Oil Spill Dispersants Efficacy and Effects (2005) / Chapter Skim
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3 Dispersant-Oil Interactions and Effectiveness Testing
3 Dispersant-Oil Interactions and Effectiveness Testing
Pages 51-134
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From page 51... ...
. Reduction of the interfacial tension between oil and water by addition of a dispersant promotes the formation of a larger number of small oil droplets when surface waves entrain oil into the water column.
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From page 52... ...
The following sections address dispersant chemistry, the physical and chemical interactions of dispersants with oil slicks and droplets, oil chemistry and weathering behavior and how they affect the window of opportunity for effective dispersant applications, and the importance of turbulence for introducing the energy necessary to entrain oil droplets into the water column as well as their subsequent transport by dispersive and advective processes. Next is a discussion of effectiveness testing and related issues, including laboratory systems, wave-tank tests, field studies, and studies involving spills of opportunity.
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From page 53... ...
. Natural components that promote the for dispersant application droplet formation and entrainment oil droplets dispersed in blow up of water column surfactant-coated oil droplet hydrophilic group surfactant molecule lipophilic group FIGURE 3-1 Mechanism of chemical dispersion: surfactant accumulates at oil-water interface, facilitating formation of small oil droplets that become entrained in the water column.
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From page 54... ...
Such a balance will promote formation of stable oil-in-water dispersions (entrained oil droplets in the water column) because the dominant hydrophilic group of the surfactant mixture favors the formation of oil droplets in water.
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From page 55... ...
The major nonionic surfactants include ethoxylated sorbitan mono- and trioleates and sorbitan monooleate; the major ionic sur
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From page 56... ...
THE PHYSICAL CHEMISTRY OF DISPERSANT-OIL INTERACTIONS AND THE ENERGY REQUIREMENTS FOR EFFECTIVE OIL-DROPLET ENTRAINMENT AND DISPERSION The objective of an oil-spill dispersant application is to lower the oil/ water interfacial tension to enhance entrainment of small oil droplets into the water column at lower energy inputs. Entrainment of small oil droplets into the water column (by either physical or chemical means)
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From page 57... ...
Turbulent energy is the environmental parameter most responsible for generating and transporting dispersed oil droplets in the ocean. Delvigne and Sweeney (1988)
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From page 58... ...
Droplet-size distributions describe the relative abundance of droplets of various sizes, which may range from <1 µm to >100 µm in diameter. These distributions can be based on either droplet number or volume, although the volume distribution may be most informative, because the relationship between droplet volume and oil mass is constant regardless of droplet size (i.e., the proportionality constant is the density)
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From page 59... ...
has sug gested that dispersant effectiveness tests should be conducted in labora tory-scale systems and wave tanks that generate microscale turbulence similar to that which prevails in surface seawater, because such similarity suggests that the droplet-formation mechanisms will also be similar. There fore, effectiveness testing should include measurement of droplet-size dis tributions, preferably in the presence of turbulent mixing energy, so that the observed size distribution will not be affected by size fractionation.
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From page 60... ...
on the droplet-size distribution produced when crude oil (Forties) was dispersed at sea.
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From page 61... ...
. Energy dissipation rates can be expressed in units of energy loss per volume per time, e (J-m3-s1)
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From page 62... ...
. In a laboratory flask, column, or tank, the rate of energy dissipation can also be determined indirectly by the rate of energy input by assuming that all input energy turns into turbulence.
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From page 63... ...
and surface spreading of the oil was limited by the walls of the tank, the changes in oil chemistry and rheological properties that occurred in this oil over time were remarkably similar to those that were observed in the Alaska North Slope crude oil released from the T/V Exxon Valdez oil spill in Prince William Sound, Alaska (Payne et al., 1991a)
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From page 64... ...
5000 wave tanks INTERFACIAL TENSIONS (dynes/cm) 40 30 1000 oil/air water/oil 20 100 10 10 1 3 6 9 12 2 4 6 8 10 12 4 months 9 months TIME (days post spill)
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From page 65... ...
Viscosity (cP) Water 20 1.0 Ethyl alcohol 20 1.2 Olive oil 40 36 Fresh Prudhoe Bay crude oil (PBCO)
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From page 66... ...
. Although the predictions of this model are reasonably accurate, it is not always possible to predict whether a particular oil will emulsify under specified environmental conditions in the field and what the final water content will be.
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From page 67... ...
, but no studies on leaching of surfactants from the oil phase have been conducted at realistic oil-to-water ratios and under different energy regimes to test this hypothesis. In particular, the effects of surfactant leaching on the effectiveness of initial oil dispersion and the potential for dispersed oil droplet coalescence should be understood better.
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From page 68... ...
However, there were no large capacity application packages (e.g., ADDS pack) in Alaska, and only a single helicopter bucket spray system was stored in Kenai (Alaska Oil Spill Commission, 1990)
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From page 69... ...
Between 1999 and 2004, dispersants were used seven times to combat oil spills in the Gulf of Mexico. In six of these cases, dispersants were used under the existing pre-approval plan for oil spills greater than 3 nautical miles offshore and in waters of greater than 10 m depth.
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From page 70... ...
Chemical analysis of water samples collected from the area of one of the treated slicks on the second day of dispersant operations showed only low concentrations of dispersed oil in the water column. British Petroleum estimated that approximately 160 tonnes (15 to 31 percent)
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From page 71... ...
operational effectiveness, which describes the encounter probability of the dispersant application and the ability of the dispersant to become incorporated into the floating oil, (2) chemical effectiveness, which is measured by the fraction of treated surface oil that becomes stably entrained as small droplets in the water column, and (3)
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From page 72... ...
Chemical effectiveness has been investigated in the laboratory, in wave tanks, and at sea. In many of these studies, effectiveness was defined based on chemical effectiveness, which was quantified as the mass fraction of oil that was measured in samples collected from the water column or the mass fraction that was not recovered from the water surface as floating oil.
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From page 73... ...
small wave tanks for dispersant effectiveness tests can be significant. Planned field studies and spills of opportunity also have many similarities, but the advantages and disadvantages are sufficiently different that they are considered separately.
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From page 74... ...
As a result, these comparisons are often conducted in bench-scale systems, but more limited testing has also been conducted in wave tanks and at sea. Effectiveness tests may also be used in fundamental investigations of the mechanisms that control natural or chemically enhanced dispersion of oil into water (Belk et al., 1989; Fingas et al., 1991; Blondina et al., 1999; Canevari et al., 2001; Chandrasekar et al., 2003)
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From page 75... ...
Ideally, dispersant effectiveness would be an output of a mathematical model, and the inputs would be factors such as oil characteristics, weather conditions, and other operational factors (e.g., dispersant type, effective DOR)
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From page 76... ...
Although these principles can be applied at all scales at which dispersant effectiveness can be tested, time and financial constraints will limit the degree to which they can be implemented as the scale of the test system increases. Such practical limitations, however, make clear definition of objectives and careful experimental design more -- not less -- important with increasing scale.
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From page 77... ...
In general, this aspect of dispersant effectiveness, which would be considered operational, is not adequately characterized or controlled in most existing effectiveness tests at any scale. Wave tanks provide the most appropriate system for investigating the relationship between dispersant penetration and oil characteristics, because these systems are large enough to use realistic dispersant application systems (e.g., spray booms with typical nozzles)
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From page 78... ...
For dispersant effectiveness testing, the endpoint is often defined to be the percent of added oil that is dispersed into the water column. For larger-scale systems, such as wave tanks and field studies, the water-column sample collection protocols can affect the observed effectiveness because the distribution of dispersed oil droplets is likely to be heterogeneous (Brown et al., 1987; Brown and Goodman, 1988; Lewis and Aurand, 1997)
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From page 79... ...
As a result, more generally useful information would be obtained if effectiveness tests measured droplet-size distribution in addition to the mass fraction of oil dispersed into the water column or remaining on the water surface. An objective of dispersant effectiveness testing at all levels is to determine whether addition of a chemical dispersant to a floating oil slick will increase the amount of oil that is transferred into the water column as small droplets relative to the amount that would be transferred from an untreated oil slick or from a slick treated with a different dispersant.
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From page 80... ...
When an experimental design requires tests to be conducted over a prolonged period of time or by different analysts, precautions should be taken to ensure that results are comparable. That is, a mechanism should BOX 3-2 Basic Principles of Experimental Design Dispersant effectiveness is often quantified by measuring the amount of oil that is transferred to the water column or remains on the surface (or both)
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From page 81... ...
variations in experimental conditions or mea surement technique, will reduce the likelihood that two independent mea surements of dispersant effectiveness will produce the same result even when they are made under nominally identical conditions. For example, small variations in the energy input or dispersant-to-oil ratio may cause the measured extent of dispersion to be different in replicate effectiveness tests.
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From page 82... ...
Although it is generally recognized that these results cannot be used to quantitatively predict dispersant effectiveness in the field, their use for the objectives described above implies a belief that they provide reliable relative rankings. Although the results of several bench-scale effectiveness tests may be weakly correlated, this assumed relationship has not been thoroughly investigated or subjected to rigorous peer review (Fingas et al., 1994; Fiocco et al., 1999)
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From page 83... ...
. Note that the basis for this conclusion was that the standard error of dispersant effectiveness in replicate MNS apparatuses was 9 percent versus 3 percent in replicate swirling flask tests (described below)
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From page 84... ...
84 Bath cooler Sampling tube Orifice meter Oil containment ring 4.5 cm Disc valve 29 cm 50 cm ID = 3.2 cm Singer vortex blower I D = 29 cm 60 cm FIGURE 3-5 Schematic diagram of Mackay-Nadeau-Steelman (MNS) apparatus.
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From page 85... ...
In the EXDET procedure, however, mixing energy is provided by a wrist-action shaker, which is also available in many FIGURE 3-6 Picture of Warren Springs Laboratory (WSL; a.k.a., Labofina) dispersant effectiveness testing apparatus.
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From page 86... ...
. So, EXDET has an advantage over many other dispersant effectiveness tests in that it does not require specialized equipment.
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From page 87... ...
of six oil-dispersant combinations. The most common method for quantifying dispersion effectiveness in the swirling flask test is measurement of the absorbance of long-wave ultraviolet light (e.g., averaging the absorbance at 340 nm, 370 nm, and 400 nm)
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From page 88... ...
at the bottom of the flask that increase turbulence during mixing by preventing development of a vortex due to the swirling motion of the gyratory shaker. The baffled flask Baffles Stopcock FIGURE 3-8 Photograph of the glassware used in the baffled flask test.
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From page 89... ...
Recently, the energy dissipation rates in the swirling flask and baffled flask tests were compared using a Hot Wire Anemometer to characterize the turbulence characteristics (e.g., the velocity gradient, G, and the energy dissipation rate per unit mass) of both systems (Kaku et al., 2002)
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From page 90... ...
As described above, laboratory studies measure only chemical effectiveness; effectiveness tests conducted in wave tanks have the potential to also include some level of operational effectiveness. In particular, dispersant application equipment that produces dispersant droplets with size distributions and impact velocities that are similar to those encountered in spill-response operations can be used in tank tests.
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From page 91... ...
In this regard, the formation of water-in-oil emulsions is particularly important in inhibiting dispersant effectiveness. To date, large wave tanks have not been used to examine the performance of dispersants on water
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From page 92... ...
Dispersant application efficiency is affected by dispersant droplet size and velocity. If this aspect of operational effectiveness is to be investigated in wave tanks, the dispersant application system should simulate the droplet-size distributions and impact velocities that are characteristic of specific application methods (e.g., aircraft, helicopter, vessel)
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From page 93... ...
Description of Wave Tanks Available for Mesoscale Dispersant Testing This section provides brief descriptions of some facilities that are available for testing of dispersant effectiveness in wave tanks. These descriptions focus primarily on the physical facilities and the tools available for measuring experimental conditions and results.
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From page 94... ...
. Oil for dispersant tests has typically been added to the water surface in an approxi
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From page 95... ...
Note, however, that the entire volume of the tank is potentially available for dilution of the dispersed oil plume. In dispersant tests that have been conducted at OHMSETT -- beginning in March 2002 -- the test oil has been applied to the water surface through a manifold mounted to the leading edge of the main bridge (Figure 3-10)
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From page 96... ...
The large size of the OHMSETT tank offers advantages to experimenters wishing to investigate certain aspects of operational effectiveness (e.g., the dispersant application equipment can produce dispersant droplets with realistic size distributions) and hydrodynamic effectiveness (e.g., the facility allows dispersed oil to be transported in a relatively large volume of water)
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From page 97... ...
Finally, the size of the OHMSETT tank and its associated equipment is likely to increase the difficultly of closing mass balances through collection of non-dispersed surface oil, measurement of the concentration of dispersed oil droplets in the water column, and quantification of the oil that escaped the boomed test enclosure or adhered to the boom itself. The addition of a secondary containment boom outside the north end of the 10,000 ft2 experimental area has significantly improved collection of surface oil that splashes out of the test enclosure (it is then included with the other non-dispersed oil collected from the water surface within the test area)
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From page 98... ...
seawater in this tank is small enough that it can be replaced relatively quickly between tests, reducing concerns about the build-up of dispersant or dispersed oil concentrations between runs. A disadvantage of the small tank volume is that it precludes investigation of hydrodynamic effectiveness (e.g., dilution of the dispersed oil plume by turbulent mixing)
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From page 99... ...
The SERF wave tanks have the ability to simulate nearshore environments by constructing sand beaches, including a flat back-beach area just above the high-tide line. The tanks can be operated with a high-tide water depth of 2.0 m and a tidal range of about 0.6 m.
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From page 100... ...
To this end, investigators at the SERF measure oil concentrations in several compartments, including the water surface, the water column, the shoreline sediments, and the tank walls (Bonner et al., 2003)
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From page 101... ...
101 DISPERSANT-OIL INTERACTIONS AND EFFECTIVENESS TESTING FIGURE 3-14 S.L. Ross wave tank, Ottawa, Ontario, Canada.
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From page 102... ...
. Design of Effectiveness Tests in Wave Tanks The primary advantage of wave-tank studies over laboratory-scale tests is the ability to investigate some components of operational effectiveness and introduce the energy that drives formation of small oil drop
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From page 103... ...
. Whenever possible, the design of mesoscale dispersant effectiveness tests, including hydraulic flumes and wave tanks, should incorporate these factors.
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From page 104... ...
. In addition to quantifying the energy dissipation rate, the fraction of added oil that becomes entrained in the water column should be measured in wave-tank studies.
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From page 105... ...
. Some attempts have been made to close mass balances during dispersant effectiveness tests in wave tanks (Brown et al., 1987; Brown and Goodman, 1988; Bonner et al., 2003)
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From page 106... ...
Although this control is desirable because it allows the experimenters to limit or, at least, to identify and measure the uncontrolled variables, it also introduces artificiality into the test. From a more fundamental perspective, the motivation for studying dispersant effectiveness in field studies derives from the opportunity to study phenomena that cannot be addressed at the smaller scale of laboratory and wave-tank systems.
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From page 107... ...
Carefully executed field studies can inform these conceptual models by testing the suspected cause-and-effect relationships that control dispersant effectiveness. In addition, field studies can be used to calibrate model parameters by providing measured dispersed-oil concentration distributions for specific well-characterized initial and boundary conditions that can be compared to model output.
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From page 108... ...
Design of field studies should involve principles similar to those used in the design of any experiment. In particular, a primary objective should be to obtain an unbiased estimate of the variation that exists between two experimental units (i.e., oil slicks)
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From page 109... ...
In addition to the inability to control weather and, therefore, to set mixing energy as an independent variable, field studies are subject to an additional important technical limitation: the inability to quantitatively measure effectiveness for use as an endpoint in statistical comparisons of treatments. Dispersant effectiveness in sea trials has been monitored by measuring surface oil and dispersed-oil concentrations in the water column, but neither method produces satisfactory results.
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From page 110... ...
For example, operational effectiveness will be unrealistically high due to application of dispersant from a boom mounted close to the oil discharge position, and the short time period between oil discharge and dispersant application allows for no weathering and limited spreading of the slick. Also, the hydrodynamic effectiveness will be artificially high, because this experimental design lays down a very narrow (initially 1-m wide)
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From page 111... ...
Review of Past Field Studies A number of controlled field trials of dispersant effectiveness have been conducted in Canada and Europe since the 1989 NRC review (McDonagh and Colcomb-Heiliger, 1992; Lunel and Lewis 1993a,b; Brandvik et al., 1995, 1996; Walker and Lunel 1995; Lunel 1993, 1994a,b, 1995a,b; Lewis et al., 1995a,b, 1998a,b; Lunel et al., 1995a,b,c; StromKristiansen et al., 1995; Walker and Lunel, 1995; Lunel and Davies, 1996; Fiocco et al., 1999)
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From page 112... ...
. Based on the monitoring results from field studies and actual spills, it can be concluded that it is difficult to estimate average concentrations under treated slicks because of the significant heterogeneity both horizontally and with depth into the water column (Brandvik et al., 1995; Lewis et al., 1998b)
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From page 113... ...
1m 5 4 3 4m 2 0.5m 1m 1m 0.5m 0.5m 4m 1 0 0 200 400 600 800 1000 1200 Distance (meters) FIGURE 3-17 Dispersed oil concentrations under an approximately 27 m3 surface slick of Forties crude oil (a)
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From page 114... ...
Dispersant Application: 1990 Test -- Objective was to have a steady-state oil discharge so that rep licate measurements could be made of the dispersed oil concentrations under the treated slicks to better quantify dispersant effectiveness. Four continuous releases of 50 liters per minute of MFO+GO, with dispersant application 1215 minutes later at a dispersant:oil ratio of 1:20, using OSR 5, Slickgone NS, and 1100X, and no dispersant as a control.
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From page 115... ...
Monitoring Results: Effectiveness: In all tests, remote sensing aircraft equipped with SLAR, video, ultraviolet, and infrared cameras were used to track the behavior of surface slicks. Oil concentrations in the water column were monitored us ing field fluorometers towed through the slicks at multiple transects at dif ferent depths and distances downcurrent.
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From page 116... ...
After dispersant application, oil concentrations were typically 110 ppm down to 5 m, with a maximum concentration of 25 ppm. There was a 16-fold increase in the volume of dispersed oil under the treated slick compared to the control.
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From page 117... ...
energy regimes encountered. There is a clear ranking in percentage of oil that the different formulations successfully dispersed into the water column in the field as the encountered energy regime increased; however, it should be noted that the overall percent dispersed values were relatively low.
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From page 118... ...
Effectiveness Testing Using Spills of Opportunity In the arena of public opinion, no test can hope to have the positive impact of an actual success in using dispersants during a real spill. There are several areas around the country where the volume of crude oil traffic is so large that small spills are somewhat common.
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From page 119... ...
Should it be approved and incorporated into a spill response plan (including identification and pre-placement of sampling equipment, and stand-by contracts for personnel) , it will provide considerable advantage in marshaling all the components necessary to adequately sample and monitor the results from dispersant applications when and if they occur in the designated areas.
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From page 120... ...
The distribution of dispersed oil droplets was very heterogeneous and reflected the patchy distribution of oil on the water surface before dispersant application. Maximum concentrations of dispersed hydrocarbons in the center of the treated zone were 22 mg/L for total aliphatics (primarily dispersed droplets)
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From page 121... ...
, suggesting that they represented a background, steady-state concentration of very fine, physically dispersed oil droplets that were formed by natural dispersion of the slick during the six days before the dispersant tests began. The ratio of the concentrations of aliphatic to aromatic hydrocarbons showed no evidence of significantly enhanced dissolution of lower- and intermediate-molecular-weight aromatics as a result of chemical dispersion.
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From page 122... ...
Each day, the thickest parts of the fresh oil slicks were repeatedly sprayed until they had been dispersed, then larger patches of more weathered oil offshore were sprayed. The last oil release occurred on day 7, and dispersant applications were terminated on day 8 when it was determined that they were no longer effective on the emulsified oil.
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From page 123... ...
Fluorometry and visual observations from boats were used to docu ment that dispersant application on emulsified oil did increase the oil con centrations and depth of oil mixing into the water column. The first dispersant application appeared to break the emulsion, whereas subsequent applications increased the concentrations of dispersed oil into the water (Lunel et al., 1997a)
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From page 124... ...
Depending on the circumstances, ground-truth information on fate and effect may or may not be required. Dispersant effectiveness is a phrase that has been interchangeably used to describe how well the product performs both in the laboratory
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From page 125... ...
Depending on many factors, the field effectiveness for a product may range from 0 percent to 100 percent. Effectiveness of a dispersant application in the field has been defined as "the amount of the oil that the dispersant puts into the water column compared to the amount of oil that remains on the surface" considering the total amount of the oil that was treated (Fingas, 2002a,b; 2003; Lewis, 2004)
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From page 126... ...
Because there is no truly quantitative method to determine dispersant effectiveness in the field, the best that can be done is to qualitatively estimate if the dispersants are working (Henry, 2004)
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From page 127... ...
, the addition of confirmation fluorometer readings will help substantiate visual observations that there has been an increase in the amount of oil entrained into the water column under treated slicks. Table 3-5 contains guidelines to assist in determination of effective/ineffective application.
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From page 128... ...
. SMART only outlines how to determine if the dispersant application is working, but provides no guidance on how to determine a percent dispersant effectiveness.
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From page 129... ...
This protocol is not intended to provide quantitative estimates of dispersant effectiveness, real-time estimates of water-column dispersed oil concentrations, or estimates of oil mass balance. This protocol attempts to monitor the dispersed oil plume by locating the water-column sampling stations and the in-situ fluorometry transect relative to drogues that drift with subsurface currents (usually at 2-m depth)
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From page 130... ...
The database of oil component acute toxicity is much better than the knowledge of the bioavailability of dispersed oil components in the water column. Unfortunately, most of the measurements on concentrations of TABLE 3-6 Guidance on the Additional Monitoring Data or Samples Pre-application Application Post-application Name of dispersant Spray time Volume of dispersant Dispersant lot number Pump rate remaining onboard Sample of each dispersant Speed during application lot number Volume of dispersant Spray height during onboard application Platform (aircraft or vessel)
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From page 131... ...
DEVELOPING ADEQUATE UNDERSTANDING OF DISPERSANT EFFECTIVENESS TO SUPPORT DECISIONMAKING As discussed in Chapter 2 and shown in Figure 2-4, the potential effectiveness of dispersants is a key consideration at several steps in the
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From page 132... ...
Future effectiveness tests should measure chemical effectiveness over a range of energy dissipation rates to characterize the functional relationship between these variables. Finally, evaluation of chemical effectiveness should always include measurement of the droplet-size distribution of the dispersed oil.
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From page 133... ...
The concentration of oil should be measured in all identifiable compartments to which it could be transferred when dispersant effectiveness is investigated in wave tanks. This includes, but may not be limited to, the water surface, the water column, the atmosphere, and wave-tank surfaces.
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From page 134... ...
Alternatively, oil dispersion should be measured after dispersant is applied and incubated with floating oil under calm conditions to determine the effect of surfactant leaching from a surface oil film on dispersant effectiveness. Although careful and controlled research in the laboratory or test tank will be important to developing tools to support decisionmaking, the results of dispersant application during real spills will be the most important indicator of whether or not the dispersant application was effective.
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