Geotechnical Aspects Of Bucket Foundations Replacing Piles For The Europipe 16/11-E Jacket

Abstract

ABSTRACT A major departure from traditional foundation piles for jackets is planned for the Europipe 16/11-E riser platform in the North Sea. Scheduled for installation in the summer 1994 in 70 m water depth, the jacket will have no piles. The permanent foundation will consist of a 12 m diameter mudmat at each of the 4 corner legs fitted with skirts projecting down from its periphery. This new concept will use suction to achieve the fill penetration depth of 6 m. The structure will not rely on gravity platform principles only, but will use passive suction generated by tension load to resist the overturning moment from environmental forces. INTRODUCTION Jacket structures have always been supported and tied down to the sea bottom by piles. Only one known steel structure is different, the Maureen steel gravity structure, ref. Agostoni et al. (1980). With the exception of Maureen, alternatives to piled steel jackets have not been available. The riser platform now under construction for the Europipe gas pipeline in block 16/11 has its piles replaced by bucket foundations, see Fig, 1. In each corner of the four legged jacket the mudmat has been equipped with 6m steel skirts projecting down into the soil. Baerheim (1994) presents further structural details. Even though skirted mudmats are not a new type of foundation since it has been used for a number of jackets to secure on bottom stability, they have always represented a temporary solution prior to pile installation, probably because of a reluctance to accept that tension capacity is available in soils. Tension capacity refers here to the uplift resistance of an object being retrieved from the sea bottom within a limited time period. This uplift capacity is well documented in clay and is the basis for the skirted concrete foundation units used for the Snorre TLP, ref. Andersen (1992). For sand the same uplift capacity is less accepted in the industry, but development work supporting the Europipe jacket foundations has given confidence in this new concept, called in this paper bucket foundation. The adequateness of a bucket foundation, from a geotechnical point of view, for supporting a jacket on strong soils can be shown in two example calculations for clay and sand: Given a steel plate with dimensions 2×20m and 25rnm thickness. This plate can theoretically provide bearing capacity in three different ways:A vertical plate penetrated into the soil,The same plate rolled as a pile andThe plate laying flat on the ground. For a clay with undrained shear strength of 100 kPa. For a dense North Sea sand, the capacities correspond to the values in Fig. 3(available in full paper). From a theoretical standpoint, the flat plate resting on the surface is the obvious choice. Since the non-stiffened flat plate is not the practical way of supporting structures, a structural compromise can be the inverted bucket which has the highest capacity of all the alternatives.