Method for operating a gas turbine and control device

Method for operating a gas turbine (10), which is operated with a gaseous fuel (A) consisting of a gaseous mass flow ( m gas ) and/or with an oil fuel (B) consisting of an oil mass flow ( m oil ), wherein a change between an operating mode with gaseous fuel (A) and an operating mode with oil fuel (B) is undertaken during load operation of the gas turbine (10), wherein a water addition (11) of a water mass flow ( m H2O ) is provided at least in the operating mode with oil fuel (B), and wherein the ratio (© norm ) of the added water mass flow ( m H2O ) to the fuel mass flow during the change between operating modes is determined according to