Two soil investigations were performed in soft, lightly overconsolidated clays in 1300m of water depth. This paper reviews and compares the results of these investigations and the subsequent onshore laboratory testing. It focuses in particular on the quality of the samples and the performance and efficiency of the seafloor based drilling system that was utilised. The laboratory testing results clearly demonstrate that the lower plasticity clays encountered at Site L of a Norwegian gas field are more susceptible to sample disturbance, compared to the high plasticity clays from Site C offshore West Africa. Previous soil investigations were also conducted in the vicinity of both these fields, and they utilised conventional vessel based drilling techniques. Additional geotechnical information was available from seafloor based systems operated in non-drilling mode in similar ground conditions. The two sites are both deepwater lightly overconsolidated soft clay sites and are geotechnically homogeneous. Given the similarity in water depth, they offer a unique opportunity for comparison of performance and sample quality.

1. Introduction

Two deepwater soil investigations were performed in 2009 and in 2010 utilising a remotely operated seafloor based drilling system - the Benthic Geotech Portable Remotely Operated Drill 1 (PROD 1) (Kelleher and Hull, 2008). The first campaign was at a Norwegian gas field (Site L), and the second was for a development offshore West Africa (Site C). Both sites are remote from shore and from existing field installations, located in ~1300m of water where the soil conditions consist of soft, lightly overconsolidated clays. The detailed field programme at these two sites was aimed at resolving all early requirements for regional and local seabed data for both geohazards and foundation studies. Samples were taken to depths in excess of 40m and complemented with in situ testing.

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