Nonlinear Wave Mechanics of Steep Sea-States, Refraction and Breaking

Lead Research Organisation: University of Oxford
Department Name: Engineering Science

Abstract

Offshore wind turbines, as well as wave energy devices, must be designed to withstand the loads imposed by ocean waves. In shallow water, a wind turbine is typically embedded into the ocean floor while, in deep-water, a wind turbine can be installed on a floating island. In both instances excessive wave loading can result in catastrophic failure. An understanding of wave mechanics is, thus, necessary for the development of offshore wind sites. Similarly, shoreline wave-energy devices experience fluctuating loads due to the complex wave formations caused by coastal obstacles as well as variations in coastal depth. An understanding of wave mechanics is, thus, also necessary to maximise the energy output and life expectancy of a wave-energy device.

Although the governing equations which dictate the mechanics of ocean waves are inherently nonlinear, previous research has shown that wave evolution typically occurs in a linear fashion up until the point of "breaking". Linear approximations of the governing equations have been used by engineers to estimate the loads experienced by offshore structures and such approximates have proven reliable in estimating the ling-term time-averaged statistics of ocean waves. However, more recent research indicates that the nonlinearity of the governing equations can result in short-term transient events which deviate significantly from the long-term time-averaged statistics, In the vernacular, such events are termed "rogue" or "freak" waves, which represent a significant risk to offshore structures. The formation of extreme waves can be mitigated by the phenomenon of wave "breaking". However, the process of wave breaking is itself nonlinear. "Weakly" non-linear approximates of governing equations have, thus, been developed for the analysis of nonlinear wave mechanics. However, the nonlinear mechanisms involved in the formation and breaking of extreme waves have not been conclusively determined. The fidelity of the "weakly" nonlinear approach, thus, remains unproven.

This project seeks to identify nonlinear mechanisms which could cause extreme wave events, including ocean currents, in deep waters, as well as seabed topology, in shallow waters. The fidelity of the "weakly" nonlinear approach shall then be assessed by direct comparison with numerical simulations of the fully nonlinear potential flow equations, in collaboration with HR Wallingford, Oxford, and the Technical University of Denmark (DTU). Although experimental wave-tank facilities are not available at Oxford University, experimental validation of the results may also be performed with partner laboratories in the United Kingdom, at Plymouth University, and China, at Shanghai Jiao Tong University, with complimentary field data from a variety of ocean sites.

Ultimately, this project aims to provide insight into a crucial sustainability topic by advancing the state-of-the-art in the analysis of nonlinear wave mechanics. This project, thus, falls within the EPSRC Energy research area with particular relevance to the subtheme of Renewable Energy.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/R512333/1 01/10/2017 30/09/2021
2261366 Studentship EP/R512333/1 01/10/2017 30/09/2021 Dylan Barratt