Fig. 9 Averaged autospectra. a: propeller accelerations (Ax = axial, Ay = transversal, Az = vertical), b: slewing bearing accelerations, c: relative shaft displacements at propeller bearing (St = transversal, Sv = vertical), d: torque of the thruster body around steering axis (T_u) and hydraulic pressure of the steering system (Pr1_Pr2) Operating Deflection Shapes (ODS) at frequencies of most significant peaks in the averaged spectra were determined. It was found that these modes resembled natural mode shapes of the thruster. Operational Modal Analysis (OMA) was also carried out in order to identify natural modes of the thruster during operation. Measured accelerations of 6 points were used and PolyMAX algorithm was applied for modal parameter identification [24]. Identified modal parameters are given in Table 2. Natural modes in air obtained from modal testing in dry dock are also presented in the table for comparison. Lowest rotor bending modes were estimated by separate analysis. Before this analysis all measured accelerations were integrated twice and relative shaft displacement signals were summed to absolute bearing displacements in order to get absolute shaft displacements. Modal parameters of rotor bending modes were then estimated by PolyMAX algorithm by using these signals. It should be noted that the used OMA method does not include assumptions of nonlinearities and whirling modes. Therefore, obtained modal parameters of rotor bending modes contain uncertainties. However, it is possible that the natural frequencies obtained may be reasonably reliable. 11
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