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

吸式沉箱基础由钢瓶组成，钢瓶顶部由钢板封闭。它们通过对沉箱内部施加吸力的方式安装到海底。海上风力涡轮机（OWT）可以建立在单个（单沉箱）或多个（多沉箱）的沉箱上。与传统基础（例如单桩基础、重力基础）相比，安装吸力沉箱速度更快，噪音更少，对海洋生物有利。此外，与传统基础相比，吸式沉箱需要的材料更少，并且在其使用寿命结束时更容易解构。OWT的基础受到风和浪的高循环载荷，这可能导致土壤变形的积累，从而导致OWT的倾斜，最终可能失去使用能力。对于这种荷载条件下吸力沉箱基础长期变形的预测，目前还没有行之有效的方法。因此，本研究项目的目的是验证高周积（HCA）模型，该模型已经在其他类型的OWT基础上得到了验证，用于吸力沉箱基础。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和离心机模型试验的反分析。在有限元模拟中，采用u-U公式的特殊单元来捕捉沉箱、土体和孔隙流体之间复杂的相互作用。模型试验中使用的两种砂的HCA模型参数是基于排水循环三轴试验标定的。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。模拟在考虑沉箱安装和不考虑沉箱安装的情况下进行，以便量化安装引起的土壤状态变化的影响。数值方法，特别是其中使用的HCA模型，可以在模拟和模型试验测量之间令人满意的情况下得出结论。在敏感性研究中，探讨了不同参数（包括土的状态和本构模型参数）的变化对模型试验数值预测的影响。最后，将验证的数值模型应用于吸力沉箱基础的实际参数化研究。在此基础上，建立了可用于各种边界条件下吸力沉箱基础长期变形估计的简图。

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.