A floating skimmer is a device used to remove oil and other contaminants from the surface of water. It works by using a floating head or disc that sits on top of the water. The head has an intake that draws in the polluted surface water. This water passes through an oil/water separator inside the skimmer which removes most of the oil and contaminants. The cleaned water is then pumped back out into the body of water while the separated oil and debris collects inside the skimmer head for later disposal.
How does the skimming head float and move?
The skimming head floats due to buoyancy provided by built-in floats or pontoons. The floats are sealed, watertight chambers filled with air or foam. They provide enough buoyancy to keep the head floating on the water’s surface. The skimmer is usually tethered by ropes or cables connected to a mounting point onshore or on a boat. This allows the skimming head to move around a contained area as needed to collect oil across the surface.
Many floating skimmers are designed with a weighted bottom that hangs down into the water below the floating head. This weighted skirt helps stabilize the head against wave action and prevents debris from washing underneath. The motion of waves and currents naturally push the head around to skim across slick areas. But most skimmers also have propulsion mechanisms to actively move and direct the head as needed. Small propellers, jets, or thrusters powered by onboard electric or hydraulic motors allow precise maneuvering of the skimmer head.
How does the skimmer head intake work?
The skimmer head has an intake area that draws in the oil-covered surface water. This is typically an opening or gap all the way around the floating disc. The area of the intake opening determines the flow rate – larger openings allow more water to enter the skimmer. Common intake flow rates range from 200 gallons per minute (GPM) for smaller skimmers up to 6000 GPM for heavy-duty systems.
The intake flow is generated by a built-in water pump which continuously pulls water into the head and through the separator. The pump is usually a centrifugal type impeller driven by electric or hydraulic motors. In some cases, the motion of the skimmer vessel itself provides enough force to push water into the head without needing an onboard pump.
Adjustable weirs or baffles around the intake opening help control water flow rates. Raising or lowering the weirs exposes more or less of the intake area to match the pump capacity against the thickness of the oil slick. This optimizes the amount of water and oil drawn into the separator.
How does the oil/water separation work?
After water enters through the intake, it passes through an internal oil/water separator within the floating head. There are several types of separation systems used:
- Gravity separation – Oil floats above denser water in a calm collection tank due to buoyancy. Allows bulk oil to separate and rise up to a collection point.
- Parallel plates or discs – Oily water passes horizontally between angled plates or discs. Oil sticks to the surface while clean water passes through.
- Oleophilic discs/belts – Rotating discs or belt coated with oil-attracting materials. Oil adheres as water spins off.
- Coalescing plates – Mesh plates that cause small oil droplets to join together into larger droplets for easier separation.
The specific method depends on the skimmer design and its intended oil recovery capacity. In general, the separator slows the intake flow velocity to allow oil to separate while water remains suspended. Floating oil collects at the top of the separator and is continuously pumped out via a pickup tube. The de-oiled water exits the bottom of the separator back into the body of water.
Gravity Separation
Gravity separation is a simple but effective technique used in many small floating skimmers. The intake water enters a calm collection tank inside the floating head. Here the flow velocity slows dramatically allowing oil droplets and globules to float up. Separated oil accumulates at the top of the tank where a pickup tube removes it for storage. Clean water exits the bottom of the tank back into the environment.
Internal baffles and weirs help prevent turbulence and channel separated oil to the pickup point. The tank may have a boot or drop-pipe that extends down from the oil outlet. This lets the skimmer pump out the thick oil layer without pulling up additional water.
Parallel Plate Separators
This type of separator uses a series of angled parallel plates installed inside the skimmer head. The plates are typically made of smooth metals, plastics, or fiberglass. As oily water flows horizontally between the plates at a controlled velocity, oil droplets impinge and adhere to the plate surface. Clean water passes through the gaps between plates with minimal turbulence.
The plates are often zig-zag shaped to provide more surface area for oil adhesion. After flowing past multiple plates, the water has much less oil content. The separated oil accumulates on the plates and drains or is wiped into a collection well. Parallel plate separators can efficiently process large intake flow rates with minimal space requirements.
Oleophilic Disc and Belt Separators
These separators use rotating discs or a continuous belt made of materials that attract oil. Common oleophilic (oil-loving) materials include polypropylene, stainless steel mesh, and patented composites. As the oily intake water contacts the rotating components, oil sticks to the surface while the water spins off. Scraper blades remove the clinging oil deposits which then drain into a collection area.
Oleophilic separation systems are highly efficient and can handle both light sheens and heavy crude oils. The discs or belt provide extensive surface area for oil removal in a compact unit. Self-cleaning and low maintenance requirements make these separators ideal for larger skimming vessels or responses.
Coalescing Plate Separators
Coalescing plate separators contain stacked layers of plates made from materials like perforated metal, plastic mesh, or fabrics. The intake water flows between the plates at controlled velocities that encourage small dispersed oil droplets to combine into larger drops. This process is called oil droplet coalescence. Larger oil drops rise faster and separate more completely due to buoyancy. Coalescing separators are often used as a pre-treatment before another primary separation stage.
Additional plate features like electrostatic charging, hydrophobic coatings, and demulsifiers help speed up the droplet coalescing. As oil droplets grow in size, the floating oil layer builds up and can be recovered through slots or drain holes in the plates. This type of separator works best for oils that readily form emulsions – like crude oils and lubrication oils.
How does the recovered oil collect and store?
The oil separated within the skimmer head continuously accumulates in a collection well or recovery line. A pickup pump then transfers the recovered oil into onboard storage tanks or directly to external storage vehicles. Smaller skimmers may have 50-100 gallon onboard storage tanks. Larger recovery vessels can hold thousands of gallons of recovered oil and debris.
Many systems have an intermediate collection tank to buffer surges in oil recovery rates before pumping to main storage. Recovery pumps are typically positive displacement types like diaphragm, rotary lobe, or progressive cavity pumps. Slow pumping rates prevent turbulence that could re-mix recovered oil and water.
Skimmer storage tanks have drain ports to periodically drain out any collected water. High level sensors turn off recovery pumps when the tanks are full. Large skimming vessels can also have oil/water separation systems onboard to further purify the recovered oil/water mixture from the skimmer head.
Where direct offloading of recovered oil is possible, some skimmers pipe oil directly to shoreside storage tanks, tanker trucks, or other recovery vessels rather than using onboard storage. This allows continuous high-volume oil collection.
What are the main skimmer components?
While skimmer configurations vary by design and capacity, most floating skimmers consist of these key components:
- Buoyant floating head or disc
- Oil/water separator mechanism
- Intake flow pump and adjustment weirs
- Recovered oil pickup and transfer pumps
- Onboard oil/debris storage tanks
- Hydraulic power pack (if hydraulically driven)
- Control and monitoring systems
Smaller skimmers may have just the floating head/disc along with manual recovery and storage options. Larger high-capacity systems incorporate extensive pumping, processing, and storage equipment onboard dedicated oil recovery vessels.
What types and sizes of floating skimmers are available?
Floating skimmers range greatly in size, capacity, features, and cost:
Small Portable Skimmers
- 1-6 feet wide floating head
- 50-300 GPM recovery rate
- Manual control and collection
- Used for small spills in marinas, rivers, etc.
Mid-Range Skimmers
- 6-15 foot wide head
- 200-800 GPM capacity
- Self-propelled and adjustable
- Onboard storage and power
- Good for facility contingency planning
Large Heavy-Duty Skimmers
- Up to 30 feet wide or larger
- 2000-6000+ GPM rates
- Hydraulic control and pumping
- Direct transfer to storage barges
- Used in major open water responses
In addition to size, skimmer heads are designed to handle different oil types and thicknesses:
- Conventional oils – Crude oils, fuels, lubricants. Use gravity separation.
- Viscous oils – Heavy or weathered crude oils. Heated surfaces prevent sticking.
- Emulsions – Crude and refined product emulsions. Coalescers and demulsifiers enhance separation.
How are floating skimmers deployed and operated?
Small portable skimmers can be deployed right at the leak source or towed from piers or small craft. Mid-range skimmers are usually operated from open powerboats, landing crafts, or barges. Large skimming vessels have the equipment onboard to continuously process and store recovered oil:
- Pumps, hydraulics, and generators to power the skimmer
- Cranes and hoists to deploy/retrieve the skimmer head
- Oil/water separators to purify recovered oil
- Heated storage tanks and transfer lines
- Accommodations for crew and spill personnel
Dedicated skimming vessels range from 90 to over 300 feet long. They use hydraulics, cables, or A-frames to lift and control the skimmer head. Larger vessels can launch and operate multiple skimmer heads at once for expanded coverage of a spill area.
Skimmers have open water maneuverability but perform best in calm protected areas. Wave height over 3-4 feet significantly reduces skimming effectiveness. Currents above 1 knot also impede many designs. Booms are often used to corral and concentrate slick areas for more controlled skimming.
What are the advantages of floating skimmers?
Floating skimmers offer these recovery advantages:
- Designed specifically to handle oil spills on water
- Can remove thin sheens up to thick crude oils
- Effective in calm harbors, protected shorelines, rivers
- Continuous operation capability
- High recovery rates for portability
- Simple and passive operation
- Minimal secondary waste generation
Their ability to actively move around a spill and recover oil across a wide area make skimmers a primary response tool. High-volume skimming vessels provide vital oil removal capacity for major spills offshore or in busy ports.
What are the disadvantages and limitations?
Floating skimmers also have these inherent limitations:
- Limited effectiveness in rough seas, strong currents
- Requires relatively calm operating conditions
- Large units need infrastructure/vessels for support
- Captures water and debris along with oil
- Incomplete oil removal in single pass
- Slow transit speeds to spill site
- Oil storage capacity is finite
While vital for oil recovery, skimmers cannot be relied on as the sole response tool. Booming, dispersants, sorbents, and manual cleanup supplement skimmer capabilities. Careful staging, support logistics, and waste handling are critical to achieve maximum skimming productivity during spill responses on the water.
What are some new technological advances in skimmers?
Ongoing research seeks to expand the operability and efficiency of skimmers. Some areas of development include:
- Improved oleophilic materials – New material coatings attract oil faster with less fouling.
- Dynamic positioning systems – GPS and vector thrusters to hold skimmer on slick in rougher conditions.
- Onboard oil/water separation – Better purification of recovered oil/water mixes.
- Demulsifiers and de-emulsifiers – Chemical additives break down crude oil emulsions.
- Expanded onboard storage – Higher capacity holds from larger skimmer heads.
- Lighter, flexible booms – Boom arrays to corral and concentrate oil slicks.
While small incremental gains continue, giant leaps in skimmer productivity rely on breakthroughs in materials science and adaptation from other industries. Costs, maintenance, and operating skill remain barriers to widespread use of more advanced but complex skimming systems.
Conclusion
Floating skimmers provide a mobile, flexible oil recovery capability that many spill scenarios rely upon. When staged and supported properly, high-capacity skimmers can remove thousands of gallons of spilled oil per hour. Careful matching of skimmer performance to conditions, oil types, and containment strategies optimizes their effectiveness during response operations.
While floating skimmers have limitations in rough water and with emulsified oils, they remain an essential response tool. Ongoing technology developments may expand the operating envelope and efficiency of skimmers. But most advances target affordability, durability, and ease-of-use – making skimmers more available to smaller facilities and improving productivity during major offshore spills.