Ormen Lange and the Storegga slide

Tle Ormen Lange gas field lies near the headwallof the enormous Storegga submarine landslide that took place for over 8200 years ago. NGI's contribution to the project has been to ensure that no such gigantic landslide is about to occur again.

The Ormen Lange gas field is the second largest gas field in Norway after the Troll gas field. For almost 8200 years ago enormous quantities of stones, gravel, sand and clay material slid out from the Norwegian shelf towards the Storegga area. The landslide scarp is about 300 km long and studies have shown that parts of the landslide scar is more than 800 km out into the deep ocean.

This landslide has also resulted in the destruction of Stone Age communities in Western Norway which was devastated by 10-15 m high waves (tsunami). There are also found marks from this event on land in both Scotland and the Faroe islands. The consequences of a new landslide of this magnitude to the pipeline system and underwater production facilities on the sea floor would be catastrophic.

 
Overview map (left) showing thearea affected by the Storegga submarine slide into the Norwegian Sea and the location of the Ormen Lange field. To the right is shown the results from numerical analysis of the tsunami which was generated by the enormous slide.

Norsk Hydro, which was the operator of this gas field, initiated a large project to evaluate the risk of a new landslide in the area, and to investigate what actually caused the Storegga landslide. Before the new Ormen Lange gas field could be developed, reassurances were needed that the chances for a new landslide were minimal and that activities associated with the development and operation of the field would not increasing the risk for a new devastating landslide. Innovative design and construction of the seabed systems were required for the installation on the extremely uneven seabed that was created after the Storegga landslide.

Comprehensive site investigations
Thorough seismic and geotechnical investigations were performed from 1999 until 2006. In situ testing as well as several boreholes with soil sampling down to 200 m below sea floor were made. The water depth varied between 250 and 1600 m. NGI also developed a new pore pressure measuring device that could be placed in the bottom of the boreholes. The device could be placed down to 1100 m below the sea level.
NGI performed laboratory testing on sampled material and evaluated the results from all the site investigations. In addition NGI had the responsibility for several projects in connection with planning of the exact pipeline routes and placement of the pipelines on the extremely uneven sea bed. This also included the crossing of the slide rim.

Cause of Storengga landslide
In order to understand the Storegga slide, we must first consider the facts during the last ice age which ended about 10,000 years ago. When the ice age was at its climax, the sea level was about 120 to 130 m below the sea level today. The melting of the ice, which started about 18,000 years ago, led to movement of enormous quantities of stones, gravel, sand and clay material were dug out from the inner parts of Scandinavia. These masses were transported by the moving icebergs and finally formed gigantic moraine deposits in the so called North Sea Fan (Nordsjøvifta).

This series of photos show (upper) seabed from the tip of the landslide based on bathymetric investigations   (middle) results for seismic investigations that shows stratification and fragmented sea bed and (bottom) Numerical simulation  of the landslide that results in the same type of the fragmented sea bed that was detected by the seismic measurements.

The sedimentation process took place relatively quickly, and the water in the sediments (pore water) was not drained out. This resulted in increasing pore pressure in the sediments, significantly higher than the hydrostatic pressure, as the sedimentation process continued. High pore pressure leads to less particle contact in the soil, and the material thereby has less strength. The combination of high pore water pressure, low shear strength and inclination made the area unstable. It is likely that an earthquake was the triggering mechanism for the Storegga landslide. But since this whole area was so unstable, the slide could have been  triggered by less drastic factors.

When the Storegga landslide took place, it also resulted in the mass movement of moraine material that was deposited during several different ice ages in history. Six or seven different slide surfaces with low friction have been identified. The lowest sliding surface lies close to the bottom of the landslide area, which means that the landslide propagated backwards as it continuously ¿jumped¿ to the next sliding area. The landslide is about 700 m deep. Which means that huge quantities of soil mass was in motion. It is very likely that this landslide process took place during some hours, or maximum during a couple of days.

The Ormen Lange gas field was discovered in 1997. Gas production started Thursday 13th September 2007 with gas transport through the pipeline system to the processing unit at Aukra outside Kristiansund in Western Norway. The investigations of the Storegga landslide has shown the necessity for similar type of investigations in other parts of the world. Several oil companies has therefore contacted NGI and requested assistance in connection with the evaluating for submarine landslides and geohazards along other continental shelves.