Polyamide 6 fibre ropes are of interest for floating offshore wind turbine mooring lines but their fatigue durability represents a key aspect to characterize and validate on the new constructions and coatings developed for this long term application. This paper presents a fatigue study on laboratory scale polyamide 6 sub-ropes which were wetted before testing. A T-N curve describing ranges of [2%, 70%] of MBL and [102,105] is obtained. An investigation of the damage mechanisms, using X-ray tomography analysis and SEM images, is performed and highlights the complexity of the mechanical response of twisted ropes, due to their hierarchical multi-scale construction. Cyclic loading changes significantly the sub-rope aspect and architecture. Finally, this study describes the use of a heat build-up measurement protocol, for a rapid evaluation of the fatigue properties of ropes. The cyclic dissipated energy was determined from temperature measurements by an infrared camera. A first prediction using this method is proposed and shows that it could provide a powerful solution, to overcome the very long testing times required for fatigue studies on synthetic sub-ropes.