Modelling Marine Renewable Energy Devices; Designing for Survivability

Lead Research Organisation: Imperial College London
Department Name: Civil & Environmental Engineering

Abstract

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.

Planned Impact

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.

Publications

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Henry A. (2014) A two dimensional experimental investigation of slamming of an Oscillating Wave Surge Converter in Proceedings of the International Offshore and Polar Engineering Conference

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Henry A. (2013) The Characteristics of Wave Impacts on an Oscillating Wave Surge Converter in Proceedings of the International Offshore and Polar Engineering Conference

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Latheef M (2013) A laboratory study of wave crest statistics and the role of directional spreading in Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences

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Latheef M (2017) A laboratory study of nonlinear changes in the directionality of extreme seas. in Proceedings. Mathematical, physical, and engineering sciences

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Ma Z (2014) A compressible multiphase flow model for violent aerated wave impact problems in Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences

 
Description Three specific areas are key to delivering our impact: (i) the development of standardised protocols for the development of wave energy devices, (ii) providing design guidance to the wave energy industry and (iii) embedding state-of-the-art research methodologies into day-to-day engineering consultancy. The development of tank testing guidelines is achieved through the framework of IEC-PT62600-103 "Guidelines for the early stage development of wave energy converters", where Spinneken (Imperial College) acts as the UK principle expert. Spinneken (UK) and Holmes (Ireland) took the lead in progressing the international contributions into a first Committee Draft (CD); this CD being expected to be published in Q3 2016. IEC-PT62600-103 gives guidance towards the development of any new wave energy technology. Based on a three-stage development, targets and "Stage Gates" are provided, reducing the potential for a device concept failure at a late commercial stage. These guidelines are believed to lead to cost reductions of new technologies, and to increase investor confidence by adopting a structured development approach. Work undertaken by Queen's University Belfast (physical model testing) and Manchester Metropolitan University (numerical modelling) feeds directly into the design of surging flap-type wave energy converters. Findings obtained are used to suggest important decisions in the design process in terms of reducing and mitigating the most severe extreme loads. The development of accurate and precise physical testing facilities has also led to improvements in how extreme load testing will be undertaken in the future. The project team works closely with WavEC Offshore Renewables to place the research findings into a day-to-day consulting context. This has helped the development of partially nonlinear and computationally efficient models fit for engineering design. To accelerate this process, a WavEC team member was seconded to Imperial College; this four-month placement having been supported through additional EU KIC funding. Throughout this project, a significant amount of progress was made in relation to extreme wave loading. The methodologies established reach well beyond the wave energy sector, with applications to many static and dynamic marine structures. New experimental techniques suitable for the measurement of extreme loads have been developed. Numerical models suitable to impact loading cases were also advanced. In addition the project has provided: - Inputs to a new Joint Industry funded Project (JIP), and - Revised design guidance in respect of the relevant met-ocean conditions, particularly the occurrence of breaking wave loads
First Year Of Impact 2014
Sector Aerospace, Defence and Marine,Energy
Impact Types Economic,Policy & public services

 
Description EPSRC research grant: Towards a unified approach for the hydrodynamic modelling of WEC
Amount £126,641 (GBP)
Funding ID EP/M019977/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 10/2014 
End 06/2015
 
Description Industrial CASE Award with BP. Wave breaking and the effect of wave-current interactions
Amount £107,560 (GBP)
Funding ID Industrial CASE: voucher number 15220049 
Organisation BP (British Petroleum) 
Department BP Exploration Company
Sector Private
Country United Kingdom
Start 09/2016 
End 03/2020
 
Description Industrial CASE Award with Shell. Wave vessel interactions and the occurrence of wave impacts
Amount £107,560 (GBP)
Funding ID Industrial CASE voucher number 16000155 
Organisation Shell Global Solutions International BV 
Department Shell Research Ltd
Sector Private
Country United Kingdom
Start 10/2016 
End 04/2020
 
Description Industrial funding from BP: combined wave-current flows
Amount £120,000 (GBP)
Organisation BP (British Petroleum) 
Department BP Marine
Sector Private
Country United Kingdom
Start 12/2014 
End 12/2016
 
Description Industrial funding: LOWISH JIP
Amount £155,000 (GBP)
Organisation Shell Global Solutions International BV 
Department Shell Research Ltd
Sector Private
Country United Kingdom
Start 10/2014 
End 09/2016
 
Description Royal Society International Exchange Grant: The occurrence and intensity of surface wave breaking in realistic seas
Amount £12,000 (GBP)
Organisation The Royal Society 
Sector Charity/Non Profit
Country United Kingdom
Start 03/2015 
End 03/2017
 
Description Joint Industry Project: Limits on waves in shallow water (LOWISH) 
Organisation Shell Global Solutions International BV
Department Shell Research Ltd
Country United Kingdom 
Sector Private 
PI Contribution Laboratory observations of extreme waves in shallow water Improved understanding of wave breaking
Collaborator Contribution Provision of field data
Impact New descriptions of shallow water wave statistics New laboratory data base concerning large waves in shallow water Improved models for shallow water wave kinematics
Start Year 2014
 
Description Wave breaking: 
Organisation Scripps Research Institute
Country United States 
Sector Charity/Non Profit 
PI Contribution Improved methods of wave generation Improved models of wave breaking and energy dissipation Insights into deep water crest height statistics
Collaborator Contribution Flow visualisation techniques Bubble plume analysis methods Linkage between the white cap foam area and the energy dissipation associated with breaking.
Impact New models for wave breaking and the associated wave energy dissipation
Start Year 2015
 
Description Industrial seminar series 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Keynote address on wave-in-deck loading, organised by WS Atkins. First in a series addressing key issues in Offshore Engineering. Guest audience of 150 professionals and thereafter available on the Web.
Year(s) Of Engagement Activity 2014