Application of API RP2A-LRFD to a North Sea Platform Structure
Authors: Thomas G.A.N. , Snell R.O. 
Published: October 2008

This paper is about the application of API RP2A-LRFD (Draft) in the North Sea. The preliminary design of a fried steel platform for BP's Andrew Field development had been performed to the 18th Edition of API RP2A. The platform was redesigned using LRFD, but using partial load factors applicable to North Sea conditions. The effect on total structure weight and the differing effect on various categories of structural element are summarised.

This paper also discusses the different partial load factors applicable to gravity loads when improved accuracy of weight control is effected, and applicable to environmental load when 50-yearor 100-year return period extreme environmental criteria are used in the North Sea.

To conclude, this paper makes the connection between the work reported here and the international effort to develop an ISO Standard for offshore structures, in which partial factors appropriate to regional conditions will feature.

INTRODUCTION

API RP2A is the primary worldwide design practice used for offshore structural design. Its use in Europe is supplemented by additional local regulatory requirements and performance criteria but the designmethodology, basic code checking equation sand safety levels for ultimate strength are primarily related to API RP2A.

The Working Stress Design version of API RP2A is Soon approaching its Twentieth Edition. However 1992 is expected to see the publication of the Load and Resistance Factor Design version as an AP1-approved alternative.

Inherent in the design methodology of API RP2A-WSD are the safety levels developed for US waters. The WSD methodology has been successfully applied largely without change within Europe and worldwide. But with the introduction of the LRFD methodology it has become possible to rationally account for the variance between US regional conditions and other localities. Partial factors are used to account explicitly for the variance of criteria, and can therefore be calibrated to provide more consistent safety levels.

This paper describes the work undertaken to derive partial factors for use of API RP2A-LRFD in European waters and to quantify the economic impact ofintroducing the LRFD format. Sets of partial load factors that can be recommended as a basis for design in the North Sea with API RP2A-LRFD First Edition are presented.

This paper does not discuss the very important but less quantifiable concepts of structural redundancy and the ability to redistribute loads from non-performing structural elements. These aspects assume increasing importance as element design is further refined. There results of this work will form a significant contribution to the work currently in progress to develop an International Standard for Offshore Structures which, for filed steel platforms, will be based on a development of API RP2A-LRFD.

API RP2A-LRFQ

In 1989, API published for public comment the Draft First Edition of API RP2A-LRFD (l). This was based onthe Eighteenth Edition of API RP2A but in place of the Working Stress Design method included a different design method (Load and Resistance Factor Design) for evaluating component strengths and the effect of platform loads.