Abstract: The invention relates to a method of assembling and installing a device for exploiting an oil-field. Said device comprises an upper structure comprising two superposed platforms provided with a vertically slidable column, and a lower structure comprising at least two tanks connected to the upper structure by means of support legs. Such a device allows its elements i.e. each of the platforms and the tanks to be fabricated separately, and the assembly to be performed only in floating conditions and not in a dry dock. Furthermore the fabrication can therefore take place in a yard designed for constructions of small size compared with that of the completely assembled device.