Rogue Waves the FOURTEENTH ‘AHA HULIKO’A HAWAIIAN WINTER WORKSHOP
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MEETING REPORT Rogue Waves THE FOURTEENTH ‘AHA HULIKO’A HAWAIIAN WINTER WORKSHOP BY PETER MÜLLER, CHRIS GARRETT, AND AL OSBORNE “It came from nowhere, snapping giant of the Fourteenth ‘Aha Huliko’a Hawai- Shackleton (1919) gave a particularly ships in two. No one believed the survi- ian Winter Workshop convened January gripping account of an extreme wave en- vors … until now” screamed the cover of 25-28, 2005, at the University of Hawaii, countered during his epic 1300-km voy- New Scientist magazine on 30 June 2001, with the support of the Offi ce of Naval age in the 7-m whaler James Caird from introducing an article entitled “Monsters Research and the participation of 25 in- Elephant Island to South Georgia in May of the Deep.” The power and danger sug- vited speakers. 1916. During a day with tremendous gested by the term “rogue” to describe The practical importance of large cross-seas he recalls: extreme ocean waves add to the public ocean waves (such as that in Figure 1) At midnight I was at the tiller and sud- fascination with these waves. They also is clear: they affect naval and civilian denly noticed a line of clear sky between presented important and challenging sci- shipping and can damage offshore oil the south and south-west. I called to the entifi c problems and provided the topic platforms and coastal structures. Ernest other men that the sky was clearing, and then a moment later I realized that what I had seen was not a rift in the clouds but the white crest of an enormous wave. Dur- ing twenty-six years’ experience of the ocean in all its moods I had not encoun- Figure 1. An extreme wave, a so called “white wall,” approach- ing a merchant vessel in the Bay of Biscay (from http: //www. tered a wave so gigantic. It was a mighty photolib.noaa.gov/historic/nws/wea00800.htm). Th e visual upheaval of the ocean, a thing quite apart classifi cation of extreme waves also includes “singular wave from the big white-capped seas that had towers” assumed to be generated by refraction and focusing in currents and the “Th ree Sisters” that bear some similarity been our tireless enemies for many days. to wave patterns created by a ship. I shouted, “For God’s sake, hold on! It’s got us!” Then came a moment of sus- pense that seemed drawn out into hours. Peter Müller ([email protected]) is Professor of Oceanography, University of Hawaii, Honolulu, HI, USA. Chris Garrett is Lansdowne Professor of Ocean Phys- ics, University of Victoria, Victoria, British Columbia, Canada. Al Osborne is Director, Nonlinear Physics Section, Physics Depart- ment, Università di Torino, Torino, Italy. Th is article has been published in Oceanography, Volume 18, Number 3, a quarterly journal of Th e Oceanography Society. Copyright 2005 by Th e Oceanography Society. All rights reserved. Reproduction of any portion of this article by photo- 66 Oceanography Vol. 18, No. 3, Sept. 2005 copy machine, reposting, or other means without prior authorization of Th e Oceanography Society is strictly prohibited. Send all correspondence to: [email protected] or Th e Oceanography Society, PO Box 1931, Rockville, MD 20849-1931, USA. White surged the foam of the breaking 15 sea around us. We felt our boat lifted and fl ung forward like a cork in breaking surf. 10 We were in a seething chaos of tortured water; but somehow the boat lived through 5 it, half-full of water, sagging to the dead (m) Elevation 0 weight and shuddering under the blow. Other mariners have been less fortu- -5 nate. On 12 December 1978, the cargo 800 850 900 950 1000 ship München with 27 people on board Time (s) sent out a garbled May Day message, and Figure 2. Wave record of November 17, 1984 from the Gorm platform in the central North Sea (Sand et al., 1990). Th e wave that stands out, though only having a crest height of mere was never heard from again. Pieces of 11 m, clearly exceeds the signifi cant wave height of the background by a factor of two. An- wreckage that were recovered included other famous wave record is the Draupner wave or “New Year’s Wave” where a rogue wave an un-launched life boat with a pin that of 25.6-m height was measured in a 10.8-m sea on New Year’s Day 1995. had been twisted by an enormous force. The lifeboat was stored 20 m above the water line. There are many more ac- counts of extreme waves hitting cruise waves. Much of the scientifi c interest, if the wave is steep—immense forces ships, passenger liners, container ships, however, stems from evidence that un- can be exerted by a breaking wave on tankers, and fi shing boats. The fate of the usually large waves occur more frequent- the side of a ship, or on an offshore or fi shing boat Andrea Gail was the basis for ly than would be expected from simple coastal structure (Figures 3 and 4 and, the movie The Perfect Storm. The bare statistics, sometimes appearing within a for example, Welch et al., 1999). statistics are that, over the last 20 years, group of very much smaller waves (Fig- We would also like to predict the per- 200 supertankers and container ships ure 2). More dynamically minded people sistence of an extreme wave or the wave longer than 200 m have sunk in severe often reserve the term rogue for these group in which it may occur. Can a “wall weather, extreme waves certainly being a waves that require more than mere sta- of water” be spotted enough in advance main suspect. The highest wave ever re- tistical superposition. Here we stay with to allow time for safety measures? Can corded—quoted in many oceanography the operational defi nition. one identify and track a group within textbooks—was the 34-m wave observed The quantity of interest depends on which a rogue wave might suddenly in the Pacifi c by the U.S. Navy-chartered the user. For bottom-fi xed structures appear? Do extreme waves appear in tanker Ramapo in the 1920s. The height used by the oil industry, it is the crest groups (the “Three Sisters” of mariners’ was obtained by lining up the wave crest height that matters, whereas for mari- lore)? Are there particular oceanograph- with a line from the top of the foremast ners the height from trough to crest ic conditions in which waves more than to the bridge. is more important. The direction of twice the signifi cant wave height are For operationally minded people, any propagation is a signifi cant factor; a more probable than suggested by simple large waves can be called rogues. More ship typically heads into large seas and statistics? precisely, the term rogue is sometimes uses its large pitch moment of inertia to Topics covered at the meeting included reserved for waves more than some mul- limit the response. A ship is vulnerable the basic physics (such as wave-current tiple (perhaps 2 or 2.2) times the signifi - to large waves coming from a different interactions) that leads to large waves, cant wave height, defi ned as the average direction as the roll moment of inertia is statistical considerations, and the nonlin- height of the highest one-third of the much less. This is particularly important ear physics that can provide surprises. We Oceanography Vol. 18, No. 3, Sept. 2005 67 800 700 Date: 31/01/04 Time: 1800 600 500 400 300 Pressure (kPa) Pressure 200 100 0 -100 0 100 200 300 Time (s) Figure 4. Pressure measurement of an extreme wave recorded at the Admiralty Breakwater, Alderney, Channel Islands, which is exposed to waves from the Atlantic Ocean. Th e peak pressure 2 Figure 3. Th e Wilstar after being hit by a rogue wave in the Agulhas current. To devel- is 745 kPa, which corresponds to a weight of 74.5 tonnes/m . op standards for the design of ship hulls one must predict not only the likelihood of Many ocean-going ships are designed to withstand 15 tonnes/ 2 2 rogue waves but also the forces that they exert on the hull. Photo credit: DLR. m and may break at 30 tonnes/m . Figure courtesy of Howell Peregrine and Charlie Obhrai, University of Bristol, England. also discussed wave breaking as a conse- dominant waves present but rather into Steady winds over large fetches are, quence of large waves and possibly as a short waves (Gemmrich et al., 1994). of course, rare, so models have been de- limiting factor on their amplitude. This energy is mostly lost by breaking, veloped to predict wave parameters in and this is how energy (and momentum) general conditions (Komen et al., 1994). BASIC PHYSICS is transferred to the current fi eld. The The forecasts are very valuable for deter- When the wind blows on a calm ocean remaining energy is passed by nonlinear mining safe and economical routes for it fi rst generates short ripples, which interactions to lower frequencies, caus- merchant shipping. The models allow are affected by surface tension as well ing the shift of the peak, and to higher for energy input by the wind, dissipation as gravity. Given suffi cient time and/or frequencies, where it is dissipated by by wave breaking, and the interactions distance offshore, longer waves become breaking (Komen et al., 1994). that shape the spectrum. In mathemati- dominant. The wave spectrum describes Various formulae have been fi tted to cal terms, these interactions rely on how wave energy is distributed with fre- the signifi cant wave height, the period of phase locking between different waves, quency, and the peak of this spectrum the dominant waves, and the frequency but this was traditionally thought to be gradually moves to a lower frequency at spectrum that are typically observed weak enough that it was reasonable to which waves propagate faster.