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UBC Theses and Dissertations

Geotechnical considerations for offshore gravity type structures with emphasis on foundation stability under storm wave loading Gaard, Thomas C.


A thorough discussion of offshore gravity type structures presently being used, or considered for use in the near future by the oil industry, is presented, along with a brief summary of the major types of structures now used offshore. Factors affecting the stability of offshore gravity type structures are discussed, from the evaluation of a suitable site and the selection of soil parameters, through installation and short-term foundation safety. A case study of the Ekofisk tank is included to show how geotechnical concepts are applied offshore. A thorough description of wave loading on offshore gravity structures is presented, including a discussion on how the design storm is used in geotechnical analyses. Existing stability methods are reviewed. The merits and shortcomings of each method are discussed with respect to their application offshore. Procedures for analyzing the stability of offshore gravity type structures subjected to storm wave loading are developed based on the method of slices. Both Janbu's (1973) Generalized Procedure of Slices and Sarma's (1973) method are adapted for offshore analyses. The latter method is modified to perform pseudo-three-dimensional analyses. A computer program GRAVSTAB developed for this purpose is described and applied to several example problems. The versatility of the method of analysis is demonstrated and results are compared with existing methods.

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