An oil skimmer is a device designed to physically recover spilled oil floating on the surface of water or other liquids. These systems are a primary tool in oil spill response, serving to contain and remove the pollutant before it spreads or causes further environmental damage. The purpose of a skimmer is to separate the oil from the water for collection, allowing the recovered product to be pumped to storage for later processing or disposal. Skimmers are deployed in various environments, from open ocean spills to industrial wastewater treatment facilities. They are not intended as water purification devices but rather as a means to remove the bulk of the contaminant from the surface layer.
The Physics of Oil and Water Separation
The ability of a skimmer to separate oil and water relies on fundamental differences in the physical properties of the two liquids. The most influential factor is the difference in density. Because most hydrocarbon oils are less dense than water, they naturally float, forming a distinct layer on the surface that can be targeted for removal. This natural process of gravity separation is why skimming is a feasible cleanup method.
Skimming technology also exploits the difference in surface tension between the two substances. Oil molecules have an affinity for certain solid surfaces, a property known as oleophilicity, or “oil-attracting.”
Water has a high surface tension and is generally repelled by oleophilic materials. Skimmers are engineered to leverage this oil-attracting, water-repelling characteristic to achieve high efficiency in recovery. The goal is to capture the oil while minimizing the amount of water collected, which reduces the volume of liquid that must be stored and processed later.
Categories of Skimming Equipment
Skimming equipment is broadly divided into three categories, each utilizing a different mechanical action to achieve oil recovery.
Weir Skimmers
The first category is the weir skimmer, which operates on the principle of overflow, similar to a dam or adjustable spillway. These devices feature a lip or weir set precisely at or slightly below the oil-water interface. Oil floating on the surface flows over this adjustable lip and into a central collection sump, driven by gravity. Weir skimmers are high-capacity systems suitable for thicker oil slicks. However, this method is less selective and tends to recover a higher percentage of water along with the oil, particularly in rougher sea conditions.
Oleophilic Skimmers
The second category is the oleophilic skimmer, which relies on adhesion to selectively remove oil from the surface. These skimmers incorporate rotating elements made of materials that oil sticks to, such as:
- Discs
- Drums
- Belts
- Continuous mop chains
As the element rotates through the oil slick, the oil adheres to the surface and is then scraped or squeezed off into a recovery tank. Oleophilic skimmers are highly selective, often recovering oil with only a small percentage of entrained water. Different designs are optimized for various oil viscosities; for instance, brush skimmers trap thicker oils, while disc skimmers are effective for medium-viscosity materials. The rope mop skimmer uses a continuous loop of sorbent material, proving effective even when debris or ice is present.
Suction Skimmers
The final category is the suction skimmer, which operates much like a large, floating vacuum cleaner. This device uses a wide floating head to draw the surface layer of oil and water into a pump and then transfers the mixture to storage. While efficient in removing oil layers, suction skimmers are highly vulnerable to clogging from debris and trash. They are most effective when operating in calm, smooth water where the oil has been concentrated against an artificial barrier.
Operational Considerations for Deployment
The successful deployment of any oil skimmer is heavily influenced by external factors. Weather and sea state are significant operational challenges, as choppy or rough water dramatically reduces efficiency. Waves cause the skimmer to recover a greater volume of water instead of oil, diluting the collected material and slowing the overall cleanup effort.
The oil’s viscosity also dictates the appropriate skimming technology. Fresh, light crude behaves differently from weathered, heavy oil. As oil remains on the water, it undergoes weathering, which increases its viscosity and can lead to the formation of a dense, water-in-oil emulsion often termed “mousse.” This change in physical properties may necessitate switching from a high-volume weir skimmer to a more robust oleophilic system suited for viscous material.
Skimmers rarely work in isolation and require ancillary equipment, primarily containment booms, to function optimally. Booms are deployed to corral and concentrate the oil slick into a thicker layer, which significantly improves the skimmer’s recovery rate and oil-to-water ratio. The logistics of handling the recovered mixture must be considered, as the final product is not pure oil but a combination that requires storage capacity and eventual separation to maximize the salvage value of the recovered oil.