Page 41 - Practical Design Ships and Floating Structures

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0 KRISTAL - a 27 year old product tanker that sunk due to cargo overloading.

0 BALU - a 24 year old chemical tanker that sunk without a known cause.
Many of these recent casualties were old tankers, and there is now a renewed concern about the
structural safety of aging tankers.
5.1 Aftermath -More Tanker Regulations?
With so many structural failures in older ships, the impact on the call for regulatory change is as great,
if not greater, than that which occurred after the AMOCO CADIZ grounding in 1977. Now ten years
after the passage of OPA-90 in the U.S., IMO is now considering accelerating the phase out scheme of
single hull tankers to be more in line with OPA-90 in the U.S.

6 BULK CARRIER SAFETY

Whereas the tanker safety problems have been largely focused on the prevention of accidental oil
pollution to the marine environment, the safety problems being experienced by bulk carriers has
focused on the large number of seafarer lives being lost.

There have been numerous casualties in the late 1980’s and continuing to the present. What the
casualty statistics tell us is that bulk carriers are at higher risk as they get older. Single side skin bulk
carriers carrying high density cargoes such as iron ore are at a higher risk than when carrying other
lower density cargoes, and many of the casualties were related to flooding or structural failures in No.
1 hold.

During the period fiom 1983 to 1998,48 bulk carriers were lost with many seafarer lives also lost. A
possible loss scenario is that water enters a cargo hold as a result of fractures in the side shell, or loss
of side shell plating in the forward hold in heavy weather. Seawater fills the hold causing the
transverse bulkhead to fail with progressive flooding of an adjacent hold, resulting in the sinking of the
ship. Under this scenario the bulk carrier losses can be attributed to an initial failure of the primary
watertight barrier, such as shell plating or hatch cover - followed by failure of the secondary watertight
barrier, such as a transverse bulkhead. A key issue regarding the bulk carrier casualties has been the
effect of cargo hold flooding on the structural survivability of these ships, which has not been covered
by existing rules and regulations.
Why many bulk carriers suffer damage to the side shell plating can be explained by considering the
stiffness of its cross section. The lower portion of a bulk carrier’s cross section consists of a double
bottom and hopper tanks that are very rigid and stiff. The upper portion is also very stiff due to the
upper wing ballast tanks. Both upper and lower portions are connected by the side frames, which are
the weakest links in the structure. When carrying heavy iron ore, the bottom deflects causing the side
frames to deform inboard. The external action of sea pressure and waves causes further deformation
of the frames inboard. In addition if there is ballast water in the upper wing tanks, there is further
bending and compressive loads on the frames. The upper and lower ends of the side h e
attachments to the topside and hopper ballast tanks are therefore very critical areas affecting bulk
carrier safety.