Soil conditions and slope stability in the Ormen Lange area Tore J. Kvalstad a, * , Farrokh Nadim a , Amir M. Kaynia a , Karl Henrik Mokkelbost a , Petter Bryn b a Norwegian Geotechnical Institute, P.O. Box 3930, Ullevaal Stadion, N-0806 Oslo, Norway b Norsk Hydro, N-0246 Oslo, Norway Received 2 December 2003; received in revised form 15 June 2004; accepted 29 October 2004 Abstract The Ormen Lange gas field is located in 850–1100 m water depth in the slide scar of the enormous Storegga Slide. This slide occurred about 8200 years ago and left steep and high scarps above and below the planned field development area. Today’s stability of these scarps has been a major concern for the field development work. This paper gives an overview of soil investigations, evaluation of potential trigger mechanisms and stability analyses performed to demonstrate adequate conditions. The area under consideration is large with a limited number of geotechnical borings, and integration of geological, geophysical and geotechnical information was required to develop a geotechnical model of the area. The stress history and the soil conditions are strongly influenced by rapid deposition during glacial periods, by the loss of overburden from the Storegga Slide and glacial loading along the shelf edge. Natural and project induced triggering sources with potential for generation of renewed slide activity were evaluated with respect to magnitude and location and possible influence area relative to the major scarps. Stability analyses were carried out for critical sections of the scarps under gravity loading and under combination of gravity and potential triggering sources applying drained analyses for long-term gravity loading and undrained analyses considering gravity combined with short term load effects. Project induced load effects like weight of equipment and rockfills, anchor loads and effects of drilling and production activities were found to have insignificant effect on the large scale stability of the headwalls, and local, surficial slope failure can be handled by adequate engineering and lay-out. With respect to natural triggering sources it was concluded that an extremely strong earthquake with a very low probability of occurrence is the only realistic mechanism for triggering of new slides in the Ormen Lange area. q 2004 Elsevier Ltd. All rights reserved. Keywords: Slope stability; Soil strength; Pore pressure; Gravity; Earthquake 1. Introduction The Ormen Lange gas field is located in the Norwegian Sea about 120 km WNW of Kristiansund in the slide scar of the Storegga Slide (Fig. 1). The Storegga Slide is one of the largest submarine slides in the world (Bugge, 1983; Solheim et al., 2005; Bryn et al., 2005) and occurred about 8200 calendar years ago (Haflidason et al., 2005). The slide scarp forming the upper headwall has a total length of about 290 km. The soil mass involved in the slide was about 3000 km 3 . In the deep, upper part of the slide scar where the Ormen Lange field is located, the headwall slide scarps are steep, with heights from 100 to 250 m (Fig. 2). The stability of these scarps and the potential for future debris flows into the field development area or pipeline corridors have been of major concern for the field development work. In order to estimate the risk related to slope instability in the Ormen Lange area, failure mechanisms, triggering sources and location of the most critical areas relative to the field installations had to be determined. This was done through a thorough evaluation of site conditions, develop- ment of a geotechnical model of the area and deterministic slope stability analyses. The selected approach and the main results of this work are described in this paper. 0264-8172/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2004.10.021 Marine and Petroleum Geology 22 (2005) 299–310 www.elsevier.com/locate/marpetgeo * Corresponding author. E-mail address: tk@ngi.no (T.J. Kvalstad).