Offshore wind energy has the potential to generate substantial electricity production compared to onshore locations, due to the high-quality wind resource. Offshore wind turbines must endure severe offshore environmental conditions and be cost effective, in order to be sustainable. As a result, load mitigation becomes crucial in successfully enabling deployment of offshore wind turbines. A direct approach to reduce loads in offshore wind turbines is the application of structural control techniques. So far, the application of structural control techniques to offshore wind turbines has shown to be effective in reducing fatigue and extreme loads of turbine structures. However, the majority of previous research regarding the application of structural control to offshore wind turbine noted the needs for the high-fidelity analysis for structural control using a computer aided engineering (CAE) tool, such as FASTv8. In this dissertation, a structural control module coupled with FASTv8 is developed to meet the needs for high-fidelity analysis of structural control techniques for various OWTs. In addition, the developed control module is updated to analyze various structural control devices operating both passively and semi-actively. The dynamics of an omni-directional pendulum-type tuned mass damper and orthogonal tuned liquid column dampers (TLCDs) are mathematically modeled and incorporated into the structural control module. With the developed control module, several structural control devices are optimized through a variety of techniques (parametric study, exhaustive search and multi-objective optimization). Solving optimization problems not only provides the parameters for each control device that can be applicable to other multi-megawatts offshore wind turbines, but also provides insight into the effects of design variables on the control performance. Site-specific meteorological and oceanographic data that consists of a combination of wind and wave data are processed and compiled in order to establish key design load cases. With the optimal designs of structural control devices, non-linear fully-coupled time marching simulations are conducted by running a series of design load cases in order to investigate the impacts of passive and semi-active structural control on improving fatigue and extreme behaviors of fixed-bottom and floating offshore wind turbines. The simulation results demonstrate the effectiveness of various structural control techniques on reducing fatigue and extreme loadings.