GEOTECHNICAL ASPECTS OF FOUNDATION INVESTIGATIONS FOR PRODUCTION PLATFORMS AT NORTH RANKIN

Abstract

Production platforms required for the development of the North Rankin gas/condensate field will be founded on deep deposits of loose to weakly cemented calcareous material of predominantly marine origin. Although the data base for foundation design in these materials is extremely limited, experience has shown that calcareous sediments exhibit unusual engineering characteristics and that in situ load tests appear to be the most reliable indicator of soil-bearing potential.

The foundation investigations performed at North Rankin, in 1978, integrated conventional soil sampling techniques with cone penetrometer testing (CPT), which is a small-scale load test that provides a semicontinuous profile of soil resistance. This being the first time that CPT had been used extensively in calcareous sediments, a series of innovative prototype in situ load tests were performed to provide data for calibration of the core resistance profile.

A modified carbonate classification system was introduced into the programme and, together with detailed geological analysis of specimens, it provided an insight into subtle variations in lithology and cementation throughout the profile.

These new developments and the comprehensive nature of the investigation produced a large amount of data that are relevant to other locations where similar sediments exist.

As a result of these investigations and subsequent studies, Woodside Petroleum Development has been able to progress confidently in the definition of a suitable foundation system and the design of the North Rankin platforms.