Warning: Rogue Wave Ahead — Predicting the Ship Killing Waves
Rogue waves are real sea monsters. Rising many times higher than surrounding waves, they have the power to sink ships and to cripple offshore platforms. Recently, engineers from MIT have developed a new means of predicting the formation of rogue waves, which may give mariners a two to three-minute warning before one of the monster wave hits.
Whether or not the new application proves to be practical, our understanding of rogue waves has undergone a remarkable and rapid evolution over the last twenty years – from a widespread denial that rogue waves even existed to a growing understanding of how to cope with these infrequent but all too often deadly waves.
For centuries, those sailors who survived an encounter with a rogue wave told of the incredible size and crushing power of these monsters. But in most cases, no one believed them. Captains, who had never seen such a wave, were skeptical and thought that the accounts were just tall tales or another captain making excuses for poor ship handling.
In the 20th century, science only added to the skepticism. Oceanographers’ mathematical wind and wave models were generally very accurate and they predicted that such extreme waves would happen only once every ten thousand years or so. Obviously, the sailors’ tales of rogue waves were exaggerations or just wild sea stories. Rogue waves were officially a myth.
Then, the world changed on New Year’s Day, 1995, when a huge wave struck the Draupner Platform in the North Sea. The platform was equipped with a downward-pointing laser sensor which accurately recorded the height, shape and speed of the wave. The wave the sensor recorded was exactly the size, shape and speed of the rogue wave that sailors had described for so many years. Scientists announced what many mariners already knew. Rogue waves are indeed real.
Within about five years, scientists learned that while rogue waves are not common, they are by no means rare. Around 2004, the European Space Association’s (ESA) European Remote Sensing (ERS) satellites observed more than ten rogue waves during a three-week period, while the satellites were scanning ten-by-five-kilometer (six-by-three-mile) patches of the sea surface every 200 kilometers.
But how are these waves created? According to the wind and wave model used by most oceanographers, they simply do not and cannot exist. And yet, they obviously do. Physicists turned away from the well behaved Newtonian physics of the wind-wave model and looked to wilder realms of quantum mechanics, specifically a derivative of a nonlinear Schrödinger wave model. Schrödinger may be best remembered for his famous cat, but one version of his wave model could be used to recreate the size and behavior of rogue waves. The physicists found that, in a wave train, one wave under certain circumstances could take energy from the adjacent waves and grow to remarkable heights, becoming a ship-killing rogue wave.
Once the mechanism for the creation of rogue waves was better understood, a group of engineers at MIT began working on a way to predict their occurrence. Initially, it looked like the calculations were far too complex and complicated to provide useful data for mariners. Now a team of MIT engineers led by Themis Sapsis, an Assistant Professor of Mechanical Engineering at MIT and former postdoctoral student, Will Cousins, have developed a simpler way to estimate which wave groups have the highest probability of developing a rogue wave.
“Using data and equations, we’ve determined for any given sea state the wave groups that can evolve into rogue waves,” Sapsis says. “Of those, we only observe the ones with the highest probability of turning into a rare event. That’s extremely efficient to do.”
The algorithm is said to be capable of providing a several minute warning to officers on ships or drilling platforms that a rogue wave is forming. To make it work, the ship or platform would need high-resolution scanning devices such as LIDAR or advanced radar to scan the surrounding waves. Additional research and testing will be required, but the initial work appears promising.
Sapsis and Cousins have recently published their results in the Journal of Fluid Mechanics. Their research was supported in part by the Office of Naval Research, the Army Research Office, and the American Bureau of Shipping.
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