近年來全球暖化及能源耗竭的問題，使各國積極發展再生能源，離岸風電為我國發展再生能源的關鍵項目之一。離岸風機支撐結構影響風機運轉穩定甚鉅，且為我國積極自主建置之離岸風電產業鏈供應元件。目前我國先導型風場採用之支撐結構型式包括大口徑單樁基礎以及套管基礎。離岸風機轉子及塔架結構受到風、波、流造成之週期性載重，作用於上部結構之側向力傳遞至下部樁基礎，於套管基礎支撐結構，將對迎風側與背風側之樁基礎分別產生軸向拉力及壓力。反覆拉力將造成基礎承載力降低，影響風機運轉穩定性。本研究以有限元素模型將打樁效應及反覆拉力下樁周正向應力納入考量，計算套管基礎受靜態及反覆拉力下之拉拔力，評估風機運轉年限內之支撐結構穩定性。
基樁受拉力作用時，軸向承載力主要由樁身摩擦力提供。對於套管樁基礎，API規範中建議之樁身摩擦力計算經驗公式並未考量打擊樁於安裝過程對樁土系統之影響。本研究以有限元素樁土系統數值模型進行基樁受拉力下之承載力分析，並與Jardine and Standing(2000)之現地打擊樁受靜態及反覆載重試驗成果比對，探討打樁效應對基樁靜態拉拔力之影響。由數值模擬成果可知，若以土壤靜止土壓係數考量打擊樁所受之樁周正向力，則計算所得之軸向承載力會遠小於現地試驗結果。為將打擊樁設置過程中樁周土壤側向壓力增加之反應納入拉拔力計算，數值模擬將側向土壓增加，以計算基樁之靜態拉拔力。對於基樁受反覆拉力後之承載力衰減行為，本研究則以土壤體應變衰減模型將樁周土壤受反覆剪力作用時，土壤體積隨拉力作用次數增加而減小，造成樁周土壤對基樁間正向應力逐漸降低之反應納入考量，以達到樁身摩擦力隨反覆作用次數增加而衰減之目標。模擬結果顯示，當數值模型考量打樁效應時，基樁於反覆拉力作用下之拉拔力與Jardine and Standing(2012)透過現地試驗建立之互制關係圖評估衰減趨勢吻合。
本研究以9組不同樁長與樁徑數值模型參數分析，評估基樁尺寸對基樁靜態拉拔力之影響。分析結果顯示，當樁徑固定時，增加樁長將使基樁靜態拉拔力提高；當樁長固定時，樁徑越大，則基樁靜態拉拔力越大。當反覆拉力作用時，反覆拉力振幅比越大，基樁反覆拉拔力比隨作用力次數衰減幅度越顯著。而平均反覆拉力比則對基樁反覆拉拔力衰減程度之影響較不顯著。另外，考量打樁效應之反覆拉拔力衰減程度遠小於未考量打樁效應之計算結果；且考量打樁效應之反覆拉拔力於Jardine and Standing(2012)互制關係圖衰減趨勢吻合。

The pilot offshore wind farm will be built at western coast of Taiwan, for Changbin and Fuhai offshore wind farm, the support structure type of the offshore wind turbines is selected to be jacket foundation, which is high stiffness. The moment caused by the periodic wind and wave force applies on the whole offshore wind turbine constantly. As a result, the piles of support structure are subjected to cyclic axial tension or compression loads. And cyclic tension loads degrade the capacity of the foundation. A finite element model is established to simulate the behavior of pile under cyclic tension loads in this research. The field tests of Jardine and Standing (2000) is documented, and the result of field tests is used to calibrate the numerical model. With considering the effect of pile driving and soil volume degradation due to cyclic tension loads, the result shows that the model can simulate the behavior of the pile under axial tension loads. If the numerical model without considering the effect of pile driving underestimates pullout capacity. When the pile is subjected to cyclic tension loads, the pullout capacity of pile decreases as the number of cycle and amplitude of the cyclic loads increase.

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