Abstract: The present invention concerns a realization procedure of a modular system particularly suitable for use off coasts, this system comprising a set of cylinders (A.sub.1, A.sub.2, A.sub.3 . . . ) rigidly linked together, this set including at least one central cylinder (A.sub.1) and a plurality of peripheral cylinders (A.sub.2, A.sub.3 . . . ).This procedure includes the following successive stages:the assembling together at the quay (8) of at least two peripheral first cylinders (A.sub.4 and A.sub.5),a joining of the central cylinder (A.sub.1) to these two peripheral cylinders (A.sub.4 and A.sub.5),the placing in water of the unit constituted by the said central cylinder (A.sub.1) and the said peripheral first cylinders (A.sub.4 and A.sub.5) and, the said unit being placed in the water, the joining of any remaining peripheral cylinders (A.sub.2, A.sub.7, A.sub.8) to the said unit (A.sub.1, A.sub.4, A.sub.5).The invention can be advantageously used for the realization of a petrol production platform off coasts.