GB2266326A - Foundation of an offshore platform - Google Patents

Abstract

A foundation for an offshore platform (1), in particular for a re-usable platform or a jackable platform is made up of several modules (5) designed to be stacked onto each other. The modules preferably have a hollow design with open bottom, and are completely or partly filled with a filling material (6) when arranged on the seabed. The modules (5) may be equipped with an adaptor (8) being shaped in concordance with the adjacent module so that a plurality of modules stacked onto each other make up a stable foundation unit. The foundation may be erected to different heights, and, therefore, both the platforms and the foundations may be reused on locations having different depths. <IMAGE>

Description

FOUNDATION OF AN OFFSHORE PLATFORM The present invention relates to a foundation for offshore platforms, and in particular a foundation for platforms suitable for re-use, however the invention may also be employed as a foundation for jackable platforms. The foundation itself may also be suitable for re-use and may also be built on or moved, partly or completely.

To obtain an adequate exploitation of small, subsea oil or gas reservoirs the need for re-usable platforms has come into being. The reason for this is that short production periods require high interest on the investments. However platforms being installed have to be removed when used. This applies both when the platforms have to be discarded and when they are to be re-used.

It is in particular expensive when traditional platforms of steel and based on poles are to be removed, as the poles must be shot off before removing the platforms. If such traditional platforms are to be re-used by means of present technique, they must be transported to the shore and there be equipped with new guides for the poles. Therefore it is very expensive to re-use conventional pole based steel platforms.

So far all platforms have been projected for a particular water depth and therefore they have to be extensively rebuilt before being used on a new oil field. By means of the present invention a re-usable platform may be projected for different water depths.

Similar conditions exist for traditional jackable platforms. The range of depths where jackable platforms may be used is continually being extended due to new technology, but all the same it will represent a much more economic solution to produce a flexible foundation resulting in the possibility for using one particular jackable platform at different sites having considerable differences in water depths, and even at depths for which the jackable platform was not originally adapted.

As an example of an earlier known movable platform the solution according to Norwegian patent application No.

91.2449 may be mentioned. In this application the lower portion of the platform consists of a watertight caisson leading from the foundation and up above the mean water level. This caisson carries an easily removable upper part which again supports a platform module. Thus the upper part of the construction has to be adapted to the different locations represented by different water depths while the lower part is maintained without any changes.

From Norwegian patent No. 164.997 it is also earlier known to build a foundation of a jackable platform by arranging each platform leg is arranged within a tube being depressed into the seabed and filled with gravel to a desired level. The platform legs are then arranged on this gravel filling before the jacking process is started.

According to the earlier known solution for supporting jackable platforms a rather limited possibility of variation is obtained when the height of the foundation is considered.

Those solutions do not at all try to solve this problem. The earlier known solution aims at the possibility of having the jackable platform suitable for a slightly greater depth of water than it is projected for, and at the possibility of removing the foundation and using it on a new site.

The main object of this invention is to provide a modulized foundation system for jackable platforms according to which the foundation itself as well as the platform may be re-used even at locations having a considerable difference in depth and with other seabed formations than at the original building cite.

A further object of this invention is to modulize the foundation into standard modules adapted to build foundations for platforms to a required height.

Yet another object of the present invention is to provide a modulized foundation system suitable for the re-use of wellhead platforms, and finally it may be mentioned that a foundation according to the present invention makes re-use of the platforms and the foundations possible without any necessity for bringing the equipment to shore for control, adjustment and/or maintenance.

These objects are met by building the foundation according to the claims below.

To give a more clear understanding of the present invention it is referred to the detailed description of embodiments given below and to the acompanying drawings in which: Fig. 1 shows an example of an earlier known technique using only one foundation module, Fig. 2 shows a typical embodiment of a foundation module according to the present invention, Fig. 3 shows how more than one module according to Fig. 2 may be built together to form a complete founda tion, Fig. 4 shows a further embodiment of the foundation mo dules, and Fig. 5 shows still a further embodiment of the modules.

In the drawings elements having the same function are also given the same reference number when practical. In addition it should be mentioned that all the figures represent sections through one or more modules, along their longitudinal axis. The shape of the modules perpendicular to the shown sections are of no relevance as it does not have any effect on the principle. Thus, there is no requirement for the design to be symmetrical. The only requirement is that modules which are to be stacked on to each other have complementary end surfaces which fit together and give an even weight distribution.

In Fig. 1 there is shown a truss platform 1 of previously known construction. Each leg 3 may be equipped with a small box 21 having downward directed skirts 2. Those skirts 2 may be pressed, or alternatingly pressed/sucked, down into the filling material 6 which is filled into separately arranged foundation tubes A, which in Fig. 1 are shown arranged below each main leg 3. The tube sections A may also be provided with one or two downward directed skirts 2', 2". These skirts protrude into the seabed and ensure stability. They also ensure that filling material should not be washed out below the foundation due to wave effects.

In the figure the seabed is shown as an even surface and further it is shown that the tube sections A are completely filled up with filling material 6. If the seabed is more uneven the conditions may be enhanced as the single sections A are filled to a different degree with filling material 6, so that the filling material 6 in all sections A reach the same level d1 under the surface.

In Fig. 1 the sections A are shown having an open bottom and an open upper part, while the boxes 21 are shown with an open tops but with closed bottoms. However A and 21 are both equipped with skirts 2, 2' and 2''. The bottoms 23 in the boxes 21 ensure an even weight distribution over a larger area of the filling material 6.

Using the shown design the load on the side walls of the tube sections A will not be very high as the walls of the modules are exposed only to the horisontal components of pressure of the filling material 6. The highest load will be the vertical pressure components acting on the seabed below each module.

In Fig. 2 there is shown a stackable foundation module 5 according to the present invention which has a somewhat different design. On the first hand the module 5 now principally comprises two components, the tube shaped module 15 itself as well as an adaptor 8 The adaptor 8 is designed in such a manner that the bottom surface 13 fits the upper surface 14 of the next module exactly. The adaptor 8 may be considered as a part of the module 5 as at least one such adaptor 8 belongs to each module 15. However, the foundation module 5 may also be designed without such adaptors 8, but then the upper/lower surfaces are of complementary design to fit directly onto each other.

In the embodiment shown in Fig. 2 the foundation module 5 is in addition equipped with an impermeable partition 4 which subdivides the interior of the module 5 in two compartments. This partition must not necessarily be flat but can have the shape of a dome or may be equipped with strengthening ribs. Then a filling material 6 may be filled in above the partition 4 to auxiliarate the pressure distribution from modules and the platform arranged higher up in the system.

This is also the reason why the adaptor 8 may be equipped with a smooth but somewhat downward protruding sentral portion 10. This sentral portion 10 will, when more modules are erected on top of each other, give an even pressure distribution. This is more clearly shown in Fig. 3 where two modules are shown in an assembled condition. Still referring to Fig.

2 it should also be mentioned that the adaptor 8 is provided with downward protruding skirts 2 acting in a similar manner as the skirts 2' and 2" in Fig. 1, so that the foundation module arranged on the seabed is forced down into the ground.

In addition the module 5, when manufactured from steel plates, may be provided with internal and/or possibly external reinforcements 7 (not shown) in the shape of ribs or similar structural elements 2 to increase the modules' capability of resisting outer mechanical stress in different directions. It should here be noted that the module 5 shown in Fig. 1 represents a simplified version of the foundation module shown in Fig. 2, and that modules shown in Fig. 2 alternatively may be manufactured as open tube sections which may be filled with filling material or sand as shown in Fig.

1, but in such a manner that they may be stacked onto each other and mounted together as shown in Fig. 3 and Fig. 5.

It should be noted that each single module 5 may be provided with buoyancy chambers 9 or may have hollow walls or wall portions which may be filled/emptied through valves 11.

This will to a very high degree simplify both installing and removing of the foundation modules 5. The volume of the buoyancy chambers 9 may be determined according to the weight of the modules and probably also according to the weight of the filling material arranged onto the module. (The latter condition is in particular viable when the module is closed at its lower side as shown in fig. 2). The buoyancy chambers 9 may be designed as separate elements, may be integrated in the walls of the module and/or may consist of the chamber itself within the central body of the module.

In fig. 3 there is shown how two modules of the kind being shown in detail in to fig. 2 are to be mounted together. The figure is substantially self-explanatory, however, corresponding reference numbers are used in both modules so that references to the uppermost module is given by an added '. Further, it may be mentioned that the skirt 2' on the adaptor 8' corresponds exactly to the upper part 14 of the module 5. It should also be noted that the platform or the platform legs 3 either may be placed directly on the top of the filling material 6' or into a corresponding box as already mentioned in connection with fig. 1.

Finally it should be mentioned with reference to fig 3, that the modules 5 and 5' do not necessarily have the same height. In practical implementation modules of mutually adapted lengths may be produced and stored so that a complete foundation having the desired height may be obtained by combining two or more different modules.

By means of the solution shown in fig. 3 the total volume of the chambers 9, 9' may be used as buoyancy chambers without any further remedies than possibly a valve 11 introduced in the wall of the module.

In fig. 4 still another kind of stackable foundation modules are shown. The same reference numbers are again used for components with similar functions as in the previously described figures. The main difference is now that the modules are not quite identical as the lowermost modules are the biggest ones, being shaped as truncated cones or pyramids. The principle and implementation is otherwise as already explained in connection with the previous figures.

The lengths of the skirts 2' may either be shorter than the length of the skirts 2, they may be flexible to allow bending, or of a material which can be cut, or there may be arranged recesses for the skirts 2' at the top of all modules 5. As a possibility only the lowermost module may be provided with skirts.

Finally it is referred to fig. 5 in which still a different embodiment is shown within the scope of the present invention. Here each single foundation consists of tubeshaped modules 5,5',5" which all have the same diameter so that all the modules may be directly mounted onto each other.

The skirts 2,2',2" are now preferrably arranged as an extension aligned with the internal surface of each single module.

During the mounting process the skirts of the lowermost modules 5 may first be forced down into the seabed whereupon filling material 6 is put within the module until the level h is reached. Thereupon the next module 5' is arranged with its skirt 2' forced down into the filled material 6 and at the same time some material will flow out between the contacting surfaces of the modules to distribute the load. Then further material 6 is filled into the foundation up to the level H, and in this manner the erection of the platform leg 3 is completed until the leg 3, possibly comprising a box 21, is arranged into the loosely packed material in the uppermost module 5''. Also when this embodiment is used adaptors may be arranged between the sections in a similar manner as already shown in fig. 3.Those adaptors then may have the shape of open annulus having brims adapted to the upper/lower adjacent module.

When a platform according to this invention is to be removed, sand which may be found in the box 21 first can be removed by water jets. Then the platform 1 is lifted and removed whereupon each single module 5, 5',5'' has its corresponding valves ll,1l',ll'' closed and the buoyancy chambers 9,9',9'' are emptied for water or rather filled with air.

Then the modules may be lifted up to the surface of the water one at a time.

If the modules are open at the bottom as shown in fig. 5 the material or sand 6 which is arranged within the modules will fall out when the modules are lifted from the seabed.

During the construction of the modules there may be provided jet units at the end of the skirts 2 (not shown in the figure) for alternating washing and sucking. It should also be mentioned that all truss platforms or other platforms have to be inspected for possible maintenance between each re-use.

The foundation modules may have many different shapes and modifications without leaving the scope of the present invention. Thus, the material in the modules may be chosen among all known materials which can endure the stress for which the modules are constructed. Different materials may also be used in different parts of the modules. One solution, therefore, involves adaptors 8 manufactured from reinforced concrete while the walls of the modules may be manufactured from steel tube sections. Similarly the cross section of the modules may be modified within wide limits and the modules do not necessarily have to be symmetrical. Both circular, connical, square or triangular cross sections may be used. It must also be mentioned that the foundation itself according to the present invention is suitable for reuse. In this connection the complete foundation or portions of the foundation may be removed and built up on a new locality. Similarly the complete foundation or parts of it may be built on to be used in connection with a different platform.

Claims (12)

1. A foundation for supporting a leg or a leg extension of offshore platform comprising a series of stackable foundation modules for location on the seabed, each module having a lower surface which is complementary to the upper surface of another module, so that the modules may be stacked upon each other to form a stable foundation of a predetermined height, the contacting surface between two consecutive modules at least partly comprising a filling material which provides an equalisation of the load, the filling material of the uppermost module being arranged to receive the leg.

2. A foundation as claimed in Claim 1, in which each module is subdivided in its longitudinal direction into two compartments by means of a massive partition arranged within the module at a certain distance from its upper edge.

3. A foundation as claimed in Claim 2, in which the partition is in the form of an arched dome.

4. A foundation as claimed in any preceding Claim, in which the modules have different lengths.

5. A foundation as claimed in Claim 1, in which each module is open at its bottom as well as at is top so that the filling material is filled up from the sea bottom to the desired level.

6. A foundation as claimed in any preceding Claim, in which at least some of the modules are completed at one end by an adapter comprising a pressure equalising portion adapted to the adjacent module.

7. A foundation as claimed in any preceding Claim, in which the modules have different lengths.

8. A foundation as claimed in any preceding Claim, in which each module is equipped with at least one buoyancy chamber which may be filled and/or emptied via a valve or a similar device.

9. A foundation as claimed in any preceding Claim, in which one or more flushing devices are arranged at the lowermost portion of each module.

10. A foundation as constructed and arranged substantially as herein specifically described with reference to and as shown in Figures 2 and 3 or Figure 4 or Figure 5 of the accompanying drawings.

11. An offshore installation comprising a plurality of foundations as claimed in any preceding Claim, the legs or leg extensions of the platform being arranged to rest in the filling material in the uppermost modules of the foundations.

12. An offshore platform installation comprising a single foundation as claimed in any of Claims 1 to 10, within which several or all of the leg extensions of the platform rest.