Base frame for an offshore platform and method for installing such a base frame

A method for installing a base frame for an offshore platform (1) comprising the steps of:

• building a base frame (2) having a self-floating lower building structure having a stable trim and at least one tank in the lower building structure to allow the lower building structure to sink when the at least one tank is flooded,
• the lower building structure having a horizontal underframe consisting of several frame parts which are connected to each other on corners of the underframe, the lower building structure being configured as buoyancy body which has several hollow chambers and a mechanism for bearing piles which comprises towers that project upward from the lower building structure, each tower having a lower and an upper opening, a passage between the upper and the lower opening, and an upper and a lower bearing for one of the piles, each of the towers having one further tank, each of the towers having closed side walls, a closed bottom wall with a lower opening of the lower bearing therein, a pressure seal in the lower bearing and a cavity in each tower around the pile that is borne therein, so that water cannot enter into the cavity of the into the cavity of the tower through the lower opening, the cavity forming the further tank,
• transporting the base frame to a set-up location on the sea in a self-floating manner,
• flooding the base frame (2) and letting the base frame down to the sea floor (3), and anchoring the base frame (2) on the sea floor (3) by piles (4), with each of the towers projecting upward from the lower building structure, and the further tank acting as a further buoyancy cell such that when the further tank is not filled with water and the lower building structure is let down by flooding the at least one tank of the lower building structure, an unstable condition of the lower building structure is avoided by the further tank when the lower building structure dips into the water.