During operation, it was found that high levels of vibration were noted at the location of both forward and rear generators. The problem was identified as a first order vibration imbalance which excited a natural frequency of the frame with a mode shape which caused both ends of the frame to vibrate opposite to the engine and at increased levels.

At the location of the diesel a vibration level of approximately 20mm/s was measured, a very acceptable level which indicated their was nothing wrong with the engines. At the location of the generators, the effect of resonance increased the levels to 25-30mm/s at both ends but most severe at the front where the generator was mounted on a small subframe.

Although not excessive, the generator manufacturer considered these levels unacceptable.

The parties involved had been trying to lower the vibration levels for some time, experimenting with balancing, couplings and elastic supports. When Techno Fysica was brought in, it took the approach of creating a finite element model of the frame, including the weights and inertias of diesel and generators. This model was then tuned to match the actual situation by using the data from the vibration measurements. With this tuned finite element model the effects of modifications were evaluated, including a forced response calculation with input data based on the measured level of vibration. The need for more trial and error was thus eliminated.

The calculation indicated the effect of modifications, which consisted mainly of additional stiffness at critical locations.

The predicted decrease in the levels of vibration by over 50% was confirmed by overall vibration measurements which were performed after the modifications had been made.

MJ Information No: 18909