This paper investigates random Froude-Krylov (FK) force on a rectangular structure. It is a key parameter in the design process of some maritime structures. Indeed, the exciting force on a large floating body is commonly determined by a contribution due to the incident wave field (FK) and by a contribution due to the diffraction of sea waves.

The work is based on results of a small-scale field experiment at NOEL (Natural Ocean Engineering Laboratory) in Reggio Calabria, Italy. First, field experiment is described, with characteristics of the selected sea states. Then, FK forces are analytically derived in the context of linear random waves. Frequency spectrum of the FK force is derived and it is discussed the occurrence of zeros in frequency domain.

Extreme FK forces are determined by Quasi-Determinism theory. The theory enables to derive the analytical expression of the FK force when a large wave (either a large crest height or a large crest-to-trough wave height) occurs at any given point of the wave field, in a fixed time instant. Time domain representation allows investigating the wave force and extreme wave pressure. It is shown that the wave force is highly width dependent in time domain. Further, time histories are not quasi-impulsive. This characteristic is well-rendered in large structures (large with respect to the dominant wave length), where the wave group crossing gives rise to a time history “protraction” in time domain.

In the last part of the paper theoretical results are supplemented by comparison with experimental data.

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