GB2124684A - Offshore platform - Google Patents

Abstract

A structure 1 for installation on the sea bed, comprising a base section 2 which is piled, a plurality of slanting legs 3 and preferably one or more vertical columns 4, which legs 3 and columns 4 are joined together in a riegel 12, the whole structure to be towed to the field in vertical position, preferably using auxiliary cells 18, 28. <IMAGE>

Description

SPECIFICATION Offshore platform The present invention relates to an offshore marine structure to be placed on the sea bed. The invention includes a concrete structure which is fabricated ashore, towed to the field in an upright position, lowered to the sea bed and fastened by means of piles. The structure has a tower and a plurality of slanting supporting legs.

Concrete structures placed on the sea bed are known, e.g. from Swedish patent no. 392 632, Norwegian patent publication no. 136 122 and Norwegian patents 135677,140431 and 142005.

Also piled offshore platforms are known, e.g. the well known steel jacketwhich isthe most common of all offshore fixed platforms.

Neither is the combinatin of concrete platforms and piling unknown. However, such platforms are known only in particular variants. None of these concepts are suited as platforms for deep waters (more than 200 m water depth).

A concrete platform usually comprises a large caisson. The caisson gives the advantage that the platform can be constructed ashore and towed to the field in an upright position, even with a large deck load preinstalled. However, the large volume will cause large wave forces when the platform is installed, and foundationing is therefore difficult when the soil is poor.

To construct a concrete platform of the truss or "jacket" type has never been a success. The advan tage of building such a platform of concrete rather than steel, is questionable. In addition, such a structure would be rather unusual and not proven as a concrete structure, and this is a disadvantage.

The purpose of the present invention is to find a platform which has the advantages of the known concrete platforms, and which in addition can be installed on a poor sea bed and in deep waters. The structure is preferably piled.

A structure according to the invention will have a simple base section, which preferably is piled to the sea bed. From the base section upwards, hollow columns are stretching, of which at least some are slanting. Some distance above the sea bed, the columns are tied together to one structure. Usually this joint section, the riegel will be below the sea surface. From the riegel upwards, the structure will continue as one or a plurality of towers. Above the sea surface, the structure will usually have a deck section.

A platform as described will have less buoyancy and stability in the tow-out condition than a platform of the caisson type. The ability to carry a heavy deck structure is therefore limited in this condition. To improve the stability in floating condition, the sturcture therefore can be equipped with auxiliary cells, which can be removed when they are no longer needed.

Other features of the invention will be discussed below in connection with an embodiment given as preferred, but non-limitative examples.

Figure 1 is a vertical section through the structure.

(Scale 1:2000).

Figure 2 is a horizontal section at elevation +20 through the structure.

Figure 3 is the same section as in Figure 1, but with the auxiliary cells and their supports included.

Figure 4 is the same section as in Figure 2, but with the auxiliary cells included.

Figure 5 is a horizontal section of the structure with the auxiliary cells, at elevation +250. (Scale 1:1000).

Figure 6 is a horizontal section of the structure with the auxiliary cells, at elevation +210.

Figures 7-9 are illustrating details of the present embodiment.

Figure 10 is an auxiliary cell.

Elevations are measured from the bottom slab of the structure (or from the sea bed when the structure is installed).

Elevations are in meters.

The structure 1, shown as an example, is a drilling/production platform, and consists of a base section 2, three slanting legs 3, a centercolumn 4 and a deck 5. The platform is placed on the sea bed 6 and is equipped with piles 7. The deck structure is above the sea surface 8.

The base section 2 consists of three main girders 9, three secondary girders 10 and three footings 11.

The latter are constructed as conical enlargements of the legs 3. The slanting legs 3 have a vertical portion at the top, so that this portion lies adjacent the centercolumn 4. The joint section, the riegel 12, will consequently in the exemplary embodiment consist of a centric cylinder 4 togehter with three other cylinders 13 on the periphery which touch the central cylinder (Figure 5). In the touching region between the centercolumn 4 and the periphery cylinders 13 there will be a solid part 14 for transfer of forces and moments. In addition there are vertical walls 15 between the centercolumn 4 and the periphery cylinders 13 for further increase of the capacity of the joint. The triangular cells 16 which are created can be filled with foamed material (e.g. styrofoam) to increase the buoyancy in floating condition.The triangular cells 16 are covered with horizontal slabs at the upper and lower end (el. +260 and el. +235).

The centercolumn 4 extends from the sea bed to the deck. Some parts are conical, as shown on Figure 1. Inside, there is another cylinder 33, called the utility shaft, where the ballast system is placed.

The legs 3 are shown cylindrical, but could also be conical or change diameter.

Both the centercolumn and the legs arse divided by slabs or domes.

The footnigs 11 and the centercolumn 4 are equipped with skirts 17. Their main purpose is to stabilize the platform in the unpiled condition, before the piling is carried out.

The fabrication of the platform is carried out as usual for concrete platforms. The base section 2 is carried out in a dry dock. It is towed out and moored in deep water. Then legs, centercolumn and utility shaft are made through slip-forming. Then the deck is mounted, the platform is towed to the field in upright position and it is lowered to the sea bed.

Finally, the platform is piled.

During tow-out the draft of the platform may be restricted. In the exemplified embodiment maximum draft is 220 m. With a deck load during tow-out of 30 000 tons one will immediately realize that there will be present problems as regards buoyancy and stability. To improve this the platform is equipped with three temporary auxiliary cells 18, called bucket cells. These are placed on circular foundations 19 which are an integral part of the base section 2. The foundation 19 is equipped with a pressure resistant dome 20. The wall above the dome 20 ist also pressure resistant (able to carry the full water pressure).

The bucket cells 18 are cylindrical, and are equipped with a conical top slab 21, a domed bottom slab 27, and domes 22 for volume division. inside there is a utility shaft 23 for the ballast system. The utility shaft is extended up to el. +290.

The bucket cells 18 are placed on the fundation 19 without use of fastening means, but the joint 24 is slanted, so that transverse movement is impossible (sse Figure 9). At the upper end (at el. +210) each bucket cell has supports 25 on the legs 3 (see Figure 8) and one support 26 on the centercolumn (see Figure 7). The supports 25 are only for compression, as the support 26 is formed to take both tension and compression. The joints are sloping slightly so that the buckets 18 can be lifted without problems. Utility shaft is also fastened (at its upper end (el. +280) with fastening means 34. For this purpose a conventional connection, e.g. with bolts, can be used.

On the top of the cells 13 there are also placed some auxiliary cells 28, called minicells. These are placed on a circular foundation 29, with a sloped joint 30, corresponding to the joint 24. The minicells are terminated upwards by a dome 31.

All auxiliary cells should have a ballast system for seawater. They may also have a system for compressed air. They may be partly filled with a formed material, e.g. styrofoam.

All auxiliary cells are cast of concrete as a part of the-platform, using slipforming etc. Until removal these cells can be considered as a part of the platform. Using this fabrication technique one is guaranteed adaptation in the joint and the auxiliary cells will also contribute to give stability in possible damaged condition during fabrication.

The auxiliary cells are partly kept in place by their own weight, but will. usually have a net uplift. The volume 32 between bucket cell and foundation, and the minicells, are filled with air under atmospheric pressure. Thereby, the uplift will not act on the auxiliary cells themselves, but on the underlying foundation. The auxiliary cells will therefore not be lifted, but rather "sucked" to the foundation. One will have the same effect even if pressure is higher than the atmospheric, if it is just lower than the surrounding water pressure. Further, it is of no importance wether the volumes mentioned are filled with air or water, if just the pressure inside the joint is kept sufficient low.

The use of the auxiliary cells mentioned in the exemplified embodiment shall be discussed step by step.

1. The platform with auxiliary cells are fabricated as usual for concrete platforms. The final works are carried out with the waterline (WL) at el. +280.

2. The platform is lowered to a freeboard of 6 m.

(WL el. + 362). During this operatin the utility shaft 23 is closed and there is connection between utility shaft 23 and the platforms utility shaft 33. The bucket cells 18 and possibly also the minicells 28 are filled with compressed air to reduce the load on the walls.

3. The deck structure is floated over the platform and fastened in the usual way.

4. The platform is deballasted to WL el. +210.

5. The platform is towed to the field atWL. + 210 (draft 220 as the skirts are 10 m high). When the shallow part of the tow-out route is passed, the WL may be increased to e.g. +240.

6. Upon arrival at the field, the platform is lowered to WL. +280.

7. The bucket cells 18 are filled with water to zero uplift and with the volume 32 open to the sea. The fastening means 34 are disconnected (above water).

Thereafter the bucket cells are deballasted until they are floating, they are lifted some meters and towed away. If the bucket cells 18 are sticking to the base, an over-pressure is introduced in the volume 32 and the cells are disconnected by hydraulic pressure.

Jacks could also be used. After this operation, the platform with the minicells is self-floating. WL = 280.

8. The platform is lowered to the sea bed, the skirts 17 penetrate the sea bed and lowering continues until the bottom slab is resting on the sea bed.

9. The minicells 28 are filled with water, loosened, lifted to sea surface f.ex. with a crane, and removed. If partly filled with foamed material, the submerged weight can be chosen freely. If the minicells are sticking to the foundation, they can be detached by means of similar means as for the buckets cells.

One will realize that the removal of the auxiliary cells can be carried out without use of divers.

Some details in the exemplified embodiment will be discussed.

The piles are steel tubes of aprox. 2 m in diameter and having a wall thickness of 50 mm. The piling will usually be carried out as under water piling, but if the legs 3 are extended almost to the surface, the piling could also be carried out from above water in the usual way.

Conductors, risers and J-tubes can be placed in the annulus between the centercolumn wall and the utility shaft 33. Alternatively they can be placed outside the centercolumn and possibly alongside the legs.

The platform could be used as oil storage. In particular the legs 3 could be utilized for storing, but also the centercolumn 4 and the base section could be used. Another possibility is to extend the foundation 19 and have the joint 24 e.g. at el. +100. The foundation could thereby be formed as a long, hollow tube and be utilized as oil storage.

The inside of the structure, whether used as oil storage or not, could be arranged to have an underpressure to keep the concrete permanently in compressed state. The oil storage could also have an overpressureto simplify the oil ballasting system.

This solution would create tension in the concrete structure and necesCitate prestressing.

It is obvious that the exemplified embodiment discussed is merely used to describe principles. In practice, figures and details may vary considerably according to the conditions of each separate operational case.

Some modifications to the exemplified embodiment shall be discussed.

The number of legs 3 could be two or more. A possible modification could be one vertical piled column and two slanting legs horizontally spaced got. Further the number of columns could be any figure. From the riegel and downwards there does not have to be any column.

The piles may be of concrete. One can fabricate the pile ashore, e.g. floating in verical position, by means of slipforming technique. Then the pile can be transported horizontally to the field and lowered through openings in the footings as for steel piles.

The pile could be constructed as an open tube, 2-4 m in diameter and with wall thickness 10-30 cm.

The bucket- and minicells mentioned do not have to be used together. Each type can be used independently. The fastening and detaching means and procedures can be used in any combination for all types of auxiliary cells.

One variation is to use only bucket cells 18, but to extend the utility shaft 23 to above sea surface (el.

340). In this case the bucket cells could be detached after installation of the main structure.

All cylinders could be made somewhat conical, preferably within the practial limits of slipforming technique. Currently this means a maximum slope approx. 6:1 relative to vertical.

Parts of the structure, preferably the legs 3, could be permanently air filled to reduce weight, and thereby the number of piles.

The riegel 12 could be placed above the sea surface 8. In this case the centercolumn 4 could be omitted. The piling could be carried out alongside the legs 3.

If the buckets 18 are removed after installation of the main structure, the utility shaft expansion 23 may be very long. In the exemplified embodiment it will be approx. 130 m. (Figure 10). With a diameter of e.g. 3m, it should be supported by a particular support structure 35 which forms a part of the bucket structure. Figure 10 shows a support structure consisting of three sloping columns 36, each approx.

2 m in diameter. The support structure may also be a steel truss.

Claims (5)

1. A structure to be installed on the sea bed, comprising a base section, a plurality of slanting legs and preferably one or more vertical columns, which legs and columns are joined together in a riegel.

2. A structure according to claim 1, where the base section is fastened with piles.

3. A structure according to claim 1, where the riegel is comprising a central cell, and a plurality of periphery cells.

4. A structure according to claim 1, to be floated to the field in vertical position, using auxiliary cells 18or28.

5. A structure to be installed on the sea bed, substantially as described with reference to the drawings.