Techno cases
Techno Fysica BV is a Dutch-based engineering company, highly specialised in solving all kinds of problems related to the dynamic loading of machinery and installations. For this, several measuring and analysis techniques are used. In a series of articles, we will give examples of what can go wrong in the engineering and operating of vessels, and what errors lie at the root of the damages observed.
On a RoRo (Roll on, Roll off) ferry, damage occurred to the bearings of intermediate shafts of both propulsion installations, each consisting of two 12500 kW engines on a common gearbox, driving CP propellers.
The damage consisted of wiping of the bearing material and leakage of oil from the bearings.
In order to solve the problem, a series of vibration measurements at trial conditions were conducted. The measurement program consisted of torsional- and lateral vibration measurements, in order to describe the shaft behaviour of the location of the bearings, given an indication of bearing load and of the position of the shaft relative to the bearings. These measurements revealed that near the nominal (constant) speed of the installation, a natural frequency of the shaft line and it’s bearings was exited, which lead to high vibration levels due to lateral vibrations of the shaft. The vibration mode of concern was dominated by the low stiffness of the bearing pedestals. At these locations, large translations in the horizontal direction were measured, indicating that the bearing supports deflected at these conditions. Especially pedestal #2 was weak due to the fact that big holes had been cut in the pedestal to allow for the routing of pipes.
A finite element model was set up to be able to visualize and gain control of the problem, and the model was fine-tuned to match the measured vibration modes. The results of this finite element calculation and the resulting modal shapes of the deflected shaft line indicated that the frequencies and the mode shape were dominated by the low stiffness of the bearing supports.
Since a tuned and matching model was available, it was relatively easy to predict the effect of modifications. The advantage of combining measurements and calculations for problems like these, is the accuracy with which the effect of modifications can be predicted. In this case, the solution was aimed at restoring the original stiffness of the bearing pedestals, without conflicting with existing pipes. Although the possibilities were limited, it was possible to engineer stiffeners that increased the lateral stiffness enough to shift the natural frequency outside the area of excitation frequencies and thus prevent resonance, which caused the problem in the first place.
This could be accomplished, and, after performing the modifications, final measurements confirmed the positive effect of the modifications.
Operational Deflection Mode shape of the natural frequency of 24.3 Hz in the original configuration.
