隨著全球能源轉型與淨零碳排政策的推動，天然氣作為低碳排過渡能源，其穩定供應對台灣能源結構至關重要。台灣本島地狹人稠，土地資源有限，海底管線成為運輸天然氣的重要途徑，目前承擔超過50%的輸氣量。然而，海底管線長期暴露於複雜海洋環境中，易受海流沖刷導致局部懸空，引發渦激振動（Vortex-Induced Vibration, VIV），影響結構穩定性與輸氣安全，甚至造成天然氣洩漏，引發環境污染與經濟損失。台灣海峽地形多變，海流湍急且變化莫測，近年氣候變遷加劇海洋環境的複雜性，常導致海床淘刷，使海底管線局部懸空，進一步引發結構不穩定、挫曲變形，甚至破裂，對國家能源安全與海洋生態構成威脅。因此，深入探討海底管線沖刷過程中的流場特性，對確保供氣安全與環境永續具有重要意義。
本研究針對海底管線沖刷過程中其懸空高度變化對流場行為的影響，設計五階段沖刷坑演化模型，並使用 FLOW-3D 軟體進行二維定床數值模擬。模擬採用0.5 m/s、1.0 m/s及1.5 m/s三種流速下之來流條件，並分析各階段沖刷坑之流場特徵、渦度、回流帶速度及管線上下游壓力差變化。
研究結果顯示，渦脫落週期隨沖刷坑發展而延長，且隨流速增加而縮短，反映出間隙比變化與入流速度對渦旋生成頻率的顯著影響。隨著沖刷坑演變，尾流正負渦脫落之強度逐漸對稱，流場逐漸趨向穩定。管線下方渦度受到間隙射流與海床邊界層約束，於初期相對較低，隨後隨間隙比增加先上升後下降，顯示沖刷主導機制自間隙射流轉向渦流作用，最後達成平衡。
回流帶位置穩定生成於沿主流方向距離管線中心x/D=1.2~1.3之間，並隨正負渦的週期性脫落同步擺動。隨流速增加，回流速度明顯提升，但隨著沖刷發展與間隙比增加，回流速度呈現下降趨勢。管線上下游壓力差則與流速呈正相關變化，在沖刷初期最為明顯，隨沖刷坑發展趨於平衡，惟在高流速情境下，仍可能引發滲流與管湧現象，進一步促進局部沖刷與結構不穩定風險。
本研究揭示了海底管線沖刷演變過程中，流速與間隙比對渦度、渦脫落、回流速度與管線上下游壓力差的綜合影響，並為海底管線設計、鋪設及防護工法提供了相關數值基礎與理論參考。

With the global promotion of energy transition and net-zero carbon emissions, the share of gas-fired power generation in Taiwan has steadily increased. Due to geographic and regulatory constraints, the installation of onshore pipelines has become increasingly limited, making submarine pipelines an increasingly important means of transporting natural gas, now accounting for over 50% of Taiwan’s total gas transmission.
Submarine pipelines are typically buried beneath the seabed to prevent direct exposure to ocean currents and reduce the risk of third-party damage. However, under sustained ocean current action, local seabed scour may occur, causing sections of the pipeline to become exposed or suspended. The interaction between ocean currents and a suspended pipeline alters the surrounding flow field, potentially leading to vortex shedding. This phenomenon not only intensifies local scour but also poses a risk to pipeline structural integrity, potentially leading to deformation, damage, and reduced service life.
In this study, numerical simulations were conducted using FLOW-3D to investigate flow field variations around submarine pipelines. A series of five seabed configurations were developed to represent different scour stages, and simulations were performed under three different flow velocity. Results show that the pressure difference across the pipeline increases with flow velocity. Additionally, flow velocity significantly influences the period and intensity of vortex shedding. The gap ratio has a marked effect on the vortex shedding pattern and vorticity magnitude.

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