EXTENDED ABSTRACT

Until very recently, standard procedure in North Sea oil investigations has been to perform mostly simple index tests on soil samples in the offshore field laboratory. More sophisticated laboratory testing on samples is carried out in the onshore laboratory with the purpose of determining soil design strength and deformation parameters for foundation design. These onshore laboratory tests are normally carried out one to several weeks, sometimes months, after sampling (see Andresen et al., 1979).

It has not been clear what effects the transport of the samples to the onshore laboratory and the age of samples have on the derived soil properties. Until recently this unknown effect has been considered both theoretically and in the laboratory (e.g. Kirkpatrick and Khan, 1984)

Over the last couple of years several major North Sea Soil investigations have been carried out with a comprehensive programme of offshore laboratory testing including:

  1. radiography of samples in the sample tubes;

  2. consolidated and unconsolidated un-drained triaxial tests;

  3. consolidated constant volume direct simple shear tests; and

  4. constant rate of strain consolidation tests.

Radiography, direct simple shear tests and constant rate of strain tests have been performed in special containers built at NGI and brought on board the drilling vessel (Lacasse et al., 1984: Sandbaekken et al., 1985). Offshore triaxial testing has been provided by McClelland and Fugro.

For one particular job at the Troll East Field in the Norwegian Sector of the North Sea all of the above special tests were performed both offshore and onshore on parallel samples. For this job performed in the Summer of 1984 Norsk Hydro contracted NGI to be main soil investigation consultant while McClelland Ltd was subcontracted by NGI to perform sampling offshore triaxial testing and in situ testing.

Special care was taken to test samples offshore quickly after sampling and to perform onshore tests on selected parallel samples (Amundsen et al., 1985).

  1. check quality of samples offshore so that, if necessary, sampling procedures and equipment could be modified

  2. facilitate presentation of preliminary soil design parameters during and shortly after the end of field work;

  3. check the effect of transport and storage time on laboratory-determined strength and deformation soil parameters;

  4. define stress history and strength characteristics so that the consolidation stresses and procedures to be used for the onshore testing are as correct as possible. Usually very little time is available from end of the field work to when the laboratory report must be submitted. In the present paper emphasis is placed on the effect of storage time.

Based on a comparsion between results of onshore and offshore tests it can be concluded that it has not been possible to find any significant differences between the offshore and the onshore test results with regard to the following parameters:

  • water content;

  • volume change in consolidation to po in the CRSC tests;

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