Dealing with oil spills on water and at land-water boundaries presents a different set of challenges compared to cleaning up oil spills on dry ground. Whereas oil pollution on dry ground during the dry season is usually limited to the local area of the spill, the high mobility of water makes it possible for water-borne oil pollution to rapidly affect wider and more distant areas. Also, depending largely upon the density of the oil and on the water temperature, a water-borne oil pollution plume can  remain either above or below the surface of the water.

On dry ground with a porous sandy or loamy soil, light oils tend to penetrate and sink down into the soil, while on water, light oils generally float above the water surface. Conversely, high-density/high-viscosity oils on dry ground tend to remain aboveground, but tend to sink after entering a body of water.

Mechanical removal, dispersal of the oil in the water column, and incineration of floating oil are the three most common ways of dealing with spilled oil on water. At the shoreline, mechanical removal and chemical dispersion are the preferred cleanup methods. Floating spills on rivers are usually recovered mechanically using booms, skimmers, pumps, filtering weirs and dams, etc. Floating and submerged spills on seas and lakes are managed using practically identical techniques; namely, mechanical removal, or chemical dispersal, or in-situ incineration.

These three standard oil-spill removal options that are typically applied today are essentially the same methods used 20 or so years ago, with only relatively minor improvements in recovery/removal capabilities. Labor-and-equipment-intensive mechanical recovery at sea, then as now, can reclaim only about 20 percent of the total volume of spilled oil. But mechanical removal at sea using booms, skimmers, and oil-transfer pumps is most often easier and more efficient than mechanically removing oil that has reached the shoreline. Shoreline recovery becomes very difficult as the oil can “stick” to rocks, sand, and vegetation. Strong wave action and the presence of loose solid materials (sand, rocks, seaweed and other coastal vegetation) can make skimmers and pumps practically useless at the shoreline. The use of sorbent materials and mechanical dispersal with pressurized seawater or freshwater are more effective when it comes to oil that has reached the shore; but it is generally the case that one ton of oil that reaches land results in 10 tons of bulky waste materials (mostly as oil-soaked sorbents) that need to be carted away and processed for recycling, incineration, or permanent disposal in a landfill.
 
Mechanical removal is the most environmentally friendly method for dealing with water-borne oil pollution; but, as already mentioned, it has a rather low recovery/removal rate. The mechanical method utilizes various devices such as containment booms, grabs, skimmers, sweeping arms, sorbents, and temporary storage vessels, containers, or “bladders.” Booms are used to gather floating oil and then confine it to a small area where it can be skimmed and/or pumped to temporary storage. Booms filled with solid floats (including those filled with sorbent materials) and air-and-water-filled booms are used mostly on shoreline areas, while air-filled booms are typically deployed in the open sea. The oil gathered by the boom is concentrated at the apex of the V, U, or J formation and is mechanically removed by skimmers or floating pumps (or pump suction heads) that are attached to this end of the boom. Fire-resistant, air-inflated booms are used simply to corral buoyant oil films prior to in-situ incineration.

Mechanical skimmers recover spilled oil that is floating above, at, or very near the surface of the water. There are four basic types of skimmers. Oil-transfer pumps are used for suction-type skimmers that work like household vacuum cleaners. These devices often include floating suction heads that look and function in much the same way as household vacuum-cleaner intake nozzles. These vacuum/suction devices work best on spills in calm or still waters such as on freshwater lakes and swamps or salt marshes. Oleophilic skimmers come in the form of belts, brushes, discs, drums, ropes, etc. The oil adheres to the surfaces of these devices. The oil is then scraped or squeezed off the collecting device and then pumped to a container. Oleophilic skimmers work best on fresh oil spills that have not yet been degraded by heat, exposure to air and water, or bacterial or chemical action. Oleophilic skimmers work even in relatively rough sea conditions. Weir-type skimmers plow through the water to separate the floating oil layer from the water layer. There are a number of types including the advancing weir, the auger weir, the screw weir, the self-leveling weir, and the vortex weir. Finally, sweeping-arm skimmers combine the containment and recovery functions in just one device, much like a boom-and-skimmer setup. This type of skimmer is attached either at the front or at the sides of a vessel. The arm/s collect the oil and direct it to a chamber that has one or more pumps. The pumps transfer the oil from the collecting chamber into an on-board storage tank. This type of skimmer can recover oil in rough seas wherein ordinary boom-and-skimmer combinations no longer work; but because the combined length of the sweeping arms is much shorter than the typical lengths achievable with booms, this type of skimmer is more suitable for narrow, elongated oil slick ribbons or windrows.

Mechanical grabs include clamshell dredgers, buckets, nets, scoops, and any other machine-mounted or human-wielded device that can scoop up or gather very viscous semi-solid masses of extra heavy grade oil (such as clarified slurry oil, bitumen, tar oil, etc.) or liquid oil that has been treated with solidifiers (gelling agents). Solidifiers are polymer compounds that incorporate the oil within their structures, turning the liquid oil into a rubbery solid or semi-solid chunks, mats, or blobs. Solidifiers are also used on light oils spilled on land. The amount of solidifiers needed to treat a spill is about three times the volume of spilled oil. Developing a non-toxic, biodegradable solidifying compound that is effective even at low concentrations is a promising spill remediation avenue to look into.   

The most recently developed mechanical oil-recovery equipment that is already at the production stage utilizes a vessel-mounted “moon pool” skimmer system in conjunction with “oil-trapping towers.”

Adsorbent materials that are used on onshore spills can also be applied on water-borne spills. Sorbents are best used on shorelines for soaking up oil that has reached land because bulky sorbent materials can be difficult to mechanically recover at sea, and they can take up substantial storage space on a vessel. These materials can also interfere with skimmers and clog up oil-transfer pumps. Clay-based polymer aerogels are a new class of very porous, lightweight, and mechanically stable materials that both adsorb and absorb oil molecules. Aerogels may soon replace the non-biodegradable oil-adsorbent materials that are manufactured from synthetic polymers.  

After mechanical removal, dispersant chemicals and in-situ incineration are the two other commonly used response options for water-borne oil spills. The choice of either method depends upon the type and condition of the oil and on the sea/water and weather conditions. Dispersant chemicals can be applied on both surface and submerged plumes, whereas in-situ burning is suitable only on floating oil spills that can still be ignited.

Dispersants reduce the adhesion between oil molecules and cause the oil to emulsify (turn into smaller droplets which are surrounded by water molecules). The oil droplets can then sink into the water column and be dispersed by water currents. Dispersants are more effective on lighter and freshly spilled oils because the low viscosity and density of the light oil allow the dispersant and water to thoroughly mix with the oil. More viscous/denser oils (2000 centistokes or more), on the other hand, are more difficult to penetrate with dispersants. Dispersants can be sprayed from aircraft and boats at the surface and can be delivered underwater by ROVs. The use of dispersants require the permission of regulatory agencies as these chemicals can be toxic to marine life and their use causes a sudden increase in the amounts of dissolved oil in the water column, which can also be potentially harmful to local marine life.

In-situ burning can remove 90 percent or more of floating spilled oil and generally does not harm marine life beneath the surface. Burning produces a lot of smoke pollution, which makes it a less-desirable method for dealing with nearshore oil spills that happen to occur in close proximity to coastal communities. Also, this method works best for freshly spilled, lighter-grade oils that still contain sufficient ignitable volatile fractions. Thicker oils are harder to burn and/or tend to sink.



Sources:
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2.http://tinyurl.com/cbpkj9h
3.http://www.megator.com/drum_oil_skimmer.htm
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15.http://www.epa.gov/oem/docs/oil/fss/fss06/usher.pdf