Innovation in Concrete Gravity Substructures: The Ravenspurn North Platform and Beyond
Authors: Roberts J. 
Published: October 2008

The Raven spurn North Concrete Gravity Sub-structure (CGS) is the first of a new type of platform installed in the North Sea. The concept was selected because it was the most economic solution. Construction of the sub-structure was completed entirely within the dry dock using conventional civil engineering technology and readily available materials. The CGS was towed directly from the dock and installed.

Subsequent design development has shown that the concept is applicable to CGS' s in water depths ranging from 20m to 150m.

INTRODUCTION

The largest concentration of deep water concrete sub-structures for oil and gas production is in the central and northern North Sea. To date nineteen have been installed. In the early 1970's the main reason for adopting concrete was that it made possible the mating of the sub-structure with the completed deck inshore and so minimized offshore hook-up and commissioning costs. At that time the capacity of offshore crane age was restricted to approximately 1,000te and therefore a large topsides required the separate lifting of many relatively small modules.

Several of the concrete gravity sub-structures were designed to store oil within the structure. The early 1970s was a period of unprecedented high demand for large platforms in the North Sea and the adoption of concrete was also seen as a means of reducing pressure on steel fabrication yards.

This first generation' of concrete gravity substructures (CGS's) was a technically elegant, logical response to the particular set of technical and economical circumstances which prevailed at that time. However CGS' s were not adopted for the shallower water fields in the southern North Sea. Here separate steel jackets could be provided economically to support functionally discrete topsides equipment.

In Europe the first recognition that a eGS could be an economically viable alternative to a pile steel jacket for shallow water applications was the installation in 1983 of two small monotones CGS's in the Schwedeneck-See area of the Baltic for Texaco Winter shall (Reference 1). These unmanned platforms have small topsides and were installed offshore in water depths of 15m and 25m respectively. The reason for choosing a concrete sub-structure for this field development was greatly influenced by the need for the sub-structures to resist the loading from sea ice.

In 1986 Hamilton Brothers Oil & Gas Limited were considering development options for the Raven spurn North field in Block 43/26 of the southern North Sea. After carrying out a number of feasibility studies, Hamilton Brothers realized that if the decks of a CGS were installed offshore its size would be substantially reduced (Reference 2). If a deck is mated with a concrete sub-structure inshore a significant proportion of the structure is required only to provide adequate floating stability during tow out and installation. Conversely if offshore deck installation is adopted the substructure is likely to become sufficiently small to be built entirely in a dry dock. Together these factors led Hamilton Brothers to the conclusion that a eGS was the most economic solution for their central gas process1ng platform.