The weakening of the steel in a structure due to
constant flexing, under the repeated cycles of stress
may result in structural fatigue failure. The concern
about fatigue failure is that it occurs without any
apparent forewarning (eg deformation of a structure
that results in a crack).
Fig: Fracturing at transverse bulkhead
Fatigue usually begins at
welded joints, notches, discontinuities in structures
and areas of high rigidity in particular. However,
variations in the size, shape and design of each
component and the conditions that the ship operates
mean this may not necessarily result in a structural
failure. Areas where extra vigilant inspection is
- The brackets at the connection of frames to the
upper and lower wing tanks
- the upper and lower connection of corrugated
- corners of the hatch coamings where they are
joined to the main deck.
Bulk carriers in particular become progressively
weaker due to continuous corrosion. In addition, the
repetitive cycles of changing loads and the resulting
stresses due to hogging, sagging, panting, pounding
and vibration all increase fatigue.
High tensile steel
(which is stronger than mild steel) is used in all areas
likely to experience high levels of stress. It means
that scantlings can be reduced but the vessel will still
have higher strength and resistance to stresses, eg
slamming due to heavy pitching that may cause fatigue
on the forward section of the hull.
It is recommended that, as soon as any cracks are
seen, arrangements are made immediately to repair
them. Where possible, a crack arrestor hole should be
drilled at each end of the crack before any temporary
repair is made. If the extent of the crack is not evident,
a detector dye can be used to establish this. As soon
as possible, Class should be called for a survey to
make a permanent repair because a crack that is
overlooked may become a central point for localised
stress resulting in structural failure.
A crack may
also damage protective coatings such as paintwork,
creating an `open' area for corrosion. While cracks
may not initially be apparent, corrosion in any area
should be carefully checked for signs of minor cracks,
particularly if there are dents in the structure.
Corrosion and fatigue will gradually weaken the hull
over time. This can be increased by variations in
loading patterns and particularly heavy density cargoes
such as iron ore.
Another factor that gradually weakens a ship's
structure is the abrasive and corrosive nature of bulk
cargoes such as coal, which can cause unintentional
damage to cargo hold coatings. Areas such as welded
frame joints with tanktop or deck plating are very likely
to develop corrosion and subsequently crack if the
coatings are damaged.
Other factors include:
- Liquefaction of cargoes, caused by water ingress
or moisture in the cargo, can cause cargo shift
during the voyage
- movement of ballast water in partly filled ballast
water tanks or holds can cause damage and
create corrosion. To avoid this, tanks and holds
should be completely filled.