On the reliability of electrical cable and optimal design of ocean energy arrays

A clear relationship exists between component reliability, the need for maintenance operations, and the resulting levelised cost of energy (LCOE) of offshore renewable energy arrays. To model this relationship, complex combinations of low-level components can be captured to provide the reliability of higher-level sub-systems. The open source, ocean energy array modelling tool “DTOcean” uses reliability block diagrams to represent higher-level sub-systems as networks of low-level components. Subsequently, failures of the higher-level sub-systems, based on the combined reliability of their components, can be simulated, resulting in LCOE estimates which are highly responsive to changes in array configuration. The operating environment in which a sub-system is deployed may also impact on its reliability. Considering the electrical network, it has recently been reported that subsea cable reliability reduces with the length of the cable deployed. By modifying DTOcean to include failure rate adjustment factors that model the effects of cable length, this study investigates the impact of cable length on the reliability of the electrical network and the most economical design of ocean energy arrays. It was found that for an array of 20 tidal energy converters, using the adjustment factors did not reduce the LCOE of the optimal array, but did offer a broader “design envelope” and may reduce overestimation of environmental impact when fixed cable reliability values are used.