A Comparative Study Using API RP2A-LRFD

Abstract

ABSTRACT This paper assesses the benefits offered by API RP2ALRFD. With reference to the Goodwyn 'A' platform, it contrasts the requirements of the draft LRFD version of RP2A with those of the current version and those of NPD. It demonstrates that improvements in reliability offered by API RP2A-LRFD can be achieved without significant change in structure weight. 1.0 INTRODUCTION 1.1 GENERAL The issue for industry review, just last December of a 'Load Resistance Factor Design' version of API-RP2A (Ref I) marks a major milestone in the history of the API Recommended Practice for Fixed Offshore Platforms. Since it was first issued in 1969, the working stress code RP2A (Ref 2) has become an internationally accepted standard for fixed platforms. The issue of a load resistance factor design version reflects the worldwide trend away from working stress design to reliability based methods. This paper presents a comparative study using API RP2ALRFD. As a reliability based alternative, the load resistance factor design (LRFD) method offers a more consistent approach to platform safety than does the existing RP2A working stress design (WSD) method. The study is directed to assessing the benefits of RP2A-LRFD. In the .context of the Woodside Goodwyn'A' platform, comparisons are drawn between RP2A-LRFD requirements and those of RP2A-WSD and the Norwegian Petroleum Directorate (NPD). The work is confined to the in-place strength of the jacket and module support frame (MSF). The study is based on work performed for Woodside as part of the substructure design for the Goodwyn 'A' Platform. The reliability of the Goodwyn 'A' substructure has been a major focus of the design and results are presented showing the platform reserve capacity, computed using non-linear inelastic ductility analyses. 1.2 RP2A - LOAD RESISTANCE FACTOR DESIGN One of the great merits of the existing API RP2A, now in its 18th edition, has been the way that sound engineering practices have been presented in a flexible manner such that the designer's judgment and creativity are not inhibited. This is achieved by providing a basic framework for design, accompanied by guidance on problem areas lying outside this framework. These same hallmarks apply also to the RP2ALRFD draft. Although fundamental differences exist between the WSD and LRFD versions, the two documents remain remarkably similar with the LRFD draft containing most of the carefully phrased clauses which are familiar to many practicing engineers. LRFD however, is an improved approach to structural design which takes into account the possible uncertainties in the applied loads and component resistances. The designation 'load resistance factor design' derives from the concept of factoring both loads and resistances as opposed to the working stress design approach where only the resistance is divided by a factor of safety.