Diffraction of ocean waves around a hollow cylindrical shell structure

In recent years, there has been renewed interest in problems of diffraction and radiation of ocean waves around structures, in relation to “green” power generation by Oscillating Water Column (OWC) devices. In this paper we present a first-order analytical solution for the diffraction of ocean waves around a hollow cylindrical shell structure suspended in an ocean of finite depth. By revisiting work done by Garrett (1970) on the problem of a bottomless harbor, but adopting a different and more direct method, we obtain the solution for the diffracted wave potential. Using the new approach, we analyze the dependence of the solution upon various parameters, as well as the rate of convergence of the series solution. Apart from some problems we observed with matching the boundary condition at the edge of the cylinder, we find good agreement with Garrett’s results. Furthermore, we analyze the accuracy of the solution as a function of cylinder submergence. Finally, we briefly discuss the extension of the method to the related problem of radiation of surface waves by an oscillating surface pressure inside a hollow suspended cylindrical shell structure. The results presented in this paper show that even a simple hollow cylinder, which only captures the most essential feature of an OWC, can produce very complicated patterns of diffracted waves. This clearly demonstrates the complexity associated with using OWC devices to convert ocean wave energy to electricity and the necessity of further fundamental research needed in this area before we can realistically adopt this new technology to efficiently produce renewable energy in practice.