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Monitoring of Seaway Variability on WEC Performance

Date: October 27, 2013 at 18:48 GMT

One of the major issues that arise when moving from the early phases of wave energy device development conducted in hydraulic facilities to initial sea trials is the loss of control of the excitation conditions. This imposition requires that the test programmes and data analysis techniques must be much more rigorous and expansive to ensure understanding and appreciation of the large amounts of information that should be in the process of being generated. This situation exists whether the early sea trials are being conducted at benign medium scale sites (circa l?¼) such as Galway Bay and Nissum Bredning or have advanced to full prototype size exposed locations such as EMEC, or the Portuguese’s pilot zone or the many other demonstration sites proposed around Europe for the future. 

The real sea is inherently unpredictable, so device developers have to take into serious consideration a number of factors. One of these is the accurate measurement of the impinging ocean waves if any unravelling of how this affects the device performance is to be expected. In the relative comfort of hydraulic facilities excitation waves can be programmed on demand and repeated with reasonable fidelity as required. Besides classical seaways any mix of sea and swell combinations should be possible, producing twin peaked spectra in the frequency domain, directional bimodality or both simultaneously. Test schedules investigating the various aspects of a WEC design effecting performance can be drawn up and run to a pre-determined timetable. This convenience is not available once at sea and situations must be exploited when they become available. To achieve this control to any degree of satisfaction the appropriate sea state conditions must be identified implicitly. Simply knowing the summary statistics of the conditions is no longer really satisfactory. Knowledge of the excitation forces is essential before understanding of the response can be expected. 

This paper aims to look at some of the aspects, in relation to both resource and engineering that must be addressed when considering body response and power production of wave energy converters at sea. The expected electrical output supply of a device can be considerably different when the concurrent sea state is of the form of a multi-modal sea. This is especially the case if an energy trough exists between the wind sea and swell components that coincides with the eigen frequency of the device. Juxtapose to this, greater energy may be derived in a narrow banded JONSWAP type sea state when the peak harmonic occurs at the resonant period of the primary degree of freedom (e.g. heave) of the device. 

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