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Comparison of ISO and API Seismic Design Requirements for Offshore Structures

Bor-Feng Peng, Ph.D., P.E.Department of Structural Engineering, J. Ray McDermott Engineering, LLC.

Houston, Texas, USA

G Abdel Ghoneim, Ph.D., P.E.Det Norske Veritas (USA), Inc.

Houston, Texas, USA

ABSTRACT

This paper presents a comprehensive comparison of API and ISO

seismic design requirements for offshore structures. The selectedcritical API and ISO seismic requirements, which have significantimpact on the structural design of the offshore platform, includes thesimplified design response acceleration spectrum, the detailed

probabilistic seismic hazard analysis, the uniform seismic hazardresponse spectrum, the tubular member strength and stabilityrequirements, and the tubular joint strength requirements. An existing

platform had been selected to illustrate the stress ratio comparison ofthe platform per WSD and LRFD design approaches.

KEY WORDS: Seismic; Spectrum; PSHA; Offshore; Structure;ISO; API.

INTRODUCTION

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. Althoughthe newly updated API RP 2A-WDS (21st Edition) with supplement 3was issued in 2007, the seismic design requirements are generally thesame as those specified in the 20th edition of 1993. However, the

seismic design and analysis technology and resources have beenimproved significantly during the past decade with more advancedcomputer hardware and software. Another fact is that more offshore

platforms are recently designed at seismic-active locations due to long-

term worldwide energy demand.

The seismic requirements for offshore structure design in ISO 19901-2and 19902 are updated after 2004. The design concept applies the

LRFD approach which fundamentally considers the seismic riskuncertainties and probabilistic seismic hazard analysis with morespecific and detailed guidelines for engineers to follow. The strengthand stability requirements in the code check formulae for the design of

structural members and joints are revised based on the most updatedresearch and information data base. Recently, a comprehensivecomparison of the seismic guidelines in ISO and API (Chang, etc.,2005) had been carried out. It was shown that the dynamic base shears

and moments of three existing platforms would be similar by using the

simplified design response spectrum specified by ISO and API. Th

comparison of ALE requirements had been presented by Peng (2008) oChang (2005). In this paper, the design response spectra, the mor

detailed seismic hazard analysis procedures, and the code checformulae are addressed by comparing the requirements per ISO an

API. The results also provide the input to the ongoing updates of APRP 2A-LRFD in conjunction with the ISO 19901-2 and 19902. Thconservative load factors for hydrostatic pressures specified in th

LRFD methodology may be required to be carefully reviewed oredefined for application in relatively deepwater platform design.

SIMPLIFIED RESPONSE ACCELERATION SPECTRUM

The most widely used seismic input parameter for the seismic desigand analysis of offshore structures is the design response accelerationspectrum. For offshore platforms classified per ISO as seismic riskCategory 2 or 3, the simplified seismic hazard analysis procedure may

be followed. By the simplified seismic hazard analysis procedure i

ISO, the design response spectrum (or so called simplified desigresponse spectrum) can be estimated and defined based on the soil typethe location of the platform, and the type of foundation system (shallow

or deep foundation) in lieu of detailed probabilistic seismic hazaranalysis (PSHA). If the platforms seismic risk category is 3 or highethe detailed seismic analysis procedure and probabilistic seismic hazaranalysis shall be performed to derive the so-called uniform hazar

spectrum. The ISO and API simplified response design spectra with 5%of critical damping are compared as follows and shown in Figs. 1~2.

The ISO 1000-yr Response Acceleration Spectrum defines the spectra

acceleration SA as,

sec0.2Tfor)4.03( 2.0, += aaA SCTS (1

sec4.0T0.2for2.0,0.1, = TSCS avA

where T is the oscillator period; Sa,0.2 is spectral response acceleratiowith oscillator period of 0.2 seconds; Sa,1.0 is spectral respons

acceleration with oscillator period of 1.0 seconds; the coefficients Cand Cv are dependent of the site classification or soil type, the type othe foundation, and the spectral response accelerations Sa,0.2 and Sa,1.For deep pile foundation, Ca and Cv are dependent of soil type only

Note that Sa,0.2 and Sa,1.0 are provided in ISO Annex B for variouseismic regions around the world. From Fig. 1, it is shown that the ISO

Proceedings of the Nineteenth (2009) International Offshore and Polar Engineering Conference

Osaka, Japan, June 21-26, 2009

Copyright 2009 by The International Society of Offshore and Polar Engineers (ISOPE)

ISBN 978-1-880653-53-1 (Set); ISSN 1098-618

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