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

S0300-A6-MAN-060Spilled oil is acted upon by natural processes that occur more or less simultaneously, although theprocesses proceed at very different rates. These processes lead to the removal of oil from thewater surface and its eventual accommodation by the environment. The main processes acting onspilled oil are:• Spreading• Evaporation• Dispersion• Dissolution• Emulsification• Oxidation• Microbial degradation• Sinking• ResurfacingNearly all the processes cause chemical or physical changes in the oil. The natural processes andresulting changes in the oil characteristics are collectively called weathering. Many of the changesfrom weathering help response efforts. Weathering of oil on water is much more complex thanweathering of oil on land because of the dynamics of water movement and exposure to the elements.The specific processes that contribute to weathering of spilled oil are described in the followingparagraphs. Figure 2-2 is a graphical representation of the weathering processes. Figure2-3 shows the relative impact and time span of these natural processes.Oils may be categorized as either persistent or nonpersistent. Persistent oils remain on the sea surfacefollowing a spill and require response activity for removal. Nonpersistent oils tend to dissipatewithout assistance, requiring less human intervention following such a spill. Crude oils andrefined residual oils are persistent oils. Nonpersistent oils include gasoline; naphtha; kerosene;Diesel Fuel, Marine (DFM); and other diesel fuels. Nonpersistent oils are typically more toxic tomarine life than persistent oils and safety concerns regarding vapors given off from nonpersistentoils may limit offensive response measures.2-3.1 Spreading. The initial and very visible process of spreading of spilled oil can last as longas 10 days for a very large spill. Spreading thins the oil slick to a few millimeters or less. Rate ofspreading depends upon the volume of oil spilled, the duration of the spill and physical characteristicsof the spilled oil, particularly pour point, viscosity and surface tension.Low-viscosity oils spread more rapidly than high-viscosity oils. Spilled oils at temperaturesbelow their pour point spread very little.2-8