|
|
marinediesels.co.uk
Horror Stories
Piston Skirt
Fatigue Failure
|
****JavaScript based drop down DHTML menu generated by NavStudio. (OpenCube Inc. - http://www.opencube.com)****
|
|
|
|
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
|