Can oil gravity-based platforms be converted into viable oscillating water columns? A preliminary hydrodynamic study

Wave energy converters (WECs) offer a sustainable solution to produce electricity without greenhouse gas emission. This study examines a patented WEC design, utilizing decommissioned gravity-based structures with oscillating water column (OWC) principles. The design involves creating holes in the concrete cylindrical structure, causing water levels to oscillate and drive a bidirectional turbine to generate electricity. To assess energy potential, specific platforms were selected, and wave energy availability was evaluated using the NORA-10 hindcast. The efficiency of the energy conversion is estimated assuming a single opening, long-crested waves, and simplifying the geometry as two dimensional. This allows a systematic, careful study of the relevant wave-body parameters and paves the way for more realistic three-dimensional investigations of the concept. A three-step strategy was employed. First, two linear potential-flow semi-analytical solutions examined the effects of size and submergence of the opening, respectively, on 1) the wave transmission and on 2) the WEC efficiency. The latter approach incorporated the turbine characteristics and was obtained adapting the solution by Evans and Porter (1995) to the present scenario using specific eigenfunctions from Porter (1995) to define the spatial velocity potential. Then, CFD simulations were examined using an open-source solver within the OpenFOAM platform, neglecting turbulence and the damping effects of the turbine. Occurrence of resonance phenomena for the internal water was examined, both within the potential-flow and the real-flow scenario; in the latter case, information from air-water interface and local real-flow features complemented the analysis. The studies within linear regular waves were used as basis to estimate the WEC efficiency as a function of WEC parameters and wave period, as well as extracted power and energy within long-term analyses for a selected platform column. Turbine influence has also been examined. Based on the results, the proposed WEC concept is potentially a competitive candidate but it is expected that the 2D simplification is optimistic and simple estimations suggest that, in realistic 3D scenarios, more holes would be needed to ensure similar performance levels.