The Ormen Lange gas field is located in about 900 to 1100 m water depth in the slide scar of the enormous Storegga Slide that occurred about 8000 years ago. The slide left steep and high headwalls above and below the planned field development area. Today's stability of the headwalls is a major concern for the field development work The area under consideration is large and has been mapped extensively with 2D and 3D seismic profiling The number of geotechnical borings is limited and integration of geological, geophysical and geotechnical information was required to develop a geotechnical model of the area. Stability analyses have been carried out for critical sections of the headwalls. These involved long-term drained analyses under gravity loading and undrained analyses considering the effects of earthquake-loading and possible Influence from field installations like rockfill supports for pipelines and anchors. Focus has been set on explanation of slide mechanisms involved in the Storegga slide and comparison of the stress-strain-strength conditions in the headwall at the tune of the slide and today. Work is still ongoing and under review and the conclusions presented here are thus to be considered as preliminary.
The Ormen Lange gas field was discovered m 1997 within the slide scar of the Storegga Slide The recoverable gas reserves are 400 billion sm3 and production is planned to start in 2007. Norsk Hydro is the operator for development and construction, and Norske Shell wll be the operator for the production. The other partners in the Ormen Lange license are Statoil, BP, ExxonMobil and Petoro. Three papers are presented on the Ormen Lange and Storegga Slide topic at the 2002 International Conference on Offshore Site Investigation and Geotechnics. These are
Establishing a geological model for the regional understanding of the Storegga Slide, by P Bryn et al.
Ormen Lange geobonngs by T.I Tjelta et al.
Ormen Lange slope stability (this paper) by T J. Kvalstad et al.
The Ormen Lange gas field is located in the Norwegian Sea about 130 km WNW of Kristiansund. Figure 1 shows the location and the approximate extension of the Storegga Slide The Storegga Slide is one of the largest submarine slides m the world (Bugge, 1983 and Bryn et. al., 1998) and occurred about 8200 calendar years ago. The upper headwall scar has a total length of about 290km. The estimated soil masses involved were (according to Bugge, 1983) in the order of 5600 km3 More recent updates indicate somewhat lower volumes Evidence of a tsunami generated by this slide event has been found along the coast of Norway, Scotland and the Faeroe Islands. Figure 2 shows an exaggerated bathymetric picture of the central, upper part of the slide showing the upper and lower headwalls of the slide.