This State of the Art report deals with the experience from the foundations of the concrete gravity oil or gas platforms which have recently been installed in the North Sea. The report was prepared by a Working Group formed in November 1976 by the FIP Commission on Concrete Sea Structures. Members of the group represent a cross-section of the foundation engineering interests which have been involved in this recent experience.

Through a number of working sessions and the preparation of original papers, the Working Group has distilled the state of the art into four main themes:

• Geotechnical investigations
• Design procedures
• Installation methods
• Operational experience

The benefit of this work has been considerable for those involved in its preparation, and FIP has published the report for general information to other engineers who may be interested in this experience.

Since the installation of the Ekofisk concrete oil storage tank by Phillips Petroleum in 1973, there has been a rapid development of design and construction technology in applying concrete structures to other locations in the North Sea. In dealing with these offshore locations, many severe problems had to be resolved, chiefly arising from the severity of the wind and waves in this area. For foundation engineers, this presents particular problems due to the innovative concepts of design and their location on deep-water, unprepared seabed sites, for which there was limited experience of soil response to this type of structure.

The soil conditions at the sites, consisting of stiff clays and dense sands, are being observed to be capable of supporting the loads introduced by concrete gravity platforms. It is satisfactory to be able to report that the foundations of all the concrete structures are performing without difficulty.

The highlights of the conclusions contained in this report relate to the following points:

• The quality of information obtained from geotechnical investigation techniques under severe offshore conditions has improved with the development of new equipment.
• Many of the theoretical analyses of geotechnical design problems related to foundation behaviour have been confirmed by observed experience.
• The reliability of installation procedures has been demonstrated, and careful monitoring has provided further data which has improved construction methods.
• The interaction between the foundations and the installation and operation of platform equipment for conductors and well drilling has been observed, with methods devised to minimise disturbance to seabed foundation soils.

The report has been divided into sections, and each section has been written by individual members together with the collaboration of colleagues in their organizations. The complete report has been reviewed by all members of the Working Group, who have endorsed it in the form in which it is to be published. The Chairman wishes to acknowledge the considerable voluntary effort made by all concerned. Their cooperation in the preparation of original work for this report has led to a significant advance in published information being made available to others from the unique experience they have gained in the North Sea.