A “Toolbox” for Forensic Engineers

Failure of Storage Vessels 249
Dam
Design
Barrel
Design
Figure 7.34 Schematic sections of two contrasting tank designs; at left, a dam
design with increasing wall thickness to resist increasing hydrostatic pressure
from contents and at right, the barrel design of the failed tank.
in thickness from top to base to resist the water pressure, and the same principle
should apply to any fluid reservoir.
By adding hoops, the top band could be redundant, and the lower bands
might not be sufficient to resist the much greater pressures toward the base
of the structure. Such a hypothesis would explain why the failure occurred
in a lower, unreinforced panel. It would not explain why only two welds failed
from pinholes, but the calculation of hoop stress above shows that the singlethickness
panel is having to resist a large hoop stress. Doubling the wall
thickness would halve the applied stress, while having three panels here would
give a stress of only about 1.15 MN m –2 . Even if pinholes occurred in the
weld, failure would be much less likely with such substantial lowering of the
hoop stress. The general conclusion of the stress analysis was that the design
itself was faulty: in order to resist hydrostatic pressure, the tank should have
been designed like a dam wall, rather than like a barrel (Figure 7.34).
But why should just two of the four welds have shown cracks? The answer
to the problem came when the welding stage was inspected directly. The hoop
of panels for such tanks is made sequentially by hot fusion welding, that is,
by melting the surfaces of two panels and pushing them together. This is fine
for three of the welds where the flat panels are joined, but difficult for the
final joint when the ends have to be brought together by bending the sheet
into a cylinder (Figure 7.35). It would certainly explain the lower quality of
one weld, and the low quality of another weld was probably caused by similar
problems in bringing two large flat sheets together. The quality of such welds
is tested for through-the-thickness holes using a spark tester, a method that
will not detect partial pinholes. One rather disturbing aspect of the process
is that the hoop so formed is under a bending stress, so the outer surface of
the final wall will be in tension. This will of course make failure much more
likely, and is akin to the frozen-in strain problem of the radiator box already