Case Study of an Integrated Geophysical and Geotechnical Site Investigation Program for a North Sea Platform

Abstract

Abstract As Operator of the North Brae Field, Marathon Oil U.K. Limited needed to determine:a suitable location for a large production platform in Block 16/7a of the U.K. sector of the North Sea, andthe foundation conditions at the selected location. Previous experience in this region indicated that the soil conditions were extremely variable over distances that were even shorter than the lateral dimensions of the platform due to the presence of deep in filled glacial channels. As a result of this variability, it was critical to carefully locate the platform, particularly when the ultimate axial loads on individual piles were envisaged to be as high as 11,000 kips. This paper describes all the components of an integrated geophysical and geotechnical site investigation programmer used to accomplish the stated objectives. The details of the initial planning of the high-revolution geophysical survey, using the available geophysical and geotechnical data, are described. The choice of geophysical equipment, both analogue and digital and computer based navigation systems are discussed with respect to resolution, reliability and limitation of the available equipment. The results of the geophysical survey and the manner in which they were utilized to select the location for site specific geotechnical investigations are presented. The navigational requirements for the soil investigation vessel and the equipment used during the geotechnical phase of the programmed are discussed. The paper indicates how well the soil layers revealed by the soil borings agreed with the geophysical interpretations and predictions. Finally, the benefits and need of an integrated site investigation programmed such as the one described in the paper are emphasized with regard to future development programmers. Introduction Early in 1981, Marathon Oil U.K. Limited commenced a site investigation programmed aimed at selecting a suitable location for a large drilling and production platform in the North Brae Field in Block 16/7a of the U.K. sector of the North Sea. The primary objective of the programmed was to identify a location where the foundation sediments showed the most favorable foundation conditions to support a piled structure for which the ultimate axial loads on individual piles were envisaged to be as high as 11,000 kips. The programmed consisted of a detailed engineering geophysical survey in order to select: a suitable location for site specific geotechnical investigations after interpretation of the geophysical survey data. The work was performed during Spring-Summer 1981. The North Brae Field is located approximately 155 miles north east of Aberdeen, Scotland. Previous experience in the southern part of the field indicated that the foundation sediments were extremely variable over distances even shorter than the lateral dimensions of the platform, this being due to the presence of deep in filled channel features. The possibility of encountering such soil conditions in the northern part influenced both the planning and the selection of equipment for the geophysical investigations.