A Practical and Interdisciplinary Framework to Develop Post-Earthquake Evacuation and Inspection Thresholds for Offshore Platforms

Abstract

Abstract After a strong earthquake occurs near an offshore platform, taking timely actions, such as evacuation of personnel and inspection of the platform structure, is important to minimize the potential for loss of lives. The objective of this paper is to present a practical framework for developing post-earthquake inspection and evacuation thresholds based on information that will be available immediately after the earthquake, such as an estimate of earthquake magnitude and epicenter location from the United States Geological Survey (USGS) website. Assessing the structural performance of an offshore platform in a strong earthquake is truly an interdisciplinary effort. Seismologists and geologists review the regional seismo-tectonic environment in the vicinity of a platform and characterize the platform's seismic hazard at the bedrock or competent soil level using ground motion prediction equations (GMPEs), commonly known as attenuation relationships, in a process called Probabilistic Seismic Hazard Analysis (PSHA). Geotechnical earthquake engineers then perform a nonlinear site response analysis to capture the local soil effects and develop the seismic ground motion time histories and response spectra that will shake the platform at its foundation. Structural engineers then perform seismic structural analyses to assess the platform response under the earthquake excitations and evaluate its performance for the life safety of the personnel who will need to ride out the earthquake because its timing cannot be predicted in advance. This interdisciplinary effort can be expanded beyond examining structural response to ground motions to develop a tool for post-earthquake response, the seismic action chart. Seismic action charts showing evacuation and inspection thresholds have been developed for a number of offshore platforms in seismically active regions around the world, such as California, Indonesia, and Trinidad and Tobago. These seismic action charts have been used successfully to date without any known issues. To demonstrate the concept and application, the development of a seismic action chart using the Next Generation Attenuation (NGA) relationships and subduction zone attenuation relationships for an example platform is presented in this paper. The evacuation and inspection thresholds outline three distinct regions on the earthquake magnitude vs. epicentral distance chart. Below the inspection threshold, no action is required. Between the inspection and evacuation thresholds, targeted inspections of critical structural members and joints based on seismic analyses are recommended as soon as practically possible. Above the evacuation threshold, immediate and safe evacuation of platform personnel is advisable.