Oil Spill Dispersants Efficacy and Effects (2005) / Chapter Skim
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Appendix E: Analysis of the Sensitivity of Dispersed Oil Behavior to Various Processes
Appendix E: Analysis of the Sensitivity of Dispersed Oil Behavior to Various Processes
Pages 355-378
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From page 355... ...
While some sensitivity was discussed analytically in earlier parts of Chapter 4, an integrated sensitivity analysis can only be performed with a comprehensive computer model that includes all of the relevant processes. The sensitivity modeling analysis discussed here was conducted for 14 cases with various oil types (crude oil and light and heavy refined oil)
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From page 356... ...
; both simulate most mechanisms needed to assess dispersant use, except dispersant effectiveness itself and changes in the oil droplet sizes (which are user inputs)
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From page 357... ...
. Oil Types Three types of oil were selected: Alaska North Slope crude oil, Intermediate Fuel Oil 300 (IFO 300)
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From page 358... ...
. The latter option was used here, resulting in the following values: TABLE E-2 Distillation Cuts of the Three Oils Used in the Modeling Sensitivity Analysis Alaska North Slope Intermediate Crude Oil Fuel Oil 300 Diesel Fuel Oil Weight Weight Weight Fraction, Fraction, Fraction, Oil Cut wt Temperature, wt Temperature, wt Temperature, °C °C °C Number percent percent percent 1 1.0 42 1.1 180 1.1 120 2 4.0 98 1.1 200 1.1 140 3 5.0 127 6.4 250 1.1 160 4 5.0 147 9.4 300 3.2 180 5 5.0 172 7.2 350 5.2 200 6 10.0 216 8.1 400 20.4 250 7 10.0 238 6.0 450 31.9 300 8 5.0 247 3.0 500 25.5 350 9 5.0 258 4.9 550 9.7 400 10 5.0 265 9.8 600 1.0 450 11 5.0 272 14.7 650 -- -12 10.0 282 10.7 700 -- -13 30.0 >282 17.4 >700 -- - SOURCE: Data from Environment Canada, 2005.
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From page 359... ...
These four oil droplet-size distributions, each including seven droplet sizes, are shown in Figure E-1. Figure E-2 shows the representative rising velocity of an oil droplet of Alaska North Slope crude oil in 20° C (roughly 68° F)
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From page 360... ...
Modeling Results and Evaluation Sensitivity analyses were performed by combining the three oil types, three wind speeds, three wave heights, three horizontal diffusion coefficients, and three vertical diffusion coefficients with and without dispersant application, as well as four oil droplet-size distributions with dispersant applications. Table E-3 shows the 14 test cases for the sensitivity analysis.
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From page 361... ...
Simulation results for evaporation, natural dispersion, chemical dispersion, amount floating, and amount beached at 14, 24, and 48 hours are summarized in Table E-4. Oil volume percent may be converted to weight percent using oil density.
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From page 362... ...
For Case 1 without a dispersant application, Figure E-4 shows the location of the predicted oil plume 24 hours after the spill and the oil spill location marked by "+." Black spots represent oil floating on the water surface, that traveled about 50 km from the spill site over 24 hours. The colored areas show different ranges of oil concentrations in the top 1 m of the water column; note that oil plume in the water column is following a different trajectory than the oil on the surface.
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From page 363... ...
cValues are at 48th simulation hour. 70 60 Oil volume as percent 50 Case 1 Case 2 40 30 20 10 0 Evaporation Chemical Floating Natural On Beach Dispersion Dispersion FIGURE E-3 Predicted oil distributions at 24 hours after the release of Alaskan North Slope crude oil with (Case 2)
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From page 364... ...
Natural dispersion caused by wind, waves, and current is the only mechanism in Case 1 to disperse oil into the water column. One aspect of complexity comes from the fact that oil consists of a wide range of hydrocarbons (see Table E-2)
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From page 365... ...
Additional compounds present in cuts 15 (e.g., be 50 Floating oil as volume percent 0 Hour 40 6 Hours Case 1 9 Hours 30 12 Hours 24 Hours 20 10 0 10 1 11 2 12 3 13 5 4 6 7 8 9 Oil cut number FIGURE E-5 Predicted composition of floating Alaskan North Slope crude oil with a dispersant under 10-m/s wind at various times (Case 1)
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From page 366... ...
Furthermore, the operational effectiveness of a dispersant application to hit a targeted oil spill area should also be considered in setting the final dispersant effectiveness value. Predicted oil movements with a dispersant application are shown in Figure E-6 after 24 simulation hours.
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From page 367... ...
Although these results are understandable, it is difficult to quantitatively estimate the time-varying oil concentration at the point of interest to assess the potential environmental impact of the oil spill without modeling it.
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From page 368... ...
, but they also reflect the diffusivity (which increases at higher 60 Floating oil as volume percent 50 0 Hour 6 Hours 40 Case 2 9 Hours 30 12 Hours 24 Hours 20 10 0 12 10 13 11 9 7 8 6 3 1 4 5 2 Oil cut number FIGURE E-7 Predicted composition of floating Alaskan North Slope crude oil with a dispersant under 10-m/s wind at various times (Case 2)
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From page 369... ...
of the Alaska crude oil floating on the water surface. The combined effects of wind (thus wave energy and diffusion)
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From page 370... ...
Note that oil viscosity var ies as oil weathers. The simulation results without dispersant application indicate that the viscosity of Alaskan North Slope crude oil floating on the water surface (Case 1)
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From page 371... ...
These oil viscosity changes have a significant effect on how oil spreads and on how effective a dispersant would be in dispersing it. 80 70 Oil volume as volume percent 60 Case 8 50 Case 10 40 30 20 10 0 Evaporation Chemical Floating Natural On Beach Dispersion Dispersion FIGURE E-11 Predicted oil distributions at 14th hour after the release of diesel oil with (Case 10)
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From page 372... ...
These results reveal the importance of oil type, oil properties, and emulsification on oil dispersion. Cases 2, 9, and 10 with dispersant use correspond to Cases 1, 7, and 8 without dispersant use (Cases 1 and 2 are with Alaska North Slope crude oil, Cases 7 and 9 are with IFO 300, and Cases 8 and 10 are with marine diesel)
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From page 373... ...
Thus, transport and fate models should be used to assist decisionmakers to take appropriate remedial actions during an actual oil spill. Effect of Oil Droplet Sizes A dispersant application is expected to result in entrained oil composed of many more small droplets, which rise more slowly, as discussed previously (see Figure E-2)
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From page 374... ...
, these four cases were run with oil spilled on the surface of 200-m deep water, further offshore along the Florida coast, and constant diffusivities were assigned over a mixing depth of 200 m. Figure E-14 presents predicted oil migration on the water surface and in the top 1-m water column 48 hours after the oil spill for Case 11.
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From page 375... ...
Currently the effect of chemical dispersant application on oil droplet sizes is a model input, but the ability to predict droplet size should be developed and incorporated into oil transport and fate codes. These results again indicate that it is very difficult to integrate all of the interacting and sometimes competing transport and fate processes and
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From page 376... ...
Note: There was an 80 percent reduction in oil droplet size due to dispersant application. oil types/properties to predict when and how much oil will move to specific areas with and without dispersant application during an actual oil spill event.
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From page 377... ...
The ADIOS2 and 3-D GNOME codes, and possibly other codes, may support emergency response to provide "rough-cut" predictions within hours of oil spills.
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