In this context, the term "production" means all phases of the work and equipment which are associated with the extraction of oil and/or gas, including drilling and testing, as well as the installation and operation of equipment, etc. for the extraction of oil and/or gas from the formation under the sea bed.

The production of oil and gas at sea takes place from two main types of structures, either floating structures, such as floating platforms or ships, or fixed installations (structures) which rest on the sea bed.

The floating structures have the major advantages that they can be moved (removed) when production in the field in which they are located ceases, and that they can be used at greater depths of the sea. However, greater risks and technical problems are associated with the use of floating structures.

Thus, as these structures are anchored to the sea bed via anchor lines, there will always be the risk of the anchors breaking, which can lead to damage in the worst cases.

Furthermore, there will always be the risk of fractures or leakages occurring in the freely suspended risers which are used to transport the oil/gas to the surface which can cause major or minor pollution.

All the equipment involved in oil/gas production is also positioned on the deck of the platforms, which means that the platforms must be designed and dimensioned to tolerate the loads of the equipment. Moreover, when positioned on the deck, the equipment is exposed to the wind and weather, which may result in corrosion and other damage or situations which lead to a suspension of operations.

These conditions concerning the equipment being positioned on the deck also apply to the fixed structures.

The fixed structures are, however, solid and safe and have long lives, although they are generally expensive to build and use. However, there are exceptions, namely the small steel platforms which can be jacked up. These platforms are not expensive and can easily be moved. However, they are subject to the major restriction that they can only be used in relatively shallow seas.

The present invention represents an intermediate stage between a floating and a fixed structure. Intermediate because it is easy and inexpensive to build and use and it can easily be moved, but it rests on the sea bed.

In further detail, the present invention is characterised by a body which is designed to be lowered and placed on the sea bed and which extends from the sea bed above the surface of the sea. As a minimum, the lower part of the structure contains a space designed to hold equipment for the production of oil and/or gas and the upper part, as a minimum, contains a passage or column/shaft for the installation of hoists, pipes, cable paths, etc. and for communication with the surface.

The present invention will be described in further detail in the following by means of examples and with reference to the drawings, where: Fig. 1 shows a perspective view of a structure in accordance with the present invention.

Fig. 2 shows an outline of a preferred embodiment of the structure in accordance with the present invention.

Figs. 3 and 4 show cross-sections of the structure along the lines A-A and B-B in Fig.

2 respectively.

Fig. 5 shows a sequence of pictures of the structure shown in perspective.

a) shows the structure under construction. b) shows how an annular equipment and operation part is positioned on the base part of the structure. c) shows the structure being towed out and d) shows the structure in an operative situation.

Fig. 6 shows an alternative embodiment of a structure according to the invention.

Fig. 7 shows, in enlarged scale, a cross section along lines C-C of the structure shown in Fig. 6.

Fig. 8 shows the structure in Figs. 6 and 7 under installation.

As stated, Figure 1 shows a structure 1 for the production of oil and/or gas in accordance with the present invention. The structure is designed to be placed on the sea bed and extends from the sea bed up to a level above the surface of the sea.

The lower part 2 of the structure comprises a preferably circular, voluminous part with an internal space/internal spaces which is/are designed to hold the necessary equipment 6 for the production of oil and gas and which is/are indicated in part in the area 5 of the structure which has been cut open.

The upper part 3 of the structure 1, which is connected to the lower part, comprises a preferably circular, column-like part which is designed to hold hoists, pipes and cables, etc. 7 which is shown in part in another section 8 of the structure which has been cut open.

It may be expedient to arrange a helicopter deck 9, an area (not shown in detail) for taking on supplies, or pipes, valves, etc. and other equipment for discharging oil/gas (not shown either) in connection with the upper part of the structure.

In this connection, it must be noted that the present application will not deal in detail with the location and use of the equipment and technical installations but will only describe how the main principles for structures in accordance with the present inventions are designed and function.

The present invention, as it is described in the above and shown in Fig. 1, is thus a structure in which the equipment for the production of oil and gas is placed in a cavity/cavities in the structure directly above the sea bed and can be operated and monitored (maintained) under "atmospheric" conditions, as the cavity/cavities is/are in direct communication with the atmosphere above the surface of the sea.

Such a solution represents considerable advantages over both floating and fixed structures of the prior art.

Advantages over fixed installations of the prior art: - The structure in accordance with the present invention can easily be moved to a new oil/gas field as, using a ballast system (not shown in detail), it can be lifted and then towed to the desired location.

- It is much cheaper to build and operate than the solutions of the prior art.

- The load on the structural part which breaks the surface of the sea is lower because it does not carry production equipment above the water.

Advantages over floating structures: - The present invention is much more reliable as the equipment is placed directly above the sea bed and there is no need for risers.

- It is easier to move.

- It is cheaper to build and operate than floating structures.

Fig. 2 shows an outline and Figs. 3 and 4 show horizontal sections of a preferred embodiment of a structure in accordance with the present invention. In this case, the structure 1 consists of a column-shaped (pipe-shaped) part 13 which is connected to a lower foundation part 12 which is designed to rest on the sea bed. The column-shaped part 13 and the foundation part 12 are built expediently as an integrated section, for example of concrete in accordance with conventional principles (see Fig. 5 a) ). Furthermore, the structure consists of an annular part 11 with an internal space/internal spaces which is/are designed to hold the necessary production equipment (not shown in detail). The ring part 11 can also expediently be made of concrete and has an internal diameter which corresponds to the external diameter of the column 12. It is made as a separate section and is placed on the foundation 13 by threading it down over the column either after the column and foundation have been lowered over the oil/gas field in question or before the structure is towed out (see Fig. 5 b)).

The column 12 and the annular part 13 are provided with corresponding, initialiy closed openings (not shown) and, after the ring part has been positioned, the openings are opened in order to achieve free passage from the ring part 11 to the surface via the column 12. Both the lower part of the column and the foundation and the ring parts are provided with ballast tanks (not shown) so that the buoyancy can be regulated while the respective parts are being lowered.

Fig. 5 shows a sequence of pictures which illustrate the construction, assembly, towing out and operative positioning of a structure in accordance with the present invention. In the example shown here, the structure is constructed in concrete at a suitable location in accordance with a conventional method with sliding falsework, see Fig. 5 a). After the construction of the ring part 11, the foundation 13 and the column 12 has been completed, the structure is assembled as shown in Fig. 5 b).

This is done by first lowering the foundation and the column 12, 13, then placing the ring part 11 on a lighter or similar and towing it into position above the column 12, and finally lowering the ring part down over the column 12. While it is being towed to its destination, the ring part is kept in a position on the surface of the sea so as to create sufficient buoyancy and stability during the towing operation, as shown in Fig.

5 c).

Finally, as shown in Fig. 5 d), when the structure has been towed to its destination over an oil/gas field, the necessary ballast is placed in the respective structural parts so that the ring part can be lowered to the foundation and the structure as a whole can be placed on the sea bed.

When the structure is subsequently to be moved to a new field, the ballast water is pumped out of the respective structural parts so that the structure again assumes the position shown in Fig. 5 c) for towing.

Fig. 6 shows an alternative embodiment of the structure according to the invention. It comprises, as with the previously described structures, a column-like part 14 with an upper helicopter or service deck 15, and a lower base part 16. The base part is, however, in this embodiment provided with individual fixed 17 or releasable 18 (see also Fig. 8) ballasting or production cells. The fixed cells 17 are an integral part of the base, whereas the releasable cells are designed to be submerged and attached to the base 16 after having placed the structure 14,16 on the sea bed. Preferably the base 16 may be provided with an upwardly open cavity 19, whereby the cell 18 has a complementary shape to fit in the cavity 19. Openings (not shown) may further be provided between the cells 17,18 and column 14.

Fig. 7 shows, in enlarged scale, a cross section of the base along iine C-C in Fig. 6.

As can be seen, the base in this example is provided with totally six cells, three fixed cells 17 and three releasable cells. Preferably the releasable cells 18 may be provided with production oil/gas equipment before being submerged and attached to the base 16. The fixed cells may be used for ballasting. Of course, the number of fixed and releasable cells may be of different number than what is shown in this figure.

Fig. 8 shows a situation of the construction 14,16 according to Figs. 6 and 7 where a cell 18, under installation, is being submerged by means of pull line 20, which pulls the cell 18 towards the cavity 19 in which it is designed to be attached to the base 16. To avoid collision with the column 14, a towing barge 21 is holding the cell 18 in position under the installation process.

Instead of being circular with egg-shape or the like, the cells 17,18 may be of segment shape.

It must be noted that the present invention, as it is described in the claims, is not restricted to the examples shown in the drawings and described above. Thus, the structure can be made of materials other than concrete, for example steel.

Furthermore, the construction, towing and installation of the structure can take place in other ways. For example, if it is constructed in steel, it can be towed to its destination as a horizontal structure on which the ring part has been positioned before towing out.

Moreover, the structure can be fitted with two or more ring parts arranged above one another instead of just one ring part.