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Ken Lee: 2013 Sandia National Laboratoies Wind Plant Reliability Workshop

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Sandia National Laboratories: Energy & Climate: Renewables
Sandia National Laboratories: Energy & Climate: Renewables

Current wind turbine component design and certification methods may underestimate the effects of low-cycle fatigue (LCF), contributing to premature structural failures. LCF refers to high-amplitude stresses that occur at low cycle counts and can induce crack growth faster than predicted. Guidelines do not fully account for transient loads from emergency stops or faults, which analysis shows can significantly reduce fatigue life. Future research is needed to better model LCF effects, manufacturing defects, and increase load spectra resolution to improve structural design and certification methods.

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Underestimated Low-Cycle Fatigue as a Contributor to Premature Failures Ken T. Lee, MSAE, Blade Design Technical Lead Amir Bachelani, Blade Structural Engineer Cody Moore, Loads & Dynamics Engineer Kyle K. Wetzel, Ph.d., CEO/CTO Sandia Blade Reliability Workshop 2013 Albuquerque, New Mexico
Outline  Wind Turbine Component Damage/Failure  Low-Cycle Fatigue – Premature Failures  Design Loads for Certification  Fatigue Analysis for Certification  Recommendations for Future Research Copyright © 2013 Wetzel Engineering, Inc. All Rights Reserved.
WT Component Damage/Failure Share of main components of total number of failures Damage/failure occurrences attributed to: electrical systems rotor blades gearbox Structural in nature sustained early in the life of these components (sometimes within 3 or 6 months to a year)  continuous and costly maintenance and repair
WT Component Damage/Failure Wind Turbine subassembly reliability: WT reliability & downtime varies across subcomponents of WT Failure frequencies of blades and gearboxes  lower compared to other subcomponents Downtime per failure for rotor blades and gearbox  higher compared to other subcomponents Pitch mechanism & electrical system shown to be significant sources. Failure or fault-induced shutdowns
Common WT Component Damage/Failure Common Types of Rotor Blade Damage/Failure: Crack propagation due to inherent design or manufacturing defects Blade damage due to extreme load buckling or blade-tower strike Failure of adhesive bonds  leading edge/trailing edge splitting Blade failure at the blade root connection  blade throw
Causes of WT Component Damage/Failure Operating loads exceed design loads: Underestimated Design Loads (Ultimate, Low-cycle fatigue, or High-cycle fatigue)  Mistakes in applying standard methods  Shortcomings in standard methods Malfunctioning control safety systems Improper site assessment  Insufficient assessment of structural integrity: Mistakes in applying standard methods Shortcomings in standard methods
Low-Cycle Fatigue (LCF) Possible Contributor to Premature Wind Turbine Component Failures Focus: LCF – Low-cycle fatigue characterized as: • High amplitude alternating stress • Low cycle counts (approx. less than 104) • Fatigue failure: propagation-dominated
LCF - Overview Components designed to 20 years of life (according to GL, IEC, DNV) Premature failure of WT subcomponents: Rotor Blades Gearbox Structural design & analysis of WT Components, Current Provision: Fatigue design focused on high-cycle fatigue (HCF) (i.e. S-N curve methods). High amplitude loading events that occur at low cycle counts induce crack growth rates that far exceed that predicted by S-N curve analyses.
LCF - Overview Design Loads Simulations, Current Framework: Does not capture high-amplitude transient events (less than 104 – 504 cycles) Insufficient resolution of the “tails” of the loads spectra  “Normal-operation” during turbulent conditions Under-reporting of loads induced by transient events, coherent inflow conditions, fault- induced shutdowns. Blade Load Spectra as per Current Framework for Certification Design Loads: Average Normalized Cumulative Blade Load Spectra with/without LCF:
Design Loads for Certification Case Study on the Impact of Emergency Stops on Blade Fatigue Life
GL -Design Load Cases for Fatigue Analysis •Power Production Load Case: DLC 1.1; NTM; Vin < V < Vout •Idling or Parked Load Case: DLC 6.4; NTM; V<Vin and Vout < V < Vref
GL -Design Load Cases for Fatigue Analysis •Transient Load Cases, Faults during Operation: DLC 1.4; NWP; Grid-loss/E-stop – 20/year DLC 1.8; NWP; Iced Conditions – 24 hours/year DLC 2.1; NWP; Fault Occurrence – 24 hours/year DLC 3.1; NWP; Start Up - 1000/year @ Vin, 50 @ Vr & Vout DLC 4.1; Normal Shutdown - 1000/year @ Vin, 50 @ Vr & Vout
Possible Shortcomings in Current Guidelines • WT components could possibly be failing prematurely from fatigue. • Current guidelines do not account for higher frequency rates of E-stops and/or Fault-Induced shutdown procedures  • E-stops due to grid loss could have a higher impact on fatigue life • E-stops of more than 20/year, averaging 1 E-stop/day (360/year)  Early life of WT operation, debugging of control and safety systems Control system malfunction in response to abnormal inflow conditions • Impact of an E-stop or a pitch control system fault during a gusting wind (with or without turbulent inflow) not considered.
Case Study of Impact of Increased Frequency of E-Stop Procedures • Fatigue life of two turbines analyzed  the impact of stricter provisions for Emergency Shutdown (E-stops)  20 cases/year of DLC 1.1 NTM (GL- 2010), grid loss timed at highest point of My  Increase cycle counts of DLC 1.4 (NWP) Estops from 20/year to 360/year (7200 in turbine lifetime)  1 case/year of DLC 1.5 EOG1 (GL- 2010), grid loss timed at highest point of My  21 cases/year of grid loss during a gusting wind were added to the analysis
Results: 100 kW Turbine – 13m Blade Variable Speed-Pitch Regulated
Results: 2.0MWTurbine – 45m Blade Variable Speed-Pitch Regulated
Fatigue Analysis Methods for Certification
Current Fatigue Analyses Guidelines Fiber-Reinforced Composite Laminates: Characteristic S-N curve established for laminate. (GL 5.5.3.3.1) Goodman diagram constructed using this curve. (GL 5.5.3.3.1) Fatigue damage calculation using Miner’s rule (IEC 61400-1, 7.6.3.2) 95 % survival probability with a confidence level of 95 % used as basis for SN- curve (IEC 61400-1, 7.6.3.2) Adhesives:  3 samples (being representative for the jointed components in geometry and material)  Minimum number of load cycles of N=106. (GL 5.5.6.10)
•HCF estimation is captured. LCF response of WT subcomponents/blade composites should also be considered in guidelines. •Testing on pristine laminates. Manufacturing defects not tested. •Structural damage/failures due to: Manufacturing that is out of QC Failure to design for manufacturing quality that can be realistically controlled. •Current Safety Factors (SF’s) result in excessively conservative designs in areas not really the source of problems. •Heavy reliance on SF’s still fails to address on-going problems industry has with blade reliability. Current Fatigue Analyses Guidelines
Types of Damage & Defects • Delamination •Voids & Cracks in Laminates/Adhesive Bonds • Wavy fiber plies, bridging of fibers • Porosity, discontinuities in laminates
Crack Growth in Fiber-Reinforced Plastics •Region I – Matrix Cracking •Region II – Matrix-Fiber Interface Cracking •Region III – Fiber Cracking
Crack Growth Modes in Adhesives •Mode I – Opening Stresses (Peel Stress) •Mode II – Shear Stresses •Mixed Mode Loading of I and II
Low Cycle Fatigue (LCF) - Unidirectional Composites • Normal S-N curve does not properly capture LCF. • A bi-linear S-N curve can be used as a basis to capture structural response from LCF.
Crack Growth - Adhesives
Crack Growth - Adhesives Crack Length Range, mm Best Fit Equation, mm/cycle 0 – 20 0.99 . 20 – 40 2.77 . 40 – 80 6.70 .
Future Research Directions & Recommendations Design Fatigue Loads Estimation: Fatigue loads estimations methods should analyze: Increased probability of high amplitude transient events resulting from:  Fault-induced shutdown procedures  Control system fault or Emergency shutdown during a gusting wind and/or coherent inflow with wind directional changes  Rare occurrences of extreme oblique inflow  Increased cycle counts of high amplitude loading to capture extreme statistics where peak load will occur during the early stages in the operating life of WTG.  Increase resolution of the tails of fatigue loads spectra.
Future Research Directions & Recommendations Structural Design for Fatigue: Structural influences of manufacturing defects, response to LCF  Crack growth modeling  Fracture mechanics Improved S-N curve analyses:  Non-pristine laminates  Adhesive bonds  Sandwich cores Anticipated manufacturing tolerances on structurally critical members Analyses should define tolerances expected of the manufacturing QMS Establish SF’s that rely less on testing laboratory coupons to establish material strength and properties, more on testing of components and subsystems that more closely reflect actual QC to establish.
Wetzel Engineering, Inc. http://www.wetzelengineering.com/ info@WetzelEngineering.com +1 785 856 0162 (office) 1310 Wakarusa Drive, Suite A Lawrence, Kansas 66049 U.S.A. Thank you!

More Related Content

Ken Lee: 2013 Sandia National Laboratoies Wind Plant Reliability Workshop

  • 1. Underestimated Low-Cycle Fatigue as a Contributor to Premature Failures Ken T. Lee, MSAE, Blade Design Technical Lead Amir Bachelani, Blade Structural Engineer Cody Moore, Loads & Dynamics Engineer Kyle K. Wetzel, Ph.d., CEO/CTO Sandia Blade Reliability Workshop 2013 Albuquerque, New Mexico
  • 2. Outline  Wind Turbine Component Damage/Failure  Low-Cycle Fatigue – Premature Failures  Design Loads for Certification  Fatigue Analysis for Certification  Recommendations for Future Research Copyright © 2013 Wetzel Engineering, Inc. All Rights Reserved.
  • 3. WT Component Damage/Failure Share of main components of total number of failures Damage/failure occurrences attributed to: electrical systems rotor blades gearbox Structural in nature sustained early in the life of these components (sometimes within 3 or 6 months to a year)  continuous and costly maintenance and repair
  • 4. WT Component Damage/Failure Wind Turbine subassembly reliability: WT reliability & downtime varies across subcomponents of WT Failure frequencies of blades and gearboxes  lower compared to other subcomponents Downtime per failure for rotor blades and gearbox  higher compared to other subcomponents Pitch mechanism & electrical system shown to be significant sources. Failure or fault-induced shutdowns
  • 5. Common WT Component Damage/Failure Common Types of Rotor Blade Damage/Failure: Crack propagation due to inherent design or manufacturing defects Blade damage due to extreme load buckling or blade-tower strike Failure of adhesive bonds  leading edge/trailing edge splitting Blade failure at the blade root connection  blade throw
  • 6. Causes of WT Component Damage/Failure Operating loads exceed design loads: Underestimated Design Loads (Ultimate, Low-cycle fatigue, or High-cycle fatigue)  Mistakes in applying standard methods  Shortcomings in standard methods Malfunctioning control safety systems Improper site assessment  Insufficient assessment of structural integrity: Mistakes in applying standard methods Shortcomings in standard methods
  • 7. Low-Cycle Fatigue (LCF) Possible Contributor to Premature Wind Turbine Component Failures Focus: LCF – Low-cycle fatigue characterized as: • High amplitude alternating stress • Low cycle counts (approx. less than 104) • Fatigue failure: propagation-dominated
  • 8. LCF - Overview Components designed to 20 years of life (according to GL, IEC, DNV) Premature failure of WT subcomponents: Rotor Blades Gearbox Structural design & analysis of WT Components, Current Provision: Fatigue design focused on high-cycle fatigue (HCF) (i.e. S-N curve methods). High amplitude loading events that occur at low cycle counts induce crack growth rates that far exceed that predicted by S-N curve analyses.
  • 9. LCF - Overview Design Loads Simulations, Current Framework: Does not capture high-amplitude transient events (less than 104 – 504 cycles) Insufficient resolution of the “tails” of the loads spectra  “Normal-operation” during turbulent conditions Under-reporting of loads induced by transient events, coherent inflow conditions, fault- induced shutdowns. Blade Load Spectra as per Current Framework for Certification Design Loads: Average Normalized Cumulative Blade Load Spectra with/without LCF:
  • 10. Design Loads for Certification Case Study on the Impact of Emergency Stops on Blade Fatigue Life
  • 11. GL -Design Load Cases for Fatigue Analysis •Power Production Load Case: DLC 1.1; NTM; Vin < V < Vout •Idling or Parked Load Case: DLC 6.4; NTM; V<Vin and Vout < V < Vref
  • 12. GL -Design Load Cases for Fatigue Analysis •Transient Load Cases, Faults during Operation: DLC 1.4; NWP; Grid-loss/E-stop – 20/year DLC 1.8; NWP; Iced Conditions – 24 hours/year DLC 2.1; NWP; Fault Occurrence – 24 hours/year DLC 3.1; NWP; Start Up - 1000/year @ Vin, 50 @ Vr & Vout DLC 4.1; Normal Shutdown - 1000/year @ Vin, 50 @ Vr & Vout
  • 13. Possible Shortcomings in Current Guidelines • WT components could possibly be failing prematurely from fatigue. • Current guidelines do not account for higher frequency rates of E-stops and/or Fault-Induced shutdown procedures  • E-stops due to grid loss could have a higher impact on fatigue life • E-stops of more than 20/year, averaging 1 E-stop/day (360/year)  Early life of WT operation, debugging of control and safety systems Control system malfunction in response to abnormal inflow conditions • Impact of an E-stop or a pitch control system fault during a gusting wind (with or without turbulent inflow) not considered.
  • 14. Case Study of Impact of Increased Frequency of E-Stop Procedures • Fatigue life of two turbines analyzed  the impact of stricter provisions for Emergency Shutdown (E-stops)  20 cases/year of DLC 1.1 NTM (GL- 2010), grid loss timed at highest point of My  Increase cycle counts of DLC 1.4 (NWP) Estops from 20/year to 360/year (7200 in turbine lifetime)  1 case/year of DLC 1.5 EOG1 (GL- 2010), grid loss timed at highest point of My  21 cases/year of grid loss during a gusting wind were added to the analysis
  • 15. Results: 100 kW Turbine – 13m Blade Variable Speed-Pitch Regulated
  • 16. Results: 2.0MWTurbine – 45m Blade Variable Speed-Pitch Regulated
  • 17. Fatigue Analysis Methods for Certification
  • 18. Current Fatigue Analyses Guidelines Fiber-Reinforced Composite Laminates: Characteristic S-N curve established for laminate. (GL 5.5.3.3.1) Goodman diagram constructed using this curve. (GL 5.5.3.3.1) Fatigue damage calculation using Miner’s rule (IEC 61400-1, 7.6.3.2) 95 % survival probability with a confidence level of 95 % used as basis for SN- curve (IEC 61400-1, 7.6.3.2) Adhesives:  3 samples (being representative for the jointed components in geometry and material)  Minimum number of load cycles of N=106. (GL 5.5.6.10)
  • 19. •HCF estimation is captured. LCF response of WT subcomponents/blade composites should also be considered in guidelines. •Testing on pristine laminates. Manufacturing defects not tested. •Structural damage/failures due to: Manufacturing that is out of QC Failure to design for manufacturing quality that can be realistically controlled. •Current Safety Factors (SF’s) result in excessively conservative designs in areas not really the source of problems. •Heavy reliance on SF’s still fails to address on-going problems industry has with blade reliability. Current Fatigue Analyses Guidelines
  • 20. Types of Damage & Defects • Delamination •Voids & Cracks in Laminates/Adhesive Bonds • Wavy fiber plies, bridging of fibers • Porosity, discontinuities in laminates
  • 21. Crack Growth in Fiber-Reinforced Plastics •Region I – Matrix Cracking •Region II – Matrix-Fiber Interface Cracking •Region III – Fiber Cracking
  • 22. Crack Growth Modes in Adhesives •Mode I – Opening Stresses (Peel Stress) •Mode II – Shear Stresses •Mixed Mode Loading of I and II
  • 23. Low Cycle Fatigue (LCF) - Unidirectional Composites • Normal S-N curve does not properly capture LCF. • A bi-linear S-N curve can be used as a basis to capture structural response from LCF.
  • 24. Crack Growth - Adhesives
  • 25. Crack Growth - Adhesives Crack Length Range, mm Best Fit Equation, mm/cycle 0 – 20 0.99 . 20 – 40 2.77 . 40 – 80 6.70 .
  • 26. Future Research Directions & Recommendations Design Fatigue Loads Estimation: Fatigue loads estimations methods should analyze: Increased probability of high amplitude transient events resulting from:  Fault-induced shutdown procedures  Control system fault or Emergency shutdown during a gusting wind and/or coherent inflow with wind directional changes  Rare occurrences of extreme oblique inflow  Increased cycle counts of high amplitude loading to capture extreme statistics where peak load will occur during the early stages in the operating life of WTG.  Increase resolution of the tails of fatigue loads spectra.
  • 27. Future Research Directions & Recommendations Structural Design for Fatigue: Structural influences of manufacturing defects, response to LCF  Crack growth modeling  Fracture mechanics Improved S-N curve analyses:  Non-pristine laminates  Adhesive bonds  Sandwich cores Anticipated manufacturing tolerances on structurally critical members Analyses should define tolerances expected of the manufacturing QMS Establish SF’s that rely less on testing laboratory coupons to establish material strength and properties, more on testing of components and subsystems that more closely reflect actual QC to establish.
  • 28. Wetzel Engineering, Inc. http://www.wetzelengineering.com/ info@WetzelEngineering.com +1 785 856 0162 (office) 1310 Wakarusa Drive, Suite A Lawrence, Kansas 66049 U.S.A. Thank you!