The Art of Dredging

Dredging and shipping

Rogue waves  .... (also known as "freak waves")

The movie "The Poseidon Adventure" gives a true-to-life picture of what a rogue wave can do to a large ship.

(I'm talking about the first part of the movie. In the second part, the fantasy of the scriptwriters becomes downright pathetic.) 

Rogue waves, also known as "freak waves" were long thought to be just a seafarers myth, waves of 20 metres high, and sometimes much more are now being seriously studied and know to be much more frequent than once thought. 

 It’s hardly surprising that as many as 200 ships have been sunk over the past 20 years by these giant waves.

Such a wave broke windows on the 10th deck of the 92,250 gross tonnes cruise ship "Norwegian Dawn" in April 2005 (read the story here) .

Something similar took out the whole forepeaktank of the Norwegian tanker "Wilstar", 1974.

 
This rare photo (hereunder) of a rogue wave was taken on July, 21st, 2004 by first mate Philippe Lijour aboard the supertanker Esso Languedoc, during a storm off Durban in South Africa in 1980. The mast seen starboard in the photo stands 25 metres above mean sea level. The wave approached the ship from behind before breaking over the deck, but in this case caused only minor damage. The mean wave height at the time was between 5-10 metres.

 

 

Rogue waves, also known as freak waves, monster waves, extreme waves are  large ocean surface waves that are a threat even to large ships

Therefore rogue waves are not necessarily the biggest waves found at sea; they are, only, surprisingly large waves for a given sea state.

Ancient mariners already told about rogue waves, and damages inflicted on ships suggested they occurred; however, their scientific measurement was only positively confirmed following measurements of the "Draupner wave", a rogue wave at the Draupner oil platform, in the North Sea on January 1, 1995.

The last twenty years, and more so since the Draupner wave, the rogue wave phenomenom has been studied more scientifically.

In the "MaxWave" project, researchers collected data satellite, and identified a large number of rogue waves.

Many years of research have confirmed that waves of up to 35 meters in height are much more common than previosuly believed. 

In fact, rogues seem to occur in all of the world's oceans many times every year. This has caused a re-examination of the reason for their existence.

A rogue wave is not the same as a tsunami.

Tsunamis are mass displacement generated waves which propagate at high speed and are more or less unnoticeable in deep water; they only become dangerous as they approach the shoreline and do not present a threat to shipping (the only ships lost in the 2004 Asian tsunami were in port). A rogue wave, on the other hand, is a localized event that most frequently occurs far out at sea.

 

The phenomenon of rogue waves is still a matter of active research, so it is too early to say clearly what the most common causes are.

The areas of highest predictable risk appear to be where a strong current runs counter to the primary direction of travel of the waves; the area near Cape Agulhas off the southern tip of Africa is one such area.

 

Suggested mechanisms for the formation of freak waves include the following:

  • Nonlinear effects — It seems possible to have a rogue wave occur by natural, nonlinear processes from a random background of smaller waves. , a simple matter of resonance.

  • Normal part of the wave spectrum — Rogue waves are not "freaks" at all but are part of normal wave generation process, albeit a rare extremity.
  • Wind waves — While it is unlikely that wind alone can generate a rogue wave, its effect combined with other mechanisms may provide a fuller explanation of freak wave phenomena. As wind blows over the ocean, energy is transferred to the sea surface.
  • Focusing by currents — Storm forced waves are driven into an opposing current. This results in shortening of wavelength, causing shoaling (i.e., increase in wave height), and oncoming wave trains to compress together into a rogue wave.

  

http://coast.gkss.de/projects/maxwave/

http://www.math.uio.no/~karstent/waves/index_en.html

http://www.gresham.ac.uk/event.asp?PageId=45&EventId=729

   

Marc Van de Velde

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