Method for operating a gas turbine plant with reduced NOx emissions

The gas turbine plant (1) comprises a compressor (2), a first combustion chamber (6) followed by a high pressure turbine (4), a second combustion chamber (10) and a low pressure turbine (5). In the first combustion chamber (6) a first gaseous fuel (8) is injected and combusted generating first hot gases (9) that are at least partly expanded in the high pressure turbine (4), and in the second combustion chamber (10) a second fuel (11) is injected in the at least partly expanded hot gases generated in the first combustion chamber (6) and combusted, generating second hot gases (12) that are expanded in the low pressure turbine (5). In the first combustion chamber (6) also water is injected to increase the gas turbine plant power output. In the first combustion chamber (6) a water flow rate is injected compatible with a prefixed flame stability limit. In the second combustion chamber (6) the flame temperature is regulated to burn the non-combusted products generated in the first combustion chamber (6) to bring their amount below a prefixed emission limit, without exceeding a prefixed NO x emission limit.