Saving Ships from Extreme Waves
How an oceanographic research vessel sailed into a Perfect Storm, and inspired the study of extreme and rogue waves.
Written by Science & TechnologyFiled under
“I saw the movie The Perfect Storm. Liked it. But I never thought I would live it,” says Dr. Penny Holliday of the National Oceanography Centre in Southampton, England. Holliday is referring to her experience aboard RRS Discovery, a British oceanographic research vessel, which in late January 2000 ventured out into the North Atlantic, part of an ongoing effort to study the wind conditions and currents in that part of the ocean. The ship encountered violent weather near Rockall, 250 km west of Scotland, which prevented the assembled scientists from conducting their planned research. Fortunately, the 295-foot Discovery was equipped with a Shipborne Wave Recorder MkIV, which recorded what turned out to be the largest waves ever measured by scientific instruments in the open ocean. The collected data—discussed in a 2006 paper in Geophysical Research Letters—resulted in a surge of interest in extreme and rogue waves, and a renewed emphasis on protecting ships and offshore structures from their destructive power.
But in the midst of the aforementioned storm, the scientists on board were mostly concerned with simply getting home safely. “The captain [Keith Avery] kept the ship pointing into the waves [hove to], so we would ride up the steep point of a wave, teeter on the top for a little while, and then dive down the other side to this big hole in the ocean,” recalls Holliday. This went on for days, during which time Holliday and her colleagues were periodically thrown across their cabins, and several of her shipmates suffered cracked ribs. At 4 a.m. one morning the starboard lifeboat came loose and repeatedly clanged against the side of the ship until crew members could secure it—a challenging and dangerous chore considering the gale force winds and waves that measured up to 29.1 meters (95.5 feet) high from crest to trough.
Of course, the Discovery isn’t the only seaborne vessel to be battered by extreme waves—or a rogue wave. On April 16, 2005, a seven-story high wall of water crashed over the bow of the Norwegian Dawn while the cruise ship was en route from Miami to New York, resulting in damage to the vessel’s superstructure and broken windows as high as the 10th deck, just below the pilothouse window. The passengers were so unnerved by the freak wave (which seemed to come out of nowhere) that five hundred of the 2,590 on board disembarked during the ship’s unscheduled repair stop in Charleston, South Carolina.
Even more notable was the 84-foot rogue (measured by a down-looking laser band wave sensor) that hit the Draupner platform in the North Sea on January 1, 1995. The so-called New Year’s Wave was far larger than expected in light of the 35-footers battering the structure at the time.
Over the centuries, there have been countless accounts of freak waves that were blamed for the loss of ships, yet oceanographers long remained skeptical. If nothing else, the Draupner event provided confirmation of the existence of rogue waves, which are distinct from extreme waves in that they are disproportionally large as compared to their brethren.
“The big waves you get in a hurricane or storm are extreme waves,” says Peter Challenor, also of the National Oceanography Centre, who specializes in satellite remote sensing and environmental statistics. “Rogue waves might not be as big as extreme waves, but the general sea state is relatively low and then you get one really enormous one. In many ways that is much more dangerous for a ship because if you’ve going along and you’ve got a sea state of one to two meters and suddenly a nine meter wave hits you, you’re not prepared at all.”
“It’s not just the height of the waves that you’ve got to worry about, it’s the steepness,” continues Challenor, who admits he tends to do his oceanography via computer, which is fortuitous considering his tendency to get seasick. “If the wave is really steep, it’s like falling off a cliff,” he says. A steep wave may well have doomed the München, a German barge carrier that sank under mysterious circumstances in the Atlantic in December 1978.
But it was the measurements made by Discovery and at Draupner—in combination with the loss of an inordinate number of large ships—that led to more in-depth study of both extreme and rogue waves. On December 1, 2000, the first MaxWave meeting was held in Brest, France, where members of the wave science community assembled in hopes of developing forecast criteria for rogue waves, and using improved knowledge of freak waves to make adjustments to ship design.
The MaxWave Project continued for three years and “revealed new and ground breaking knowledge about extremes of ocean waves, their crest heights and trough lows, their spectral shapes, their groupiness and their classification according to wind patterns,” noted the minutes of the final MaxWave meeting, which was held in Geneva in October 2003.
Of course, it behooves the shipping and cruise industries to have the best information possible. By one estimate more than 200 supertankers and container ships longer than 650 feet were lost in severe weather between 1985 and 2005, though certain parts of the ocean seem especially likely to produce ship-swamping events.
According to Challenor, the biggest waves in the world occur in the North Atlantic in winter, especially in the Rockall region of the northeast Atlantic, which is famous for its frequent strong winds and high seas. Prior to Discovery, the highest instrument-recorded wave in the region was 26.3 m (84 feet), measured in December 1972 by the Ocean Weather Ship Weather Reporter. In the case of the Holliday cruise, the highest waves were recorded on February 8-9, 2000, with the largest arriving approximately twelve hours after local wind speeds peaked. “It was a case of there being a long fetch. The wind and waves were moving across the entire North Atlantic at about the same speed. So the wind had been continually putting energy into the waves, and they just got bigger and bigger,” recalls Holliday.
Naturally, the North Atlantic isn’t the only part of the ocean famous for its dangerous waves. Going around the Cape of Good Hope off the coast of South Africa is also treacherous—year-round—and rogue waves are apparently not uncommon. “You’ve got a lot of swell coming up from the Southern Ocean and you’ve got the Agulhas current coming down from the east coast of Africa,” begins Challenor. “It comes round the bottom of the Cape and meets these waves that are coming up from the opposite direction. The waves steepen up and ships (particularly if they try to surf the current) will hit these steep, nasty waves.”
But in spite of advances in technology, scientists still don’t have a good picture of how many rogue waves occur worldwide each year. The classic way to measure waves are with a buoy or ship, but there are just a few dozen measurement buoys scattered around the world. In his research, Challenor uses a radar altimeter on a satellite. “It beams down a small radar echo and it looks at the smearing of that radar echo, and that’s related to the roughness of the sea surface,” he says. “We can’t measure individual waves. But we can measure significant wave height, which is the general roughness of the sea.”
Notably, the satellites measure wonderfully spatially but not terribly well temporally. “The two satellites we use at the moment—one comes over every ten days and the other every thirty-five days,” notes Challenor. “And they only measure directly beneath the satellite. So we get a really good picture of what’s happening in the ocean over a whole ocean basin. But if you want to know what is happening to waves at specific spots”—like where a ship has foundered—“we don’t get a good picture at all,” he admits.
Still, the satellite data have already been of assistance to offshore operators. “When they put in an oil rig, they have to have the ability to withstand a sea that you see once in fifty years. And then they put a safety factor on top of that,” says Challenor.
The data has also allowed for the production of maps that calculate where and when extreme waves are most likely to be encountered. “And we do have wave models where we try forecasting. Most people do try to direct ships to avoid storms, and most modern ships, if they are not caught unawares, will be fine. Most of the accidents happen when operators are desperately trying to get to port to make a deadline,” he begins. “The classic example is the [bulk carrier] Derbyshire, [which sank off the coast of Japan after what is believed to have suffered a massive structural failure]. If they had taken different action, I believe they would have gotten away with it.”
As for Dr. Holliday, she’s just grateful that the captain and crew of Discovery behaved prudently and were able to return herself and her colleagues to safety, especially after she experienced descending into a few so-called holes in the ocean. “It’s quite scary if you’re wondering if you are going to carry on floating once you get to the bottom [of one of those holes],” she reports.
She’s also grateful that the Discovery cruise yielded highly-compelling data, even if it wasn’t the data she hoped to come back with. “With the help of my colleagues who know something about waves, we were able to have something productive come out of a cruise that didn’t reach its scientific objectives.”