海底邊坡在地震作用下的數值模擬分析

Abstract

近年來，隨著全球對能源需求的提升與可再生能源技術的快速進展，海洋資源的開發活動愈發活躍，特別是在石油與天然氣開採以及離岸風電等領域。這些工程設施多建置於大陸棚及其邊緣區域，而這些區域常伴隨著地質條件複雜的海底邊坡。海底邊坡若因天然或人為因素發生破壞，不僅會對離岸風力機基礎、油氣輸送管線與其他設施造成嚴重損壞，進而導致工程延誤與重大經濟損失，更會因海底修復工作的困難度高及環境危險性強，使得災後復原作業代價昂貴且耗時。因此，深入瞭解海底邊坡的破壞機制，並建立可靠的破壞模式與分析方法，對於海洋工程設施的風險管理與設計安全性而言具有相當關鍵的意義。 本研究採用有限元素軟體ABAQUS對海底邊坡進行穩定性分析。首先，在靜態條件下，針對不同坡度的海底邊坡幾何，利用強度折減法（Strength Reduction Method）計算其安全係數。分析中同時進行總應力分析（Total Stress Analysis, TSA）與有效應力分析（Effective Stress Analysis, ESA），以探討不同分析方法對安全係數結果的影響，並與陸域飽和土坡之行為進行對比，了解海水與孔壓對於邊坡穩定性的潛在影響。此外，為模擬地震條件下海底邊坡的動態行為，本研究進一步於 ABAQUS 中進行動態分析，並引入地震加速度歷時作為輸入地震。分析中特別考慮地震動的振幅大小與頻率組成等特性，藉此評估其對邊坡反應的影響。材料行為方面，使用簡化非線性運動硬化模型（Simplified Nonlinear Kinematic Hardening, SNKH Model），以模擬土壤於反覆剪應變歷程下之非線性與循環硬化特性。最後探討在不同地震歷史、材料強度、地盤種類下邊坡位移及剪應變情形，以及地震動放大比分析（Ground Motion Amplification Ratio, GMAR），結果顯示GMAR並不與坡角、地盤種類、地震最大加速度有絕對關係，而是和地震主頻大小、土壤強度參數等等有密切關係，說明邊坡因低頻率地震產生的共振效應是動態分析中導致地震動放大和破壞的主因，本研究最後進一步與陸地彈性邊坡比較其地震動放大效應，發現絕大部分GMAR值皆小於陸地彈性邊坡，說明邊坡產生塑性破壞的能量耗散使得地震波無法有效轉換成地表加速度。

In recent years, the development of offshore resources such as oil and wind energy has become increasingly active. However, submarine slope failures can severely impact the construction and operation of offshore infrastructure such as wind turbine foundations and pipelines, resulting in substantial losses. Moreover, the repair of submarine facilities is extremely challenging, not only due to high costs but also because personnel are exposed to hazardous working environments. Therefore, understanding the failure mechanisms of submarine slopes and conducting failure analyses are critically important. In this study, stability analyses of submarine slope were conducted using the ABAQUS software. Factors of safety for submarine slope with various slope angles under static condition were computed using the strength reduction method. Total stress analysis (TSA) and effective stress analysis (ESA) were carried out to compare the factors of safety corresponding to saturated slopes on land. For the evaluation of seismic submarine slope stability, dynamic analyses were performed in ABAQUS. Effects of ground motion characteristics (such as amplitude, frequency content) on the seismic behavior of submarine slope were studied. Nonlinear material model (Simplified Nonlinear Kinematic Hardening, SNKH Model) is used to simulate the nonlinear and cyclic behavior of soils under repeated shear loading. The study examines displacement, shear strain, and ground motion amplification ratio (GMAR) under various seismic inputs, soil strengths, and bedrock types. Results indicate that GMAR is more influenced by the dominant frequency and soil strength than by slope angle, ground type, or PGA, highlighting resonance from low-frequency earthquakes as a key factor in amplification and failure. Compared to elastic land slopes, this study shows lower GMAR due to energy dissipation from plastic deformation.