Offshore wind turbines and their foundations are designed for a lifetime of 20 to 30 years. Once this design life has passed, the standard procedure is that the operation of the structure can be prolonged if it is shown that the loading was lower than anticipated during the design phase. This is only possible if accurate monitoring has been performed, and it also neglects the possibility that the structure has actually a larger fatigue resistance than assumed in standards. In the FlexWind proposal, we will investigate the possibility to prolong the life of an aged offshore foundation of an offshore wind turbine by re-assessing the fatigue resistance of critical joints of the substructure. This will be done by innovative methods combining inspection of aged joints (scanning the geometry of critical joints, assessment of the surface degradation due to corrosion), numerical analysis of accumulated fatigue damage (backward modelling) and remaining useful lifetime (forward modelling), probabilistic tools to account for the inherent stochastic nature of a wide range of parameters affecting fatigue resistance and representative experiments on aged large-scale joints of offshore jackets.