This report represents a summary of a larger report that describes a substantial programme of work concerning the nonlinear potential flow loads acting on single and multiple column structures. In particular, the report addresses the issue of structural ringing, or transient structural deflections occurring at frequencies well above the incident wave frequencies. The work has involved both theoretical modelling and experimental observations.
The key findings of the work are that both single and multiple column structures may be subjected to unexpected high-frequency forces. These are entirely dependent upon conditions at the water surface, and are associated with the unexpected scattering of high-frequency waves that cannot be predicted by existing diffraction solutions. The nonlinear forcing includes significant force components up to the fifth harmonic of the incident wave. However, with the scattering of high-frequency waves in large part controlled by the movement of fluid around the circumference of the structure, a harmonic analysis based solely on the characteristics of the incident waves neglects important time-scales and is unlikely to be universally applicable. The magnitude of the nonlinear forces is critically dependent upon both the steepness and the period of the incident waves, with the largest forces occurring in peak periods around 12-14 seconds. This suggests that the effects may represent important local design conditions, but are unlikely to be significant for maximum global loading conditions. The link between nonlinear forcing and scattering also suggests that localised vertical jetting is a related phenomenon; the latter having implications for the setting of an effective air-gap.
This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy.
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