U.S. Navy Ship Salvage Manual Volume 6 - Oil Spill Response

S0300-A6-MAN-060arrangements is twofold—first, to identify environmentally sensitive areas where dispersant use isappropriate and second, to determine appropriate conditions for dispersant application. By thiscontingency planning, an RRT, in response to an actual spill, makes a timely recommendation tothe FOSC about dispersants, so that the dispersants will have a high probability of being effective.Without preapproval, the time spent by the RRT discussing dispersants may extend beyond thetime that dispersants can be effective.There are several models that can assist in deciding whether dispersants are an applicableresponse tool. Two primary models are the EPA Computerized Spill Response Decision Tree(Figure 3-1) and the American Petroleum Institute Oil Spill Control Decision Diagram (Figure3-2).Other models usually consider specific activities, such as offshore drilling or oil terminal operationsand areas to be protected. They also provide information about effectiveness of various spillresponse options.6.2.5 Field Application of Dispersants. Dispersants may be applied by hand, from vessels orfrom aircraft. Time, cost and environmental considerations dictate the choice of method.In general, dispersants must be applied shortly after an oil spill. If the spill is miles at sea, aircraftare often the most effective response vehicle. Vessels may be utilized to apply dispersants to spillsclose to shore.The costs of using dispersants include transportation to deployment site, labor, training anddeployment of the application system and demobilization of the system. Although dispersantapplication by boats is less expensive, it is also less effective than by aircraft, particularly if thespill covers a large area or if the dispersant must be applied quickly to protect an environmentallysensitive area.Prevailing weather may either dictate the method of application or prohibit dispersants. Smallboats are ineffective in rough weather. Spotters in aircraft require good visibility to see spilled oil.6.2.5.1 Theory of Application. A sufficient volume of dispersant in properly sized droplets mustbe applied. Ideal droplet diameter is 300 to 1,000 microns. Smaller droplets turn to mist and driftwith the wind. Larger droplets tend to pass through an oil slick without dispersing it. Nozzlediameter and air shear generated by aircraft are the major influences on droplet size. Other factorsinfluencing droplet size include dispersant viscosity, system flow rate and operating pressure.A proper dispersant spray system takes these factors into account.6.2.5.2 Application Rates. First- and second-generation hydrocarbon-solvent-based dispersantsare applied undiluted at a ratio between 1:1 and 1:3—one part dispersant to one to three parts oil.Third-generation dispersants are highly concentrated and may be applied undiluted or mixed with6-6