This paper discusses a possible alternate Foundation system for TLP platforms in deep water. It was developed through Tecnomare for Agip during a feasibility study related to a site located in the Southern Adriatic Sea having a water depth of 827 m. The system is a combination of a pile-gravity foundation, consisting of relatively short (=20 m) steel piles of very large diameter (6to12 m) called "superpiles", closed at the top and open at the bottom and installed in soft soil under the effect of self-weight and "active" suction, The permanent tension of the tendons is equilibrated by self-weight only; tension due to wave action is equilibrated by the weight of the soil inside the superpiles. In fact a pulsating tension applied to the superpiles generates a suction keeping the soil plug inside the cylinder and preventing the cylinder from being extracted from the (impervious) foundation soil.
The paper describes the results of analyses of the installation and in service behaviour of superpiles, based on conventional principles of soil mechanics and related to a typical profile of clays at the site.
Installation problems are briefly discussed; then the stability in service is examined, and the capacity of superpiles in compression and in tension and the associated factors of safety are evaluated. Finally, considering the pore pressure gradient generated by the applied tension inside the soil plug, the flow rate end related deformations during storms and the expected design life of the TLP are evaluated.
In conclusion, the proposed system appears to be feasible, easy to install, and economical. Further dtudy and experimental research is needed to optimize its full potentiality.
The Tension Leg Platform (TLP) is a compliant platform concept consisting of a floating structure supporting the production plant, and connected to the seabed by tethers. The connection to the well is performed through small diameter tensioned risers. The TLP is currently recognized to be the most promising solution for very deep water production platforms.
Tecnomare has carried out for AGIP an extensive research project to assess the feasibility of the TLP concept for application to an oil field located in Southern Adriatic Sea having a water depth of 827 m. The structure that resulted is composed of 4 columns mutually braced and anchored to the seabed by means of 16 prestressed high quality steel tubular tendons. Its installed displacement is 35000 tons and allows for a deck weight of 11000 tons (Fig. 1). The tendons are connected to preinstalled (piled) templates and prestressed by deballasting the columns.
The logical support for the tendons is a dead weight or gravity foundation. But if the tension forces are too high or the soils are too soft to safely support such a weight, then long piles in tension (usually driven, although drilled and grouted piles have also been considered) must be used. Surprisingly, relatively little is known about the long term performance of long piles, especially when subjected to cyclic tensile loads; thus the design of TLP pile foundations is somewhat uncertain, especially at soft clay sites where the piles would have to be very long to support both static and dynamic loads.
