| 摘要: | 離岸風機的成本受環境載重與地質條件的影響甚大，而外海地質調查困難且成本高 昂，很難有完整充分的地質與地工資訊，因此地質不確定性對離岸風機基礎的建造成本 有顯著的影響，凸顯地質不確定性對離岸風機基礎設計影響研究的重要性。本研究以彰 濱離岸風場可行性階段的少數探查孔為依據，使用馬可夫隨機場產製風場之人工地層模 型，以總熵量化地層模型的不確定性。進而探討給定不同數量探查孔資料所產製人工模 型的精確度。結果顯示對於本研究案例風場之簡單地層分布，規劃 9 個均布探查孔，即 可得較為精確的地層模型。應用上述人工地層模型，探討地質不確定性對於特定位置上 離岸風機單樁基礎垂直承載力之影響，亦發現使用 9 個均布探查孔所得到的設計樁長與 實際地層設計樁長幾乎相同。將上述結果結合蒙地卡羅模擬法可清楚分別或整體量化地 層與地工參數不確定性對單樁基礎垂直承載力破壞機率之影響。本研究採用類似的方法， 亦可瞭解離岸風機單樁基礎設計主頻於地質模型不確定性下之分布範圍。上述結果可作 為離岸風機基礎性能設計參考之用。
因離岸風機支撐結構系統之主頻為服務性能狀態之重要檢核項目，本研究為地質不 確定性對主頻之影響程度，首先進行地工參數與結構參數對離岸風機單樁基礎主頻之敏 感度分析，以掌握各種參數對主頻之影響程度，並探討各種簡易評估法與有限元素法分 析結果之差異。研究結果顯示對於結構參數，其影響程度由大至小之排列為:塔高、塔 柱及單樁之樁徑、集中質量、樁長、壁厚。對於地工參數，剪力波速為最主要之影響因子，相對密度影響輕微。在本研究使用之 4 MW 與 6 MW 案例風機時，使系統主頻達穩 定值之單樁基礎臨界長徑比之範圍約在 1.5〜1.7 之間，顯示海床下 2 倍樁徑內淺層土壤 之剪力波速對主頻影響遠較深層土壤為大，因此淺層土壤剪力波速之調查精度相當重要。 本研究建議之等值樁頭勁度法與等值懸臂梁法，都可大量簡化樁土互制作用之數值分析工作，並在計算系統主頻上具有足夠的精度(最大誤差在 2%以內)，建議可應用於工程實務。
最後，本研究以一個實際風場的探查資料，研究不同方式所建置之波速剖面與不同 調查孔地層的差異來分別探討地工參數不確定性與地質模型不確定性對風機系統主頻 的影響。主頻的評估方法採用精確的有限元素分布彈簧法，分析軟體採用 SAP2000。結 果顯示以各鑽孔位置之 PS Logging 與其回歸模型之波速剖面所計算主頻明顯較 CPT- based 與其回歸模型之波速剖面所計算之主頻為高;而以各風機位置之 CPT-based 的波 速剖面較能反應地質不確定性對各風機位置主頻之影響。;The cost of offshore wind turbines is greatly affected by the environmental load and geological conditions, and offshore geological surveys are difficult and costly. Thus it is difficult to have complete and sufficient geological and geotechnical information. Therefore, geological uncertainty has a significant impact on the construction cost of offshore wind turbine foundations. This highlights the importance of research on the influence of geological uncertainty on the foundation design of offshore wind turbines. This study is based on a small number of exploration holes in the feasibility stage of the Changbin offshore wind farm. The geological uncertainty is quantified by the total entropy of the artificial geological model of the wind farm produced by the Markov random field. Furthermore, the accuracy of the artificial model produced by given different numbers of exploration hole data is discussed. The results show that for a simple stratum distribution, an accurate stratum model can be obtained by planning 9 evenly distributed exploration holes. Using the artificial stratum model above, the influence of geological uncertainty on the vertical bearing capacity of the monopile foundation of offshore wind turbines at a specific location is investigated. It is also found that the design pile length obtained by using 9 uniformly distributed exploration holes is almost the same as the design pile length of the actual stratum. Combining the above results with the Monte Carlo simulation method can clearly quantify the influence of stratum and geotechnical parameter uncertainty on the failure probability of the vertical bearing capacity of monopile foundation separately or as a whole. Using a similar method, this study can also quantify the failure probability of the design natural frequency of the monopile foundation of offshore wind turbines.
Since the natural frequency of the support structure of offshore wind turbine is an important inspection item for the serviceability limit state, this study focuses on the influence of geological uncertainty on the natural frequency. First, the sensitivity analysis of the geological and structural parameters to the natural frequency is carried out in order to grasp the degree of influence of various parameters on the natural frequency, and the differences between the evaluated results of various simplified methods and finite element method are also investigated. The research results show that for the structural parameters, the order of influence from large to small is: tower height, diameter of tower column and monopile, concentrated mass, pile length and wall thickness, and for geotechnical parameters, the shear wave velocity is the most important factor, and the relative density has a slight influence. The critical length-to- diameter ratio of the monopile foundation that stabilizes the natural frequency of the system is in the range of 1.5 to 1.7, which reveals that the shear wave velocity of the shallow soil within 2 times the pile diameter under the seabed has a greater influence on the natural frequency than the deep soil, so the survey accuracy of soil shear wave velocity in the shallow soil is very important. The equivalent pile head stiffness method and the equivalent cantilever beam method proposed in this study can greatly simplify the numerical analysis of pile-soil interaction, and have sufficient accuracy (the maximum error is within 2%). It is recommended to be applied to engineering practice.
Finally, this study uses the exploration data of an actual wind farm to study the influence of wave velocity profiles constructed in different ways (geotechnical uncertainty) and strata differences in different hole (stratum uncertainty) on the natural frequency of the wind turbine system. The accurate distributed spring method by finite element is used to evaluate the natural frequency and the analysis software uses SAP2000. The results show that the natural frequency calculated by the wave velocity profile of PS Logging and its regression model at each drilling position is significantly higher than that calculated by the wave velocity profile of CPT-based and its regression model. The CPT-based wave velocity profile of each wind turbine location can better reflect the influence of geological uncertainty on the natural frequency of each wind turbine location. |
