The Superpile System: A Feasible Alternate Foundation for TLP in Deep Water

Abstract

This paper discusses a possible alternative foundation system for TLP platforms in deep water, which was developed by Tecnomare for Agip during a feasibility study for 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 (6 to 12 m) called 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 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 and related deformations during storms and the expected design life of the TLP are evaluated. Economy and ease of installation are the two main features of the system. Further study and experimental research is needed to optimise its full potential.