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Marine Loss Prevention Circular No

 

Marine Loss Prevention Circular No. 02-00

SHIP TYPE:

Panamax Bulk Carrier

YEAR OF BUILD:

1981

   

EVENT: Main Engine Medium Speed Crankshaft Damage

Course of Events
We have recently experienced two severe consequential damages to main engine crankshafts after failure to big end connecting rod bearings. The sequences of events have, in both cases, been the same. Alarms were given for the oil mist detector fitted in the crankcase indicating over heating of internal parts. The engine was slowed down and continued running on reduced load for a period between 20 to 45 minutes before it was finally stopped. The crankcase was then opened for inspection of crankshaft and bearings.

Clear evidence was found of a damaged big end connecting rod bearing that had rotated inside the connecting rod. In order to reach a safe port the piston with connecting rod was dismantled and cylinder blanked off. Preparation was made to grind the crankpin in-situ to remove the scores and to fit an undersize connecting rod bearing.

Extent of Damage
The crankpin in both cases was found with considerable scores and small cracks, but these could have been removed by in-situ grinding of the crankpin. Serious damage was discovered when testing the Brinell hardness of the crankpin. The long period with running the engine with a damaged big end connecting rod bearing had transferred such amount of heat to the crankpin that the steel had hardened to an unacceptable level. To fully restore the crankpin, the diameter had to be reduced by more than 8 mm. Consequently, the engine would have to be de-rated.

This was not acceptable for the shipowner. The result in both cases was a complete dismantling of the engine and fitting of new crankshaft.

The approximate costs for each of the repairs were about USD 1,800,000. and total off-hire days excluding slow steaming or towing to repair yard, about 2 months depending on delivery time for new crankshaft.

In comparison, had the main engine been stopped after the alarms indicated the overheating, the damages could have been limited. The total costs for a normal in-situ grinding of the crankpin including new undersize connecting rod bearing and other spares would have been in the range of USD $50,000.

Cause of Damage
It has not been possible to establish the cause for the breakdown of the big end connecting rod bearings, but it is most likely to be related to the cleanliness and/or supply of lubricating oil. Small scores could be seen on other main journals and crankpins.

Lessons to be Learned

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When overheating is detected and/or the oil mist detector alarm is sounding, stop the engine immediately if possible.

–

The engine should not be restarted before the reason for the overheating is identified and corrected.

–

Keep the lubricating oil as clean as possible using continuously the lubricating oils purifier at the maximum recommended temperature (normally above 90 deg. C.). Maintain the lubricating oil filters in clean and proper condition by frequent daily routines or as necessary.