长期反复荷载作用对海上风电单桩基础的影响分析

Effects of long-term cyclic loadings on offshore wind turbine monopile foundation

国内外已建成的海上风电通常采用大直径单桩基础,需承受多种复杂外部荷载,如风荷载、波浪荷载和潮流荷载等。在长期反复荷载作用下,单桩基础产生累积变形,侧向刚度发生变化,而这些性能的变化会对海上风电整体安全性产生重要影响。对于这个问题,目前设计中采用规范给出的简化设计方法考虑长期反复荷载的影响,即在传统的p-y曲线上乘以一个降低系数,而没有针对海上风电大直径单桩基础给出具体的设计方法。鉴于p-y曲线是基于较小直径桩试验结果所得,对于大直径桩是否安全可靠,还需要验证。鉴于此,采用文献提出的桩周边土体在反复荷载作用下变化的简化模型,建立了整体三维有限元模型,考虑长期反复荷载作用对桩周边土性能的影响,以一实际海上风电案例为背景,研究长期反复荷载对海上风电基础自振频率和桩体变形性能的影响以及参数敏感性分析,分析结果对海上风电单桩基础设计有一定的参考作用。

Offshore wind turbines are often supported on large diameter monopole foundation suffering various complex external loadings, especially, wind and wave loads. These long-term cyclic loadings cause the monopile foundation to have accumulated deformation and changes in its lateral stiffness,they have important effects on offshore wind turbine's safety. For considering the effects of long-term cyclic loadings on offshore monopole foundation,the current deign codeis to use the p-y curve method adopted in the American Petroleum Institute Standards. The p-y curve is obtained from field tests with less than 200 cycles loads and pile's diameter range up to 0. 6 meters,and may not be applicable to monopile with large diameter and subjected to much more cycles loading. Here,an overall 3-D FE model was built to study displacements of an offshore wind turbine on a monopile in a sandy seabed under long-term cyclic lateral loading. A simplified model to account for the influence of long-term cyclic loading on the soil surrounding the pile was adopted based on the model test results in the literature. A real wind turbine on a monpile foundation was taken as a study object,the effects of long-term cyclic loading on the foundation 's natural frequencies and the pile 's deformation characteristics were analyzed in detail. The results provided a reference for the design of offshore wind turbine's monopole foundation.