One immediate challenge for the commercial development of floating Marine Renewable Energy Converters is reducing the weight and associated costs of mooring lines in deep water (>75m). Synthetic fibre ropes offer already a solution to the weight problems of using steel lines in deep-water offshore oil and gas installations as they have a very low weight in water. The present study focuses on the performance of fibre ropes in shallow waters, subjected to laboratory tests based on loads measured at sea in addition to (time) accelerated tests. Determining fatigue life is one of the most important aspects of long-term mooring analysis. At present, the fatigue analyses are usually based on S-N or T-N curves that are obtained with regular loads even when these loads are completely different to the ones measured at sea by a wave energy converter. The differences between the standard fatigue test and the real life of a mooring system are mainly the rate in which the loads are applied and the profile of the loads. Here, these two elements are analysed to get the fatigue damage, obtaining important differences in this measure.