G.R. GAMSAKHOURDIA, M.Y. SHOKALSKIY, A.A. SVERTILOV Non-profit Scientific Technical Centre «Fugro-Jacques», Russia

Offshore engineering - geological surveys, are earned out for objects of the assigned geotechnical risk. First of all on the Arctic shelf it is projects on exploration and field development of gas-oil fields. Experience of such researches is generalized in the SP 33-101-00 «Engineering surveys on a shelf for construction of marine oil-gas field facilities».

At offshore researches, geophysical methods are most effective for revealing and contouring zones of development of dangerous geological processes and phenomena.

In these zones is necessary to use "direct" methods for a quantitative estimation of geotechnical soils properties. These are field tests and. first of all. Cone Penetration Test (CPT) [1].

For many years Company "Fugro" develops and introduces in the researches technology of field tests and sampling in sea conditions. The structure of researches and a technique depend on a stage of engineering survey, complexity of engineering - geological conditions, depth of the sea and capacity of investigated thickness. Researches carry out from the specialized courts, in two (and more) a stage (geophysical-hydrographic and geotechnical). Geotechnical researches carry out by results of hydrographic and geophysical works. The optimum combination of "direct" and "indirect" methods determines reliability of engineering - geological model of the investigated site.

Studying of soils properties is the important part of geotechnical researches. Soils structure is possible to define precisely on the selected tests by laboratory methods. Geotechnical properties depend on an intense condition and other natural fields: therefore they always change after extraction soil from a massif. Any mine working also changes a natural field of massif stress, therefore borehole methods (Pressuremeter Test etc.) analysis soils in somewhat different changed condition. In addition to it is necessary to drill a borehole and to make installation and dismantle of the equipment, in the beginning and the end, each investigated interval. Methods without borehole logging services or sounding when probes take root (pressed) into a soil massif supplied by sensor measuring «in situ» strength, deformation, electric, optical, thermophysical, acoustic soil properties, and also their humidity, contents in pore solutions of oil hydrocarbons, etc. Undoubtedly, most the wide circulation in practice of engineering researches was received with a method of Cone Penetration Test (CPT).

The equipment and technology of carrying out of tests in sea conditions substantially differs from overland. Exception is represented with works from ice, on small depths, in this case it is possible to use overland installations, but tests are necessary for carrying out with beading of "trailing" bars to prevent their bend.

Specificity of engineering - geological researches besides deadlines of field works (hydrometeorological conditions) consists in the Arctic pools in presence of extensive cryolieete zone, including seasonal cryopegs. Therefore geotechnical soils characteristics by results of CPT is impossible without the temperature characteristic of a structure of tests. This problem successfully is solved with the help of temperature static sounding (TCPT). Experience of carrying out of such measurements on the Arctic shelf of Russia is generalized in works АМИГЭ [2,3].

In presentation the complex approach to studying a working platform is submitted. The complex allows to reveal the dangerous phenomena and to develop the reliable geotechnical decision. The review of the sea geotechnical equipment for tests of a ground in a file is made.

References

1. Gritsenka LI., Bondarev V.N., Loktev A.S. et al. Sub-bottom permafrost and shallow gas in Pechora sea. //Offshore Technology Conference. OTC'97, USA. Houston . May 5-7, 1997.- Paper OTC 8324.

ARCTIC SHELF OIL AND GAS CONFERENCE 2004