Towards the Worldwide Calibration of API RP2A-LRFD

Abstract

ABSTRACT API RP2A-LRFD[l] is soon to be adopted by API for the design of fried offshore steel platforms, and will be available for use worldwide. The paper describes the work under taken to adapt LRFD for use in the North Sea. Sets of partial safety factors have been calibrated to suit specific environmental conditions, and design and operating practices using a reliability-baaed optimisation procedure. INTRODUCTION API RP2A[2] (henceforth denoted RP2A-WSD for Working Stress Design) forms the basis of steel jacket design throughout the world. It is widely used and has proved a popular design standard since fiit issued in October 1969. API have continually revised and updated it as a matter ofpolicy to include both new information and research, and to reflect current trends. RP2A-WSD is now on its 19th Edition. Throughout most branches of structural engineering theinternational trend in design standards is to move from the working stress, or permissible stress, design format, typified by RP2A-WSD, towards a multiple factor format with partial safety factors applied to both loads or loading effects, and toresistances. By careful adjustment or calibration of the partial factors to reflect the uncertainty and variability in the different types of loads and resistances a much more consistent level ofcomponent reliability can be achieved. The partial factor format also offers more opportunity for influencing and improving design. In 1979 API set up the PRAC-22 Committee to develop a partial factor format for RP2A; their work is reported inreference [3]. Following considerable development a draft Recommended practice 2A-LRFD was finally published in December 1989 and has been available for comment. The two year review period has now ended and RP2A-LRFD is soon expected to be issued as an API Recommended Practice. In common with RP2A-WSD, LRFD has been written anddeveloped for use specifically in the Gulf of Mexico, the waters off Alaska, and the Pacit3cand Atlantic seaboards of the USA. When RP2A has been specified for design use in other areas of the world it has been subject to regional modifications and supplemented by additional guidance. When LRFD is internationalised it is hoped and intended that regional modification will be incorporated into Annexes for specific areas. In addition to area specific guidance, modified partial factors may be included which account for regional differencein environmental conditions, and in design and operating practices. This ability to tailor the safety factors to suit specific conditions is a major advantage over the working stress approach and should have significant benefits for platform design and the internationalisation of RP2A-LRFD. The calibration of the partial factors is undertaken using a reliability-based procedure. The general principles of the code calibration procedure are widely accepted. Firstly a set of one or more target reliabilities are assessed from a range of representative components which have been designed to an swept.ed code (usually the previous WSD version).