Numerical hydrodynamic analysis of wave energy converter arrays with inter-body mooring connections, with the spar-buoy OWC as a case study

Abstract

This PhD thesis explores the hydrodynamics and the performance of a moored array of Wave Energy Converters (WECs). A comparison is made between the performance of an array with bottom mooring lines and the performance of an array that uses shared mooring lines (i.e. inter-body mooring connections). The fundamental equations for arrays with shared moorings are developed in the frequency-domain where simple spherical devices moving in the three translational modes are considered. The numerical model uses linear hydrodynamic and hydrostatic forces, while the mooring connections are linearised using perturbation theory. This is further elaborated upon by considering cylindrical devices, so that the three rotational modes may be considered, as well as their hydrodynamic coupling with the three translational modes. This moored array of cylindrical devices moving in all six Degrees-of-Freedom (DoF) is used as a foundation to explore the hydrodynamics and the performance of an array of floating Oscillating Water Column (OWC) devices known as spar-buoy OWC. Real fluid viscous effects are accounted for by using a linear approximation. The frequency-domain numerical model of the moored array of spar-buoy OWCs is used as a basis for a stochastic model, where the array's performance for a Portuguese wave climate is assessed. The performance of arrays with shared mooring connections VS arrays with bottom mooring connections is explored in both the frequency and stochastic domain models for the spar-buoy OWC array. Both models have confirmed that there is minimal difference in performance between the two mooring systems, which is desirable because the one has been suggested as a more economically viable solution than the other due to drastic reductions in the amount of mooring cables and anchors.