Enhancing wave energy developments through mooring system reliability assessment
Gordelier, Tessa Jane
Thesis or dissertation
University of Exeter
This Thesis is available for use on the understanding that it is copyright material and that no quotation from the Thesis may be published without proper acknowledgement.
Wave energy generation is a promising renewable energy source but it faces certain challenges before it can become commercially viable. In comparison to conventional energy generation it is expensive, furthermore it has been plagued by reliability challenges due to the harsh operating demands of the marine environment. This Thesis investigates the reliability of wave energy devices, and specifically focuses on mooring system reliability. Two major themes are developed: Firstly, an assessment is conducted on a conventional mooring component, reviewing safety factors suggested in mooring system design guidelines and investigating whether there is a potential to reduce these safety factors (and in so doing, reduce system costs). Numerical modelling, laboratory testing and field testing demonstrate that excessively large safety factors are published in design guidance for static loading scenarios. However, when considering fatigue loading regimes (a critical aspect of wave energy generation), the proposed safety factors are found to be appropriate. In fatigue design, the importance of selecting an appropriate stress concentration factor for use with generic S-N curves is highlighted. These findings indicate the publication of additional stress concentration factors and a standard approach for mean stress adjustment would be a valuable addition to mooring system design guidance for fatigue. The second theme introduces a novel mooring component, The Exeter Tether, designed to reduce mooring loads and thus reduce system costs. The introduction of any novel technology brings new reliability considerations, and a reliability assessment of the tether and sub-components is presented in this Thesis. Following a failure modes and effects analysis, a bespoke range of physical tests is developed to investigate reliability concerns unique to this novel component. Laboratory testing of the tether assembly shows promising fatigue performance, however field trials highlight concerns regarding bio-fouling and marine debris ingress. Sub-component testing of the EPDM (Ethylene propylene diene monomer) polymer core suggests an increase in material stiffness with both marine ageing and repeated compression cycles. This finding supports results from assembly trials in the laboratory and at sea, where tether assembly dynamic axial stiffness is observed to increase over time. The overarching design philosophy behind the Exeter Tether is to reduce mooring system loads, so establishing the `worked' operating profile of the tether is crucial for the design intentions to be realised without compromising the reliability of the overall mooring system. Trials on the anti-friction membrane establish optimum performance when using two layers of UHMWPE (Ultra high molecular weight polyethylene) tape. Further areas requiring research are highlighted, and suggestions are made to improve the reliability of future design iterations of The Exeter Tether. The two reliability approaches presented demonstrate the potential for cost reduction in mooring system design and highlight the importance of physical component testing, both in the field and in laboratory conditions, to optimise component design whilst ensuring overall system reliability.
The Engineering and Physical Sciences Research Council (EPSRC) through the SUPERGEN UKCMER programme.
The European Commission through the MARINET programme also funded two weeks test work to facilitate some of the work presented in this Thesis.
Thies, P. R., Johanning,L., Gordelier, T., Vickers, A., Weller, S. "Physical component testing to simulate dynamic marine load conditions." ASME 2013 32nd International Conference on Ocean, Offshore and Arctic Engineering. American Society of Mechanical Engineers, 2013.
Gordelier, T., Parish, D., Thies, P., Johanning, L. "A Novel Mooring Tether for Highly-Dynamic Offshore Applications; Mitigating Peak and Fatigue Loads via Selectable Axial Stiffness." Journal of Marine Science and Engineering 3.4 (2015): 1287-1310.
Weller, S. D., Thies, P.R., Gordelier, T.G., Johanning, L. "Reducing Reliability Uncertainties for Marine Renewable Energy." Journal of Marine Science and Engineering 3.4 (2015): 1349-1361.
Weller, S. D., Thies, P.R., Gordelier, T.G., Davies, P. and Johanning, L. "The Role of Accelerated Testing in Reliability Prediction." 11th European Wave and Tidal Energy Conference 2015. September 6-11, 2015, Nantes, France. (2015).
Weller, S. D., Thies, P.R., Gordelier, T., Parish, D., Harnois, V., Johanning, L. "Navigating the valley of death: Reducing reliability uncertainties for marine renewable energy." ASRANet International Conference on Offshore Renewable Energy, September 15-17, 2014, Glasgow, UK. (2014).
Gordelier, T., Johanning, L. and Thies, P.R. "Reliability verification of mooring components for floating marine energy converters." SHF Marine Renewable Energy Symposium, October 9-10, Brest, France, 2013.
Thies, P.R., Johanning, L., and Gordelier, T. "Component reliability testing for wave energy converters: Rationale and implementation." 10th European Wave and Tidal Energy Conference, Aalborg, Denmark, 2013.
PhD in Renewable Energy