Wave forces on a fixed, vertical circular cylinder calculated by using the Morison equation and the wave diffraction theory are compared with each other, and the wave force calculation for the transition from drag to inertia dominated region is studied. To calculate the Morison equation, the inertia and the drag coefficients obtained by Chakrabarti (1980) in the wave tank tests are used, and the depth integration of the Morison equation is then used to obtain the wave forces acting on the structure. Wave forces on large circular cylinders are calculated by using MacCamy and Fuchs I wave diffraction theory wherein viscous effects and wave nonlinearities are neglected in the theory. various forms of the Morison equations are examined and compared to study the extent the simplifications made in the wave force calculation will effect. The drag force and the inertia force components in the Morison equation are compared with each other to investigate their relative importance in the wave ranges. A scaling analysis of the Morison equation is derived to show the order of magnitude of the drag and the inertia force components. To show the compatibility of different approaches, the inertia force component of the Morison equation is also compared with the wave force calculated by the wave diffraction theory with or without the same wave phases. It is to study if the two different theories can compensate with each other and eventually can be combined together to be a unified formula which could include flow phenomena that have not been acounted for in the individual theory. It is shown that the drag force can be comparable to the inertia force only for D/L<0.01, otherwise, the inertia force is dominant and the drag force is only a small portion of the total wave force.

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