近年來離岸風力發電快速發展，單樁式基礎因具有結構型式簡單、施工便利及經濟效益佳等優點，已成為淺水域離岸風場常見之基礎型式。然而，單樁結構長期受波浪、潮汐與流速等複合水動力作用影響，樁周局部淘刷現象將可能造成基礎埋置深度降低、結構穩定性下降及長期安全風險增加。臺灣西部彰濱海域具有明顯潮差，且於季風與颱風期間會受到不同強度之波浪與流速作用，因此有必要針對海水面變化條件下之單樁鄰近海床淘刷行為進行系統性探討。
本研究以彰濱海域離岸風場之環境條件為背景，於國立成功大學水工試驗所風波流試驗水槽進行縮尺物理模型試驗，探討離岸單樁式結構在不同波浪、流速及潮位條件下之海床淘刷發展特性。試驗條件包含季風波、季風波加固定流速及颱風波加固定流速，並進一步區分為變動潮位與固定潮位兩類情境。其中，變動潮位試驗用以模擬實際海域中潮位升降對淘刷歷程之影響；固定潮位試驗則分別以季風波條件下之平均潮位及颱風波條件下之最高潮位作為代表水位，其他波浪與流速條件則與變動潮位試驗保持一致，以比較不同潮位型態對樁周淘刷行為之差異。
試驗結果顯示，單純季風波作用下，單樁周圍淘刷發展較為緩慢，沖刷坑主要集中於樁前迎波側，坑形多呈橢圓且相對對稱，屬於波浪主導之淘刷型態。當季風波疊加固定流速後，波流耦合作用使近床流速與床面剪應力增加，樁前下衝流及馬蹄形渦流增強，導致淘刷深度與範圍均較單純波浪條件增加，且沖刷坑逐漸向下游偏移，呈現較明顯之非對稱分布。於颱風波加固定流速條件下，由於波高與流速共同提高，使床面剪應力長時間維持於較高水準，淘刷發展最為快速，沖刷坑亦呈現較深、較廣且非對稱性更強之特徵，顯示極端波流條件將顯著降低單樁周圍底床穩定性。
在潮位條件比較方面，固定潮位試驗之淘刷發展多呈現單調漸進並逐漸趨近平衡之趨勢，沖刷歷程較為平滑，主要受固定水深下持續波流能量輸入控制。相較之下，變動潮位條件下之淘刷歷程則具有階段性變化，當水位由高轉低或進入較低水位階段時，有效水深降低，使近床波浪粒子速度及床面剪應力增加，進而造成局部淘刷再活化現象，使沖刷曲線出現斜率增加或分段成長之特徵。綜合比較結果顯示，颱風波加固定流速於變動潮位條件下為本研究中較不利之淘刷情境，其最終淘刷深度明顯大於固定潮位條件，表示若工程設計僅以固定水位作為分析基準，可能低估實際海域潮位轉換下之單樁淘刷風險。
整體而言，本研究證實波浪能量、流速強度與潮位變化為控制離岸單樁周圍海床淘刷發展之主要因子，其中變動潮位所造成之水深改變與淘刷再活化效應，對最終沖刷深度及地形分布具有重要影響。研究成果可作為臺灣西部離岸風場單樁式基礎防沖刷設計、基礎裸露風險評估及後續數值模式驗證之參考依據。未來建議可進一步針對颱風波、流速與固定低潮位等極端組合進行補充試驗，以建立更完整之在地化離岸風電基礎淘刷評估資料庫。

In recent years, offshore wind power has developed rapidly as an important renewable energy source. Among various offshore wind turbine foundation types, the monopile foundation has been widely used in shallow-water wind farms because of its simple structural form, construction efficiency, and economic advantages. However, monopile foundations are continuously subjected to complex marine environmental forces, including waves, currents, and tidal variations. These hydrodynamic actions may induce local seabed scour around the pile, reduce the embedded depth of the foundation, and further affect the long-term stability and safety of offshore wind turbine structures. In the western coastal waters of Taiwan, especially near the Changbin offshore area, significant tidal variation, seasonal monsoon waves, typhoon waves, and current effects are commonly observed. Therefore, it is necessary to systematically investigate the scour behavior around offshore monopile foundations under sea-level variation and combined wave-current conditions.
This study focuses on the Changbin offshore area as the representative marine environment and conducts a series of physical model experiments in the wind-wave-current flume at the Hydraulics Laboratory of National Cheng Kung University. A scaled monopile model was used to investigate the development of seabed scour under different wave, current, and water-level conditions. The experimental cases included monsoon waves, monsoon waves combined with a steady current, and typhoon waves combined with a steady current. In addition, both variable water-level and fixed water-level conditions were considered. The variable water-level tests were designed to simulate the influence of tidal variation on scour development, while the fixed water-level tests were conducted as comparison cases. For the fixed water-level tests, the monsoon wave-current condition was set at the mean water level, and the typhoon wave-current condition was set at the highest water level. Other wave and current parameters were kept consistent with the corresponding variable water-level tests.
The experimental results show that under monsoon wave-only conditions, scour developed relatively slowly around the monopile. The scour hole was mainly concentrated in front of the pile on the wave-facing side, and its shape was generally elliptical and relatively symmetric. This indicates that the scour process was mainly controlled by wave-induced flow and horseshoe vortices. When a steady current was added to the monsoon wave condition, the combined wave-current action increased the near-bed velocity and bed shear stress. As a result, the scour depth and scour range increased significantly, and the scour hole gradually shifted downstream, forming a more asymmetric horseshoe-shaped pattern. Under typhoon wave-current conditions, the higher wave energy and steady current caused a rapid increase in bed shear stress, resulting in the most severe scour development among the tested conditions. The scour hole became deeper, wider, and more asymmetric, while a clear downstream deposition zone was also observed.
The comparison between variable and fixed water-level conditions indicates that water-level variation plays an important role in the scour process. Under fixed water-level conditions, scour development generally followed a monotonic trend and gradually approached an equilibrium state because the effective water depth and hydrodynamic forcing remained relatively stable throughout the test. In contrast, under variable water-level conditions, the scour process exhibited staged development. When the water level decreased, the effective water depth became smaller, which enhanced the near-bed wave orbital velocity and bed shear stress. This caused the scour process to be reactivated locally, leading to additional deepening of the scour hole and changes in the slope of the scour development curve. Therefore, the scour behavior under variable water levels was not simply controlled by wave and current intensity, but was also strongly affected by the timing and transition of water-level changes.
Overall, this study demonstrates that wave energy, current velocity, and sea-level variation are the three major factors controlling seabed scour around offshore monopile foundations. Among the tested cases, the typhoon wave combined with steady current under variable water-level conditions produced the most unfavorable scour response. The results suggest that using only fixed water-level conditions in engineering design may underestimate the actual scour risk in tide-dominated offshore environments. The findings of this study can provide experimental reference for scour protection design, foundation exposure assessment, and numerical model validation for offshore wind turbine foundations in the western coastal waters of Taiwan. Future research is recommended to further investigate extreme conditions such as typhoon waves combined with steady current under fixed low-water-level conditions in order to establish a more complete database for offshore monopile scour assessment.

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