The interaction of sinusoidal waves with a large open bottom structure near the ocean floor is investigated. The wave diffraction problem is formulated within the framework of linearized potential theory and solved numerically on a digital computer. The analysis is confirmed by the results of wave tank tests on a hemispherical shell and underwater storage tank models. It is found that both the horizontal and vertical forces acting on an open structure are lower than those on the corresponding sealed structure, the vertical force, in particular, being an order of magnitude smaller over the practical range of wave periods. The inside pressure on the structure when slightly open correlates very well with the mean bottom pressure on the structure when sealed, so that the results for the slightly open case can be obtained from the results for the sealed case.
Increased offshore activities in recent years have led to the placement of a variety of structures on or near the ocean bottom. These structures are subject to surface wave forces in both the horizontal and vertical directions and in many instances must be held by piling in addition to their own weight. Frequently, a piled structure is scoured out at the bottom, leaving its underside exposed to the wave action. Indeed, for a shell-like structure the entire 'inside' will be opened to the wave action. The large Khazzan oil storage tanks (each one half million bbl in capacity), situated 59 miles offshore Dubai, fall into this category, for recent diver inspections have revealed sizable openings under these structures within the framework of linear wave theory this paper examines the effect of such openings upon the wave forces experienced by the Khazzan tanks and similar structures.
Different Green's function formulations of the potential problem describing the wave interaction with a solid body, defined by a closed surface, have been presented by John (5), Wehausen and Laitone (10), and Black (1), and numerical solutions for particular shapes have been obtained by a number of investigators (2, 4, 6, 9). In this study, however, we are primarily interested in the wave forces on a shell, defined by an open surface. The formulation of this problem is similar to that for a solid, except that the solution is contained in a singular integral equation, rather than a regular Fredholm equation. The numerical solution of such an equation has been carried out earlier for a hemispherical shell (8). Here, this solution is extended to several open bottom structures of practical design, and the wave forces are compared to those for the corresponding sealed structures. Wave tank tests conducted on models of these structures show good agreement with the analysis.
In general, the analysis indicates that the pressure inside a slightly open shell matches closely the mean pressure at the bottom of the corresponding sealed body, and this result is borne out by experimental tests on a slightly open hemisphere.
