Storegga Slide Risk Assessment

Abstract

Abstract The Ormen Lange gas field is located at water depths of about 850 to 1,100 m. The field is situated approximately 100 km off the northwest coast of Norway, within the scar of the gigantic prehistoric Storegga Slide (Fig. 1). The slide, which took place about 8,200 years ago, is one of the largest known submarine slides in the world. Evidence of flood waves (tsunamis) generated by the Storegga Slide, have been found along the coast of Norway, Scotland, Shetland and the Faroes. This paper describes the risk analysis framework and the quantification of natural and project-generated slide risk for both third and first party. The main part of the work focused on the probability for new submarine slides in the Ormen Lange area and their potential consequences. Another important aspect of the work was to understand and explain how the prehistoric Storegga Slide could occur. The developed methodology provides a rational framework for quantifying the risk from submarine slides in other offshore areas. A regional geological model was required to explain the Storegga Slide. The slide was most likely initiated 150 km downslope from the Ormen Lange gas field and developed as a retrogressive slide. The unstable sediments in the area disappeared with the slide 8,200 years ago. A new ice age with infilling of glacial sediments on top of marine clays in the slide scar would be needed to create a new unstable situation. The slopes around the Ormen Lange field are stable and slope stability will not change due to the Ormen Lange activities. The positive conclusion of this work made it possible to obtain approval for field development from partners and authorities, which again gave access to the second largest gas field offshore Norway. Fig. 1. Location map showing the Storegga submarine slide on the Mid-Norway margin.(Available in full paper) Introduction The Ormen Lange gas field was discovered in 1997 and is the second-largest gas reservoir in Norway with estimated 397 billion Sm3 recoverable gas reserves. The field is located within the scar from the Storegga Slide (Fig. 1) and close to the steep headwall of the slide (Fig. 2). This gigantic submarine slide occurred 8,200 years ago. The headwall of the slide is about 300 km long and the total run-out distance was close to 800 km. The estimated slide volume was 3,000 km3. The slide caused large waves (tsunamis) that also reached the coasts of Norway, Scotland, Shetland and the Faroes. The heights of the tsunamis were up to 10-12 m along the coast of western Norway and locally up to about 25 m on Shetland (Ref. 1). Fig. 2 Location of the Ormen Lange gas field in relation to the steep headwalls of the Storegga Slide.(Available in full paper) Problem definition. When the Ormen Lange license was awarded, knowledge about the Storegga Slide was limited and it was uncertain if the seabed in this area was stable. The timing and number of slide events were also uncertain. However, one large event around 8,000 years ago was known from previous work (Ref.2).