Response Of Concrete Gravity Platforms To Earthquake Excitations

Abstract

ABSTRACT The earthquake response of typical concrete gravity structures in a 100 to 200m water depth range was evaluated using response spectrum and Fast Fourier Transform solution techniques. A series of parametric studies showed:The analysis method has a marked effect on magnitude and distribution of computed foundation and structural forces.Correct modeling of the foundation compliance, especially radiation damping, is essential.At the lower end of the soil stiffness range, foundation instability governs and at the upper, structural strength. INTRODUCTION One of the specialty sessions at the 1975 OTC was dedicated to the subject of earthquake engineering offshore. It was evident from the state of the art presentations at that time, that few if any investigations had addressed the question of the response of concrete gravity type platforms to earthquake influences. This is hardly surprising since the first production platforms of this kind were installed in the North Sea in the summer of 1975 and to the authors' knowledge, none has yet been ordered for an offshore region characterized by high earthquake risk. One of the chief advantages of the concrete gravity platform is its potential for carrying payload to location and the very limited amount of offshore work required. The advantage of this in a short installation weather window, like that of the North Sea, is only now becoming fully appreciated. The Gulf of Alaska has similar weather characteristics to the North Sea but also is associated with a high earthquake risk. This paper presents a part of an overall investigation into the feasibility of constructing and installing concrete gravity platforms in the Gulf of Alaska. This was conducted from September 1974 through April 1975. At the time the study commenced, there were sound geological reasons for anticipating the presence of suitable founding conditions at or close to the mudline but only limited numerical data existed. OBJECTIVES It would be logical to assume that the intensive nuclear power development programme in the 1960's would have generated ample means for evaluating earthquake problems experienced by massive concrete structures. While this is undoubtedly true with respect to an understanding of the basic phenomena and the provision of some of the methodology, certain factors must be remembered. In the first instance, the size in terms of both mass and geometry of a typical gravity production platform is significantly greater than that of a nuclear reactor installation. Secondly, the superstructure of the former is relatively flexible. Thirdly, the interaction with the surrounding water represents a further level of complication in the case of a submerged structure. Finally, in a typical nuclear reactor design situation the emphasis is on the detailed evaluation of a particular site whereas the type of investigation undertaken in this Gulf of Alaska study was geared more to the identification of which type of site would be satisfactory from all view points.