Using Oil Spill Dispersants on the Sea (1989) / Chapter Skim
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5 How Dispersants Are Used: Techniques, Logistics, Monitoring, and Application Strategies
5 How Dispersants Are Used: Techniques, Logistics, Monitoring, and Application Strategies
Pages 215-238
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From page 215... ...
5 How Dispersants Are Used: Techniques, Logistics, Monitoring, and Application Strategies DESIGN OF DISPERSANT APPLICATION SYSTEMS Application systems are designed to meet, within practical limits, the following basic criteria that were discussed in Chanter 2: The dispersant must be sprayed on the oil. -sea - The dispersant must mix with the of} and move to the oilwater interface.
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From page 216... ...
Oil thickness or other factors do not influence the design of the apparatus (Exxon Chemical Company, 1985~. Application rate per unit area is usually varied by changing pump rate, boat or aircraft speed, or number of spray nozzles, or by repeat application (Before, 1985; Chau et al., 1986~.
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From page 217... ...
Mechanical system design factors that strongly affect droplet size distribution are nozzle orifice diameter, number of nozzles, fluid pumping rate, and aircraft air speed (Lindblom and Cashion, 1983~. These factors affect the shear acting on the fluid as it passes through the nozzle and enters the air stream behind the aircraft.
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From page 218... ...
Orifice, flow rate, and the number of nozzles used are chosen to keep the mechanical shear below 10,000 sec~i (Exxon Chemical Company, 1985~. Fortunately, the range of droplet sizes naturally produced with more viscous formulations under typical air shear is in the range recommended above, 350 to 500 am (Lindblom and Cashion, 1983~.
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From page 219... ...
Extensive testing by API and the Exxon Chemical Company has shown that depositional efficiency drops off markedly if clifferential velocity is more than about 64 m/see (Lindblom, 1987; Smediey, 1981~. Anything that causes a decrease in median droplet size, such as a decrease in dispersant viscosity, increase in differential velocity, or increase in nozzle shear, will decrease depositional efficiency and accuracy.
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From page 220... ...
Dispersant Type Four physical properties of a dispersant formulation affect droplet size during aerial dispersant spraying (I.indblom and Cashion, 1983~: · Viscosity is by far the most important. Volatility is not likely to be important for any concentrate dispersant formulations, but can be significant for hydrocarbon-based formulations.
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From page 221... ...
(Chapter 2~. Undiluted chemicals require a lower flow rate, and nozzles must be properly chosen and sized accordingly (Becker and Lindblom, 1983~.
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From page 222... ...
Recent tests of depositional accuracy were made using Kromekote cards to obtain total amount deposited per unit area and the percentage of total fluid pumped that reached the target (Lindblom, 1987~. Data from these tests indicate that 45 to 90 percent depositional efficiency can be expected (most of the data were in the 65 to 85 percent range)
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From page 223... ...
A seawater pump allows the dispersant to be added to the water stream where it is mixed with the water and applied through the nozzles of a spray boom mounted as far forward as possible. This is done to avoid the bow wave, which may push of} out of reach of the spray at typical boat speeds of 2 to 10 kn (Exxon Chemical Company, 1985~.
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From page 224... ...
Cone-shaped spray patterns are less efficient because they concentrate the dispersant unevenly along the edges rather than in the middle (Spraying Systems Co., 1984, 1985~. In a properly designed system, the effect of wind on the spray pattern, spray-boom location relative to the bow wave and distance above water, vessel speed, nozzle design, pump rate, and pressure are considered (Exxon Chemical Company, l9SS)
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From page 225... ...
tanks, as shown in Table 5-2. For dispersant spraying, agricultural spray aircraft need to be fitted with adequate pumps, meters, and aft-facing nozzles.
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From page 226... ...
The most important advantages of helicopters are maneuverability and convenience- they do not need a nearby airport. They can also be cost-effective for small spills, especially where offshore platforms provide staging areas and landing sites, although local ordinances that prohibit helicopters from carrying sling loads across highways may pose potential problems in some areas.
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From page 227... ...
At one time, dispersant spraying gear was carried aboard the Canadian Coast Guard's cargo-carrying hovercraft Voyageur as a possible response to oil well blowouts, but it was never used (Gill,
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From page 228... ...
Dosage from boat-mounted systems is determined by swath width, boat speed, and water or chemical pump rates (Exxon Chemical Company, 1985~. Published pump curves, tables of nozzle output versus pressure, eductor percentage settings, and even calculated swaths
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From page 229... ...
. Water can be used to calibrate units designed for diluted dispersants and for aerial application systems (Exxon Chemical Company, 1985~.
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From page 230... ...
Distribution of oil in the water column during oil spill experiments off the coast of Norway in June 1985 was determined by use of instruments that detected light scattering by of} droplets in water and the ultraviolet fluorescence of dissolved of! components (Genders, 1986~.
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From page 231... ...
visual and Near-Visible Observation Reflection, absorption, and scattering from water with and without spiked oil, and the ability of oil to affect the interference and polarization of reflected light allow visual and near-visible observation of oil. During experiments in the North Sea, for example, observers aboard aircraft noted that ~ day after treatment a chemically dispersed slick had disappeared, but an untreated control slick was still clearly visible (Lichtenthaler and Dating, 1985~.
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From page 232... ...
Infirared Sensing Because oil absorbs and retains more heat (from solar infrared radiation) than does water, it is possible with the use of infrared-sensing devices to detect of} on the water surface by means of temperature differences.
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From page 233... ...
from the water surface when treated by dispersants. Summary of Monitoring Techniques The best method of surveying appears to be from an aircraft using SLAR for initial mapping, followed by infrared line scans to determine relative slick thickness and ultraviolet line scans to produce a picture that can be correlated with visual observation (Fast, 1985, 1987~.
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From page 234... ...
. STRATEGY OF DISPERSANT APPLICATIONS Development of contingency plans for spill response requires a great deal of thought in advance of any incident.
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From page 235... ...
The por-tions of the slick most effectively treated with dispersant are about 0.02- to 0.2-mm thick (Exxon Chemical Company, 1985~. The state of California Contingency Plans require clear evidence of a leading edge (an unusual configuration for a large spill)
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From page 236... ...
That question cannot be answered without a great deal of additional knowledge about water circulation, exchange with the open ocean, and sedimentation and resuspension rates in that area. Aerial Spraying Strategy When flying directly into the wind, the effective swath width is roughly T.2 to 1.5 times the overall length of the spray boom.
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From page 237... ...
During the early stages of an offshore continuous discharge, the oil can be treated initially, at least during daylight hours, by aircraft and boat spraying. Oil that is released during the night and moves as a slick away from the platform can be sprayed the next day.
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From page 238... ...
Application techniques in adverse weather conditions are included in some contingency plans. (EPA Regions OX and X plans allow consideration of dispersant application when mechanical means are impossible.)
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