Long term numerical simulation of offshore wind turbine monopile foundations

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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.