Oil spills: the cleanup operation

We've all seen the appalling photographs of the effects of oil spills on the sea and coastline.
But once the cameras have gone, how does the clean-up process work?


Oil in the sea

When oil enters the sea, different natural processes start acting on it. These will help to reduce the severity of the spill and accelerate its decomposition.

Weathering is a series of chemical and physical changes that cause spilled oil to break down and become heavier than water. Weathering is a progressive series of changes in oil characteristics caused by physical, chemical and biological processes.

Spreading occurs during the early stages of most spills, because oil is lighter than water, it floats and spreads out to a thin film. Spreading depends on factors such as viscosity, weather conditions etc. and it will decrease steadily as time passes.

Sedimentation takes place because the force of gravity will cause some of the oil to travel through the water and settle on the bottom. Very few oils are dense enough to sink, sinking happens by the adhesion of particles of sediment or organic matter to the oil.

Dispersion of the oil happens because of the mechanical action of the sea. Waves and turbulence act on the slick to produce different size droplets that remain in suspension mixed into the water column, which can enhance other processes such as biodegradation.

Evaporation occurs when the lighter hydrocarbons in the oil mixture become vapors and leave the surface of the water and transfer into the atmosphere. This process leaves behind the heavier, thicker, more viscous residue, which may have serious physical and chemical impacts on the environment, it may undergo further weathering or may sink to the ocean floor. Evaporation is a very important process since it reduces the toxic effect of the oil spill on the marine environment.

Oxidation occurs when the water and oxygen combine with the oil to produce water-soluble compounds. This process affects oil slicks mostly around their edges. Thick slicks may only partially oxidize, forming tar balls. These dense, sticky, black spheres can collect in the sediments or wash up on shorelines long after a spill.

Biodegradation occurs when microorganisms such as bacteria feed on oil. The main factors affecting the rate of biodegradation are the temperature and availability of oxygen and nutrients. To sustain biodegradation, nutrients such as nitrogen and phosphorus are sometimes added to the water to encourage the microorganisms to grow and reproduce. Many species of marine bacteria, fungi and yeasts feed on the compounds that make up oil. Hydrocarbons consumed by these microorganisms can be partially metabolized or completely metabolized to carbon dioxide and water.

Emulsification is the tendency of some oils to absorb water to form an emulsion. Emulsions are formed by wave action and can increase the volume of the pollutant by a factor of between three and four. The emulsions are very viscous and stable and create the phenomenon called “chocolate mousse” that make the cleanup processes much harder. Oil and water emulsions cause oil to sink and disappear from the surface, which give the false impression that it is gone and the threat to the environment has ended.

Contingency Plans

Contingency plans are necessary to cope with an oil spill by putting into place a well organised plan, that has been tested, so that the risk from a major pollution incident is minimised.

The first step in developing the contingency plan is to gather as much data about the area as possible. Contingency plans normally include information about responsibility, scope, assessment of spill risk, fate of oil, resources at risk, priorities for protection and a clean-up strategy.

Important information such as sensitive physical and biological resources, important habitat areas required by particular species for spawning, feeding or migration, tides, currents and local climatic conditions, shoreline characteristics, proximity to roads, airports, trained response personnel, oil spill clean-up equipment, etc. is normally compiled on sensitivity maps.


Response Techniques

Many different forms of response techniques are available for containing oil spills and therefore protecting the marine environment. To be as effective as possible its important to use the equipment best suited to the type of oil and the conditions at the spill site.
Containment and recovery is the best way of preventing oil spreading. It includes equipment such as booms, skimmers, pumps and sorbents.


Booms

Booms are used for oil collection, deflection, containment and protection.
Collection booms are normally towed by boats in a particular configuration (with a U or V shape) to an area where oil is floating and then one of the boats starts collecting it and removing it, this is normally done with a skimmer.
Deflection booms are used to divert the oil to certain point where it can be recovered by skimmers or pumps.

Booms are also used for oil containment to prevent it from spreading across the water surface. This is especially important when the oil has collected naturally in small bays, coves etc. When used in this way booms must be fixed with retaining anchors.
Booms are also used to protect marsh areas, mudflats, bird sanctuaries and amenity areas.

Although there can be variations in the design of booms, they generally share the following four basic elements:
1. A freeboard to contain the oil and to help prevent waves from splashing oil over the top of the boom.
2. A flotation system.
3. A skirt to prevent the oil from passing under the boom.
4. A tension line, usually a chain or cable running along the bottom of the skirt, that strengthens the boom against wind and wave action.

While most booms perform well in gentle seas with smooth, long waves, they can present certain limitations normally related to the sea conditions; for example if the current speed perpendicular to the boom is greater than 0.7 knots oil will be lost under the boom. Waves also have an important role in the performance of a boom since they can force oil over the top of the boom's freeboard or even flatten the boom into the water, causing it to release the contained oil. Generally, booms will not operate properly when waves are higher than one metre.


Skimmers

A skimmer is a device for recovering spilled oil from the surface of the water. Their efficiency will depend on sea conditions, oil thickness and viscosity, presence of debris etc. The main application for skimmers is in shelter waters. They can be used in open waters but if the waves are higher than two meters the efficiency is very low. They are normally used with containment booms to maximise the recovery efficiency. Pumps are used with skimmers. They lose efficiency as the oil viscosity increases.

There are different types of skimmers depending on their operating principles:
Weir skimmers use a dam positioned at the oil/water interface. Oil floating on top of the water will spill over the dam and be trapped. The trapped oil and water mixture can then be pumped out through a pipe or hose to a storage tank. Floating debris can clog these skimmers.
Oleophilic skimmers use 'oil loving' materials in belts, disks, or mops to recover the oil from the water surface. The oil is then squeezed out or scraped off into a recovery tank. They are good because they are easy to use and will skim all kinds of oil, even oil emulsions. The disadvantages are they only work well in calm conditions and recover a lot of water.
Vacuum skimmers use vacuum pumps to suck oil from the water surface into a containment tank. Although they are generally very efficient, they easily become clogged by debris.
Mechanical skimmers use metal tooth discs, grab buckets and drum separators, the collected oil is then pumped into a tank, and they are ideal for retrieving heavy oils.


Sorbents

Sorbents are materials that recover oil by absorption (oil penetrates the pores of the material) or adsorption (the oil is attracted to the sorbent's surface and adheres to it). Sorbents can be natural materials such as ash, straw, peat (these normally a absorb three to six times their weight in oil and are non toxic and biodegradable) or synthetic materials like polyethylene, polypropylene (they have a high recovery ratio but they are not biodegradable).


Chemical Dispersants


Dispersing agents are chemicals that consist of two components: a blend of surfactants and a solvent that carries the surfactant.
The job of the dispersant is to emulsify the oil into the water column to form oil droplets small enough for them to remain below the surface and not reform as a slick. In the water column they are subjected to natural processes, such as waves and currents, that aid to break them down further. This helps to clear oil from the water's surface, making it less likely that the oil slick will reach the shoreline.

Unfortunately dispersants will not work effectively in all circumstances. If the oil is too viscous (thick, like syrup) the dispersant solvent will not be able to penetrate it and the oil will not break up into droplets. Dispersants work more effectively applied on fresh oils since weathering may increase viscosity. They can be sprayed from ships, aircrafts, helicopters or the beach.

Environmental factors, including temperature, salinity and conditions at sea also influence the effectiveness of dispersants. It is important not to rely on dispersants to combat oil spills since they can produce long term environmental effects. At the present there is a lot of research in this field to improve the application and to reduce the toxicity of dispersants.


Biological agents

Biological agents are organisms that increase the rate at which natural biodegradation occurs. Biodegradation is the breaking down of the oil by microorganisms such as bacteria, fungi, and yeast into simpler products. It does occur naturally but its very slow process.

Scare tactics are used to protect birds and animals by keeping them away from oil spill areas. Devices such as propane scare-cans, floating dummies, and balloons are often used, particularly to keep away birds.


Burning

Setting fire to the oil on the surface of the water is a fast and effective way of removing it from the surface of the sea. This method can get rid of large quantities of surface oil very quickly, limiting the damage to wildlife and coastlines, but it creates huge plumes of smoke and emits large quanitites of CO2 into the atmoshpere.

Find out more about conserving the marine environment:

Natural disasters
Oil spills
The cleanup operation
A fishy business...

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© NOCS
February 2007