Each oil spill in the marine environment is unique and challenges responders, who must balance decisions to account for immediate and potential long-term human health, socioeconomic, and environmental impacts. Oil spills at sea may result from a variety of incidents, including an oil well blowout, a vessel collision or grounding, or a leaking pipeline. Additionally, the location, time of year, duration of the spill, water depth, environmental conditions, affected biomes, potential community impact, and available resources may also vary significantly.
The unique context of each spill requires that responders have access to a variety of response options that can be applied based on the specific conditions of the spill. Having a variety of response options available in the “tool kit” provides responders with alternatives in the face of operational limitations. Marine oil spill response methods include mechanical recovery of oil through skimmers and booms, in situ burning of oil, monitored natural attenuation of oil, and dispersion of oil by dispersants. Booms and berms may also be employed at the shoreline to minimize the impact of oil on shoreward resources, or to divert oil from a more sensitive area of shoreline to another, less sensitive area. Natural attenuation and biodegradation processes can substantially contribute to a reduction in the volume of oil from a spill.
Each response method has advantages and disadvantages. For example, the containment and mechanical recovery of oil has the advantage of removing the oil from the environment, but it is a very slow process that is limited by weather. In situ burning has the potential to remove significant quantities of oil from the sea, but ignition generally requires that the oil slick be reasonably fresh and sufficiently thick. Dispersants have the advantage of being able to treat large areas/volumes of oil, but they rely on other processes, such as biodegradation by microbes, to remove the oil from the environment. Several other factors also play a role in determining which response techniques will be most effective on their own or in combination with other approaches. It is often a combination of tools and adaptability based on circumstances that affords the optimal response outcomes. This report focuses on the factors that contribute to the decision as to whether to use dispersants as a response tool for any given marine oil spill. In oil spill response decision making, it is important to understand specific scenarios where a net benefit may be achieved by using a particular tool. With regard to dispersants, the primary objective is to prevent or reduce the formation or thickness of
surface oil slicks. Dispersants accomplish this by reducing the oil-water interfacial tension, and, with sufficient mixing energy, increasing the formation of small droplets that become or remain entrained in the water column with minimal recoalescence and slow resurfacing.
Modern dispersant formulations (see Box 1.1) contain one or more surface active agents (surfactants) that align at the oil-water interface allowing for wave action or other turbulence to cause the formation of droplets on the order of 70 microns (µm) or less. The dispersed droplets retain the initial buoyancy of the bulk oil itself (i.e., they remain less dense than the surrounding water in most cases) but rise more slowly through the water column by virtue of physical processes associated with their small size. Conceptually, a key potential advantage of these oil microdroplets1 is that the increased surface area-to-volume ratio provides more substrate with which microorganisms may interact, thus enhancing oil biodegradation, assuming no other limitations imposed by the environment (see Chapter 2). These smaller droplets are susceptible to colonization by naturally occurring oil-degrading microorganisms and may potentially biodegrade more quickly compared to oil in a floating slick, emulsified oil, or oil stranded on the shoreline. Similarly, the increase in surface area may promote greater dissolution.
Dispersant use also offers the opportunity to respond rapidly to large-scale, offshore marine surface or subsurface spills, especially with the recent advent of subsea dispersant injection (SSDI) capabilities and the use of jet aircraft delivery platforms. These advances in technology expand the operational window of opportunity, which was formerly more limited by hours of daylight, weather conditions, distance, and remoteness of a spill site (Chopra and Coolbaugh, 2016). These advances in dispersant application technology provide the opportunity to respond to a spill before oil weathers to the point where most other response options become less effective. Furthermore, subsurface application of dispersant may reduce responder exposure to volatile organic compounds (VOCs) known to be hazardous to human health.
When reading this report, it is important to consider the circumstances for which dispersants would be considered as a potential response option. For example, for small spills or in particular sea state conditions, it may not be logistically feasible to mount a dispersant operation. Similarly,
1 The committee recognizes that the term “microdroplets” is loosely defined, but in this report the term typically means droplets that are approximately 70 microns or less. A notable exception is in Chapter 3.
in the United States, preauthorization zones for dispersant use are generally limited to areas greater than 3 nautical miles from shore and in depths greater than 10 m. In other parts of the world, these zones may differ. Also, although a few freshwater dispersant products are available on the market, they are not currently approved for use in freshwater in the United States. Therefore, the committee interpreted the Statement of Task as limited to marine oil spill scenarios in which dispersants would be considered a potential response option.
One of the first major incidents where chemicals were used to disperse oil in the marine environment was on March 18, 1967, when the SS Torrey Canyon ran aground off the southwest coast of the United Kingdom. In that case, however, the chemicals used to respond to the Torrey Canyon were not specifically formulated for oil spill response and were not designed to minimize environmental damage. In fact, the products used during that response consisted of chemical degreasers with high levels of aromatic compounds that could be harmful to aquatic organisms but were very effective at transferring floating slicks into the water column. Since that time, a number of products have been developed that are much less toxic and are more effective on a wide range of oils.
Just over two decades later, in 1989, the T/V Exxon Valdez struck a reef in Prince William Sound, Alaska. One result of the ensuing oil spill was passage of the Oil Pollution Act of 1990 (101st Congress H.R. 1465, 1990), which had a tremendous impact on positioning the maritime community to better prepare for marine oil spill response. The act mandated vessel and facility response plans with specific minimum equipment and personnel capabilities for oil containment and recovery. The act also called for national and regional response teams to develop guidelines for spill preparedness and response strategies. This resulted in some regions in the United States identifying zones where dispersants and in situ burning are “pre-authorized” for use.
The U.S. Coast Guard (USCG) published a Final Rule on September 30, 2009 (74 FR 45003), titled Vessel and Facility Response Plans for Oil 2003 Removal Equipment Requirements and Alternative Technology Revisions. The Final Rule updated the requirements for spill response equipment associated with vessel response plans and marine transportation-related facility response plans. It provided additional requirements for new response technologies and modified response methods and procedures for marine and aquatic spills within the jurisdiction of the United States. This Final Rule clarified requirements for response capabilities, including effective daily application capacity for dispersants, using an Office of Natural Resources Revenue, the National Oceanic and Atmospheric Administration (NOAA) dispersant planning calculator known as Dispersant Mission Planner 2.
Since the Torrey Canyon, dispersants have been applied in the United States approximately 20 times (Bejarano, 2018) and are routinely used internationally, including during the 1979 Ixtoc I spill and the 2009 Montara spill (described below).
The Ixtoc I spill off Campeche, Mexico, was a shallow-water (54 m water depth) marine blowout that persisted for more than 9 months (Soto et al., 2014). The spill released about 3.3 million barrels (bbl) of crude oil and was the first spill in which large quantities (approximately 9,000 metric tons) of dispersants (mostly Corexit® products) were used via surface application (Jernelöv and Lindén, 1981; Linton and Koons, 1983).
In Western Australia, seven different dispersants (totaling 48,000 gallons) were applied at the surface during the Montara wellhead blowout in 2009. This spill involved the continuous release of approximately 30,000 bbl of a waxy crude oil into the Timor Sea over 10 weeks. Although this spill was a subsea blowout, the platform remained intact and the oil from this spill was released at the surface. The extent of dispersant effectiveness and overall potential impacts from this spill are still being litigated within the Australian federal courts.
A recent use of an unprecedented amount of dispersants in a major marine incident came as a result of the Deepwater Horizon (DWH) oil spill (also referred to as the Macondo oil spill), which occurred in the Gulf of Mexico in 2010. The DWH spill started as a well blowout and explosion from a mobile offshore drilling unit, followed by the collapse and sinking of the platform to the seafloor, resulting in a continuous release of oil and gas from the subsea well for 87 days (National Commission, 2011). During the DWH spill, the use of dispersants on the surface was preauthorized under the Gulf Coast Area Contingency Plan; and, with the oil release taking place more than 40 miles offshore, responders quickly commenced the application of dispersants on the surface.
This was followed by an unprecedented subsea injection at the wellhead, which required a difficult decision, because there was an “absence of information on the effects of dispersants in the deepwater environment” (National Commission, 2011). In weighing the trade-off decision, responders reasoned that subsea injection might reduce the overall volume of dispersants needed; worker safety would be improved on the surface due to less VOCs in the vicinity of the ongoing well control work; and less oil would reach the sensitive and fragile Gulf Coast shoreline (National Commission, 2011). The National Commission’s report noted that the decision for subsea injection was appropriate at the time based on all the factors considered.
Since the 2010 DWH spill response, the petroleum industry has invested significantly in the purchase of the most studied modern products (Dasic Slickgone NS, Finasol® OSR 52, Corexit® EC9500A) and their placement in strategic global locations to facilitate rapid response in an event where dispersants represent a viable response option (see Figure 1.1).
While a variety of dispersant products are available globally, regulatory considerations are key to their potential use. Figure 1.2 lists the countries where dispersants were considered as either a primary or a secondary response option as of 2013. The list of countries is likely to change over time.
There are many perspectives and perceptions surrounding the impact that dispersants and dispersed oil have on the environment and on human health. The decision to use dispersants to prevent oil from reaching the surface or to transfer surface oil into the water column is often seen as a difficult decision which involves consideration and evaluation of trade-offs with other response options.
Since the 1970s, approaches to environmental trade-off analysis for spill response planning have evolved. These approaches, collectively known as Net Environmental Benefit Analysis (NEBA), help decision makers select the most appropriate response option(s) to minimize the net impacts of oil spills on the environment. The U.S. Environmental Protection Agency (EPA) describes NEBA as a method for identifying and comparing the environmental benefits associated with alternative management options in spill response. As described in IPIECA-IOGP (2015) and ASTM (2019), NEBA does not include human health, but its scope varies among different countries. In other countries, the process may include an analysis of net benefits to people, such as the consideration of socioeconomic sensitivities and costs (IPIECA-IOGP, 2015).
For planning purposes, a NEBA needs to consider a broad range of geographic areas, ecological habitats, environmental, oceanographic, and climatological information because it is unclear exactly when or where an actual oil spill might occur. Similarly, an effective NEBA accounts for the fact that an ongoing spill event is highly unpredictable, and the range of ecological receptors potentially affected can be enormous. This requires that NEBA processes be highly flexible and use a comparative risk process that can be adapted in “real-time” to align with changing field conditions.
Three tools that support the NEBA conceptual approach include:
- Consensus Ecological Risk Assessment (CERA)
- Spill Impact Mitigation Assessment (SIMA)
- Comparative Risk Assessment (CRA)
Each process involves a structured approach used by the response community and stakeholders to compare the impact mitigation potential of candidate response options. Additionally, these three NEBA tools all consider realistic response measures and identify the best overall set of actions that will promote the most rapid recovery. The three tools can each be adapted to fit various regulatory
and environmental contexts. Distinct differences in these approaches exist in terms of the degree and timing of stakeholder engagement as well as the type and complexity of environmental analysis, such as the extent to which numerical models support the process. A more comprehensive discussion of the NEBA methodology is provided in Chapter 6.
As mentioned previously, the use of dispersants is not a novel approach to oil spill response. To that point, the National Research Council (NRC) released two previous reports, in 1989 and 2005, that focused on the use of dispersants at sea in response to a spill. The NRC report titled Using Oil Spill Dispersants on the Sea (1989) was commissioned to “review the state of knowledge in toxicity, effectiveness of application techniques, and effectiveness of commercially available dispersants” (NRC, 1989). At that time, much research on dispersant use had been conducted by industry in the United States and abroad, and the report assessed the state of knowledge and practice about the use of dispersants. That report concluded that the use of dispersants can be an effective spill response and control method, especially to minimize environmental damage caused by the presence of surface slicks, but the method for applying dispersants is a critical factor.
Shortly after the 1989 NRC report was completed, the Oil Pollution Act of 1990 was adopted. In the late 1990s, a series of workshops conducted by the USCG further examined the trade-offs associated with multiple response options, including dispersants. In 2003, a multiyear rulemaking process commenced to enhance the oil spill contingency planning regulations. This prompted the former Minerals Management Service (now the Bureau of Ocean Energy Management and the Bureau of Safety and Environmental Enforcement), NOAA, the USCG, and the American Petroleum Institute to request that the National Academies form a committee to examine the state of science on dispersants. The committee was tasked with considering the adequacy of existing information and ongoing research regarding the efficacy and effects of dispersants as an oil spill response technique in the United States (NRC, 2005). That request resulted in the NRC 2005 report titled Oil Spill Dispersants: Efficacy and Effects.
This current report builds on the two previous reports by incorporating the tremendous amount of subsequent research on dispersants. The DWH spill and the resulting funds from litigation and penalties (see Figure 1.3) have led to a rapid increase in the volume of science and literature surrounding oil spill response and dispersant use in particular.
The use of SSDI in the DWH spill raised new questions and challenges focused on the fate and effects of dispersant and dispersed oil, especially in the deep ocean. As the studies prompted by this spill are in various stages of completion, an understanding of the impacts of dispersant use as well as the potential limitations and benefits—particularly in scenarios similar to the DWH—is continuing to develop.
In light of this expanded body of knowledge since the previous National Academies publications on dispersants, this report highlights and synthesizes new information on the topic. The committee recognizes that this is an area of ongoing research, and it strives to provide as much complete and current information as possible to inform decision makers and other stakeholders. While most literature cited in this report has been released since the 2005 report, this was not a requisite criterion, and, where appropriate, the committee does cite earlier literature as well. Similarly, the committee acknowledges that much of the recent literature focuses on the DWH oil spill; however, this report is not intended to be a retrospective evaluation of that event. Instead, the committee intends for this report to be forward looking and applicable to future offshore marine spill scenarios. Where possible, the committee relied on peer-reviewed publications; however, the committee also recognized the value of other sources of information, including and not limited to industry reports, conference proceedings, and guidance documents.
Addressing the Statement of Task (see Box 1.2) requires consideration of the objectives of an oil spill response, the factors that contribute to response decision making, the trade-offs associated with the use of dispersants, and the processes available for assessing these trade-offs.
Chapter 2 focuses on the first task by considering processes associated with the fate and transport of oil, dispersed oil, and dispersants in the marine environment. Chapter 3 addresses the second task and discusses aquatic toxicity and ecological consequences of exposure to oil, dispersed oil, and dispersants. Next, Chapter 4 answers the fourth task by exploring the potential human health concerns associated with oil spill response and the use of dispersants, with a particular focus on occupational health, community psychosocial impact, and seafood safety. In Chapter 5, the committee partially responds to the sixth task and reviews the tools available and the information necessary for evaluating risk and making decisions regarding the use of dispersants and other response options. Drawing from the previous chapters, Chapter 6 compares the benefits and limitations of using dispersants to other response methods, as called for in the third task. Finally, and in accordance with the fifth task, the committee also considers the research protocols and standards that would increase the applicability and comparability of field and laboratory research in Chapter 7. Throughout the report, the committee further responds to the sixth task by identifying information necessary for decision making and additional research and modeling needs.2
2 Since the release of the prepublication version, the text was edited for clarity and references have been checked and modified as necessary.