Seabed gouging by ice

Seabed gouging by ice is a process that occurs when floating ice features (typically icebergs and sea ice ridges) drift into shallower areas and their keel comes into contact with the seabed.King 2011Palmer & Been 2011Barrette 2011 As they keep drifting, they produce long, narrow furrows most often called ''gouges'', or ''scours''.Wadhams 2000, p. 72Weeks 2010, Ch. 13 This phenomenon is common in offshore environments where ice is known to exist. Although it also occurs in rivers and lakes,Noble and Comfort 1982Grass 1984 it appears to be better documented from oceans and sea expanses.Palmer & Been 2011Wadhams 2000, p. 72Weeks 2010, Ch. 13

Seabed scours produced via this mechanism should not be confused with strudel scours. These result from spring run-off water flowing onto the surface of a given sea ice expanse, which eventually drains away through cracks, seal breathing holes, etc. The resulting turbulence is strong enough to carve a depression into the seabed. Seabed scouring by ice should also be distinguished from another scouring mechanism: the erosion of the sediments around a structure due to water currents, a well known issue in ocean engineering and river hydraulicsSee Annandale 2006, for instance – see bridge scour.

Historical perspective and relevance

It appears Charles Darwin speculated in 1855 about the possibility that icebergs could gouge the seabed as they drifted across isobaths.Weeks 2010, p. 391 Some discussion on the involvement of sea ice was brought up in the 1920s, but overall this phenomenon remained poorly studied by the scientific community up to the 1970s. At that time, ship-borne sidescan sonar surveys in the Canadian Beaufort Sea began to gather actual evidence of this mechanism. Seabed gouges were subsequently observed further north, in the Canadian Arctic Archipelago, and in the Russian Arctic as well. Throughout that decade, seabed gouging by ice was investigated extensively.

What sparked the sudden interest for this phenomenon was the discovery of oil near Alaska's northern coastlines, and two related factors: 1) the prospect that oilfields could abound in these waters, and 2) a consideration that submarine pipelines would be involved in future production developments, as this appeared to be the most practical approach to bring this resource to the shore. Since then, means of protecting these structures against ice action became an important concern.Pilkington and Marcellus 1981Woodworth-Lynas et al. 1985Woodworth-Lynas et al. 1996Clark et al. 1987 An oil spill in this environment would be problematic in terms of detection and clean-up.McHale et al. 2000

Scientists in fields of research other than offshore engineering have also addressed seabed gouging. For instance, biologists have linked regions of the seabed reshaped by seabed gouging by ice to the formation of black pools, seabed depressions filled with anoxic high-salinity water which are death traps for small marine organisms.Kvitek et al. 1998 However, much of it appears to have been documented from an offshore engineering perspective, for the purpose of oil exploration.Weeks 2010, p. 403

Seabed survey for gouges

Seabed gouging by ice is an eminently discreet phenomenon: little sign of it can be observed from above the water surface – the odd evidence includes sea floor sediments incorporated into the ice. Information of interest on these gouges includes: depth, width, length and orientation.Sonnichsen & King 2011, for instance Gouging frequency – the number of gouges produced at a given location per unit time – is another important parameter. This kind of information has been gathered by means of seabed mapping with ship-borne instrumentation, typically a fathometer: echo sounding devices such as a side-scan and a multi-beam sonar systems.Weeks 2010, p. 392 Repetitive mapping involves repeating these surveys a number of times, at an interval ranging from a few to several years, as a means of estimating gouging frequency.Blasco et al. 1998Sonnichsen et al. 2005

Gouge characteristics

Seabed gouges produced by drifting ice features can be many kilometers in length. In Northern Canada and Alaska, gouge depths may reach .Been et al. 2008 Most, however, do not exceed 1 meter (3 feet). Anything deeper than 2 meters is referred to by the offshore engineering community as an ''extreme event''. Gouge widths range from a few meters to a few hundred meters.Héquette et al. 2008Oickle et al. 2008 The maximum water depths at which gouges have been reported range from , northwest of Svalbard in the Arctic Ocean.Weeks 2010, p. 395 These are thought to be remnant traces left by icebergs during the Pleistocene, thousands of years ago, when the sea level was lower than what it is today. In the Beaufort Sea, Northern Canada, a 50 km (30 mi) long gouge was shown to exist, with a maximum depth of and in water depths ranging from .Blasco et al. 1998 The gouge is not always straight but varies in orientation. This event is thought to be about 2000 years old. Recent episodes of grounding, gouging and fragmentation of large Antarctic icebergs have been observed to produce powerful hydroacoustic and seismic signals that further illuminate the dynamics of the process.Martin et al. 2010

The ice features

In the offshore environment, the gouging features are made up of two kinds of ice: glacial ice and sea ice.

Glacial ice

Physically and mechanically, glacial ice is akin to lake ice, river ice and icicles.Hobbs 1974 The reason is that they all form from freshwater (non saline water). Ice sheets, ice caps and glaciers essentially consist of glacial ice. Since glacial ice spreads sideways and down-slope (as a result of gravity),By means of a mechanism known as creep. in some areas this ice reaches the coastline. Where this happens, depending on topography, the ice may break up into pieces that fall in the sea, a mechanism called ice calving, and drift away. Alternatively, ice sheets may spread offshore into extensive floating ice platforms called ice shelves, which can ultimately also calve. The features produced by these calving processes are known as icebergs and may range in size from meter to kilometer scale. The very large ones, referred to as ''ice islands'',Weeks 2010, p. 399 are typically tabular in shape. These may be responsible for extreme gouging events.

Sea ice

Sea ice is the outcome of freezing seawater. It is porous and mechanically weaker than glacial ice. Sea ice dynamics are highly complex.Haas 2003Weeks 2010, ch. 12 Driven by winds and currents, sea ice may ultimately develop into pressure ridges, a pile-up of ice fragments, or ''rubble'', making up long, linear features. These are a very common source of seabed gouges. Pressure ridges are often enclosed inside expanses of drifting pack ice, such that gouging activity from sea ice ridge keels is closely related with pack ice motion. Stamukhi are also pile-ups of broken sea ice but they are grounded and are therefore relatively stationary. They result from the interaction between fast ice and the drifting pack ice. Stamukhi can penetrate the seabed to a considerable depth, and this also poses a risk to subsea pipelines at shore approaches.

Gouging dynamics

Keel reaction

Because of the differences in the nature of glacial ice and pressure ridges, gouging events from these two types of ice are also different. In both cases, the ice-soil interface is expected to retain a certain equilibrium angle, called the ''attack angle'', during which the gouging process achieves a ''steady state''. Icebergs may adjust to this angle by rotation. Sea ice ridges may do so through the rearrangement of the rubble at the keel-seabed interface or through keel failure.Croasdale et al. 2005

Seabed reaction

Seabed reaction to the gouging process depends on the properties of both the ice and the seabed. Assuming the former is stronger than the latter, and the ice driving force is sufficient, a gouge will form in the seabed. Three zones within the seabed are distinguished on the basis of soil response.Palmer et al. 1990Palmer 1997Løset et al. 2006Nobahar et al. 2007 ''Zone 1'' is the gouge depth, where the soil has been displaced by the ice feature and remobilized into side berms and front mound ahead of the ice-seabed interface. ''Zone 2'' is where the soil undergoes some displacement. In ''Zone 3'', little or no displacement takes place, but stresses of an elastic nature are transmitted from the zone above.

Arctic offshore oil & gas

The area north of the Arctic Circle may hold a significant amount of undiscovered oil and gas, up to 13% and 30%, respectively, according to the USGS.Gautier et al. 2009 This resource probably lies in continental shelves at water depths below , which makes up about one third of that area. Also, more than 400 oil and gas fields had been identified up to 2007, most of them in Northern Russia and on the North Slope of Alaska.

A challenge for offshore engineering

Access poses a challenge.Mørk 2007 An offshore production scheme necessarily aims for safe and economical operation throughout the year and the full lifespan of the project. Offshore production developments often consist of installations on the seabed itself, away from sea surface hazards (wind, waves, ice). In shallower waters, the production platform may rest directly on the seabed. Either way, if these installations include a submarine pipeline to deliver this resource to the shoreline, a substantial portion of its length could be exposed to gouging events.Palmer & Tung 2012

Protecting submarine pipelines from gouging events

According to recent reviews on the subject,Cardenal et al. 2022 adequate protection against gouging activity may be achieved through pipeline burial. Placing the pipeline in Zone 3 would be the safest option, but the costs for this option are deemed prohibitive. Instead, current design philosophy envisages pipe location within Zone 2, which is still below the gouge depth, but where the soil is expected to move as a result of a gouging event above it. This implies that the pipeline must undergo a certain amount of bending and consequent deformation, or strain, of the pipeline wall. For the currently operating North Star production site, “ e minimum pipeline depth of cover (original undisturbed seabed to top of pipe) to resist ice keel loads was calculated based on limit state design procedures for pipe bending”.Lanan et al. 2011, p. 3 For that particular site, “ seabed soil displacements beneath the maximum ice keel gouge depth (3.5 ft) yielded a 7-ft minimum depth of cover for pipe bending strains up to 1.4%”.

This design philosophy must contend with at least three sources of uncertainty:
*''The maximum expected gouge depth'': Based on the past gouging regime (gouge depth distribution and gouging frequency, especially), one must rely on probability analyses to estimate the likely maximum gouge depth at the planned pipeline deployment site during its full operational lifespan (e.g. 20–40 years). This type of analysis is not unusual in civil engineering – textbooks are written on this subject.''e.g.'' Jordaan 2005 But changing climate patternsComiso 2002Kubat et al. 2006 are an added source of uncertainty, since it is uncertain how climate change will affect future gouging regimes.
*''Subgouge deformation'': Seabed gouging by ice is a relatively complex phenomenon, depending on a number of parameters (keel dimensions and properties, soil response, etc.). Even if the maximum gouge depth can be ascertained, it is difficult to assess the amount of soil displacement below it, a parameter considered when establishing what a safe pipeline burial depth should be.
*''Pipeline strain'': Another source of uncertainty is the amount of strain that the pipeline is likely to see at a given depth below the gouge.

Environmental issues

Oil and gas developments in Arctic waters must address environmental concerns through proper contingency plans. Parts of the Arctic are covered with ice most of the year. During the winter months, darkness prevails. If an oil spill occurs, it may go undetected for several months.Timco & Davies 1996DF Dickins 2000 Assuming this spill is located, clean-up procedures are likely to be impeded by ice cover. Furthermore, these are remote locations, such that logistical issues would come into play. Arctic ecosystems are sensitive – a timely response is required to mitigate the consequences of an oil spill.

See also

* Drift ice
* Iceberg
* Offshore geotechnical engineering
* Submarine pipeline
* Pressure ridge (ice)
* Stamukha
* Strudel (ice)
* Bloop

References

Bibliography

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Sea ice
Glaciology
Oceanography