On my new container vessel, crew noted strange vibration on piping system in engine room area, even if ME is stopped. Any advice?

In May 2015 we received similar question from our Client. It was decided to send two of our service engineers to carry out an investigation using multichannel global hull vibration measurements.  

Our service engineers made a visual inspection of engine room area, looking for local resonances and to choose the best available measuring points.  Basing on our experience, the points were chosen as follows: stern tube bearing, middle bearing, AFT ship centerline, steering gear room, main engine and wheelhouse wings.

During visual inspection there were observed some local resonances in ER. The highest one was observed on STBD side of the ER, below generators level. What was interesting, high vibrations of the FW piping system and pumps occur even when ME was not running, and even when FW pumps were stopped. Basis on these observations, following points were chosen: one of FW cooling pump suction pipe support, purifier room and FW cooling pipe in ME area. Local investigation and detailed measurements show that dominant frequency of the vibrations in this area is 12Hz, which correspond to generators running speed which is 720 RPM (12Hz).

 

Above are presented results for one of local resonance points. As shown, when ME is running in area of 60 RPM, the influence of ME vibration order harmonic and blade vibrations order harmonic are interfering with generators running speed and local natural frequencies causing excessive vibrations. Such high results for vibration velocity (up to 50 mm/s) suggest that further exploitation of this system in such conditions will lead to fatigue of the material (pipe, connecting bolts) and failure of the system.

After collecting the data and pre-analysis, our service engineers decided to investigate case of high vibrations generated by DG. Using 2-channels vibration measurements, resilient mounts of DG were checked and results showed that there is damping with very good results but main forcing frequency is not dumped in the same relation. Additionally there was noted that support frame of DG does not cover all length of Alternator, but only 50% of it.

 

After final analysis, we decided to present following recommendations to prevent future failures:

  • Installation of supports for FW cooling piping system in ME area and FW central cooling pumps
  • Installation of supports for FW central cooling pumps
  • Installation of the pipe vibration compensators at their connecting flanges
  • Checking with project requirements for the static and dynamic load on the DG floor and/or improving its support
  • Checking the vibration resilient mounts of the DGs
  • Consideration of replacement vibrations resilient mounts with better dumping in lower frequency range
  • All support changing should be recalculated with proper analyze for good results
  • Consideration of extending the support frame for generator to full-length of alternator
  • It was noticed that two AFT ME Top bracings are misaligned. For better damping properties it should be corrected.

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