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
 
Description Nonlinear interactions between ocean waves are dominated by different processes in deep or shallow waters. The intermediate range of depths (between "shallow" and "deep") have been thought to suppress nonlinear interactions, reducing the likelihood of extreme wave formation. We have observed a form of nonlinear interaction which persists at intermediate depths. While most offshore wind turbines are currently installed at water depths of less than 30m, next generation wind turbines will likely be installed in deeper waters. The nonlinear interactions we have been observing remain active for depths of 35-60 m and, thus, may be important for the design of next generation offshore wind turbines. Furthermore, our simulations have shown that the results are highly sensitive to the "spectral equilibrium" of the initial conditions. Simulations and experiments performed with simplified initial conditions may, thus, produce spurious results. We have shown that a commonly form of simplified initial condition leads to drastic overestimation of certain nonlinear features of ocean waves. Lastly, we are exploring a method of simulating ocean waves which is faster and less computationally expensive than fully-nonlinear simulations but which captures the most important features, necessary for load calculations.
Exploitation Route The design of offshore structures at intermediate depths should consider the effects we have observed. We also warn against the use of simplified initial conditions when performing simulations / experiments in the field of ocean wave mechanics and demonstrate the importance of realistic initial conditions. Our method of load analysis is currently under development but may prove useful for the design of structures in the ocean, including offshore wind turbines. Floating turbines installed in deep and intermediate waters may also be effected by the nonlinear wave mechanics we have observed.
Sectors Energy

 
Description DeRisk Project 
Organisation Technical University of Denmark
Country Denmark 
Sector Academic/University 
PI Contribution The DeRisk project focuses on new design methods for extreme ocean wave loads, aiming to reduce the uncertainty and risk in the analysis. The design methods are aimed at the development of offshore wind energy. Our team performed high-fidelity simulations of extreme ocean waves, using the OceanWave3D code, and published a paper about the simulations. We have identified some likely conditions for the formation of extreme ocean waves and also analysed the effect of the water depth on wave formation. We are currently developing a tool to calculate the wave loads in a faster way, with less computational expense.
Collaborator Contribution Professor Harry Bingham at the Technical University of Denmark provided us with the OceanWave3D numerical simulation tool and helped teach me to use the code. I spent two weeks at the Technical University of Denmark (DTU) in January 2018, being trained to use the code. We have performed our simulations using the High Performance Computing (HPC) cluster of DTU, with access provided by Professor Bingham and the DeRisk project.
Impact I presented our findings at the "Industrial Dissemination" workshop, 22-24 Aug 2018 in Copenhagen, Denmark, as part of the DeRisk project. A number of industry representatives attended the workshop including members of Dong Energy and Statkraft. The findings have also been presented at the 34th International Workshop on Water Waves and Floating Bodies (IWWWFB34) held in Newcastle, Australia during 7-10 April 2019. This is a small but prestigious conference in our field, managed and led by fellow academics.
Start Year 2017