Slick spreading

Equations (20.24) and (20.25) were developed for spills of constant volume, constant surface tension, and low viscosity on calm water. The effects of wind and currents on spreading rates are not well studied and are difficult to estimate. Therefore, the quantifiable uncertainty in the spreading rate lies in the estimation of the parameters used in Eqs. (20.24) and (20.25). The transition from a viscous spread, i.e., Eq. (20.25) to a surface tension spread, i.e., Eq. (20.23) occurs rapidly for most spills, and the spreading rate is described by Eq. (20.24). Since the density and viscosity of water can be estimated fairly confidently, most of the uncertainty in the spreading rate lies in the estimation of the net surface tension, specifically in the estimation of the air-oil surface tension and the oil-water surface tension. There is also an uncertainty in the applications of the slick-spreading model to a cross-sectional nonuniform velocity profile, where the nonuniformities would add to the spreading. In this case, the slick would experience a longitudinal dispersion in addition to the water. This phenomenon is not a component of the sensitivity analysis. 

Contemporary concern about pollution has made it important to dispose of oil slicks from spiUs. The suitable use of surfactants may reverse the spreading of the slick, thereby concentrating the slick for easier removal. 

Oil spreads on water to form a film about 100 nm thick (two significant figures). How many square kilometers of ocean will be covered by the slick formed when one barrel of oil is spilled (1 barrel = 31.5 U.S. gal) ... 

Diesel-like products (jet fuel, diesel. No. 2 fuel oil, kerosene) are moderately volatile products that can evaporate with no residue. They have a low-to-moderate viscosity, spread rapidly into thin slicks, and form stable emulsions. They have a moderate-to-high (usually, high) toxicity to biota, and the specific toxicity is often related to type and concentration of aromatic compounds. They have the ability to penetrate substrate, but fresh (unoxidized) spills are nonadhesive. 

As oil enters the environment, it begins to spread immediately. The viscosity of the oil, its pour point, and the ambient temperature will determine how rapidly the oil will spread, but light oils typically spread more rapidly than heavy oils. The rate of spreading and ultimate thickness of the oil slick will affect the rates of the other weathering processes. For example, discharges that occur in geographically contained areas (such as a pond or slow-moving stream) will evaporate more slowly than if the oil were allowed to spread. Most of this process occurs within the first week after the spill. 

Some special problems arise at sea. When crude oil is spilled on the ocean, a slick is formed which spreads out from the source with a rate that depends on the oil viscosity. With sufficient energy an O/W emulsion may be formed, which helps disperse oil into the water column and away from sensitive shorelines. Otherwise, the oil may pick up water to form a water-in-oil emulsion, or mousse ( chocolate mousse ). These mousse emulsions can have high water contents and have very high viscosities, with weathering they can become semi-solid and considerably more difficult to handle, very much like the rag-layer emulsions referred to above. The presence of mechanically strong films makes it hard to get demulsifiers into these emulsions, so they are hard to break. See Chapter 9. 

Marine oil spills can cause significant environmental damage. When a crude oil is spilled at sea several processes can occur (Figure 9.5) [552]. Immediately following a spill, a slick is formed which spreads out from the source [553,554]. Studies of the spreading of the spilled oil have been carried out by a number of researchers [553,555-561], Next there is the potential for the formation of two quite different kinds of emulsions. 

Surface-Active Material and Surface Pressure. If a slick or any surface-active film is to spread and increase in area it must exert a horizontal surface pressure. The relation between surface pressure and surface tension is (both measured in dynes/cm) ... 

The spreading of oil spilled on water is discussed in this section. Oil spreads to a lesser extent and more slowly on land than on water. The spreading of oil on land is described in Chapter 12. Oil spilled on or under ice spreads relatively rapidly but does not spread to as thin a slick as on water. On any surface other than water, such as ice or land, a large amount of oil is retained in depressions, cracks, and other surface irregularities. 

The rates of spreading under ideal conditions are shown in Figure 8. As a general rule, an oil slick on water spreads relatively quickly immediately after a spill. The outer edges of a typical slick are usually thinner than the inside of the slick at this stage so that the slick may resemble a fried egg. After a day or so of spreading, this effect diminishes. 

Winds and currents also spread the oil out and speed up the process. Oil slicks will elongate in the direction of the wind and currents, and as spreading progresses, take on many shapes depending on the driving forces. Oil sheens often precede heavier or thicker oil concentrations. If the winds are high (more than 20 km/h), the sheen may separate from thicker slicks and move downwind. 

Oil moving along these zones is alternately concentrated and spread out by the circulation currents to form ribbons or windrows of oil rather than continuous slicks. In some locations close to shore, zones of convergence and divergence often occur in similar locations so that oil spills may appear to have similar trajectories and spreading behaviour in these areas. 

In addition to their natural tendency to spread, oil slicks on water are moved along the water surface, primarily by surface currents and winds. If the oil slick is close to land and the wind speed is less than 10 km/h, the slick generally moves at a rate that is 100% of the surface current and approximately 3% of the wind speed. In this case, wind does not generally play an important role. 

As previously discussed, oil can be burned on water without using containment booms if the slick is thick enough (2 to 3 mm) to ignite. For most crude oils, however, this thickness is only maintained for a few hours after the spill occurs. Oil on the open sea rapidly spreads to an equilibrium thickness, which is about 0.01 to 0.1 mm for light crude oils and about 0.05 to 0.5 mm for heavy crudes and residual oils. Such slicks are too thin to ignite and containment is required to concentrate the oil so it is thick enough to ignite and burn efficiently. 

Chemical barrier — Chemicals that act as surface tension modifiers to inhibit the spread of an oil slick on water. When placed on the water surface next to an oil film, these chemicals push away the oil as a result of their surface tension. Chemical barriers work only with fresh oils, however, and their effect lasts only a few hours. (See also Surface tension.)... 

Containment boom — A floating mechanical structure that extends above and below the water surface and is designed to stop or divert the spread or movement of an oil slick on the water. Booms consist of floats, a freeboard member to prevent oil from flowing over the top of the boom, a skirt below the water surface to prevent oil from being swept under the boom, and one or more tension members to support the entire boom. Booms are an integral part of virtually all cleanup programs after oil spills on water. (See also Boom failure, Critical velocity, Freeboard.)... 

Fire-resistant booms — Floating containment structures constructed to withstand high temperatures and heat fluxes, used when burning oil on water. These booms restrict the spreading and movement of oil slicks while increasing the thickness of the slick so the oil will ignite and continue to burn. 

Sinking agent — A material that is spread over the surface of an oil slick to adsorb oil and cause it to sink. Common sinking agents include treated sand, fly ash, and special types of clay. These materials are rarely used, however, because they provide a purely cosmetic approach to oil spill cleanup and may cause considerable damage to bottom-dwelling organisms. 

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