Infrastructure for Regional Development in Deep Water
Authors: Marshall P.W. 
Published: October 2008

Exploration in deep water has revealed a number of finds in the 0.5-1.0 billion barrel range, plus more in the 100-500 million barrel range. Surprisingly few of the latter will be viable as stand-alone developments. Comparative economic feasibility studies will be described, seeking to quantify the benefits of providing infrastructure for coordinated regional of these significant resources, rather than stand-alone development on a field-by-field basis. Realistic generic examples will be presented, rather than announcement of actual development plans.

The studies cover west of Shetlands, drawing on concepts developed for turbidite trend in deep water Gulf of Mexico. The approach could also be applicable to other deep-water areas, e.g. West Africa. Central platform water depths are 400- 900 m, with satellite fields in much deeper water being within reach using current technology. Design requirements include heavy payloads (40,000 tons), long life (80 years), and large mirnbers of preferably hardpipe risers (up to 100, both wells and pipelines). Adaptability to unpredicted and changing operational requirements is featured. This work presents an alternative to the Deepstar approach, in that it covers sites beyond the reach of shallow-water facilities, and does not presume that 'gunky crude? problems on the petroleum engineering side, which have been intractable for 100 years, will suddenly be solved.

INTRODUCTION

To date there have been ten oil or gas discoveries west of Shetlands, five of which are considered significant, with combined reserves of 0.7 to 1.6 billion barrels of oil equivalent. Estimates of the total potential of the province range from 2.5 billion BOE upwards. Although intial development of the BP/Shell Foinaven field is going ahead as a shipshape floating production system with subsea wells, it lies in a cluster of 3 or 4 fields which could form a nucleus for regional development (Foucart 1994). Compliant structures, such as tension leg platforms and compliant towers, hold promise as permanent structures for long-term production facilities in the deep water (400-900 metres) west of Shetlands. They can economically support heavy playloads, as well as large numbers 'of topside? "wellheads"and hard-piped risers for importing production from satellite fields and exporting oil and gas to shore. Even where largely subsea development is contemplated, riser-friendly permanent structures will be attractive as a central facility to support the necessary infrastructure.

For these compliant structures, the complex problems of in-place dynamic analysis and fatigue are now understood. Time domain random wave analysis is able to handle the nonlinear applied forces in the crests of large steep waves, which are important to both types of structure.

The demanding issues of detailing construction, Loadout, transport, launching, flotation, mating, and underwater pile driving have also beenmastered. Like fried platforms, the 900-m compliant towers, described in Marshall et al. (1992), are inherently reliable passive structures, with no moving parts, and their construction relies n tried and trusted methods. For example, the method of stacking, as used for the world-record Cognac platform in 1978, was originally conceived as a way of dealing with harsh installation environments like the North Sea and Gulf of Alaska.