Transcript Trondheim Tekniske Fagskole

What is a Rogue Wave ?
The “Design Wave Philosophy’’
Calculation of the design wave
Wave forces on semi-submersible platforms
Wave forces and bending moments in FPSO-ships
Platform movements in large waves
Examples of heavy weather damage
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What is a Rogue Wave ?
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Why, where and when ?
Shall we design against Rogue and Freak Waves ?
What can a platform master do against Rogue and Freak Waves ?
Remote-sensing of sea conditions
Search And Rescue and emergency operations
Decision making in an emergency
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What is a rogue wave ?
Trondheim Tekniske Fagskole
What is a rogue wave ?
• The various definitions
• What are the problems for design ?
• What are researchers working on ?
Trondheim Tekniske Fagskole
What is a rogue wave ?
Let’s start with “What is a Wave ?”
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What is a rogue wave ?
In practice, you know
it was a rogue wave
once you saw it.
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What is a rogue wave ?
A good excuse when
crew failed to install
port hole storm
covers ?
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What is a rogue wave ?
A 15° roll of the
camera angle ?
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What is a rogue wave ?
Just something that happens ?
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What is a rogue wave ?
In physics and/or
statistics, there are
several definitions:
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What is a rogue wave ?
Some say a wave that
is more than twice the
significant wave
height, but that may
not be a reliable
definition.
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What is a rogue wave ?
If significant wave height was constant and equal to 10 meters, one would encounter a wave
of:
• 18.5 m every 3 hours
• 21.5 m every day
• 25.0 m every month
• 27.5 m every year
• 30.0 m every 20 years
• 31.5 m every 100 years
• 33.1 m every 1000 years
• 34.8 m every 10000 years
The maximum in one day is also the maximum in the 20 minutes or 3
hours surrounding it, so twice the significant wave height is by no
means abnormal: it happens every day !
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What is a rogue wave ?
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Second order sea
10-5
Cumulative probability – Gumbel scale
One of those
waves in a [large]
set of a given
height that make
the set more
numerous than it
should be from
conventional
theory.
10-3
Possible effect of
The freak phenomenon
Gaussian sea
Fullt non-linear sea
Annual maximum crest height
What is a rogue wave ?
From the figures above, theory states that there should
be 500 waves above 2Hs in a year.
If one measures 1000 of them, they are rogue waves:
about 500 freak and 500 normal extremes and of course
we can’t distinguish easily which are which.
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What is a rogue wave ?
A wave of unexpected
severity given the
prevailing sea
conditions at the time
it occurs
Trondheim Tekniske Fagskole
What is a rogue wave ?
A wave of unexpected severity given the prevailing sea
conditions at the time it occurs.
Unexpected: A good deal of damage can be avoided if
those exposed are fully aware there and then.
Severity: Steepness, Front steepness and other
characteristics may be more damaging than mere height.
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What is a rogue wave ?
The famous “Draupner wave”
Reconstructed water surface elevations over a 1000 m span, from T-30s (blue)
to T (red) for the New Year Wave.
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What is a rogue wave ?
• The various definitions
• What
are the problems for design ?
(calling again on Sverre Haver)
• What are researchers working on ?
Trondheim Tekniske Fagskole
Air gap
If waves do not reach the deck, wave forces are
rather mild since
waves go through a
rather transparent
structure, but if
they hit the deck…
How high above
sea-level should
the deck sit ?
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Are rogue waves a problem for structural design?
Could be – but that depends very much what is meant by rogues waves.
At least two options:
a) ”Classical” extreme waves
i.e. rare members of a population of waves defined by modelling the
surface process as a piecewise stationary and homogenous slightly
non-Gaussian process.
This type of extreme waves are presently accounted properly for by the
offshore industry provided some accidental wave load scenario is implemented.
Traditional shipping may have some room for improvements.
However, waves that are likely to cause structural failure do exist within this
population, but their annual probability of occurrence should be lower than
say 10-5.
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Are rogue waves a problem for structural design?
b) Freak (extreme) waves;
defined as typical members of a population being in agreement with
physical mechanisms well beyond those under a).
This type of extreme waves are outside our design events. If such a
population exists, it may challenge present design recipes if it effects
the 10-3 – 10-5 annual probability of exceedance level at a given site.
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Suggested definition of freak waves
A freak wave event is an event that is governed by mechanisms and/or
phenomena well beyond those underlying the piecewise stationary
and homogenous second order model of the sea surface.
A second order model is selected as a reference population for non-freak
events because that is the most sophisticated model available for routine
design.
A factor definition is not very useful since it may – with low probability –
be exceeded even under Gaussian assumption for the sea surface elevation.
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Can freak waves represent ”an ugly” mechanism?
YES – IF THEY MAKE THE UPPER TAIL (10-3 – 10-5) FOR THE ANNUAL
EXTREME VALUE DISTRIBUTION AT A GIVEN SITE SIGNIFICANTLY FATTER
Second order sea
Cumulative probability – Gumbel scale
10-5
10-3
Possible effect of
The freak phenomenon
Gaussian sea
Fullt non-linear sea
Annual maximum crest height
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The Challenge!
KNOWN THREAT
Major threat of safety – but can
be accounted for.
Wave-parallel:
”Classical” extreme waves
UNKNOWN THREAT
Difficult to approach in a rational
way – but can be crucial regarding
safety.
Wave-parallel:
Freak extreme waves
Does a separate freak wave
population exist????
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What is a rogue wave ?
• The various definitions
• What are the problems for design ?
• What
are researchers working on ?
Trondheim Tekniske Fagskole
Scientists work
Two main approaches:
• Look for theoretical models that enable to
construct mathematical rogue waves that are as
similar as possible to measured ones.
• Analyse occurrences to find characteristics of
the wave/sea state/storm that distinguish them
from innocuous ones.
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