我國近年積極發展再生能源，經濟部能源局規劃於2030年完成開發3GW之離岸風場總裝置容量，離岸風力發電機開發成本中，風機支承結構之花費占總成本約1/5以上，且肩負離岸風機之安全性及穩定性，因此基礎支承結構的設計與分析為開發離岸風場的重要關鍵。多樁基礎型式之樁徑較小，為我國之海事工程施工能量較可自主施工之基礎型式之一。由於離岸風機需長期承受風力及波浪之週期性側向作用力，當側向載重經上部結構傳遞至多樁基礎之下部結構時，將對基樁產生軸向作用力，並且具有週期性特性。當多樁基礎受反覆軸向拉力及壓力時，基樁之受力變形行為會造成基礎傾斜，當基礎傾斜角度超過風機運轉容許傾斜角度(0.5°)時，將影響風機運轉之穩定性。
樁基礎受軸向反覆拉力下之受力行為，由於樁土間之交互關係並未明確，且現有現地樁載重試驗十分稀少，因此，目前於工程界並無明確之評估方法，現有較常用於評估分析方法為收集樁載重試驗成果所繪製之穩定性關係圖(Stability diagram)或稱互制關係圖(Interaction diagram)。於穩定性關係圖中，僅能得到樁基礎受反覆載重比條件下之容許作用力次數，樁基礎之尺寸條件無法納入考量，且無法得到樁基礎受反覆載重下之極限拉拔力及樁身位移量。
本研究針對樁基礎受軸向拉力之受力行為進行研究，應用Abdel-Rahman & Achmus (2011)提出利用有限元素法數值模型分析，考量樁-土系統之互置關係，將土壤受反覆作用下之體應變導入數值模型中，針對不同樁基礎尺寸及受不同反覆拉力條件比下之受力變形行為，藉此得到樁基礎受反覆拉力下之極限拉拔力及樁身位移量。 本研究利用數值模型分析樁基礎受反覆拉力下之行為反應，總共進行9組樁基礎尺寸分別受6組反覆拉力條件之受力行為分析。反覆拉力條件包含反覆拉力振幅比及平均反覆拉力比。根據分析結果顯示，反覆拉力振幅比越大，極限拉拔力隨反覆作用力次數衰減幅度越顯著，樁身位移量增加幅度亦越大。相較於反覆拉力振幅比，當平均反覆拉力比增加時，極限拉拔力及樁身位移量隨反覆作用次數之變化幅度較小。

In recent years, Taiwan have been energetic to envelop renewable energy. Offshore wind energy plants will be installed and be planned to reach total capacity of 3 gigawatts before 2030. The foundation of offshore wind turbine plays an important role in the stability of the wind turbine structures. Multi-pile foundation such as the jacket type foundation and the tripod foundation is the most likely to be selected in Taiwan. The wind turbine foundation standing in the seabed will under the wind and waves action whole the operation lifetime (about twenty years). The pile foundation will be subjected to cyclic axial loading including cyclic tension loading and cyclic compression loading. However, in current engineering practice theses effects are hard to be concern because we have no clear description of the interaction relationship. This study apply Abdel-Rahman & Achmus (2011) published method using Finite Element Method to model the pile under cyclic axial tension loading in cohesionless soil the pile deform behavior and pile ultimate pull-out capacity with the cyclic loading numbers. A two-dimensional numerical model was established. The pull-out capacity of pile under static axial loading are almost the same as the results calculated with AIP (2007) method. The deformation responses of pile under cyclic tension loading are compared with the results from field axial cyclic loading experiments on pile. As to be expected, the result shows that the pile pull-out capacity decrease with the number of cyclic tension load cycles, and the pile displacements increase with the number of cyclic tension load cycles. Besides, the pile pull-out capacity reduction rate increase with the cyclic tension load level, and the pile displacements increase rate raise with the cyclic tension load level.

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