Abstract
This paper concerns experimental measurements of the interaction of a taut moored floating body, representing a wave energy converter in survivability mode, with extreme waves. Focussed wave groups, based initially on NewWave theory, are used to generate the extreme waves, with crest amplitude exceeding the mooring׳s design capacity. Two data sets are presented and discussed. In the first the influence of wave steepness on model response and mooring load is investigated using non-breaking focussed wave groups. In the second the influence of wave breaking location is investigated using a plunging breaking wave. Both data sets exhibit snatch loading as the extension of the mooring is exceeded. The magnitude of this loading is not found to be strongly dependent on wave steepness, while the following motion response of the body is. Breaking location has a much greater effect than wave steepness on the magnitude of the mooring load, while significant influence of the body motion and displacement on the mooring load is demonstrated. Evidence is provided that the use of individual focussed wave groups is inadequate to assess fully the extreme loads experienced by a taut moored WEC due to the demonstrated dependence of mooring load on the body׳s motion and displacement.
Highlights
Regardless of their location in the ocean, wave energy converters (WECs) have two primary requirements: to provide efficient conversion in small to moderate seas and to survive storm conditions (Barstow et al, 2008, p. 52)
This mooring arrangement is similar to that used by Eriksson et al (2006) during full scale tests on a cylindrical buoy designed for use as a wave energy converter
In order to assess the effect of wave steepness on the WEC response, the steepness of the wave group was increased by multiplying the original peak frequency used to generate the NewWave (0.356 Hz) by a particular factor
Summary
Regardless of their location in the ocean, wave energy converters (WECs) have two primary requirements: to provide efficient conversion in small to moderate seas and to survive storm conditions (Barstow et al, 2008, p. 52). One approach that can be used to estimate the response is by conducting scaled experiments of the WEC in storm sea conditions This requires long duration runs before a statistically reliable assessment of the extreme loading can be made. By measuring the dependence of motion and loads on both wave steepness and breaking point an assessment could be made of the validity of using single focussed wave groups to measure extreme wave impacts on a single taut moored floating body. The data is being used to validate a CFD numerical model of a floating WEC These measurements were conducted as part of the EPRSC X-MED project (Extreme loading of marine energy devices due to waves, current, flotsam and mammal impacts). They were the first measurements to be conducted at Plymouth University's ocean basin (COAST Lab, 2013)
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