1. Offshore wind turbine design Addressing uncertainty drivers
Sten Frandsen
Niels Jacob Tarp-Johansen
Erik Asp Hansen
Michael Høgedal
Lars Bo Ibsen
Leo Jensen
Risø National Laboratory
DHI – Water and Environment
Vestas Wind Systems
Aalborg University
ELSAM Engineering

2. Utilising experience from demonstration projects
Objectives of work presented:
To get the most out of the two Danish demonstration projects
in terms of structural design

3. North Sea: Horns Rev - monopile

4. ….and Nysted: Demonstration project in the Baltics
Concrete, gravity foundations ( km )
Nysted

5. 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

6. The design process
Response integration

7. Measurements COMPONENTS: FOCAL POINTS: Model 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

8. Measurements on offshore wind farms
measurements wind and wakes waves and current soil conditions aero/hydro-response synthesis

9. Instrumentation, Horns Rev
Wind turbine
Wind
Waves
measurements wind and wakes waves and current soil conditions aero/hydro-response synthesis

10. 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
measurements wind and wakes waves and current soil conditions aero/hydro-response synthesis

11. Extreme gust during normal operation
measurements wind and wakes waves and current soil conditions aero/hydro-response synthesis

12. 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.
measurements wind and wakes waves and current soil conditions aero/hydro-response synthesis

13. Soil conditions COMPONENTS Design drivers in general: FOCAL POINTS:
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.
measurements wind and wakes waves and current soil conditions aero/hydro-response synthesis

14. 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
measurements wind and wakes waves and current soil conditions aero/hydro-response synthesis

15. Handling and synthesis of response calculations
1st edition of standard for design of offshore wind turbines IEC 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
measurements wind and wakes waves and current soil conditions aero/hydro-response synthesis

16. Load cases – who many do we need?
measurements wind and wakes waves and current soil conditions aero/hydro-response synthesis

17. Computational needs with present draft standard
A complete set of simulations:
runs of each 10min duration
measurements wind and wakes waves and current soil conditions aero/hydro-response synthesis

18. Extreme load cases Normal operation Special load cases
measurements wind and wakes waves and current soil conditions aero/hydro-response synthesis

19. 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.
measurements wind and wakes waves and current soil conditions aero/hydro-response synthesis

20. Suggested structure of design process
Aim is to optimize structural design by
reducing uncertainty of component models
reducing uncertainty in load systhesis
rationalising load cases
tune load case synthesis
Thus, main design components:
Site assessment
Dynamic analysis
Load cases and synthesis
measurements wind and wakes waves and current soil conditions aero/hydro-response synthesis

21. Site assessment Measurements and hindcast
Joint probability density function (JPDF)
ways of estimating its uncertainty
measurements wind and wakes waves and current soil conditions aero/hydro-response synthesis

22. 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
measurements wind and wakes waves and current soil conditions aero/hydro-response synthesis

23. 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
Extreme response from
Fatigue from
measurements wind and wakes waves and current soil conditions aero/hydro-response synthesis

24. 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