Direction de la Technologie Marine et des Syst

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The Design Wave Philosophy
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 What is a Rogue Wave ? 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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The Design Wave Philosophy
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The Design Wave Philosophy

• Standards, class societies, rules and
  regulations
• Consequence-based design, safety factors,
  reliability
• Design wave vs. design sea state
• Statistics and extreme value theory
• Meaning of the 100-year (or 10000-year) wave

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The Design Wave Philosophy

• Everything started in a way similar to the
  Oklahoma rush in the Conquest of the West, but
• Hurricane Anita in the Gulf of Mexico
• The design wave increased by 1 meter each year
  from return of experience in the late 70s North
  Sea
• The Alexander Kielland accident occurred in
  1980

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The Design Wave Philosophy

• National requirements and shipping regulations
  from a large amount of actors
• National Agencies (Oljedirektoratet, HSE, ...)
• Classification Societies (DNV, API, Lloyds, BV,
  , IACS)
• Standardisation bodies (ISO, Bnpé, DIN, )
• Professional bodies (OGP)

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The Design Wave Philosophy
ended up into a philosophy for design. In the
North Sea, design is determined by extreme waves,
and at the time (80s), for fixed platforms with
quasi-static response, by the single largest wave
that would break the platform. At that time, one
would compute what happens with a 100-year wave
and add a safety margin.
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The Design Wave Philosophy
A 100-year wave is the wave height that is
exceeded in average once every century over a
large number of centuries. It is NOT exactly the
same as having a 100-year average interval
between two exceedances, and NOT AT ALL the same
as being able to expect a duration of the order
of magnitude of 100 years before the next after a
given exceedance.
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The Design Wave Philosophy

• Standards, class societies, rules and
  regulations
• Consequence-based design, safety factors,
  reliability (some points made by Markku Santala -
  Exxon)
• Design wave vs. design sea state
• Statistics and extreme value theory
• Meaning of the 100-year (or 10000-year) wave

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Design Method Effectiveness

• Identification of controlling design conditions
• Failure to identify controlling conditions may
  impact project schedule or lead to unacceptable
  performance
• Design practices that are over-conservative may
  not be cost effective
• For floating systems the maximum environment is
  not always sufficient for design
• Maximum environment ? maximum response
• Response-based methods provide an approach for
  identification of controlling design conditions
• Implementation details key to effectiveness

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Traditional procedures and limitations

• Fixed Platforms
• Response f(Hmax) secondary contributions (ws,
  v)
• Specifying the 100-year wave plus associated
  parameters leads to the 100-year response
  approximately.
• Floaters
• Responsef(Hs, Tp,, ws, v, ?) secondary
  contributions
• Specifying the 100-year wave (or any other single
  parameter) plus associated parameters DOES NOT
  necessarily lead to the 100-year response.
• Example limitations
• In central GoM where offset can be dominated by
  Loop Current in a VIV lock-in condition.
• In western GoM responses can be dominated by wind
  plus associated conditions

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Common Patches

• Specify a set of 100-year cases and look for the
  dominant response. Minimal specification might
  include
• 100-year significant wave associated wind and
  current
• Range of associated spectral wave periods
• 100-year wind associated wave and current
• 100-year current associated wind and wave
  
• Develop contours in Hs-Tp, Hs-ws, ws-v space to
  search for dominant responses.
• Multi-dimensional parameter contours though
  theoretically possible are not necessarily
  practical or sufficient.

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Response-Based Approach

• Methodology
• Determine limit state for critical systems
• Formulate response functions for each critical
  system element
• Realistic characterization effects of wind, wave,
  and current
• Computationally efficient
• Develop long-term characterization of the
  environment
• Simulate long-term response time history
• Evaluate extreme response statistics
• Identify environments that produced design
  response
• Assess design for controlling environments
• Consideration
• Factors other than environmental conditions may
  have comparable contribution

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Traditional 100-yr Environments(as per ISO
regional annexes)

• West Africa GoM central N. Sea
• Hs 3.9 m 12.6 m 13.6 m
• Tp, associated 15-17 s 14.6 s 15.5-19.4 s
• ws, 1hr,10m 8 m/s 46 m/s 35 m/s
• 3-second gust is 30m/s. (due to West Africa
  squall conditions)

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Why is the issue different for W. Africa?

• Response may be highly resonant near its natural
  frequency.
• In the Gulf of Mexico, which is a semi-enclosed
  sea, there are no long period waves to excite the
  heave resonance.
• In environments like West Africa where there are
  long period swells it may be possible to excite
  this resonance.
• This comparison shows a heave response more than
  10 times greater in a 1m, 25s swell than in the
  100-year GoM hurricane.

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Long-Term Characterization for Environment
45-Year Wave Hindcast

• Assembling a long-term environmental database can
  be problematic.
• Wind and Waves - Hindcast data provided a 45-year
  time history of continuous 6-hourly normal
  winds and waves.
• Squalls Only one year of measured wind data on
  the seasonal frequency and intensity.
• Currents - A long-term synthetic time-series of
  current based on a year of measurements.
• For this region, squalls and currents have little
  correlation to the swell dominated wave
  environment.
• Assembling long-term databases would be more
  straight-forward in mature areas such as the GoM
  or N. Sea but must still be done with care.

45-Year Squall Distribution
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Simulate Long-Term Response Time History
Initialize load environmental database
Analyze next seastate
Compute mean forces moments
Compute offset resulting mooring stiffness
Compute slow-drift, wave-frequency and
wind-induced motions at the keel
Compute min/max stroke in seastate
no
Last seastate ?
yes
Archive results as input to extreme value analysis
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Extrapolation of Response to Extremes
Peak-Over-Threshold Analysis

• With a 45-year sequence of responses,
  extrapolation to a 100-year extreme is
  straightforward.
• If our response functions were perfect we could
  use the results of the analysis directly.
  However, the response model used was an
  approximation and we can only use the analysis as
  a screening tool to determine input conditions.
  
• In past analyses in the GoM where we have used
  extremely long synthetic time-series (500
  years), the 100-year response can simply be
  picked out of the input database.
• In this case we need to back out conditions
  which lead to the 100-year response.

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Determining the 100-Year Stroke Input Condition

• To determine the environmental conditions which
  give rise to the 100-year response we examine the
  conditions which generated the largest peak
  responses.
• None of the responses occurred in the region of
  the 100-year Hs plus the conservative range on
  the associated Tp. In fact the 100-year response
  was more than 50 greater than the response in
  the worst part of the 100-year Hs and associated
  Tp range.
• In this case the top ten responses were all
  caused by conditions with long wave periods,
  modest wave heights and negligible winds and
  currents.
• The environmental conditions driving the 100-year
  stroke response were backed out of the region of
  the top ten responses using the response
  function.

• This result could have also been
• determined by examining 100-year
• Hs-Tp contours. And, for this case with a
• known sharp resonance, a prudent design
• team would explore this option in the
• absence of having performed a response
• analysis.

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Design Cycle Considerations

• The conditions determined by the response
  analysis are dependent on the system
  configuration.
• In a subsequent design cycle where the DDCV
  geometry and mass distribution was changed the
  response analysis was re-run.
• A case unrelated to swells emerged as the peak
  case. A large tilt response to extreme wind
  caused a large pull-down (right).

• Here simply using Hs-Tp contours does not yield
  the critical response. Relying contours requires
  examining other contour dimensions to ensure
  identification of other conditions that may
  govern the extreme response.

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Summary

• Traditional methods based on SPJ experience are
  clearly dated and most of industry has made some
  effort to move ahead with specifications of
  metocean conditions more appropriate for
  floaters.
• Specifying a limited set of cases (e.g.
  wind-dominated, wave dominated etc) in the
  absence of any knowledge of the structure to be
  used is a first step but does not guarantee that
  the 100-year response of every critical system
  element has been considered.
• Judicious use of environmental contours and
  careful consideration of system resonance and
  damping on various components of the system may
  lead to an acceptable range of design cases. In
  cases where damping or VIV lock-in are an
  important part of the response it is not assured
  that the contour approach will identify the
  critical cases.
• Response-based analyses require designers and
  metocean specialists work together in a
  collaborative (rather than sequential) mode to
  identify critical cases. Success requires
• the appropriate responses being screened,
• a good input database,
• good response models,
• appropriate updates of response analysis as
  design matures.
• Satisfying the above conditions is not easy and
  requires a non-trivial analysis and data
  gathering effort.

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The Design Wave Philosophy

• Main problems with the 100-year wave safety
  factor approach
• Failures occur for sub-extreme wave height
  combined with other factors
• Actual level of safety is not known, not
  consistent over different structures, and with
  sometimes costly overconservativeness and
  sometimes dangerous unconservativeness

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The Design Wave Philosophy

• New goal-based approaches
• Define target levels of reliability
• Probability of failure Overall probability
  that simultaneously stochastic action exceeds
  stochastic resistance
• Targets
• 10-2 yearly unmanned, no danger to environment
• 10-3 yearly evacuatable, no danger to
  environment
• 10-4 yearly manned, or danger to environment
• 10-4 yearly is similar to a 10000-year wave, it
  is also different.

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The Design Wave Philosophy

• Standards, class societies, rules and
  regulations
• Consequence-based design, safety factors,
  reliability
• Design wave vs. design sea state
• Statistics and extreme value theory
• Meaning of the 100-year (or 10000-year) wave

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The Design Wave Philosophy
For many kinds of structures, wave height is not
the only wave characteristic leading to
failure. Steepness, wavelength, wave groups,
ringing, springing, beam waves, etc. lead to
consider one or several sea states (durations of,
say, 3 hours) as the design conditions.
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The Design Wave Philosophy

• Two ways to arrive to the design wave
• Extrapolate the maximum waves measured in each
  sea state
• Find the distribution of the largest Hss, and
  perform convolution with the distribution of the
  ratio Hmax/Hs
• The two methods should yield the same final
  value if assumptions are verified and database
  is sufficient.

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The Design Wave Philosophy

• Standards, class societies, rules and
  regulations
• Consequence-based design, safety factors,
  reliability
• Design wave vs. design sea state
• Statistics and extreme value theory
• Meaning of the 100-year (or 10000-year) wave

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Statistics and extreme value theory
How can one extrapolate a few years of data to
yearly probabilities of occurrence of 10-4
? Extreme values theory is a very powerful tool
Using measured or hindcast data of a few decades,
and the independent identically distributed
assumption, it allows to determine the likely
distribution of 10000 year extremes
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Statistics and extreme value theory
Extreme values theory is a very powerful tool
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Statistics and extreme value theory
..., and not forgetting the independent
identically distributed assumption, ...
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Statistics and extreme value theory
What does independent identically distributed
mean ? Independent, in practice, means that a
single event should not be counted more than
once. Designers are very concerned about
independence, and tend to accept higher
uncertainties in order to ensure independence.
Often, they use POT (Peak Over Threshold) to
retain only one value per storm, and may even
consider that 2 storms 3 days apart should be
taken as a single one. In fact, statisticians
have shown that many kinds of slight dependence
do not spoil extreme value extrapolation.
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Statistics and extreme value theory
What does independent identically distributed
mean ? Identically distributed means that events
are of a single kind. A typical case where it is
not verified is locations where hurricanes occur
once in, say, 10 years. Extrapolation from the
main bulk of measurements is thus
useless. Identically distributed is very
difficult to verify, so designers have assumed it
in many cases. Hence the question whether rogue
waves are normal extremes or ones from
nowhere, and its crucial importance.
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The Design Wave Philosophy

• Standards, class societies, rules and
  regulations
• Consequence-based design, safety factors,
  reliability
• Design wave vs. design sea state
• Statistics and extreme value theory
• Meaning of the 100-year (or 10000-year) wave
  (Some points made by Sverre Haver - Statoil)

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Jacket structure in the North Sea
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Target Safety Level of Offshore Structures

• By designing according to Norwegian Rules and
  Regulations, it is tacitly
• assumed that the nominal annual probability of
  structural failure is
• 10-4 10-5 or lower.
• ? A structure should resist all wave events or
  wave induced load events
• corresponding to an annual exceedance probability
  of 10-4 with a proper
• margin (i.e. in worst case some local damage
  damage may be experienced).
• ? Quantity of concern regarding ultimate safety
  is therefore the very, very
• upper tail of the annual distribution function of
  wave events and loads.

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Target Safety Level of Offshore Structures

• Regarding overload failures, industry aims to
  fulfill target by the followingdesign
  controlsi) Ultimate Limit State
  (ULS)Component based control ensuring that the
  10-2 annual probability loadsmultiplied by a
  load factor are lower than a low percentile of
  the elasticcomponent capacity divided by a
  material factor. ii) Accidental Limit State
  (ALS)System based control ensuring that the 10-4
  annual probability load is smallerthan the the
  system capacity.

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Governing limit state (introducing the ugliness
property)
ALS governs design
ULS governs design
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If freak waves exist what is the problem?
For ship and platforms, freak waves will mainly
represent a problem if their crest hits a
structural element which is not designed for wave
loads.