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Wave energy converters (WECs) are commonly designed and analyzed using numerical models that combine multibody dynamics with hydrodynamic models based on the Cummins equation and linearized hydrodynamic coefficients. These modeling methods are attractive design tools because they are computationally inexpensive and do not require the use of highperformance computing resources necessitated by high-fidelity methods, such as Navier-Stokes computational fluid dynamics. Modeling hydrodynamics using linear coefficients assumes that the device undergoes small motions and that the wetted surface area of the devices is approximately constant. WEC devices, however, are typically designed to undergo large motions to maximize power extraction, calling into question the validity of assuming that linear hydrodynamic models accurately capture the relevant fluid-structure interactions.In this paper, we study how calculating buoyancy and Froude-Krylov forces from the instantaneous position of a WEC device changes WEC simulation results compared to simulations that use linear hydrodynamic coefficients. First, we describe the WEC-Sim tool used to perform simulations and how the ability to model instantaneous forces was incorporated into WEC-Sim. We then use a simplified one-body WEC device to validate the model and to demonstrate how accounting for these instantaneously calculated forces affects the accuracy of simulation results, such as device motions, hydrodynamic forces, and power generation.Other aspects of WEC-Sim code development and verification are presented in a companion paper [1] that is also being presented at OMAE2014. INTRODUCTIONWave energy is the most abundant source of marine hydrokinetic energy in the United States and is a plentiful resource around the globe [2]. Recent estimates indicate that the U.S. wave energy resource is 2,600 TWh/year [3]. If it is possible to extract even a small fraction of this energy, there is potential to satisfy a significant amount of U.S. electricity demand [4]. This finding has stimulated commercial and governmental interest in developing wave energy converter (WEC) technologies, and indicates that wave energy could play a significant role in the world's renewable energy portfolio for years to come. Nevertheless, WEC devices are at an early stage of development, corresponding to technology readiness levels (TRLs) 3 through 5, and are not yet a commercially viable technology.Over the past several decades, open-source numerical modeling tools have helped the wind turbine industry achieve commercial viability by enabling the rapid development, analysis, and certification of system designs. The recent emergence of the WEC industry has created a need for a similar set of WEC design and analysis tools that enable the advancement of WEC technologies. Several companies have developed WEC modeling tools, such as WaveDyn, OrcaFlex, and AQWA, that meet many of the needs of the WEC research and development community. Previous experience at the National Renewable Energy Laboratory (NR...

This paper presents a recent study on the design and analysis of an oscillating surge wave energy converter (OSWEC). A successful wave energy conversion design requires balance between the design performance and cost. The cost of energy is often used as the metric to judge the design of the wave energy conversion (WEC) system, which is often determined based on the device’s power performance; the cost of manufacturing, deployment, operation, and maintenance; and environmental compliance. The objective of this study is to demonstrate the importance of a cost-driven design strategy and how it can affect a WEC design. A set of three oscillating surge wave energy converter designs was analyzed and used as examples. The power generation performance of the design was modeled using a time-domain numerical simulation tool, and the mass properties of the design were determined based on a simple structure analysis. The results of those power performance simulations, the structure analysis, and a simple economic assessment were then used to determine the cost-efficiency of selected OSWEC designs. Finally, we present a discussion on the environmental barrier, integrated design strategy, and the key areas that need further investigation.

The paper presents results from the Collaborative Computational Project in Wave Structure Interaction (CCP-WSI) Blind Test Series 2. Without prior access to the physical data, participants, with numerical methods ranging from low-fidelity linear models to fully non-linear Navier–Stokes (NS) solvers, simulate the interaction between focused wave events and two separate, taut-moored, floating structures: a hemispherical-bottomed cylinder and a cylinder with a moonpool. The ‘blind’ numerical predictions for heave, surge, pitch and mooring load, are compared against physical measurements. Dynamic time warping is used to quantify the predictive capability of participating methods. In general, NS solvers and hybrid methods give more accurate predictions; however, heave amplitude is predicted reasonably well by all methods; and a WEC-Sim implementation, with CFD-informed viscous terms, demonstrates comparable predictive capability to even the stronger NS solvers. Large variations in the solutions are observed (even among similar methods), highlighting a need for standardisation in the numerical modelling of WSI problems.

Extreme loads are often a key cost driver for wave energy converters (WECs). As an alternative to exhaustive Monte Carlo or long-term simulations, the most likely extreme response (MLER) method allows mid- and high-fidelity simulations to be used more efficiently in evaluating WEC response to events at the edges of the design envelope, and is therefore applicable to system design analysis. The study discussed in this paper applies the MLER method to investigate the maximum heave, pitch, and surge force of a point absorber WEC. Most likely extreme waves were obtained from a set of wave statistics data based on spectral analysis and the response amplitude operators (RAOs) of the floating body; the RAOs were computed from a simple radiation-and-diffraction-theory-based numerical model. A weakly nonlinear numerical method and a computational fluid dynamics (CFD) method were then applied to compute the short-term response to the MLER wave. Effects of nonlinear wave and floating body interaction on the WEC under the anticipated 100-year waves were examined by comparing the results from the linearly superimposed RAOs, the weakly nonlinear model, and CFD simulations. Overall, the MLER method was successfully applied. In particular, when coupled to a high-fidelity CFD analysis, the nonlinear fluid dynamics can be readily captured.