The Lessons From 8 Years' Operation Ot A First-Generation North Sea Platform And Planning To Its End-Ot-Field Lite

ABSTRACT Thistle design was started in 1974 in the days of oil shortages and rising prices. When installed, it was in the deepest water and the most northerly of the Brent area developments. Production drilling commenced from the platform early in 1978. With eight years operating experience, there are two clear lessons; firstly that a facility cannot be operated in the Northern North Sea without further investment for a twenty-five year life, and secondly, that a plan is required to ensure a cost effective engineering programme through the field's declining production years. The paper considers the major capital outlays incurred since installation. It then presents how this and other experience has been developed in a "Life-of-Field" study aimed at reducing future engineering, maintenance and diving costs. INTRODUCTION Britoil currently operates two North Sea fields (four platforms) and one 'onshore terminal. A further field (Clyde) is scheduled to start production in 1987. The Thistle field was discovered in July 1973. The field, Iodated in the East Shetland Basin, was then the most northerly and in the deepest water of any North Sea field. Thistle design was undertaken in parallel with other early Northern North Sea platforms, ahead of any other installations. Thistle 'A' is therefore very much a "first generation" northern North Sea platform. Although it did not use any obviously new technology, it did involve untried extensions of technology in many areas. The field was developed by a single fixed steel platform with facilities for:-–60 wells–2 drill rigs and drilling services–4 separator trains (rated at 68,000 bpd per train)–2 subsea crude oil storage tanks (35,000 bbls per tank)–crude export by tanker (via a SALM buoy) 5 water injection pumps (rated at 50,000 bpd per pump)–3 produced water treatment flotation cells (rated at 80,000 bpd each)–gas compression (for condensate recovery and gas disposal at 3,500 psig)–2 gas turbines (rated at 26 MW per set)–accommodation for 180 men–3 future oil risers–1 future gas riser–8 future flowline riser bundles (for subsea wells tie-in) Figure 1 shows Thistle's geographical location and its connections to pipeline systems. Figure 2 is a diagramatic view of the platform facilities, highlighting additional facilities. Like many other platforms in the North Sea, Thistle has now reached a stage where production is in decline. However, the facilities are not only required to remain in service for a further 15 years or more, but must also accommodate profiles somewhat different from the orginal design parameters. Recognizing that these early North Sea platforms are almost certainly required to remain in service for twenty years or more, it was decided to review past history and, in anticipation of future requirements, attempt to construct a programme which would lead to a more cost-effective engineering, maintenance and diving programme for the remaining life of the field.