Decommissioning strategies for offshore wind turbine foundations

Decommissioning refers to the planned process of safely removing and dismantling infrastructure, facilities, or equipment that have reached the end of their operational life. In the context of offshore wind farms, decommissioning occurs when the wind farm reaches the end of its operational life or when components become unsafe or inefficient; regulatory requirements and contractual agreements often dictate the timing. Considering the foundations, decisions as to whether partially remove (cutting foundations below the seabed) or completely remove (lifting out entire foundations) the foundations have to be made. Factors that influence this decision include environmental regulations, recycling feasibility, and site-specific conditions. The decommissioning involves developing a plan that considers environmental impact, safety, and cost-effectiveness, and minimizes disturbance to marine ecosystems during removal.

Currently, monopiles represent more than 80% of offshore wind turbines foundations, and the most common removal technique consists of dredging around the monopile and cutting it at some depth below the seabed. While partial removal may seem convenient, it is essential to carefully weigh the environmental impact, safety risks, and financial implications when deciding on decommissioning strategies for monopiles. In particular, a significant amount of steel is left in place, which becomes more challenging as piles grow in diameter and reach greater depths. The seabed around partially removed monopiles may experience changes in sediment dynamics and habitat availability. Partially cut monopiles pose safety risks during navigation and fishing activities, as protruding steel can damage vessels and fishing gear. In deep-water locations, cutting underwater and below the seabed tends to be underestimated. External cuts require specialized equipment, which may not significantly reduce costs compared to full removal.

On the other hand, full removal allows for complete restoration of the seabed and marine ecosystem, which can return to its natural state, supporting biodiversity and ecosystem health. In an alternative scenario, it ensures a seabed ready for new installations of new foundations for larger OWT’s at the same strategic location, contributing this way to the continuously increasing targets for clean energy demands. In both cases, it allows to recycle a large amount of steel used for those foundations, and it further allows for compliance to strict regulations (at some regions) that require full removal of the decommissioned infrastructure. In summary, while full removal offers environmental benefits and safety, it comes with some technical challenges. Some preliminary works have been done to enable full removal such as vibratory extraction or overpressure at the top of the monopile which creates upward force. However, the efficacy of decommissioning methods remains uncertain under different soil conditions. Additionally, a comprehensive understanding of the underlying mechanisms that governing optimal extraction processes is yet to be fully understood.

At the moment offshore engineering companies face compelling reasons to prioritize decommissioning efforts. As offshore assets age and offshore structures reach the end of their productive lives, must be safely dismantled, recycled, or disposed. To mitigate long-term liability and advance sustainability, an efficient and cost-effective removal of the entire foundation if of paramount importance. This PhD project aims to investigate the monopile removal processes by means of experimental (small scale prototype tests at the UCLouvain Laboratory) and theoretical (computational models) developments, and thus contribute to the understanding of the extraction mechanisms, addressing a timely question of the offshore energy industry. The project will facilitate the development of guidelines for future full-scale applications, and the findings from the proposed research will greatly help understand and adapt extraction/decommissioning strategies in the harsh offshore environments.