Modelling Marine Renewable Energy Devices; Designing for Survivability

The primary aim of the project is the assessment of the extreme wave loads on WECs using numerical models validated against experimental observations and full-scale prototype data. The project team combines institutions with significant experience in research into extreme waves (Imperial College), wave energy converters (Queen's University Belfast) and numerical modelling (Manchester Metropolitan University), forming a strong and well-balanced consortium. They will be supported by a steering committee comprising a number of key industrial practitioners and stakeholders, bringing in a wide range of backgrounds from device developers, certifying bodies and the offshore industry.

In designing wave energy converters (WECs), scientists and engineers face the challenge of having to compromise between two competing criteria. The power take-off, with all associated mechanical and electrical components having to be optimised for an annual average or nominal sea state. At the same time all these components will have to withstand large storm events, where the applied fluid loads are substantially higher compared to the nominal sea state. A successful design is inevitably characterised by one that balances these two criteria. Identifying such a balance at an early design stage (prior to expensive small or large scale physical model testing) requires accurate, reliable and efficient numerical models appropriate to both design criteria. Survivability defines the long term success of a WEC, and must be addressed by design.

Water waves exhibit inherent nonlinearities, which are functions of the wave steepness. In severe sea states, linear models fail to predict the fluid kinematics. As a result, the numerical modelling of wave loading in severe sea states is challenging; the loads being directly affected by the underlying fluid kinematics. Further, the occurrences of wave impacts, wave breaking and air entrainment pose additional challenges. An accurate description of wave nonlinearities, combined with the ability to model local loading effects, is key to the success of the numerical modelling. The project team brings in world-leading expertise in the development of numerical models. In fact, these models have now reached a level of sophistication where a direct comparison with experimental data is practical.
The integrated research programme builds upon

(i) The latest advances in Met-Ocean, providing a realistic input to both the numerical and the experimental modelling

(ii) Numerical modelling based on a hierarchical approach, ranging from linear and fully nonlinear potential flow models to fully nonlinear viscous flow solvers

(iii) Extensive experimental investigation using state-of-the-art wave testing facilities appropriate to both shallow and intermediate / deep water conditions

(iv) Comparisons with field data relating to loading of prototype WECs

The results of the numerical models will be analysed to provide guidance on the appropriateness of particular models, as well as issues associated with the scaling of extreme loads. This will enable an estimation of the uncertainty in extreme loads based on the modelling technique adopted. The research programme initially focuses on two generic device types, and guidelines for the application of the models to other WECs will be developed. In summary, the project is defined by a twin-track approach, combining advanced numerical models and careful experimental practice; the results of which will help to facilitate the large-scale deployment of wave energy converters.

In matching our future energy demand it is clearly unsustainable to rely on fossil fuels alone. The related greenhouse gas emissions and associated problems, such as climate change and sea level rise, are alarming. A solution to this global dilemma must be twofold: by reducing our current energy consumption and by tapping into renewable energy sources. The UK has signed the EU Renewable Energy Directive targeting 15% of energy generation from renewables by 2020. It is widely acknowledged that this ambitious target must be met by a balanced mix of energy sources, including wind, hydro, biomass, solar, tidal and wave energy. The coastal waters of the British Isles receive some of the world's largest wave energy levels. As a result, this renewable has the potential to make a significant contribution.

After some enthusiastic pioneering work in the 1970s and 1980s, the wave energy sector has seen further significant developments in the past decade. The first full-scale prototypes are now being tested and it is vital to foster this recent R&D momentum. Currently the UK is the world leader in this emerging technology. It is important to capitalise on this extensive engineering experience, at a time when large scale deployment of the technology is likely to take place over the next two decades. Mass production of wave energy converters will provide a sustained source of employment, and has the potential to emerge as a high-tech export technology. Adopting cutting-edge tools (both numerically and in the laboratory), the proposed research programme will assist device developers with methods that are often unavailable to small and medium sized businesses. The key deliverable of the work is a suite or hierarchy of validated numerical tools appropriate to the modelling of a wide range of wave energy converters.

With some forty years of experience in the field it is now recommended to develop any WEC concept using a five-staged approach. At least the first two stages take place in the laboratory environment using scaled models, accompanied by numerical simulations. These must be complete before further tests take place at a benign sea site and in the open sea. As an example, the Pelamis wave energy converter (one of the leading device technologies) has undergone over a dozen experimental test rounds and extensive numerical modelling efforts, before a full scale prototype was successfully launched. The costs that inevitably arise as a result of extensive model testing are significant. Typically this may be in the order of £10M from inception to full scale prototype deployment. As such, a numerical assessment procedure, leading to reduced costs and a more rapid development programme to prototype testing, would be widely welcomed by industry.

The principal beneficiaries will be the device developers, who will be able to assess the adequacy of modelling techniques to provide an estimation of the extreme loads on their devices and the uncertainty associated with this extreme value. A second group of key beneficiaries will be the standards and certification agencies, who will be able to use the analysis of the modelling techniques to refine their guidelines for the design of WECs. Secondary beneficiaries include investors, both public and private, who will be able to use the assessment of modelling techniques to determine the adequacy of a particular technique and its appropriateness for the particular stage of device development. The increased understanding of extreme loads will increase confidence in WEC designs leading to increased investment in the industry. Beneficiaries outside the field of marine renewable energy will include companies and researchers with an interest in extreme loads on offshore structures, for which the modelling techniques may be equally valid. The coupling of two sophisticated wave models will also have academic beneficiaries, allowing new classes of fluid flows to be modelled numerically.