Seismic Exposure and Site Response Characteristics for Offshore Platform Design

Abstract

Abstract Members of Panel 5 (P5) of the International Standards Organization (ISO) Working Group 3 (WG3) have developed earthquake design criteria for inclusion in 1S0's design guidelines for fixed offshore platforms. One of the components of these criteria was a procedure to determine the shape and level of the elastic design spectrum to be used in the computation of the design earthquake loads. The shape of the spectrum depends not only on the local geology at the platform site but also on the seismotectonic zone of the site region. The level of the spectrum depends on the seventy of the ground-motion seismic hazard. Introduction Various working groups and technical panels under the 1S0 have been working during the last several years to develop guidelines for the design of fixed offshore platforms worldwide. Several panels were convened to address aspects of earthquake resistant design including:seismic exposure and site response characteristics,structural and foundation design considerations,operation and performance issues, andprobability based earthquake load and resistance factor design concepts. The members of WG3/P5 were responsible for developing the earthquake loading and response criteria within the framework of the Load and Resistance Factor Design (LRFD) approach, which has also been incorporated into the API RP2A guidelines for fixed offshore platform design.' A summary of the earthquake LRFD guidelines developed for the initial edition of the 1S() guidelines can befound in a paper by Bea3, published in these proceedings. A necessary component of the earthquake guidelines was the determination of the earthquake loads, which involved specification of elastic designs spectra. The procedure to specify the spectra is the subject of this paper. Background A primary ingredient in earthquake resistant design of offshore platforms is the specification of elastic design spectra that are used to calculate the earthquake loads. In the API RP2A guidelines' a simple procedure is provided to obtain these spectra in lieu of a site-specific study. According to these guidelines, the only parameters required to obtain the 5% damped elastic design spectrum for a particular offshore site are the local soil type at the site and the seismic zone in which the site is located. The local soil types in the API RP2A guidelines are (1) bedrock (Soil Type A), (2) shallow stiff soil (Soil Type B), and (3) deep stiff soil (Soil Type C). A site dependent spectral shape is presented in the API RP2A guidelines for each of these soil type. Seismic zonation maps for the offshore areas of the United States are also provided in these guidelines. Zones are designated as O, 1, 2, 3, 4, or 5, where Zone O represents negligible seismic hazard (e.g. Gulfof Mexico) and Zone 5 represents the highest seismic hazard (e.g. Gulf of Alaska). The API RP2A guidelines provide a table listing the ground accelerations for these zones, which range from O(Zone O) to 0.40g (Zone 5) in uneven increments.