a) Mechanical Containment a1) floating booms, a2) bubble barriers and a3) current barriers b) Mechanical Recovery b1) weirs and suction devices, b11) floating weirs, b12) suction heads, b13) free vortex, b2) lifting surfaces, b21) rotating discs, b22) rotating drums and b23) moving belts c) Application Agents c1) dispersants, c2) sinking agents, c3) collecting agents, c4) herding agents, c5) burning agents and c6) biodegradation
Listing of Oil Sinking Agents See Table 8.5.2
CHARACTERISTICS AND COMPOSITION Spreading Rate of Oil Spills
Several factors need to be vectorially combined to define the oil spill spreading pattern. The current will drift the unrestrained oil at about the same velocity as the water. Wind adds a component of about 3-4% of the wind velocity, and natural spreading acts concentrically to disperse the slick. This is initially caused by the oil's hydrostatic head balanced by the oil's inertia. Typically, for an impulsive 500-2,500,000 gal spill of 0.9 specific gravity (SG) oil, this acts for about 1/4-1 hr until the oil reaches about ]/4in thickness and a 500-3,000 ft diameter. At this point, pressure spreading is primarily balanced by viscous drag in the underlying water, and the slick diameter grows at about 300 ft/hr. As the gravity head decreases, the net surface tension spreading pressure (water to air-oil to air-oil to water), usually about 20 dynes per centimeter, continues to disperse the oil until typically a 0.01-0.001 in thickness is reached.
Variations in spreading rate depend on the oil's specific gravity, surface tension, characteristic evaporation, solubility in water, emulsification of water into the oil, and pour point. In a confined area, natural surface active agents in the oil can spread into a monomolecular film holding up to Af in of even low specific gravity oil.
Oil spill specific gravities range from 0.75 to 1.03. The lower values represent highly refined products such as gas oline, kerosene, and diesel fuels. The upper values represent residual oils. Crude oils have specific gravities between 0.8 and 1.0; however, this increases rapidly when the light ends evaporate. Also, with a low sea state, crudes and oils containing asphaltines readily form water-in-oil emulsions, raising pollutant specific gravity from 0.85 to 0.95 in several days.
An oil spill's buoyant hydrostatic head is inversely proportional to the difference between the water and oil specific gravities. The lower specific gravity oils spread rapidly, but once captured by a containment boom their buoyancy resists entrainment into a current stream passing under the oil and boom. Also, specific gravity can limit removal of the oil from a contained pool. In this case, recovery is proportional to the recovery device frontal length and the gravity-inertial feedrate per unit length (Q).
Ho = Oil Depth, g = gravitational constant, A = water SG — oil SG
Thus, between the specific gravities of 0.75 and 1.0 the recovery feedrate will vary thirty-fold for a given thickness. Conversely almost a tenfold increase in thickness is required to have the same effect on recovery feedrate.
Spill viscosities range from 0.7 to over 20,000 centistokes (cst). Residual oils, weathered emulsions and high pour point crudes can even reach a semisolid state. The emulsions strongly deviate from Newtonian characteristics, frequently exhibiting very high viscosities at low shear rates and much lower viscosity at higher shear rates, such as those generated in transfer pumps. Viscosities for crudes weathered for up to a day and emulsified by moderate seas are between 300 and 1000 cst. There is no direct relationship between viscosity and specific gravity. However they tend to increase together (Figure 8.5.2).
Water-in-oil emulsions are unstable and difficult to form in highly refined oils. However, most crudes and all residual oils contain asphaltines, resins, cresols, phenols, organic acids, metallic salts, and other surface-active agents that concentrate at the interface between entrained water droplets and the oil. A crude spill can become a 40% water emulsion in a single day due to open sea action. In 5 days, this can increase to 80%. Increased shearing rates and action decreases water droplet size and increases emulsion stability. Pumping emulsions with free water may re-
suit in up to 98% water in the oil emulsions which are so formed.
Crudes frequently contain 30-70% gasoline and benzine, presenting a significant fire and explosion hazard. These conditions are rapidly mitigated by evaporation and mass transport when the wind is blowing, due to their large surface area. Residual oils and weathered emulsions present only a minimal fire hazard.
MECHANICAL CONTAINMENT Floating Containment Barriers (Booms)
Desirable characteristics for containment booms include low cost; compact storage; easy and rapid deployment; durability; easy cleanup; and performance compatible with the environment. Boom construction materials must be protected from extended immersion in water or oil. Relatively compatible plastic materials include polyethelene, polyurethane, polyvinylchloride, polypropylene, epoxies, polyesters, nylon, and neoprene.
Even in calm, still water, the boom's draft and freeboard must be adequate and balanced to account for the oil's buoyant head. Likewise, although there is a pressure balance across the boom at the bottom of the contained slick, the boom must be strong enough to hold the hydrostatic head differential above that point.
The flat plate drag, D, of a containment boom's immersed area, A, normal to a current of density, p, and velocity, v, is given by equation 8.5(2), where CD is the drag coefficient and D equals the drag tension loads that result. Test measurements indicate that CD can exceed 3 for a 2-ft draft boom filled with 0.9 SG oil in a 1-kt (knot) current. The boom's rolling moment about its lower edge is given by equation 8.5(3), where X is the distance to the center of pressure, and X/d < 0.5 without oil and < 0.7 when filled with 0.9 SG oil. This moment must be counteracted by ballast and/or roll flotation. A large number of commercially available booms have the requisite strength and stability to operate in up to a 2-kt current in calm water.
Was this article helpful?