Abstract. Offshore Wind Turbine (OWT) support structures need to be designed against fatigue failure under cyclic aerodynamic and wave loading. The fatigue failure can be accelerated in a corrosive sea environment. Traditionally, a stress-life approach called the S-N curve method has been used for design of structures against fatigue failure. There are a number of limitations in S-N approach related to welded structures which can be addressed by the fracture mechanics approach. In this paper the limitations of the S-N approach related to OWT support structure are addressed, a fatigue design framework based on fracture mechanics is developed. The application of the framework to a monopile OWT support structure is demonstrated and optimisation of in-service inspection of the structure is studied. It was found that both the design of the weld joint and Non-destructive testing techniques can be optimised to reduce In-service frequency. Furthermore, probabilistic fracture mechanics as a form of risk-based design is outlined and its application to the monopile support structure is studied. The probabilistic model showed to possess a better capability to account for NDT reliability over a range of possible crack sizes as well as providing a risk associated with the chosen inspection time which can be used in inspection cost benefit analysis. There are a number of areas for future research. including better estimate of fatigue stress with a time-history analysis, the application of framework to other types of support structures such as Jackets and Tripods, and integration of risk-based optimisation with a cost benefit analysis.