Numerical study on removal of offshore wind turbine monopile foundations using hydraulic pressure

Abstract

Many offshore wind farms installed in the beginnings of the 90 s and 2000s are approaching the end of their design lifetime, and the need of decommissioning these structures has become imminent. This study introduces a numerical approach for the complete removal of monopiles using hydraulic extraction. The numerical modeling of this study mimics the process of water injection into a pile with a sealed joint at its top, which results in increased internal pressure that ultimately moves the pile upward. A Coupled Eulerian-Lagrangian (CEL) approach within ABAQUS/Explicit is employed in the analysis. Using this approach, water injection process is simulated, and deformations of soil are captured. Moreover, to examine the movement of the pile during the removal process in fully saturated dense sand, modified Mohr-Coulomb (MMC) model is utilized. The MMC model, chosen for its ability to capture the non-linear pre-peak hardening and post-peak softening of dense sand, covers limitations in the conventional Mohr-Coulomb (MC) model. These two tools have been used to analyze pile-soil-water interaction. A parametric study is carried out on water injection rate to assess its effects on extraction rate. Breakout pressure which is required to trigger pile movement is determined for dense sand ($I_D=0.7$) soil. The trend of the extraction rate based on the water injection rate seen in this numerical study for dense sand is in good agreement with the published experimental results.