Measurements During Ship Operation Vibrations of azimuthing thruster were measured during normal operation by accelerometers. Accelerations of propeller bearing housing as well as thrust bearing housing were measured by triaxial accelerometers. Vibrations of slewing bearing near the outer race were measured at four locations by uniaxial accelerometers. Shaft displacements relative to the bearing housings were measured by eddy current displacement sensors. Radial displacements at propeller bearing and thrust bearing were measured in two perpendicular directions. In addition axial displacement was measured at propeller bearing. Hydraulic pressure of the steering system was also measured. Several most important operational parameters of azimuthing thrusters and ship were also recorded, for example rotation speeds of the propellers and azimuthing angles. During normal operation in ice conditions the most significant excitation came from the ice block impacts. When the propeller penetrated in the ice, the rotation speed of the propeller varied widely due to ice loads. Therefore the blade frequency and other harmonic excitation frequencies also changed, exciting the natural modes of the thruster. In this driving condition the excitation resembled periodic chirp excitation signal [23]. A typical 28 min long period where the vessel proceeded in ice was selected for the vibration analysis. Spectral map (vibration spectra as a function of time) for transversal acceleration of the propeller bearing is presented in Fig. 8. Fluctuation of the blade frequency component can be detected in the figure. The rotation speed varied between about 0.5…1 times the nominal speed. The first transversal mode of the azimuthing thruster at about 0.7 fi/fref can be can be seen in the map. Averaged autospectra are presented in Fig. 9. The number of averages was about 1700. On the averaging process the rotational speed related vibration components were averaged out due to relative high fluctuation of rotation speed. Most of the significant peaks in the averaged autospectra are natural frequencies of the thruster. Fig. 8 Spectral map of transversal acceleration of propeller bearing. 10
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