Probability Based Earthquake Load & Resistance Factor Design Criteria For Offshore Platforms
Authors: Bea Robert G. 
Published: October 2008

Abstract
This paper describes a probability reliability based formulation to determine earthquake Load and Resistance Factor Design (LRFD) parameters for conventional, steel, pile supported, tubular membered platforms that is proposed as a basis for earthquake design criteria and guidelines for offshore platforms that are intended to have worldwide applicability. The formulation is illustrated with application to platforms located in five areas: offshore California, Venezuela (Rio Caribe), the East Coast of Canada, in the Caspian Sea (Azeri), and the Norwegian sector of the North Sea.

Introduction
Approximately 100 conventional, steel, pile supported (template-type) platforms have been installed in high activity earthquake regions such as offshore California, Alaska, New Zealand, Japan, China, and Indonesia. New areas in high activity earthquake regions such as offshore Venezuela, Trinidad, Indonesia, and the Caspian Sea are under development.
A Seismic Design Methodology (SDM) should represent a readily applied process and set of parameters that will guide the engineering of a platform system to have acceptable performance characteristics during its intended lifetime12. The primary objectives of a SDM are to assure that the platform system will have sufficient strength and ductility to satisfy its intended purposes without undue expense or risk2. The designer must seek an appropriate balance between costs and safety.
The primary concern of the platform designer is with the response that develops after first significant yielding occurs in the structural and foundation elements that comprise theplatform structural system, and the damage states that can led to collapse of the platform. The engineer's objective is to provide an acceptable degree o! safety against undesirable performance during intense seismic events.
During the past 20 years, advanced earthquake design guidelines have been developed by the American Petroleum Institute (API) for steel, template-type offshore platforms.45 This development has had a substantial element of its background founded in the framework of prob.. bilistic seismic exposure and reliability based design and decision .m analysis methods. 67@ The International Standards Organization (1S0) has undertaken an effort to translate the API seismic design guidelines to an international standard bas d on an LRFD approaches8 This development has introduced several new aspects to the API approach. The first is the need to define consequence based criteria that will enable an appropriate balance to be developed between the reliability required, for a platform and the potential consequences associated with undesirable performance of the platform. 9–12 The second is the need to prescribe a methodology that would have global applicability to different soil and geologic conditions, seismotectonic regions, ad platform elements. The third is the need to prescribe a methodology that is information sensitive and capable of accounting for changes in the biases and uncertainties in platform elements and site specific characteristics. 12–14 The approach that is proposed and illustrated in this paper addresses these needs.

Probability Based Methodology
The probability based earthquake LRFD procedure is outlined in Fig. 1. The procedure is based on the API LRFD formulation for design of the elements that comprise offshore platforms: 4,7