scope:

INTRODUCTION

The American Petroleum Institute (API) has initiated a research project for the analysis of the response of offshore structures under earthquake loading conditions (API PRAC project 80-26A). The scope of the investigation entails three dimensional time history dynamic analyses of a typical offshore platform under design conditions comparable to American Petroleum Institute recommended practice (API RP 2A) earthquake design provisions for Southern California offshore environment.

The API RP 2A earthquake design provision provides for strength and ductility requirements. The strength requirements are intended to provide a platform, which is adequately sized for strength and stiffness to insure no significant structural damage for the level of earthquake shaking that has a reasonable likelihood of not being exceeded during the life of the structure. The ductility requirements are intended to insure that the platform has sufficient energy absorption capacity to prevent its collapse during rare intense earthquake motion, although structural damage may occur.

In the absence of a detailed site-specific seismic exposure study, the strength level earthquake is described through a normalized major horizontal component response spectra (soil dependent), an effective ground acceleration design coefficient (G-scaling factor) as a function of zone relative seismicity, and provisions for the orthogonal weaker horizontal and vertical components as a fraction of the specified major component. The major horizontal component (applied along a major axis of the platform) consists of the response spectrum (appropriate to the specific soil condition) shown in Figure 1 and scaled by a design coefficient appropriate to the various seismic zones presented in Figure 2. The minor horizontal component (acting in an orthogonal direction to the major axis of the structure) is taken as two-thirds of that assigned to the major horizontal component. A response spectrum of one-half the major component is assigned to the vertical component.

The ductility requirements are defined in terms of performance goals to insure that the platform has adequate energy absorption capacity to prevent collapse under a rare intense earthquake. However, for the present study, the ductility requirements will be defined in terms of response spectra having ordinates twice those defined for the strength requirement.

This report summarizes Woodward-Clyde Consultants' effort conducted for the API on this project to evaluate earthquake time histories appropriate to characterize design earthquake provisions of the API RP 2A strength and ductility requirements for offshore Southern California environment.