應用不同方法計算離岸風機單樁基礎所受波浪力之適用性探討

APPLICABILITY OF DIFFERENT METHODS FOR COMPUTING WAVE FORCES ON MONOPILE FOUNDATION OF OFFSHORE WIND TURBINE

The wave load of an offshore wind turbine foundation is determined by water depth, waves, ocean currents and seabed geology, whether it is a monopile, tripod, jacket, or gravity type foundation. In this paper, different computational methods are used to compute the wave load of the offshore wind turbine monopile foundation. Fifteen wave conditions are set by three periods and five wave heights which gradually increase from small waves to breaking waves, and the force per wave amplitude is defined as a comparison parameter. For the hydrodynamic analysis of the foundation, we use three methods to compute, they are self-developed Morison equation program, potential flow program WAMIT, and the viscous flow RANS software StarCCM+. Since the physics of viscous flow method is closer to the real physical phenomenon, it is possible to completely simulate linear and nonlinear forces with the method, but it takes a lot of computational time and cost to conduct. In addition to the comparison of the computational results of three methods, the results are also compared with the analytical solutions by MacCamy and Fuchs. The results show that in the case of non-breaking wave, the wave height and force relationship computed by the three methods generally grow linearly, while the force per wave amplitude decreases with the increase of the period, which is consistent with the analytical solutions. However, in the case of breaking waves, the force computed by the RANS method increases sharply, and the force per wave amplitude cannot be accurately observed due to the indeterminate nature of the position of the breaking point. After comparing the results, it is recommended one of the three methods applied for various sea conditions.

離岸風機的基礎結構物，不論是單樁、三腳管式、套管式或是重力式等，其受力必須考慮到水深、海浪、洋流以及 海床地質的影響。本文應用不同的計算方法計算離岸風機單樁基礎結構豎立於海中承受的波浪負荷，波浪條件計算了三 種週期及五種波高，從較小的波高逐漸增加到產生碎波的波高，組合而成十五種不同的波浪條件，並定義了單位波高的 受力作為比較基準。水動力學分析方法上，我們應用三種方法進行計算，一是求解莫理森方程式 （Morison equation），二 是使用勢流計算軟體 WAMIT，三是應用黏性流的 RANS 方法。莫理森方程式與 WAMIT 僅能用於線性分析，無法呈現 非線性波浪條件下的分析。黏性流 RANS 方法應用商用軟體 STAR-CCM+進行計算，由於接近真實的物理現象，因此可 以完整模擬線性與非線性的受力，但需要耗費大量的計算時間和成本。本文利用三種方法和十五種不同的波浪情況進行 離岸風機單樁基礎計算，除了三種方法計算結果的比較之外，並與文獻 MacCamy & Fuchs 方法的結果進行驗證，結果顯 示在非碎波情況時，三種方法計算的波高和受力關係大致呈線性成長，而單位波高的受力會隨著週期增加而減少，和文 獻的趨勢一致；但在碎波情況時，RANS 方法計算所得之受力會急遽增加，而單位波高的受力由於無法確定碎波點的位置，趨勢沒有辦法準確觀察。在比較結果後，本文中所應用的三種方法各有其優點與缺點，因此需要針對不同海況選擇最適當的方法。