On the influence of mooring systems in optimal predictive control for wave energy converters

Wave energy conversion systems have a massive potential in securing a reliable renewable energy mix. In their development, a crucial role is that of optimal control (OC) algorithms. Such systems are able to maximize the wave energy converter (WEC) power extraction, while respecting the corresponding of technological constraints. State-of-the-art OC techniques, such as Model Predictive Control (MPC), rely on mathematical models of the device to control, in a predictive fashion, the WEC system, thus maximizing the power production. Nonetheless, to date, control algorithms are usually developed and assessed on the basis of free floating WEC models, i.e. which neglect the mooring system influence. As a matter of fact, the anchorage introduces nonlinear dynamics in the device motion. Consequently, to test the idealized potential of a control strategy, such system is commonly neglected. Moorings are a fundamental WEC component, and have the potential to influence significantly the associated system dynamics. Neglecting this element can lead to deceptive results, either in terms of device theoretical productivity, and control strategy effectiveness. This paper proposes a systematic procedure to include, in the model used to synthesized such OC strategies, a linear representative model of the mooring system, presenting its benefits by discussing the consequent MPC loop development and corresponding performance assessment. Such procedure consists in retrieving an estimate of the frequency response of the moored system, using properly designed input conditions, and identifying the associated input–output linear system. A main objective of this study is, hence, to assess the difference between an MPC strategy designed and synthesized, with and without the proposed mooring control-oriented representation, always using as simulation system a high-fidelity numerical model for performance evaluation, which incorporates a full account of the mooring effects.