Hydroelastic behaviors of large aircushion supported elastic floating structures in regular waves

Abstract

A pontoon type very large floating structure (VLFS) has elastic deflections in ocean waves. The deflection is larger than that of a semi-submergible structure. Thus, a pontoon type VLFS can be installed in the shallow water field enclosed by breakwaters. However, a semi-submergible one will be applicable to development of an offshore field. The authors have developed a pontoon type VLFS with an OWC (oscillating water column) type wave energy absorption system. This can be installed in the offshore field being relatively deep water. Such a VLFS can reduce not only the elastic deflection but also the wave drifting forces. However, it is very difficult to reduce the wave drifting forces effectively because the effectiveness of the reduction depends on the wave energy absorption. Therefore, the authors propose an air-supported type of VLFS. This idea has already been proposed. However, it was not a flexible structure. Such an air-supported structure allows transmission of many waves. Therefore, the wave drifting forces may not increase. In addition, the elastic deflection may decrease because the pressure distribution due to the incident waves becomes constant at the bottom of the structure, i.e. the pressure is constant in the same air cushion. We developed the program code for the analysis of the hydrodynamic forces on the VLFS with the air cushion. The potential flow theory is applied and the pressure distribution method is used for the analysis of the wave pressures. Zero-draft is assumed in this method. The pressures and volume change of the air cushion is linearized. In this paper, basic characteristics of the elastic deflections of the large air cushion-supported elastic floating structures are examined. We examine the hydroelastic behavior of the structures in not only head-on sea conditions but also oblique sea conditions. We confirm the effectiveness, and discuss the behavior of the waves around the floating structure. However, the examinations are only carried out concerning the hydrodynamic characteristics.