Long term numerical simulation of offshore wind turbine monopile foundations

目前，海上基础设施在能源生产系统的发展中发挥着主导作用。这种基础设施的一个关键因素是在整个设计寿命期间连续加载的电缆、管道基础和锚固系统。对于海上风力涡轮机系统的基础，大直径桩基础仍是较好的解决方案。值得注意的是，2018年安装的海上风力涡轮机中有74.5%由风力发电机支撑，而该系统的成本约占总成本的30%。最新的岩土工程研究工作集中在以下几个方面：a）桩土相互作用，强调系统的基频，B）土壤阻尼，c）冲刷和孔隙压力的演变，以及d）基础的长期性能。本论文的目的是涵盖这一工程问题的后一方面，特别是，大型桩基础的长期响应分析，现在在国家的实践技术，众所周知的p-y曲线法似乎低估了桩的能力，因为它已被最近的研究表明。这是因为这些方法是针对具有较高L/D比的较小直径桩得出的。高级有限元分析可用于改进现有的p-y曲线，因为可以捕获该问题的许多方面。此外，可以研究位移的累积以及导致稳定、亚稳定或不稳定长期响应的条件。具有相对小的纵横比（L/D）的大直径桩被称为“刚性”或“短”桩。在这样的系统中，土壤特性对于所产生的响应是非常重要的。然而，这些属性不断交替的数量施加的周期的负载，导致恶化的性能的桩基。在此之前，在大型桩基的安装过程中，土体的性质被破坏，导致密实或松散的区域。从过去的研究中可以很好地确定，具有不同相对密度的无粘性材料的降解速率是不同的。因此，这是一个关键的方面，需要考虑在模拟的周期响应的桥梁。为了分析考虑安装效应的桩基的循环响应，需要建立两种不同的模型，采用两种不同的本构关系。第一个模型将是一个适合捕捉高应力条件和安装过程中空隙率变化的模型，而第二个模型捕捉砂的长期性能和降解。这样就可以对桩基的循环响应进行严格的计算。

Offshore infrastructure is currently undertaking a leading role in the development of energy production systems. A key factor in this infrastructure refers to the continuously loaded cables, pipelines foundations and anchoring systems throughout their design life-time. Emphasising on the foundation of offshore wind turbine systems, large diameter piled foundation still seem to be the preferable solution. It is remarkable that 74.5% of the installed offshore wind turbines in 2018 are supported by monopiles, while the cost of this system is approximately 30% of the total. Up-to-date geotechnical engineering research efforts focusing on the following aspects: a) pile-soil interaction emphasising on the fundamental frequency of the system, b) soil damping, c) scour and evolution of pore-pressures, and d) long-term performance of the foundation. The aim of this thesis is to cover the latter aspect of this engineering problem, specifically, the long-term response analysis of large piled foundations.Looking now at the state-of-practice techniques, the well-known p-y curve method seems to underestimate the capacity of monopiles, as it has been illustrated by relatively recent research studies. This is because these methodologies are derived for smaller diameter piles which higher L/D ratios. Advanced Finite Element Analyses can be used to improve the existing p-y curves, as many aspects of this problem can be captured. In addition, the accumulation of displacements and the conditions which lead to a stable, meta-stable or unstable long-term response can be investigated. Large diameter piles with relatively small aspects ratios (L/D) are well-known as "rigid" or "short" piles. In such systems, the soil properties are of a great importance for the resultant response. However, these properties continuously alternate with the number of the applied cycles of loads resulting in the deterioration of the performance of the piled foundation. Prior to this effect, during the installation of the large piled foundations, the properties of the soil mass are disrupted, leading to densified or loosened zones. It is well-established from past research that the rate of degradation of cohesionless materials with different relative density is different. Therefore, this is a key aspect that needs to be considered in the simulation of the cyclic response of monopiles. For the purpose of analysing the cyclic response of the piled foundations considering the installation effects, two different models need to be developed with two different appropriate constitutive laws. The first one will be a model suitable to capture the high stress conditions and the changes in the voids ratio during installation, while the second model captures the long-term performance and degradation of sands. In this way, the rigorous computation of the cyclic response of piled foundations will be carried out.