Rogue wave


Rogue waves are unusually large, unexpected and suddenly appearing surface waves that can be extremely dangerous, even to large ships such as ocean liners.
Rogue waves present considerable danger for several reasons: they are rare, are unpredictable, may appear suddenly or without warning, and can impact with tremendous force. A wave in the usual "linear" wave model would have a breaking pressure of. Although modern ships are designed to tolerate a breaking wave of, a rogue wave can dwarf both of these figures with a breaking pressure of.
In oceanography, rogue waves are more precisely defined as waves whose height is more than twice the significant wave height, which is itself defined as the mean of the largest third of waves in a wave record. Therefore, rogue waves are not necessarily the biggest waves found on the water; they are, rather, unusually large waves for a given sea state. Rogue waves seem not to have a single distinct cause, but occur where physical factors such as high winds and strong currents cause waves to merge to create a single exceptionally large wave.
Rogue waves can occur in media other than water. They appear to be ubiquitous in nature and have also been reported in liquid helium, in nonlinear optics and in microwave cavities. Recent research has focused on optical rogue waves which facilitate the study of the phenomenon in the laboratory. A 2015 paper studied the wave behavior around a rogue wave, including optical, and the Draupner wave, and concluded that "rogue events do not necessarily appear without a warning, but are often preceded by a short phase of relative order". A 2012 study confirmed the existence of oceanic rogue holes, the inverse of rogue waves, where the depth of the hole can reach more than twice the significant wave height.

Background

Rogue waves are an open water phenomenon, in which winds, currents, non-linear phenomena such as solitons, and other circumstances cause a wave to briefly form that is far larger than the "average" large occurring wave of that time and place. The basic underlying physics that makes phenomena such as rogue waves possible is that different waves can travel at different speeds, and so they can "pile up" in certain circumstances, known as "constructive interference". However, other situations can also give rise to rogue waves, particularly situations where non-linear effects or instability effects can cause energy to move between waves and be concentrated in one or very few extremely large waves before returning to "normal" conditions.
Once considered mythical and lacking hard evidence for their existence, rogue waves are now proven to exist and known to be a natural ocean phenomenon. Eyewitness accounts from mariners and damage inflicted on ships have long suggested they occurred. The first scientific evidence of the existence of rogue waves came with the recording of a rogue wave by the Gorm platform in the central North Sea in 1984. A stand-out wave was detected with a wave height of in a relatively low sea state. However, the wave that caught the attention of the scientific community was the digital measurement of the "Draupner wave", a rogue wave at the Draupner platform in the North Sea on January 1, 1995, with a maximum wave height of with a peak elevation of. During that event, minor damage was also inflicted on the platform, far above sea level, confirming that the reading was valid.
Their existence has also since been confirmed by video and photographs, and satellite imagery and radar of the ocean surface, by stereo wave imaging systems, by pressure transducers on the sea-floor and notably by oceanographic research vessels. In February 2000, a British oceanographic research vessel, the RRS Discovery, sailing in the Rockall Trough west of Scotland encountered the largest waves ever recorded by scientific instruments in the open ocean, with a SWH of and individual waves up to. "In 2004 scientists using three weeks of radar images from European Space Agency satellites found ten rogue waves, each or higher."
A rogue wave is a natural ocean phenomenon that is not caused by land movement, only lasts briefly, occurs in a limited location, and most often happens far out at sea. Rogue waves are considered rare but potentially very dangerous, since they can involve the spontaneous formation of massive waves far beyond the usual expectations of ship designers, and can overwhelm the usual capabilities of ocean-going vessels which are not designed for such encounters. Rogue waves are, therefore distinct from tsunamis. Tsunamis are caused by massive displacement of water, often resulting from sudden movement of the ocean floor, after which they propagate at high speed over a wide area. They are nearly unnoticeable in deep water and only become dangerous as they approach the shoreline and the ocean floor becomes shallower; therefore, tsunamis do not present a threat to shipping at sea. They are also distinct from megatsunamis, which are single massive waves caused by sudden impact, such as meteor impact or landslides within enclosed or limited bodies of water. They are also different from the waves described as "hundred-year waves", which is a purely statistical prediction of the highest wave likely to occur in a hundred-year period in a particular body of water.
Rogue waves have now been proven to be the cause of the sudden loss of some ocean-going vessels. Well-documented instances include the freighter MS München, lost in 1978. A rogue wave has been implicated in the loss of other vessels including the Ocean Ranger, which was a semi-submersible mobile offshore drilling unit that sank in Canadian waters on 15 February 1982. In 2007 the United States National Oceanic and Atmospheric Administration compiled a catalogue of more than 50 historical incidents probably associated with rogue waves.

History of rogue wave knowledge

Mythical status

In 1826, French scientist and naval officer Captain Jules Dumont d'Urville reported waves as high as in the Indian Ocean with three colleagues as witnesses, yet he was publicly ridiculed by fellow scientist François Arago. In that era it was widely held that no wave could exceed. Author Susan Casey wrote that much of that disbelief came because there were very few people who had seen a rogue wave, and until the advent of steel double-hulled ships of the 20th century "people who encountered 100-foot rogue waves generally weren't coming back to tell people about it."

State of knowledge prior to the 1995 Draupner wave

Unusual waves have been studied scientifically for many years, but these were not linked conceptually to sailors' stories of encounters with giant rogue ocean waves, as the latter were believed to be scientifically implausible.
Since the 19th century, oceanographers, meteorologists, engineers and ship designers have used a statistical model known as the Gaussian function to predict wave height, on the assumption that wave heights in any given sea are tightly grouped around a central value, known as the 'significant wave height'. In a storm sea with a significant wave height of, the model suggests there will hardly ever be a wave higher than. It suggests one of could indeed happen – but only once in ten thousand years. This basic assumption was well accepted. The use of a Gaussian form to model waves had been the sole basis of virtually every text on that topic for the past 100 years.
The first known scientific article on "Freak waves" was written by Professor Laurence Draper in 1964. In that paper which has been described as a 'seminal article' he documented the efforts of the National Institute of Oceanography in the early 1960s to record wave height and the highest wave recorded at that time which was about. Draper also described freak wave holes.
However, even as late as the mid 1990s, most popular texts on oceanography such as that by Pirie did not contain any mention of rogue or freak waves. Even after the 1995 Draupner wave, the popular text on Oceanography by Gross only gave rogue waves a mention and simply stated that "Under extraordinary circumstances unusually large waves called rogue waves can form" without providing any further detail.

Draupner Wave

In 1995, strong scientific evidence for the existence of rogue waves came with the recording of what has become known as the Draupner wave. The Draupner E is one structure in a gas pipeline support complex operated by Statoil about offshore and west by southwest from the southern tip of Norway. The Draupner E platform is the first major oil platform of the jacket-type attached to the seabed with a bucket foundation instead of pilings and a suction anchoring system. As a precaution, the operator fitted the platform with an extensive array of instrumentation. The instruments continuously check the platform's movements in particular any movement of the foundations during storm events. The state-of-the-art instrumentation fitted to the platform was able to continuously measure seven key parameters:
The rig was built to withstand a calculated 1-in-10,000 years wave with a predicted height of and was also fitted with a state-of-the-art laser wave recorder on the platform's underside. At 3 p.m. on 1 January 1995 it recorded a rogue wave i.e., taller than the predicted 10,000-year wave, that hit the rig at. This was the first confirmed measurement of a freak wave, more than twice as tall and steep as its neighbors, with characteristics that fell outside any known wave model. The wave was recorded by all of the sensors fitted to the platform and it caused enormous interest in the scientific community.

Modern knowledge since 1995

Following the evidence of the Draupner wave, research in the area became widespread.
The first scientific study to comprehensively prove that freak waves exist, which are clearly outside the range of Gaussian waves, was published in 1997. Some research confirms that observed wave height distribution in general follows well the Rayleigh distribution, but in shallow waters during high energy events, extremely high waves are more rare than this particular model predicts. From about 1997 most leading authors acknowledged the existence of rogue waves with the caveat that wave models had been unable to replicate rogue waves.
Statoil researchers presented a paper in 2000, collating evidence that freak waves were not the rare realizations of a typical or slightly non-gaussian sea surface population, but rather they were the typical realizations of a rare and strongly non-gaussian sea surface population of waves. A workshop of leading researchers in the world attended the first Rogue Waves 2000 workshop held in Brest in November 2000.
In 2000 the British oceanographic vessel RRS Discovery recorded a wave off the coast of Scotland near Rockall. This was a scientific research vessel and was fitted with high quality instruments. The subsequent analysis determined that under severe gale force conditions with wind speeds averaging a ship-borne wave recorder measured individual waves up to from crest to trough, and a maximum significant wave height of. These were some of the largest waves recorded by scientific instruments up to that time. The authors noted that modern wave prediction models are known to significantly under-predict extreme sea states for waves with a significant height above. The analysis of this event took a number of years, and noted that "none of the state-of-the-art weather forecasts and wave models — the information upon which all ships, oil rigs, fisheries, and passenger boats rely — had predicted these behemoths." Put simply, a scientific model to describe the waves encountered did not exist. This finding was widely reported in the press, which reported that "according to all of the theoretical models at the time under this particular set of weather conditions waves of this size should not have existed".
In 2004 the ESA MaxWave project identified more than ten individual giant waves above in height during a short survey period of three weeks in a limited area of the South Atlantic. The ESA's ERS satellites have helped to establish the widespread existence of these "rogue" waves. By 2007, it was further proven via satellite radar studies that waves with crest to trough heights of to, occur far more frequently than previously thought. It is now known that rogue waves occur in all of the world's oceans many times each day.
Thus acknowledgement of the existence of rogue waves is a very modern scientific paradigm. It is now well accepted that rogue waves are a common phenomenon. Professor Akhmediev of the Australian National University, one of the world's leading researchers in this field, has stated that there are about 10 rogue waves in the world's oceans at any moment. Some researchers have speculated that approximately three of every 10,000 waves on the oceans achieve rogue status, yet in certain spots — like coastal inlets and river mouths — these extreme waves can make up three out of every 1,000 waves, because wave energy can be focused.
Rogue waves may also occur in lakes. A phenomenon known as the "Three Sisters" is said to occur in Lake Superior when a series of three large waves forms. The second wave hits the ship's deck before the first wave clears. The third incoming wave adds to the two accumulated backwashes and suddenly overloads the ship deck with tons of water. The phenomenon is one of various theories as to the cause of the sinking of the SS Edmund Fitzgerald on Lake Superior in November 1975.
Serious studies of the phenomenon of rogue waves only started about 20–30 years ago and have intensified since about 2005. One of the remarkable features of the rogue waves is that they always appear from nowhere and quickly disappear without a trace. Recent research has suggested that there could also be "super-rogue waves", which are up to five times the average sea-state. Rogue waves has now become a near universal term given by scientists to describe isolated large amplitude waves, that occur more frequently than expected for normal, Gaussian distributed, statistical events. Rogue waves appear to be ubiquitous in nature and are not limited to the oceans. They appear in other contexts and have recently been reported in liquid helium, in nonlinear optics and in microwave cavities. It is now universally accepted by marine researchers that these waves belong to a specific kind of sea wave, not taken into account by conventional models for sea wind waves.
In 2012, researchers at the Australian National University proved the existence of rogue wave holes, an inverted profile of a rogue wave. Their research created rogue wave holes on the water surface, in a water wave tank. In maritime folk-lore, stories of rogue holes are as common as stories of rogue waves. They follow from theoretical analysis but had never been proven experimentally.
In 2019, researchers succeeded in producing a wave with similar characteristics to the Draupner wave, and proportionately greater height, using multiple wavetrains meeting at an angle of 120 degrees. Previous research had strongly suggested that the wave resulted from interaction between waves from different directions. Their research also highlighted that wave-breaking behavior was not necessarily as expected. If waves met at an angle less than about 60 degrees, then the top of the wave "broke" sideways and downwards. But from about 60 degrees and greater, the wave began to break vertically upwards, creating a peak that did not reduce the wave height as usual, but instead increased it. They also showed that the steepness of rogue waves could be reproduced in this manner. Finally, they observed that optical instruments such as the laser used for the Draupner wave might be somewhat confused by the spray at the top of the wave, if it broke, and this could lead to uncertainties of around 1–1.5 metres in the wave height. They concluded "that the onset and type of wave breaking play a significant role and differ significantly for crossing and non-crossing waves. Crucially, breaking becomes less crest-amplitude limiting for sufficiently large crossing angles and involves the formation of near-vertical jets".

Research efforts

There are a number of research programmes currently underway focussed on rogue waves including:
Because the phenomenon of rogue waves is still a matter of active research, it is premature to state clearly what the most common causes are or whether they vary from place to place. 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; the warm Agulhas Current runs to the southwest, while the dominant winds are westerlies. However, since this thesis does not explain the existence of all waves that have been detected, several different mechanisms are likely, with localized variation. Suggested mechanisms for freak waves include the following:
;Diffractive focusing :According to this hypothesis, coast shape or seabed shape directs several small waves to meet in phase. Their crest heights combine to create a freak wave.
;Focusing by currents :Waves from one current are driven into an opposing current. This results in shortening of wavelength, causing shoaling, and oncoming wave trains to compress together into a rogue wave. This happens off the South African coast, where the Agulhas Current is countered by westerlies.
;Nonlinear effects :It seems possible to have a rogue wave occur by natural, nonlinear processes from a random background of smaller waves. In such a case, it is hypothesized, an unusual, unstable wave type may form which 'sucks' energy from other waves, growing to a near-vertical monster itself, before becoming too unstable and collapsing shortly after. One simple model for this is a wave equation known as the nonlinear Schrödinger equation, in which a normal and perfectly accountable wave begins to 'soak' energy from the waves immediately fore and aft, reducing them to minor ripples compared to other waves. The NLS can be used in deep water conditions. In shallow water, waves are described by the Korteweg–de Vries equation or the Boussinesq equation. These equations also have non-linear contributions and show solitary-wave solutions. A small-scale rogue wave consistent with the nonlinear Schrödinger equation was produced in a laboratory water tank in 2011. In particular, the study of solitons, and especially Peregrine solitons, have supported the idea that non-linear effects could arise in bodies of water.
;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.
;Constructive interference of elementary waves :Rogue waves can result from the constructive interference of elementary 3D waves enhanced by nonlinear effects.
;Wind wave interactions :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. When strong winds from a storm happen to blow in the opposing direction of the ocean current the forces might be strong enough to randomly generate rogue waves. Theories of instability mechanisms for the generation and growth of wind waves—although not on the causes of rogue waves—are provided by Phillips and Miles.
;Thermal expansion :When a stable wave group in a warm water column moves into a cold water column the size of the waves must change because energy must be conserved in the system. So each wave in the wave group become smaller because cold water holds more wave energy based on density. The waves are now spaced farther apart and because of gravity they will propagate into more waves to fill up the space and become a stable wave group. If a stable wave group exists in cold water and moves into a warm water column the waves will get larger and the wavelength will be shorter. The waves will seek equilibrium by attempting to displace the waves amplitude because of gravity. However, by starting with a stable wave group the wave energy can displace toward the center of the group. If both the front and back of the wave group are displacing energy toward the center it can become a rogue wave. This would happen only if the wave group is very large.
The spatio-temporal focusing seen in the NLS equation can also occur when the nonlinearity is removed. In this case, focusing is primarily due to different waves coming into phase, rather than any energy transfer processes. Further analysis of rogue waves using a fully nonlinear model by R. H. Gibbs brings this mode into question, as it is shown that a typical wavegroup focuses in such a way as to produce a significant wall of water, at the cost of a reduced height.
A rogue wave, and the deep trough commonly seen before and after it, may last only for some minutes before either breaking, or reducing in size again. Apart from one single rogue wave, the rogue wave may be part of a wave packet consisting of a few rogue waves. Such rogue wave groups have been observed in nature.
There are three categories of freak waves:
The possibility of the artificial stimulation of rogue wave phenomena has attracted research funding from DARPA, an agency of the United States Department of Defense. Bahram Jalali and other researchers at UCLA studied microstructured optical fibers near the threshold of soliton supercontinuum generation and observed rogue wave phenomena. After modelling the effect, the researchers announced that they had successfully characterized the proper initial conditions for generating rogue waves in any medium. Additional works carried out in optics have pointed out the role played by a nonlinear structure called Peregrine soliton that may explain those waves that appear and disappear without leaving a trace.

Reported encounters

Many of these encounters are only reported in the media, and are not examples of open ocean rogue waves. Often, in popular culture, an endangering huge wave is loosely denoted as a rogue wave, while it has not been established that the reported event is a rogue wave in the scientific sense — i.e. of a very different nature in characteristics as the surrounding waves in that sea state and with very low probability of occurrence.
This section lists a limited selection of notable incidents.

19th century

The loss of the MS München in 1978 provided some of the first physical evidence of the existence of rogue waves. München was a state-of-the-art cargo ship with multiple water-tight compartments and an expert crew. She was lost with all crew and the wreck has never been found. The only evidence found was the starboard lifeboat, which was recovered from floating wreckage some time later. The lifeboats hung from forward and aft blocks above the waterline. The pins had been bent back from forward to aft, indicating the lifeboat hanging below it had been struck by a wave that had run from fore to aft of the ship and had torn the lifeboat from the ship. To exert such force the wave must have been considerably higher than. At the time of the inquiry, the existence of rogue waves was considered so statistically unlikely as to be near impossible. Consequently, the Maritime Court investigation concluded that the severe weather had somehow created an 'unusual event' that had led to the sinking of the München.
In 1980 the MV Derbyshire was lost during Typhoon Orchid south of Japan along with all of her crew. The Derbyshire was an ore-bulk-oil combination carrier built in 1976. At 91,655 gross register tons, she was — and remains — the largest British ship ever to have been lost at sea. The wreck was found in June 1994. The survey team deployed a remotely operated vehicle to photograph the wreck. A private report was published in 1998 that prompted the British government to reopen a formal investigation into the sinking. The government investigation included a comprehensive survey by the Woods Hole Oceanographic Institution, which took 135,774 pictures of the wreck during two surveys. The formal forensic investigation concluded that the ship sank because of structural failure and absolved the crew of any responsibility. Most notably, the report determined the detailed sequence of events that led to the structural failure of the vessel. A third comprehensive analysis was subsequently done by Douglas Faulkner, professor of marine architecture and ocean engineering at the University of Glasgow. His 2001 report linked the loss of the Derbyshire with the emerging science on freak waves, concluding that the Derbyshire was almost certainly destroyed by a rogue wave.
In 2004 an extreme wave was recorded impacting the Admiralty Breakwater, Alderney in the Channel Islands. This breakwater is exposed to the Atlantic Ocean. The peak pressure recorded by a shore-mounted transducer was. This pressure far exceeds almost any design criteria for modern ships and this wave would have destroyed almost any merchant vessel.
Work by Smith in 2007 confirmed prior forensic work by Faulkner in 1998 and determined that the Derbyshire was exposed to a hydrostatic pressure of a "static head" of water of about with a resultant static pressure of. This is in effect of green water flowing over the vessel. The deck cargo hatches on the Derbyshire were determined to be the key point of failure when the rogue wave washed over the ship. The design of the hatches only allowed for a static pressure of less than of water or, meaning that the typhoon load on the hatches was more than ten times the design load. The forensic structural analysis of the wreck of the Derbyshire is now widely regarded as irrefutable.
In addition fast moving waves are now known to also exert extremely high dynamic pressure. It is known that plunging or breaking waves can cause short-lived impulse pressure spikes called Gifle peaks. These can reach pressures of for milliseconds, which is sufficient pressure to lead to brittle fracture of mild steel. Evidence of failure by this mechanism was also found on the Derbyshire. Smith has documented scenarios where hydrodynamic pressure of up to or over 500 metric tonnes per could occur.

Design standards

In November 1997 the International Maritime Organization adopted new rules covering survivability and structural requirements for bulk carriers of and upwards. The bulkhead and double bottom must be strong enough to allow the ship to survive flooding in hold one unless loading is restricted.
It is now widely held that rogue waves present considerable danger for several reasons: they are rare, unpredictable, may appear suddenly or without warning, and can impact with tremendous force. A wave in the usual "linear" model would have a breaking force of. Although modern ships are designed to tolerate a breaking wave of 15 t/m2, a rogue wave can dwarf both of these figures with a breaking force far exceeding 100 t/m2. Smith has presented calculations using the International Association of Classification Societies Common Structural Rules for a typical bulk carrier which are consistent.
Peter Challenor, a leading scientist in this field from the National Oceanography Centre in the United Kingdom, was quoted in Casey's book in 2010 as saying: "We don’t have that random messy theory for nonlinear waves. At all." He added, "People have been working actively on this for the past 50 years at least. We don’t even have the start of a theory."
In 2006 Smith proposed that the International Association of Classification Societies recommendation 34 pertaining to standard wave data be modified so that the minimum design wave height be increased to. He presented analysis that there was sufficient evidence to conclude that high waves can be experienced in the 25-year lifetime of oceangoing vessels, and that high waves are less likely, but not out of the question. Therefore, a design criterion based on high waves seems inadequate when the risk of losing crew and cargo is considered. Smith has also proposed that the dynamic force of wave impacts should be included in the structural analysis.
The Norwegian offshore standards now take into account extreme severe wave conditions and require that a 10,000-year wave does not endanger the ships integrity. Rosenthal notes that as at 2005 rogue waves were not explicitly accounted for in Classification Societies’ Rules for ships’ design. As an example, DNV GL, one of the world's largest international certification body and classification society with main expertise in technical assessment, advisory, and risk management publishes their Structure Design Load Principles which remain largely based on the 'Significant Wave height' and as at January 2016 still has not included any allowance for rogue waves.
The U.S. Navy historically took the design position that the largest wave likely to be encountered was 21.4 m. Smith observed in 2007 that the navy now believes that larger waves can occur and the possibility of extreme waves that are steeper is now recognized. The navy has not had to make any fundamental changes in ship design as a consequence of new knowledge of waves greater than 21.4 m because they build to higher standards.
There are more than 50 classification societies worldwide, each with different rules, although most new ships are built to the standards of the 12 members of the International Association of Classification Societies, which implemented two sets of Common Structural Rules; one for oil tankers and one for bulk carriers; in 2006. These were later harmonised into a single set of rules.

Footnotes

Extreme seas project