Soil conditions and slope stability in the Ormen Lange area

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.