The sustainability of offshore renewable energy structures, for periods of up to 15 years, remains a major challenge to the sector. The manufacture of these structures is often based on combinations of materials assembled with structural bonding. The long-term performance of such hybrid structures (resistance to the marine environment and fatigue) is still unclear and requires more research. This lack of knowledge leads to the use of exaggerated safety factors, or limits the introduction of innovative multi-material structures. It is therefore essential to study the lifespan of these bonded multi-material structures and particularly to in-service monitor the health of the glue seal.