Tidal current energy converters (TCECs) represent a promising source of predictable, renewable energy, but their commercial deployment depends critically on the reliability of key drivetrain components operating in harsh subsea environments. Infinitely variable transmissions (IVTs) have emerged as an enabling technology for TCECs because they can continuously adjust speed ratios under high-torque, low-speed conditions using all-gear contact forces, thereby improving energy capture efficiency while avoiding the sliding losses associated with conventional friction-based transmissions. However, the reliability and availability of IVTs in tidal applications have received limited attention, and a structured assessment is essential for supporting the technology qualification process.

This paper presents a failure mode and effects analysis (FMEA)-informed reliability framework for the IVT gearbox in a TCEC system. The IVT gearbox consists of four primary subsystems: a non-circular gear pair (NCG) for speed modulation, planetary gear sets (PGS) for speed combining and control, rack-pinion sets for motion conversion, and output gears for final torque delivery. Each subsystem is subject to distinct failure modes arising from the mechanical demands and environmental exposure of subsea operation, including seawater corrosion, biofouling, abrasive contamination, and cyclic loading.

The FMEA is conducted following a systematic seven-step process covering component specification, process function definition, failure mode identification, failure mechanism analysis, risk scoring, risk priority number (RPN) ranking, and mitigation strategy development. Severity, frequency, and detectability scores are assigned based on the Gearbox Reliability Database and engineering judgment, reflecting the limited experimental data available for this new IVT design. The analysis identifies root bending fatigue as the dominant failure mechanism across multiple gear components, yielding the highest RPN scores and indicating that tooth-root strength and load uncertainty are primary design concerns. Additional critical mechanisms include abrasive and corrosive wear, debris-induced scuffing, and seal degradation leading to water ingress.

The FMEA results are then used to inform two complementary assessments. For the non-repairable case, a fault-tree-based reliability model captures the serial dependence of the NCG, PGS, and motion conversion module (MCM), providing a direct estimate of overall IVT reliability. For the repairable case, a continuous-time Markov chain (CTMC) model is developed to represent the degradation and maintenance states of the IVT, including normal operation, degraded performance, derated output, forced outage, preventive maintenance, and corrective maintenance. Steady-state availability is derived analytically from the CTMC, and the framework identifies the transition rates requiring quantification through future physics-informed or data-driven approaches.

This work provides a structured foundation for IVT qualification and identifies the data collection and modeling priorities needed for full reliability and availability quantification in support of TCEC commercialization.