Braving Rogue Waves with the Power of Knowledge

By June 22, 2016 Article, Marine News No Comments
1 - Seconds Before a Ship Hits a Large Wave

^Seconds Before a Ship Hits a Large Wave

Image Courtesy of MIT News & YouTube at http://news.mit.edu/2016/prediction-tool-rogue-waves-0225 & https://www.youtube.com/watch?v=xgALuj6WUbk&feature=youtu.be

 

From Myth to Reality

Once sidelined to the stuff of fairy tales that sailors passed around for amusement during long voyages, the rogue wave revealed itself to the world on January 1, 1995. The year certainly began with a fresh bang for oceanography.

An 84-foot tall wave crashed into the Draupner Platform in the North Sea at 3:20 PM. Heavy weather was already shooting 39-foot waves on the platform and authorities had ordered the crew to stay indoors at 3 PM for safety reasons. Precisely why none witnessed the wave in real time.

Personnel who first noted the reading on the laser-based, wave-height detector dismissed it as erroneous. But when other, independent instruments on the platform confirmed the monstrous height, the world woke up to a new reality – rogue waves were real, very real.

Thereafter, many other research efforts have recorded rogue waves. The most notable evidence came in 2004. European Space Agency Satellites recorded ten rogue waves of heights hitting or exceeding 82-foot in a brief span of three weeks in a limited area of the South Atlantic.

Some investigations have identified the mechanism via which rogue waves break and / or drown a ship. They have suggested modifications in ship design, but research on rogue waves is still in its infancy. At present, predicting rogue waves is the best defense against them.

Fundamentals of Rogue Waves

Constructive Interference Adds the Amplitudes of Individual Waves to Create a Large-Amplitude Wave  Image Courtesy of Haade at https://en.wikipedia.org/wiki/File:Interference_of_two_waves.svg

Constructive Interference Adds the Amplitudes of Individual Waves to Create a Large-Amplitude Wave
Image Courtesy of Haade at https://en.wikipedia.org/wiki/File:Interference_of_two_waves.svg

Rogue waves those whose height exceeds that of the significant wave height (SWH) by twice or more. Simply speaking, they are two times taller than those around them. They can be eight times as tall though.

These waves typically strike with a breaking force of 100 MT/m2. Most ships are designed to withstand only 15 MT/m2. Plus, rogue waves form rapidly and strike without warning. Now, that’s a fatal combination!

Research has pinpointed three types of freak waves:

  • Single Waves: are formed during storms and do not last long
  • Walls of Water: can travel up to 10 km in oceans
  • Three Sisters: group of three serial waves common in the Great Lakes of America

Given that the research on rogue waves is nautical miles away from completion, it would be premature to ascribe the cause. Even otherwise, research is a never ending process. Nevertheless, some probable causes include:

  • Modulational Instability / Nonlinear Effects: wherein an unstable wave parasitically absorbs energy from surrounding waves and becomes a very high and huge, a rogue wave
  • Diffractive Focusing: by the shape of the seabed or the coast causes the crests of waves to meet in phase

Such combination is called constructive interference wherein the crests (high points) and troughs (low points) of numerous waves meet in sync to create a wave larger than the individual waves

  • Wind-Current Interaction: when stormy winds create huge waves that run opposite to existing ocean currents, the resultant instability generates rogue waves
Shoaling: Wave Height Increases as they Enter Shallow Waters Image Courtesy of Regis Lachaume at https://en.wikipedia.org/wiki/File:Propagation_du_tsunami_en_profondeur_variable.gif

Shoaling: Wave Height Increases as they Enter Shallow Waters
Image Courtesy of Regis Lachaume at https://en.wikipedia.org/wiki/File: Propagation_du_tsunami_en_profondeur_variable.gif

  • Ocean Current Focusing: currents flowing in opposite direction shorten the wavelengths of the waves they drive. This creates shoaling or increase in wave height

Such shoaling is regularly observed near the South African coast where the westerlies run opposite to the Aghullas Current. Extreme shoaling creates rogue waves

  • Thermal Expansion: when a large, stable wave group moves from cold water to hot, it expands. The amplitudes and wavelengths of the waves increase

If waves from the front as well as the rear of the wave group are expanding towards the center, the resultant compression effect can create a rogue wave at or near the center

How Rogue Waves Sink Ships

According to Hugo Montgomery-Swan in Heavy Weather Powerboating, the bow of the ship sticks in the lower part of the rogue wave when the ship is accelerating downwards on the back of the wave that comes just before the rogue wave.

Sketch: Fore Part of Ships is Particularly Exposed to Destruction by Rogue Wave

Sketch: Fore Part of Ships is Particularly Exposed to Destruction by Rogue Wave

With the ship so positioned, the rogue wave hurls thousands of tons of water on the fore part of the ship. The structure of the ship is critical here because it determines whether the ship will fail or not.

According to Professor Faulkner (who investigated the sinking of the MV Derbyshire as we shall see later), bulk carriers are particularly vulnerable to failure when hit by rogue waves.

This is because their high inertias and natural pitch periods make them bury into waves, not rise with them. This exposes them to the thousands of tons of falling water and the shattering force that comes with it.

Coaming or hatch covers can fail rapidly as the descending fore end promotes flooding of large holds. Natural pitch period of a ship depends very much on the ship’s length between perpendiculars (LPP) – it increases with LPP.

Investigating Rogue Wave Disasters

Exhaustive investigation of shipwrecks remains an exception in the maritime industry. Perhaps this is a reflection of the fact that around the world there are over 50 classification societies for shipping, each with a different set of standards.

Things started to change for the better with the MV Derbyshire, the largest British ship lost at sea till date. Typhoon Orchid that hit the south coast of Japan sunk this ore-bulk-oil carrier in 1980. All its 44 crew perished. The ship was built in 1976 and as such was fairly young.

After the wreck was found in 1994, a survey team used a drone to photograph it. This prompted the British government to re-investigate the episode. The enquiry engaged the Woods Hole Oceanographic Institution that took 135,774 pictures of the wreck.

Rotational Motions of a Ship Image Courtesy of Jmvolc at https://en.wikipedia.org/wiki/File:Rotations.png

Rotational Motions of a Ship
Image Courtesy of Jmvolc at https://en.wikipedia.org/wiki/File:Rotations.png

Most importantly, the report pinpointed the sequence of events that culminated into the structural failure of the vessel. The crew was no longer blamed of mismanagement, the culprit was structural failure.

Then came the third report on this incident in 2001, that of Professor Douglas Faulkner of the marine architecture and ocean engineering department at the University of Glasgow. He linked the sinking to freak waves.

Subsequent investigations confirmed his findings. Professor Faulkner called for the inclusion of radically new Survival Design considerations to ship design in addition to existing ones. As of 2016, no one has contradicted his recommendations. No one has implemented them either.

In 2007, Craig Smith in Extreme Waves and Ship Design established that the MV Derbyshire had sunk because the typhoon had exerted a static pressure of 201 kN/m2 on its deck cargo hatches, over ten times the design capacity of 17.1 kN/m2.

Raging waves can also create short-lived dynamic pressure spikes of 200 kN/m2. There were signs of such failure aboard the MV Derbyshire and Craig Smith has recorded incidences when such dynamic pressure can hit an astounding 5,650 kN/m2.

Numerous researchers have recommended a rectification of design standards that makes ships capable of withstanding taller waves instead of the present values of wave heights:

  • Craig Smith suggested the International Association of Classification Societies (IACS) revise its design wave height from 36 feet to 65 feet
  • Although the US Navy has acknowledged the existence of waves higher than 70 feet, it still premises its design on 70-foot waves
Caledonian Star was Hit by a 98-Foot Freak Wave in 2001 in the South Atlantic   Image Courtesy of Stan Shebs at https://en.wikipedia.org/wiki/File:Caledonian_Star_in_Paradise_Bay.jpg

Caledonian Star was Hit by a 98-Foot Freak Wave in 2001 in the South Atlantic
Image Courtesy of Stan Shebs at https://en.wikipedia.org/wiki/File:Caledonian_Star_in_Paradise_Bay.jpg

Even DNV GL, one of the largest international classification agencies in the world with tremendous specialization in technical assessment and risk management, prescribes design standards based on significant wave heights only. Rogue waves can rise eight as high as this criteria.

Prediction & Understanding of the Finer Aspects of Rogue Waves

With only limited knowledge on rogue waves and with a host of classification agencies each with its diverse design standards, unanimity in ship design against rogue waves remains elusive. Predicting rogue waves therefore is the best defense against them. At present, at least.

MIT Algorithm: according to the website MIT News, researchers from the Massachusetts Institute of Technology (MIT) have in February 2016 developed a tool that may provide sailors a 2-3 minute warning of an impending rogue wave. Forewarned truly is forearmed.

First, the tool locates numerous groups of waves around the ship or platform. The algorithm of the tool considers the length and height of the waves in the cluster to calculate the probability of the cluster of waves joining ranks to form a rogue wave.

It provides the possible location of the rogue wave and the probable time that it may strike. In an emergency situation, even this brief window of opportunity can prove invaluable as it may allow the crew to change course and shut down essential operations.

Scientists have conventionally relied on trying to simulate every wave in the ocean when predicting rogue waves as also when determining the sea state. This is an infinitely complicated task as the ocean surface is an intricate blend of constantly changing data points.

Although accurate, this approach requires numerous computers for calculations. This makes it expensive and, most importantly, slow. And therein lays its weakness, for time is of great essence here. What good is a precise prediction if it comes after the rogue wave has taken its toll?

Supported by the Office of Naval Research, The American Bureau of Shipping, and the Army Research Office, the MIT research team premised its prediction algorithm on two parameters – the length and height of a wave group.

Image Courtesy of MIT News at http://news.mit.edu/2016/prediction-tool-rogue-waves-0225

MIT’s Algorithm Computes the Probability of a Wave Group Turning Rogue: Red for High Probability, Yellow for Medium, & Green for Low Image Courtesy of MIT News at http://news.mit.edu/2016/prediction-tool-rogue-waves-0225

Probability, Yellow for Medium, & Green for Low

Image Courtesy of MIT News & YouTube at http://news.mit.edu/2016/prediction-tool-rogue-waves-0225 & https://www.youtube.com/watch?v=xgALuj6WUbk&feature=youtu.be

According to the group’s observation, most ocean waves move independently of each other. Some waves however combine in unidirectional wave fields and exchange energy. It is this dynamic addition of wave energy that eventually creates a rogue wave.

But to pinpoint the precise mechanism that forms rogue waves and create an algorithm for the same, the MIT researchers integrated non-linear analysis of the underlying water wave equations with ocean wave data (mainly height and length) as gathered by ocean buoys.

Offshore platforms and ships have to install high-resolution scanning devices such as radars and LIDAR to gather data on the surrounding waves. They can feed such data to this algorithm and predict a rogue wave some minutes before it actually forms.

University of Oxford & Western Australia: in December 2015, research teams from the UK University of Oxford and the University of Western Australia refuted some of the earlier held beliefs on rogue waves after simulating hundreds of randomly selected waves through mathematical models based on non-linear physics.

MS Louis Majesty was Stuck by Three Successive 26-Foot Waves in 2010 while Plying in the Mediterranean Sea   Image Courtesy of Jebulon at https://en.wikipedia.org/wiki/File:Louis_Majesty_Rhodes.jpg

MS Louis Majesty was Stuck by Three Successive 26-Foot Waves in 2010 while Plying in the Mediterranean Sea
Image Courtesy of Jebulon at https://en.wikipedia.org/wiki/File:Louis_Majesty_Rhodes.jpg

According to them, rogue waves do not arrive after a series of progressively higher waves, but appear suddenly and are preceded by smaller waves. This is because large waves usually move to the front part of the wave group.

They also proved that rogue waves have crests longer than of the smaller waves around them. These deductions more or less confirm the anecdotal evidence on rogue waves – that these waves arrive out of nowhere and are substantially long.

University of Leeds: in March 2016, technicians from the University of Leeds, UK developed a mathematical model that accurately maps the forces that a rogue wave inflicts on a moving ship.

Engineering and Physical Sciences Research Council (EPSRC) has supported the creation of this unprecedentedly precise simulation that will be instrumental in improving the safety design of fast ships in the near future.

Fast ships can hit 30 knots and serve time-critical missions viz. anti piracy, search and rescue, anti-drug, and transporting oil-gas personnel. They are particularly vulnerable to rogue waves. Each year, rogue waves sink around 100 such ships causing about 2,500 casualties.

Bernoulli Principle: Pressure-Velocity Variation for Constant Elevation

Bernoulli Principle: Pressure-Velocity Variation for Constant Elevation

Of particular note in this case is the Funnel Effect. This is the narrowing of moving waves that spikes up their velocity. In extreme cases, funneling transforms not very high waves into lethally tall waves – rogue waves.

We have seen how the researchers have modeled ocean waves to predict rogue waves. That is complex enough. Adding the interaction of a moving boat to the (modeled) waves and then modeling the complete effect of these waves on the ship is a whole new ballgame altogether.

After successful laboratory trials, researchers now plan to completely incorporate the motion of ships into the model over the next three years. This will create a tool that maritime designers and ship engineers can use from a very early stage of design.

Funnel effect is the increase in velocity of fluids when made to pass through narrow sections. The rise in velocity is accompanied with a drop in pressure in accordance with the Bernoulli’s Principle.

Bernoulli’s Theorem applies to the flow of incompressible fluids along a streamline. It states that the total energy of a flowing system remains constant. The theorem is the extension of the Principle of Conservation of Energy to fluid flow.

Total energy consists of the energy on account of static pressure (pressure energy), velocity (kinetic energy), and elevation (potential energy). For flow at constant elevations, any increase in velocity is accompanied with a drop in static pressure and vice versa.

Finally

New concepts often irk us because they push, nay shove, us out of our comfort zones. The case of the rogue wave has not been very different. But if we don’t venture out of our comfort zones, we risk stagnation and decline.

Now that we have accepted the existence of rogue waves, it is time to act. In a nuanced manner that is.

For more such interesting stuff on weird oceanic phenomena, visit our blog.

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