Floating offshore platforms consisting of pontoons, supporting columns and decks, are known in a number of different embodiments and are used both with slack and tight anchoring. It is known to use both separate, longitudinally extending"catama- ran"pontoons and ring pontoons with different types of deck structures. As an example of a platform of the above-mentioned type, reference can, e. g., be made to NO 160 249.

With the existing concepts the transfer of forces between the different parts of the platform will often be complicated and give rise to large stress variations. Further, with the known solutions, it may be difficult to adapt the rigidity relations between the individual platform members, and the members get a complicated design with respect to internal stiffening, etc.

It is a main objective of the invention to provide a floating offshore platform of the stated type having a construc- tion resulting in an advantageous transfer of forces between the different parts of the platform.

A simultaneous object is to provide such a platform giving an advantageous distribution of rigidity in general, and between pontoon and deck structure in particular, to achieve thereby a more optimized construction and more easily avoid fatigue problems.

Another object of the invention is to provide a particularly redundant platform structure which thereby tolerates large damages on the supporting structure without collapsing.

An additional object of the invention is to provide a platform structure of the relevant type wherein the main supporting elements are situated in the outer part of the platform members, so that a flexible space utilization of the members of the platform is obtained.

For achievement of the above-mentioned objects there is

provided an offshore platform of the introductorily stated type which, according to the invention, is characterized in that the deck has main supporting elements including an annular box structure having a shape at least partly corresponding to that of the pontoon and having side walls constituting main supporting elements, and that the columns at their upper and lower ends have side walls directly connected to side walls belonging together of the box structure of the deck and the pontoon, respectively, so that the main supporting system of the platform is constituted by the side walls of the pontoon, the corresponding side walls of the columns and the main supporting elements of the deck.

An advantageous embodiment of the platform is charac- terized in that the directly connected side walls of the pontoon, the columns and the box structure lie in the same vertical plane.

Further, it is advantageous that the pontoon and the box structure of the deck are square in plan view, and that a column is located at each corner of the pontoon, so that the outer circumference of the pontoon and the box structure in the region of the columns is aligned with the outer side walls of the columns.

Since the main supporting system of the platform is constituted by the side walls of the pontoon, the corresponding side walls of the columns and the main supporting elements of the deck which are arranged in the stated manner in connection with the columns, there is obtained a favourable transfer of acting forces between the different platform parts, and a favourable distribution of the rigidity of the structure, so that an optimized construction can be obtained and fatigue problems more easily can be avoided. Since the main supporting elements in all essentials are situated in the outer parts of the platform, there is also obtained a possibility for maximum area utilization on the deck for a given column spacing.

The platform construction further results in a redun- dant structure, so that the supporting structure tolerates large damages without collapsing. This is a substantial advantage in such an offshore structure wherein loads of the type fire, explosions and ship shocks will be relevant.

The invention will be further described below in connection with an exemplary embodiment with reference to the

drawings, wherein Fig. 1 shows a side view of a platform according to the invention, shown with relevant installations and equipment placed on the platform deck; Fig. 2 shows a side view of the platform, as viewed in the direction of the arrow A1 i Fig. 1, on a reduced scale and without installations on the platform deck; Fig. 3 shows a side view of the platform, as viewed in the direction of the arrow A2 in Fig. 2; Fig. 4 shows a plan view of the top of the pontoon; Fig. 5 shows a plan view of the deck of the platform; Fig. 6 shows a typical cross-section of a pontoon member; Fig. 7 shows a view of a watertight bulkhead in the pontoon, with an opening for an excess tunnel; Fig. 8 shows a view of a transverse frame in the pontoon; Fig. 9 shows a partly sectioned side view of a column as it appears to the left in Figs. 2 and 3; Fig. 10 shows a typical cross-section of the column; Fig. 11 shows a cross-section along the line XI-XI in Fig. 9, in a region provided with a cofferdam; Fig. 12 shows a section along the line XII-XII in Fig.

11; and Fig. 13 shows a section along the line XIII-XIII in Fig. 11.

The platform 1 shown in the drawings is an offshore platform of the semi-submersible type, and consists of an annular pontoon 2, four supporting columns 3 and a deck structure 4. The pontoon and the columns may be regarded as a lower hull (LH) and the deck structure as an upper hull (UH) of the platform.

As shown in Fig. 1, different installations and equipment which are necessary on offshore platforms of the relevant type, are placed on the upper deck or main deck of the platform. These installations do not constitute any part of the invention, and will therefore not be further described. It may only be mentioned that, at one end of the platform deck (the right end in Fig. 1), there is arranged a balcony 5 to provide a space for risers 6, manifolds and some process equipment. At

the opposite side of the platform there is, inter alia, shown to be arranged an accomodation quarter 7 for the crew, a helicopter deck 8 and lifeboats 9 (only one is visible in the figure).

As appears from Figs. 2 and 3, the upper hull (UH) comprises a main deck 10 and a lower deck 11. The lower deck is built as a double bottom hull which has a planar underside and is integrated into internal parts of the main supporting elements of the deck structure 4. Such a double bottom is favourable since it minimizes operational and maintenance costs.

As appears from Fig. 4, the annular pontoon 2 in the illustrated embodiment is square, and more specifically quadra- tic, and a supporting column 3 is placed in each corner at the intersection between the four pontoon members 13. The pontoon members has a rectangular cross-section without rounded corners, as shown in Fig. 6, something which results in easy production.

The supporting columns or corner columns 3 have a square, more specifically quadratic cross-section with rounded corners, as shown in Figs. 10 and 11. Both the pontoon and the columns are constructed from stiffened steel plates, and such steel plates form outer and inner side walls 14 and 15, respec- tively, of the pontoon members 13 and side walls 16 of the columns 3.

The load-carrying main element of the shown embodiment of the deck structure 4 is an annular box structure 17 having a shape corresponding to that of the pontoon 2, and thus conse- quently is quadratic, and having side walls 18,19 constituting main supporting elements in the deck. Thus, the deck comprises eight main girders.

In accordance with the fundamental principle of the invention, the side walls 16 of the columns 3 at the upper and lower ends of the columns are directly or integrally connected respectively to the main supporting elements of the deck 4, in the illustrated embodiment accordingly to the side walls 18,19 of the box structure, and to the side walls 14,15 of the pontoon 2. Further, in the illustrated embodiment, directly connected side walls belonging together of the pontoon, the columns and the box structure lie in the same vertical plane, to achieve thereby a particularly favourable transfer of forces between the platform members. Further, in the region of the columns, the outer

circumference of the pontoon and the box structure is aligned with the outer side walls of the columns.

The annular box structure in its inner region supports longitudinally and transversely extending bulkheads 20 and 21, respectively, and the deck girders in this region are arranged as a grid system. The side walls 18,19 of the box structure consist of stiffened plate panels, except that an outer side wall 18 in the region of the aforementioned process equipment consists of a lattice structure 22, as shown in Fig. 2. The lattice structure allows a natural ventilation in the process area.

The constructive embodiment of the pontoon members 13 is further shown in Figs. 6-8. As mentioned above, the pontoon members have a rectangular cross-section, the members being constructed from planar steel plates which are stiffened as suggested in Fig. 6 which shows a typical pontoon cross-section.

As appears, there is provided an access tunnel 23 extending along the interior of each pontoon member. A number of transversely extending, watertight bulkheads are arranged at suitable intervals along each pontoon member, to form a corresponding number of watertight compartments 24 (suggested in Fig. 2). Such a watertight bulkhead 25 is shown in Fig. 7. The access tunnels give a direct access to all the watertight compartments in the pontoon members.

Further, between said bulkheads 25, there is arranged a number of transverse frames, preferably at regular intervals.

A typical transverse frame 26 is shown in Fig. 8.

The constructive embodiment of the columns 3 is further shown in Figs. 9-13.

As mentioned above, the columns have a square cross- section with rounded corners, as shown in Figs. 10 and 11. These pass into square cross-sections in the direct connections with the pontoon members 13 and the main supporting elements 18,19 of the deck. As shown in Fig. 4, however, the outer corner portions of the pontoon 2 are rounded, and the rounding of the columns corresponds to the rounding of the pontoon, so that the column cross-section here retains its rounded shape.

As mentioned above, also the columns are constructed from stiffened plate structures, and the columns are provided with horizontal transverse frames 27 having a suitable mutual

distance, and vertical struts spanning between the frames.

Further, the columns are divided into a number of watertight compartments by means of horizontal decks 28 and a cofferdam 29 formed along at least a part or parts of the side walls of the columns inside thereof, as shown in Fig. 12. The cofferdams suitably may extend from the top of the pontoon and up to a distance above the current region for ship shocks. Advantageous- ly, the cofferdam may be load-carrying.

In the columns there are also arranged access shafts 30 leading from the upper hull of the platform down to the pontoon.

Further, there are arranged chain lockers (not shown), elevator shafts (shown at 31), cable shafts and hatch shafts which also lead from the upper hull down to the pontoon.

Even if the columns in the illustrated platform embodiment are placed at the corners of the pontoon, they may possibly be placed at a distance from the corners. When required, there may also by provided more than four columns. In their central region the columns may also have another cross-sectional shape than at the end regions, and for example have a circular cross-section along a part of their length. The pontoon may have rounded corners.