Power output: 6 750kW=4500kW Location: Top of Taikoyama, Kyoto prefecture, Japan. Terrain condition: Complex mountainous area

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2 Introduction Power output: 6 75kW=45kW Location: Top of Taikoyama, Kyoto prefecture, Japan m 45.94m Fracture section Terrain condition: Complex mountainous area m History: Nov, 21: Power generation began; Mar, 213: Nacelle of No.3 turbine collapsed Complex mountainous terrain results in high turbulence intensity 2/ Different from the situation in Europe 13

3 Questions 1. Wind turbine design life is 2 years; However this turbine nacelle collapsed only 12 years after construction. How could it collapsed at a middle age of its life time? 2. The No.3 turbine s high-tension bolts broke three times in 28, 212 and 213. After the last bolts replacement in 212, the nacelle collapsed within one year, although there was no damage detected according to the periodical inspection; Why the bolts broke frequently? Questions must be answered 3/ 13

4 Flow chart Wind condition investigation High turbulence; Ambiguous explanation is dangerous: 1) Solving problem; 2) Dividing responsibility. Fracture section investigation Material strength no problem; Fatigue cracks propagated at the tower tube inner surface. Quantitative analysis required Aerodynamic modelling; Understanding the force; Fatigue life evaluation. Spatial effect & stress concentration Accurate FEM model; Clarify the relationship between nominal stress and bolt stress Root cause 4/ 13

5 Objectives 1. Aerodynamic model is necessary in order to understand the response at the fracture section; 2. Filed and observation should be conducted to understand the complicated nacelle structure; 3. FEM model should be built accurately in order to recreate the complex strain distribution at fracture section, and to evaluate the fatigue life of high-tension bolt; 4. Bolts broke frequently. Root cause must be clarified. 5/ 13

6 Turbulence Intensity Moment (knm) Field Moment Ave moment, std moment and max moment Occurrence frequency West wind predominates Turbulence intensity ε s M SN ε W θ M total M EW ε E Nacelle direction ε N Moment calculation schematic diagram α Std moment Ave moment Max moment Plot of moment WNW W WSW 3% N NNW NNE NW 2% NE SW SSW 1% % S SE SSE ENE E ESE Occurrence frequency I p = U max U mean 1 P, P = 1 2 ln T t Extrapolated value for high wind speed (>17m/s) Measurement NTM(9% quantile) Bin (9% quantile) I v 1.I u I w.7i u Slope with gradient over 2 degree Turbulence intensity (W) Numerical topographic map High turbulence 6/ 13

7 Moment (knm) Moment (knm) Moment (knm) Power output (kw) Rotor speed (rpm) Pitch angle (deg) Power output (kw) Rotor speed (rpm) Pitch angle (deg) Aerodynamic modelling & modification Modelling Tower part Engineering drawings Blade shape and aerodynamic coefficient NREL airfoil family Modification of real turbine Power output: 75kW 63kW; Rotor speed: 33rpm 26rpm; Pitch angle: different Modification of model No information from manufacturer Pitch angle error 5 degree; Reduce both torque and thrust simulation designed simulation modified simulation modified 1 simulation 5deg Power output Power output simulation 5deg Ave moment Rotor speed Aerodynamic model is built based on real situation simulation designed simulation modified Rotor speed simulation modified simulation 5deg simulation 5deg Std moment simulation modified simulation 5deg simulation designed simulation modified Pitch angle Pitch angle simulation 5deg Max moment 7/ 13

8 Strain (με) Strain distribution at top flange joint Strain distribution should be understand Spatial effect & stress concentration Strain gauges arrangement Around 45.94m, CH9~CH32 Strain distribution 45.94m CH19 Fracture section Bolt Flange 2mm 2mm 6mm 1mm CH19 Dissatisfy the plane section assumption Nacelle investigation Nacelle weight transmits in three paths 3 Yaw motors 16 Yaw brakes 129 Ball bearings + m Rotor direction FEM model is necessary to recreate the strain distribution at fracture section CH19 CH1 Nacelle rotating plane graph Strain gauges layout at stage # Plain section assumption Measurement Nacelle ratational degree (deg) CH19 strain distribution Yaw motor Ball bearing Strain gauges installment Yaw brake Local view of yaw bearing 8/ 13

9 Strain (με) FEM modelling FEM model Solid element Ball bearing, pinion gear; Shell element Nacelle Spring element Yaw brake, ball bearing Beam element Bolts Strain distribution Good agreement Mechanism Yaw deformation is elliptical; Tower strain direction aft side is large; Tower strain in fore direction is small; Nacelle side view Concentrated mass Thrust force Pinion gear Yaw brake Ball bearing Upward view FEM Measurement fore aft Nacelle ratational degree (deg) Tower strain distribution 45.94m Fore Aft Yaw bearing deformation FEM model shows good agreement and necessary 9/ 13

10 Stress range (MPa) High tension bolt fatigue life (y) Nominal stress (N/mm 2 ) Bolt (N/mm2) Blot pre-tension stress (N/mm 2 ) Bolt fatigue life evaluation Relationship between nominal and bolt stress Nominal stress (45.94m): σ n =N/A-M/Z Nominal stress increases, gradient increases as pre-tension decreases Steeper gradient when pre-tension decreases Fatigue life Rainflow counting, S-N curve, miner s rule Max -1 Average Min Nominal stress (45.94m) % 8% 6% 4% 2% % Nominal stress (N/mm2) Nominal stress vs bolt pre-stress Pre-tension 2% Pre-tension % Time (s) Bolt pre-tension stress (14m/s) Life time drops drastically when pre-tension decreases Pre-tension is 8%: fatigue life 28 years ---- safe Pre-tension less than 8%: fatigue life < 2 years ---- failure Pre-tension less than 4%: fatigue life only a few days E+6 5.E E+5 1.E+6 1.E+7 1.E+8 1.E+9 Cycles Ni S-N curve DC % % %.97 4% 2%.17 Pre-tension force Bolts fatigue life vs. pre-tension (IEC A).4 % High turbulence is not the reason but reduction of bolts pre-tension 1/ 13

11 Pre-force (kn) Reason of bolt pre-tension reduction Pre-tension force is applied by torqueing F1T-M24 Torque: 85Nm Axial force: 265kN Indoor experiment Different wrenches and wrench If well lubricated: Pre-tension can be guaranteed with 1% variability Filed experiment Same with indoor Bolt force reduction ratio R = 1 L 15 (%) Re-torqueing is important Reference force 265kN 8% Well lubricated F1T-M24 1% variability Data logger Face ring lubricant Adhesive Strain gauge Female screw lubricant L Torque Power wrench Normal wrench Hydraulic wrench Bolt reduction ratio R (5 h after operation) Bolt No. L [mm] R [%] Torque (kn m) Indoor experiment Re-torqueing was not applied 5 hours after bolt replacement caused the reduction Bolt 11/ 13

12 Conclusions 1. Due to high turbulence, control method was adjust by manufacturer. Modification was applied to aerodynamic model ; 2. The strain distribution at fracture section is not abiding by plain section assumption due to complex nacelle structure and yaw motors constraint. FEM model was built accurately and shows good agreement with result; 3. The root cause of this accident is not the matter of high turbulence or design, but the reduction of bolts pre-tension. When pre-tension force is less than 8% the fatigue life decreases; 4. Re-torqueing is important to prevent pre-tension reduction. Communication between manufacturer and operation workers should be more effective. 12/ 13

13 This project is funded by the Kyoto Prefecture Government, New Energy and Industrial Technology Development Organization. I wish to express my deepest gratitude to the concerned parties for their assistance and contribution to this project