This thesis explores the feasibility of retrofitting ageing offshore wind farm (OWF) foundations with airborne wind energy (AWE) systems as a sustainable alternative to decommissioning or wind turbine (WT) refurbishment. These OWFs, starting their operational life between 1995 and 2003, face the challenge of reaching the end of their 20 to 25-year lifespan. Decommissioning incurs costs and environmental concerns, while refurbishment with larger WTs is increasingly expensive due to rapid technological advancements. The study conducts a structural assessment of retrofitting offshore foundations with a 500 kW AWE system, covering the ultimate limit state (ULS) and fatigue limit state (FLS) evaluations. ULS calculations confirm that the foundations can withstand new AWE-generated wind and wave loads without exceeding design limits. Fatigue assessments demonstrate substantial expected foundation lifespans, even with a 99% initial damage assumption, suggesting AWE retrofitting preserves structural integrity. Other AWE retrofitting scenarios are considered as well. Retaining the tower and mounting the 500 kW AWE system atop the tower is deemed possible, resulting in higher capacity factors. Calculations using a 2MW AWE system are performed as well. This is structurally possible, but the AWE technology of that size still faces technological challenges. The economic feasibility of AWE system retrofitting is assessed through income and cost evaluations, comparing it to repowering with larger WTs. Results indicate competitive LCoE values for tower-mounted AWE compared to WT repowering, offsetting decommissioning costs and promising sustainable energy generation. Notably, 2 MW AWE systems exhibit economic potential in various scenarios. This research contributes valuable insights into the viability of AWE retrofitting for ageing OWFs with AWE technology, offering a sustainable pathway forward and highlighting both the possibilities and challenges of this approach.