The aim of my research is to optimise the fatigue design and life assessment of offshore wind turbine support structures. Probabilistic assessments will be carried out to demonstrate the reliability level based on advanced two-stage fatigue models, which consider crack initiation and crack propagation phases explicitly. Previously disregarded effects particularly related to very long welds of large diameter structures will be incorporated. As an additional element of the probabilistic framework, weld defects are included and their influence on the fatigue life of the support structures will be thoroughly analysed. This will be combined with an analysis of the strengths and weaknesses of the existing non-destructive evaluation (NDE) techniques for detection of different sizes and locations of weld defects.