![• Gulf of Maine
• East coast in the United States of North
America and closely linked to the
medium severity site as defined in
Lifes50+ [Krieger et al., 2015]
• Water depth 130m
• Wind environment (as derived by
measurement buoy)
• Weibull scale coefficient of 6.2
• shape coefficient of 1.7
8
Reference environmental conditions
Portland
https://oceanservice.noaa.gov/
Gulf of Maine, U.S.A](https://image.slidesharecdn.com/omae2018-78244rfgforonline-180625123459/85/Simulation-requirements-and-relevant-load-conditions-in-the-design-of-floating-offshore-wind-turbines-8-320.jpg)
![• In order to determine the critical environmental conditions across a wide range of values, a sensitivity
analysis can be used
• Sampling methods help reduce amount of simulations: Sobol sampling method is used as described in
[Müller,2017]
18
3- Sensitivity to environmental parameters
Simulation conditions for power production case (here
power production for fatigue design DLC1.2)
Three load ranges differentiate between controller
mode:
• LR1: below rated wind speed
• LR2: around rated wind speed
• LR3: above rated wind speed
3 seeds simulated with average used for evaluation Scatter plot of environmental
yellow: LR1, red: LR2, green: LR3.](https://image.slidesharecdn.com/omae2018-78244rfgforonline-180625123459/85/Simulation-requirements-and-relevant-load-conditions-in-the-design-of-floating-offshore-wind-turbines-18-320.jpg)
![Environmental condition Values
LR1
wind speed [m/s] 4 : 0.1 : 10.2
wave height [m] 0.3 : 0.1 : 3.1
wave period [s] 2.5 : 0.1 : 10.7
LR2
wind speed [m/s] 10.2 : 0.1 : 13.8
wave height [m] 0.3 : 0.1 : 4
wave period [s] 2.5: 0.1 : 10.7
LR3
wind speed [m/s] 13.8 : 0.1 : 25
wave height [m] 1.2 : 0.1 : 6.6
wave period [s] 4 : 0.1 : 10.7
Turbulence intensity [%] Class C
Wind direction [°] 0
Wind seeds / wave seeds 3
Wave direction [°] 0
Current speed [m/s] 0
Current direction [°] 0
Simulation length
4,600 (3,600 for
statistics)
Total number of simulations 2799 19
3- Sensitivity to environmental parameters
Note: methodology could be
expanded to include more
environmental parameters
that vary
Environmental boundary conditions for sensitivity study
based on DLC1.2](https://image.slidesharecdn.com/omae2018-78244rfgforonline-180625123459/85/Simulation-requirements-and-relevant-load-conditions-in-the-design-of-floating-offshore-wind-turbines-19-320.jpg)
![• Wind and wave environment are stochastic: how many seeds need to be simulated to
obtain an accurate fatigue estimation?
23
4- Number of seeds needed
One set of environmental conditions for Power production
case with large DEL was chosen (rated wind)
1000 simulations were performed
Example distribution of tower base
bending moment DEL values.
Case: change wind
and wave seeds
Run in time later
removed [s]
600
Total simulation length
[s]
4,200
Simulations 1,000
Wind speed [ms-1] 12
Turbulence intensity [%] 14.6
Wave height [m] 6.3
Wave period [s] 7.8](https://image.slidesharecdn.com/omae2018-78244rfgforonline-180625123459/85/Simulation-requirements-and-relevant-load-conditions-in-the-design-of-floating-offshore-wind-turbines-23-320.jpg)
![• DNV offshore standard for design of floating wind
turbines [Det Norske Veritas, 2013]
• For operating conditions, a minimum of 3 hours are
to be simulated “to adequately capture effects such
as nonlinearities, second order effects, and slowly
varying responses, and to properly establish the
design load effects.”
• But it may good to split a simulation into a larger
number of shorter simulations
• E.g. parallelise computation
• 3 hours wind files are too large
26
5- Effect of simulation length
Case: change wind
and wave seeds
Run in time later
removed[s]
600
Total simulation length [s] 4,200
Simulations 1,000
Wind speed [ms-1] 12
Turbulence intensity [%] 14.6
Wave height [m] 6.3
Wave period [s] 7.8
Simulation settings
• Simulation study: 1000 simulations, each with
different wind and wave seed ( periodic wind files)
• statistical value of one simulation is compared to the
option of combining a multitude of shorter
simulations.](https://image.slidesharecdn.com/omae2018-78244rfgforonline-180625123459/85/Simulation-requirements-and-relevant-load-conditions-in-the-design-of-floating-offshore-wind-turbines-26-320.jpg)
Simulation requirements and relevant load conditions in the design of floating offshore wind turbines
- 1. Stuttgart Wind Energy (SWE) @ Institute of Aircraft Design Simulation requirements and relevant load conditions in the design of floating offshore wind turbines Ricardo Faerron 1, Kolja Müller 1, Po Wen Cheng 1, Luca Vita 1-University of Stuttgart International Conference on Ocean, Offshore & Arctic Engineering June 2018, Madrid
- 2. • The industry has little experience compared to fixed offshore and onshore turbines • Limited specifics when it comes to guidelines for simulation • Many different floating platform concepts • Different dynamics than fixed offshore turbine, lead to longer simulation times • 1 to 3 hours per seed 2 Why investigate the simulation requirements and relevant load conditions for floating wind turbines? Need to investigate methodologies and best practices for simulations
- 3. Overview • Wind Energy Research at University of Stuttgart • Introduction to the LIFES50+ Project • Reference model • Design requirements of floating wind turbines 3
- 4. Research Groups at SWE 1- Testing and Measurement 2- Control, Optimization and Monitoring 3- Conceptual Design and System Simulation 4 Wind Energy Research at the University of Stuttgart • Aero-Servo-Hydro- Elastic MBS simulation of WT • Detailed Modelling of subcomponents • Code development • Floating platform design
- 5. OBJECTIVES • Optimize and qualify to Technology Readiness Level (TRL) of 5, two substructure designs for 10MW turbines. • Develop a streamlined and KPI (Key Performance Indicator) based methodology for the evaluation and qualification process of floating substructures. 5 Investigation within the LIFES50+ Project
- 6. 6 Reference design turbine and platform Set up of model Publicly available turbine and platform models • DTU 10MW reference wind turbine • Dr. techn. Olav Olsen AS floating platform • Anchored to the sea bed with 3 catenary steel mooring lines with additional ballasting weights • SWE controller Time domain simulation with NREL’s FAST8 • Aerodyn v14 module for aerodynamics • Hydrodyn module with first order potential flow theory with Morison drag forces for hydrodynamic • Dynamic model of mooring lines with Moordyn • Periodic 10 min full-field turbulent wind fields www.olavolsen.no/
- 7. What do we want to analyse? • Ultimate loads • Fatigue loads (damage equivalent loads or DEL) • Assumes a straight S-N curve (stress vs number of cycles) on a log-log scale • Wohler coefficient 4 7 Blade Flapwise bending moment Fairlead tension Tower fore-aft bending moment www.olavolsen.no/ Load sensors
- 8. • Gulf of Maine • East coast in the United States of North America and closely linked to the medium severity site as defined in Lifes50+ [Krieger et al., 2015] • Water depth 130m • Wind environment (as derived by measurement buoy) • Weibull scale coefficient of 6.2 • shape coefficient of 1.7 8 Reference environmental conditions Portland https://oceanservice.noaa.gov/ Gulf of Maine, U.S.A
- 9. 1 - Pre-simulation initial conditions 2- Initial transient effects and run-in time 3 - Sensitivity to environmental parameters 4 - Effect of simulation length on DEL 5 - Number of seeds needed 6 - Influence of wave peak shape parameter (JONSWAP spectrum) 9 Simulation requirements for floating turbines Note: due to the wide range of possible platform geometries and dynamics, emphasis should be given to the methodology
- 11. • Initial conditions as inputs for the simulations are important to reduce the simulation times by reducing the transient behaviour during start-up 11 1- Pre-simulation initial conditions Simulation requirements for floating turbines Simulation conditions for Power production case (DLC 1.2) • Deterministic wind: 4-24m/s • No waves Analysis of the mean value for different 600s time frames Example simulation results for platform heave for determination of initial conditions values𝝁 𝑛𝑜𝑟𝑚 𝑡 𝑠𝑡𝑎𝑟𝑡, 𝑣 = 𝝁 𝒕 𝑠𝑡𝑎𝑟𝑡, 𝒗 𝝁 3001𝑠, 𝒗 𝒕𝑖,𝑠𝑡𝑎𝑟𝑡 = 1𝑠, 101𝑠, … , 2901𝑠 𝒕𝑖,𝑒𝑛𝑑 = 𝒕𝑖,𝑠𝑡𝑎𝑟𝑡 + 600𝑠 t1 = 0-600s
- 12. Important degrees of freedom: • Rotor speed • Generator speed • Blade pitch angle • Tower top fore-aft displacement • Platform surge • Platform heave • Platform pitch 12 1- Pre-simulation initial conditions Simulation requirements for floating turbines Convergence of normalized mean value for platform pitch DOF, all wind speeds (each colour is a different wind speed)
- 13. • The platform heave takes the longest to converge • This is due to the fact that the mean value of the final time frame of some simulations is almost 0 • Already reaches a value with less than 10% deviation from the expected value with a starting time of 1101s for the time window. • Implies total simulation time for starting conditions would be of 1701 13 1- Pre-simulation initial conditions: Results Simulation requirements for floating turbines Convergence of absolute maximum deviation for all analysed sensors taking into account all wind speeds
- 14. 14 2- Initial Transient effects and Run-in Time
- 15. 𝜎 𝑛𝑜𝑟𝑚(𝑡) = 𝜎 𝑡 𝜎 𝑡 = 2000 t T t = 2000 T 𝜎 = 𝑠𝑡𝑎𝑛𝑑𝑎𝑟𝑑 𝑑𝑒𝑣𝑖𝑎𝑡𝑖𝑜𝑛 • Check for stationarity of the load sensor: Initial transients time need to be disregarded • Tested different approaches: moving average, convergence of frequency response, forward statistic convergence, • Standard deviation 𝜎 of the remaining time series was chosen 15 2- Initial Transient effects Run-in Time Simulation study • 13698 Simulation • 8 varying environmental conditions (case of severe sea state as in DLC1.6) • Total simulation length T = 11800 seconds • Check convergence of the normalized standard deviation
- 16. Longest to settle at a stable standard deviation • Platform surge • Platform sway Results Stationarity can be seen after 600s, were the STD of the simulations is within 5% of the expected value, with 1000s being a conservative value. Further investigation could include Heidelberger and Welch's convergence diagnostic 2- Initial Transient effects Run-in Time PtFm=platform,TT=Towertop,Dsp=displacement Run-in-time evaluation for DLC 1.6.
- 17. 17 3- Sensitivity to environmental parameters
- 18. • In order to determine the critical environmental conditions across a wide range of values, a sensitivity analysis can be used • Sampling methods help reduce amount of simulations: Sobol sampling method is used as described in [Müller,2017] 18 3- Sensitivity to environmental parameters Simulation conditions for power production case (here power production for fatigue design DLC1.2) Three load ranges differentiate between controller mode: • LR1: below rated wind speed • LR2: around rated wind speed • LR3: above rated wind speed 3 seeds simulated with average used for evaluation Scatter plot of environmental yellow: LR1, red: LR2, green: LR3.
- 19. Environmental condition Values LR1 wind speed [m/s] 4 : 0.1 : 10.2 wave height [m] 0.3 : 0.1 : 3.1 wave period [s] 2.5 : 0.1 : 10.7 LR2 wind speed [m/s] 10.2 : 0.1 : 13.8 wave height [m] 0.3 : 0.1 : 4 wave period [s] 2.5: 0.1 : 10.7 LR3 wind speed [m/s] 13.8 : 0.1 : 25 wave height [m] 1.2 : 0.1 : 6.6 wave period [s] 4 : 0.1 : 10.7 Turbulence intensity [%] Class C Wind direction [°] 0 Wind seeds / wave seeds 3 Wave direction [°] 0 Current speed [m/s] 0 Current direction [°] 0 Simulation length 4,600 (3,600 for statistics) Total number of simulations 2799 19 3- Sensitivity to environmental parameters Note: methodology could be expanded to include more environmental parameters that vary Environmental boundary conditions for sensitivity study based on DLC1.2
- 20. 20 3- Sensitivity to environmental parameters Exemplary scatterplots : tower base bending moment DEL
- 21. 21 3- Sensitivity to environmental parameters • The chi-squared test is calculated for each environmental parameter and load range • The test results in a p-value which gives the probability that the correlation between two observed variables is random • Data can then be ranked Ranking table • Importance • Useful for early design stages to understand the system • When used for more parameters one can determine the most important ones which drive the loads
- 22. 22 4- Number of seeds needed
- 23. • Wind and wave environment are stochastic: how many seeds need to be simulated to obtain an accurate fatigue estimation? 23 4- Number of seeds needed One set of environmental conditions for Power production case with large DEL was chosen (rated wind) 1000 simulations were performed Example distribution of tower base bending moment DEL values. Case: change wind and wave seeds Run in time later removed [s] 600 Total simulation length [s] 4,200 Simulations 1,000 Wind speed [ms-1] 12 Turbulence intensity [%] 14.6 Wave height [m] 6.3 Wave period [s] 7.8
- 24. • Bootstrap analysis with increasing number of seed based on 5000 draws with replacement • The median, 1st, 5th, 95th and 99th percentile values are determined • Largest scattering: blade root flapwise bending moment • For it, in order to be within 5% of the expected mean 95% of the time, one needs to perform 8 simulations (4 seeds to be within 6%) 4- Number of seeds needed: Results Plot of Lifetime DELs bootstrap evaluation (Red horizontal lines indicate median, box borders 95th percentile and whiskers 99th percentile values. Normalized against the Lifetime DEL of all 1,000 simulation)
- 25. 25 5- Effect of simulation length
- 26. • DNV offshore standard for design of floating wind turbines [Det Norske Veritas, 2013] • For operating conditions, a minimum of 3 hours are to be simulated “to adequately capture effects such as nonlinearities, second order effects, and slowly varying responses, and to properly establish the design load effects.” • But it may good to split a simulation into a larger number of shorter simulations • E.g. parallelise computation • 3 hours wind files are too large 26 5- Effect of simulation length Case: change wind and wave seeds Run in time later removed[s] 600 Total simulation length [s] 4,200 Simulations 1,000 Wind speed [ms-1] 12 Turbulence intensity [%] 14.6 Wave height [m] 6.3 Wave period [s] 7.8 Simulation settings • Simulation study: 1000 simulations, each with different wind and wave seed ( periodic wind files) • statistical value of one simulation is compared to the option of combining a multitude of shorter simulations.
- 27. 27 5- Effect of simulation length 2- Box with 1000 simulations of same length (t seconds) 3- Pick X number of simulations from box to make up 3600s ( e.g. 6 * 600s) 5- Repeat process 5000 times to obtain statistics of DEL 4- Calculate Lifetime DEL of combined X simulations 1- For each of the 1000 simulation, different time frames are taken t1 t2 t3 t…
- 28. 28 5- Effect of simulation length: results 5th and 95th percentile of Lifetime DEL with varying simulations length. (normalized to the lifetime DEL of the 1000 simulations with 3600s) Result show: • Decrease on simulation length does not effect statistics of the sampled DELs for the fairlead tension and the tower base fore-aft bending moment • Changes in statistics of blade root flapwise DEL • Remembering that wind field is cycled every 10 min (i.e. periodic) • Using shorter simulation times means more wind seeds are considered in the individual DEL calculation • Lead to less spreading
- 29. • Simulation setting should not be overlooked as important factors for calculation of ultimate and fatigue loads • Good interpretation and sometimes further review of results is important to come up with an appropriate analysis of the data • Findings can be platform concept dependent (excitation frequencies and hydrodynamics) and focus should be given to the methodology • Continuing work • expansion of the considered environmental conditions • analysis of second platform concept (Nautilus platform) • Future report: Deliverable 7.7 Lives50+ 29 Summary and Remarks
- 30. e-mail phone +49 (0) 711 685- fax +49 (0) 711 685- University of Stuttgart Thank you Questions? Ricardo Faerron 68243 faerron@ifb.uni-stuttgart.de The research has received partial funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 640741 (LIFES50+). We are grateful to Dr. techn. Olav Olsen AS for the permission and contribution to the public semi-submersible design based on their OO Star Wind Floater (www.olavolsen.no).
- 31. • A. Krieger, G. K. V. K. Ramachandran, L. Vita, P. G. Alonso, G. G. Almeria, J. Berque and G. Aguirre, "LIFES50+ D7.2 Design Basis," 2015. • Det Norske Veritas, “Design of Floating Wind Turbine: DNV-OS-J103”, Det Norske Veritas AS, 2013. • K. Müller and P.W. Cheng, "Application of a Monte Carlo Procedure for Probabilistic Fatigue Design of Floating Offshore Wind Turbines," Wind Energ. Sci., 3, pp. 149-162, 2018 • K. Müller et. al., "LIFES50+ D7.7 Identification of critical environmental conditions and design load cases," 2018, to be published. 31 References