Seabed gouging by ice

As an ice feature drifts into shallow waters, it comes into contact with the seabed, and produces a linear gouge while it keeps drifting. The ice is here represented by an iceberg), but gouging is also caused by pressure ridges.

When floating ice features (typically an iceberg or sea ice) drift into shallower waters, their keel may come into contact with the seabed. As they keep drifting, they produce long, narrow furrows most often called gouges, or scours (Wadhams 2000, p. 72, Weeks 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 1982, Grass 1984), this phenomenon appears to be better documented from oceans and sea expanses.

Gouges produced via this mechanism should not be confused with strudel scours. These results 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. When that happens at shallow water depths, the resulting turbulence generates a depression into the seafloor (Abdalla et al. 2008, Fig. 5). 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 hydraulics (e.g. Annandale 2006) – 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-born 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 (Wadhams 2000, p. 72). 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 (Weeks 2010, p. 391): 1) the prospect that oilfields could abound in these waters, and 2) a consideration that pipelines would be involved in future production developments, as this appeared to be the most practical approach to bring this resource to the land. Since then, means of protecting subsea pipelines against ice action became an important concern (King 2011, Barrette 2011). 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, because of the costs involved in gouging-related data production, the bulk of that work was financed by the oil & gas industry, and much of it was documented from an offshore engineering perspective.

Seabed survey for gouges

Illustration of an echo sounding operation, here with a multibeam sonar used to map seabed bathymetry.

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 (Weeks 2010, p. 391). Information of interest on these gouges includes: depth, width, length and orientation (King 2011, Barrette 2011). Gouging frequency – the number of gouges produced at a given location per unit time – is another important parameter. To this day, 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 (e.g. Blasco et al. 1998, Sonnichsen 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 5 metres (16 ft) (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. 1995, Oickle et al. 2006). The maximum water depths at which gouges have been reported range from 450 to 850 metres (1,480 to 2,790 ft), 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 kilometres (31 mi) long gouge was shown to exist, with a maximum depth of 8.5 metres (28 ft) and in water depths ranging from 40 to 50 metres (130 to 160 ft) (Blasco et al. 1998). The gouge is not always straight but varies in orientation. This event is thought to be about 2000 years old.

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. The reason is that they all form from freshwater(normal, non saline water). Glacial ice is essentially what ice sheets, ice caps and glaciers consist of. Since glacial ice spreads sideways and downslope (as a result of gravity), 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.

When the ice keel gouges the seabed, three zones are known to exist inside that seabed: Zone 1 is where the soil is removed (to form the gouge), zone 2 is characterized by large soil displacements, zone 3 sees minimal displacement.

Sea ice

Sea ice is frozen seawater. It is porous and mechanically weaker than glacial ice. Sea ice dynamics is highly complex. 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 pack ice, such that gouging activity from sea ice ridge keels is closely related with pack ice motion.

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 (Barrette 2011, Fig. 5). For instance, 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 are expected to do so through the rearrangement of the rubble at the keel-seabed interface.

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 (e.g. Palmer 1997, Nobahar 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 large displacements. In Zone 3, little displacement takes place, but stresses of an elastic nature are transmitted from the above layer.

The near shore North Star production site in the Alaskan Beaufort Sea under open water conditions (summer) is an example of a production facility that relies on a subsea pipeline to carry the resource to land – see Lanan & Ennis 2001, Lanan et al. 2011 for more details.

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). According to that source, this resource probably lies in continental shelves at water depths below 500 metres (1,600 ft), 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

Getting to this resource poses a challenge. What offshore production scheme will ensure a safe and economical operation year-long and over 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 subsea pipeline to deliver this resource to the shoreline, a substantial portion of its length could be exposed to gouging events.

Pipeline buried below the seafloor to avoid direct impact with a seabed gouging ice feature.

Protecting subsea pipelines from gouging events

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 can accommodate a certain degree of bending, which is represented by the amount of strain. As explained by Lanan et al. (2011, p. 3) for the currently operating North Star production site, “The 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”. For that particular site, “Predicted 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%” (Lanan et al. 2011, p. 3).

This design philosophy has to contend with at least two sources of uncertainties (Barrette 2011):

• The maximum expected gouge depth: This is based on the past gouging regime (gouge depth and gouging frequency, especially) at the site of interest. Seabed mapping is used for that purpose. But how representative is this number of the gouging regime that will actually be taking place during the operational lifespan of the pipeline, especially given changing climate patterns (Comiso 2002, Kubat et al. 2006)?
• Seabed behaviour as a result of the gouging event: Seabed gouging by ice is a relatively complex phenomenon. Soil reaction is only partly understood. Even if the maximum gouge depth can be ascertained, how much confidence do we have in determining the amount of soil displacement, given that this depends on a number of parameters (e.g. keel shape and size, soil properties and dynamics)?

Environmental issues

Oil and gas developments in Arctic waters must address environmental concerns through proper contingency plans. The sea is 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 1996, DF Dickins 2000). And assuming this spill is located, clean-up procedures is likely to be impeded by the ice cover. Furthermore, these are remote locations, such that logistical issues would come into play. Arctic ecosystems are sensitive – a timely response is paramount.

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