Extreme waves under significant wind stress
File(s)
Author(s)
Gray, Alexander
Type
Thesis
Abstract
This study concerns the design and undertaking of a novel series of experiments, to ascertain the effect of both a co-flowing and an adverse wind over large or 'extreme’ ocean waves. Such waves may arise under severe storm conditions with significant wind forcing and have the potential to severely damage or overload offshore platforms. As such, the accurate calculation of extreme crest height statistics is of critical importance to engineers. The present work successfully replicates a range of wind and wave conditions associated with such extreme events in a laboratory setting, and measurements of extreme waves are made with a high degree of accuracy.
Both deterministic (focused) and random wave conditions have been considered, and the measured data compared to the existing body of experimental research, which usually takes place in the absence of wind. Comparisons are also drawn with the statistical distributions which are commonly used in engineering design.
The results of this study demonstrate a clear wind effect on the statistics of the largest crests, consideration of which is lacking from the existing body of research and best design practice. Both co-flowing and adverse wind were shown to reduce the probability of extreme crest heights for all target sea-states. Clear mechanisms are identified for these effects, using rigorous spectral and geometric analysis of the largest waves. In particular, the dominant role of wind-induced wave breaking is clearly identified as an issue of engineering significance.
Experiments concerning wind over deterministic `focused’ waves also demonstrate a clear departure from the behaviour observed in previous research; this earlier work carried out under zero-wind conditions. Most notably, the introduction of overlying wind leads to significant frequency amplitude and phase changes within the focused wave group. For co-flowing wind and waves, the most analogous to real sea conditions, the magnitude of these changes is far greater than those observed the equivalent random wave cases. This finding highlights the need for an improved approach to the use of focused waves as design wave cases, which are commonly used to typify extreme random waves, when considering waves under significant wind stress.
Both deterministic (focused) and random wave conditions have been considered, and the measured data compared to the existing body of experimental research, which usually takes place in the absence of wind. Comparisons are also drawn with the statistical distributions which are commonly used in engineering design.
The results of this study demonstrate a clear wind effect on the statistics of the largest crests, consideration of which is lacking from the existing body of research and best design practice. Both co-flowing and adverse wind were shown to reduce the probability of extreme crest heights for all target sea-states. Clear mechanisms are identified for these effects, using rigorous spectral and geometric analysis of the largest waves. In particular, the dominant role of wind-induced wave breaking is clearly identified as an issue of engineering significance.
Experiments concerning wind over deterministic `focused’ waves also demonstrate a clear departure from the behaviour observed in previous research; this earlier work carried out under zero-wind conditions. Most notably, the introduction of overlying wind leads to significant frequency amplitude and phase changes within the focused wave group. For co-flowing wind and waves, the most analogous to real sea conditions, the magnitude of these changes is far greater than those observed the equivalent random wave cases. This finding highlights the need for an improved approach to the use of focused waves as design wave cases, which are commonly used to typify extreme random waves, when considering waves under significant wind stress.
Version
Open Access
Date Issued
2020-09
Date Awarded
2021-05
Copyright Statement
Creative Commons Attribution NonCommercial NoDerivatives Licence
Advisor
Swan, Chris
Christou, Marios
Publisher Department
Civil and Environmental Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)