Global ambitions for floating offshore wind energy are rapidly escalating, with an ambitious target of 220 GW capacity by 2050, requiring the installation of approximately 550 units of 15 MW floating offshore turbines annually. The industry faces significant challenges in meeting this demand, as planned suitable wind sites are increasingly located deeper and further offshore, and the number and scale of turbines continue to grow. Current installation methods, primarily reliant on the towing of fully assembled turbines, may be limited in handling the increasing complexity and scale of future projects. This PhD dissertation developed the novel onsite installation method initiated by the SFI MOVE project for floating offshore wind turbines. It addresses the critical challenge of current installation methods and provides alternative installation solutions for the floating offshore wind industry. By employing a catamaran to transport, lift, and install fully assembled wind turbines onto pre-positioned spar foundations, this research offers a promising alternative to conventional towing methods. Comprehensive numerical modelling and dynamic analyses were conducted to assess the feasibility and optimise the onsite installation system. The study validated the numerical models and software, investigated the effects of various hydrodynamic and environmental factors and identified critical system responses. A novel mechanical damping system was developed and implemented to mitigate the challenges posed by relative motions between the installation vessel and the floating foundation, enhancing operational safety and efficiency. Furthermore, op timal positioning strategies were identified and incorporated by investigating the influence of mating position on relative motions, resulting in improved system operability and stability. Finally, the study developed a methodology for determining the weather window limiting criteria based on extreme responses and threshold exceedance analysis, providing a foundation for developing a robust guidance system to ensure safe and efficient mating operations. The findings of this study contribute significantly to the advancement of floating offshore wind installation technology and offer valuable insights for industry stakeholders.