Permanent Mooring System for the Vega Field: Design, Fabrication, and Installation

ABSTRACT I n mid 1987 a 250 000 DWT converted tanker will be connected, through a rigid mooring yoke, to an articulated column which was installed during October 1986 in the VEGA field offshore from Sicily. This ARC-YOKE Mooring System is equipped to handle an estimated production of 60 000 barrels per day with a system allow in addition of difluent to the crude circulation in order to avoid clogging. The Paper gives a complete description of the system and some details of the fabrication of the most noteworthy components, such as t h e large - size universal and tri - axial joints. A large part of the paper is also devoted to column installation, which resorted to original procedures in order not to require the need underwater assistance. INTRODUCTION The VEGA field, which is located 12 miles off the southern coast o f Sicily, was discovered in 1980. Subsequent drilling in 1983 established it to be the largest oil deposit lying under Italian waters. The field is operated by SELM, a subsidiry of the Italian group MONTEDISON, on behalf o f a consortium in which SELM has a 30 % participation. The other participants are AGIP with 40 %, CANADA NORTH WEST w i t h 20 % and PETRO MARINE with 10 %. The first phase of the development will consist of a single platform for extracting the crude oil which is to be pumped directly in t o a floating storage unit in the form o f a 250.000 DWT tanker permanently moored 1.5 mile from the platform. The ARC-YOKE Mooring System, comprising a gravity base, an articulated column and a mooring arm, is designed t o operate for 25 years. Shuttle tankers will be employed to transport an estimated production o f 75 000 barrels per day t o there fineries. Onshore mating o f the two major components of the mooring system, i.e. the gravity base and column, was carried out in September, 1986, and the combined unit installed at its location in mid-October, 1986. The storage tanker, complete with its mooring arm, will be brought to the site and connected to the column in April, 1987, after completion of conversion works in Sicily. Initial production is expected t o commence in early Sumer 1987. FIELD DATA Site Data The water depth at the location of the ARC-YOKE Mooring System is 116.6 m and the one hundred year return conditions are as follows: (Table available in fullpaper)