The Forties Field has posed very special problems with regard to thefoundation investigations. This was not only the result of the great waterdepth and the unprecedented type and dimensions of the proposed structures, butwas also caused by the soil conditions encountered. Soft normally consolidatedclays are found to great depth over much of the area. They are underlain bysands and silts and again clays in stratified deposits, that exhibitconsiderable variations in hardness and thickness.
Every effort was made to obtain accurate data about the soil conditions toensure that adequate bearing capacity for the piles would be available and toeliminate driving problems during installation. The investigations continuedover a period of three years from 1971 to 1973. Traditional techniques provedinadequate and new experimental techniques were introduced.
These included precision positioning with seabed transponders, in-situtesting by means of penetrometer tests and radioactive borehole logging, and athorough use of acoustic profiling. The paper presents an outline of the soilconditions in the North Sea and a detailed description of the soils in theForties Field. A review is given of the various stages of the investigationsand of the different techniques used, followed by a critical comparison of theresults. Finally a summary is presented of the driving records of the first twostructures installed in 1974. They provide a check on the accuracy of theassumed soil profiles and soil properties.
The overall result of the soil explorations has proved to be satisfactoryand the work has added greatly to the success of the installation. The studiesof the Forties Field have resulted in a new approach and have set a pattern forall present offshore investigations in the North Sea area.