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Horror Stories

Piston Skirt Fatigue Failure

 

 

In this unfortunate sequence of events, on a vessel fitted with twin Wartsila Vasa 32 engines, the piston skirt on No1 unit on the port main engine failed due to a fatigue crack which initiated from a casting flaw. The connecting rod then flailed about, smashing the entablature and finally becoming detached at the bottom end. The photograph shows the position it was found in after the incident. Because the main engines and generators were on a central cooling system, the massive loss of water from the smashed entablature caused the generators to shut down on high temperature thus blacking the ship out.

 

Smashed entablature cooling space

 

The Vessels maintenance records indicated that the number one unit on the port main engine had been last overhauled at 53 901 engine hours, a scheduled 24 000 hour service. At the time, a new cylinder liner was fitted in addition to the work normally performed at 12 000 hour intervals. The work included the fitment of a reconditioned piston, cylinder head and new bottom end bolts and bottom end bearing shells. The engine hours at the time of the failure were approximately 61 900, which meant that the reconditioned piston assembly had run for approximately 8 000 hours since the previous overhaul.

 

Pieces of the skirt (right) were collected together and sent for analysis together with remains of the piston, liner, connecting rod, bottom end bolts and bottom end bearing shells.

 

The report noted that sections of the piston where the piston skirt attaches to the gudgeon pin socket via two webs did show evidence of progressive failure, ie fatigue crack growth. Well developed fatigue growth fronts were observed on the web section of one gudgeon socket, while less well defined fatigue crack growth fronts were observed at a similar location in the second gudgeon socket of the piston. In both instances, the cracking was associated with the central region of the two webs at the casting ‘flash’ at the web/gudgeon socket transition.

 

 

port main engine failed catastrophically when the piston assembly fitted to number one unit failed. It is likely that the casting inclusion found by ETRS in the piston skirt initiated the formation of the fatigue crack adjacent to the gudgeon pin socket with the evidence suggesting that the crack had grown over a period of time.

 

 

 

Microscopic examination of the surface around the gudgeon pin socket showed the presence of a large casting defect in the form of entrapped dross/shrinkage. The fatigue crack started from this point, and led to ultimate failure.

 

The bottom end bolts were also examined and it was ascertained that the fracture surfaces on the bolts indicated that they had failed over a relatively short time and the report concluded that the piston fractured first and that during the secondary failures that resulted in bending of the piston rod and damage to the counter weights etc, the out of balance and uneven loading has resulted in the impacting of the nuts, fatigue of one stud and then subsequent failure of the second stud by overload, separating the piston rod from the crankshaft.

 

The failure emphasises the need to carry out careful examination of the piston skirt at engine overhauls, especially around the gudgeaon pin socket. The use of crack detection sprays would be beneficial in these examinations.

 

Information of this event was taken from the Australian Transport Safety Bureau website. The full report can be downloaded here in pdf format

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