Offshore platforms are increasingly being planned and installed in calcareous soils of marine origin. Experience shows these materials to exhibit unique mechanical characteristics and variable cementation. However, very few data are available for foundation design In dealing with the design of major structures in these particular soils, three major problem areas can be identified:
An objective soil classification system that incorporates degree of cementation is not available.
There are no published data or experience of the performance of calcareous soils under lateral load.
It is not possible to correlate the results of conventional laboratory tests with the few existing load test data.
Foundation investigations for platform structures have been carried out at the North Rankin gas field, located on the North West Shelf, Western Australia. Foundation soils at this site are loose to well-cemented calcareous silt and sand sized deposits. These investigations specifically addressed the above problems and new techniques and equipment were developed for the project. These new developments and the comprehensive nature of the investigation produced a large amount of data which are relevant to other areas of the world where similar soils exist.
Conventional sampling, laboratory studies and geophysical surveys were supplemented by in-situ cone penetration, plate load and pressure meter testing. In addition, eleven axial load tests and four lateral load test series were performed in-situ on small scale steel piles. The cone penetrometer data, in combination with both visual and laboratory assessment, provided a superior and objective classification system which was substantiated by the geophyscial data. The in-situ load tests produced a consistent set of results which correlated well with the available pile load data. However, conventional laboratory strength testing was again found to be an unreliable method of prediction of load capacity. The paper describes the field investigations, including the developed techniques and classification system. Typical results are shown and are used to illustrate the conclusions. The investigation produced an increased understanding of the behaviour of calcareous soils, and has enabled pile design to proceed confidently.
The initial offshore production platforms for the North West Shelf Development Project will be located on the North Rankin gas/condensate field (Fig. I), situated 130 km off the north-west coast of Western Australia. The first of these structures, Platform ‘A’ will support a maximum topside load of 20,000 t and will be located in approximately 125 m of water. The platform support structure will weigh about 16,000 t (excluding piling) and will be subjected to cyclonic conditions which will impose high axial and lateral loads on the foundation.
The structure will be supported on four main legs, each with eight 1.83 m diameter piles driven to a distance of 119 m below the mudline. The seabed material at the site is composed of loose, weakly cemented calcareous sediments of marine origin. Engineering experience of this type of material is extremely limited and the few data available generally indicate inferior load capacity compared with siliceous deposits.
