This thesis project takes place in the context of heavy-duty gas turbines production, evolution and upgrades. Given the trends going more and more towards higher output powers in order to satisfy the energy demand, increased efficiencies and lower emissions to deal with the stringent regulations imposed by the Countries concerning environment on the one hand and the necessity to guarantee a machine life as long as possible to amortise its capital cost on the other one, the partial re-design of old generation machines to make them competitive on the market with new generation ones becomes predominant. The focus of the present thesis is centred on a machine, the FIAT TG20 B7/8, whose original design dates back to the Seventies, moreover many turbo-gas plants all over the world still present this machine.