Accurate prediction of nonlinear wave structure interaction remains a challenge for the design of large wave energy converter (WEC) arrays. This thesis develops and validates a weakly nonlinear frequency domain (NLFD) framework for modelling multi device point absorber arrays in realistic irregular seas. The formulation embeds nonlinear restoring, viscous drag, and power take off (PTO) dynamics directly within the frequency domain (FD) balance while retaining linear hydrodynamic coupling from HAMS-MREL. This enables fully coupled array simulations at a fraction of the computational cost of time domain (TD) approaches.

The solver was benchmarked against a nonlinear MATLAB TD reference and verified against the linear HAMS-MREL solution. Nonlinear heave dynamics were reproduced with amplitude errors of 11 to 17% and near identical phase, and the model collapses to the linear limit when nonlinear terms are disabled. These validations establish a robust foundation for irregular sea analyses.

A systematic study across four sea states, three spacings of 5D, 10D and 15D, and four incident headings quantified the mechanisms governing array performance. Nonlinear effects broaden spectral peaks without shifting the dominant frequency. Spacing redistributes absorbed power between devices while leaving the array averaged interaction factor close to unity. Hydrodynamic coupling is weak beyond roughly ten diameters, but nonlinear and directional effects still introduce measurable differences between rows at 15D. Directional incidence imposes the strongest control: long side headings between 60 degrees and 90 degrees maintain radiative synchronisation and produce mild constructive interaction, whereas oblique headings reduce phase coherence and suppress amplification.

A device level case study shows that array behaviour is governed by radiative phase alignment rather than geometric proximity. Devices experiencing radiative softening exhibit larger response and absorbed power, while those subject to radiative stiffening show reduced motion despite nearly identical incoming excitation. This establishes the link between local impedance modification and array scale performance.

Overall, the results show that weakly NLFD methods capture the essential physics of large WEC arrays with clarity, computational efficiency, and controlled accuracy. The framework supports phase based layout, directional alignment, and PTO tuning strategies for early stage design of future wave energy farms.