This study focuses on the operational lifetime assessment of floating offshore wind turbines (FOWTs) using the Gaidai risk evaluation method. The research aims to address the challenges of environmental loads and extreme structural dynamics that can cause fatigue damage and structural failure in FOWTs. The study employs the FAST software, which combines aerodynamics, hydrodynamics, control, and electrical system dynamics models, to numerically assess the in situ environmental loads acting on FOWTs. The Gaidai risk evaluation method, known for its ability to handle high-dimensional dynamic systems, is used to evaluate the reliability and lifetime of FOWTs. The method involves simulating a range of potential meteorological conditions and analyzing the system's critical components to estimate the target cumulative distribution function (CDF) of the system's lifetime. The study demonstrates the effectiveness of the Gaidai method through a case study of three critical components of an FOWT system: the tower's fore-aft bending moment, the blade's root out-of-plane bending moment, and the anchor's tension. The results show that the Gaidai method can provide reasonably narrow confidence intervals, making it a reliable tool for assessing the lifetime of complex, multidimensional dynamic systems. The study concludes that the Gaidai method is a promising approach for improving the design and reliability of FOWTs, particularly in extreme weather conditions.This study focuses on the operational lifetime assessment of floating offshore wind turbines (FOWTs) using the Gaidai risk evaluation method. The research aims to address the challenges of environmental loads and extreme structural dynamics that can cause fatigue damage and structural failure in FOWTs. The study employs the FAST software, which combines aerodynamics, hydrodynamics, control, and electrical system dynamics models, to numerically assess the in situ environmental loads acting on FOWTs. The Gaidai risk evaluation method, known for its ability to handle high-dimensional dynamic systems, is used to evaluate the reliability and lifetime of FOWTs. The method involves simulating a range of potential meteorological conditions and analyzing the system's critical components to estimate the target cumulative distribution function (CDF) of the system's lifetime. The study demonstrates the effectiveness of the Gaidai method through a case study of three critical components of an FOWT system: the tower's fore-aft bending moment, the blade's root out-of-plane bending moment, and the anchor's tension. The results show that the Gaidai method can provide reasonably narrow confidence intervals, making it a reliable tool for assessing the lifetime of complex, multidimensional dynamic systems. The study concludes that the Gaidai method is a promising approach for improving the design and reliability of FOWTs, particularly in extreme weather conditions.