One of the key engineering challenges for the installation of floating marine energy converters is the fatigue of the load-bearing components. In particular the moorings which warrant the station-keeping of such devices are subject to highly cyclic, non-linear load conditions, mainly induced by the incident waves. To ensure the integrity of the mooring system the lifecycle fatigue spectrum must be predicted in order to compare the expected fatigue damage against the design limits. The fatigue design of components is commonly assessed through numerical modelling of representative load cases. However, for new applications such as floating marine energy converters numerical models are often scantily validated. This paper describes an experimental approach, where load measurements from tank tests are used to estimate the lifecycle fatigue load spectrum for a potential deployment site. The described procedure employs the commonly used Rainflow cycle analysis in conjunction with the Palmgren-Miner rule to estimate the accumulated damage for individual sea states, typical operational years and different design lives. This allows the fatigue assessment of mooring lines at a relatively early design stage, where both information from initial tank tests and the wave climate of potential field sites are available and can be used to optimise the mooring design regarding its lifecycle fatigue conditions.