The Storegga Landslides: Catastrophic Underwater Natural Methane Explosions

Methane is a small molecule made up of a single carbon atom surrounded by four hydrogen atoms, which exists in an ice-like form variously called “methane hydrate”, “methane ice” or “methane clathrate”. Clathrate means “cage”, which describes the structure of methane ice: a cage of water molecules around methane gas molecules, allowing high methane concentrations. One unit volume of methane hydrates contain over 160 volumes of methane gas and less than one unit of water at surface pressures and temperatures. (1) Methane ice burns when it meets fire.

Methane ice was originally thought to occur only in the outer regions of the solar system. Today it is well known to occur abundantly in earth’s marine and Arctic permafrost sediments. For example, the United States has as much as 200,000 trillion cubic feet of methane in hydrate systems in (Alaskan) permafrost regions and surrounding waters, which is over a hundred times greater than the estimated conventional US methane gas resource. (2)

Methane ices are known to pose risks for 1) marine safety and 2) seafloor stability, as follows:  

1. Marine Safety and Methane

Arctic and marine hydrates can cause problems during drilling and production of conventional hydrocarbons. “Difficulties include gas release during drilling, blowouts, casing collapse and well-site subsidence. These problems are generally the result of dissociation of gas hydrates caused by the heat of circulating drilling fluids or flow of warm production fluids. Pipelines carrying warm fluids may suffer loss of support due to underlying hydrates,” according to the Department of Energy’s “Strategy for Methane Hydrates Research and Development.” (3) Little is known about the long term impacts on seafloor stability and safety due to methane gas production from methane ice (see more below).

2. Seafloor Stability, Methane Outgassing, and the Storegga Submarine Landslides

Methane ices can break down at certain temperatures and pressures, permitting the gas in the clathrate cages to be released. This process is what experts believe triggered the ancient catastrophic “Storegga” submarine landslides off the coast of Norway. The Storegga landslide complex is a world-class geographic feature and one of the largest areas of known slope failure anywhere in the world.

The complex consists of three very large underwater landslides known to have taken place during the last 100,000 years. The landslides departed from the destabilized slope and “flowed” into the deep ocean crevasses below. The Second Storegga Slide was large enough to have caused a megatsunami around 7,100 years ago that triggered widespread coastal flooding in Scotland, Norway and other coastlines bordering the eastern North Atlantic and North Sea. (4) For example, at a number of localities near the eastern coast of Scotland is a sand deposit as deep as 25 feet above sea level that has been dated to about 7,000 years ago. One researcher in 1989 proposed that this sand is a megatsunami deposit resulting from the sediment displacement associated with the Second Storegga Slide. (5)  

Bathymetry and acoustic seafloor imagery of the Storegga Slide have identified seafloor depressions or “pockmarks” up to 1500 feet in diameter and less than 15 feet in depth, which are associated with the presence of gas. (6) The pockmarks are consistent with the remnants of old methane gas explosion sites that triggered the landslides at Storegga.

Extensive seafloor mapping following the December 2004 Indonesia earthquake/tsunami was conducted to identify submarine landslides and pockmarks similar to those at the Storegga Slide complex. (See “Dramatic Direct Visuals of the December 2004 Bay of Bengal Epicenter Earthquake Rupture Zone” at: http://www.semp.us/biots/biot_187.html; accessed November 26, 2005.)

Commercial Production from Deep Sea Methane Fields

The Storegga Slide was discovered while companies were searching for oil and gas in the North Sea Northern. “Norsk Hydro”, one of the world's largest offshore oil companies, discovered the “Ormen Lange” natural gas (methane) field in the Storegga Slide depression in 1997. The company has committed to the commercial production of natural (methane) gas from the Storegga Slide depression even though at a depth of approximately 2,700 feet, the site is so deep that the 24 wellheads and 120 km pipelines to shore must be built using underwater robot technology and advanced installation techniques. (7) No conventional offshore platforms will be used. A direct pipeline will be built from the Norway refinery to England.

Other challenges confronting Norsk Hydro in the commercial production of methane from the methane fields in the Storegga Slide area are:

1. Subsea temperatures below zero, which cause the methane gas and water in the pipelines to form brash methane ice, blocking the pipes if production were to stop. Ensuring good flow in the pipelines is critical, so Hydro’s solution is to add continuous anti-freeze at the wellheads to prevent the well stream freezing. The anti-freeze will be separated out and reused when it arrives onshore.
2. Mountainous seabed topography, requiring difficulty routing of pipelines through peaks that rise between 90 feet and 180 feet. The pipelines must be routed through this rocky underwater landscape in such a way that unsupported pipe spans don’t become too long, allowing unacceptable vibrations, or ensnare fishing trawler’s nets.
3. Some of the strongest underwater currents anywhere in the world, and some of the stormiest weather conditions.
4. Requirement for well stream pipelines to ascend an 1800 feet high underwater cliff created by the massive Storegga Landslide 7,000 years ago.
Danger of Commercial Drilling of Methane Fields, Seafloor Destabilization, and Tsunamis
Before Norsk Hydro began its commercial production of methane gas, it obtained expert risk analysis of the probability that drilling and removing methane gas would destabilize the seafloor, cause more landslides, and huge tsunamis. The experts concluded that this would not happen. (4)

Sources:

1. US Department of Energy and Office of Fossil Energy: “A Strategy for Methane Hydrates Research and Development,” p. 10, at: http://www.netl.doe.gov/scngo/NaturalGas/hydrates/pdf/98hydratestrategy.pdf; accessed November 26, 2005.

2. Ibid, p. 1.

3. University of Wisconsin: “Chemical of the Week: Methane” at: http://scifun.chem.wisc.edu/chemweek/methane/methane.html; accessed November 26, 2005.

4. The Tsunami Initiative: “Tsunami Risk in the Northeast Atlantic: The Storegga Slides.” At: http://www.nerc-bas.ac.uk/tsunami-risks/html/HSE1Storegga.htm; accessed November 26, 2005.

5. Dan Evans: The BGS deep-two boomer meets the Storegga Slide” in The Edinburgh Geologist, Issue no. 28, Autumn 1995, available at: http://www.edinburghgeolsoc.org/z_28_04.html; accessed November 26, 2005.

6. JP Foucher: “Fluid Escape Structures on the Storegga Slope.” Geophysical Research Abstracts. European Geophysical Society, 2002. Available at: http://www.cosis.net/abstracts/EAE03/11149/EAE03-J-11149.pdf; accessed November 26, 2005.

7. “Hydro” website: “Boiling tea with gas from chilly seas” at: http://www.hydro.com/en/press_room/features/ormen_tea.html; accessed November 26, 2005.