The Ormen Lange Geohazard Experience: Best Practice for Geohazard Evaluations of Passive Continental Margins

Abstract

Abstract The Ormen Lange gas field is developed in the slide scar left after the giant Storegga slide that happened only 8200 years ago. The slide event caused a major tsunami hitting the costlines around the North Sea. The risk process experience gained through the Ormen Lange project forms the basis for best practice recommendations included in this paper. This paper describes the type of data to be considered included in a database for a geohazard study. The value of a regional geological model as input to a slide risk evaluation is demonstrated. The slide risk conclusion for the Ormen Lange project is to a large degree based on a logic explanation of why a new slide cannot happen until the pre slide conditions are re-established (after a new glacial period). Best practice for the slide risk approach is described. This includes use of a standard risk system analyses framework and formal requirements to risk acceptance criteria's. Use of a hazard identification process (HAZID) for slide explanation and for the present day stability evaluation is documented. The HAZID process is used to identify requirements for natural and project generated triggers/driving forces and destabilizing factors that should be documented as part of the risk process. A risk estimation flow chart is presented. Further the main methods of approach for the geosciences documentation is illustrated with emphasis on input to the slide risk estimation process. Further the complexity of such a slide risk project is demonstrated and the typical topic included in such a risk process is documented. Introduction The Ormen Lange gas field is located in the deepwater margin outside Mid Norway approximately 100km from the coast. The first deepwater licences in Norway were awarded in 1996 and the field was discovered in 1997. The Ormen Lange gas reservoir represents the second largest gas field in Norway. The field is located close to the steep headwall of the slide scar left after the giant Storegga slide that happened about 8200 years ago (Fig. 1). Fig. 1. Location map showing the Storegga submarine slide scar and the Ormen Lange gas field on the Mid-Norway margin (available in full paper) Limited knowledge about the slide was available at the time of the award of the licence blocks in 1996. A common work for all the new deepwater licences was started to acquire new seafloor - and geotechnical data to get a better understanding of the geohazards in the area with special emphasis on the slide issues. In addition to the Storegga slide another major slide was known in the area, namely the Trænadjupet slide. However, little was known about the timing of the slides, the triggering mechanisms, slide frequency, slide morphology and the pre slide and present soil conditions. Existing age information about the Storegga slide indicated several events of which one major event was dated to around 8000 years BP. Similar age of tsunami sediments found and dated onshore indicated that the Storegga slide, or parts of the slide, had caused a major tsunami hitting the coastline around the North Sea with wave heights at least up to 12m -15m.