Abstract

A silnificant upgrade to the topsides facilities of the Beryl Alpha platform has recentiv been undertaken by Mobil North Sea Limited (MNSL). Structural reassessment of the deck structure was performed to demonstrate its continued integrity under the resultant increase in topsides loads. Previous papers [1, 2] describe how this was achieved with only minor strengthening. The reanalysis has now been extended to incorporate an assessment of the concrete gravity base substructure which is of the Condeep design. MNSL's purposes for undertaking this reanalysis were to:

  • reassess the structure taking into account the platform's operational history and the increased deck loads:

  • review the implications of extending the operational life:

  • investigate how the reinforced, prestressed concrete design performs against contemporary rules and codes:

  • provide a basis for future inspection:

  • provide a rapid reanalysis capability in the event of accidental damage or freak environmental loads.

Many aspects of the substructure design were evaluated, thus providing a unique opportunity to review the performance of a Condeep and its foundation after eiqhteen years of operation. The analysis included the development and validation of hydrodynamic and all other loads. A particular feature was the integrated use of both a simple skeletal simulation of the platform and a detailed finite element model. Extensive concrete code checking has been performed to a number of limit states using specialist software.

This paper presents the philosophy employed in, and discusses the preliminary results of, this analysis.

Introduction

Beryl Alpha, the first Condeep Production Platform, was installed in Block 9/13 of the United Kingdom Continental Shelf in 1975. It is illustrated in Figure 1 and comprises a concrete gravity base structure, itself consisting of sixteen storage cells and three shafts, supporting a steel deck which contains production and other facilities within its depth. The deck also supports the drilling-equipment and accommodation. Beryl Alpha is operated by MNSL: their co-venturers are Amerada Hess Limited. Enterprise Oil plc. BG North Sea Holdings Limited and OMV (UK) Limited.

A major programme of refurbishment and upgrade has been undertaken to establish the platform as the hydrocarbon production and export centre for Block 9/13 and the surrounding areas of the North Sea. The modifications have resulted in an increase in topsides load of 6,300 tonnes to a total of 34,200 tonnes. This weight increase has been concentrated in the South West and South East corner and along the East face of the platform. A comprehensive reanalysis of the deck structure was undertaken to satisfy both MNSL and the Certifying Authority of the structure's integrity [1, 2] under the revised loading.

The logical extension of the previous work was to undertake a reanalysis of the substructure and its foundation to confirm their integrity. Unlike the deck, no suitable model was available which could be modified to accommodate MNSL's analysis requirements, therefore two new FE models were developed and analysed using the ASAS program:

  • a skeletal representation suitable for examining and understanding the global behaviour and response of the structure:

  • a detailed finite element model for ascertaining linear elastic stresses for subsequent use in code checking. This incorporated considerable detail and the resultant model was large, necessitating extensive use of component or substructuring methods.

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