The Basics of Oil Spill Cleanup - Chapter 6

CHAPTER 6 Containment on Water Containment of an oil spill refers to the process of confining the oil, either to prevent it from spreading to a particular area, to divert it to another area where it can be recovered or treated, or to concentrate the oil so it can be recovered, burned, or otherwise treated. Containment booms are the basic and most frequently used piece of equipment for containing an oil spill on water. Booms are generally the first equipment mobi-lized at a spill and are often used throughout the operation. This chapter covers the types of booms, their construction, operating principles and uses, as well as how and why they fail. It also covers ancillary equipment used with booms, sorbent booms, and special-purpose and improvised booms. The topic of fire-resistant booms for use when burning oil on water is covered in Chapter 10, In-Situ Burning. TYPES OF BOOMS AND THEIR CONSTRUCTION A boom is a floating mechanical barrier designed to stop or divert the movement of oil on water. Booms resemble a vertical curtain with portions extending above and below the water line. Most commercial booms consist of four basic components: a means of flotation, a freeboard member (or section) to prevent oil from flowing over the top of the boom, a skirt to prevent oil from being swept underneath the boom, and one or more tension members to support the entire boom. Booms are constructed in sections, usually 15 or 30 m long, with connectors installed on each end so that sections of the boom can be attached to each other, towed, or anchored. A section of a typical boom is shown in Figure 13. Some typical commercial booms are illustrated in Figure 14. The flotation members or floats determine the buoyancy of the boom and keep it floating on the water surface. They are located along the centre line, outboard, on one side, or on outriggers. Booms either have solid floats or the boom itself is inflatable. Solid floats are usually made of a plastic foam such as expanded poly- ©2000 by CRC Press LLC Figure 13 Basic boom construction. urethane or polyethylene and are segmented or flexible so the boom can ride the surface of the waves. Inflatable booms are either self-inflating or are inflated using a powered air source. They require little storage space, but are generally less rugged than booms with fixed floats. The freeboard member is the portion of the boom above the water, which prevents oil from washing over the top of the boom. The term freeboard is also used to refer to the height from the water line to the top of the boom. The skirt is the portion of the boom below the floats or flotation that helps to contain the oil. It is usually made of the same types of fabric as the freeboard member and the covering of the floats. Typical materials include polyvinyl chloride (PVC), polyester, nylon, or aramid, sometimes coated with a spray-on protector or another covering such as PVC, polyester, polyurethane, nitrile, and polyether urethane to resist degradation from oil. Most booms are also fitted with one or more tension members that run along the bottom of the boom and reinforce it against the horizontal load imposed by waves and currents. Tension members are usually made of steel cables or chains but sometimes consist of nylon or polyester ropes. The boom fabric itself is not strong enough to withstand the powerful forces to which booms are subjected, except in protected waters. For example, the force on a 100-m-long section of boom could be as much as 10,000 kg, depending on sea conditions and the construction of the boom. Booms are sometimes constructed with ballast or weights designed to maintain the boom in an upright position. Lead weights have been used for this, but steel chain in the bottom of the boom often serves as both ballast and tension member. A few booms also use a chamber filled with water as ballast. Many booms nowadays are constructed without ballast, however, and their position in the water is maintained by balancing the forces on the top and bottom of the boom. Another construction feature common in larger booms is the addition of “stiffeners” or rigid strips, often ©2000 by CRC Press LLC Figure 14 Typical containment booms. consisting of plastic or steel bars, which are designed to support the boom and keep it in an upright position. The three basic types of booms are fence and curtain booms, which are common, and external tension member booms, which are relatively rare. Booms are also classified according to where they are used, i.e., offshore, inshore, harbour, and river booms, based on their size and ruggedness of construction. The fence boom is constructed with a freeboard member above the float. Although relatively inexpensive, these booms are not recommended for use in high winds or strong water currents. Curtain booms are constructed with a skirt below the floats and no freeboard member above the float. Curtain booms are most suitable for use in strong water currents. External tension member booms, which are constructed with a tension member outside the main structure, are used in strong currents and in water containing ice or debris. ©2000 by CRC Press LLC Photo 49 This is a small section of a typical general purpose boom. (Environment Canada) The characteristics of booms that are important in determining their operating ability are the buoyancy-to-weight ratio or reserve buoyancy, the heave response, and the roll response. The buoyancy-to-weight ratio or reserve buoyancy is deter-mined by the amount of flotation and the weight of the boom. This means that the float must provide enough buoyancy to balance the weight of the boom with the force exerted by currents and waves, thereby maintaining the boom’s stability. The greater a boom’s reserve buoyancy, the greater its ability to rise and fall with the waves and remain on the surface of the water. The heave response is the boom’s ability to conform to sharp waves. It is indicated by the reserve buoyancy and the flexibility of the boom. A boom with good heave response will move with the waves on the surface of the water and not be alternately submerged and thrust out of the water by the wave action. The roll response refers to the boom’s ability to remain upright in the water and not roll over. Uses of Booms Booms are used to enclose oil and prevent it from spreading, to protect harbours, bays, and biologically sensitive areas, to divert oil to areas where it can be recovered or treated, and to concentrate oil and maintain an even thickness so that skimmers can be used or other cleanup techniques, such as in situ burning, can be applied. Booms are used primarily to contain oil, although they are also used to deflect oil. When used for containment, booms are often arranged in a U, V, or J configu-ration. The U configuration is the most common and is achieved by towing the boom behind two vessels, anchoring the boom, or by combining these two techniques. The U shape is created by the current pushing against the centre of the boom. The critical ©2000 by CRC Press LLC Photo 50 Photo 51 Booms can retain oil in calm waters. (Al Allen) The current on this river is too fast to allow containment by a boom placed directly across the flow. Loss of containment in this case is by several failure modes. (Environment Canada) requirement is that the current in the apex of the U does not exceed 0.5 m/s or 1 knot, which is referred to as the critical velocity. ©2000 by CRC Press LLC ... - --nqh--