Geotechnical Design of Bucket Foundations

Abstract

ABSTRACT The successful installation of the Europipe 16/llE Riser jacket in July 1994 and the coming installation of Sleipner T jachzt in spring 1996 proves that suction installed skirted plate foundations (buckets), may not only be a competitive alternative to piles, but also a complementaryfoundation solution both for noncohesive and hesive soils. This paper presents the background for the d.exign analyses of the Europipe 16/llE and Sleipner T foundations in terms of the performed field and model tests, as welt as the theoretical models and practical analysis tools utilized for design. INTRODUCTION This paper is a joint presentation made by the designers of the Europipe 16/1lE jacket, Aker Engineering, and thedesigners of the Sleipner T jacket, Kvaerner Ed and wright. The active development of bucket foundations for steel jackets was initiated by Statoil's chief foundation engineer Tor Inge Tjelta and lead structural engineer Morten Baxheirn at the end of 1989. The successfid installations of the Veslefrikk jacket, the deep skirted concrete platform Gullfaks C and the suction installed tether References included at end of Part 1 and Part 2 anchors at Snorre played a vital part in this development. However, soil conditions for the two jackets in question are dominated by dense to very dense sands, while the previous skirted foundations were installed in soft to from clays. Two novel and important issues were identified as critical with respect to the design of the bucket foundation namely.Penetration of the skirts through the very dense sandTension capacity, particularly under cyclic loading To assist in the development of practical design tools, Statoil initiated field testing comp-rising penetration and capacity tests at the actual offshore sites in late 1992. These tests formed the basis for the Europipe platform bucket design which was completed in late 1993. At the end of 1993, a comprehensive model test program was initiated aimed specifically at improving the knowledge and understanding of tension load capacity and clic loading effects for inclusion in the Sleipner T bucket design. Also included in this program were a number of penetration tests for verification of the penetration analyses performed for the Europipe buckets. These tests have been filly utilized during detailed design of the Sleipner T platform. The design is now complete, and fabrication of the jacket is under way. In the following, the background and design of the Europipe 16/1lE and Sleipner T bucket foundations are presented separately. Key elements from field and model testing arc described and mechanisms of behaviour are presented. This experience is coupled with theoretical models and transformed into design tools for the prototype foundations designed for the Europipe 16/1lE and Sleipner T platforms. PART 1- EUROPIPE 16/llE Key design data Europipe 16/1lE is a riser platform for the Europipe transport system, situated in Block 16/11 in the Norwegian sector of the North Sea. Key parameters for the site and structure are water depth 71 m, topside weight 5000 and dry support structure weight 3800 t. A 3D view of the structure is presented in Fig. 1.1.