This paper shows the influence of the method of modelling the support structure, i.e. the casings, support struts and skid, on the rotor dynamics and forced response in a gas turbine structure. Numerical examples, based on the conceptual design of GTX100 in the simple cycle configuration, are given for the blade loss case.

The standard method of analysing the rotor dynamics of a stationary gas- or steam-turbine rotor train, with hydrodynamic bearings, is based on beam theory. The bearings are modelled as a system of linear springs and dampers and are in some cases modelled as if there is no cross-coupling between the bearings. The support structure is normally based on a simple FE-analysis. This method is normally sufficient for the analysis of rotor dynamics characteristics at normal running if the stiffness of the bearings are much lower than the stiffness of the support structure.

In analysing the case of blade loss, the dynamic characteristics of the casing and the support structure have a much stronger influence on the rotor dynamics and the forced response in the structure. At the high unbalance forces present at a blade loss, the stiffness of the bearings will be of the same magnitude as that of the structure.

Results are given and discussed for the analysis of the rotor dynamic response based on coupled 3D-FE models, and on beam theory with the dynamic characteristics of the support structure described by various FE models.

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