Prestige forliser utenfor spanskekysten i 2003

Was the oil tanker Prestige delt the final blow by a giant wave? Photo: Wikimedia

Did a giant wave provoke an environmental disaster?

Was a giant wave involved in the sinking of the oil tanker Prestige off the Spanish coast? We don't know, but there have been speculations on the possibility of a giant wave due to a phenomenon called crossing sea. Norwegian researchers have participated in a project to estimate the probability of a giant wave.

Oljerydding etter foliset av oljetankeren Prestige

On the 13th of November 2002, the oil tanker Prestige loaded with 77.000 tons of heavy fuel oil suffered damage in its hull off the coast of Finisterre, Spain, known as the “Coast of Death”.

The attention from Spanish mass media has been extensive due to the environmental damage following the accident. 

The oil caused an estimated damage of some four thousand million euros, and the bill ended up in the hands of Spanish taxpayers.

A lawsuit to decide who was responsible was finished on 13th of November 2013, on the 11th anniversary of the accident, but the ruling was appealed and the final outcome has not yet been decided.

New research on crossing sea

For the first time, a group of researchers from the University of Oslo, the University of Alcalá and the French Meteorological Institute (Météo-France) have determined the probability of a giant wave. The research resulted in an article published this year in the  Journal of Geophysical Research: Oceans

In order to perform this study several mathematical and numerical models were employed. Central in this work was professor Karsten Trulsen at the Department of Mathematics. He is an expert on wave theory.

Professor i matematikk Karsten Trulsen.

– I became interested in the phenomenon of crossing sea after hearing rumors from workers on oil platforms in the North Sea. They had observed that dangerous waves could arise under certain wind and wave conditions.  Essentially under conditions of crossing waves, Trulsen explains.

It is common for the «Coast of Death», where the accident occurred, to have extreme wave conditions with waves coming simultaneously from different directions, a phenomenon called crossing sea.  The researchers have by means of meteorological data tried to determine the conditions at the moment of the accident.

By studying the air pressure globally and regionally, one can determine the wind direction and speed during the hours prior to and after the accident. These wind fields can be used to compute the wave direction and sea state in the area of the accident.

Difficult task

To compute an exact description of waves on the ocean is not easy.  It is necessary to combine «big data» from the meteorological conditions with wave theory and statistics. This is not an exact science, but the research group claims their work so far provides the best assessment if a giant wave could have occurred at the moment of the Prestige accident.

– What we try to find out is if the crossing sea and the existing weather conditions at the given time could have caused exceptionally large waves. What we are looking for are waves that are much bigger than they normally would have been expected to be, Trulsen says.

This was achieved by means of models for the propagation of wave fields, including interactions between individual waves propagating in different directions.

In order to determine the probability of an extreme wave one must in the end use statistics.

We cannot say what happened, we can only determine the probability that an extreme wave could have occurred, and the expected height of the largest waves, Trulsen explains.

Normal probability

Bilde av spaniakysten der oljetankeren Prestige forliste

The conclusion of the research group is that there was not an unusual probability for the occurrence of extreme waves. An extreme wave could have hit the Prestige, but the probability is neither greater than or smaller than normal.

The Coast of Death has deserved its name. This area often has crossing sea and there can be large waves and violent seas more often than at other locations. The point is that the crossing sea that occurred at this particular moment did not imply a probability for giant waves different from what is normal.

The conclusion may sound as a lack of any conclusion, but it is rather important for a heated debate in Spain.

It means that from a scientific point of view, it is not possible to conclude one way or the other. Indeed, there have been other researchers participating in the Spanish debate who have suggested otherwise.

This is so far the most complete study of the weather and wave conditions and the possibility for extreme waves at the time of the accident.  New studies may come in the future, but for the time being these are the most accurate results that should be taken into account, says Trulsen.

Translated from Norwegian by Karsten Trulsen.

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Professor Karsten Trulsen at Department of Mathematics at University of Oslo