Dept. Ocean �Engineering (URI)�

Weather information systems�Effects of severe weather: freak waves�

By Stéphan Grilli�Department of Ocean Engineering �

University of Rhode Island, Narragansett, USA ��

Ferry Safety + Technology Conf., NY, April 16-17, 2015

Dept. of Ocean�Engineering, URI � Ferry Safety�

•  Many Ferry accident (particularly in developing countries) are related to severe weather events (53%; see Abigail Golden’s talk).�

[additional causes are overloading affecting stability, and human errors] �•  In the ocean, severe weather causes large waves (Hs, Tp), winds (Uw) and

currents (Uc), which in combination affect ferry stability and can cause structural damages.�

•  It has been reported that, over the past 20 years, severe weather has “sunk” more than 200 supertankers/container ships exceeding 200 m.�

•  Climatic changes will cause a higher frequency of stronger (hence potentially more damaging) storms (such as tropical cyclones) => high winds (tornados) and waves (including highly damaging freak waves).�

•  Hence the importance of accurate, timely and real time weather forecast (and onboard forecasting tools) will become even more prevailing (see J. Sienkiewicz‘s and M. O’Brien’s talks).�

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Overview

Dept. of Ocean�Engineering, URI � Ferry Safety�

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Past events

Dept. of Ocean�Engineering, URI � Ferry Safety�

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Photo Gallery

•  Many accidents caused by freak waves impacting large ferries have been reported.�

•  Ex: 2006: Pont Aven Ferry (off Brest, Brittany), 185 m long, 1100 passen-gers, 500 cars, was hit sideways by three 8 m waves in calm weather�

=> 2 deaths and 6 severly injured passengers, 15 windows broken and � 150 cabins inundated�

Dept. of Ocean�Engineering, URI � Ferry Safety�

Freak wave definition

•  Freak waves: extreme ocean waves of height Hmax that are under-predicted by traditional (Gaussian) wave probability distributions.�

•  Typical statistical definition: Hmax > 2.25 Hs or ηc > 1.2 Hs � (ex.: Daupner wave in the North Sea, 01/01/95, Hs = 11.9 m)���������•  Probable cause: 2 large intersecting sea states�" " " " " "from the N and NW�

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ηc ~ 18.5 m� ~ 1.55 Hs �H ~ 25.6 m� ~ 2.14 Hs �

Dept. of Ocean�Engineering, URI � Ferry Safety�

Freak wave generation mechanisms

•  Freak waves waves are highly localized in space and time.�•  They can be observed by satellites but are hard to forecast.�•  Under Gaussian statistics, they would only occur once every 1,000

to 10,000 years, but they are observed to occur 1,000 times more frequently => this is due to nonlinear interactions.�•  Radars from Daupner and other platforms in the North Sea measured 446

freak waves, meeting the definition over 12 years.�•  Maxwave Program (ESA’s ERS-1 & 2 satellites) has confirmed the worldwide

occurrence of freak waves, particularly where there are large currents, eddies, fronts, but also in the absence of any of those (see Wave Atlas).�

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Dept. of Ocean�Engineering, URI � Ferry Safety�

Freak wave generation mechanisms

•  Freak waves result from a local focusing of wave energy, in which nonlinear effects are key, due to multiple mechanisms : �  Wave-current interactions (e.g., South Africa) �  Modulational instability (“Benjamin-Feir” instability), 2D, 3D ->

energy from different frequencies self-focuses on another frequency.�

  Directional focusing : different wave components with different phases, directions, (and frequency) can nonlinearly superimpose in a small region of space/time to produce a much larger wave.�

  Effects of bottom topography (shoals, ridges,…), shoaling and refraction �

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Dept. of Ocean�Engineering, URI � Ferry Safety�

Freak waves due to wave-current interaction

•  Example: Agulhas Current (South Africa) makes Western swells grow into a succession of freak waves�

Dept. of Ocean�Engineering, URI � Ferry Safety�

-> Nonlinear process in which energy is transferred (focused) over a small area of space for a brief time, causing the emergence of a very large waves���

[Brandini and Grilli, 2001; 2D and 3D BF instability] �

Freak waves due to modulational instabilities

Dept. of Ocean�Engineering, URI � Ferry Safety�

Freak waves due to 3D spatial energy focusing �

[Brandini and Grilli, 2001; Fochesato, Grilli, Dias (2004,2005,2007)] �

-> Computer model simulating nonlinear focusing wave processes: ��

Dept. of Ocean�Engineering, URI � Ferry Safety�

Freak wave by 3D spatial focusing �

Dept. of Ocean�Engineering, URI � Ferry Safety�

Freak wave - max. horiz. velocity�

[Fochesato, Grilli and Dias, 2007] �

Dept. of Ocean�Engineering, URI � Ferry Safety�

��-> Computer model simulating nonlinear processes of shoaling/breaking: ���

[Fochesato, Grilli, Dias, 2004; Guyenne and Grilli, 2006] �

Freak waves due to bottom topography

Dept. of Ocean�Engineering, URI � Ferry Safety�

•  Wind generation of ocean waves by storms: -> wave modeling is done at a variety of scales : -> Large scale non-phase resolving, including nonlinear wave-wave interactions (e.g., NOAA’s WAVEWATCH III,…); grid of order 10 km resolution => good prediction of Hs, Tp, spectrum, but no of freak waves. �

Operational wind-wave models (NOAA)

Dept. of Ocean�Engineering, URI � Ferry Safety�

•  Wind generation of ocean waves by storms: -> wave modeling is done at a variety of scales : -> Phase resolving fully nonlinear models (e.g., HOS) can predict the occurrence of freak waves on grids of order of 10 m. Statistics of freak waves and of their parameters can be made.�

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[Ducrozet et al., 2006; ECN] �

Dept. of Ocean�Engineering, URI � Ferry Safety�

2D self-focusing of irregular waves�

•  The tail of the surface elevation η distribution is non-Gaussian, due to extreme wave events�

•  Similarly, the wave height H distribution’s tail does not fit Rayleigh’s distribution�

⇒ Nonlinearity -> non-Gaussian distribution of large waves�

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Hmax /Hs (nonlinear)

-15 -10 -5 0 5 10 150

0.05

0.1

0.15

0.2

0.25

η[m]

p(η)

-15 -10 -5 0 5 10 1510-10

10-5

100

η[m]

log(p(η))

0 5 10 15 20 250

0.05

0.1

0.15

0.2

H[m]

p(H)

0 2 4 6 8 10 12 14 16 18 2010-10

10-5

100

H[m]

log(p(H))

η (nonlinear)

H (nonlinear)

H/Hs > 2.25�

Dept. of Ocean�Engineering, URI � Ferry Safety�

2D self-focusing of irregular waves�

H (linear)

[Brandini, 2000; �Brandini and Grilli, 2001] �

Dept. of Ocean�Engineering, URI � Ferry Safety�

2D self-focusing of irregular waves�

H nonlinear

� -> Wave height distri-bution is non-Rayleigh�-> large waves occur 100 to 1000 times more frequently �[Brandini, 2000; �Brandini and Grilli, 2001] �

Dept. of Ocean�Engineering, URI � Ferry Safety�

Ferry overloading combined with waves �

-> After tilting : stable for yM > yG and unstable for yG > yM

Dept. of Ocean�Engineering, URI � Ferry Safety�

Warning/prediction of extreme sea states

•  Ferries (particularly larger sea-crossing units) need to have access to the most accurate and timely weather forecast, including predictions of see state parameters (such as issued by NOAA based on WAVEWATCH III). Evasive measures must be taken if necessary.�"-> need for onboard equipment (internet access) and crew training �•  Ferries have to strictly respect guidelines for freight and

passengers and, in particular, avoid overloading �"–> need more crew training/insurance requirements�•  As new technologies are developed based n HF radars and Lidar

and become economically accessible, larger ferries could be equipped with forward looking sensor measuring waves in its path.�