Oil spill scenarios should be considered that span the range from worst case to most probable case.  Potential spill sources include exploration, development, production and transportation activities.  In the United States, spill size categories of minor, medium, and major are established in the National Contingency Plan, with threshold volumes of less than 40 m³ (10,000 gallons), 40 to 400 m³  (510,000 to 100,000 gallons)  and greater than 400 m³ (100,000 gallons), respectively.  Other organizations, such as International Petroleum Environmental Conservation Association (IPIECA) and the American Petroleum Institute use a three tiered, non-quantified classification system, where Tier 1 is the least environmentally significant or complicated oil spill event, and Tier 3 is the most significant.  Tier 1 spills can usually be responded to using local resources provided by the responsible party, whereas Tier 3 requires external response resources from sources such as national stockpiles or international cooperatives and significant involvement by local and national authorities. 

Various national regulations require plans that can be good sources of information on spill scenarios and associated response strategies and equipment.  Examples are the Facility Response Plans (FRP), Vessel Response Plans (VRP), and Oil Spill Response Plans (OSRP) required in the U.S.  When utilizing scenarios from plans required by regulations, it is important to ensure that the NEBA workgroup agrees that the scenarios are realistic, and address worst case spill situations in order to maintain the credibility of overall NEBA assessment.  Historical records can also be used to identify applicable spill scenarios.  In the U.S., Oil Spill Reports can be accessed through the National Response Center at www.nrc.uscg.mil.  Additional accidental release data sources are the DNV Worldwide Offshore Accident Database (Woad 2008) and the SINTEF Blowout and Well Release Characteristics and Frequencies database (SINTEF 2010).  In areas such as the Arctic, where oil exploration and production is increasing, and there have been few historical spills, the professional judgment of oil industry and regulatory emergency response professionals may need to be relied on to identify applicable scenarios.  However, across all nations, the historical record of spills from military vessels in the Arctic is likely to be insufficient and full reporting of releases from smaller fishing or shipping vessels may not be reliable. 

Estimating the fate and effect of oil from various spill scenarios can be accomplished by use of spill trajectory models, and applicable case histories.  Oil spill models commonly used include GNOME (NOAA), OILMAP and SIMAP (ASA), and OSCAR (SINTEF).  Use of these models requires historical weather data, ocean current data, and identification of likely oil types and their physical and chemical characteristics.  In the Arctic, trajectory modeling can be severely affected by sea ice. Use of computer models to predict oil spill trajectories under ice conditions is difficult, but there are coupled ice-ocean models that may be used to provide trajectory input for oil spill models.  Coupling sea ice forecasting models with oil spill models is an emerging area of science, and rapid progress is expected in the next 5-10 years. In many cases, combinations of best modeling practices, local knowledge and professional judgment may provide the best available information.

Once project-relevant planning scenarios have been developed, potentially applicable OSR strategies must be identified.  The OSRs that are typically evaluated include but may not be limited to:  1) natural attenuation (monitoring only), 2) mechanical containment and/or recovery using booms, skimmers, and sorbents; 3) in-situ burning using fire-proof containment boom, natural booming structures such as ice structures, or chemical herders; 4) oil dispersion using chemical dispersant formulations or application of oil-mineral aggregates, and 5) manual clean-up of oiled shorelines and debris.  Response strategies must be evaluated based not only on their anticipated effectiveness, collateral impacts, and availability as well as regulatory approval and technical feasibility.  Sea ice is likely to be an extremely limiting factor on the feasibility of some response options.  Mechanical recovery using booms and skimmers may be effective under open sea conditions, but its effectiveness decreases rapidly with increasing ice coverage, however the applicable response time may be longer than in open water conditions.  In-situ burning may be difficult in open water conditions due to wind and wave heights, but may be very effective in open areas between ice packs, or on solid ice surfaces.  Any response method that involves movement of large equipment overland may be difficult during summer months due to the lack of hard surfaced roads.  Manpower intensive operations may be hampered at any time due to lack of infrastructure to support large numbers of personnel (i.e. lodging, transportation, food, etc.).  Safety of response personnel must be considered in all scenarios, and waterborne operations will likely be restricted at times due to heavy weather and/or visibility constraints.  Aerial applications of dispersant may have significant limitations under certain icing conditions, but may in fact be the only response option that can be safely employed under other response condition.  Estimating the efficiency of any response method in ice conditions is complicated by the icing extent, seasonal factors, and geographic location; however, Evers et al. (2006) produced a chart of operational limits for various response techniques that may be useful for NEBA purposes (Table 9-1). This basic and widely distributed information has been updated based on findings from the SINTEF Oil in Ice JIP report regarding the potential for use of dispersants in ice (Lewis and Daling 2007, Sørstrøm et al. 2010, Faksness et al. 2011).  These studies evaluated new methodologies and strategies for dispersion of oil in high ice coverage (80-90%) which included maneuverable dispersant spray units and use of energy and turbulence to complete the dispersion process (see Figure 9-2; also discussed in Daling et al. 2010).

Table 9-1. Indication of expected operational limits of different response methods as a function of ice coverage.

Figure 9-2. Advanced technology for application of chemical dispersants in ice extends operational window.

Selection of response methods and resources for environmental conditions under which they  are unlikely to be available or not technically feasible will do little to aid in the development of effective or credible response plans, and could create unrealistic expectations for response capabilities.  On the other hand, if methods and resources are identified that could produce desired environmental and socioeconomic outcomes capturing such information could help guide future response capacity improvement efforts. Assessment of available and potentially effective response resources could be facilitated by the development of a database that would capture resource information and their capabilities under a range of arctic conditions from existing industry, local, regional, and national response plans.