Monster of the deep - Rogue waves

Monsters of the deep

Authored by: Graham Lawton

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Tales of murderous rogue waves were once dismissed as seafaring myths, but it turns out the sailors were right all along. Graham Lawton fears for those in peril on the sea "We were in a storm and the tanker was running before the sea. This amazing wave came from the aft and broke over the deck. I didn't see it until it was alongside the vessel but it was special, much bigger than the others. It took us by surprise. I never saw one again." Philippe Lijour, first mate of the oil tanker Esso Languedoc, describing the huge wave that slammed into the ship off the east coast of South Africa in 1980.

LIJOUR and his shipmates are lucky to be alive. They were struck by a rogue wave-a monstrous wall of water that rose out of nowhere and slammed onto the deck like the fist of god. Ships often don't survive an onslaught like that. Many sink before anyone on board knows what's hit them. Lijour had another stroke of luck that day. As the wave crashed into the ship, he managed to grab his camera. The photograph he took, is one of the few images we have of a rogue wave. It shows a monstrous wall of foam-flecked water, much bigger that anything else on the sea at the time, smashing into the ship's starboard bow. By comparing it to the ship's masts, Lijour estimates that the wave was around 20 meters high. In truth it was probably bigger. Rogue waves are often preceded by a deep trough, so viewed from the sea surface shortly before it struck, the wave could have towered 30 meters or more. It would have been like being hit by a department store. 

Others have not lived to tell their tale. In the past 30 years, hundreds of ships have gone down in mysterious circumstances, taking thousands of lives with them. Naval architects now believe that a large number of these were sunk by rogue waves. Last year an inquiry into the fate of the Derbyshire, a 295-metre, British-owned bulk carrier which disappeared off the coast of Japan in 1980, concluded that a wave had probably cracked open the main hatch and flooded the hold. It
takes a very large, very powerful wave to do that. No one can be sure-because not one of the 44 people on board survived-but the Derbyshire was probably lost to a rogue.

Freakish waves have been part of marine folklore for centuries. Seafarers speak of walls of water, or of holes in the sea, which appear without warning in otherwise benign conditions. Until 20 years ago such reports were dismissed as mariners' tales, about as credible as stories of krakens and mermaids. But since the sinking of the Derbyshire, oceanographers have started to believe them. Observations gathered by the oil and shipping industries suggest there really is something out there-a true monster of the deep that devours ships and sailors without mercy or warning. Complex mathematical models have started to lend credence to these observations, showing that huge waves can blow up out of nothing. What's more, massive waves seem to be surprisingly common, suggesting that there's something dangerously naive about the way we build our ships. The cruel sea is much crueler than we ever imagined.

Apart from tsunamis, which are caused by seismic events such as tremors on the ocean floor, all waves start life as tiny wind-blown ripples on the ocean surface. On a calm day these don't develop further because surface tension pulls them back into the water. But in breezes over force 2 on the Beaufort scale (4 knots and above), the mind dumps enough energy into the ripples to form real waves, or swell.

If the wind carries on blowing, the swell will grow. The height a wave can reach depends on three factors-the wind speed, how long it blows, and how much open water there is. Oceanographers measure wave size using a figure called "significant wave height", the mean of
the largest 33 per cent of waves on the ocean at the time. A hurricane-force wind (force 12) blowing for an hour across the widest point of the Pacific Ocean-17,700 kilometers from Panama to Malaysia-would generate waves with a significant wave height of 4.2
meters. After 24 hours, that would grow to 14.1 meters. Eventually, they would build to 20.7 meters-the height of a 7-storey building.

But these are only the average figures. Waves vary in height, and some get very, very big. In February 1933, the US Navy steamship Ramapo ploughed into a Pacific storm en route to Manila from San Diego. The wind howled at an unremitting 60 knots-force 11-for seven days, lifting the sea into huge 15-metre swells. On the morning of 7 February, the ship encountered a monster. It came from behind, tossing her into a deep trough then lifting her stern-first over a mountain of foamy brine. As the stern of the 146-metre ship hit the bottom of the trough, the officer on watch triangulated the wave against the crow's nest. The figure he came up with was 34
meters-about as tall as an 11-storey building. It remains the biggest wave ever reliably measured.

A huge wave like this-anything more than twice the significant wave height-is classified as a rogue. According to orthodox oceanography, such waves are so rare that no ship or oil platform should ever expect to encounter one. But as the shipping lanes fill with super carriers and the oil and gas industry explores ever-deeper parts of the ocean, rogue waves are being reported far more often than they should. 

In 1997, marine engineer Jesper Skourop of DHI Water & Environment in HØrsholm, Denmark, published a record of wave action measured by radar over 12 years in the Gorm oilfield in the North Sea. He observed 466 waves bigger than anything orthodox theory would predict. The
biggest was 17.5 meters from trough to crest.

Skourop's study is widely accepted as the best systematic, long-term report on rogue waves, but there have been other reliable sightings in the past few years. In August 1998, the US Office of Naval Research in Arlington, Virginia, scrambled a research airplane into the heart of Hurricane Bonnie off the east coast of the US. The winds whipped up seas to a significant wave height of 7 meters, but contour maps recorded by the plane also revealed an unmistakable feature: waves
up to 15 meters high lurching out of the water like ghouls.

The most spectacular sighting of recent years is probably the so-called January Wave, which hit Statoil's Draupner gas platforms in the North Sea on New Year's Day 1995. The significant wave height at the time was around 12 meters. But in the middle of the afternoon the platform was struck by something much bigger. According to measurements made with a laser, it was 26 meters from trough to crest.

So where do the freaks come from? Until recently, oceanographers assumed that they formed in a straightforward, linear process. According to this view, big waves are simply the product of constructive interference-small waves joining forces and adding up.

That's certainly true in some places. The waters off Cape Agulhas, the southernmost tip of Africa where the Atlantic and Indian oceans meet, are a good example. Vessels rounding the cape are regularly hit by enormous waves whipped up when the fast-flowing Agulhas current collides with westerly winds blowing in from the Southern Ocean. The water slows down and waves start to pile into each other. The result is huge waves. Other rogue wave hotspots, notably the Gulf Stream, the Kuro Shio current south of Japan and the notorious seas off Cape Horn, also have the fast currents and countervailing winds that produce big waves.

But there are two problems with invoking this mechanism to explain all rogues. For starters, it doesn't account for the large numbers of rogues in places such as the North Sea where there are no fast-flowing currents.

The other problem is that, even where interference does occur, rogue waves still shouldn't be so common. Interference effects ought to produce a bell-shaped distribution of wave heights with the vast majority close to the average height, some taller, some shorter. Outliers can occur, but they're rare. Freaks more than twice the average height would only crop up once in a lifetime.

But this bears no resemblance to reality. What oceanographers are seeing suggests that the majority of waves are smaller than the mean and that the true giants rise up more frequently than anyone imagined. Orthodox oceanography has been holed below the waterline.

Chaotic oceans

Faced with facts that their standard theory can't explain, oceanographers and mathematicians are scrambling to produce an alternative. Linear modeling-based on the idea that waves simply add up-has clearly failed. So maybe some kind of non-linear interaction, also known as chaos, is the cause.

Many systems, such as the weather or the financial markets, can follow chaotic patterns which create outlandish swings in behavior. Tiny changes in initial conditions-a single trader deciding to sell dollars, for example-can have disproportionate consequences. Perhaps, oceanographers reasoned, the same is true of the ocean, with just small changes in wave height, speed or direction suddenly producing freakish effects.

To explore this possibility, researchers have started experimenting with non-linear mathematical models to see if they produce roguish behavior in ocean waves. At the moment there is no consensus as to which ones work best. Most researchers just pick the non-linear equation they're most familiar with. But the early results suggest they're on to something: computer simulations using non-linear math to govern the development of the sea surface are throwing up huge waves left, right and center.

Al Osborne, a physicist at the University of Turin in Italy who has worked on rogue waves for the US Office of Naval Research, uses the non-linear SchrÖdinger equation to generate his waves. Though originally developed to describe the quantum behavior of electrons in an atom, it has since been modified to describe how all kinds of waves evolve in space and time. Using this formula, Osborne has seen how waves up to four times the average height can rear up from nowhere, then die down just as suddenly-exactly as in the mariners' tales.

Other researchers use different equations to do the same thing. Efim Pelinovsky of the Institute of Applied Physics in Nizhny Novgorod, Russia, favours the Korteweg-de Vries equation. Krystian Dysthe, a mathematician at the University of Bergen in Norway, has discovered another non-linear interaction between four colliding waves that, he says, can build a monster.

No one knows which approach best represents reality. Maybe they all do. That, at least, would account for the diversity of rogue wave sightings, ranging from walls of water that are spotted on radar long before they come into view, to steep crests that appear suddenly from nowhere and disappear just as quickly. Some non-linear models produce groups of three, four, or even five huge waves in a row, giving credence to the fabled and feared "three sisters"-three massive
waves in quick succession. "When the seas get steep, you get a lot of non-linearity," says oceanographer Linwood Vincent of the US Office of Naval Research.

But researchers aren't just interested in explaining rogue waves. They  want to save ships and lives. To that end, the European Union has set up a new research project called Maxwave to observe, model and eventually forecast rogue waves. The US Office of Naval Research has a similar program.

Naval architects, in particular, are starting to take notice. They have always worked on the assumption that their vessels are extremely unlikely to encounter a rogue. The new research suggests that's wrong and has cost lives. Douglas Faulkner, a marine consultant and former professor of naval architecture at Glasgow University, has compiled a catalogue of recent disasters that might have been caused by rogue waves. Working with Jens RÖmeling of the Danish Maritime Institute and Brian Corlett of Manx maritime consultants Burness, Corlett & Partners, he has identified an extraordinary number of cases where super carriers-huge cargo ships more than 200 meters long-have sunk suddenly due to flooding of the main hold. "In a high proportion, the cause was probably a wave beyond the ship's design limit," says Faulkner.

Out of 60 super carriers lost to sudden flooding between 1969 and 1994, they reckon rogues were behind 22 lost ships (see Diagram). A total of 542 lives were lost as a result. And that's just super carriers. If you include trawlers and yachts the figures will be much, much higher.

Offshore platforms are also vulnerable to rogue waves. That's almost certainly what claimed the Ocean Ranger, a drilling rig run by Mobil Oil on the Grand Banks of Newfoundland, 170 miles east of St John's. On 15 February 1982, a giant wave smashed the windows and flooded the control room. Shortly afterwards the rig capsized and sank killing all 84 people on board. This remains the worst rogue wave disaster on record.

But it probably won't be the last, and marine architects are now calling for changes. They want drilling platforms to be raised by as much as 20 per cent, and ships' hatches to be strengthened. Lives are plainly at risk, since many of the vessels plying the world's shipping lanes were designed and built before anyone even believed in rogue waves. Almost everything on the sea is sailing under the false assumption that rogue waves are, at worst, vanishing rare events. May the luck of Philippe Lijour sail with them.

 

The ones that got away

1943, North Atlantic. Cruise liner Queen Elizabeth ploughs into a trough and is hit by two massive waves in succession. The impacts shatter the bridge windows 28 meters above the waterline.

1944, Indian Ocean. British Royal Navy cruiser Birmingham plunges into a deep hole then takes a huge wave over her bows. The commander reports wading through knee-high water on a deck more than 18 meters above sea level.

1966, North Atlantic. Italian steamship Michelangelo is hit by a 21-metre wave en route to New York. The water smashes through the bridge and into the first class compartments, killing two passengers and a crew member.

1995, North Sea. Statoil floating rig Veslefrikk B is severely damaged by a rogue wave. One crew member describes a "wall of water" visible for several minutes before it strikes.

1995, North Atlantic. The QE2 encounters a hurricane on a crossing to New York. She takes a 29-metre wave over her bow. "It looked as if we were going into the White Cliffs of Dover," says Captain Ronald Warwick.

1998, North Atlantic. Schiehallion, a BP Amoco floating production platform, is struck by a wave which smashes the fo'c'sle 18 meters above the waterline.

2000, North Atlantic. British cruise liner Oriana is hit by a 21-metre wave while answering a mayday call from a yacht 600 miles west of Cork, Ireland.


From New Scientist magazine, vol 170 issue 2297, 30/06/2001