The Use of Dispersants in Marine Oil Spill Response (2020) / Chapter Skim
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2 FATE AND TRANSPORT
2 FATE AND TRANSPORT
Pages 23-66
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From page 23... ...
While the primary focus is on the capacity of dispersants to alter oil's fate and transport, the chapter also considers feedbacks between processes with a leaning toward the subsurface, owing to studies that followed the Deepwater Horizon (DWH) oil spill (also known as the Macondo spill)
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From page 24... ...
The combination of source-specific compositional variability with complex and variable industrial processing leads to a complex terminology for oil spill responders that includes functional classification of industrial products, functional descriptions of chemical composition, specific methodologies used to derive chemical composition, and multiuse terminology. Because any of these products can spill to the environment, nuances of the terminology are relevant to the issue of dispersant application.
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From page 25... ...
Subsea well blowouts such as occurred in the DWH, the 1969 Santa Barbara, and the Ixtoc I oil spills occur at seafloor temperature and pressure conditions and involve the unprocessed reservoir fluids that may include natural gas, reservoir water, carbon dioxide, and crude oil. More so than for surface spills, the circumstance of such blowouts -- particularly the gas composition (see Box 2.1)
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From page 26... ...
FIGURE 2.1 Summary of the important components (bold font) of an oil spill and the processes (normal font)
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From page 27... ...
Natural gas is typically separated from crude oil following extraction and prior to transport; as a result, it is a nonissue for many oil spills. The major exception is for well blowouts.
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From page 28... ...
. The rapid exsolution of natural gas from the liquid phase at or prior to the point of environmental discharge introduced uncertainty in the estimation of liquid oil flow rate.
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From page 29... ...
, which complicates assessment of flow rate and droplet size distribution. Degassing of oil in the rising plume has also been hypothesized to accelerate rise velocity for dual-phase droplets (Pesch et al., 2018)
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From page 30... ...
. OIL FATE Evaporation Evaporation of lighter surface oil components occurs rapidly after an oil spill, which causes the loss of smaller, more volatile petroleum compounds with boiling points typically lower than that of n-pentadecane (Stout et al., 2017)
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From page 31... ...
For example, high wind speeds increase evaporation rates; but, they also promote dissolution of oil in the water column and diffusion of VOCs in the air, muddling the overall effect of wind (Crowley et al., 2018)
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From page 32... ...
However, laboratory studies have shown that the application of dispersants to surface oil slicks can increase the number of aerosol particles produced by breaking waves by one to two orders of magnitude compared to untreated oil slicks (Afshar-Mohajer et al., 2018)
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From page 33... ...
Photooxidation can produce tar and gum residues from surface oil when higher molecular weight products are produced through the condensation of peroxide and other free-radical intermediates (NRC, 1985, 2003) , while some laboratory studies showed insignificant differences in oil density following irradiation (NRC, 2013; Short, 2013)
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From page 34... ...
. The application of dispersant to oil, leading to dispersion in the water column, can reduce atmospheric exposure and increase aqueous exposure; thus, dissolution remains relevant for considering the fate of dispersed oil.
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From page 35... ...
Each concentric circle represents the calculated distance from the well that surface oil traveled before photooxidation decreased dispersant effectiveness to < 45%, assuming high irradiance and slow transit speed (red inner circle) , mean irradiance and transit speed (green intermediate circle)
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From page 36... ...
of hydrocarbon mass flows in the marine environment; values are calculated for June 10, 2010, in millions of kgs per day. This figure illustrates the molecular fractionation associated with dissolution and evaporation as occurred during the Deepwater Horizon oil spill.
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From page 37... ...
In the DeepSpill field experiments, where oil was released as a plume at a depth of about 840 m, hydrates were not observed, even though it was noted that the methane dissolution rate in the water column was roughly half the value for clean bubbles which suggested that hydrate skinning might have been important (Johansen et al., 2003)
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From page 38... ...
; conversion to petroleum-derived bacterial flocculent (Bælum et al., 2012; Hazen et al., 2010; Valentine et al., 2014) ; and flocculation with marine snow or other would-be sediment particles (Passow, 2014)
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From page 39... ...
The relationship across these terms is depicted in Figure 2.7. Oil-particle interactions alter the buoyancy of oil droplets because the OPAs formed are negatively or near neutrally buoyant, allowing their transportation in the water column and eventual sediment deposition (Bragg and Owens, 1995; Lee et al., 2003)
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From page 40... ...
. The interactions of oil droplets and particles following oil spills is now considered to be an important process in the natural attenuation of oil spilled at sea (Bragg and Owens, 1995; Bragg and Yang, 1995; Lee, 2002)
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From page 41... ...
OPA formation has been associated with previous oil spills such as Tsesis, Ixtoc I, and others (Jernelöv and Lindén, 1981; Johansson et al., 1980; Teal and Howarth, 1984; Vonk et al., 2015)
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From page 42... ...
. For a 4- to 5-month period during and after the oil spill, Brooks et al.
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From page 43... ...
and marine snow they produced (Fu et al., 2014; Hatcher et al., 2018; Passow, 2016; van Eenennaam et al., 2016) , although some inhibition of OPA formation in the presence of weathered crude oil has also been observed under certain conditions (Passow et al., 2012, 2017)
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From page 44... ...
(2018a) recently completed a comprehensive review of literature related to marine snow studies following the DWH oil spill, with a focus on the use of oil spill dispersants and the formation, fate, and transport (i.e., sedimentation)
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From page 45... ...
FATE AND TRANSPORT 45 Multiple studies have investigated the ecological impacts of enhanced mass accumulation of sedimented oil on deep-sea benthic organisms. Declines in macro- and meiofauna (Montagna et al., 2013; van Eenennaam et al., 2018)
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From page 46... ...
Evidence preceding the onset of SSDI is similarly consistent with low percentages of liquid oil in the deep intrusion layers. These observations are consistent with a scenario in which SSDI reduced the droplet size distribution sufficiently to slow droplet rise rates and thus affect surfacing location, but not so much as to trap more than approximately 5% of the liquid oil as suspended microdroplets.
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From page 47... ...
What happened to the dispersant from SSDI? Samples collected from the DWH intrusion layers contain the anionic surfactant DOSS, which is aqueous soluble and thought to have dissolved from the oil droplets to the ocean commensurate with intrusion layer formation (Kujawinski et al., 2011)
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From page 48... ...
rose through the water column, reaching the surface. The aqueous solubility of the nonionic surfactants may have allowed some components of the dispersant to reach the surface with the oil, consistent with on-scene reports that the behavior of the surface slick near the wellhead changed following the onset of SSDI.
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From page 49... ...
Droplet size is an important consideration for biodegradation, and reduction in droplet size is the primary objective of dispersant application. Assuming no other limitation on microbial growth, increased surface area is expected to enable greater colonization of oil, which has been observed in laboratory studies (Brakstad et al., 2014, 2015; Ribicic et al., 2018)
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From page 50... ...
Figure 2.10 indicates substantial delays in oil surfacing time for smaller droplet sizes. Very small droplets and dissolved oil lack buoyancy to reach the surface, and they can become trapped in deep intrusion layers (see Figure 2.10)
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From page 51... ...
, including deep-diving marine mammals foraging at depth. Because the primary objective of dispersants is to promote the formation of smaller droplets, understanding the implications of varying droplet size distributions is critical to making decisions and predictions regarding dispersant use.
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From page 52... ...
For any particular spill, unforeseen conditions may impact droplet size formation and complicate reconstruction of the actual conditions, such as ancillary small-aperture fissures or changes in pressure, fraction of methane, and others. Understanding the dynamics associated with droplet formation and transport is important to forecasting and preparing for the impacts of an oil spill.
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From page 53... ...
The bimodal distribution with this range in the presence of dispersant has been observed in the literature of oil release from jets (Gopolan and Katz, 2010; Murphy et al., 2016) and from oil spills on the water surface (Li et al., 2009b)
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From page 54... ...
At the same time, these approaches risk biasing results due to droplet fractionation. Observations of the physicochemical characteristics of oil that reached the surface following the DWH spill have been used to estimate the rise velocity of oil droplets and droplet size distribution (Ryerson et al., 2011, 2012)
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From page 55... ...
, much smaller than the resolution of ocean circulation models; hence, these features are simulated using sub-models specifically designed to capture fine-scale dynamics. Because of the plume effect, droplets rise more rapidly below the intrusion layer than they do above it, where they rise as individual droplets.
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From page 56... ...
Disadvantages are that they cannot resolve unsteady flow features or the complex processes of detrainment, intrusion formation, and weak plume dynamics above the detrainment point. Recently, large-eddy simulation models have been developed to treat complex oil and/or gas plumes in stably stratified conditions (Fabregat et al., 2015, 2016; Fraga et al., 2016; Yang et al., 2016a)
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From page 57... ...
The intrusion thickness also reflects the droplet size, with larger droplets producing a thicker intrusion due to their faster rise velocity (Murphy et al., 2016a)
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From page 58... ...
The committee notes that Equation 2 describes a DSD based on the number of droplets, rather than mass, as described earlier in connection with subsurface droplet size distributions. The simplicity of Equation 2, and the fact that the model provides a way for entraining droplets into the water column, has contributed to the popularity of the DS model.
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From page 59... ...
Their approach forms the basis of most oil spill models. However, the assumption of an infinitesimally thick oil slick floating on a stagnant water column does not apply well at sea due to the presence of wind, currents, and waves.
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From page 60... ...
They also reported the relative absence of oil droplets smaller than 70 µm from the surface layer. Various investigators found that "particles" released at the water surface tend to propagate downward.
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From page 61... ...
For a Lagrangian stochastic models, oil is typically represented by numerical particles, or Lagrangian elements, that are advected by mean ocean currents and droplet rise velocity, diffused by ambient turbulence, and transformed by a host of physical, chemical, and biological processes, including dissolution, biodegradation, sediment particle interactions, etc. While oil transformations are important for determining oil fate, we focus here on transport and mixing.
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From page 62... ...
While measurements to date suggest little impact above the intrusion layer, the impact below the intrusion layer remains uncertain. Farfield tracking models generally treat horizontal and vertical turbulent mixing using random walk (or similar)
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From page 63... ...
Finding: Droplet size is a major factor determining the fate and transport of oil spilled at depth. Both natural processes and the application of subsurface dispersants affect droplet size distributions (DSDs)
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From page 64... ...
Finding: Oil transport on the water surface is different from that of neutrally buoyant material, as oil can gather in convergence zones and/or move relative to the water surface. Recommendation: Oil spill models should account for sharp vertical and horizontal variations in oil transport.
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From page 65... ...
FATE AND TRANSPORT 65 Recommendation: Additional modeling and experimental studies are needed at large scales and at high pressures to better define natural gas's impacts on subsea discharge and fate and transport processes. Finding: The dissolution of surfactants from oil droplets occurs during buoyant rise associated with SSDI.
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