New offshore structures have recently been suggested for recovering hydrocarbons from marine sediments lying beneath very deep water. One such offshore structure is a rigidly based concrete platform consisting of one or more legs for supporting the deck structure. The environmental forces are transmitted to the sea floor through a strong bottom structure, which is some embodiments have deep skirts. It is characteristic for this type of foundation that the platform obtains a first oscillating period which is substantially shorter than the wave periods exciting the structure. The dynamic response caused by the environmental forces will therefore always be higher than the result of the static forces. In deep waters this type of structure will necessitate gigantic dimensions in order to withstand the environmental forces.
Existing and known structural solutions for concrete platforms fixed to the bottom entail making a large bottom structure with a plurality of cells, with or without deep skirts to be forced into the bottom of the sea in order to obtain the stiffest possible fixation. Alternatively it has been suggested to pile this type of structure to the bottom in order to increase the fixation and prevent foundation settlements. Calculations show that this may be obtained by means of a large number of piles, but that it would require a long installation time, which could be a potential problem if the weather window is short during this operation. It is commonly known among platform designers that this platform type becomes uneconomical at large ocean depths. Compliant steel platforms have recently been developed in order to alleviate the problems of the rigidly based structures, and the tendency is that this results in less costly solutions.
The present invention consists in using a compliant pile system for supporting a concrete structure so that the structure becomes compliant with respect to the wave forces, contrary to existing concrete platforms. The pile system is generally described in U.S. Pat. No. 4,378,179 issued Mar. 29, 1983, but as a foundation system for a guyed tower. For a compliant concrete platform the pile sleeve guides and the pile sleeves will be attached directly to the concrete shell. The main attachments of the pile sleeves to the concrete shell may be anywhere along the column and must be adapted in each particular case.
The advantages of using said invention is primarily low structural weight and short fabrication time, which has been made possible by the use of the compliant pile system. The total forces that the foundation system must absorb are substantially reduced as compared to a rigidly fixed platform.