Installation of the Troll Phase I Gravity Base Platform

Abstract The TROLL Phase 1concrete Gravity Base Platform is located in 303 m of water in the Norwegian Trench some 80 kilometers northwest of Bergen. The platforms wi11be a major gas supplier to the European market far into the next century. This paper describes the major aspects in the design, preparation and performance of the platform installation operation. The extreme height of the structure was a challenge for design and performance of the marine operations, particular when passing a "shallow area" with minimum depth of237 m. The foundation soils at the Troll Field required skirt piles to record breaking 36 m depth in order to sustain the extreme North Sea weather conditions. The potential of intolerable horizontal soil pressure on the skirt walls from large soil displacements during penetration was the dominating aspect of the skirt penetration. Extensive engineering supplemented with model testing and planning were conducted in the development of systems and procedures for monitoring and manipulation of acting loads on the skirt walls. The success of the design, planning and performance of the operation is documented through comparison between observed and predicted performance of main operation parameters. Introduction Field Information, The TROLL Phase 1Gas Platform was installed in May 1995. The Troll Field is located in the Norwegian sector of the North Sea. The platform will be a major supplier of gas to the European market far into the next century. Design operating life is 70 years and the maximum production rate some 84 × 106 Sm3/day (3 BCF/day). Norske Shell was appointed as field development operator in 1988. The contract for engineering, procurement and construction of the concrete Gravity Base Structure (GBS) including tow out and installation of the platform at the Troll Field was given ?)Seconded to Norske Shell during project. Tables and figures at the end of the paper, to the Aker company Norwegian Contractors in March 1991,. Watoil will take over as production operator in June 1996. Platform Design and Site Conditions. The Troll Field is in the Norwegian Trench. At the platform location the water depth is 303 m, The upper 74 m of soil consists of more or less normally consolidated, homogeneous clay. From seabed to 16.5 m depth the clay is sofi to firm with high plasticity. This layer is called Soil Unit I. In Soil Unit 11, below 16,5 m, the plasticity becomes medium. The strength increases gradually by depth, A summary of soil data is given in Figure 1. The 472 m high structure including topsides is the tallest structure that has ever been moved on the earth's surface. An artists view of the platform is shown in Figure 2. The substructure is made of concrete and is a Co deep SP-4 -19-R design, comprising skirt piles, 19 caisson cells, one Siegel and four shafts. Figure 3 provides a general view and a cross section.