Decommissioning of offshore wind farms at the end of their operational life requires full or partial removal of foundation structures such as monopiles. Removing entire piles is more sustainable, economical, and less hazardous in terms of Health & Safety (H&S) than partial removal. This is because e.g. underwater cutting can be avoided, and no steel remains in the seabed. A promising method for full pile removal is hydraulic extraction. This method involves sealing the pile after removal of the top structure, and pressurizing water inside its void, thus forcing the pile to move upwards. Before applying this method offshore at full scale using expensive vessels and equipment, a better understanding of the pile-fluid-soil interaction in different soil types is needed. To accomplish this, a testing campaign has taken place as part of a Joint Industry Project. Tests are performed at scales of 1:20 and 1:30 for a prototype monopile with a diameter of 8 m. Four different soil conditions were used: medium dense sand, dense sand, medium stiff clay, and layered soil. The piles were installed by impact driving. During the extraction process, several parameters have been monitored including pressure, flow, pile displacement and plug displacement. This paper describes the test program and shares first insights from the results.
T. Balder, D. A. de Lange, A. S. K. Elkadi, P. J. P. Egberts, W. J. A. P. Beuckelaers, M. Coronel, J. van Dijk, R. Atkinson and H. J. Luger
Keywords: monopile, decommissioning, offshore wind, sustainability, circular economy
Publisher: DFI