1. A method for operating a wind energy installation having a tower, a nacelle arranged on the tower, and a rotor having at least one rotor blade, wherein an azimuth of the nacelle is adjustable, wherein a blade angle of the at least one rotor blade is adjustable, the method comprising: detecting and monitoring tower oscillations during operation of the wind energy installation using at least one measuring apparatus; and switching off power operation of the wind energy installation if a measurement variable of the tower oscillations exceeds a tower oscillation limit value; wherein the tower oscillation limit value is defined, at least during load operation of the wind energy installation, as at least one limit value function which is dependent on prevailing wind speed and/or a parameter associated therewith, and which has different functional dependencies in a plurality of different value ranges of prevailing wind speed and/or the parameter associated therewith.