An efficient source of renewable energy, which is increasingly the preferred solution for realising the UK's short- and long-term energy demand, is offshore wind. According to the UK's Renewable Energy Roadmap and the national target to reach up to 50 GW of installed offshore wind capacity by 2030, deployment of further offshore wind farms will be boosted over the coming decades. The dominant majority of the UK's offshore wind turbines are supported using monopile foundations which are fabricated by welding large thickness steel plates. These support structures are subjected to severe cyclic loading condition during service; therefore, they need to be designed against fatigue failure for a desired target life which is typically between 20 and 25 years. The aim of this PhD project is to evaluate the level of fatigue damage in the support structures at the end of initial design life and explore possible engineering solutions for life extension of the existing assets.