Monday, March 8, 2010

Rogue Waves

Today I ran across an interesting essay on our changing understanding of scurvy. As often happens when you learn history better, the simple narratives turn out to be wrong. And you get strange things where as science progressed it discovered a good cure for scurvy, they lost the cure, they proved that their understanding was wrong, then wound up unable to provide any protection from the disease, and only accidentally eventually learned the real cause. The question was asked about how much else science has wrong.

This will be a shorter version of a cautionary tale about science getting things wrong. I thought of it because of a a hilarious comedy routine I saw today. (If you should stop reading here, do yourself a favor and watch that for 2 minutes. I guarantee laughter.) That is based on a major 1991 oil spill. There is no proof, but one possibility for the cause of that accident was a rogue wave. (Rogue waves are also called freak waves.) If so then, comedy notwithstanding, the ship owners could in no way be blamed for the ship falling apart. Because the best science of the day said that such waves were impossible.

Here is some background on that. The details of ocean waves are very complex. However if you look at the ratio between the height of waves and the average height of waves around it you get something very close to a Rayleigh distribution, which is what would be predicted based on a Gaussian random model. And indeed if you were patient enough to sit somewhere in the ocean and record waves for a month, the odds are good that you'd find a nice fit with theory. There was a lot of evidence in support of this theory. It was accepted science.

There were stories of bigger waves. Much bigger waves. There were strange disasters. But science discounted them all until New Years Day, 1995. That is when the Draupner platform recorded a wave that should only happen once in 10,000 years. Then in case there was any doubt that something odd was going on, later that year the RMS Queen Elizabeth II encountered another "impossible" wave.

Remember what I said about a month of data providing a good fit to theory? Well Julian Wolfram carried out the same experiment for 4 years. He found that the model fit observations for all but 24 waves. About once every other month there was a wave that was bigger than theory predicted. A lot bigger. If you got one that was 3x the sea height in a 5 foot sea, that was weird but not a problem. If it happened in a 30 foot sea, you had a monster previously thought to be impossible. One that would hit with many times the force that any ship was built to withstand. A wall of water that could easily sink ships.

Once the possibility was discovered, it was not hard to look through records of shipwrecks and damage to see that it had happened. When this was done it was quickly discovered that huge waves appeared to be much more common in areas where wind and wave travel opposite to an ocean current. This data had been littering insurance records and ship yards for decades. But until scientists saw direct proof that such large waves existed, it was discounted.

Unfortunately there were soon reports such as The Bremen and the Caledonian Star of rogue waves that didn't fit this simple theory. Then satellite observations of the open ocean over 3 weeks found about a dozen deadly giants in the open ocean. There was proof that rogue waves could happen anywhere.

Now the question of how rogue waves can form is an active research topic. Multiple possibilities are known, including things from reflections of wave focusing to the Nonlinear Schrödinger equation. While we know a lot more about them, we know we don't know the whole story. But now we know that we must design ships to handle this.

This leads to the question of how bad a 90 foot rogue wave is. Well it turns out that typical storm waves exert about 6 tonnes of pressure per square meter. Ships were designed to handle 15 tonnes of pressure per square meter without damage, and perhaps twice that with denting, etc. But due to their size and shape, rogue waves can hit with about 100 tonnes of pressure per square meter. Are you surprised that a major oil tanker could see its front fall off?

If you want to see what one looks like, see this video.

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