循环荷载作用下软土中吸力锚基础变形的弹塑性分析

发布时间：2018-11-11 11:39

【摘要】：为维持海洋平台正常工作和安全稳定,吸力锚基础不仅要承受上部结构产生的工作荷载,还要承受风、浪等产生的循环荷载。因此,有必要对静荷载和循环荷载共同作用下吸力锚基础的变形失稳过程做出评价。本文依据一个能够描述循环荷载作用下饱和软粘土不排水应力应变响应的增量弹塑性边界面本构模型,采用欧拉切线算法建立了该本构模型的子增量步显式积分格式。针对该边界面模型无弹性域、无屈服面的特点,对传统子增量步显式积分算法做了以下修正:将试算应力状态回退至边界面上,克服了传统显式积分算法需要修正试算应力状态至屈服面上、而无弹性域边界面模型又没有屈服面的矛盾,提高了显式积分算法在边界面模型中的应用能力;通过试算应力增量方向与像应力点在边界面上外法线方向的夹角大小判断加卸载状态,克服了传统显式积分算法需要通过试算应力与屈服面的位置关系来判断加卸载状态、而无弹性域边界面模型又没有屈服面的矛盾,并以此确定映射中心位置。进一步利用ABAQUS有限元软件的用户自定义材料接口子程序UMAT,编制了能够反映循环荷载作用下软粘土不排水应力应变响应的子程序,并结合ABAQUS增量求解主程序,形成了一种通过跟踪循环荷载时程来分析静荷载和循环荷载共同作用下饱和软粘土变形的增量弹塑性有限元方法。利用该有限元方法对循环三轴试验和循环扭剪试验进行预测,并将预测结果与上述本构模型的理论分析结果比较,验证了在ABAQUS环境中该增量弹塑性有限元方法的正确性。利用该增量弹塑性有限元分析方法对静荷载和循环荷载共同作用下吸力锚基础模型试验进行模拟。结果表明有限元方法得到的水平破坏模式和竖向破坏模式下吸力锚系泊点沿系泊方向循环累积变形、循环变形与模型试验结果较为接近;通过将有限元方法和模型试验得到的两种破坏模式下吸力锚系泊点沿水平方向、竖直方向的循环累积变形进行比较,表明有限元预测结果与模型试验结果相比偏小,但整体趋势上基本一致。因此,该有限元方法可以在一定程度上分析静荷载与循环荷载共同作用下吸力锚基础的变形失稳过程。
[Abstract]:In order to maintain the normal operation and safety and stability of the offshore platform, the suction anchor foundation must bear not only the working load generated by the superstructure, but also the cyclic load caused by wind and waves. Therefore, it is necessary to evaluate the deformation and instability process of suction anchor foundation under both static and cyclic loads. Based on an incremental elastic-plastic boundary surface constitutive model which can describe the undrained stress-strain response of saturated soft clay under cyclic loading, an explicit integral scheme of sub-incremental step is established by using the Euler tangent algorithm. In view of the characteristics of the inelastic domain and yield surface of the boundary surface model, the traditional explicit integral algorithm of subincrement step is modified as follows: the stress state of the trial calculation is retreated to the boundary surface. It overcomes the contradiction that the traditional explicit integration algorithm needs to modify the stress state to the yield surface, while the inelastic boundary surface model has no yield surface, and improves the application ability of the explicit integral algorithm in the boundary surface model. The loading and unloading state is judged by calculating the angle between the stress increment direction and the image stress point on the outer normal direction on the boundary plane. The traditional explicit integral algorithm needs to judge the loading and unloading state by calculating the position relation between the stress and the yield surface. However, the inelastic boundary surface model has no contradiction of yield surface, and the location of mapping center is determined. Furthermore, by using the user-defined material interface subprogram UMAT, of ABAQUS finite element software, a subroutine which can reflect the undrained stress-strain response of soft clay under cyclic load is developed, and the main program is solved with ABAQUS increment. An incremental elastic-plastic finite element method is developed to analyze the deformation of saturated soft clay under both static and cyclic loads by tracking the cyclic load time history. The finite element method is used to predict the cyclic triaxial test and cyclic torsional shear test. The prediction results are compared with the theoretical analysis results of the above constitutive model, and the correctness of the incremental elastic-plastic finite element method in ABAQUS environment is verified. The incremental elastic-plastic finite element method is used to simulate the model test of suction anchor foundation under both static and cyclic loads. The results show that the horizontal failure mode and the vertical failure mode obtained by the finite element method have accumulated deformation along the mooring direction of the suction anchor mooring point, and the cyclic deformation is close to the results of the model test. By comparing the cyclic cumulative deformation of the suction anchor mooring points along the horizontal and vertical direction under the two failure modes obtained by the finite element method and the model test, it is shown that the finite element prediction results are smaller than those of the model test results. But the overall trend is basically the same. Therefore, the finite element method can be used to analyze the deformation and instability process of the suction anchor foundation under both static and cyclic loads to a certain extent.
【学位授予单位】：天津大学
【学位级别】：硕士
【学位授予年份】：2014
【分类号】：TU447;TU476

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