Since 1980, API has been developing the new Load Resistance Factored Design (LRFD) method for fixed offshore platform design. In contrast with the currently used API RP2A Working Stress Design (WSD) method in the LRFD method, platform loads and component strengths are modified by factors representing their statistical uncertainties. This results in a more uniform safety (reliability) for a wide range of load and load combinations and component types.
This paper summarizes the LRFD method development efforts sponsored by API since 1980. Results from sixteen API research projects performed to formulate the LRFD method and calibrate it against six fixed platforms in different water depths, locations and environments are outlined. Advantages of the LRFD method against the current API Working Stress Design (WSD) method are also discussed.
The development of the Load and Resistance Factor Design (LRFD) alternative to the RP2A1 was begun in 1980 with the impetus provided by Marshall, 2 Bea, 3 Stahl, 4 Fjeld, 5 existing DnV Rules,6 and the concurrent AISC work for the provision of an LRFD7 alternative to their current WSD code.8 Offshore platform design is ideally suited for probabilistic evaluation because the primary sources of loading are due to natural random processes. The oceanographic data base developed for establishing 100-year return period design events for the RP2A is directly adaptable to probabilistic evaluation. Furthermore, there is an extensive experimental data base for platform components (Table 1), In principle, the development of a revised guideline should be very simple, In practice, however, this development has been complex. The purpose of this paper is to explain the nature of the problems that have been encountered and provide some background that will help the reader better understand the provisions, the format, and the basis for the draft document. The draft RP2A-LRFD document will be issued later this year for industry review.
The draft LRFD document has been developed under the jurisdiction of what is now the API Subcommittee for Fixed-Systems Criteria. The initial RP2A-LRFD document has been prepared by an API Technical Advisory Committee (TAC-22) that has been overseen by the API Production Research Advisory Committee PRAC-22. Following the completion of this initial document, each of its sections has been reviewed and approved by the responsible API task group. This review and approval process is coordinated by the LRFD coordination task group setup by the Fixed Systems Criteria subcommittee and chaired by the second author. Professor Fred Moses, Case Western Reserve, has been the principal investigator throughout this project. His project reports are given in References 9 through 15. Subject to API approval, the draft for comment will be changed to an AP1-endorsed alternative recommended practice after its first release and review and acceptance by the offshore platform industry. At the present time, the API has not decided whether to maintain one or both documents following release of the LRFD alternative. Figure 1 provides the activity breakdown and the schedule of the RP2A-LRFD effort.
