Innovative process to extract and recycle offshore wind turbine foundations - Time effects

Details

Supervisors

Rattez, Hadrien

Faculty

Ecole polytechnique de Louvain

Degree label

Master [120] : ingénieur civil des constructions, à finalité spécialisée

Abstract

enOffshore wind energy has expanded rapidly across the North Sea over the past two decades, with thousands of large-diameter steel monopiles installed as foundations. Designed for a service life of 20 to 25 years, the earliest of these structures are now approaching decommissioning. Yet the industry remains largely unprepared for their full removal. Current practice favours cutting the pile below the mudline and leaving the lower section in place, raising concerns over site rehabilitation, marine biodiversity, and the loss of significant quantities of recyclable steel. A major obstacle to complete extraction is the substantial increase in axial resistance that driven piles develop over time through a process known as pile set-up. Field observations consistently report capacity gains of 15 to 65% per log cycle of time (in days), attributed to two coupled mechanisms: granular creep and soil ageing. The latter is suspected to involve the formation of a cemented crust along the pile shaft, driven by oxygenic corrosion of the steel in saline water and the subsequent precipitation of calcium carbonate. While pile set-up has been studied in the context of bearing capacity, its implications for extraction resistance remain largely unexplored. This master thesis investigates the physico-chemical mechanisms governing cementation at the pile-soil interface, with two main objectives: reproducing them at laboratory scale and estimating their effect on extraction forces. A series of controlled experiments was conducted to isolate the influence of four parameters - shell dissolution, steel type, bulk pH, and ion availability - on crust formation in saline conditions. Results suggest that shell dissolution after 30 days is negligible under purely saline conditions and that, although steel corrosion may trigger cementation, alkalinity is suspected to be the dominant parameter under the investigated conditions: specimens aged at pH 10 developed a distinct 3 mm carbonate crust within 50 days, while ion enrichment alone proved insufficient to accelerate the process at laboratory scale. A vibration procedure was developed to reproduce homogeneous and repeatable saturated sand conditions at a scale of 1:100. Finally, an analytical framework based on the American Petroleum Institute (API) shaft friction method was derived to predict the extraction force required for aged monopiles, accounting for both plugged and unplugged extraction mechanisms. These results provide a practical experimental basis for testing emerging extraction techniques - including vibratory and hydraulic methods - under more realistic end-of-life conditions, and represent a step toward the full, sustainable decommissioning of offshore wind foundations.