The Skirt Piled Gullfaks C Platform Installation

ABSTRACT The Gullfaks C concrete platform was successfully installed on a soft soil site in 1989. This was the first time the new foundation concept of skirt piling i.e. very deep concrete skirts acting as piles, was implemented for a gravity base structure. The platform foundation consists of 16 large diameter skirt piles penetrating 22 meters into the seabed. Installation of these "piles" was achieved by platform weight and underbase suction. The platform weight was initially carried as base contact pressures. A soil strengthening scheme enabled full weight of the platform to be carried by the skirtpiles within 3 months. Through this first skirtpiled platform installation, the concept of skirt piling should be recognized as a proven design. INTRODUCTION The Gullfaks C platform installed in May 1989 is the largest and heaviest offshore concrete structure ever built. Designed for 220 m water depth it has a displacement of more than 1.4 million tonnes and represents a total investment of $ 2 billions. Contrary to previous gravity base structures, the Gullfaks C platform was offered one of the most complex seabed conditions in the North Sea to date. The upper 45 meters of soils consist predominantly of normally consolidated soft clay and loose clayey and silty sands with interbedded dense sand layers, see Fig. 2. The combination of a heavy gravity structure and soft soils called for a new foundation solution:–skirt piles through soft soil layers, and.–strength improvement of soils below the depth that could be reached by skirt piles. The strength improvement was achieved through an acccelerated soil consolidation by means of a large soil drain system comprizing more than 800 single filters and a total filter area of 1600 m2 The system was operated with suction through the first summer season. In this period 8000 m3 of ground water was extracted from the foundation soils. The platform foundation base area amounts to 16000 m2 The foundation is made up of 1400 running metres of concrete skirts penetrated 22 m into the seabed. The skirt piles consist of 16 circular concrete cells of diameter 28 m and wall thickness 0.4 m, see Fig. 1. After soil strengthening by artificial drainage, the operational platform submerged weight of 500.000 tonnes is mainly transferred to the soil as skirt tip resistance and skirt wall friction. This will reduce the future platform settlements significantly. The skirt penetration driving force was provided by platform water ballasting and underbase suction. FOUNDATION DESIGN ASPECTS AND INSTALLATION PRINCIPLES Foundation design aspects There are four major foundation aspects of the GBS foundation. These are:–Skirt penetration resistance–Foundation stability–Foundation load distribution–Platform settlement Foundation stability of Gullfaks C could theoretically be obtained in several ways, e.g. by large base area, very deep skirts or"stronger" soil. The relationship between these parameters are shown in Fig. 3. At the design stage, it was apparent that any realistic solution would have to be based on much deeper skirts than for any platform before.