The uncertainty of theoretical predictions of hydrodynamic forces based on Morison model may be easily determined. If inertia and damping coefficients are correctly selected, the wave forces over small structural elements can be accurately estimated. The model predicts well the hydrodynamic loading, except in an intermediate region where the Keulegan-Carpenter number is between 8 and 25. This region corresponds to the situation in which both hydrodynamic coefficients are similar and strongly dependent from flow turbulence and surface roughness. The correlation between our theoretical predictions and experimental measurements earlier obtained in wavetank (see Mendes et al. [3]) reflects, inevitably, important scale effects. From the above considerations it appears convenient to conduct a deeper theoretical analysis of the fluid-structure interaction, based on higher-order wave theories, aiming to put in evidence the real non-linearities in extreme sea conditions. We must equally keep in mind that there is an important variation of the immersed parts of the structure at free-surface level, which implies a variation of water-particle kinematics at each structural element position. The computational model must also enable the possibility of including different hydrodynamic coefficients associated with each structural element, as a function of depth and age of the platform. The effect of marine fouling over the structure, consolidated by time, is in fact very important. There are very complete databases of C, and C,, based on laboratory experiments. However, some care must be taken on the extrapolation of these coefficients to the case of a real structure operating at sea. Useful hydrodynamic coefficients for such a situation can be selected from data collected in situ through the years, as a function of KC and Re.

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