A method and a suitable arrangement for installation of a deck structure at an offshore location, where the deck structure is put on a vessel at a location inshore, then transported on the vessel to the offshore location and positioned relative to legs (13) of a jacket or gravity base type support structure standing on the sea bottom, or the legs or columns of a floating substructure, the deck structure having deck legs (5) corresponding to support legs (13) on the support structure, the deck legs (5) each being provided with a jack type of mechanism with an associated piston (1) which is extended into contact with and supported by the top part (3) of the corresponding support leg (13) at the beginning of a procedure for transferring the weight of the deck structure from the vessel to the support legs (13). Said procedure comprises ballasting the vessel (12) while permitting wave induced motions of the vessel (12) to further lift the deck structure with respect to the support structure and permitting the pistons (1) to extend further below the respective deck legs (5) when a higher wave is encountered. The pistons (1) are prevented from moving into the respective deck legs (5) during the weight transfer by mechanically locking the pistons (1) in the legs by means of a one-way ratchet type mechanism.