When used appropriately, dispersants can be an effective method of response to an oil spill. They are capable of rapidly removing large amounts of certain oil types from the sea surface by transferring it into the water column. Following dispersant application, wave energy will cause the oil slick to break up into small oil droplets that are rapidly diluted and subsequently biodegraded by micro-organisms occurring naturally in the marine environment. They can also delay the formation of persistent water-in-oil emulsions. In common with other response techniques, the decision to use dispersants must be given careful consideration and take into account oil characteristics, sea and weather conditions, as well as surrounding environmental sensitivities.
Dispersants are a group of chemicals designed to be sprayed onto oil slicks to accelerate the process of natural dispersion. Significant environmental and economic benefits can be achieved, particularly when other at-sea response techniques are limited by weather conditions or the availability of resources. In certain situations, dispersants may provide the only means of removing significant quantities of surface oil quickly, thereby minimising or preventing damage to important sensitive resources. Their use is intended to minimise the damage caused by floating oil, for example to birds or before the oil may hit a sensitive shorelines. However, in common with all spill response options, the use of dispersants has limitations and its use should be carefully planned and controlled. Dispersant use will also depend upon national regulations governing the use of these products.
How chemical dispersion works
Natural dispersion of an oil slick occurs when waves and other turbulence at the sea surface cause all or part of the slick to break up into droplets and enter into the water column. The addition of dispersants is intended to accelerate this natural process.
Dispersants have two main components: a surfactant and a solvent. Surfactants molecules are made up of two parts: an oleophilic part (with an attraction to oil) and a hydrophilic part (with an attraction to water). When dispersants are sprayed onto an oil slick, the solvent will transport and distribute the surfactants through the oil slick to the oil/water interface where they arrange themselves so that the oleophilic part of the molecule is in the oil and the hydrophilic part is in the water. This creates a reduction in the surface tension at the oil/water interface and small oil droplets will break away from the oil slick with the help of wave energy. These droplets will be of varying sizes and although the larger ones may rise back to the surface some will remain in suspension and will drift apart and become degraded by naturally occurring bacteria. If dispersion is successful, a characteristic brown plume will spread slowly down from the water surface a few minutes after treatment.
Dispersants have little effect on very viscous, floating oils, as they tend to run off the oil into the water before the solvent can penetrate. As a general rule, dispersants are capable of dispersing most liquid oils and emulsions with viscosities of less than 2000 centistokes, equivalent to a medium fuel oil at 10-20ºC. They are unsuitable for dealing with viscous emulsions (mousse) or oils which have a pour point near to or above that of the ambient temperature. Even those oils which can be dispersed initially become resistant after a period of time as the viscosity increases as a result of evaporation and emulsification. For a particular oil, the time available before dispersant stops being effective depends upon such factors as sea state and temperature but is unlikely to be longer than a day or two. Dispersants can, however, be more effective with viscous oils on shorelines because the contact time may be prolonged allowing better penetration of the dispersant into the oil.
Types of dispersant
There are three main types of dispersants:
- Type 1 dispersants are based on hydrocarbon solvents with between 15% to 25% surfactant. They are sprayed neat onto the oil as pre-dilution with sea water renders them ineffective. Typical dose rates are between 1:1 and 1:3 (dispersant:oil).
- Type 2 dispersants are dilutable concentrate dispersants which are alcohol or glycol (i.e. oxygenated) solvent based with a higher surfactant concentration. Dilution is normally 1:10 with sea water.
- Type 3 dispersants are also concentrate dispersants with a similar formulation to type 2 products. However, they are designed to be used neat and typical dose rates are between 1:5 and 1:30 (neat dispersant:oil).
Type 1 and 2 dispersants require thorough mixing with the oil after application to produce satisfactory dispersion. With type 3 products, the natural movement of the sea is usually sufficient to achieve this. The lower application rates required with concentrates mean that types 2 and 3 have largely superseded type 1 dispersants for application at sea.
Methods of application at sea
Dispersants can be applied to open water by a variety of methods. In general workboats are more suitable for treating minor spills in harbours or confined waters. Large multi-engine planes are best equipped for handling large off-shore spills. Small, single-engine aircrafts and helicopters are suitable for treating smaller spills and near shore areas. Regardless of the method used, the droplet size of the dispersant is important as it needs to be sufficiently large to overcome the effects of wind and evaporative loss but not so large that it will result in the droplets being able to pierce through the oil slick. A uniform spray pattern of larger droplets, "rain drops", is required rather than a fog or a mist. Ultimately, whichever method of application is used, the key to a successful response using chemical dispersants is the ability to target the thickest part of the oil slick within a short time and before weathering or sea state render the oil undispersable.
Dispersants are usually applied from boats equipped with spray arms. In a typical spray arm system, pumps are used to pump dispersants from a storage tank through a set of nozzles calibrated to produce a uniform spray pattern of droplets.
Spray units can be portable or permanently installed on a vessel and systems are available which deliver the dispersant either undiluted or diluted with sea water. Spray arms are usually mounted as far forward on the vessel as possible to avoid the effect of the bow wave which can push the oil beyond the spray swath. Mounting the spray arms on the bow allows the vessel to travel faster and, because freeboard area is often greater at the bow also allows for longer spray arms. This combination allows optimisation of the amount of oil which can be treated (increasing the encounter rate) with a limited dispersant payload. If spray arms are not available, fire hoses or monitors are sometimes used to apply diluted concentrate dispersants. However, optimum dilution of the dispersant is difficult to achieve because of the very high flow rates and wastage of dispersant is a common problem. The high-powered jet of water also makes it difficult to apply the dispersant as a uniform spray of droplets and it frequently pierces through the oil making it ineffective. Thus fire monitors are unlikely to be an effective application tool unless specially modified for the purpose.
Vessels offer certain advantages for dispersant spraying because they are usually readily available, easy to load and deploy, have cost advantages over aircraft and can apply dispersant fairly accurately to specific areas of a slick. Nevertheless, they also have serious limitations, particularly for larger spills, because of the low treatment rate which they offer and the added difficulty of locating the heaviest concentrations of oil from the bridge of a vessel. Furthermore, when slicks become fragmented or form narrow windrows, it is inevitable that some dispersant will be sprayed onto clear sea. These problems can be partially overcome by controlling the operation from spotter aircraft.
The spraying of dispersant from an aircraft has the significant advantages of rapid response, good visibility, high treatment rates and optimum dispersant use. In addition, aircraft allow treatment of spills at greater distances from the shore than with vessels.
Two categories of aircraft are used: those designed for agricultural or pest control operations which require minor modification for dispersant application, and those which have been specifically adapted for the application of dispersant. Several types of helicopter have also been adapted to spray dispersants although most are able to carry an under slung bucket spray systems without the need for modifications. The ideal aircraft will be determined primarily by the size and location of the spill, although in reality local availability will be the crucial factor. The endurance, fuel consumption, turn around time, payload and the ability to operate from short or improvised landing strips are all important. In addition, the aircraft should be capable of operating at low altitude and relatively low speeds (50-150 knots) and be highly manoeuvrable.
Only type 3 dispersants are suitable for aerial spraying, since they require no mixing beyond that provided by the natural movement of the sea. The relatively low dose rate required also makes the best use of available payload.
Dispersants are sometimes used to remove oil from hard surfaces such as rocks, sea walls and other manmade structures, particularly during the final stages of clean-up. However, it is important to remove the bulk of the stranded oil by other means first. Shores subjected to strong wave action are often cleaned naturally and they should not be sprayed unless the oil has to be removed immediately.
Dispersants may be applied to the surface and scrubbed into the oil before flushing with sea water. The dispersed oil cannot be collected and for this reason dispersant use on the shoreline is restricted to areas of low environmental concern. Shoreline cleaners may also be used but it is important to note that their mechanism of action is different from that of dispersants. Degreasers are often carried on board ships to deal with small spillages of oil on deck but most are more toxic than dispersant and should not be used as a dispersant at sea or as a shoreline cleaner. Today it is recommended to use citrus-based chemical cleaners as these usually have a lower environmental impact than traditional chemical cleaners.
Monitoring dispersant effectiveness
It is essential that the effectiveness of chemical dispersion should be monitored continually and the response terminated as soon as the dispersant is no longer working. Successful dispersion will usually produce a coffee-coloured plume spreading under the water surface. However, visual observation of effectiveness may be impaired in poor weather conditions, in waters with high sediment content, when dispersing pale-coloured oils or in poor light. It is inappropriate to spray at night. Experience has shown that for the application of dispersants to be worthwhile, the oil will need to disperse sufficiently rapidly to effect a change in appearance of the slick and a subsequent reduction in oiled area, which should be visible from the air shortly after spraying. Conversely, if there is no change in oil appearance or coverage, and the dispersant runs off the oil to create a milky white plume in the water, the dispersant is not working. Equally, if the oil has become fragmented and widely scattered, it is unlikely that sufficient oil will be removed from the water surface by the dispersant to achieve a significant reduction in pollution damage.
Ultra-violet fluorimetry (UVF) is sometimes used to provide ‘real-time’ data on the concentration of dispersed oil in the water column during the application of dispersants. Typically, the variation in the concentration of fluorescent components is measured at least 1 metre under the slick using a fluorimeter that is towed behind a sampling boat. In open water, dispersion is demonstrated by a significant increase in the concentration of oil detected by the sensor compared with that measured prior to dispersant application. However, when used operationally, UVF does not provide a quantitative measurement of the amount of oil that is actually being removed from the sea surface and it should be used in combination with visual observations to decide whether a worthwhile response can be achieved.
The use of dispersants has in the past tended to provoke controversy since their application can be seen as a deliberate introduction into the sea of an additional pollutant into the water. Many of the first dispersants used in the 70s and 80s did show high toxicity to marine organisms. However, today there is a wealth of laboratory data indicating that modern dispersants and oil/dispersant mixtures exhibit relatively low toxicity to marine organisms.
The rapid dilution of the dispersed oil, the proximity to sensitive areas as well as the direction of currents and the mixing depths of surface waters are all factors which should be considered when deciding upon dispersant use. In the open sea, dispersed oil concentrations after spraying are unlikely to remain high for more than a few hours and significant biological effects are therefore improbable. In shallow waters close to the shore, where water exchange is poor, higher concentrations may persist for long periods and may give rise to adverse effects. However, the controlled application of dispersants may, on occasions, be beneficial in that it may reduce damage to adjacent ecologically sensitive shorelines by oiling.
The decision on whether or not to use dispersants rather than other response options will need to take into account the cost-effectiveness and conflicting priorities for protecting different resources from pollution damage. On occasions the benefit gained by using dispersants to protect coastal amenities, sea birds and intertidal marine life may far outweigh disadvantages such as the potential for temporary tainting of fish stocks. Certain resources such as water intakes, mariculture facilities or fish spawning areas are difficult to protect from dispersed oil and spraying may be decided against when near to these resources. Detailed contingency planning will aid in this decision process.