Installation Of A Gravity Structure Over Predrilled Wells
Authors: Gerbault Marcel , Pliskin Lucien 
Published: December 2007

Abstract
Experience has proved that gravity platforms could be installed with maximum accuracy with the help of tugs. The main objective is to improve this accuracy to more or less 2 inches, so as to allow installation of a structure over a template, comprising a certain number of predrilled wells.
Studies demonstrating the feasibility of such operations in complete reliability are progressing in this field.
This type of installation allows the drilling operations while the structure in under construction. When the structure is installed, the conductors connection and well completions can be achieved. The time gained by such a method in platform exploitation is about one year.

Introduction
The present techniques devised for the exploitation of undersea oil fields require huge investments and long time lags before starting production. It takes 2 to 3 years to build and production. It takes 2 to 3 years to build and equip a platform, notwithstanding the time spend in preliminary studies. preliminary studies. The platform must be installed in the location and fitted up with all relevant equipment before starting any drilling operations, which last an additional 2 years, during which time production is not possible.
The advantage of the new STC concept is that it allows both operations, building the platform and drilling the wells, to be carried out simultaneously, which means saving two years in the actual exploitation of a field.
SEA TANK CO have just completed the first stage of a study on the improvements of a gravity platform, the equipment needed and the procedure to allow the installation of such a structure on the sea bed, on predrilled wells not yet productive, with no risk of predrilled wells not yet productive, with no risk of deteriorating the wells themselves and so as to allow future connection with the surface equipment supported by the platform, specially the well-heads.
This procedure necessitates the checking of the behaviour cf the predrilled wells. Comprehensive studies show that the effect of subsequent settlements occuring after the emplacement of the platform does not induce overstressing of the conductor pipes.

CONTEMPLATED PROCEDURE
Observers have been quite impressed by the precisions with which a platform may be controlled precisions with which a platform may be controlled during immersion for the deck mating, and also by the accuracy displayed when placing some platforms with the only help of tugs.
The results published up to day, regarding the off target distance show an accuracy of less than 10 m, when a platform is installed alone in the field, and 1 to 2 m when it is installed in the neighbourhood of a preexisting platform.
This is mainly due to the good behaviour of the platforms under the action of waves and the reliability platforms under the action of waves and the reliability of the immersion system.
Some of the techniques now in use have to be adapted and improved for the installation of a gravity platform over drilled wells. platform over drilled wells.- the template, which is fixed on the sea bed and through which the wells are predrilled, has to be sufficiently protected so as to be able to withstand the contact with the platform and ensure its final centering.
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