The present invention concerns a floating concrete structure, preferably for storing oil. The floating structure can also in addition to be an oil storage be provided with for example oil and gas processing equipment and thus function as an offshore floating oil and gas platform. The structure can also function as a floating body alone, accordingly without storage function.

NO 147591 B describes a device in the form of a floating body which is formed to reduce pitching, rolling and submerging of the body. The body which can be a floating platform or an oil storage, can be formed as an annulus. The body is formed with an internal closed ring where the stored material can float freely. The annulus itself is encircled by a cylindrical wall extending above the water surface and also down below the water surface. In the bottom of the floating body substantially vertical, downwardly open and in other respect closed chambers are provided which in the floating condition are completely filled with water, whereby the mass of inertia of the body is changed.

NO 320663 B1 describes a floating cylindrical oil storage having a cylindrical centre section and tank sections which all are connected to the central section containing pipe systems. The object of this structure is mainly to optimise the pipe system connecting the tanks in a tank arrangement of this particular type. An object of the present invention is to provide a concrete oil storage which can be constructed based on effective construction methods and hence is economical competitive with today solutions. Another object is that the floating structure shall exhibit good hydrodynamic characteristics under all sea conditions. In this connection it is important to avoid that the period for heave and pitch (accordingly vertical movement and rotation) respectively is not to strongly connected together such that these movements are increased. A further object is that the floating structure is provided with a favourably shape with regard to strength such that for example differential pressures acting on walls are relatively low. Another object is that the floating structure shall be flexible, i.e. in addition to be an oil storage it shall also in a simple way could be provided with required equipment and this function as an offshore oil and gas platform.

The objects of the present invention are achieved by a floating concrete structure preferably for storing oil, characterized by comprising a substantially torus shaped shell structure having internal radial walls and a upwardly and downwardly curved dome shaped shell structure closing the open centre area of the substantially torus shaped shell structure, above and below, respectively.

Preferred embodiments of the floating structure is further described in the claims 2 - 8.

A preferred embodiment of the invention shall now be explained with reference to the attached drawings, where figure 1 depicts a floating structure according to the invention having a deck structure mounted on, figure 2 depicts a floating structure of figure 2, but now with a part of the upper shell structure removed. The floating structure is here shown without the deck structure, figure 3 depicts a plan view of the floating structure, figure 4 depicts a front view of the floating structure in a operational phase with empty tanks, figure 5 depicts a floating structure now in a operational phase with full tanks and figure 6 depicts a plan view of the floating structure where the internal tanks having another form than in figure 3. With reference to the figures a floating structure 1 of concrete is shown. The structure 1 includes storage tanks for oil and is shown with a possible deck structure in figure 1.

The floating structure 1 has a torus shaped shell structure 2 which in the shown embodiment is substantially ring formed with an outer and inner cylindrical wall 3, 5. The outer and inner cylindrical walls 3, 5 are in their upper area connected to a upwardly curved torus part 10 and in the lower area connected with a downwardly curved torus part 11 thus closing the torus formed shell structure 2. The substantially torus shaped shell structure 2 is provided with internal radial walls 15, and in this connection references are particularly made to figure 2 and 3. The torus shaped shell structure 2 is provided with a upwardly and downwardly curved dome formed shell structure 7, 8 closing the open centre area of the substantially torus shaped shell structure, above (in the upper parts) and below (in

5 the lower parts), respectively.

The outer cylindrical wall 3 of the shown embodiment has a double shape including transversal bulk heads 4. It should in this connection be mentioned that double hulls are required for such installations. The double shape of the wall 3 provides chambers which for example can be ballasted in order to adjust theo freeboard of the floating structure during different operational conditions.

In the depicted embodiment the internal radial walls 15 have a height equal to the outer and inner cylindrical walls 3, 5. It should be mentioned that the internal radial walls 15 either can start higher up or end lower down. The main object of the radial walls 15 is to provide global stiffness and to prevent "splashing" within the5 storage volume. The radial walls 15 are provided with internal transversal bulk heads 16.

Figure 6 depicts another embodiment of the internal radial walls 15 which also here has a double shape with a first and a second wall 17, 18. The walls 17, 18 are curved in each direction in relation to a radial centre axis 20. o The internal radial walls can also be provided with throughgoing openings.

The geometrical form of the floating structure 1 is a result of a number of design criteria as for example requirement to: stability, strength with regard to acting water pressure etc., safety with regard to collision from for example ship, minimal weight and draft and effective construction methods as for example tunnel5 formwork, slip casting, prefabrication of segments etc.

On the basis of the above discussed criteria the optimum solution is a floating structure 1 consisting of a substantially torus formed shell structure 2 having a upwardly and downwardly curved dome shaped shell structure 7, 8 or spherical shell closing the open centre area, above and below, respectively of theo substantially torus formed shell structure. Inside the storage itself radial walls 15 are provided which take care of "sloshing-forces" and ensure global stiffness and strength. In the shown embodiment the internal radial walls 15 terminate at lower and upper ring beam elevation, such that oil freely flows under the walls inside the torus form. The solution of the torus shape and the radial walls 15 extending not completely down allows the fluid to freely flow in the bottom of the tank such that the complete volume is to be regarded as a large tank (free liquid surface). This provides the floating structure 1 with an acceptable relation between natural period for vertical movement and rotation in floating condition whereby hydrodynamic problems are avoided and good stability is achieved. The geometrical form of the floating structure 1 with the different shell formed domes, torus and cylinders takes care of the water pressure in an effective way. Further, a very simple rotational symmetrical load distribution is achieved and accordingly reinforcing arrangement which results in a cost effective manufacture of the floating structure 1.

Figure 4 depicts the floating structure 1 in an operational condition with empty internal storage tanks. With regard to stability of the floating structure 1 the internal storage tanks will be filled with liquid up to the shown level.

Figure 5 depicts the floating structure 1 in a operational condition with full internal storage tanks.

The floating structure 1 floats high in the water when the internal storages are empty and lower in the water when the internal storages are filled up. In both cases the differential pressures across the walls are relative minor.

An embodiment of the floating structure 1 is discussed above, but it should be understood that the floating structure may have other forms, for example a

"more complete" torus shaped shell structure 2 without the outer and inner vertical walls is possible. The internal radial walls may also have another form than in the shown embodiment.