Retractable Pile: A New Solution for Jack-up Platform
Authors: Thomas P.A. , Tcherniguin N. , Maconochie A. , Oliphant J. 
Published: October 2008

Within its jack-up development framework, Technip has developed a new foundation concept that widens the range of soil conditions where a jack-up production facility can be used without compromising its self-installation capability (i.e using only minimal support).

This new design, the 'retractable (and extendable) pile' solution, is suitable for a wide range of soil conditions and thereby makes the use of a production jack-up a viable option at many more locations worldwide. It also has application where a jack-up is to be utilised on a number of developments and has to be relocated during its operational life as the leg can be disconnected from the pile.

After reviewing the foundations designs already adopted for jack-up production facilities, this paper presents the new foundation design and its, benefits over existing designs. Discussion is also given on how the new foundation design can be integrated into an overall design to produce a production platform for shallow water projects worldwide with a much reduced marine operation and installation cost.

Introduction

Historically, jack-up type platforms have been extensively used for drilling and more recently some have been used for production facilities. These represent one type of selfelevating platform and a number of similar designs are now being progressed for production duties. However, there are a number of issues that challenge the designer when considering the long-term installation of jack-ups for production facilities including:Use of appropriate design codes.Foundation fixity and associated platform dynamic behaviour.Future relocation of the facility.

On existing major self-elevating production facilities e.g. Technip's TPG 500, driven piles and very large skirted spud cans have been two of the solutions used for the foundations. This new foundation design for jack-ups not only offers a low cost alternative to the current designs, but also extends the operating range of soil conditions where a self-elevating platform can be employed.

This foundation concept can be used directly on jack-up platforms or for a new platform design where a truss deck and its associated legs are transported to site on a barge, the legs lowered to the seabed, the deck raised to the operating air gap and the 'suction piles' used to give long term fixity complying to fixed platform design codes. The overall effect is to substantially reduce the marine operation costs for installing the platform, both substructure and topsides.

The new design employs an open ended retractable caisson within each leg which is lowered once the platform is at the installation site. Associated with each caisson is a suction pump facility which is used to drive it fully into the seabed thereby substantially increasing the seabed fixity and providing sufficient capacity to meet the fixed platform design codes (i.e. extreme environmental loading return period and fatigue).

Foundation Designs of Installed Platforms

Fig 1 illustrates a number of alternative designs that have been considered for the use of foundations for the TPG 500 jack-up type production platform along with the new retractable pile design.