ABSTRACT

Chevron's Ninian Field Southern Platform is noteworthy for its rapid planning, design and fabrication, and its extremely successful installation, all of which justified the conceptual philosophy adopted early in the design stage. From the outset, the aim was to build on past experience and provide simplicity and refinement within the limits of available technology, consistent with a summer 1977 installation of a platform capable of supporting the heaviest deck load yet proposed for a piled steel structure. These goals, together with severe functional and environmental criteria, resulted in a number of interesting features.

This paper describes the process leading to the selection of a self-floating steel tower, and outlines the manner in which considerations of fabrication, transportation and installation influenced the configuration of structure and piles.

The structural design of tower and piles is described, as is the upending control system. The finite element method of analysis to verify the design of the unusual 9.2M leg nodes, is outlined and the novel cathodic protection system described.

Special study results are outlined including a summary of full scale tests to confirm the adequacy of the pile-to-sleeve grouted connection, and a computer approach to the dynamics of on-bottom stability. Installation procedure studies are listed.

Finally, the fabrication and installation record is appraised in relation to the original design concept.

SELECTION OF PLATFORM TYPE, FACTORS INFLUENCING CONFIGURATION
Introduction

It was believed that the ultimate success of the proposed program would rely as much on designing for fabrication and installation requirements as it would on structural analysis and design. In the overall development cost, the effect of saving structural steel weight by virtue of exhaustive analytical work, is insignificant; on the other hand, time spent on designing for fabrication and installation can result in very significant cost savings indeed.

Description of Tower

The Ninian Southern platform is a 4-1eg, piled, steel tower structure supporting modularized production and drilling facilities, as shown on Fig. 1. It is a self-floating unit with an integral ballasting system designed for remote controlled operation.

The deck comprises 2 levels of modules 65m by 68m in plan dimension providing three main levels, with a nominal production capacity of 150,000 B.O.P.D. Maximum anticipated deck load is 26,000 tonnes, including two drill rigs. Slots are provided for 42 conductors.

Typical dimensions are 75m by 75m center to center of the legs at the mudline tapering to 55m by 55m at the top of the module support skid-beams; overall height from mudline to skid-beams is 168.5 metres.

The tower is supported by 32 skirt piles, 1825mm O.D. by 63mm wall thickness, arranged in clusters of 8 at each leg in a 12,2m diameter circular pattern. Storage is provided in the upper parts of the 9.2m diameter flotation legs for 20,000 bbls of diesel fuel and 10,000 bbls of drilling water.

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