Tanker design rules tightened in wake of Exxon Valdez

New design standards to reduce the risk of oil pollution from tanker accidents were agreed by the International Maritime Organization (IMO) on 6 March. They will require the use of double hulls or other equivalent measures to prevent oil outflow on new oil tankers from the mid-1990s, and retrofitting of protective measures to older tankers.

The rules are a response to the Exxon Valdez oil pollution disaster in Alaska in March 1989 (ENDS Report 171, pp 11-13). The accident prompted the US administration to pass the Oil Pollution Act 1990, which required all new tankers operating in US waters to be fitted with double hulls from June 1990. Existing single-hulled tankers are to be phased out by 2010. The USA then pressed for similar legislation to be adopted by the IMO.

The IMO rules agreed in March have been incorporated in MARPOL 73/78, the global convention on prevention of pollution from shipping. The rules will come into force automatically in July 1993 unless they are rejected by one-third of the contracting parties to MARPOL, or by parties whose fleets comprise 50% or more of the world's merchant shipping tonnage.

The rules dealing with the design of tankers apply to vessels of 600 tonnes or more. The UK had wanted a threshold of 3,000 tonnes.

Different requirements will apply to new tankers of 600-5,000 tonnes and 5,000 tonnes. The larger vessels will have to be fitted with double bottoms and wing tanks extending the full depth of the ship's side.

However, an alternative to this double hull construction is also provided for. This is known as the mid-deck design, in which wing ballast tanks provide protection against collisions, while the cargo tanks are split horizontally by a deck at the waterline. This reduces the hydrostatic pressure which would force oil out of the vessel if the hull was ruptured.

Both designs were accommodated following an international study which concluded that they would provide broadly equivalent protection against oil spills. In eight out of ten groundings, the study predicted, the inner hull of a tanker with a double hull would not be ruptured, while oil losses from a mid-deck tanker would be small. The balance of advantage works the other way in groundings at higher speeds and collisions, when a rupture of the inner hull of double-hulled vessels would result in much more pollution than would be the case with a mid-deck tanker.

In addition to mandating either of these two designs, the regulations provide that other methods of design and construction may be accepted if they ensure the same level of protection against oil spills. These design methods will have to be approved by the IMO's Marine Environment Protection Committee.

For new tankers in the 600-5,000 tonnes range, the regulations require the fitting of double bottom tanks and limit the capacity of each tank to 700m3, unless the vessel is fitted with a double hull.

These standards will apply to tankers for which a building contract is placed after 6 July 1993, or the keels of which are laid after 5 January 1994, or which are delivered after 5 July 1996.

The second new regulation applies to existing crude oil carriers of 20,000 tonnes or more and existing oil product carriers of 30,000 tonnes or more. Its main provisions require existing tankers to meet the standards set for new vessels no more than 30 years after their date of delivery. However, other structural or operational safeguards against oil outflows may be accepted in future. Many older tankers which cannot be brought up to the new standards economically are likely to be scrapped.

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