The paper presents some findings of numerical investigations of torsional vibrations in steam turbine shafting in the turbogenerator abnormal operation modes (short circuit, random paralleling start, etc.). The shafting vibrations are shown to be essentially dependant on the reaction torque surge parameters (magnitude, duration, and form). For three types of the reaction torque surge a comparative hazard analysis is performed from the standpoint of initiation of cyclic damage in the rotor material and the risk of shafting failure.