The invention relates to a gas dome structure (1) intended for a sealed and thermally insulating tank. The strucutre comprises: - a sealed pipe (10) equipped with a connecting flange (18) and a lowerlongitudinal end (15) intended to pass through the tank roof; - a cover (19) that is fixed to said connecting flange (18); - a gasket (20) that is compressed between the cover (19) and the connectingflange (18); - a vapour collecting pipe (3, 17) that passes hermetically through a wall of the sealed pipe (10); - the sealed pipe (10) comprising a carrier plate (29) that supports an insulating plug(30) that seals the upper portion of the sealed pipe (10).