Reliability and sustainability of offshore wind turbine support structures in marine environments: a comprehensive review of failures, monitoring technologies, robotics, and predictive O&M

Offshore wind turbine support structures are increasingly required to operate in deeper and more demanding marine environments, where cyclic loading, hydrodynamic forcing, corrosion, scour, soil-structure interaction, and limited long-term monitoring continue to challenge structural reliability and lifecycle performance. This review synthesizes current knowledge on the reliability, degradation mechanisms, and maintenance needs of major offshore wind support-structure concepts, including monopiles, jackets, gravity-based foundations, tripods, hybrid fixed-bottom systems, spars, semi-submersibles, and tension-leg platforms. The study integrates findings from global field observations, reported failure cases, industrial practice, and recent advances in structural health monitoring, digital twins, robotics-assisted inspection, and AI-supported predictive operation and maintenance. Particular emphasis is placed on how marine environmental conditions and offshore loading processes influence structural response, fatigue behavior, corrosion-scour progression, and maintenance demand across fixed and floating systems. The review also identifies key gaps in long-term field validation, unified reliability assessment, and integrated monitoring-to-maintenance frameworks. By combining structural, operational, and marine-environment perspectives, this paper provides a consolidated reference for improving the durability, safety, and sustainability of offshore wind support structures within the broader context of marine renewable energy development.