Structural And Installation Design Of Plate Foundations For Jackets

ABSTRACT A new foundation system for offshore steel jackets has been developed by Statoil. The method is called plate foundations and it replaces the use of traditionally piled foundations. The plate foundation consists of a circular plate placed below each jacket leg. The plate is surrounded by a vertical skirt penetrating the sea bed. The plate foundation acts in principle as a gravity foundation. Due to the skirts, however, the foundation has a considerable capacity for tension loading even for sandy soil conditions. The tension capacity is due to high suction pressures as the tension loads tries to pull the plate out of the sea bed. This paper describes the development of the plate foundations, discusses the design and installation aspects of the foundations and describes typical mechanical equipment needed for the foundations. INTRODUCTION Plate foundations are in principle not new for supporting offshore steel structures. The Maureen steel gravity platform stands on similar foundations and jack-up platforms stand on either individual spud-cans or large mats. Typical for both these solutions are that they are not designed to support tension loads. Even though the Maureen platform had skirts for capacity against sliding, the main difference between the gravity foundations and the plate foundations is the design of the skirts to resist uplift loads. Experiences from concrete gravity platforms, and in particular the Gullfaks C structure with 22 metres deep skirts, have provided valuable background for the design of the skirts, the load transfer mechanisms and the planning of the installation operations. DEVELOPMENT OF THE PLATE FOUNDATIONS Statoil first investigated the ability of a gravity foundation in steel jackets to support tension loading for the Veslefrikk jacket installed in 1989. The slender jacket in 175 meters water depth had a problem with on-bottom stability during the installation. The mudmats on the jacket were rather large due to soft seabed conditions, and they had one meter deep skirts for resistance against sliding. Intuitively it was expected that the mudmats would be able to support some tension load. Little information was found on suction capacity, but based on what was available the necessary tension capacity was proven. The installation of the Veslefrikk jacket was also affected by pile driving problems, which is not a uncommon problem in offshore installations. Based on the realisation about suction capacity of mudmats and reoccurring pile driving problems, it was felt that the foundations for offshore jackets could be simplified and improved. Thus the work on developing the plate foundations started at the end of 1989. The initial work was spent on developing typical structural configurations (ref. 4) and determine typicaI area requirements and skirt depth for the foundations. The geotechnical design is a very important factor of the plate foundations. The development of design procedures and offshore and onshore test programs to develop the technology are described in more detail in ref. 1.