Abstract

Fifteen years after the installation of the Beryl Alpha Condeep platform, a programme to upgrade the topsides facilities was commenced. This upgrading increased the overall design weight of the topsides by some 17%. Due to the modular nature of the additional facilities, weight increases of several hundred tonnes each are applied at discrete points around the deck structure.

Prior to this upgrade, Mobil North Sea Limited (MNSL) Prior to this upgrade, Mobil North Sea Limited (MNSL) had commenced the development of reanalysis models of their structures, including the Beryl Alpha deck. A complete reassessment of the strength of the deck was undertaken using detailed finite element analysis and contemporary code checking methods. The methodology used in the checks combined the application of appropriate codes, recent theory, experimental results and both linear and non-linear buckling analyses. Whereas simpler techniques would have suggested a requirement for considerable stiffening, this methodology demonstrated that even the substantial additional loads from the upgrade could be carried with only minor and localised strengthening being required.

This paper discusses the methods used together with the approach adopted for the design of stiffening at minimal cost.

Introduction

Beryl Alpha was the first Condeep Production Platform and was installed in Block 9/13 in the UK Northern North Sea in 1975. It comprises a concrete gravity base substructure with 16 storage cells and 3 shafts supporting a steel deck which contains production and utility facilities within its depth. The deck also acts as a support platform for drilling equipment and accommodation units. The platform is operated by MNSL. Other co-venturers are Amerada Hess Limited, Enterprise Oil plc, B.G. North Sea Holdings Limited, and OMV (UK) Limited.

A major refurbishment and upgrade is underway to establish the platform as a petroleum production and export centre for block 9/13 and the surrounding areas of the North Sea. The refurbishment includes a rebuild and extension of the accommodation areas whilst the upgrade to facilities includes the SAGE gas export pipeline, an Additional Gas Compression Module pipeline, an Additional Gas Compression Module (AGCM) and the installation of a Riser Access Tower (RAT). The latter carries pipeline risers from nearby MNSL-operated fields, future third-party operated fields and also the SAGE Pipeline. Beryl Alpha is already used for the storage and export of crude oil produced by itself, Beryl Bravo and two subsea developments, Ness and Linnhe. Gas is presently reinjected pending completion of the SAGE facilities. The Beryl Alpha complex as extended is illustrated by Figure 1.

The platform was originally designed to carry a total topside load of some 29,100 tonnes which included an allowance of 1,200 tonnes for a possible additional module on the South West corner of the deck structure. The total additions in static load that have now been incorporated total some 6,300 tonnes.

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