Surface sediments and their geotechnical characteristics in the development area of deepwater gas field LS17-2

The gas field LS17-2, the first deep-water and high-yield gas field discovered independently by CNOOC in the Qiongdongnan Basin, South China Sea, is a large gas field with a proven reserve more than 100 billion cubic meters. Studied in this paper are the geophysical data, such as multi-beam echo sounder data, backscatter strength data, side scan sonar data, and sub-bottom profiler data as well as seabed surface sampling and drilling data, collected by the engineering survey vessels and autonomous underwater vehicles within the development zone of the gas field LS17-2 in the water depth between 200 m and 1600 m. Types, distribution patterns and geotechnical properties of subsurface sediments are described. The bathymetry data suggests that the study area of the development zone of the gas field LS17-2 could be subdivided into three subareas, i.e. the shelf subarea, gentle slope subarea and slumping subarea. The seabed sediments are predominated by clay and silty clay, with sandy deposits in some places. The geotechnical characteristics are quite different in different sampling locations. The geotechnical characteristics of seabed sediments within the gentle slope subarea and slumping subarea are typical deep-water sediments, characterized by high water content, low density, high void ratio, high liquid limit, high plasticity and low strength. The undrained shear strength of the soil layers between seabed and the layer about 0.3 m below seabed varies between 0～4 kPa, and it is conducive to the laying of submarine cables, submarine pipelines and umbilical cables. Soil conditions in the shallow part are suitable for the foundation of suction type, mud mat and grab anchor in the gentle slope subarea, and for the types of mud mat, grab anchor and driven piles in the slumping subarea. The research results are believed useful as a reference to the design and installation of subsea facilities for the coming deep-water oil and gas field development projects in the Qiongdongnan Basin.