Estimation of Soil Damping Contribution on Offshore Monopiles Using Plaxis 3D Approach

Abstract

Offshore wind has achieved many milestones considering this relatively new and emerging industry. Its huge advancement is mostly driven by lowering the overall project cost and renewable energy targets which are set by the EU. One of the potential cost savers can be found in the foundation structure design in which the monopiles are the most used choice. The area from which the design of these structures can be improved is quite complex and it can be related to the field of Dynamic Soil-Structure Interaction (DSSI). Offshore Wind Turbine (OWT) has several damping sources but in the last few years soil damping phenomena has attracted a lot of interest within the offshore wind research community because it is believed that potential benefits can be achieved if the contribution of this damping source can be estimated with reliability. Offshore wind design codes do not provide a methodology for the estimation of this damping type and therefore there is a need for the research. For the purpose of soil damping estimation, a geotechnical Finite Element (FE) software – Plaxis 3D was utilized. Firstly, the chosen soil model was verified through the three-step verification process which involved a comparison between experimental field data and numerical data from Plaxis. Once successfully verified, the soil model is utilized further in the design of the simplified OWT structure which is exposed to the dynamic wave and wind loads based on the serviceability and ultimate limit conditions. What is of the particular interest leads to the free vibration phase and monopile positioning (position of structure at the start of free vibration) from which the necessary information regarding the decay of displacement amplitudes is obtained and further analysed with the logarithmic decrement method for damping estimation. Adding on soil damping, as part of research additional topics were also considered and they relate to soil behaviour and natural frequency. The final results are compared to the other research papers and they are found to be well within the proposed literature damping range which highly depends on a certain combination of monopile geometry, soil type and loading conditions. From the research it was concluded that soil damping value increases as the wave and wind loads increase. Also, it was proven that a damping potential in offshore wind exists and that eventually soil damping should be implemented in the future offshore wind design codes.