Earthquake Ductility Study for Offshore Structures

Abstract

A two‐part study was conducted to help evaluate analytical methods used to assess the ductile behavior of offshore structures subjected to severe earthquake ground motion. The first part of the study was a comparison of analytical time‐history responses, predicted using the Inelastic Tower Response Analysis (INTRA) program, and experimental data gathered during a shake‐table experiment performed by the University of California at Berkeley in March 1981. (INTRA is a registered trademark of PMB Systems Engineering, Inc., and is owned by Shell Oil Co.) The experiment involved a series of tests on a scaled, two‐dimensional model of a prototype offshore platform subjected to various levels of simulated ground motion. The second part of the study consisted of a comparison of ductile behavior, predicted using simplified analysis methods (static pushover and impulse velocity), and ductile behavior demonstrated by the experimental and analytical time‐history results. First part results indicated that the analytical time‐history analyses predicted within reasonable accuracy the response of the test structure as measured during the experiment. The results of the second part demonstrated that the simplified methods produce conservative estimates of the test structure's energy absorption capacity and ductile behavior during intense ground motion.