A vessel has been proceeding at full sea speed for several days without any problems. One evening, the duty engineer made his routine inspection of the engine room, which had unmanned machinery space classification. He noticed that the on-line lube oil filter was showing a high pressure differential, so he switched over to the standby filter using the cock valve (1). After leaving the engine room and returning to his cabin, the main engine suffered catastrophic failure with full seizure of the crankshaft (2).
Brookes Bell was called in to investigate.
Photographs taken of the filter housing showed the in-coming filter housing had undisturbed paint coating on the vent plug (3). Both filter housing contents were taken and sent to the lab for forensic examination of the oil and filter medium. From the results, we established that no bearing failure debris existed in the outgoing filter, but there was considerable debris in the incoming filter.
The bearings (4) were examined carefully. Damage was seen to be widespread, in particular the large ends. However, there was seen to be no lack of oil in the engine and no low oil pressure alarms had activated – the alarms were tested and found to be working normally.
Can you work out the reason for the seizure? How would you continue the investigation?
Reveal the answer here
We hypothesised that something must have occurred associated with the switching of the outgoing filter to the incoming filter, since the engine had been running perfectly prior to this. Clearly it was something the duty engineer had done, or not done. But what?
Finding debris only in the incoming filter and none in the outgoing meant there was no evidence of a failure mechanism before the filters were changed over. This strongly supported our hypothesis that it was something associated with the filter changeover.
The undisturbed paint on the vent plug was a key clue. If the paint was undisturbed, then the venting of the incoming filter prior to switchover was hardly likely to have been conducted. In fact, the duty engineer later admitted he had neglected to do this.
Not venting the filter meant an aerated oil supply had entered the engine, which caused the thin oil film in the large end bearings to collapse under the extreme firing loads. The widespread damage suggested collapse of the oil film, consistent with oil starvation rather than contamination by foreign debris.
One final piece of evidence further supported the hypothesis. The low oil pressure alarms had not activated because the entrained air from the filter would have formed bubbles in the oil, and these would have been at the same pressure as the oil supply. The air bubbles would have caused the oil film to become 'compressible' when under firing loads, allowing metal on metal contact (1, 2 and 3).
