A model is suggested which is capable of providing to third order of approximation the drag, inertia and total integrated random forces acting, in the flow separation regime, on an isolated smooth vertical cylinder in directional seas. The model considers both deep water and finite depth conditions; adopts a procedure based on the Kitaigorodskii et al. self similarity shape hypothesis to indentify the surface frequency spectrum on decreasing depths, the corresponding spectrum on deep water being assigned; uses the Morison equation to obtain the sectional random forces from which the integrated ones are deduced in analytical form; specifies that equation by using the kinematics to third order of approximation already proposed by the authors; finally, gives the inline and transverse integrated random forces relevant to multidirectional random waves starting from the forces relevant to unidirectional random waves, a procedure which involves a directional spreading function in an independent frequency form being assumed. Some illustrative examples and observations on the time histories and frequency spectra of random forces are carried out and some comparisons are made with experimental data from other authors, the results obtained being quite satisfactory.
As is well-known, the prediction of the hydrodynamic forces on offshore isolated structures is an area of active research, owing to the importance for the design. The most significant theoretic and experimental investigations referred to the sectional random forces acting, in the flow separation regime, on an isolated smooth vertical cylinder on deep water in presence of unidirectional URW or multidirectional MRW random waves are summarized, among others, by Rebaudengo Landò et al. (1993). In general, the theoretic investigations are based on an extension of the Morison equation to random waves whereas the most recent field and laboratory experimental investigations are carried out in such a way as to provide the time histories of the forces which may be later on processed by different procedures.
