A New Seabed Penetrometer

Abstract There is today no operational system able to perform geotechnical measurements with standard 36 mm CPT cones in deep sea with a depth of investigation of 20 to 30 metres below the sea-bed. The advantage of the new system is that it will allow surveys in deep sea in a more cost-effective manner. GEOCEAN-SOLMARINE, a French Marine Contractor with deep water geotechnical tools capabilities and IFREMER, a French Research Institute for exploitation of the sea, joined to design and build a new seabed penetrometer which will be operated by GEOCEAN-SOLMARINE for industrial operations and by IFREMER for scientific applications. The IFREMER's version is now under assembly. The first tests at sea of the penetrometer are expected for mid-2000. Introduction The current trend for offshore oil developments is to move into deeper water. There will therefore be a growing need for reliable soil design parameters in deep water, for the design of structure foundations, for the anchoring of floating production and storage systems, for pipeline laying and for slope stability problems. Scientists are also interested in in-situ measurements for different topics: sediment characterisation, slope stability, environmental studies, hazard mapping, stratification logging, indication of stress history and lithology deposits,... The tool designed by GEOCEAN-SOLMARINE and IFREMER is a frame which is lifted over the seabed and which remains during the duration of the measurement. The penetrating system is based on a flexible rod which is pushed by friction into the soil. For scientific applications, the tool is called "Penfeld", the penetration force is limited to 40 kN which allows a penetration of 20 metres into the soft soils encountered in deep seas. Several locations can be investigated during the same deployment of the system. The industrial version called "DS7000" will be built with higher capacities. In a first step, the penetrometer will be equipped with a conventional cone including the measurements of point resistance, shaft resistance and pore pressure. Further a new cone will be developed specially devoted to high pressure environment, including differential pore pressure and complementary sensors such as a gamma-densimeter and a thermometer. Need of sea bed measurements in deep sea The offshore activities are now moving from the continental shelf to deep seas. This change modifies the needs in soil surveys. On the continental shelf, the main object of the surveys for foundation design of platforms was to obtain information on the soil for the design of the piles for the anchoring of platforms. The piles were 50 metres, 100 metres long, even more. A typical survey consisted generally in a cored borehole over all the length of the pile, and an instrumented borehole, using a wire line Cone Penetrometer Test performed by length of 3 metres. In order to perform such a survey, a geotechnical vessel with facilities in drilling was needed. These methods are operational only in water depths less than 200 metres.