The invention concerns a turbine structure for the expansion of gas and vapour, comprising a body or casing with a peripheral work fluid transit volute from an entrance to an exit passage, at least a first stator and possible subsequent stators, a turbine shaft rotating around an axis and carrying at least a first rotor and possible subsequent rotors working together downstream of the first stator and subsequent rotors, respectively, an external tube member jutting out frontally from said body or casing and coaxial to the rotating turbine shaft, and where between the tube member (18) and the turbine shaft (15) is positioned a supporting unit (19) extractable en bloc from said external tube (18) except for the shaft.