The Troll A platform in the Norwegian trench rests on a huge gravity-based structure (GBS) with 36m deep concrete skirts. The GBS was installed by skirt penetration into a seabed of normally consolidated clay in May 1995. A comprehensive structural and environmental monitoring system has provided large amounts of data on foundation and structural performance since platform installation, and is still operative. The monitoring showed unexpected pressure response in 2002 and triggered a comprehensive study including foundation behaviour from installation until today. The GBS is found to settle as predicted. Several interesting soil structure interaction aspects are studied in detail, including redistribution of stresses from the skirt to the base due to dissipation of high excess pore pressure generated during installation, cyclic loading during the winter storms and creep along the skirt walls. Temperature increase in the soil around the production wells, uneven loading between the four shafts and gas transport along a geotechnical borehole are found to be reasonable explanations for the uneven pressure distribution below the foundation.
The Troll A platform is located in the Norwegian trench at 303m water depth and is one of the main gas producers in the North Sea. The platform supports 40 wells, 39 of which produce gas from the Troll East reservoir. The steel deck of the platform rests on a huge concrete gravity-based structure (GBS), whose foundation consists of 19 concrete foundation cells. As the soil at the site is normally consolidated clay, the 32m diameter foundation cells are extended by cylindrical concrete skirt cells that penetrate 36m into the seabed in order to obtain bearing capacity for the 2820MN submerged weight of the structure, see Figure 1. The figure also identifies the triangular shaped cells between the cylindrical skirt cells; the ‘tricells’. The gross foundation area is 16 597m2, and the net cross section of the soil enclosed by skirts is 15 542m2.
The whole structure was constructed inshore, towed to the offshore location and installed by skirt penetration into seabed May 1995. The concrete skirts displaced nearly 40 000m3 of soil during installation, and it was predicted that this would create significant excess pore pressure in the soil contained within the skirt cylinders, in addition to the excess pore pressure due to soil stresses generated by the platform weight. The GBS is equipped with a structural and environmental monitoring (SEM) system. This system allows continuous surveillance of important parameters describing platform behaviour, including settlement, water pressure in the skirt compartments, earth pressure against concrete faces and subsoil pore pressure ? parameters that are essential for understanding the foundation behaviour1. The system also monitors environmental loading parameters such as wind and waves. The soil conditions at the site are presented by By and Skomedal2, and details about the platform foundations by Hansen et al.1
The SEM system has served its purpose as a tool to monitor the platform and its foundations. The pressure inside the skirt cells has been bled off when reaching operational limits, and due to the SEM system