Geotechnical Experience from the Installation of the Europipe Jacket with Bucket Foundations

Abstract

ABSTRACT The Europipe jacket with bucket foundationsreplacing piles was installed in July 1995 in 70 metres water depth in the central part of the North Sea. The bucket foundations offer a new and economically attractive alternative to piled foundations and consist of circular mudmats with 6 metre deep skirts around the circumference, one under each jacket leg, Fig. 1. They have the ability to resist both tensile and compressive loads, Installation is provided by means of a suction system that enables the skirts to be filly penetrated into the sea bed. The use of suction to install the Europipe bucket foundations is absolutely essential, The geotechnical aspects of this installation method are discussed in the paper and results from installation are provided. INTRODUCTION It is possible to use suction as an installation technique for suction piles, concrete skirt pilesand model caissons (see Ref, 1) It is also possible to install steel structures without using piles as the foundation, e.g. the Maureen platform, but never before the Europipe jacket had bucket foundations been used as the only permanent foundation with suction providing the primary foundation installation system, The need to penetrate steel skirts 6 m into very dense sand also goes beyond any experience previously obtained. Due to the novel foundation method, the installation contractor was unwilling to accept responsibility for the skirt penetration operation, Consequently this operation was planned and carried out by Statoil personnel. This detailed planning process together with the actual recorded and observed installation results are briefly presented and discussed in this paper. INSTALLATION ASPECTS The installation of bucket foundations, (penetration of steel skirts into the seabottom) can be conveniently divided into three phases:Touch down and self weightpenetration phase: Jacket lowering from the point where the skirt tip is approximately 10 m above seabottom to where the skirts have penetrated under the jacket weight, i.e. touch down followed by the self weight penetration f the jacket, This penetration is uncontrolled compared to the following penetration phases.Ballast weight penetration: The penetrationof skirts into the seabottom by increasing the jacket submerged weight. For Europipe this involved flooding the jacket legs, effectively increasing the submerged weight by 100% from 1350 tons to 2700 tons. This was a very controlled operation compared to the self weight penetration stage since the objective was to achieve a perfect sealing between skirt and seabottom.Suction penetration: Suction was used to assist in achieving final penetration since weight alone was not sufficient. Note: Suction both increase the driving force and reduced the resistance to penetration (Ref. 1). The objective of the bucket installation was to achieve 6 meter penetration of steel skirts into dense sand. Since no directly relevant experience was available, the reference to similar experience such as the Germ suction piles and the Gullfaks C concrete skirts was important, and lessons learnt on these installations had to be considered. Using this information and accounting for specific uncertainties related to bucket foundations for jacket.