Semi-submersible heavy transport vessels are used for transport of floating cargo. In general discharging takes place in sheltered and/or very benign areas, which are often not the areas of operation for these cargos. To be able to discharge in less benign areas, an R&D project was initiated to capture the problems arising at hydrodynamic interaction between two floating bodies in very close proximity. Comparison of model tests with industry standard 3D-diffraction analysis has shown large discrepancies on the motions on the vessel. Therefore, the first stage of this R&D project is to investigate the effect of the narrow gap between the loading deck and the bottom of the floating cargo, find a better analysis solution and incorporate this in a multi-body hydrodynamic interaction method. To be able to develop this method, first the single body problem in extreme shallow water is investigated. This problem was investigated by Drobyshevski [1], who assumed that a two-dimensional flow describes the flow in the narrow gap, being the under-bottom domain, for which better solution methods are available than 3D-diffraction method. His method is tested and validated with a truncated vertical cylinder with flat bottom in extreme shallow water. Since the Asymptotic Matching Program of Drobyshevski does not include 3D-diffraction for arbitrary shaped structures and is only applicable for single bodies, the new developed Multi Domain Diffraction Method is a combination of two-dimensional flow and general 3D-diffraction, both based on linear potential theory.

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