This study reports the practicalities of applying a numerical model to assess the wave energy extraction at proposed development sites in Orkney Waters, Scotland. A state-of-the-art phase averaged spectral wave model, MIKE 21 Spectral Wave, in association with the wave-structure interaction software tool WAMIT, has been employed to study the impact of energy extraction by large arrays of Wave Energy Converters (WECs) on the wave height alteration in the neighbourhood of WEC arrays. Two generic types of WEC, one representing surface attenuators (deployed in deep water) and other representing Oscillating Wave Surge Converters (deployed in shallow waters) are used for numerical modelling. The power extraction performance of the WECs are initially modelled using WAMIT and validated with data from literature. As MIKE 21 SW has limited ability in modelling complete dynamics of a moving structure, each WEC has been modelled as a generic structure but with appropriate reflection, transmission and energy absorption properties derived from WAMIT, and this methodology is found to have worked well. In total there are 198 attenuators and 120 Oscillating Wave Surge Converters are constructed within the numerical model at potential energy locations in Orkney Waters, and the wave-WECs array interactions for the year 2010 have been simulated. The results suggest that the wave farm will have an impact on the wave climate immediately in the lee of the array, although the magnitude of these effects decreases monotonically with distance from the farm.