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Most oil tankers are too old, too weak and not designed to survive accidents.
But if the world’s tanker fleet were fitted with double hulls and other
devices to prevent pollution, the fleet’s capacity would shrink by a fifth.
The price of petrol would increase by one cent a gallon in the US.

These conclusions from the US National Research Council’s Committee
on Tank Vessel Design add a powerful voice to the debate on the need for
double-skinned hulls. The committee’s report will be published in July by
the National Academy of Sciences (NAS).

The debate about double-skinned hulls reached a peak two years ago,
after the Exxon Valdez ran aground in Prince William Sound. Critics in the
US said that if the ship had had a double hull it would have spilled only
a tenth of the 35 000 tonnes that leaked out into Alaskan waters.

In the 1990 Oil Pollution Act, Congress said that double hulls must
be phased in over 20 years from 1995, for all tankers in US waters. However,
the International Maritime Organization (IMO) wants universal agreement
on tanker safety, not unilateral action by members.

The NAS report, Tanker Spills: Prevention by Design, says existing tanker
designs are ‘no longer adequate’. The accuracy of modern designs has removed
the extra strength traditionally built in to allow for unpredictable errors
and corrosion. Naval architects traditionally have not designed tank vessels
to withstand collisions and groundings, says the report.

In collisions and groundings at slow speeds, ships with double hulls
are unlikely to spill any of their cargo: ‘On the basis of cost-effectiveness,
the double hull is among the best . . . designs evaluated by the committee.’

The panel looked at 17 options for improving the safety of tankers.
Other designs include double-sided ships, double bottoms, smaller cargo
tanks, ballast water tanks placed to absorb the impact of collisions, and
sealed upper and lower holds so that the weight of cargo does not force
oil out of the ship if it is holed.

In ‘hydrostatically balanced loading’, tankers carry less than full
cargoes. If the ship is grounded and holed, sea pressure on the hull pushes
the oil back inside, so avoiding massive pollution. Although this seems
a simpler and cheaper option than replacing tankers with double-hulled ships,
it needs a highly competent crew to manage the cargo, and the deck must
be both airtight and watertight.

The International Chamber of Shipping (ICS) in London, which represents
tanker owners, says double hulls can make things worse in collisions at
speed. The 2-metre gap between the skins could flood, making a ship unstable.
Gases could build up, increasing the risk of explosion. The NAS report says
there is ‘little conclusive evidence’ of this.

John Joyce, marine manager of ICS, says not all owners are against double
hulls. But, he maintains, new performance standards set by the IMO would
be a better way of reducing pollution. These would require tankers to survive
‘typical’ collisions without spilling large amounts of oil. Shipowners would
have to find cost-efficient ways to achieve the standards.

‘Congress has imposed a double-hull standard which people believe will
be an improvement, without evidence to demonstrate that it will be,’ Joyce
says. Tanker accidents were ‘completely random’, with many different causes.
He points out that the NAS report admits ‘no design appeared superior in
all respects’.

Joyce estimates that building a ship with a double hull would increase
the cost by 18 to 20 per cent. Such tankers would be uncompetitive with
older ships, unless new IMO standards were applied universally.

Liberal Democrat MP Jim Wallace, whose Orkney and Shetland constituency
includes the Flotta and Sullom Voe oil terminals, is urging the British
government to follow the US example on double hulls. So far, Britain has
followed the IMO’s recommendations.