One of our customers asks us to help with problems with his newly built vessels. The vessels were still on guaranty period, but he had to face serious problems with diesel generators exhaust lines – especially with dampers and mounting elements. The customer complained about the necessity of frequent replacement of damping elements, which were damaged even every few months. Shipbuilder together with the owner decides to ask us for help in that matter, cause actions taken previously by them do not give satisfying results.
Our investigation starts with data collection which allow us to narrow the search field. The mentioned vessels were diesel-electric cruisers which are quite complicated to construction with four diesel (V12) generators of over 10,000 kW each. Due to the fact that our customer suggests problems with them we start with crosscheck of its condition. Vibration measurements were performed for each of generators in different working conditions, but results show no abnormalities according to international standards – that have forced us to look for problems elsewhere. The next step were to focus on the vibration dampers and mounting elements directly. We decide to crosscheck all exhaust line mounting elements and dampers to find those with possible highest readings. A portable data collector was used to spot-check measurements along the lines till the chimney. Based on the results we found that the problem exists only in the first part of the exhaust lines, from the diesel engine to the exhaust manifold.
We decided to crosscheck the condition of dampers in that area by multichannel measurements of its structure, which allow us to establish when dangerous vibration occurs and what is its source. Using a multichannel vibration data collector, we mounted sensors on each vibration damper – from both sides (engine exhaust line side and structure side). Those type of measurements allow us do not only confirm vibration level but also the damping effect of each element.
As for Diesel Generators, measurements were performed in a wide range of loads – that’s allowed us to establish which load generates the most vibrations and what their spectrum looks like. Our goal was received. We found that on some of the dampers, vibration levels were very high – over 2 times the DNV limit for cracks development in steel (45 mm/s, RMS of Velocity 4-200Hz), that’s clearly shows why dampers are damaged so frequently. The next step was to find the source of those vibrations. Detailed spectrum analysis shows that the main sources of high vibrations are frequencies 20.8Hz, 25Hz and 29.2Hz. Those frequencies are related to a few things –first, with diesel engine working conditions (they were found in measurements performed on DGs). Results were presented and discussed with a shipbuilder representative, and we found that those frequencies could be related to exhaust gas pulsation. But still was not clear why they produce high vibration, we started to suspect some resonances at those frequencies. We decide to perform the additional type of measurement – natural frequency measurements of exhaust lines in the area of interest.
That part of the work was discussed in very detail with shipbuilders – we have to cooperate to perform the best possible measurements which allow us to confirm the natural frequencies of the exhaust elements. Based on information obtained from them and our experience we performed natural frequency measurements (hammering test).
For the hammering test, we must use the modal hammers and multichannel data collector – it was very important to get results from each point at the same time, from the same hit. Thanks to that we received information on how structure responds to impacts in different directions. Hammering was performed in 3 main directions: UP-DOWN, PS-STBD and FW-AFT.
Data were recorded and delivered to the office, where special software was used to present results as vibration magnitude and phase. That type of results presentation allows us to clearly identify if frequencies of interest are excited by hammering in different directions. As were suspected we found them in collected data, each of them exists in some cases depending on the hammering position, we record for them both peaks in vibration magnitude and phase jumps. As a result, our customer received detailed reports with data presented in an easy-to-read way. Shipbuilders used that information to propose solutions.
Meantime we decide to do one more test – vibration measurements of one of the exhaust lines with mounted additional weight of 300kg. Such action (adding the weight) should allow us to separate natural frequencies from exhaust gas pulsation frequencies and reduce the vibration level on dampers. We choose one of the lines with the highest level of vibrations and performed the test at the same working conditions as previously. Performed action gave very good results – vibration level was reduced from 84 mm/s to 29 mm/s (DNV limit: 45 mm/s, RMS of Velocity 4-200Hz). Thanks to performed measurements we support our customer and shipbuilders in solving problems with premature damper failure.
The problem was found and solutions were properly indicated – all related parties were satisfied.