1 Offshore wind turbine design Addressing uncertainty drivers Sten Frandsen Niels Jacob Tarp-Johansen Erik Asp Hansen Michael Høgedal Lars Bo Ibsen Leo

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Offshore wind turbine designAddressing uncertainty drivers

Sten FrandsenNiels Jacob Tarp-Johansen

Erik Asp Hansen

Michael Høgedal

Lars Bo Ibsen

Leo Jensen

Risø National LaboratoryRisø National Laboratory

DHI – Water and Environment

Vestas Wind Systems

Aalborg University

ELSAM Engineering

i n d u s t r y r e s e a r c h u n i v e r s i t i e s u t i l i t i e s o f f s h o r e c o n s u l t i n g

i n d u s t r y r e s e a r c h u n i v e r s i t i e s u t i l i t i e s o f f s h o r e c o n s u l t i n g

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Utilising experience from demonstration projects

(km)

Nysted

Horns Rev

Objectives of work presented:

To get the most out of the two Danish demonstration projectsin terms of structural design

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North Sea:Horns Rev - monopile

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….and Nysted:

(km)

Nysted

• Demonstration project in the Baltics

• Concrete, gravity foundations

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Design components

Design in general has many important sub-components like:

•Environmental protection

•Cabling

•Preparation and construction

•etc.

– and:

•Structural loads, response and design loads

For this project the target is design loads, extremes and fatigue:

•Measurements

•Wind and wakes

•Waves and current

•Soil conditions

•Response

•Synthesis of load cases

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The design process

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Measurements

COMPONENTS:

• Verification measurements– Models

– Design calculations

• External conditions’ recording – Atmospheric measurements,

incl. wake measurements

– Wave measurements

– Geotechnical measurements

FOCAL POINTS:• Model verification measurements

– Interpretation of specific loads based on response measurements.

• Load response measurements– Extremes from measurement.

• Metocean measurements– Measurement of 2D/3D wave kinematics.

– Measurements of scour level.

• Wake measurements– Conceptual – which characteristics of the

wake affect the wake loading and how?

– Relationship between the wind turbine characteristics and flow characteristics

– Reference mean wind speed.

– Measurement of turbulence, wake width etc. to which the rotor is actually exposed.

measurements wind and wakes waves and current soil conditions aero/hydro-response synthesis

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Measurements on offshore wind farms


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Instrumentation, Horns Rev

Wind turbine Wind Waves


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Wind and wake-induced loads

COMPONENTS:• External conditions

– Gusts, mean wind climate, turbulence, 50y extreme wind, shear, spectra, air density

• Wakes– CT curves for stall and pitch-regulated

wind turbines

– Turbulence as function of separation and wind speed

– Separation of effect of turbulence and mean deficit

– Discrete flow structures in wakes

• Rotor aerodynamics– Adequacy of contemporary aero-codes

• FOCAL POINTS:• Extreme gusts during normal

operation

• CT curves vs. turbulence and velocity deficit

• Models for aerodynamic rotor loading

• Updating of models for extreme and fatigue loading


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Extreme gust during normal operation

0

500

1000

1500

2000

2500

3000

3500

0 2 4 6 8 10

Umax [m/s]

Mflap

[kN

m]


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Waves and current

COMPONENTS• Determination of the sea

states that shall be considered in design (wave height, period and direction, current, current direction, water level, etc.).

• Evaluation of combined undisturbed wave and current kinematics at the OWT position.

• Evaluation of the time varying loads from the kinematics.

FOCAL POINTS• Cases where the engineering

models are sufficiently accurate and cases where more precise modelling is required.

• Determination of kinematics and time varying loads..

– ..from steep waves superimposed on current, by use of a 3 dimensional Navier-Stokes solver, to be applied where simpler methods are not applicable. This action is parallel to attempts to derive approximate methods for implementation in aero-elastic codes.

• Contributions from loads on appurtenances.

• Determination of maximum run-up.


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Soil conditions

COMPONENTS

Design drivers in general:– Soil condition

– Water depth

– Possible erosion

– Size and type of wind turbine

– Environmental conditions (wave height, current, ice, etc.)

– Economics and politics

• FOCAL POINTS:• Lateral pile resistance

– The ULS calculations for the piles are to be compared with an elasto-plastic finite element Analysis with the same soil conditions.

• Load-deflection curves (p-y curves):

– Performance of the p-y curves for pile diameters larger than 4 meters should be investigated.

– Curvature of the p-y curves in the pre-plastic portion is typically approximated by parabolic expressions. These approximations are not useful in the small-strain area.

• Behaviour of piles under cyclic loading.


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Aero/hydro-elastic modelling

COMPONENTS• Effect of choice of structural

modelling scheme on the response modelling, viz. e.g. finite-element method or modal formulation used for structural elements

• Aerodynamic and hydrodynamic load models’ ability to determine correctly the loading, based on proper input parameters representing the external conditions

• Foundation models ability to represent correctly the soil- structure interaction for both simple and more complex foundation types and the effect of the selected implementation scheme on the response modelling

FOCAL POINTS• Compare the response from

simulations from different structural models on a generic turbine on a number of identical artificial test conditions

• Compare the response from different structural models with measurements of response and external parameters exist

• Feasibility of improving elements of the structural models

• Implement improvements and quantify reduced uncertainties by performing verification modelling


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Handling and synthesis of response calculations

• 1st edition of standard for design of offshore wind turbines IEC61400-3 is currently being issued in its final draft

• Through a lengthy process the draft was created as an extension of the onshore standard

• This implies, that relative to the onshore standard the number of load cases has increased substantially due to the addition of wave loads

• In turn, also the composition of relevant load cases has become a more difficult task


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Load cases – who many do we need?


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Computational needswith present draft standard

A complete set of simulations:

1000-1500 runs of each 10min duration


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Extreme load cases

0

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10000

15000

20000

25000

30000

35000

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Normal operation Special load cases


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What – if anything – is less than perfect in IEC61400-3?

• Not enough specific in terms of definition of loads and how to combine load cases

• The extend of load cases may signal comprehensiveness, also in terms of accuracy

• The many load cases in reality reflect uncertainty – “some extra load cases don’t hurt”

• The instructions regarding how to perform site assessment are not sufficiently specific

• Several components – viz. the descriptions given previously in this talk – may be improved

• First and last – there are too many load cases.


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Suggested structure of design process

Thus, main design components:• Site assessment• Dynamic analysis• Load cases and synthesis

Aim is to optimize structural design by

– reducing uncertainty of component models

– reducing uncertainty in load systhesis

– rationalising load cases– tune load case synthesis


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Site assessment

• Measurements and hindcast– Joint probability density function

(JPDF)– ways of estimating its uncertainty

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Dynamic analysis

• Investigate the dynamic properties of the wind turbine– adequacy of aero/hydro elastic model– Identify possible peculiarities of design

• This action should encompass special load cases

– Investigate whether structure can be subdivided into components

– Sensitivity analysis, including effect of joint-action of climate variables; theresponse function

– is to be mapped for all relevant combinations of x

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Load cases and synthesis

• Formal load cases should cover wt in operation

• Derive conditional distributions on basis on prescribed random variability of response, conditioned on external conditions

• Simulate without aero/hydro-elastic code to determine unconditional distribution

• Derive equivalent load conditioned on external conditions

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Conclusion

• Optimisation and decrease of uncertainty of the design of offshore wind turbines are sought

• Therefore, a number of design-process components that contribute the most to the aggregated uncertainty has been identified

• Considerations regarding handling of load cases and synthesis these have been presented

Documents

1 Offshore wind turbine design Addressing uncertainty drivers Sten Frandsen Niels Jacob Tarp-Johansen Erik Asp Hansen Michael Høgedal Lars Bo Ibsen Leo