A given normal water level in this case is indicated by WL.

Wires, poles, rails, or other corresponding parts could be used as an alternative for the first bracket elements depending on the prevailing support de- mands.

Figures 16-18 illustrate three alternative configurations of the unitary hollow frame structure 27 of the foundation 2 in sectional view. The sectional view is taken at the level of the connection between the bulkhead elements 21 , 22, 26 and the first bracket elements 32 as discussed above. The foundation 2 of Figure 16, i.e. the unitary hollow frame structure 27, is mainly constructed of bulkhead elements 21 in the form of corrugated steel plates. The radially arranged bulkhead elements 22 of the inner ring, adjacent the base column 3, are made of flat steel plates 22. Both the corrugated steel plate and the flat steel plate are easy to manufacture. In addition, both plate types enhance the lightweight structure of the unitary hollow frame structure 27 with the open cells 28 of the foundation 2. This embodiment corresponds to the one discussed in connection with Figures 1 -10. In Figures 17 and 18 all the radially arranged bulkhead elements 22 of the unitary hollow frame structure 27 are made of planar flat steel plates and the cir- cumferentially arranged bulkhead elements are made of planar curved steel plates 26. By varying the degree and orientation of curvature of the circumfer- entially arranged bulkhead elements, various degrees of resistance to defor- mation can be achieved in comparison to using bulkhead elements of corrugated steel plates or flat steel plates.

Thus, by minor variations, adaptation to different installation conditions is easily achieved.

Particularly in the embodiment of Figure 18, the outward oriented curvature of the bulkhead elements 26 gives increased stability by means of lesser material thickness and/or by means of plates with a larger non-reinforced area. The ballast material in the open cells 28 of the unitary hollow frame structure 27 sup- ports and stiffens the open cells formed by the curved steel plates.

In the following an advantageous manufacturing process of the offshore structure and an advantageous method for deploying the offshore structure 1 on a sea floor S will be described with reference to Figures 1 1 -13.

The manufacturing process, defined as a method for manufacturing an offshore structure, can easily be integrated into the logistics and the working processes of a shipyard, particularly if the components are made of steel. Firstly, the material for the various components used for the offshore structure in question, e.g. comprising the bulkhead elements 21 , 22, 26 including stiffened 23 and flanges 24, and possibly an internal grid (discussed below in connection with Figures 1 1 -13 in relation to the deployment of the offshore structure 1 on the sea floor S), the base column 3 and the bottom flange 31 , the first bracket elements 32, including stiffeners 33 and flanges 34 when relevant, as well as the second column 41 , the intermediate column 42, the se- cond bracket elements 51 and the sheet elements 52 can be pre-treated and cut for having modularized and standardized components. Particularly the sheet elements 52 of the ice cone 5 can be provided with a stainless steel surface. This can be followed by bending and welding the components and subjecting them to required surface treatment. If desired, the various components can then be placed in interim storage within limited space before the offshore structure 1 is assembled into one unit as discussed in connection with Figure 10. The various components, except for the intermediate column 42, and the vari- ous stages of semi-assembly are discussed more in detail above in connection with Figures 1 -9.

The various components of the modularized offshore structure can then be assembled in view of e.g. a given height and size of foundation. This can take place in one sequence or stepwise as described in connection with Figures 1 - 10.

The semi-assembled or assembled offshore structure 1 can then yet be placed in interim storage, if necessary, before loading onto a carrier in order to be taken out to a deployment site. The more or less hollow steel offshore structure 1 , with the foundation 2 formed as a unitary hollow frame structure 27 composed of bulkhead elements forming adjacent open cells 28, as discussed in this embodiment, is relatively light in comparison with known similar offshore structures, which facilitates its loading, transporting and off-loading at site.

At site, the hollow offshore structure 1 can be lowered from the carrier to the sea floor S with almost no resistance due to the unitary hollow frame structure 27. The unitary hollow frame structure 27 has an open bottom surface and an open top surface in that the unitary hollow frame structure 27 is constructed of bulkhead elements forming adjacent open cells 28 with respective open bottom surface sections and respective open top surface sections. The unitary hollow frame structure 27 will settle on the sea floor S by its own structural mass. Once on the sea floor, the next steps would be to secure the position of the offshore structure 1 with respect to the sea floor S. By providing the base column 3, the intermediate column 42, or the second column 41 with a suitable height, the offshore structure can be adapted to a given normal water level WL.

The deployment of the offshore structure 1 on the sea floor S is advantageously carried out as follows with reference to Figures 1 1 -13. Firstly, in a preparatory phase, the sea floor S, depending on its constitution, is prepared so that a concrete layer 7 can be cast at least at the bottom of a number of open cells 28 after the unitary hollow frame structure 27 of the foundation 2 has been lowered and has settled on to the prepared sea floor S. After the sea floor S has been adequately prepared, the offshore structure 1 with its unitary hollow frame structure 27 composed of the open cells 28 is lowered on to the sea floor S. After this, the concrete layer 7 is preferably cast in a subsequent phase at the bottom of the open cells 28 of the unitary hollow frame structure 27 of the foundation 2 and, if so desired, also on the bottom of the base column 3. In order to reinforce the concrete layer 7, the open cells 28 can be provided with an internal grid of iron bar that advantageously is welded in place already in connection with the manufacturing process of the offshore structure. Alternatively, so-called fiber concrete can be used, since the flanges on the bulkhead elements provide sufficient surface area for retaining the con- crete in the cells.

Furthermore, it is advantageous that at least a number of the bulkhead elements forming the open cells 28 are provided with downwards oriented skirt portions. The skirt portions provide stability against any undesired lateral movements of the foundation of the offshore structure. The skirt portions would be made in connection with the above described manufacturing process.

If the sea floor S is of sand or other corresponding soft material that is easy to penetrate, so-called mini-piles could be used as an additional stabilizing measure. The mini-piles can easily be driven into the relatively soft sea floor and fastened at their upper end to the concrete layer 7 within the relevant cells 28. When the concrete has set, at least a number of the open cells 28 are filled with a first ballast material 8, e.g. gravel or other similar material. The first ballast material 8 is advantageously granular for easy handling. The cells 28 can also be provided with a removable internal grid for carrying ballast material of courser structure. As an alternative, concrete or fiber concrete can be used as the first ballast material. The first ballast material can also be a combination of the above. The base column 3, if so desired, is filled at least partly with a second ballast material 9, e.g. sand or other similar material, preferably up to the level of the ice cone 5. The second ballast material 9 is advantageously also granular for ease of handling. As an alternative, concrete or fiber concrete can be used as the second ballast material. The second ballast material can also be a combi- nation of the above.

In the embodiments discussed above, the open cells 28 that would be filled would preferably be the open cells 28 in the outer ring. The filled cells 28 are advantageously covered by cover elements 6, which prevent erosion and also provide additional ballast weight. The cover elements 6 prevent that the granular ballast material, if used as the first ballast material, flows away from the cells. Furthermore, this allows for using only the necessary amount of ballast material needed for stabilizing the foundation 2 on the sea floor S. The cover elements 6 are preferably prefabricated concrete blocks. In order to increase the weight of the cover elements 6, material, such as waste ore or other corresponding material can be mixed with the concrete. If necessary, extra weights can be placed on the cover elements. If concrete or fiber concrete is used as the first ballast material, separate cover elements 6 for the cells 28 would not be necessary.

If a height adjustment of the offshore structure is done by means of an optional intermediate column 42, this column can also be at least partly filled with the second ballast material 9 for stability. In a corresponding manner, the second column 41 can also be at least partly filled with the second ballast material 9. The degree of the optional filling of the base column 3, an intermediate column 42 and a second column 41 with ballast material is preferably chosen with respect to prevailing stability criteria. The installation can then be finalized by providing an additional layer 10, a so- called scouring layer of e.g. very course gravel and stones around the foundation 2.

The offshore structure 1 is deployed on the sea floor S taking into account its desired use and properties in view of a given normal water level L. Practically this means using an offshore structure 1 of suitable height. Examples of deployed offshore structures 1 with respect to the given normal water level WL are shown in Figures 1 1 -15. In Figures 1 1 -14 the ice cone 5 is shown to be at the given normal water level WL, preferably so that the converging surface por- tion 55 or the converging surface portion 58 is at the given normal water level WL. This enhances the ice breaking properties of the ice cone 5. In the case with the inverted ice cone (Figure 14) this also assists in pressing the offshore structure 1 towards the sea floor S as discussed above in connection with Figure 14. Figure 15 shows the offshore structure 1 (without an ice cone 5) at the given normal water level WL. In this case it is advantageous the relatively small first brackets 32 are at the given normal water level WL for the purposes given above.

The concrete layer 7, and the optional filling of the columns, additionally pro- vide protection for a so-called J-tube containing cabling (not shown) which is drawn from the bottom of the offshore structure to above water level.

The above described method avoids using various complicated injection techniques for filling ballasting material into closed ballast compartments as known from prior art.

In case the offshore structure 1 needs to be removed from the site, the various ballast materials and the additional layer can be easily removed from the offshore structure due to their granular nature. In order to facilitate removal of the concrete layer 7 from the sea floor S, the concrete layer 7 can be provided with lifting loops or other lifting fixtures, to which lifting wires can be attached. Alternatively, bores could be drilled or pre-formed into the concrete layer 7, through which bores e.g. air or water can be directed under the concrete layer 7 in order to levitate the concrete layer 7 from the sea floor S.

Figure 19 shows yet another embodiment of the offshore structure 1 according to the invention. In this embodiment the base column 3 is arranged eccentrically with respect to the unitary hollow frame structure 27 of the foundation 2. The unitary hollow frame structure 27 is constructed of bulkhead elements 21 as discussed above. Clearly also other forms of bulkhead elements can be used, such as bulkhead elements 22 and 26.

Such an eccentric configuration is advantageous e.g. when the offshore structure 1 is arranged as a support for one leg or for one pillar of a multi-legged or multi-pillar construction as mentioned above and as very schematically illustrated in Figure 20 by way of an example.

In this embodiment the unitary hollow frame structure 27 of the foundation 2 composed of the bulkhead elements 21 forming adjacent open cells 28 is shown only very schematically. All the components of the offshore structure, such as the bulkhead elements of various design, the first bracket elements with possible stiffeners and flanges, further columns (in this embodiment the second column 41 is also schematically indicated), and ice cone, etc. can be applied also to this eccentric configuration (particularly as discussed in connection with Figures 1 -18 above). Figure 20 shows a multi-legged construction including three offshore constructions 1 each supporting one leg of a three-legged arrangement for supporting a superstructure indicated by reference numeral 1 1 . The superstructure is thus supported on the respective uppermost columns (in this embodiment the second column 41 is also indicated) of the respective offshore structures. The in- dined column portions as shown in Figure 20 are adapted to provide the common support for the superstructure 1 1 . Providing inclined column portions is considered to fall within the competence of the person skilled in the art. Clearly, each of the legs can also be vertical depending on the configuration of the superstructure. The manufacturing process of each of the offshore structures can be carried out with respect to the eccentric configuration in a corresponding manner as discussed above. The method for deploying each of the offshore structures with an eccentric configuration can also be carried out in a corresponding manner as discussed above. The advantage of the eccentric configuration is that particularly the cells 28 can be more easily be reached and filled with ballast material in connection with a multi-legged construction.

The manufacturing process can readily be rationalized and adapted to existing conditions in e.g. shipyards and other similar construction facilities, thus taking into account of the materials used for the offshore structure. The offshore structure and its components can be modularized and standardized in order to satisfy various needs of offshore structures, such as to size and load resistance, and varying water depths. The modularized and standardized components are suitable for serial production, which gives short lead-times. If the components are made of steel, this further accelerates production, since no drying or hardening time is needed, which would be the case e.g. with con- crete.

The optional ice cone is suitable for demanding ice conditions as well as for use as a docking facility for offshore supply vessels, particularly when having a form of a combined cylinder and downwards oriented truncated cone. The in- verted ice cone is suitable for more shallow waters.

Finally, the hollow and light configuration of the offshore structure allows for ease of transport to various sites, and particularly, for fast installation, which is very important considering the short installation window in arctic and semi- arctic conditions. Also, due to the configuration of the offshore structure, transport is practically immune to varying weather conditions, including strong wave and wind conditions. The offshore structure, which provides very limited resistance due to its hollow configuration, sinks down on the sea floor due to its own weight. Further, the securing of the offshore structure is easy and fast due to its hollow configuration, as described above. An offshore structure according to the invention can be equipped and outfitted in view of its intended purpose.

The drawings and the description related thereto are only intended for clarifica- tion of the basic idea of the invention. The configuration and variety, as well as the manufacturing material of the different components, as well as the measures for connecting the various components to each other, etc. may vary within the scope of the ensuing claims.