Such structures are sometimes referred to as "gravity structures", and they are used to support off-shore working platforms some 20 m above the surface of the water in depths of 200 m to 350 m, for example. The structures are generally used for housing apparatus for exploring and/or exploiting off-shore oil fields. The plan of a typical platform is a square or an equilateral triangle with a side of about 150 m. Under such circumstances, the structure supports it by means of 4 or 3 hollow columns disposed at the corners of the square or triangle. Generally the bottoms of the columns are interconnected by a base unit comprising a plurality of watertight compartments, or "caissons", resting on the sea bed.
Constructing such a structure in concrete is very different from constructing one in metal, since metal members are much lighter. Concrete structures are constructed in the following sequence of operations: prefabrication at a sheltered site, floating, towing to a point vertically above the end site, and finally sinking so that the base comes to rest on the sea floor. The floating and sinking operations are performed by filling and emptying the columns and compartments in the base by means of pumps and valves controlling the flow of compressed air or sea water.
Up to now, large-scale concrete off-shore structures have been built in the vertical position, and kept in that position during the operations of towing to their destination site, initial immersion, and final sinking.
Once the structure is finally resting on the bottom, the working platform is built above the water on the tops of the columns.
The greater the depth at which a gravity structure is to be used, the larger the construction site required by the above-described method for prefabrication, and the more powerful and expensive the lifting equipment needed. The generally held view of specialists in this form of construction is that it is not economically feasible for depths of more than about 200 meters. Great depths lead to the structure being very large and very heavy, e.g.:
displacement mass of 400,000 to 500,000 tons for structures in the 350 m range; and PA1 columns with an outside diameter of 25 m at the bottom levels. PA1 prefabricating a floating unit comprising a compartmented base with hollow columns projecting upwardly therefrom, said floating unit being prefabricated at a sheltered site by the sea; PA1 transporting said floating unit to a final site at sea; and PA1 sinking the unit over its final site by progressively filling the compartments and the columns with water, to cause the base to rest on the sea floor. The improvement involves, in the prefabrication step, constructing a succession of units each constituting one storey of the support structure, the succession including at least a bottom storey and a top storey, and each storey comprising a horizontal polygonal compartmented base with vertical hollow columns of equal height disposed at the corners of the base polygon; and the floating and transporting steps comprise floating each of the storeys at the prefabrication site; PA1 transporting each storey to an assembly site; PA1 tilting each of the storeys at the assembly site by progressively filling compartments in the base to cause the storey to float with two of its columns at the surface of the water; PA1 assembling the storeys with corresponding columns of successive storeys being aligned, and with the base of each upper storey being assembled to the tops of the columns of the next storey down; PA1 transporting the structure thus assembled in the horizontal position from the assembly site to the final site PA1 and tilting the assembled structure to bring it into the vertical before said step of sinking it.
Reference can be made to an article on this subject by Jacques BOSIO appearing in the Apr. 12, 1979 issue of the journal "Petrole Informations", and in particular to FIG. 3 thereof.
The aim of the present invention is to facilitate the construction of a rigid structure for supporting a working platform above the sea in depths of about 200 to about 600 meters, and to make it possible to transport the structure through much shallower waters.