單樁基礎之離岸風機系統於層狀土壤中的動態反應分析

Abstract

淨零碳排為全球發展趨勢，離岸風力發電因具備穩定提供永續及綠色能源的優勢而快速發展。然而，台灣離岸風場多位於砂土與黏土互層的複雜沉積地層環境，且地震活動頻繁，使砂土液化成為離岸風機設計必須面對的重要課題，對離岸風機系統之長期穩定性與使用性構成潛在威脅。 本研究利用三維、全耦合、非線性有限元素軟體OpenSees，模擬NREL-5MW離岸風機搭配單樁基礎於可液化砂土層中，受到地震與環境載重共同作用下之系統動態反應。分析模型根據彰化濱海產業園區外海CPT鑽探資料，並採用PDMY03組成律模式模擬飽和顆粒材料之動態行為。本研究考量三種載重情形：(a) 單獨地震載重；(b) 地震與極端常時風力載重；(c) 地震與諧和波風力載重，風載均施加於風機塔架頂端。地震訊號則選用無脈衝型地震與具速度脈衝之脈衝型地震，以模擬近場地震效應對離岸風機系統之影響。 分析結果顯示，脈衝型地震為觸發鬆砂層液化的主要因素，液化導致土壤有效應力下降與勁度弱化，進而降低基礎側向承載力，影響系統穩定性。常時風載亦顯著影響基礎行為，在風載單獨作用下即造成明顯傾斜，其傾角反應超出使用限度的參考標準，顯示即便非地震情境亦須審慎評估風載影響。在本案場條件下，基礎總沉陷量皆較小，推測由於部分地層在地震作用下未出現明顯勁度弱化，仍可提供基礎承載力，顯示沉陷量並非本案例的主要使用性判斷依據。針對基礎穩定性問題，本研究亦探討增大單樁直徑作為提升系統穩定性之對策，結果顯示合理增大單樁直徑有助於提升基礎抗彎矩與側推能力，並抑制彎矩變形產生。

Achieving net-zero emissions has become a global development trend. Offshore wind power is rapidly expanding due to its ability to provide stable and renewable energy. However, offshore wind farms in Taiwan are typically located in sites with interbedded sand and clay layers and high seismic activity, which present complex geotechnical conditions. Under such conditions, soil liquefaction can pose a risk to the long-term safety and functionality of the wind turbine system. Therefore, this study aims to establish a robust model of an offshore wind turbine (OWT) with a monopile foundation on a liquefiable soil site, specifically considering layers of sandy and clayey soil. It also investigates the effects of soil-structure interaction during earthquake shaking combined with various environmental loading scenarios. This study employs OpenSees, a three-dimensional, fully-coupled, nonlinear finite element analysis platform, to simulate the dynamic response of a hypothetical NREL-5MW OWT with a monopile foundation installed in stratified liquefiable soil layers under the combined seismic and environmental loading. The soil profile is derived from the Cone Penetration Test (CPT) data from the offshore area near the Changhua Coastal Industrial Park, and the PDMY03 (Pressure Dependent Multi Yield surface version 03) constitutive model parameters are adopted to simulate the highly nonlinear dynamic behavior of saturated granular soil. Three loading scenarios are considered in this study: (a) seismic loading alone; (b) seismic loading coupled with extreme constant wind loading; and (c) seismic loading coupled with sinusoidal wind loading at 1 Hz and 0.1 Hz frequecies. Both static and cyclic wind loads are applied at the rotor nacelle assembly (RNA). Both non-pulse-like (NP) and pulse-like (P) ground motions are used to assess the near-fault effects on the OWT system. The simulation results indicate that the pulse-like ground motion is the primary cause of liquefaction in loose sand layers, leading to reduced effective stress and soil stiffness degradation, and consequently decreasing the lateral capacity of the foundation. In addition, constant wind loading can cause significant tilting of the monopile, exceeding the serviceability limit state, indicating that wind effects should be considered in OWT design. Under the site-specific condition, the total foundation settlement was minor, likely due to the presence of soil layers that still retained strength during seismic loading. To enhance foundation stability, increasing the monopile diameter proves to be an effective mitigation strategy to reduce the foundation deflection. Based on these observations, this study identifies key considerations for OWT foundation design on stratified liquefiable soil layers and highlights the critical influence of soil-structure interaction on system performance under post-liquefaction conditions.