This research aims to improve the quality and availability of wave field information available to the developers and operators of wave energy converters (WECs) to aid in their design and operation. Applications relate to improving performance in varying wave climates, reducing extreme and fatigue-causing loads and reducing risk in critical marine operations through providing access to array-based and near-realtime surface elevation information. This Thesis describes a design process, leading from conceptual design, through to critical review. Design, assembly, commissioning and testing of multiple novel sensors involved technical work spanning mechanical, electrical, communications, signal processing and manufacturing disciplines. This required project management of areas including budget, procurement, IPR and programme scheduling. Three experimental procedures are outlined which were used to test the feasibility of a novel instrument conceived to meet the potential requirement for improved surface elevation data in large hydraulic test facilities and at sea. The first involves a method in the laboratory to assess the physical ("mechanical-only") surface tracking ability of long, floating, ribbon-like sensor elements that are aware of their position in two dimensions. Showing mean errors in wave height tracking of 6% and wave period tracking errors of 0.1% in irregular waves, across the widest available test range, results from surface tracking tests justify the subsequent testing of actual sensor implementations. Two approaches are taken: the first involves the modification and testing of a sensor technology comprising position-aware optical fibres with the second approach involving the design, fabrication and testing of floating sensors based on micro-electro-mechanical (MEM) sensor technology. Whilst wave period errors (individual time domain wave-by-wave comparisons) remain low for the optical fibre system at approximately 1% with standard deviations of approximately 10%, wave height errors are significant. Mean wave height error (depending on processing technique) range from -6% to 4% with standard deviations of 18% to 25% across irregular sea states. Performance is shown to be affected by wave steepness with wave trough tracking showing higher performance compared to wave crest tracking. Preliminary testing of the MEM-based sensor ribbons (in array form capable of measuring position in three dimensions) show wave height errors in regular waves to be on average 1.3% with standard deviations of relative error of 8.4%. Wave period errors and their standard deviations were below 1%. In irregular waves, mean significant wave height is under-predicted, across a range of directional seas, by 3% with standard deviation, across the tests and individual ribbons forming the array, of 7.5%. Peak wave period is under-predicted by 1.3% with standard deviation of 2.2%. Time domain statistics are not reported but it is expected that - as with the optical fibre system - performance degradation would occur when moving to irregular seas. Wave height error magnitude excludes the application of the developed sensors from small-scale tank testing where mm levels of accuracy are required. With further work, however, sensors based on this concept could potentially be used in larger scales and at sea where spatial wave field information is necessary, where wave period is critical and where other sensor techniques require baseline data.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:726480 |
| Date | January 2013 |
| Creators | Sellar, Brian Gordon |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/24305 |
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