Draft final

Discussion Note 8.2 AERIAL APPLICATION

This general aerial application guide is intended simply to highlight key issues. The FOSC will coordinate and oversee operational aspects of aerial dispersant applications.

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  Aircraft applications should always include pump-driven spray units.
  
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  Dispersant droplet size should be between 400 and 1000 microns.
  
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  Commercial aircraft spray nozzles generally range between 350 and 700 microns.
  
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  1000-micron spray nozzles may be needed for use on viscous oils.
  
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  Nozzles should achieve an application rate of 5.3 gallons per acre if using a 1:20 ratio.
  
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  Spray nozzles should be installed to discharge directly aft.
  
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  Underslung buckets on helicopters should be mounted so the pilot can see the ends of the spray booms in flight.
  
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  The altitude of the aircraft should be as low as possible.
  

From Cawthron, 2000

BOX 9 ARE THERE INDICATIONS THE DISPERSANT IS EFFECTIVE?

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  Acquire information from dispersant monitoring team (SMART team or other FOSC-designated monitors).
  
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  Review dispersant monitoring results after each dispersant application.
  
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  Determine if chemical dispersion is significantly greater than natural dispersion.
  
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  Assess whether changing application parameters could make the application more effective.
  

Decision: Are there indications the dispersant is effective?

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  Yes Go to Box 10
  
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  No See Discussion Note 9.2 and return to Box 8, or Go to Box 12.
  

Make a note of the decision in the Dispersant Decision Summary box on the Dispersant Use Flowchart

From Cawthron, 2000

Discussion Note 8.3 BOAT APPLICATION

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  Spray booms should be mounted as far forward as possible to prevent oil being moved aside by the bow wave before being sprayed. This then uses the mixing energy of the bow wave to break up the oil.
  
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  Spraying systems should be set so that the spray pattern is flat, striking the water in a line perpendicular to the direction of the boat’s travel.
  
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  The fan-shaped sprays from adjacent nozzles should be set as low as possible, overlapping just above the oil/water surface, with the inboard spray striking the hull just above the waterline.
  

Undiluted dispersants

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  Air blast sprayers and modified spray pumps can be used to apply undiluted concentrated dispersants and conventional dispersants.
  
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  Treatment rate is usually constant and determined by nozzle size and spray pressure.
  
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  Calibration and use of an appropriate droplet size is critical to effective applications.
  

Pre-diluted dispersants

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  Concentrated dispersants can be applied after pre-dilution in seawater, but will be less effective.
  
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  The dispersant : water ratio should be equal to, or greater than, 10%
  
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  Applications through ship’s fire-fighting equipment are controlled by opening or closing the dispersant supply. Vessel speed is used to control the treatment rate.
  
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  Dual pump systems for dispersant and seawater-supplying spray booms allow the dilution rate to be adjusted.
  
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  Boat speed is the main determinant of dispersant dose rate (reduce boat speed to increase the dose rate).
  
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  Boat speed should be in the order of 5 knots for fresh spills of liquid crude or fuel oil, which assumes that the oil has spread to 0.1 mm thick.
  
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  With reduced boat speeds, the required application rate per acre or km² can be maintained by reducing pump speed.
  

The following ASTM standards apply to systems involving spray arms or booms that extend over the edge of the boat and have fan-type nozzles that spray dispersant in a fixed pattern:

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  ASTM F 1413-92: Standard Guide for Oil Spill Dispersant Application Equipment: Boom and Nozzle Systems
  
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  ASTM F-1460-93: Standard Practice for Calibrating Oil Spill Dispersant Application Equipment Boom and Nozzle
  

Systems

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  ASTM F 1737-96: Standard Guide for use of Oil Spill Dispersant Application Equipment During Spill Response: Boom and Nozzle Systems.
  

Boat-based systems using a fire monitor and/or fire nozzle shall avoid a straight and narrow “firestream” flow of dispersant directly into the oil. There are no applicable ASTM standards for these systems at this time (December 2003).

In part from Cawthron, 2000

Discussion Note 9.1 ASSESSING DISPERANT EFFECTIVENESS

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  Dispersant applications must be monitored to confirm whether or not dispersant use is effective, and to determine the fate and transport of treated oil.
  
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  Dispersant applications should not be delayed simply because monitoring is not in place.
  
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  Visual observation is the minimum level of monitoring. Observations teams may use the forms in Attachment III.
  
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  There will be very few instances where a dispersant application is possible but visual monitoring is not.
  
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  Because dispersed oil plumes are often highly irregular in shape and thickness, it can be difficult to accurately estimate dispersant efficiency.
  
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  The appropriate dispersant application dose depends on the oil thickness (see Attachments III.a and III.b for common dose rates based on oil thickness). Slicks are generally not of uniform thickness, and it is not always possible to distinguish among thicker and thinner portions of the same slick. It is therefore possible to apply too much or too little dispersant to some parts of a slick. Because over- and under-dosing can lead to variations in effectiveness, these variations should be noted.
  
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  On-site monitoring of oil dispersed in the water column should support visual monitoring whenever possible. See Attachment III for additional information and forms.
  
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  Decisions to terminate operations due to poor effectiveness should ideally be based on on-site monitoring results.
  
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  A visible coffee-colored cloud in the water column indicates the dispersant is working.
  
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  A milky-white plume in the water column can indicate excessive dispersant application.
  
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  When dispersant is working, oil remaining on the water surface may also change color.
  
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  A difference in the appearance of treated and untreated slicks indicates dispersion is likely.
  
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  Absence of a visible cloud in the water column makes it difficult to determine whether the dispersant is working. When the water is turbid, you may not be able to see a plume. Oil remaining at the surface and sheens can also obscure an ability to see oil dispersing under the slick.
  
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  Successful dispersion can occur with no visible indication of dispersion.
  
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  A subsurface plume may not form instantly once dispersant has been applied. In some cases (e.g., emulsified oil) it can take several hours for a plume to form. In other cases, a visible plume may not form, and you may wish to use sampling to learn whether dispersion has occurred.
  
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  Boat wakes may physically part oil, falsely indicating successful dispersion. Mechanically dispersed oil will re-coalesce and float to the surface.
  
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  Dispersants sometimes have a herding effect on oil after initial applications, making a slick appear to be shrinking when, in fact, the dispersant is “pushing” the oil together. The effect results from the surfactants in the dispersant, which causes a horizontal spreading of thin oil films. This can cause parts of a slick to seem to disappear from the sea surface for a short time.
  

From Cawthron 2000 and NOAA Oil Spill Job Aids