Abstract
This paper concerns experimental measurements of the interaction of a taut moored floating body, representing a point absorbing wave energy converter in survivability mode, with extreme waves. The extreme waves are modelled in four ways. NewWave theory is first used to generate focused wave groups of varying steepness. Steepness is shown to have negligible effect on peak mooring loads, but causes significant differences in the resulting motion. The NewWave group is then constrained into both regular and irregular background wave trains so that the floating body has a load history caused by previous waves when interacting with the focused wave group. It is shown that an independent focused wave group is insufficient to properly model the extreme response of the floating body. However differences between the target and measured constrained time series due to non-linear wave-wave interaction limited the potential benefits of this approach. Finally the results from these tests are compared with measurements taken using irregular waves without any deterministic focused wave groups present. This comparison found cases where the floats response was greater than during any of the constrained NewWave tests, indicating that the assumption made that NewWave will generate the largest response was incorrect in this case.
Highlights
Floating wave energy converters must be designed to withstand the largest waves experienced during storms of magnitude equal to their design condition
This paper presents a systematic study into the experimental use of independent NewWave and Constrained NewWave (CNW) in the investigation of a moored floating body's response to an extreme wave
The quality of wave generation was assessed for each of the 180 irregular constrained NewWave by calculating the root mean square error (RMSE) between the signal from wave gauge 6 (WG6) with the target time series (TTS): RMSE 1⁄4 v u u tNffi1ffiffiffiffiffiX ffiiffi1⁄4Nffiffi1ffiffiffiðffiffiWffiffiffiffiGffiffiffiffi6ffiffiiffiffiÀffiffiffiffiffiTffiffiffiTffiffiSffiffiffiiffiÞffiffi2ffi; where N is the number of samples measured
Summary
Floating wave energy converters must be designed to withstand the largest waves experienced during storms of magnitude equal to their design condition. Both experimental and numerical techniques are used to model a device's response to extreme waves In both cases a deterministic focused wave group based on NewWave theory is often used to generate a time series of an extreme wave. The impact of the device's load history variation and the resulting distribution of the responses to the extreme wave can be investigated by conducting multiple simulations or experiments with the focused wave group constrained into different random background time series. Pinna and Cassidy (2004) conducted similar simulations on a fixed monopod platform using 100 CNW cases while Enderami et al (2010) modelled the extreme response of a Jacket offshore platform using 200 CNW, both with ABAQUS Both compared results with simulations of 3 h long irregular sea states and found that the maximum response were of a similar magnitude. By comparing the results from the four different sets of experiments conclusions are drawn and recommendations made about the application of independent and constrained NewWave to the experimental determination of the extreme response of a moored floating body
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