This paper aims at experimental and analytical investigation on diffraction forces acting on a spherical structure and wave diffraction. The Source Distribution Method is applied to their theoretical estimation. The effective inertia coefficient is affected by the relative diameter and submergence of the sphere as well as the diffraction parameter. The non-dimensional wave height, however, has little effect on it. The Source Distribution Method based on the linear wave theory is revealed to evaluate well the diffraction force acting on a spherical structure due to the finite amplitude waves including the spilling breakers above the submerged sphere. The Source Distribution Method is useful also in evaluating the wave height distribution around the sphere under non-breaking condition.
With increasing utilization of coastal and ocean areas, many coastal and ocean structures will be constructed in these areas. Structures with spherical shapes, like oil storage tanks, are treated here, which are considered to be one of the most suitable shapes for coastal and offshore structures against wave pressure. In designing such structures, it is necessary to accurately evaluate wave forces acting on them. The Morison equation is useful in estimating the acting wave force on a structure whose size is much smaller than the incident wavelength. If the structure is large, the Morison equation becomes rather useless and the diffraction theory should be introduced in estimation of wave forces. The number of researches conducted on the diffraction force acting on the spherical structure is less than that on the cylindrical structures. Havelock (1954) discussed the diffraction force acting on a submerged sphere under deep water wave condition. His theory, however, was derived without considering the free surface boundary condition. As a consequence, the effects of the bottom and free surface boundaries on the acting wave force cannot be evaluated.