This paper presents a comprehensive comparison of API and ISO seismic design requirements for offshore structures. The selected critical API and ISO seismic requirements, which have significant impact on the structural design of the offshore platform, includes the simplified design response acceleration spectrum, the detailed probabilistic seismic hazard analysis, the uniform seismic hazard response spectrum, the tubular member strength and stability requirements, and the tubular joint strength requirements. An existing platform had been selected to illustrate the stress ratio comparison of the platform per WSD and LRFD design approaches.
The seismic design requirements specified in API RP 2A-WSD (2000) and API RP 2A-LRFD (1993) have not been updated for some years. In particular, the API RP 2A-LRFD was first issued in 1993 and had not been widely applied in the offshore platform design industry. Although the newly updated API RP 2A-WDS (21st Edition) with supplement 3 was issued in 2007, the seismic design requirements are generally the same as those specified in the 20th edition of 1993. However, the seismic design and analysis technology and resources have been improved significantly during the past decade with more advanced computer hardware and software. Another fact is that more offshore platforms are recently designed at seismic-active locations due to longterm worldwide energy demand. The seismic requirements for offshore structure design in ISO 19901-2 and 19902 are updated after 2004. The design concept applies the LRFD approach which fundamentally considers the seismic risk uncertainties and probabilistic seismic hazard analysis with more specific and detailed guidelines for engineers to follow. The strength and stability requirements in the code check formulae for the design of structural members and joints are revised based on the most updated research and information data base. Recently, a comprehensive comparison of the seismic guidelines in ISO and API (Chang, etc., 2005) had been carried out.
