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Technical paper: Site investigations for the North Sea Forties Field

By Jaap De Ruiter and Denys A Fox, Fugro

This paper was first publsihed in GE’s September 1976 issue

Synopsis

The Forties Field has posed very special problems with regard to the foundation investigations. This was not only the result of the great water depth and the unprecedented type and dimensions of the proposed structures. but was also caused by the soil conditions encountered. Soft normally consolidated clays are found to great depth over much of the area. They are underlain by sands and silts and again clays in stratified deposits, that exhibit considerable variations in hardness and thickness.

Every effort was made to obtain accurate data about the soil conditions to ensure that adequate bearing capacity for the piles would be available and to eliminate driving problems during installation. The investigations continued over a period of three years from 1971 to 1973. Traditional techniques proved inadequate and new experimental techniques were introduced. These included precision positioning with seabed transponders. in-situ testing by means of penetrometer tests and radioactive borehole logging. and a thorough use of acoustic profiling.

This Paper presents an outline of the soil conditions in the North Sea and a detailed description of the soils in the Forties Field. A review is given of the various stages of the investigations and of the different techniques used, followed by a critical comparison of the results. Finally a summary is presented of the driving records of the first two structures installed in 1974. They provide a check on the accuracy of the assumed soil profiles and soil properties.

The overall result of the soil explorations has proved to be satisfactory and the work has added greatly to the success of the installation. The studies of the Forties Field have resulted in a new approach and have set a pattern for all present offshore investigations in the North Sea area. 

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