Containment booms are used to control the spread of oil to reduce the possibility of polluting shorelines and other resources, as well as to concentrate oil in thicker surface layers, making recovery easier. In addition, booms may be used to divert and channel oil slicks along desired paths, making them easier to remove from the surface of the water. Although there is a great deal of variation in the design and construction of booms, all generally share the following four basic elements:
- An above-water "freeboard" to contain the oil and to help prevent waves from splashing oil over the top of the boom
- A flotation device
- A below-water "skirt" to contain the oil and help reduce the amount of oil lost under the boom
- A "longitudinal support," usually a chain or cable running along the bottom of the skirt, that strengthens the boom against wind and wave action; the support may also serve as a weight or ballast to add stability and help keep the boom upright
Booms can be divided into several basic types.
- Fence booms have a high freeboard and a flat flotation device, making them least effective in rough water, where wave and wind action can cause the boom to twist.
- Round or curtain booms have a more circular flotation device and a continuous skirt. They perform well in rough water, but are more difficult to clean and store than fence booms.
- Non-rigid or inflatable booms come in many shapes. They are easy to clean and store, and they perform well in rough seas. However, they tend to be expensive, more complicated to use, and puncture and deflate easily.
All boom types are greatly affected by the conditions on the water; the higher the waves swell, the less effective booms become.
Booms can be fixed to a structure, such as a pier or a buoy, or towed behind or alongside one or more vessels. When stationary or moored, the boom is anchored below the water surface. It is necessary for stationary booms to be monitored or tended due to changes produced by shifting tides, tidal currents, winds, or other factors that influence water depth, direction, and force of motion. Boom tending requires round-the-clock personnel to monitor and adjust the equipment. The forces exerted by currents, waves, and wind may significantly impair the ability of a boom to hold oil. Currents may wash oil beneath a boom's skirt. Wind and waves 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. Mechanical problems and improper mooring can also cause a boom to fail.
Most booms perform well in gentle seas with smooth, long waves; however, rough and choppy water is likely to contribute to boom failure. In some circumstances, lengthening a boom's skirt or freeboard can aid in containing the oil. However, because they have more resistance to natural forces such as wind, waves, and currents, these oversized booms are more prone to failure or leakage than are smaller ones. Generally, booms will not operate properly when waves are higher than one meter or currents are moving faster than one knot per hour.
When a spill occurs and no containment equipment is available, barriers can be improvised from whatever materials are at hand. Although they are most often used as temporary measures to hold or divert oil until more sophisticated equipment arrives, improvised booms can be an effective way to deal with oil spills, particularly in calm water such as streams, slow-moving rivers, or sheltered bays and inlets. Improvised booms are made from such common materials as wood, plastic pipe, inflated fire hoses, automobile tires, and empty oil drums. They can be as simple as a board placed across the surface of a slow-moving stream, or a berm built by bulldozers pushing a wall of sand out from the beach to divert oil from a sensitive section of shoreline.