This paper aims to establish a systematic estimation method of wave forces acting on a submerged sphere that covers both small and large spheres. The applicable range of the diffraction theory is shown, where the linear diffraction theory is revealed to be an useful estimation method. For the estimation of wave forces on the small sphere, the Morison equation and modified Morison equation are discussed and their applicable ranges are given graphically. Also, this paper shows that the Weibull distribution is useful in evaluating the vertical wave force significantly affected by the irregular lift force, for which no estimation method has been available.
The authors of this paper have discussed the wave forces acting on a submerged sphere with large and small diameters (Iwata and Mizutani, 1989, 1990, 1991, Iwata et aI, 1989, 1990, Mizutani and Iwata 1992). They showed that wave forces are governed not only by the wave height and period but also by the relative diameter to the wave length, submergence, distance from the bottom and free surface boundaries and so on, because wave diffraction, the bottom and free surface boundary proximity effects, and the lift force due to the irregular vortex shedding affect the wave force. Owing to these effects, it seems difficult to derive a general formula to estimate such wave forces. However, from an engineering viewpoint, the establishment of a systematic estimation method of the wave force is required. Furthermore, past research fails to address the effects of irregular vortex shedding in evaluating the vertical wave force. First, the range in which the diffraction theory is available is defined based on both the numerical and experimental results. Next, the applicable, ranges of the Morison equation and alternative equations taking the boundary proximity effects into account are discussed.