Numerical modelling of vibratory extraction of monopiles from clayey seabed

Abstract

Understanding and optimizing the extraction of offshore monopiles from the seabed in decommissioning operations give both environmental benefits and cost reduction. The literature highlights the significance of utilizing a vibratory hammer for the complete removal of monopiles in granular or non-cohesive seabed, e.g., sandy soil, offering a cost-effective alternative to the conventional cutting method. However, the available knowledge for applying this method to cohesive soil is limited. The present study examines vibratory pile extraction from cohesive soil, for both soft and stiff clays, with the aim of assessing the resulting extraction force required for pile removal. Coupled Eulerian-Lagrangian (CEL) analyses with two domains of pile and soil are performed using the finite element package ABAQUS. A modified Mohr-Coulomb (MCC) soil model is coded into a VUSDFLD subroutine to properly simulate the soil softening and shear strain rate dependency of seabed soil in an Eulerian domain. The importance of shear strain and softening effects is underscored for soft clay by demonstrating an approximately 20% reduction in the force needed to initiate cyclic extraction of the monopile compared to scenarios where these effects are not included. However, incorporating these effects does not affect the behavior of stiff clay. The present numerical findings show that adopting vibratory extraction method leads to more soil resistance in cohesive soils at the beginning of the extraction. Considering the added weight og the vibratory hammer which should be clamped to the monopile top head leads to exceedance of the required force than the one required for removal of the monopile without vibration (i.e., the pulling-out method). This demonstrates that the vibratory extraction method is not an efficient method for monopile extraction from clayey seabed despite its beneficial effects reported in the literature for sandy soil.