This work was originally funded by Shell International Oil & Gas and was motivated by the unexpected occurrence of wave impact damage on the underside of a large North Sea structure. A fundamental investigation of the nonlinear wave-structure interactions identified a new mechanism for wave scattering associated with the movement of fluid around a column. This led to the scattering of a pair of non-concentric, high-frequency, wave fronts that cannot be predicted by existing diffraction theory. The interaction between these waves and the next incident wave forms part of a classical long-wave short-wave interaction that can produce a very substantial increase in the maximum local water surface elevation, hence the wave impact damage on the underside of the deck.

While the initial work was experimental, the results have now been simulated numerically, allowing the wider implications of the work to be assessed. This has established the practical importance of the high-frequency wave scattering for a wide range of offshore structures. Specifically, it provides an explanation for the increased occurrence of wave slamming on a column face. In the case of a dynamically sensitive structure, it also provides the origins for the high-frequency forcing arising above the mean water level and the consequent excitation of the structure at frequencies well above those of the incident waves, the latter commonly referred to as a “ringing” response.

The occurrence of these forces and motion modes is highly relevant to the design of offshore wind turbines, not least because their chosen location is such that they will be subject to large / steep (perhaps breaking) wave events. The work is presently being continued with funding from Hyundai Heavy Industries to consider the implications for shipping, most notably the occurrence of bow and side-shell slamming.   

Professor Chris Swan

Fluid Mechanics Section