由於雙相不銹鋼之高強度與優異的耐海水腐蝕特性，本研究分析單樁式(Monopile)與套管群樁式(Jacket)離岸風機支承結構在海水環境中受到不同載重組合作用，若兼顧結構安全與腐蝕問題的考量，採取雙相不銹鋼作為結構材料之可能性。結構分析以ABAQUS軟體建立有限元素模型，模擬兩種離岸風機結構受到結構自重、上部風力機組重、風力、海浪力之靜態載重組合作用下，結構中各元件應力與變形行為，以及整體結構之自然振動頻率及對應之變形模態。
研究結果發現，海浪分佈力對結構產生之彎矩效應不如風力明顯。在結構自重、上部風力機組重、加上風力之載重組合下，單樁式結構之基樁於海床處產生最大正向應力。當風力大小為2MN時，基樁最大正向應力接近中強度碳鋼之降伏強度；套管群樁式結構之最大應力則發生於塔架與套管連接處，此處採銲接型式，無搭接長度且斷面厚度較薄，受壓力大時容易導致局部挫屈破壞。自然振動頻率分析發現，單樁式結構之第一振動頻率高於套管群樁式結構，且較風機轉子之第一激發頻率(1P)高。在海浪力作用之頻率範圍內，兩種結構型式均存在共振頻率及對應之彎曲變形模態。由於雙相不銹鋼之楊氏模數、柏松比均與中強度碳鋼相近，將套管群樁式結構在海水環境中特別容易受腐蝕區域之套管元件採用雙相不銹鋼來取代，所得之應力分析與自然振動頻率分析結果與中強度碳鋼差異不大。因此採用雙相不銹鋼運用於離岸風機結構上預期是可行的。

關鍵字:有限元素分析、單樁式、套管群樁式、雙相不銹鋼

SUMMARY
Two offshore wind turbine structures, which are the monopile type and the jacket type, subjected to different load combinations were analyzed. The possibility of using duplex stainless steel in the wind turbine structure for its strength and excellent corrosion resistivity was considered. Our analyses were carried out using finite element model in ABAQUS. Load combination consists of the structural weight, the weight of wind turbine, the wind force, and the ocean wave force. The natural vibration frequencies of wind turbine structures and the corresponding deformation modes were also analyzed. The results show that the bending moment and the stress caused by the wind force are much larger than by the ocean wave force. For the monopile structure, the maximum normal stress occurs on the level of seabed and exceeds the yield strength of mild carbon steel, when load combination consists of the structural weight, the weight of wind turbine and the wind force above 2MN. For the jacket structure, the maximum normal stress occurs at the welded joints in the connection part, and causes local buckling when the wind force is above 3MN. The first natural frequency of the monopile structure is higher than that of the jacket structure, and is also higher than the first excitation frequency of the wind turbine rotor. For the jacket structure, the sleeve parts were selected to use duplex stainless steel instead of mild carbon steel, because they are partially immersed in seawater and partially exposed to a seawater splash and their corrosion rate is much higher the other parts. The stresses and the natural vibration frequencies of the jacket structure with duplex stainless steel sleeve are almost the same as those of the jacket structure with mild carbon steel sleeve. It is feasible to apply duplex stainless steel in offshore wind turbine structures, because of its high yielding strength and very low corrosion rate.

Key words: Finite element analysis, Monopile structure, Jacket structure, Duplex stainless steel

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