This paper presents the results of a detailed assessment of the energy consumption and gaseous emissions which would result from a range of options being considered for the decommissioning of the Heather Alpha Platform, one of the larger fixed oil installations in the Northern North Sea, for which decommissioning is imminent.
This assessment forms one constituent element in the Best Practicable Environmental Option Study performed during the planning stages of the project. The assessment considers a wide range of feasible decommissioning options including partial and full removal schemes based on toppling, refloatation, lifting emplacement and lifting removal, with disposal of resulting materials either onshore, in-situ or the deep ocean, as appropriate to the decommissioning method.
This paper provides an account of the aims of the study, the methods and boundary delineation used in the evaluation, the sources and authorities used for conversion factors and to support other judgements, and finally the results of the analysis with a brief discussion.
A number of recent reviews of the decommissioning of offshore installations have highlighted the importance of considering the comparative energy costs of different decommissioning options and the comparative gaseous or atmospheric emissions likely to result from each option (e.g. Ref. 1). The presumption is not that an operator or regulatory authority would select the option with lowest energy costs or least emissions, but rather that this information, which otherwise may not be specifically included within BPEO studies, is an important consideration which can be evaluated quantitatively and included with the other aspects of environmental impact and risk, human safety and cost in the decision-making process. While such analyses are not new they are certainly not commonplace and there are no generally accepted rules or standard accounting procedures which can be drawn on for reference for the evaluation of decommissioning options. The work presented here is based on an actual North Sea facility - the Heather Platform - which is operated by Unocal Britain Limited and is approaching the end of its production lifetime. The Heather Field is located in Block 2/5 in the U.K. Sector of the North Sea, 145km east of the Shetland Islands. The Field has been developed with a single combined drilling, production and quarters platform standing in 143m of water. 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 platform itself stands to a maximum height of 236m and consists of a modular topsides sitting on top of a deck support frame (DSF). This in turn is supported by a steel jacket substructure piled to the seafloor. The topsides consist of a relatively large number of "lift units" composed of drilling, production, utility and quarters modules, and two flare booms. The total dry weight of the topside facilities is estimated at 12,300 tonnes including DSF.
The jacket is an eight leg, tubular space frame, steel structure supported by six piles connected to each of the four corner legs. The estimated weight of the jacket including piles and grout within the pile sleeves to the mudline is approximately 17,000 tonnes. Within the jacket are located 41 well conductors which weigh an estimated 4,300 tonnes to mudline. Marine growth on the jacket and conductors is estimated to add up to a further 2,000 tonnes to the platform weight. For further information on the Heather Facility, reference is made to Ref. 2.