Abstract: System for development and exploitation of oil and or gas fields offshore, including drilling and the employment of a gravity platform (6). An elevated foundation (2) is established on the seabed (1) for placing of a predrilling template (7) on top (3) of the foundation, preferably a rock-fill foundation. One or several wells is predrilled from at least one cavity of the predrilling template (7), after which a gravity platform is placed on the foundation (2), at least one of the shaft of which platform being downwardly open and arranged for cooperation with the predrilling template (7). After installation of the platform drilling-, production- and living quarter modules (16, 17, 18) are placed on the platform deck (15). Water is pumped out of the shaft (11) and the cavity in the predrilling template (7), in this way providing a dry 1-atmospheric chamber at the base of the platform where operations like connecting wells to conductor pipes etc. may take place.