Abstract

A study is presented of a new concept for a reinforced concrete Gravity Base Structure (GBS). The primary analysis is for a 120m water depth, but the effect of 80m and 100m water depths on the design is also examined. Iceberg design loads are derived for the particular geometry of this GBS concept, using a statistical analysis based on the latest research. The GBS is designed to the Canadian Offshore Code1,2 and a constructability/cost analysis is presented. The paper concludes that new developments in the knowledge of iceberg loads combined with an innovative concept have led to a more economical GBS design for the Grand Banks of Newfoundland.

Introduction

Reinforced Concrete GBS platforms, as the name implies, depend on the forces of gravity (self-weight and ballast weight) to resist environmental loads. The GBS rests on the sea bed and can be tailored to a variety of soil conditions. The main function of a GBS is to support the topsides structure, which houses drilling, production and processing facilities for oil and/or gas development. The topsides structure is connected to the GBS at an inshore location, enabling hook-up and completion in a controlled and economical manner. Heavy lift cranes are not required for connecting the topsides to the GBS; their functions are replaced by ordinary pumps and nature's own principles. The completed GBS and topsides are then towed to the field using buoyancy from air filled compartments in the GBS. Later, the buoyancy cells may be used for oil storage, eliminating the need for a separate storage structure or export pipeline.

GBS platforms have a good track record for supporting offshore oil and gas facilities in water of moderate depths. In operational service, the GBS has proven to be a robust structure able to resist accidental loads from ship impacts, dropped objects, sea ice and fires. The concrete structure is practically maintenance-free and existing platforms have shown excellent durability after thirty years of service3. While in operation, the concrete platform is relatively insensitive to increases in topside weight, at least for moderate water depths. And finally, at the termination of the field life, the concrete platform may be removed by reversing the installation procedure.

The favourable aspects of these concrete structures and the potential for shaping them to meet demanding requirements make them cost-effective for certain offshore oil field developments. However special challenges are posed by the Grand Banks of Newfoundland which is subject not only to wave loads comparable to the North Sea environment, but is periodically invaded by large numbers of icebergs. These icebergs calve from the large glaciers in Greenland and travel down the Labrador Current through the Grand Banks, giving this area the infamous name of "Iceberg Alley". To date only one GBS (for the Hibernia field4) is operating on the Grand Banks. Subsequent to the design of the Hibernia structure, there has been significant research relating to loadings from iceberg impacts.

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