Seismic performance of offshore concrete gravity platforms

The seismic performance of concrete offshore gravity platforms is investigated. Aspects of dynamic structural analysis are discussed with particular reference to offshore gravity platforms, including soil-structure and water-structure interaction. Results of a seismic hazard analysis using currently available information are presented, and appropriate acceleration response spectra for design purposes are suggested. Site response effects are examined to investigate whether seismic hazard analysis results could be adjusted in a rational manner to account for the response of flexible nonlinear foundation materials. Artificial earthquake records are discussed and a suite of artificial accelerograms are generated to match the suggested design acceleration response spectra for offshore platforms in New Zealand. An experimental testing programme was undertaken to examine the strength and ductility characteristics of circular hollow reinforced and partially prestressed concrete members. Six columns were tested, representing scale models of members which could be used in the critical regions of possible offshore concrete platform configurations. A companion theoretical study was carried out to examine the strength and ductility of circular hollow sections. Results are presented in a design chart form suitable for use by designers. Finally, preliminary design studies were carried out for two different concrete gravity platform types to assess their likely feasibility. Of particular interest was the required distribution of strength to resist moderate earthquake effects and the ductility demands during severe earthquakes