Fatigue Failure Accident of Wind Turbine Tower in Taikoyama Wind Farm

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1 Fatigue Failure Accident of Wind Turbine Tower in Taikoyama Wind Farm Yin LIU 1, Takeshi ISHIHARA 2 1,2 Department of Civil Engineering, School of Engineering, the University of Tokyo, Tokyo, Japan Abstract One of the wind turbine nacelles at Taikoyama wind farm collapsed due to the fatigue failure of high tension bolts. Strain gauges and accelerometers were installed on the wind turbine to verify the aerodynamic model. Furthermore a FEM model was built in order to find out the relationship between tower tube and high tension bolts at the position of flange joint, where the fracture occurred. When the bolt s pre-tension force decreases, its stress range increases. Less the pretension force left, the larger the stress range will be. Hence when pretension force is %, the fatigue life is left for only a few days. On the other hand when 17 bolts are damaged, the turbine tube stress is three times larger than the stress when all the bolts are in good condition. Hence the fatigue evaluation shows that the life time rapidly decreases to less than two months compared with that of the normal life time which is 2 years. Key Words: Fatigue failure, pre-tension force, high tension bolt, nacelle collapse. 1. Introduction The Taikoyama wind farm is located at the top of Taikoyama Mountain, Kyoto Prefecture, Japan, which is surrounded by the Tango peninsular and Name Operating time Manufacturer Unit Max power output Table 1 Summary of Taikoyama wind farm Taikoyama Wind Farm 15th, November, 21 Lagerwey 6 75kW 45kW faces north to the Sea of Japan. The construction cost is approximately 12.5 million dollars and it reduces nearly 59 tons of carbon dioxide every Performance Cut-in wind speed Rated wind speed Cut-out wind speed Resistant wind speed 3m/s 12m/s 25m/s 6m/s year. The wind farm information is summarized in Table 1. In March 213 the nacelle of No.3 wind turbine collapsed[1] and the accident scene and schematic Rotor Diameter Generation rotor speed Number of blades Hub height 5.5m 13~33rpm 3 5m diagram of the wind turbine is shown in Fig. 1. Tower Height Material 46m SM4 (steel) Flange connection high-tension bolts F1T M24 Nacelle Dimensions Material W5.6 L3.3 H6.5m SS4, GFRE Wind direction control Control method Active yaw control Rated power output control Control method Pitch control 1 Presenting and corresponding author, PhD candidate, liuyin@bridge.t.u-tokyo.ac.jp

2 (b) Fracture section (a) Collapsed nacelle (c) Vertical cross section Fig. 1 Accident scene and schematic diagram The detailed structure is shown in Fig. 2. (a) Flange joint Local structur (b) Fracture section in detail Fig. 2 Detail drawing of fracture section The field investigation indicates that the wind condition satisfied the construction requirement based on the IEC 614-1[2] including annual wind speed, turbulence intensity and flow inclination angle. By observing the fracture section of the tower tube, we found that the material strength was strong enough, but evidence of fatigue crack propagation was detected at the inner surface of the tube. Furthermore, 17 broken bolts were found during the field investigation and fatigue cracks were also detected. By comparing the two aspects, fracture is considered to be preceded by a certain degree of fatigue damage caused by the reduction of bolts pretension force up to 3%~1%. Flange Welding 5m 45.94m Fracture section m 1mm below flange Fracture section The wind turbine collapsed very early in 12 years, where the expected life period was 2 years. Moreover, the accident happened only three months after the periodical inspection was carried out. Additionally, there are more than 12 wind turbines in service of the same type across Japan. Therefore, it is necessary and urgent to understand the cause of this accident, so that this kind of accident can be prevented in the future. This paper proceeds as follows: 1) Field ; 2) Aerodynamic modelling and verification; 3) Clarify the fracture section s aerodynamic characteristics; 4) Explain the relationship between nominal stress, local stress and bolt stress using FEM model; 5) Evaluate the fatigue life of both high-tension bolt and tower tube, and reveal the reason for the failure. 2. Field 2.1 Wind condition investigation All the data were measured from Feb. 2 nd 215 to Feb. 28 th 215. WNW W WSW Fig. 3 Occurrence frequency Fig. 4 Average wind speed Fig. 3 and Fig.4 indicate the occurrence frequency and average wind speed respectively. The occurrence frequency of dominate wind direction WSW, W and WNW is 9%, 27% and 15% respectively. 3% N NNW NNE NW 2% NE SW SSW 1% % S SE SSE Since the SCADA data contains only maximum wind speed and average wind speed in a time scale of 1 minute, we calculated the turbulence intensity according to reference [3] in equation (1). I p = U max U mean 1 P ENE E ESE 12 N NNW NNE NW 1 8 NE WNW ENE W E WSW SW SSW, P = 1 2 ln T t S SE SSE ESE (1)

3 The maximum wind speed U max and average wind speed U mean are derived from the 1min SCADA data, the peak factor P is evaluated by a time scale T of 6 seconds and average time t of 1 second. Consequently 1m/s bin average is calculated. Fig. 5 shows the field turbulence intensity. Because of the insufficient high wind speed data (>17m/s) during the period, the high wind speed turbulence intensity is extrapolated assuming the normal turbulence intensity in reference[2], and it is described as equation (2) σ 1 = I ref (.75V hub + b), b = 3.8 (2) I ref is the expected value of hub-height turbulence intensity at a 1 min average wind speed of 15m/s, V hub is the wind speed at hub height and σ 1 is hubheight longitudinal wind velocity standard deviation. As a result for aerodynamic simulation, a combined turbulence intensity is used: value for low wind speed ( 17m/s ) and the extrapolated value for high wind speed respectively (>17m/s). Turbulence intensity Measurement Extroplated bin average Fig. 5 Turbulence intensity in the direction of WSW+W+WNW For the turbulence spectrum, the Kaimal model is used. The lateral and vertical turbulence intensity component are considered as.8 σ 1 and.5 σ 1 according to reference [2]. 2.2 Moment Strain gauges with sampling frequency of 2Hz were installed in eight directions in order to get the moment at the height of 12.6m above tower base. Fig. 6 shows the strain gauges installment. The nacelle was forced to rotate one circle without operating for the estimation of the strain gauges installment error, and the compensation value can be calculated by the amplitude of the sin curve. Fig. 6 Strain gauges installment ε s Fig. 7 Moment calculation schematic diagram The moment was calculated following the method by Ishihara and Phuc[4]. According to Fig. 7, the East-West moment and South-North moment were given in equation (3) and (4) respectively. Where M and ε is the moment and strain at corresponding direction, EI is the stiffness of tower tube and D is the inner diameter. M EW = EI ε D = EI ε E ε W D M SN = EI ε D = EI ε S ε N D The total moment is given in equation (5). If the direction of total moment is opposite to the nacelle direction, then the total moment will be positive, otherwise it is negative. (3) (4) 2 2 M total = M EW + M SN (5) The average bending moment, maximum bending moment and standard deviation of bending moment are plotted in Fig. 8. ε W M SN θ ε E M total εm EEW ε E α ε E Nacelle direction ε E ε E ε N

4 bin average (a) Average moment bin average (b) Maximum moment bin average (c) Standard deviation of moment Fig. 8 Comparison of and bins average moment 3. Aerodynamic analysis and fatigue life investigation 3.1 Aerodynamic modelling Aerodynamic model is built to simulate the dynamic performance by GL s Bladed wind turbine modelling tool[ 5 ]. The tower section refers to the real engineering drawings. For commercial confidentiality, the blade profile is not available from manufacturer. As a result we selected airfoils from NREL s airfoil family, which are S818 for root section, S83 for primary section and S831 for tip section[ 6 ], and thickness/chord ratio, Reynolds number, lift coefficient C l and draft coefficient C d were determined. For control method, some adjustment had been applied. In case of the high turbulence intensity in the mountainous area, the wind turbine encounter over speed at times. Once it exceeds the maximum rotor speed of 33 rpm, it stops suddenly and starts to operate again when the rotor speed drops below the maximum value which causes frequent downtime. Hence the manufacturer modified the maximum rotor speed and power output to decrease the downtime. Since the details were commercial confidentiality, we adjust rated power output and maximum rotor speed according to the data. Moreover a five degrees pitch angle error is considered to eliminate the error in pitch control. With the adjustment above the power output, rotor speed and pitch angle are now close to the data as shown in Fig. 9. Power output (kw) (a) Power output Pitch angle (deg) Rotor speed (rpm) simulation modifed (b) Rotor speed (c) Pitch angle Fig. 9 Comparison of power output, rotor speed and pitch angle The proportional gain K QP and integral gain K QI for torque control, and proportional gain K SP and integral gain K SI for pitch control were calculated based on Guidelines for Design of Wind Turbine Support Structures and Foundations, JSCE[7],and optimal mode gain K opt was modified to validate the dynamic simulation results with results. Some key parameters for Bladed modelling are summarized in Table 2.

5 Table 2 Key parameters for Bladed modelling Optimal mode gain K opt Demanded generator toque (Nm) Rated Power generation (kw) Rotor speed (rpm) Error in Pitch angle (degree) Default rpm Modified rpm 5 Torque control K QP= K QI=51678 K QP= K QI= Pitch control K SP= K SI= K SP= K SI=.7715 A field test was carried out to measure the natural frequency of the tower. The damping ratio of the 1 st order frequency was applied as.5% based on the field inspection [1]. The natural frequency is shown in Table 3, which is consistent with the aerodynamic simulation result. section (45.94m) at different wind steps respectively according to simulation result. Hence the nominal stress can be calculated from equation (6), where A is the sectional area and Z is the sectional resistance moment. σ n = N M (6) A Z Table 3 Comparison of tower natural frequencies -3 Tower natural frequencies Measurement Simulation 1 st order (fore-art).515hz st order (side-side).518hz nd order (fore-art) 3.838Hz nd order (side-side) 3.832Hz Axial force (kn) Min Average Max Min Average Max Finally, Fig. 1 shows the and simulation results for moment at 12.6m above tower base were in good agreement, and the aerodynamic model is verified to be correct (a) Average moment (12.6m) (c) Maximum moment (12.6m) Fig. 1 Comparison of moment (b) Std of moment (12.6m) 3.2 Characteristics of fracture section Fig. 11 (a) and Fig. 11 (b) show simulated axial force N and bending moment M at the tower fracture (a) Axial force N (45.94m) Nominal stress (N/mm 2 ) (b) Bending moment M (45.94m) -5 Max -1 Average Min c) Nominal stress (45.94m) Fig. 11 Aerodynamic characteristics at the fracture section As shown in Fig. 11 (c), the nominal stress σ n changes and varies with the increase of the wind speed. The minimum stress turns into negative value when the wind speed is above 18 m/s. 3.3 FEM modelling The fracture section is very close to the top flange welding position, and according to the field investigation the fatigue failure propagated at the inner surface of the tower tube, so the stress concentration and spatial effect may influence the local stress σ local significantly. A 3D FEM model is T

6 built to clarify the relationship between nominal stress σ n, local stress σ local and bolt get pretension force before and after the bolts damaged. The relationship of nacelle weight, thrust force and top flange is illustrated in Fig. 12. The nacelle weighs 53.3t and it is rigidly connected to the yaw bearing. The stress concentration factor of welding geometric profile was proposed by Caccese[8]. The case for Taikomaya wind turbine is as shown in Fig. 13. Solid element is used for the modelling of yaw bearing, top flange and bolts, and shell element is used for tower tube modelling. Furthermore, contact element is considered for the contact surface of yaw bearing and top flange and the friction factor is.2. The bolts are rigidly connected to the yaw bearing. Nacelle weight (53.3t) Hub Height (=GL+5.m) Thrust force by wind N Nacelle opposite side W E ( ) S Edge of damaged Bolts (53.6 ) Fig. 14 Diagram of the damage area Thrust force is considered in seven cases from kn to 25kN to simulate different wind loading. Fig. 15 shows an example of the local stress σ local before and after 17 bolts are damaged at wind speed of 16m/s. Fig. 15 (a) implies that the cause of maximum tensile stress happens at the inner tube because of the law of lever, which is consistent with the observation of fracture face. According to Fig. 15 (b), the local stress is much larger when 17 bolts are broken. Nacelle center of gravity Yaw bearing 4m Yaw bearing Rotor side ( ) Top flange 1525mm Lee wind side (18 ) Bolt s Flange Welding Fig. 12 Force applying position relationship Yaw bearing Local stress (a) Bolts normal Tower tube Top flange Yaw bearing Fracture section Contact element Tower tube Shell element Bolt s Flange Welding Fig. 13 FEM detail at top flange position Local stress Tower tube 3.4 Investigation of the tower tube fatigue life As for the tower tube, Fig. 14 shows the cases when 17 bolts broken. (b) 17 Bolts broken Fig. 15 Comparison of the local stress (16m/s) The relationship between nominal stress and local stress considering the welding stress concentration [7] is now given as following respectively: Bolts normal σ local = σ n (7) 17 bolts broken 2 σ local = σ n +.16σ n (8) Equation (7) and (8) are plotted in Fig. 16. When 17

7 bolts are broken, the local stress is more than three times larger than bolts at normal condition. Local stress local (N/mm 2 ) Bolts normal 17 Bolts broken Nominal stress n (N/mm 2 ) Fig. 16 Local stress vs. nominal stress Local stress local (N/mm 2 ) bolts broken Bolts normal time (s) Fig. 17 Time history of local stress (22m/s) With a time period of 1 minutes, the time series simulation result is available for each wind speed combining aerodynamic model with equation (7) and (8). When the wind speed is low, the tensile stress predominates. However with increase in wind speed, compressive stress occurs and the stress amplitude increases. The case of wind speed at 22m/s is shown in Fig.17. With the time history of bolt pre-tension stress, we can investigate its fatigue life. Rain flow counting algorithm is used for fatigue analysis in order to reduce the spectrum of varying stress into a set of simple stress reversals. Goodman relation as shown in equation (9) is used to quantify the interaction of mean and alternating stresses. σ a = σ w (1 σ m /σ B ) (9) σ a is the alternating stress from rain flow counting result, σ m is the mean stress, σ w is the fatigue limit for comple`tely reversed loading and σ B is the ultimate tensile strength of the material, which is 493Mpa for SM4 steel. By using the fatigue limit for completely reversed loading σ w, S-N curve based on GL wind 25 with a detail category of 71[ 9 ], and Miner s rule, the accumulative fatigue damage D in 1 minutes is given in Equation (1), and failure is reached when D equals to 1. k n i i=1 = D (1) N i Frequency distribution of the wind speed is based on Rayleigh distribution with a mean annual wind speed of 8.5m/s. The fatigue life of tower tube is shown in Fig. 18. When the bolts are in normal condition the fatigue life is 27.5 years, which is in agreement with the design requirement. However, if 17 bolts are broken, the fatigue life decreases dramatically to.9 years, approximately one months. It is in accordance with the time interval between the last periodical inspection and the accident. Tower tube fatigue life (year) bolts broken Bolts normal bolts broken Bolts normal Fig. 18 Tower tube fatigue life 3.5 Investigation of the high tension bolts fatigue life Based on the field investigation [1], six bolts at nacelle s opposite side were found to have reduction pre-tension force reduced as shown in Fig. 19. W E Nacelle s opposite side Fig. 19 Bolts pre-tension force decreasing In order to recreate the real situation, blots pretension force is set in six different cases which were 1%, 8%, 6%, 4%, 2% and % of the design pre-tension force corresponding to 85kNm torque. The relationship between the nominal stress and bolt pre-tension stress is given as shown in Fig. 2. With the nominal stress increasing, the gradient increases as pre-tension decreases, and it is much more obvious when the pre-tension force decreases. The larger the gradient the larger the bolt stress range will be, and the bolt s fatigue load. Since the nominal N S

8 stress ranges mainly between -5N/mm 2 to 25 N/mm2 according to Fig. 11(c), the stress range may vary a lot especially when the bolts pre-tension stress drops to % as illustrated in Fig. 2. Bolt pre-tension stress (N/mm 2 ) % 2% 4% 6% -2 8% % Nominal stress Fig. 2 Nominal stress Vs. bolt pre-tension stress Blot pre-tension stress (N/mm 2 ) Pre-tension 2% Pre-tension % Time (s) Fig. 21 Time history of bolt pre-tension stress (14m/s) Fig.21 shows one example of the time history of the bolt pre-tension stress at the wind speed of 14m/s. It is clear that when the pre-tension force drops the stress range increases significantly. The fatigue life investigation follows the rules mentioned in Section 3.4. The ultimate tensile strength of FT1 bolts is1mpa and the detail category is 36. The bolts fatigue life is shown in Fig. 22. Bolts fatigue life (Y) years years Bolt pre-tension force(%) 6.95 years.22 days Fig. 22 Bolts fatigue life vs. bolt pre-tension percentage As we can see that when the pre-tension force is over 4%, the life time does not decrease. However when the pre-tension force is below 4% the fatigue life time drops dramatically as only a few days left, when the pre-tension force is %. 4. Conclusions This research is based on the collapse accident of Taikoyama wind farm No.3 turbine. The field of tower model frequency, SCADA 2 data and strain gauge data were measured. At the same time the aerodynamic model was built. In addition, the tower top FEM model was built to evaluate the high-tension bolts and tower tube fatigue life. The cause of the collapse of the wind turbine is discussed and the following conclusions were drawn: 1) Due to high turbulence intensity at site, the control of the wind turbine was modified by manufacturer. Power output and maximum rotor speed were adjusted according to data, and a five degree of pitch error was applied. With this control method the simulation results show good agreement with results; 2) For the high tension bolts, by considering the nonlinear phenomenon and stress concentration closed to welding zone, when the pre-tension force decreases, the stress range increases, especially when pre-tension force is % it is 3 times larger. The less the pre-tension force left, the larger its range is. As a result, when the pre-tension force is below 4% the fatigue life time drops drastically and it is only a few days when the pre-tension force is %; 3) Similarly, the FEM model shows that with 17 bolts broken the local stress at fracture section increases more than three times compared with the case of bolts at normal condition. This phenomenon accelerated the fatigue initiation and propagation and the fatigue life of the fracture section decreases dramatically to 1/2 of its life time. 4) The reason for the Taikoyama wind farm accident is now clearly understood in a detailed manner. It is not the matter of design or material, but was due to the fatigue failure caused by the reduction of high tension bolts pre-tension force. For the Taikoyama wind turbines high tension bolts, according to the service manual the temporary torqueing and final torqueing was applied. And at the time of 5 hours after bolt changing, the re-

9 torqueing must be applied. However at the time of periodical bolt changing operation, the re-torqueing was not applied. The wind turbine is a rotating machine system, in which the contact surface and the bolt itself plasticity deforms accompany with the wind turbine operation, and therefore the pre-tension force reduces. Moreover, according to the service manual, 5% of the bolts should be inspected per year, which means only three bolts were inspected. We should check at least 16 bolts per year in order to cover the bolts in Reference all wind direction. Besides, during the year from 25 to28, the workers only conducted the method of counter mark inspection to make sure the torque was enough. It is a serious problem between manufacturer and operator that expertise technique is not transferred accurately and efficiently. Clear rules must be made even after guarantee periods, or it may lead to devastating accident. [1] Kyoto fu, Report of the accident in Taikoyama wind farm No.3 wind turbine, Kyoto, 213. [2] International Electrotechnical Commission, (25). IEC 614-1, 3rd edition, Part 1: Design requirements. Geneva. [3] Ishizaki, H.(1983) Wind profiles, turbulence intensities and gust factors for design in typhoon-prone regions. Journal of Wind engineering & Industrial Aerodynamics, 13: [4] T. Ishihara, P.V. phuc, Yozo Fujino. A Field Test and Full Dynamic Simulation on a Stall Regulated Wind Turbine. The sixth Asia-Pacific Conference on Wind Engineering, Seoul, September 25: [5] Garrad Hassan Bladed, version 4.4, DNV-GL, 213. [6] Tony Burton, David Sharpe, Nick Jenkins. Wind Energy Handbook. John Wiley & Sons Ltd, Chichester, 21. [7] Japan Society of Civil Engineers, (21). Guidelines for Design of Wind Turbine Support Structures and Foundations. Task Committee on Dynamic Analysis and Structural Design of Wind Turbine Committee of Structural Engineering, Tokyo. [8] V. Caccese, P.A. Blomquist, K.A. Berube. Effect of weld geometric profile on fatigue life of cruciform welds mad by laser/gmaw processes. Marine Structures, 26, 19: [9] Germanischer Lloyd WindEnergie GmbH (25), Guideline for the Certification of Offshore Wind Turbines. Germanischer Lloyd WindEnergie, Hamburg.

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

Power output: 6 750kW=4500kW Location: Top of Taikoyama, Kyoto prefecture, Japan. Terrain condition: Complex mountainous area Introduction Power output: 6 75kW=45kW Location: Top of Taikoyama, Kyoto prefecture, Japan 1 2 3 4 5 6 5m 45.94m Fracture section Terrain condition: Complex mountainous area m History: Nov, 21: Power generation

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