We were yesterday at the Café scientifique in Headingley to give a talk about the mysterious freak waves.
In average, two large (>100T) ships sink in the ocean every week. This huge amount of loss comes from various reasons, such as bad weather conditions, collisions or pirates’ attacks. But among the events that cause ships to sink, one is particularly terrifying: freak waves. Considered as legendary for centuries, these sudden extreme waves started to raise the scientists’ curiosity 20 years ago.
On December 1978, the safest ship in the world, called the München, started his journey from Europe to America. But on the night of the 12th of December, she suddenly disappeared in the middle of the Atlantic ocean. Two months later, the life-boat was recovered, and the investigators concluded that a huge force had struck the ship 20m above the water level. But what this force was, was a mystery.
However the mariners had an idea about what could have hit the München at 20m height. For centuries, a lot of them came back from the ocean explaining that an unexpected huge wave, a sudden wall of water, had damaged their boat. They all described it the same way: “a single breaking wave of the size of the tower block”. From Washington Irving, even Christopher Columbus would have faced such a wave.
However, from the scientists’ point of view, this kind of sudden extreme waves couldn’t exist. In the 1990’s, Dr. Jim Gunson from the Met office used an analogy with children in a classroom to explain why such waves should not occur: in a classroom, some children are taller, some are smaller, but they all average around the same size. The chance to find a child who is three or four times taller than the others is very low.
The same happens with waves: from the linear model used at that time, extreme waves such as those described by the mariners should occur only once every 10 000 years. So scientists did not believe that waves faced by the mariners were any kind of physical event, but assumed it was simply the consequence of bad weather conditions.
But on the 1st of January 1995, an extreme wave was recorded on the Draupner offshore platform, in North sea. While all the waves measured about 12m height, one suddenly hit the structure with a height of 26m, which is more that twice the height of the waves around. From that moment, scientists started to study these waves, and a definition was given: freak waves are waves at least twice as height as the waves around.
Since then, many other events have been recorded and freak waves are a real threat for ships. While actual boats are designed to resist waves hitting them with a force of up to 30T/m2, the force applied by freak waves is estimated to be up to 100T/m2, which is equivalent to 20 locomotives on one meter square! This obviously causes many damage to boats, and is therefore a big concern for the shipping industry, as well as a threat for mariners.
Our PhD projects aim to develop a mathematical model of these waves, and understand how to generate them at a given position in a basin, in order to test their impact on models of ships or wind turbines. This will then help the mechanical engineer to design those structures better so that they can resist the load and stress applied by such extreme waves.
Note: Most of this article was inspired from the great documentary “Freak waves” published by the BBC in 2002. You can find it on that link: Freak Waves