ABSTRACT

Mathematical models are essential for effective design of wave energy converters, and hence to achieve economic viability and industrial feasibility. Despite the fact that the wave energy field is at least 45 years old, there is still a clear lack of standardization of modelling techniques and a large amount of room for increasing confidence in hydrodynamic models. The CCP-WSI project aims to define a level playing field of comparison for a plurality of models, evaluating their performance. This paper implements a computationally convenient approach to represent nonlinear Froude-Krylov forces, along with the inclusion of non-linear kinematics.

INTRODUCTION

Accurate and reliable mathematical models are imperative in modern offshore renewable and ocean engineering applications, in order to reduce margins of uncertainty that are currently affecting every stage of the design process. On the one hand, a trustworthy prediction of structural loads is essential to ensure safety of personnel and/or components, while avoiding over-sizing of the structure and excessive safety coefficients. On the other hand, in wave energy applications, the effectiveness, and hence the economic viability of the device, strongly depends on the representativeness of the mathematical model (Giorgi and Ringwood, 2017; Ringwood et al., 2018).

Although the "modelling problem" of wave energy converters (WECs) is at least 45 years old (Salter, 1974), it is still far from being settled. Early linear WEC models, naturally germinated from classic ocean engineering, are usually not fit to accurately describe the wave energy problem because the objective of WECs is to exaggerate the motion and maximise power absorption (as opposed to motion stabilization). The inclusion of nonlinearities is essential for achieving higher accuracy, but also requires an increase in model complexity, hence computational burden. In the pursuit of the best compromise between model fidelity and computational time, a large number of nonlinear models have appeared in recent years (Penalba et al., 2017a). The performance of each model strongly depends on the specific device shape (Penalba et al., 2017), dimension (Clément, 1988), installation site (Giorgi and Ringwood, 2018), conversion principle (Giorgi and Ring-wood, 2018a), and operational condition (Giorgi and Ringwood, 2017a). Furthermore, there are different accuracy/computational requirements according to the model's purpose (i.e. design, model-based control, simulation, survivability, etc.). Therefore, although comparison between different modelling approaches is a challenging task, recent years have witnessed the pressing need for consistent model evaluation and standardization (CCP-WSI, 2016). A shared effort from several players in the wave energy community led to a modelling competition (Garcia-Rosa et al. 2015), the IEA-OES project (Wendt et al., 2017), and, finally, the present CCP-WSI project, of which this work is a part.

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