Validation of a high-cycle accumulation model for suction caisson foundations

吸力式沉箱基础由钢圆筒组成，圆筒顶部由一块板封闭。通过对沉箱内部施加吸力将其安装到海床中。海上风力涡轮机（OWT）可以建立在单个（单沉箱）或多个（多沉箱）这些沉箱上。与OWT的传统基础（例如浮箱、重力基础）相比，吸力沉箱的安装速度更快，产生的噪音更小，这对海洋动物有利。此外，与传统的基础相比，吸力沉箱需要更少的材料，并且在其使用寿命结束时更容易拆除。OWT的基础受到风和波浪的高循环载荷，这可能导致土壤中变形的累积，从而导致OWT的倾斜，直至可能丧失使用性能。对于在这种荷载条件下吸力式沉箱基础的长期变形预测，目前还没有经过验证的方法。因此，本研究项目的目的是验证吸力沉箱基础的高周波累积（HCA）模型，该模型已被证明适用于其他类型的OWT基础。该验证基于在珀斯西澳大利亚大学"海上基础系统中心”（COFS）进行的1g和离心模型试验的反分析。在有限元模拟中，使用具有u-U公式的特殊单元来捕获沉箱、土壤和孔隙流体之间的复杂相互作用。模型试验中使用的两种砂的HCA模型的参数基于排水循环三轴试验进行校准。单元试验在两个不同的应力水平下进行，对应于1g和离心模型试验。为了量化安装引起的土壤状态变化的影响，在考虑和不考虑沉箱安装的情况下进行了模拟。数值方法的验证，特别是其中使用的HCA模型，可以得出结论的情况下，令人满意的协议之间的模拟和模型试验测量。在敏感性研究中，不同参数的变化的影响，其中的状态的土壤和本构模型的参数，对模型试验的数值预测。最后，将验证后的数值模型用于真实的尺度下吸力式沉箱基础的参数研究。根据这些研究结果，简化设计图表的正常使用状态，它可以用来估计长期变形的吸力沉箱基础的各种边界条件。

Suction caisson foundations are composed of steel cylinders closed by a plate at the top. They are installed into the seabed by means of a suction applied to the inner of the caisson. An offshore wind turbine (OWT) can be founded on a single (mono-caisson) or several (multi-caissons) of these caissons. Compared to conventional foundations for OWT (e.g. monopiles, gravity base foundations) the installation of suction caissons is faster and generates less noise, which is beneficial for the marine fauna. Furthermore, compared to the conventional foundations, suction caissons need less material and their deconstruction at the end of their lifetime is easier. Foundations of OWT are subjected to a high-cyclic loading by wind and waves, which may lead to an accumulation of deformations in the soil and thus to a tilting of the OWT, up to a possible loss of the serviceability. For predictions of the long-term deformations of suction caisson foundations under such loading conditions no validated method exists. The aim of this research project is thus to validate a high-cycle accumulation (HCA) model, which has been already proven for other types of OWT foundations, for suction caisson foundations. This validation is based on a back-analysis of 1g and centrifuge model tests performed at the „Centre for Offshore Foundation Systems“ (COFS) of the University of Western Australia in Perth. In the finite element simulations special elements with u-U formulation are used to capture the complex interaction between the caisson, the soil and the pore fluid. The parameters of the HCA model for both sands used in the model tests are calibrated based on drained cyclic triaxial tests. The element tests are performed at two different stress levels corresponding to the 1g and centrifuge model tests. The simulations are done with and without consideration of the caisson installation, in order to quantify the effect of installation-induced changes in the soil state. The validation of the numerical method, particularly the HCA model used within it, can be concluded in case of a satisfying agreement between the simulations and the model test measurements. In a sensitivity study the effect of a variation of different parameters, among them the state of the soil and the parameters of the constitutive model, on the numerical prediction for the model tests is investigated. Finally, the validated numerical model is used for parametric studies on suction caisson foundations in real scale. Based on the results of these studies simplified design charts for the serviceability state are developed, which can be used to estimate the long-term deformations of suction caisson foundations for various boundary conditions.