Renewed political and commercial interest in the resources of the Arctic, the reduction in the extent and thickness of sea ice, and the recent failings that led to the Deepwater Horizon oil spill, have prompted industry and its regulatory agencies, governments, local communities and NGOs to look at all aspects of Arctic oil spill countermeasures with fresh eyes. This paper provides an overview of present oil spill response capabilities and technologies for ice-covered waters, as well as under potential future conditions driven by a changing climate. Though not an exhaustive review, we provide the key research results for oil spill response from knowledge accumulated over many decades, including significant review papers that have been prepared as well as results from recent laboratory tests, field programmes and modelling work. The three main areas covered by the review are as follows: oil weathering and modelling; oil detection and monitoring; and oil spill response techniques.

Keywords: Oil spill response

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Introduction
Politics, economics and climate change are the driving forces behind the ‘industrialisation’ of the Arctic marine environment. Which of these three forces are more dominant is far from certain, but what is clear is that any increase in human activity in ice-covered waters will magnify the potential for an oil spill. Whether it be from a shipping accident, leak from a subsurface pipeline, subsurface well blowout or a cruise ship venturing further into shallow waters.

Whilst we have seen a substantial increase in Arctic fisheries and tourism, the recent slump in world oil prices combined with the need to reduce our carbon footprint in line with the legally binding Paris Climate Agreement has potentially reduced the attractiveness of investment in the Arctic. For instance, a number of major oil companies have announced the abandonment or suspension of their drilling operations in the Arctic Ocean, and trans-Arctic shipping remains at low levels. However, operations do continue, for example the newly built offshore terminals in Kara Sea region of the Russian Arctic handled (by ship) a combined 230 000 barrels a day in the second quarter of 2016 (Lee 2016). The potential for an Arctic sea route for Canadian oil-sand bitumen would create known and new oil spill response issues (Environment Canada 2013; NOAA 2013). Whilst the navigation of vessels through sea ice may be more challenging, we note that in 2016 more oil was shipped out of these Russian terminals during the sea ice season than during the previous open water season. This suggests that with the correct infrastructure and vessels, ice conditions may not be a limiting factor for the movement of oil (or indeed other valuable commodities) out of, through, or into the region. Then again, as this manuscript details, the presence of sea ice enhances the difficulty of clean-up operations should a major spill happen.

In common with oil spill response in temperate, open-ocean conditions, the purpose of conducting any oil spill response in ice-covered waters is to reduce the damage that the spilled oil might cause, both ecological and socio-economic. Knowledge of which socio-economic, ecological or cultural resources are likely to be damaged by the spilled oil at any particular location is important, so that the appropriate response strategies and methods can be used to minimise the damage that could occur. For example, cultural resource protection (e.g. remaining “secret”) requires knowledgeable representatives to participate in Shoreline Cleanup Assessment Technique (SCAT) and response operations to prevent direct oiling, accidental disturbance, illicit collecting or intentional site disturbance (Owens et al. 2005).

The degree of effectiveness of the response is the degree to which the damage by the oil is reduced when compared with a no response action. One of the challenges of accurately quantifying the damage of an Arctic oil spill is that our baseline knowledge of the Arctic system is presently limited. Over the years, there have been a number of reviews (some with recommendations) into Arctic oil spills; these include the following:

National Academy of Sciences’ report on responding to oil spills in the U.S. Arctic marine environment (NRC 2014);

JIP The Joint Industry Programme’s series of advanced research projects and reports on: dispersants, environmental effects, trajectory modelling, remote sensing, mechanical recovery and in situ burning (JIP 2016);

USGS The United States Geological Survey’s evaluation of the science needs for informed decisions on energy development in the Chukchi and Beaufort Seas (Holland-Bartels et al. 2011);

PEW Charitable Trust’s review on Arctic Standards (Pew Charitable Trust 2013); and

Coastal Response Research Center’s (University of New Hampshire) review of the state-of-science for dispersant use in Arctic waters (CRRC 2016).

An earlier, but still a comprehensive study, the Canadian Government Beaufort Sea Project (1974–1980) with 40 reports, focuses on different aspects of an oil spill in the Arctic marine environment. There have been notable experimental and unplanned oil releases in the Arctic: the Dome Petroleum Experiment (1979/1980) (Dickins et al. 1981), the Komi oil spill (1994) (Sagers 1994) and the Joint Industry Program Field Experiment (2009) (Sørstrøm et al. 2010). More recently, a review performed by the Royal Society of Canada (Lee et al. 2015) focuses on crude oil releases in freshwater and saltwater environments, but has information on the risks associated with Arctic oil spills. Much new research is coming out now through the Arctic Response Technology Joint Industry Program, which funded nine projects exploring the movement, fate, and effects of oil, detection of oil in ice, and oil recovery, in situ burning and potential use of chemical herders (JIP 2016).

This paper brings together knowledge that has been amassed over many decades, including the significant review papers mentioned above, as well as more recent laboratory tests, field programmes and modelling work. The three main areas covered by the review are as follows: (1) Weathering and modelling, (2) Oil detection and monitoring and (3) Oil Spill Response Techniques. We understand that there are omissions that given space restrictions we could not include. These include amongst others: biodegradation, effects of oil on Arctic ecosystems, infrastructure needs, logistics, training and education, indigenous communities perspectives and representation, chain of command/coordination, and the ethics, regulatory and international framework encompassing a potential Arctic oil spill.

Oil and sea ice
Depending on the season, the sea ice conditions at the time of the event and type of accident (whether it be a pipeline breach, well blowout, shipping accident, or something else), oil could be spilled on, under, or into the waters surrounding the sea ice. However, what makes an Arctic oil spill particularly challenging is the plethora of environmental scenarios that could play out and the speed in which ice conditions can change. Furthermore, the combination of natural variability and climate-forced changes in the Arctic marine system make it particularly challenging to predict the ice conditions from one year to the next. Even though a spill could happen at any time of the year, it is important to keep in mind that most Arctic marine activities, at present, are concentrated around the summer months, and generally avoid sea ice. This summer focus may change as operational experience is gained; infrastructure is enhanced, and the continued increase in the ice-free season, over the next 30 years and more, stretches into other seasons.

Oil movement in sea ice
Oil spilled on a calm ocean surface spreads into a slick due to the balance between the forces of gravity, viscosity and surface tension. In rougher water, this spreading is augmented significantly by the entrainment of oil droplets into the water column by breaking waves, and subsequent resurfacing. The trajectory or drift of the slick is governed by the forces associated with currents, winds and waves (Wang et al. 2005). Sea ice adds a new dimension to the movement of oil, and therefore, understanding how far oil spilled on sea ice-infested waters will spread is of particular importance.

In summer, the sea ice zone is a particularly challenging environment because the concentration of ice floes within a region is continuously changing. Oil spilt in these conditions will generally gather on the surface among the floes, but wind and current can move the floes together squeezing the oil between them, or drift apart allowing the oil to spread out over a larger area of the sea surface. Venkatesh et al. (1990) suggested that for low sea ice concentrations (less than 30%) oil behaved as in open water, and for ice concentrations higher than 70–80%, they found that oil drifts with ice. The gap, between 30 and 70% ice concentration, is a transition zone which requires further research. Yapa and Weerasuriya (1997) developed a theoretical model for oil behaviour under drift ice by modifying earlier work on oil under ice to allow for oil escape through cracks.

In winter, oil present in these open water regions, known as leads, is likely to be incorporated in any newly formed ice. If the lead closes, oil incorporated within the new ice will form the blocks of the pressure ridge, essentially making the oil inaccessible for clean-up operations. However, if the oil is released below the ice cover, from a sunken vessel, pipeline breach or well blowout, the oil will rise through the water column breaking down into small droplets as it rises at the transition point of the multiphase plume driven flow (Johansen et al. 2013). In the case of a blowout, it is important to remember that oil and gas will be released together. The effect of the oil/gas mixture has on the sea ice is not fully established, but when the oil itself reaches the underside of the ice most of oil droplets will coalesce to form an oil slick. As the oil layer thickens, the slick will then move outwards from the central region due to hydrostatic pressure differences. Laboratory and in situ testing under a flat ice bottom suggest that the maximum thickness range for oil free to spread is 0.5–1 cm (Dickins et al. 1975; Keevil and Ramseier 1975), depending on the oil properties.

The oil will then move outwards beyond the spill zone filling all available irregularities, but preferentially flowing towards regions of thinner ice. This movement will either be dominated by the oil spreading out in narrow rivulets (Fig. 1a) or filling up deeper and wider depressions such as those seen in Fig. 1b. When an individual depression is full, a rivulet of oil run will flow outward over the depression and into the next interconnected depression (Fingas and Hollebone 2003; Wilkinson et al. 2007).