The Storegga submarine slide area offshore Norway has been extensively investigated by ‘geoborings’, i.e. geological and geotechnical boreholes to several hundred meters below seabed Location of the boreholes was based on an extensive grid of high-resolution seismic data. The sampling and testing programme have been targeted at seismic units and horizons and include both standard geotechnical sampling and testing as well as geophysical wireline logging, porewater sampling (ambient pressure) for gas content investigation, piezometers (permanent pore pressure installations at large depths) and piezoprobe dissipation testing. In one deep borehole (450 m below seabed) rotary coring technique was applied rather than the standard geotechnical push sampling. Shipboard examination and testing of the samples and further onshore laboratory testing programmes were comprehensive. Selected examples of testing results are included in this paper. The investigations have all taken place at a glacial deepwater margin where up to 5001.11 of sediments have been removed by a submarine slide some 8200 years ago Water depths m the investigated area range from 250 to 1630 meters. The practical problems of drilling in hard glacial clays with significant boulder and stone content are discussed Installation of permanent piezometers for continuous in situ pore pressure monitoring more than 200m below seabed took place during the geoborings campaign. Methods and equipment are described and examples of m situ pore pressure measurements are shown. A range of geophysical logging tools were applied and these provided continuous information with depth, provided input to targeted sampling programmes and also enabled exact cross correlation between boreholes. The main objectives of the geoborings were to provide input to a geological model of the area including understanding of the past Storegga slide and further to provide a geotechnical database for estimating present day stability of the area. The paper discusses how these objectives were met and includes examples of the acquired data.
The Ormen Lange gas field was discovered in 1997 with the slide scar from the Storegga Slide The recoverable gas reserves are 400 billion scm and production is planned to start m 2007. Norsk Hydro is the operator for development and construction and Norske Shell will be the operator for the production The other partners m the Ormen Lange license are: Statoil, BP, ExxonMobil and Petoro
The gigantic submarine Storegga Slide occurred about 8200 years ago, and caused large waves (tsunamis) that also reached the coasts Norsk Hydro has initiated extensive work to evaluate the present stability conditions in the vicinity of the Ormen Lange gas field and to explain the prehstoric sliding. (Fig. 1 is available in full paper)
Three papers are presented on the Ormen Lange and Storegga Slide topic at the 2002 International Conference Offshore Site Investigation and Geotechnics. These are
Storegga Geomodel and its use in slide risk evaluation, by P. Bryn et al.
Ormen Lange Geoborings, (this paper) by T.I. Tjelta et al.
Ormen Lange Slope Stability, by Tore Kvalstad et al.