離岸風力機單樁基座疲勞分析;Fatigue Analysis of Monopile Foundation for Offshore Wind Turbine

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Title:	離岸風力機單樁基座疲勞分析;Fatigue Analysis of Monopile Foundation for Offshore Wind Turbine
Authors:	李易軒;Li, Yi-Syuan
Contributors:	機械工程學系
Keywords:	離岸風力機;單樁基座;順序分析法;疲勞損傷;offshore wind turbine;monopile foundation;sequential approach;fatigue damage
Date:	2020-08-13
Issue Date:	2020-09-02 19:13:08 (UTC+8)
Publisher:	國立中央大學
Abstract:	疲勞損傷是影響風力發電機壽命之關鍵因素，通常離岸風力機需具備20年之使用壽命，因此有必要計算其在服役期間的疲勞損傷值，確認結構之安全。本研究針對NREL 5-MW離岸風力發電機的單樁基座受到IEC 61400-3之DLC 1.2負載工況及DLC 6.4的負載工況作用，探討風速、潮高、環境負載角度及隨機變數對疲勞損傷的影響，並計算各種負載組合之20年累積疲勞損傷值。本研究引入順序分析法來計算單樁基座因受到環境負載所產生的響應，包含應力和位移。順序分析法的流程為在BLADED軟體建立完整的模型及環境條件，並且施加所選定的負載工況，求得基樁與塔架連接處受力的動態響應，隨後運用於ANSYS軟體計算單樁的應力和位移。完成順序分析法後會得到基樁各節面的應力歷時，針對此不規則的應力歷時，利用MATLAB軟體及雨流法計算不等振幅疲勞負載之循環數，並使用DNVGL-RP-C203所提供之S-N曲線與Goodman方程式求得對應之疲勞壽命循環數，最後使用Palmgren-Miner Rule計算疲勞損傷值。模擬結果顯示，針對20年持續發電運轉條件下，在各負載組合中，單樁基座的疲勞損傷最大值皆發生在單樁底部，原因乃類似懸臂樑結構受側向負載時，固定端會有最大的應力。WS6 (13.60 m/s)和WS7 (14.96 m/s)二個風速值會造成水平面以上的單樁結構有較大疲勞損傷，其原因為在接近額定風速時，風機會產生最大之響應。整體而言，風速WS11 (23.12 m/s)會造成單樁底部具有最大疲勞損傷，因為其產生的浪流負載較大。在基樁各節面發生最大疲勞損傷的位置與環境負載施加方向和機艙方向有關，但浪流負載較大之負載組合的施加方向將主導最大疲勞損傷發生的位置。潮位較高的負載組合會誘發較大的疲勞損傷，因為其較大的水壓會產生較大的水動力。而不同隨機變數對於疲勞損傷的影響不大，亦即不同的隨機變數會產生接近的疲勞損傷值，此乃風況頻譜在不同隨機變數下是相同的，代表風力是具有相近的能量。整體而言，本研究所探討的離岸風力機之單樁基座，其最大累積疲勞損傷值在三組不同隨機變數的環境負載作用下，皆遠小於1，顯示此單樁基座可在離岸風力機持續發電20年下或在20年服役期間經歷了待機和運轉階段，都不會因疲勞損傷而失效。上述結果顯示，本研究所建立之分析方法可適用於評估離岸風力機單樁基座疲勞損傷。 ;The accumulative fatigue damage is a key factor which leads to structural failure of offshore wind turbine (OWT). In general, the lifespan of OWT is expected to reach more than 20 years. Hence, the fatigue damage during the service time should be monitored. Fatigue damage ratio and the structural integrity for the monopile foundation of NREL 5-MW wind turbine are evaluated in this study. The Design Load Cases 1.2 and 6.4 of IEC 61400-3 are applied in this study. For discussing the uncertainties of environmental conditions, various wind speeds, tide heights, wave orientations, and ransom seeds are considered in the analyses. A sequential approach is introduced to calculate the response of monopile, including stress and deformation. In the first step, the model of 5-MW OWT, environmental conditions, and assigned design situations are specified in the BLADED code. In the second step, ANSYS code is used to calculate the stress/deformation of monopile using the output loading data from the BLADED results. The response from BLADED is extracted and applied in the ANSYS at the interface of tower and monopile. The distributed wind loading is exerted on the monopile above mean sea level (MSL), and the hydrodynamic loading acting on the monopile below MSL is inputted in the ANSYS code. The numbers of cycles for the irregular time histories are calculated using the Rainflow counting method. S-N curve listed in the DNVGL-RP-C203 and Goodman’s approach are employed to determine the corresponding fatigue life for each loading cycle. Palmgren–Miner Rule is finally employed to calculate the cumulative fatigue damage ratio. All of the above calculations related to fatigue damage ratio are performed using the MATLAB code. The results show that for all of the given loading combinations, the maximum fatigue damage ratio takes place at the bottom section of monopile. Similar to a cantilever beam subjected to lateral loadings, the maximum stress usually occurs at the fixed position. For all the wind speeds investigated, WS6 (13.60 m/s) and WS7 (14.96 m/s) generate greater fatigue damage ratios at the structure of monopile above MSL. As these two wind speeds are close to the rated wind speed, the induced response of wind loading is the largest. Among the given wind speeds, WS11 (23.12 m/s) generates the largest wave loadings and consequently the largest fatigue damage ratio. The location for maximum fatigue damage ratio is influenced by the orientation of rotor-nacelle-assembly and environmental loading. An increase in tide height leads to a greater fatigue damage ratio in the monopile. It is due to a greater hydrodynamic loading caused by the higher water pressure in deeper water. Selection of random seed barely affects the fatigue damage ratio, as a similar wind spectrum and energy is present in the given random seeds. The maximum cumulative fatigue damage ratios calculated for different random seeds are significantly less than 1. It is expected that the given monopile can last for a 20-year non-stop operation for generating power or for a 20-year service time including operating and parking. The overall results demonstrate that the methodology developed in this study is applicable to the assessment of fatigue damage ratio for the foundation of OWT.
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