Full-Scale Monitoring of Troll A Concrete Platform: A Huge Gravity-Based Structure On Soft Clay

The Troll field located in the North Sea is Europe's largest offshore gas field. The gas reserves are produced from a single concrete gravity platform resting on soft clay in 303 meters waterdepth. This is only possible since the foundation consists of skirt piles. The platform was installed in 1995. Soil conditions at the site consists of slightly overconsolidated soft clay and the concrete skirt piles penetrating 36 meters is required to achieve sufficient stiffness and strength of the foundation. In contradiction to other gravity based structures for which the performance improves continously after installation due to consolidation, the Troll platform will experience a complex balance of pore pressure regime during the lifetime between the effects of installation, cyclic loading and dissipation. Since the Troll A platform was a new design in terms of dynamic behaviour, soil type, skirt length and foundation loading, monitoring of platform behaviour in the operational phase was a requirement. A monitoring system containing sensors measuring environmental data, dynamic motions, strain in shaft and skirt, earth pressure and pore pressure at the base and skirt tip, pore pressure in subsoil, settlement and inclination was installed. The results from the geotechnical measurements during these 5 years have been summarised with special emphasize on the monitored real behaviour of the foundation of this huge concrete platform, the largest offshore stucture to date. Analysis of the data retrieved from the monitoring system installed at Troll A it can be seen that the platform is behaving as expected. There are slightly higher excess pore pressures from installation at skirt tip and down to -50 meters which will be monitored closely in the future but on the whole the long term development of the platform is satisfactory and according to design.