Trondheim Tekniske Fagskole The “Design Wave Philosophy’’ Calculation of the design...

Trondheim Tekniske Fagskole

***** The “Design Wave Philosophy’’ *****Calculation of the design waveWave forces on semi-submersible platformsWave forces and bending moments in FPSO-shipsPlatform movements in large wavesExamples of heavy weather damageWhat 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 conditionsSearch And Rescue and emergency operationsDecision making in an emergency

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



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 70’s North Sea

• The ”Alexander Kielland” accident occurred in 1980



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)



…ended up into a “philosophy for design”.

In the North Sea, design is determined by extreme

waves, and at the time (80’s), 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.



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.



• Standards, class societies, rules and regulations

• Consequence-based design, safety factors, reliability (some points made by Markku Santala - Exxon)





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


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– Response=f(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


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.


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


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)


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.

GOM – 100 YR Wave

West Africa - Swell


Long-Term Characterization for Environment

• 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 Wave Hindcast

45-Year Squall Distribution


Simulate Long-Term Response Time History

Compute offset & resulting mooring stiffness

Compute meanforces & moments

Compute min/maxstroke in seastate

Compute slow-drift,wave-frequency and wind-induced motions

at the keel

Archive results as input toextreme value analysis

Initialize & loadenvironmental database

Last seastate ?

Analyze nextseastate

no

yes


Extrapolation of Response to Extremes

• 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.

Peak-Over-Threshold Analysis


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.


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.


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.



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



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.




• Consequence-based design, safety factors, reliability

• Design wave vs. design sea state• Statistics and extreme value theory




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.



Two ways to arrive to the “design wave”:

• Extrapolate the maximum waves measured in each

sea state

• Find the distribution of the largest Hs’s, 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.






• Statistics and extreme value theory• Meaning of the 100-year (or 10000-year) wave


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



Extreme values theory is a very powerful tool



..., and not forgetting the “independent identically

distributed” assumption, ...



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.



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.







• Meaning of the 100-year (or 10000-year) wave (Some points made by Sverre Haver - Statoil)


Jacket structure in the North Sea


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.


Target Safety Level of Offshore Structures

Regarding overload failures, industry aims to fulfill target by the followingdesign controls:

i) 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.


Governing limit state (introducing the ugliness property)

0 1 2 3 4 5

- log(annual exccedance probability)

sc,ULS

1.3*sc,ULS

Load-level

Well-behaving problem

Bad-behaving problem

sc,ALS,1

sc,ALS,2

ALS governs design

ULS governs

design


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.

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Trondheim Tekniske Fagskole The “Design Wave Philosophy’’ Calculation of the design...

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Transcript of Trondheim Tekniske Fagskole The “Design Wave Philosophy’’ Calculation of the design...

Trondheim Tekniske Fagskole *****The “Design Wave Philosophy’’ ***** Calculation of the design...

Documents

Transcript of Trondheim Tekniske Fagskole *****The “Design Wave Philosophy’’ ***** Calculation of the design...

Trondheim Tekniske Fagskole The “Design Wave Philosophy’’ Calculation of the design...

Transcript of Trondheim Tekniske Fagskole The “Design Wave Philosophy’’ Calculation of the design...