RIASSUNTO
Abstract
This paper presents some of the issues which have influenced the decommissioning planning for the Heather Field facilities. The paper is based on the results of conceptual and pre-engineering work performed to date, and covers the following:
- brief background on the Heather Field development and production history, field facilities, and achievements in extending field life to date,
- considerations and constraints which have shaped Unocal's approach to the planning of the Heather Field decommissioning project, and
- discussion of the stages to be undertaken in performing the decommissioning of the field facilities, concepts and options which exists for these stages, and operational and technical considerations applicable to each stage. The stages addressed include engineering and planning, well plugging and abandonment, topside systems cleaning and preparation for removal, pipelines decommissioning, topside facilities removal and disposal, and platform structure removal and disposal.
Introduction
The Heather Field. operated by Unocal Britain Limited, is located in Block 2/5 in the U.K. Sector of the North Sea, 145km east of the Shetland Islands. Oil was discovered in the Heather Field in December 1973 and first oil was exported from the platform in October 1978. Since 1978, in excess of 110 million barrels of oil and condensate have been produced from the field, with a peak average daily production of 36,000 barrels per day being reached in 1982.
The Field has been developed with a combined drilling, production and quarters platform standing in l43m of water (Fig. 1). Processed oil and condensate is exported through a 32km 16"" pipeline to Ninian Central Platform and onward to the Sullom Voe Terminal in Shetland. Natural gas is imported through a 19km 6"" pipeline from the Welgas Pipeline for use as fuel and production wells lift gas.
The Heather Platform jacket is an eight leg, tubular space frame, steel structure supported by six piles connected to each of the four corner legs. The legs have a 1:10.824 batter in the transverse direction and are vertical in the longitudinal direction. The jacket can accommodate 43 no. 26"" diameter conductors which are laterally supported through slots provided in the conductor guide framing. In addition, two 16 risers are attached to the jacket, and nine caissons for miscellaneous services (e.g. process sump, utility sump, seawater lift caissons etc.) are supported within the jacket structure. The estimated weight of the jacket including piles and grout within the pile sleeves to the mudline is 17,300 tonnes.
The well conductor weight to mudline is estimated at 4,300 tonnes, and the marine growth on the jacket adds up to another 2,000 tonnes to the overall weight of the platform structure.
The topside facilities were prefabricated onshore and consisted of a relatively large number of ""lift units"" based on the lifting capability of the lift vessels available in the mid 1970's. There are three main deck levels, covering nearly 10,000m2, which contain all the equipment necessary for drilling and production operations together with numerous ancillary utility systems. The platform contains a skid mounted enclosed drilling derrick, two flare booms (each 52m long) and two diesel powered pedestal cranes. The total dry weight of the topsides facilities is estimated at 12,300 tonnes including the deck support frame (DSF).
The Heather Field development has always been a marginal operation, but tight cost control, focused management and a ""Fitness for Purpose"" operating culture has enabled the Heather Field operation to remain economically viable. An aggressive infill drilling programme in the 1980's enabled additional reserves to be recovered, and more recently, application of coiled tubing based well intervention and stimulation techniques have slowed down the production decline rate.
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