Allegra

Vegetable oils, mainly from Asia and America, are being used increasingly in Europe, especially in the food and cosmetics industries. It follows that shipping patterns involving this kind of oil can only increase, which means that potential occurrences of maritime casualties and spills will also increase.

On 1 October 1997, in the Channel, just off the coast of Guernsey, the Liberian tanker the Allegra was involved in a collision and subsequently spilled 900 tonnes of palm oil. The oil solidified quickly forming a slick measuring 800 by 400 metres. The slick continued to spread and turned into an immense slick, 20 km long by 4 km wide. The slick came ashore on the Channel islands and in the Cotentin where it beached at high water mark. The spill was made up of 5 to 50 cm diameter margarine-like rubbery balls with a spongy yellow core and a whitish crust.

The slick drifted and as it did it was tracked by French Customs and MPCU remote sensing aircraft using airborne SLAR (Sideways Looking Airborne Radar) systems operating in the UV and IR bands. The radar system was housed in pods under the fuselage. These methods enabled the pilots to locate and track the slick for the two days following the accident. Even though the spill had no adverse effect on the marine environment, one can easily imagine the effects the widescale landing of "margarine" balls on the beach might have at the height of the summer season. Research scientists at Cedre are currently seeking to know more about the behaviour pattern of this kind of oil. The main difference between palm and crude oil is that palm oil is solid at room temperature. Three factors are being investigated: slick drift, physical and chemical changes to the oil and its dispersion pattern in the marine environment.

The locations of the slick, as indicated by the remote sensing aircraft, were compared to computer-generated predictions designed for oil spills. However, computer modelling did not appear to be suited to dealing with this kind of oil, due to its solid state. Oil samples were collected both from the sea and from the beaches, in order to investigate the way the product had been affected by the water, however no change in the physical properties of this oil were observed.

Small scale testing was conducted at the Cedre in a bid to simulate the Allegra spill. The oil solidified almost instantaneously into very small particles, only a few millimetres in diameter, which subsequently aggregated into "margarine" balls, 5 to 10 cm in diameter. Testing showed that the oil dispersed naturally in the water column which may well explain why a large quantity of the oil spilled seemed to have disappeared.

A post-spill research programme has subsequently elucidated the fact that the physical state of the oil is of crucial importance when a spill occurs. The drift of the slick, surface behaviour patterns and response equipment and methods are radically different for solid and liquid pollutants.