Behaviour of oil at sea
From the dumping moment, the spill formed on the sea changes continuously, both in shape and in physicochemical properties. Over time, any oil spillage will dissipate into the marine environment. Unfortunately the level of environmental damage caused during natural dissipation can be significant, depending on the location of the spill and whether it is likely to reach land or other sensitive areas. The rate at which oil dissipates is greatly dependant on the type of oil, weather/sea conditions and whether the oil remains water borne or becomes land-bound. The processes involved in its evolution, collectively known as weathering, are spreading, evaporation, dispersion, emulsification, dissolution, oxidation, sedimentation and biodegradation (Jordan and Payne, 1980). To a certain degree these processes affect the spill contemporarily and even competitively. The time scale however, of their relative importance, varies from few hours to months. These processes are outlined below:|
Figure 1. Fate of oil spilled at sea showing the main weathering processes
As soon as oil is spilled into water, it begins to spread over the surface but it does not spread uniformly. Any shear in the surface current will cause stretching and even a weak wind will cause a thickening of the slick in downwind direction. Most spills quickly form a comet shape where a small black region is trailed by a much larger sheen which can be of varying color. Measurements show that most of the oil from a spill is in the black thick part, with only a small percentage of the spill oil in the sheen. A typical rule-of thumb used in the industry is to assume that the sheen represent only about 10 % of the spilled oil. How quickly oil spreads is dependant on a number of factors:
The thickness or "viscosity" of the oil.
Lighter refined "fuel oils" will spread a higher rate than heavy "crude oils".
Wind can significantly increase the spread of a slick. "Swirling" wind conditions are likely to break up a slick faster than wind from a single direction.
Rough seas will greatly enhance the rate of spread and break up the slick.
Lighter, more volatile components of the oil will evaporate. Refined "fuel oils" will evaporate significantly more than "crude oils". Once again the rate of evaporation is increased by strong winds, rough seas and higher air temperatures.
Waves, at least those that are breaking, are responsible for dispersion of oil into the water column. The entrainment rate of oil depends upon the properties of oil such as viscosity and surface tension. They are change as the slick weathers. Viscosity, in particular, increases by several orders of magnitude if the oil emulsifies, which is quite common for many crude oil. The entrainment rate depends also upon the fraction of the sea surface hit by breaking waves per unit time. In turn it is a function of wind speed. The situation somewhat alters if Langmuir cells effects take into consideration. In particular, Langmuir cells likely increase dispersion and move small oil droplets to greater depths than would otherwise be expected. Once dispersion occurs, natural biodegradation and sedimentation is more likely to occur.
Emulsification occurs when two liquids that cannot be naturally mixed combine. One liquid will become suspended in the other forming an emulsion. Emulsification can increase the volume of pollutant (at this stage it is often called "chocolate mousse") and also reduces the rate at which biodegradation will take place. Emulsions may separate back into oil/water if they become "beached" in warm weather conditions.
Oil does contain water-soluble compounds that can dissolve into the sea. It is however more likely that these compounds would have been removed due to evaporation.
Oils react to head and oxygen and break down into soluble compound or persistent forms called tars. The rate of oxidization is dependant on the type of oil and at best can be described as "slow". Heavier oils are more likely to form tars. Tar formation often occurs in the form of "tar balls" consisting of a highly weathered outer crust covering an almost liquid core. This greatly increases the persistence of the oil contained within.
Oil does not naturally sink. However, should water-borne sediments such as sand or organic material come into contact with oil particles; they may increase the weight of the oil sufficiently to cause sinking. This is more likely to occur if a slick is inshore and close to sandy beaches where there will be a high level of suspended sediment.
Sea water contains a variety of microbes that can degrade oil either partially or completely. Whether this occurs is dependant on the concentration of nutrients in the area and the type of oil involved. Heavier oils contain components that are extremely resistant to microbes and less likely to degrade.
Evolution of oil
The above processes combine to produce natural dissipation of oil into the marine environment. Spreading, Evaporation, Dispersion, Emulsification and Dissolution are most effective during early stages of a spill whilst Oxidization, Sedimentation and Biodegradation occur in the later stages. How long the process of natural dispersion takes is an almost unquantifiable figure as it is highly dependant on environmental conditions that are uncontrollable.
In terms of monitoring ship discharges with remote sensing, the most significant processes are those with dominating impact on the spill during the first few hours after dumping. Among them the most important is spreading, i.e. the rapid expansion of the spill from the point of dumping to all directions, in the form of a thin layer. This tendency which is mainly due to gravity and surface tension forces, dominates on the shaping process of the spill during its very early stages. The gravitational spreading force is proportional to the spill thickness, gradient, and to the density difference between the water and the dumped oil. All these reduce rapidly with time, thus the spill spreading due to the gravity effect tends quickly to relaxation, and so gives way to that due to surface tension effects. The later is independent of the spill thickness, and results from the difference between the air-water surface tension and the sum of the air-oil and oil-water surface tensions. It depends however on the volatile content of the spill (Fay, 1971), so that when it is removed through evaporation, the spreading due to surface tensional forces tends to termination. Evaporation also is a rapid process and is accelerated with the expansion of the spill, since the area of oil exposed to the air increases.
Therefore, a spill does not spread to infinity but up to a certain limited area, whose extent depends on the amount and the type of the spilled oil. The time a spill requires to reach its maximum breadth, depends on the spreading rate. Many oils tend to spread on sea surface at about the same rate, even though they possess different viscosities (McAuliffe, 1977). However, highly viscous oils, such as Bunker C, will not spread as rapidly as less viscous ones, especially in cold waters. Furthermore, the spreading is not uniform, since large lateral variations of thickness are frequently observed within a spill, especially of oils possessing higher viscosities.
The wind and the sea currents have a strong impact on the lateral variability of the spill thickness and on it transport. In many models, the total transport of oil particles is calculated as the sum of the wind drift, surface current and turbulent diffusion parameter. The wind-induced speed of the slick is usually estimated as 3% of wind speed. Observations of actual oil spills and controlled experiments indicate the wind drift can range from 1% to 6% of wind speed. Spread in data is likely due to subsurface oil droplets and shear in the near surface current. Langmuire circulation is an additional process which can result in wind drift variability.
For many years, observers have reported oil slicks breaking up and aligning into windrows or Langmuir cells (LC). This phenomenon has significant implications for oil cleanup and remote sensing of spills because the spilled material is not evenly distributed on the sea surface. At present, LC is not incorporated into most oil spill behaviour and trajectory models but research in this area has progressed and suggests that LC can be parameterized for inclusion into future modelling efforts.
(Review of behaviour of oil at sea was submitted by using information from http://www.oilslickharvester.com/spill.asp as well as from Pavlakis et al., 2002)