The Design, Fabrication, And Installation Of The Alba Northern Jacket

Abstract

ABSTRACT This paper discusses aspects of special interest during the design, fabrication and installation of the Alba Northern Jacket. The reasons for selecting large mudmats, high strength steel and anti-fouling coatings are among the technical topics covered. The contracting strategy and approach to administering the fabrication contract are discussed in the context of recent initiatives in the UK offshore industry. INTRODUCTION The Alba Field is located approximately 130 nautical miles North East of Aberdeen in block 16/26 in the U.K, sector of the North Sea, as shown in Figure 1. The reservoir has an elongated shape which is oriented on a North West/South East axis, and will be developed with two facilities - Alba North and Alba South. Because of the uncertainties associated with the reservoir, and the production and processing of viscous crude at Alba, the field will be developed in two phases commencing with Alba North. Consistent with the phased approach, the design of the Northern platform has been based on the provision of minimum facilities. It is intended that operational experience and further geological information will determine the type of facility for the southern part of the field. Oil will be exported through a subsea pipeline to a floating storage unit (FSU), and transported to shore via shuttle tankers. The installation of the Alba Northern platform was successfully completed at the beginning of July 1993. Early studies for the Alba Northern Platform considered the possible use of a lift-installed steel jacket. However, as the definition of the topside facilities and reservoir engineering requirements improved, increases in the topside load and number of wells meant that the jacket lift weight exceeded the lift capability of the DBI 02 and M7000 semi-submersible crane vessels (SSCVS). Although an eight-legged barge-launched jacket was eventually selected, its design incorporated some of the features studied for the lifted jacket, most notably the use of high strength steel and anti-fouling coatings. Many aspects of the Alba Northern Jacket are typical of the latest generation of structures installed in the North Sea. There are, however, some features of special interest. This paper describes these features and their significance during design, fabrication and installation. CONFIGURATION The jacket stands in 138 metres of water at lowest astronomical tide (LAT). It was designed to support a topsides load of approximately 20,000 tonnes, and was estimated to have a launch weight of around 17,000 tonnes. The jacket accommodates slots for 24 conductors, and has been installed over a 9-slot drilling template. As well as an export riser from the platform to the FSU, provision has been made on the jacket to support import risers from Alba South and future discoveries in the area. The configuration for the eight-legged jacket comprises two longitudinal frames, four transverse frames and six plan levels, and is shown schematically in Figure 2. The tops of the legs provide direct support for an integrated deck. Within the context of barge-launched jackets installed in the North Sea since 1988, it is recognised that the jacket has essentially a conventional configuration.