Development and Implementation of Code Check Methodology for Plated Steel Structure

ABSTRACT A codecheck methodology, suitable for checking the behaviour of stiffened steel plate structures, is proposed in this paper. The methodology adopts the most appropriate, complementary aspects of existing codes, supplemented by pertinent recent research. A case history of the Beryl Alpha platform is presented. This demonstrates the benefit of applying the methodology to the post processing of a finite element analysis of the deck. With minimal, but effective stiffening, an overall 17% increase in deck load is now carried. The additional capacity was used for safety enhancements and improvements necessary to extend the platform's economic life. INTRODUCTION Although code checking of framed and simple plated structures (e.g. large diameter stiffened tubulars) is normally highly automated, the opposite is true of complex stiffened plate structures. Such construction is commonplace in the offshore industry; it is featured in modules, plate girders, semi-submersibles, jack-ups, TLPs and tankers/FSUs. The behaviour of these structures is less well understood thanthat of framed structures due to the great variety of possible load paths and failure mechanisms. Development of a unified set of rules describing the behaviour of the components of complex plated structures has yet to be concluded. This leaves the offshore industry in an unenviable position. At a time when considerable attention is being paid to the intrinsic safety of such structures, the codes of practice are lagging behind the state-of-the art knowledge embodied in recent research. To overcome this limitation, it is possible to combine the best features of existing codes/rules/standards together with the published or even unpublished results of this research. The code check methodology described in this paper is justsuch a combination. It covers strength, serviceability and fatigue limit states and is particularly suited to the post processing of finite element (FE) analyses. One particular example of stiffened plate construction is the deck of Beryl Alpha. This platform, installed in 1975, comprises a three legged concrete substructure supporting the topsides structure and equipment. The platform is operated by Mobil North Sea Limited (MNSL); the other coventurers are Amerada Hess Limited, Enterprise Oil plc, B.G. North Sea Holdings Ltd and OMV (UK) Ltd. Due to the continuing research and development of various codes of practice and improvements in FE analysis capability since the Beryl Alpha design, MNSL believed that there wasconsiderable scope for increasing the allowable deck load on the platform. A number of upgrades to the platform were planned between 1988 and 1992 and consequently an additional 5,000 tonnes have now been added. The integrity of the structure under this new loading was demonstrated using detailed FE analysis and the above codecheckmethodology. This paper is organised with a review of formal codes and rules followed by the development of appropriate strength and buckling check methodologies and their computerimplementation. A description of the application of the techniques to Beryl Alpha and a discussion of the results complete the paper.