Sampling in deep water (>500 m) is challenging. Gravity core type samplers are frequently used due to operational aspects giving more disturbance than thin wall steel tube piston sampling. Additional disturbance due to large total stress relief can occur when the samples are brought to ship deck. These disturbances may be very large, especially when dissolved gas is present in the pore fluid. During stress relief this gas may come out of solution and expand.

This paper presents the first phase of a joint industry project. The objectives of the project have been to examine the problem of sample disturbance in deep water sampling practice. The problems with dissolved gases are not addressed in this first phase.

NGI has carried out a comprehensive laboratory test programme on two clays. The low plastic Lierstranda clay (Ip = 10–17%) and the medium to high plastic Bothkennar clay (Ip = 45%). Benchmark reference soil parameters have been obtained from CAUC triaxial and CRS oedometer tests on high quality block samples. The same laboratory tests have been repeated on samples obtained with piston tube sampling and samples subjected to extra disturbance to simulate thick wall gravity core sampling. A series of special tests have also been carried out where sampling in 1500 m water depth have been simulated in the laboratory.

Based on these test series it has been possible to quantify the amount of disturbance caused by thick wall tube sampling, and the also by large stress relief which occurs when sampling in 1500 m of water. The results have been used to develop a new criteria for sample disturbance and will also be used to establish procedures for correcting results of CAUC and CRSC tests to arrive at soil parameters that are as close as possible to in situ conditions.

Problem definition

For deep water soil investigations it is highly probable that many of the obtained samples will be more disturbed than those obtained m shallow water or onshore. The main reasons are:

  • the material may be more sensitive to disturbance due to various geological factors, such as the presence of very brittle ooze material

  • control of the sampling process will be reduced due to large water depth the samples may be obtained using simple sampling equipment from general survey vessels

  • stress relief dung recovery of the sample cause expansion and disturbance; sampling from 1500 m of water may give as much as 0.7% volumetric expansion even If no gas is present. With gas dissolved in the pore water large expansions can occur

  • possible gas hydrates can melt, expand and cause disturbance to the sod structure.

These potential disturbances may have a serious influence on the quality of the soil parameters obtained from laboratory tests on these samples. Statoil took the initiative to carry out a study into these effects Subsequently this study led to a larger Joint Industry Project supported by a number of oil companies. The objectives of the study have been to:

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