The Design of the Ravenspurn North Concrete Gravity Substructure: An Innovative Application of Conventional Technology
Authors: Jackson G. , Bell T.A. 
Published: October 2008

This paper describes the concept and design methods employed for a concrete substructure to a central gas processing platform located in 42m of water in the southern North Sea. The structure was designed for complete construction and mechanical outfitting in a dry dock using conventional on-shore civil engineering construction methods.

The project has demonstrated the economic and technical viability of adopting established technology, which is readily available from the civil engineering industry as an alternative to the use of steel jackets for the support of offshore installations.

INTRODUCTION

The Raven spurn North Concrete Gravity Substructure (CGS) is illustrated in Figure 1. It was installed on 5th August 1989 in block 43/26 of the North Sea 80 km east of the Yorkshire coast. The function of the platform is to process incoming gas from a number of satellite wellhead platforms and to export it to shore via the neighboring Clifton facility. Future gas compression will be provided by a module which has not yet been installed on the third shaft. The CGS was a direct substitute for two steel jacket substructures and was selected largely on economic grounds.

The conceptual design of the CGS commenced in March 1987. In the following 14 months a complete detailed design was prepared, competitive construction tenders sought and the construction contract awarded.

Construction commenced on 1 May 1988. The Piper Alpha disaster in July 1988 resulted in the riser routing being switched from one shaft to another. External pipe work had thus to be redesigned whilst construction was in progress. The design of the concrete substructure, however, remained unchanged and modifications to the mechanical outfitting were able to be absorbed without affecting the completion date of the construction contract.

DESIGN REOUIREMENTS

The CGS was designed to fulfill requirements:Support safely all functional, environmental, transformational and installation forces.Provide sufficient on-bottom weight to ensure adequate factors of safety against foundation failure.All construction to be completed prior to float out in any of seven existing dry docks in the UK.Provide sufficient buoyancy and stability during float out, transportation, installation and removal.Provide on board systems which would remain operable .at the end of the platform's useful life to permit relocating for abandonment.

DESIGN APPROACH

At the outset of the project a number of the design problems were identified:The selection of an efficient structural form which would prove sufficiently light to enable the whole structure to be floated out from a dry dock;The determination of hydrodynamic loading on shafts with a heavy concentration of external pipe work;The manner in which load would be transmitted into the caisson from the shaft and the degree of pesters needed to meet serviceability and fatigue criteria in this condition;A means of justifying the design of the concrete slab elements subjected to in-plane loads and out of- of-plane moments,The development of a support system for external pipe work (risers, caissons and J-tubes) which was functionally flexible and would facilitate changes in configuration both during design development and after installation throughout the platform's operational life.