本研究分為兩個部分，第一部分針對NREL 5MW OWT風力機基座進行裂縫應力分析，選擇IEC 61400-3中屬於需分析極限負載的工況，並配合台灣西部沿岸之海洋參數進行模擬，模擬軟體為GH-Bladed，GH-Bladed用於計算作用在基座上的實際載荷，包括力和力矩，最後將轉換後的負載輸入ANSYS Workbench，接著挑選局部應力最大的地方建立裂縫，接著將裂縫尖端的應力強度因子結合失效評估圖即可判斷裂縫是否會造成結構失效，結果顯示在裂縫深度4mm左右時，結構可能會因為裂縫附近有過大的塑性變型而導致結構失效。第二部分則是選擇IEC 61400-3中屬於需分析疲勞負載的工況，進行NREL 5MW OWT風力機基座裂縫疲勞分析，首先選擇合適的風況浪況之後進行模擬並提取10分鐘之環境負載，再將轉換後的應力輸入至ANSYS Workbench，進行應力分析，之後同樣在擁有最大拉伸應力處建立裂縫，提取裂縫尖端的應力強度因子歷時以後，將該歷時等效為等振幅的應力強度因子歷時，再通過另一幾何形狀較為單純的有限元素模型，在該模型上建立一個相同的裂縫，並在該模型上施以一等幅應力負載使其的應力強度因子振幅與之前計算出的等振幅的應力強度因子歷時的振幅相同，便可使用ANSYS Workbench中的 Smart Crack Growth功能來預測疲勞工況的循環次數與裂縫尺寸的關係，當裂縫尺寸成長至無法承受極限負載工況時，便判斷為該情況有危險。

This research studied the offshore wind turbine NREL 5MW OWT and focused on the mechanics of the crack in the foundation. First, to analyze the ultimate load, we select specific design conditions in IEC 61400-3 with marine parameters corresponding to the western coast of Taiwan. The wind turbine software BLADED is used for finding the actual loadings, including forces and moments, acting on the foundation. Then, these loadings were used in FEM software ANSYS to perform detailed stress strength and fatigue analysis. The position of the crack was selected, where the local stress was maximum. The stress intensity factor at the crack tip was combined with the failure evaluation diagram to determine whether the crack would cause structural failure. The second part of the research is to select the working condition corresponding to fatigue load in IEC 61400-3. With appropriate wind and wave parameters, BLADED can provide 10-minute loadings to the foundation. Then, its stresses were found using ANSYS. A crack is established at the bottom of the base with the largest tensile stress. For fatigue analysis, the transient stress intensity factor duration was transformed to an equivalent stress intensity factor duration with constant amplitude. Then, the same crack was established on a simplified model. The “Smart Crack Growth” function of ANSYS was used to predict the relationship between the number of cycles of fatigue conditions and the growth of the crack size. The situation is dangerous when the crack size grows to the extreme. This research can provide a systematic method of analyzing the failure of the wind turbine foundation.

[1] 廖建榮,殷菘偉, 2020 ,“ACFM 離岸與海下結構檢測”，Eddyfi Technologies.
[2] Lacalle, R. Cicero, S. Álvarez, J.A. Cicero, R. and Madrazo, V., 2011, “On the analysis of the causes of cracking in a wind tower,” Engineering Failure Analysis, Vol. 18, pp. 1698–1710.
[3] Hassanzadeh, M., 2012, ”Cracks in onshore wind power foundations: causes and consequences,” Elforsk report 11:56.
[4] 章子华,周易,诸葛萍, 2014,”台风作用下大型风电结构破坏模式研究,” 振动与冲击, Vol. 33, pp. 143-148.
[5] Shankar, V. Gill, T. P. S. Mannan, S. L. and Sundaresan, S., 2003, “Solidification cracking in austenitic stainless steel welds,” Sadhana, Vol. 28, pp. 359–382.
[6] “Strength analysis of welded structures,” 網路資料, https://forcetechnology.com/en/services/strength-analysis-of-welded-structures.
[7] Jacob, A. L. M., 2019, “The Influence of Residual Stresses on Structural Integrity of Renewable Energy Marine Structures,” PhD thesis, Cranfield University, UK.
[8] Jacob, A., & Mehmanparast, A., 2021. “Crack growth direction effects on corrosion-fatigue behaviour of offshore wind turbine steel weldments.” Marine Structures, 75, 102881.
[9] Mendes, P., Correia, J. A., De Jesus, A. M., Ávila, B., Carvalho, H., and Berto, F., 2021, ” A brief review of fatigue design criteria on offshore wind turbine support structures.” Frattura ed Integrità Strutturale, Vol. 15, pp. 302-315.
[10] Alonso, T. R., & González Dueñas, E., 2014. “Cracks analysis in onshore wind turbine foundations,” IABSE Symposium: Engineering for Progress, Nature and People, Madrid, Spain, 3-5 September, pp. 1086-1092.
[11] 李易軒, 2020,離岸風力機單樁基座疲勞分析,國立中央大學機械工程學系碩士論文.
[12] Biswal, R., Al Mamun, A., & Mehmanparast, A., 2021. “On the performance of monopile weldments under service loading conditions and fatigue damage prediction.” Fatigue & Fracture of Engineering Materials & Structures, 44(6), 1469-1483.
[13] Perry, M., McAlorum, J., Fusiek, G., Niewczas, P., McKeeman, I., and Rubert, T., 2017.“Crack monitoring of operational wind turbine foundations.” Sensors, 17(8), 1925.
[14] Fujiyama, C., Koda, Y., and Sento, N., 2018. “Evaluation and stability analysis of onshore wind turbine supporting structures.” In Stability control and reliable performance of wind turbines. IntechOpen..
[15] Mehmanparast, A., Brennan, F., and Tavares, I., 2017. “Fatigue crack growth rates for offshore wind monopile weldments in air and seawater”: SLIC inter-laboratory test results. Materials & Design, Vol. 114, 494-504.
[16] Seitl, S., Pokorný, P., Miarka, P., Klusák, J., Kala, Z., & Kunz, L., 2020, “Comparison of fatigue crack propagation behaviour in two steel grades S235, S355 and a steel from old crane way,” MATEC Web of Conferences (Vol. 310, p. 00034). EDP Sciences.
[17] Ziegler, L., Schafhirt, S., Scheu, M., and Muskulus, M., 2016, “Effect of load sequence and weather seasonality on fatigue crack growth for monopile-based offshore wind turbines.Energy Procedia,94, 115-123.
[18] Shi, K., Cai, L., Chen, L., and Bao, C., 2014, “A theoretical model of semi-elliptic surface crack growth. ” Chinese Journal of Aeronautics, 27(3), 730-734.
[19] 風機組成部分,網路資料, https://www.energy.gov/eere/wind/photos/wind-gallery
[20] Miceli F., 2012, “Offshore wind turbines foundation types,” http://www. windfarmbop.com/tag/monopile/
[21] Passon, P., Branner, K., Larsen, S. E., and Jørgen Hvenekær Rasmussen , 2015, “Design of Offshore Wind Turbines,” Chapter 2 in Offshore Wind Turbine Foundation Design, DTU Wind Energy, Copenhagen, Denmark .
[22] Wang, J. K., 2012, “Settlement of Gravity Foundations under Vertical Loads,” M.S. Thesis, National Cheng Kung University, Tainan, Taiwan.
[23] Yu, H., Zeng, X., Li, B., and Lian, J., 2015, “ Centrifuge modeling of offshore wind foundations under earthquake loading. ” Soil Dynamics and Earthquake Engineering, Vol. 77, pp. 402-415.
[24] Malhotra, S., 2007, “Design and Construction Considerations for Offshore Wind Turbine Foundations,” in Proceedings of the 26th International Conference on Offshore Mechanics and Arctic Engineering, San Diego, California, USA.
[25] Chen, D. Huang K., Bretel, V., and Hou, L., 2015, “Comparison of Structural Properties between Monopile and Tripod Offshore Wind-Turbine Support Structures,” Advances in Mechanical Engineering, Vol. 5, Article ID 175684.
[26] IEC 61400-1, 2005, International Standard Wind Turbines- Part 1: Design Requirements, Third Edition, International Electrotechnical Commission, Geneva, Switzerland.
[27] IEC 61400-3, 2009, International Standard Wind Turbines- Part 3: Design Requirements for Offshore Wind Turbines, First Edition, International Electrotechnical Commission, Geneva, Switzerland.
[28] Farahmand, B., 2001, “ Fracture mechanics of metals, 2001, composites, welds, and bolted joints: application of LEFM, EPFM, and FMDM theory,” Springer, N.Y., USA.
[29] Dowling, N. E., 1988, Mechanical Behavior of Materials, Fourth ed., Pearson, UK.
[30] S355ML材料特性, 網路資料, https://www.salzgitter-flachstahl.de/fileadmin/mediadb/szfg/informationsmaterial/produktinformationen/warmgewalzte_produkte/deu/S355ML.pdf.
[31] The UK standard BS 7910 s, 2013- Guide to methods for assessing the acceptability of flaws in metallic structure.
[32] 崔海平, 2018, 離岸風電場址風況、海洋參數及負載分析技術研究,金屬工業研究發展中心研究報告.
[33] DNV GL, and Garrad Hassan & Partners Ltd, 2016, “Bladed User Manual Version 4.8,”.
[34] 洪浚傑, 2019, 離岸風力機負載分析與結構應力分析, 國立中央大學機械工程學系碩士論文.
[35] Offshore Code Comparison Collaboration (OC3) for IEA Task 23 Offshore Wind Technology and Deployment.
[36] Passon, P., 2015, “Damage Equivalent Wind-Wave Correlations on Basis of Damage Contour Lines for the Fatigue Design of Offshore Wind Turbines,” Renewable Energy, Vol. 81, pp. 723-736.
[37] MADDOX, S. J., 1975, “The effect of mean stress on fatigue crack propagation a literature review,” international Journal of Fracture, Vol. 11, pp. 389-408.