Development and Structural Design of the Bucket Foundations for the Europipe Jacket

Abstract

ABSTRACT The installation of the Statoil Europipe 16/11 E jacket with bucket foundations in the summer of 1994 represent the dawn of a new era for jacket foundation engineering. With traditional piled foundations giving way to the novel alternative termed "bucket" foundations, the installation of the Europipe jacket provides the first real opportunity to examine properly the benefits of this solution. The bucket foundations are circular plates with skirts installed at the base of each jacket leg, and as such they represent a radical departure from traditional piled foundation solutions. The bucket acts as gravity foundations with the skirts increasingthe vertical compression and shear load capacity aswell as providing considerable tension load capacity. This paper outlines the development of the bucket foundations, describes the structural design aspects and compares the results from the structural monitoring system installed on the jacket with the installation structural analysis results for the foundations. INTRODIJCTION The idea of the bucket foundations a rose from fabrication cost and installation problems with the piled foundations on the Statoil Veslefrikk jacket (Baerheim et al. ref 4) and geotechnical experience from the Statoil Gullfaks South gravity base structure (Tjelta et al. ref. 5). The bucket foundation system was used for the first time on the Europipe riser jacket and will be used again on the Statoil Sleipner Vest Treatment platform due to be installed in 1996. The bucket foundations (also called plate foundations) for the Europipe jacket are 12 metre diameter plates placed under each leg of the four legged jacket. The plates have 6.5 metres deep skirts around the circumference. This paper describes the development of the bucket foundations and the structural design of the foundations for their first use on the Europipe jacket. The jacket was equipped with a structural monitoring system on members near the buckets. The results from this will also be described and compared to the results of the structural analysis of the installation operation Specific issues of the installation of the jacket are described by Rusas et al. in ref. 1. Bye and Erbrich et al. presents the geotechnical design of the bucket foundations in ref 2 and the geotechnical experience from the installation of the Europipe jacket are described by Tjelta in ref. 3. General aspects of bucket foundation design have been described by Baerheim in ref 7. This paper summarises this and dwells more in detail on the latest analysis development THE DEVELOPMENT FOUNDATIONS OF THE BUCKET During the design of the Statoil Veslefiikk jacket the on-bottom stability before piling was a design challenge, in particular with respect to up-lifi. The mudmats were equipped with 1 metre deep skirts due to sliding and the ability of skirted plates in soft clay to withstand uplift forces was investigated. Based on the limited information available it was found that suction forces would occur. The problem was to determine the magnitude. The installation time for the jacket was also long due to pile handling and problems with the pile hammers.