The mooring systems of Marine Energy Converters (including wave energy, tidal energy and offshore floating wind systems) are critical elements and a thorough understanding of their long term durability is essential to guarantee the survivability of these devices. Synthetic fibre ropes offer advantages compared to steel in terms of handling, and can provide a wide range of mechanical and damping properties. However, their long-term behaviour is less well known. This paper will first present results from recent studies conducted for the offshore industry and discuss these in terms of renewable energy devices. A reliability-based approach will then be described, and a framework for qualifying synthetic ropes, adapted to the specific requirements of ocean energy devices, will be presented. New data from measurements at sea will be provided to support the methodology.