Hydraulic Pile Extraction Scale Tests (HyPE-ST): Experimental Design & Preliminary Results-Paper

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The HyPE-ST project investigated hydraulic extraction as a potential method for fully removing offshore wind monopiles during decommissioning. The technique seals the pile and pumps water into its interior, using pressure to push it upward. Full removal could avoid underwater cutting and excavation, reduce safety and cost risks, and leave no steel in the seabed.<br /><br />To study pile–soil interaction before full-scale offshore use, researchers conducted scale tests representing an 8 m diameter monopile embedded 40 m. Tests used 1:20 and 1:30 piles, installed by impact driving in medium-dense sand, dense sand, medium-stiff clay, and layered soil. Pressure, water flow, pile and soil-plug displacement, and pore pressure were monitored during extraction. An analytical force-balance model was also developed to estimate the pressure needed to initiate movement.<br /><br />Preliminary results showed that hydraulic extraction was feasible at both tested scales and across the tested soil configurations. Break-out pressure—the pressure required to start upward movement—varied substantially with scale and soil conditions. Soil layers with low permeability could strongly affect the required pressure. In sand, higher installation energy, associated with greater density, corresponded to higher break-out pressure. Partly excavating a pile substantially reduced this pressure; adding bentonite after excavation brought it closer to that of unexcavated piles. Longer setup time, from one to six weeks after installation, produced no significant difference in break-out pressure.<br /><br />The findings support further investigation at larger, more representative scales and potentially offshore. The authors note that hydraulic extraction may become especially relevant as monopile diameters increase, but further work is needed to assess full-scale performance and cost-effectiveness.

Keywords

hydraulic extraction

offshore wind monopiles

decommissioning

pile-soil interaction

break-out pressure

scale tests

sand and clay soils

pressure and pore monitoring

monopile removal

bentonite backfill