Response of Concrete Gravity Platforms to Earthquake Excitations

Abstract

The earthquake response of typical concrete gravity structures in 100 to 200 m of water was evaluated. Response spectrum and fast Fourier-trans-form solution techniques were used. Parametric studies showed that the analysis method markedly affects magnitude and distribution of computed foundation and structural forces and that correct modeling is essential. Introduction One specialty session at the 1975 Seventh Annual Offshore Technology Conference was about offshore earthquake engineering. Few state-of-the-art presentations then addressed the response of concrete gravity presentations then addressed the response of concrete gravity platforms to earthquake influences. This is not surprising platforms to earthquake influences. This is not surprising because the first production platforms of this kind were installed in the North Sea only in 1975 and to the author's knowledge, none have been ordered yet for an offshore region characterized by a high earthquake risk. Among the chief advantages of the concrete gravity platform are its potential for carrying payload to location platform are its potential for carrying payload to location and the limited offshore work required. In a short-installation weather window like that of the North Sea, these advantages only now are appreciated fully. The Gulf of Alaska has weather characteristics similar to the North Sea, but also has a high earthquake risk. This paper presents part of an over-all investigation into the presents part of an over-all investigation into the feasibility of constructing and installing concrete gravity platforms in the Gulf of Alaska. The investigation was platforms in the Gulf of Alaska. The investigation was conducted from Sept. 1974 through April 1975. When the study began, there were sound geological reasons for anticipating suitable foundation conditions at or close to the mud line; however, only limited numerical data existed. Objectives One can assume that the intensive nuclear power development program in the 1960's generated ample means to evaluate earthquake problems of massive concrete structures. While this is undoubtedly true with respect to understanding the basic phenomena and developing methodology, certain factors must be remembered. First, the mass and geometry of a typical gravity production platform are significantly larger than those of a nuclear platform are significantly larger than those of a nuclear reactor. Second, the superstructure of a platform is relatively flexible. Third, the inter-action with the surrounding water represents another complication in a submerged structure. Finally, the emphasis is on the detailed evaluation of a particular site in a typical nuclear-reactor design, whereas this Gulf of Alaska study was geared more to identifying sites that would be satisfactory from all viewpoints. For these reasons, this study was considered only preliminary and emphasis was placed on the following objectives:to assess the importance of foundation compliance, added mass effect from the water, damping behavior, and method of analysis,to quantify typical design forces in the structure and its foundation, andto assess the design implications for the major parts of the structure. These objectives were accomplished within 6 months. JPT P. 318