Pile-Soil Interaction during Hydraulic Overpressure Extraction for Offshore Monopile Decommissioning

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4. Pile-Soil Interaction during Hydraulic Overpressure Extraction for Offshore Monopile Decommissioning

This paper examines the pile-soil interaction during the decommissioning of open-ended pipe piles (e.g., monopiles) through the application of hydraulic overpressure within the pile. The primary objective is to analyse stress variations in the shear zone induced by pressure loading and fluid flow through the pile. Large-scale model tests were conducted utilizing novel total stress transducers, facilitating precise measurements of physical processes in the shear zone at the pile-soil interface, which was previously unattainable due to sensor limitations.

Results reveal a significant correlation between fluid flow rate and successful pile extraction. Elevated flow rates induce a reduction in contact stress within the shear zone, attributed to hydrodynamic soil rearrangement, particularly under conditions of gradual pressure application and sustained pressure maintenance. This stress reduction mitigates substantial increases in pile resistance that would otherwise result from enhanced horizontal effective stress ('_h) along the pile shaft.

The findings underscore the importance of monitoring stress reduction over time for accurate estimation of required pressures and the probability of successful pile decommissioning. While previous studies have documented stress increases under pressure loading, they lacked evidence of significant stress reductions within the pile. The newly developed transducers enable direct detection of stress changes at the pile-soil interface, providing valuable insights for future investigations in geotechnical engineering, particularly regarding pile load-bearing behavior and extraction efficiency. This research presents novel opportunities for addressing long-standing challenges in pile decommissioning and advances our understanding of pile-soil interactions during extraction processes.

5th International Symposium on Frontiers in Offshore Geotechnics (ISFOG2025)

6 - Pile Installation Challenges and Decommissioning in sands and clays: monopiles, anchor piles, pin piles