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

S0300-A6-MAN-060within a few hours, so there is little need to apply dispersants to light oils except to prevent a fireor fume hazard. Lubricating oils are usually difficult to disperse chemically because of their additives.The type of oil, weathering, method of application and environment all influence dispersantperformance.6.2.3.1 Viscosity. Oils with a viscosity of less than 2,000 centistokes (cSt) react well with chemicaldispersants. Dispersants are less effective for oils with viscosities in the 2,000 to 5,000 cStrange. Dispersants are ineffective for oils having a viscosity greater than 5,000 cSt. Oils at temperaturesbelow their pour point become very viscous and are difficult to disperse. Table 6-1 listssome crude oils that are not amenable to treatment by dispersants in temperate waters because ofhigh viscosity or pour point.Because viscosity increases as oil cools, dispersants effective in warm weather may be ineffectiveon the same oil in a cold environment. Oil viscosity also increases with weathering. A dispersanteffective shortly after a spill may be much less effective later.6.2.3.2 Field Testing. Dispersants and oils are chemically complex. Selected dispersants shouldbe tested on the spilled oil to prove the effectiveness of the dispersant and the method of applicationbefore large applications are attempted. Testing should take place early in the response toallow large scale application before the oil weathers significantly.6.2.3.3 Method of Application. Application must be timely and deliver the dispersant in theproper droplet sizes and amounts. Proper application requires great attention to detail. Dispersantsmay be applied from aircraft or vessels during periods of good visibility.6.2.3.4 Prevailing Weather. After dispersants are applied, mixing energy is required to completethe chemical reactions that must take place. Wave action generated by winds of Beaufort Force 3or higher is usually sufficient.6.2.3.5 Environmental Considerations. Both long-term and short-term environmental impactsmust be taken into account in determining if dispersants are appropriate.Dispersants are recognized as an effective measure to prevent oil from stranding on an environmentallysensitive shoreline. However, the dispersed oil may do more long-term damage than notapplying dispersants and allowing the oil to drift ashore.The sensitivity of the local environment where dispersants may be applied must be considered.While such consideration is on a case basis, some generalities are helpful in these evaluations.• Dispersed oil has the same effects on marine life as nondispersed oil. If oil is dispersedchemically in shallow water, some oil settles on the bottom where it affects marine lifeadversely. Oil that settles into bottom sediment takes much longer to biodegrade thanoil in the water column. Consequently, it impacts benthic organisms for a longer time.Dispersed oil may penetrate beach sand and gravel deeper than nondispersed oil.6-4