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SIX large oil tankers, constructed according to a modern design using
high-tensile steel, have been recalled for emergency repairs to cracks by
the Japanese shipbuilder Mitsubishi Heavy Industries. A recent report from
Lloyd’s Register saying that such ships may be more susceptible to metal
fatigue than more traditional designs has raised fears that oil could leak
from them and cause widespread pollution.

The tankers, all with a dead weight of around 250 000 tonnes, were made
in the late 1980s to a design in which stronger high-tensile steel replaced
mild steel. A tanker built of high-tensile steel is about 15 per cent lighter
then one made of mild steel, thus enabling designers to cut the costs of
shipbuilding.

The shipping newspaper Lloyd’s List reported late last month that one
tanker, the Tagawa Maru, has spent four weeks in a dry dock in Dubai undergoing
repairs. The tanker was only built in 1985. The paper reported that two
other Mitsubishi tankers were in the Dubai shipyard and a further three
were to be inspected for cracks and, if necessary, repaired.

The company is to monitor four other tankers which have been delivered
in the last 18 months. All the ships were built to a similar design. Mitsubishi
has another six tankers on order.

Early last month Lloyd’s Register, which carries out structural surveys
on many of the world’s ships, warned that tankers constructed with high-tensile
steel were vulnerable to fatigue cracks. These are caused by the stress
of continued movement over long periods. The design of these tankers was
supervised by the Japanese counterpart of Lloyd’s Register, Nippon Kaiji
Kyokai.

In a report on modern construction techniques in tankers, Lloyd’s issued
a coded warning that tankers built with high-tensile steel would not have
long lives. It said high-tensile steel ‘does not exhibit a significant gain
in fatigue over mild steel’. Lloyd’s suggested that particular care was
needed with design, since the stress in structual components will be greater
because the components themselves will be smaller.

John Ferguson, technical development manager at Lloyd’s Register, said
that as a rough rule of thumb there is an inverse cube relationship between
stress and fatigue life. So if designers increased the stress on a component
by a half, then the fatigue life would be cut to about 30 per cent of its
original value.

The cracks in Mitsubishi’s tankers are in supporting steel members inside
the hull. If unchecked, the cracks could spread to internal watertight bulkheads
and the hull. In tanker designs using high-tensile steel there are fewer
supporting members and the hull takes more of the stress. The most likely
site for the cracks in the hull to occur would be around the waterline.

If the problems with new designs take a large number of tankers out
of service, this could cause considerable problems for the shipping industry.
Tanker building declined sharply after the oil crisis of the 1970s, so a
majority of today’s tankers are old and many are reaching the end of their
working lives. Of the 538 oil tankers flying European Community flags, 85
per cent are more than 10 years old.