|BRITAIN'S OFFSHORE OIL & GAS
Construction and Installation
The scale of offshore oil and gas construction projects is vast, especially for the oilfields of the northern North Sea. The large fields discovered in the early 1970s took an average of five years from the beginning of development to the date of production start-up, and each cost over a billion pounds in 1987 prices. The UK Continental Shelf has received £190 billion (at 2000 prices) of investment since exploration began. A further £100 billion has been spent on operating costs which, at £4 billion per year, now makes up half of the total annual expenditure - a level which is expected to continue for several years. In the ten years up to 2000, the industry accounted for 17 per cent of all UK capital investment. Over the last 20 years the bulk of the investment expenditure has been made in Britain.
As soon as field appraisal has shown that development would be a commercial success, orders for the necessary hardware and associated pipelines go out. Initially this routinely involved the fabrication of platforms - some of which have been built in Scotland. Fabrication of a platform jacket can take up to two years to complete (F90). The finished jacket must be towed out to the field in calm weather, usually during the 'fine weather window' of the summer months. It is launched off its barge and up-ended into position by the controlled flooding of ballast tanks in its legs. After piles are driven to secure the jacket to the seabed, barges bring the deck support and production modules to be lifted into position on the jacket (F89). Specially designed crane barges can lift over 10 000 tonnes. At this stage of the project, rough weather can cause serious delays. Concrete platforms have been built in deep, sheltered fjords or sea lochs. As new concrete is poured, the structure gradually sinks. The deck and modules are placed on the legs close to shore, then the platform is towed out to the field. With ballast water pumped into the storage tanks, the platform settles firmly on the seabed. Inside the platform, the hook-up and testing of equipment ready for drilling and production may take another season, and require up to a thousand installation workers, called 'bears', at any one time. Finally, after completion of the first of the wells, the platform comes on-stream, beginning a producing life of at least twenty years.
More recently, however, there has been less emphasis on new production platforms when new fields are being developed. This is because most of the new fields are relatively small and do not merit a stand-alone platform. In these cases the preparatory design and fabrication focuses on seabed production units, including sophisticated remotely controlled pumps, with links to existing platforms or pipelines. Sometimes such developments can now be completed in months rather than years.