Wave Energy Converter Mooring System Power Conversion Chain External Fluid Dynamics

Survivability is by no means a new concept to ocean engineering; ships must remain stable and structurally intact in violent sea states; the same is true for offshore oil and gas structures. While knowledge from the ship and offshore sectors can be valuable for designing wave energy converters (WECs) for survival in rough seas, the unique scale, siting and operational characteristics of WECs pose a distinct set of engineering challenges. This paper seeks to provide a review of methods for modeling the loading and dynamic response of WECs and analogue marine structures, such as ships and offshore structures, in large nonlinear waves. We identify current knowledge gaps in our understanding of WEC survivability and provide recommendations for future research to close these gaps. INTRODUCTION Thus far, most WEC research and development has focused on operational performance (optimizing energy capture and conversion efficiency). The commercial viability of a WEC is equally dependent on its ability to operate, with minimal downtime for repairs, for many years. This second aspect comprises two components: reliability and survivability. While the terms reliability and survivability are occasionally used interchangeably, the recognition of two unique definitions is essential to the function of a more systematic WEC design process. The development of standardized definitions for these terms was presented by Brown et al., in which the authors used wind turbines as a proxy to illustrate the distinction between reliability and survivability [1]. Wind turbines have relatively few survival issues, as their ability to pitch their blades into strong winds has proven to be very effective at limiting loads during ∗Corresponding Author: rcoe@sandia.gov storms. Conversely, reliability continues to be an issue for wind turbines, as gearbox failures from regular wear-and-tear exceed expectations. Drawing on inferences from other engineering disciplines, Brown et al. defines WEC survivability as “the ability of a marine energy system to avoid damage, during sea states that are outside of intended operating conditions, that results in unplanned down time and the need for service. ” It is important to note from this definition that survivability pertains specifically to extreme sea states, but not other statistically infrequent events (e.g., boat strikes, rogue waves or tsunamis). This restriction is critical to utility of survivability as metric in WEC design and we therefore adhere to this definition in the following review. Even when we restrict our focus to the survival of WECs in extreme sea states, the definition of an extreme sea state is, as it pertains to WEC survival, somewhat ambiguous. While structural loading in a device will tend to grow with wave height, the trend is also likely to possess a number of local maxima associated with certain wave period and height combinations. These local peaks could be points of resonance or simply scenarios that causes intense loading of the device. A priori knowledge of which sea states will cause the highest levels of loading in a specific WEC is not often available. WEC survival analysis must, therefore, be an iterative process, such as that shown in Figure 1. This diagram illustrates a process in which extreme sea state characterization, modeling and failure analysis are combined to assess and understand the unique survivability concerns for a particular WEC taxonomy, design and deployment site. This paper’s focus is mainly restricted to the response modeling component of the WEC survival analysis process. In that interest, the remainder of this paper is organized into sections on extreme sea state characterization, response modeling, special modeling considerations and a discussion of the how WEC development could benefit from further research in the area of survival analysis.