Limitations of Containment & Recovery
The use of booms to contain and concentrate floating oil prior to its recovery by specialised skimmers is often seen as the ideal solution to a spill since, if effective, it would remove the oil from the marine environment. Unfortunately, this approach suffers from a number of fundamental problems, not least of which is the fact that it is in direct opposition to the natural tendency of the oil to spread, fragment and disperse under the influence of wind, waves and currents. In rough seas, a large spill of a low viscosity oil such as a light or medium crude oil can be scattered over many square kilometres within just a few hours. Oil recovery systems typically have a swath width of only a few metres and move at slow speeds whilst recovering oil. Thus, even if they can be operational within a few hours, it will not be feasible for them to encounter more than a fraction of a widely spread slick. This is the main reason why containment and recovery at sea rarely results in the removal of more than a relatively small proportion of a large spill, at best only 10 - 15% and often considerably less.
A common difficulty when deploying booms and skimmers to recover oil is controlling the movements and activities of vessels and directing them to the thickest areas of oil. This can be overcome by using aircraft equipped with air to sea communications. Overall, containment and recovery operations at sea require extensive logistical support, which should not be underestimated.
The limitations that poor weather and rough seas impose on operations at sea are seldom fully appreciated. Handling wet, oily, slippery equipment on vessels which are pitching and rolling is difficult and can place personnel at risk. Winds, currents and wave action seriously reduce the ability of boom to contain and of skimmers to recover oil. In practice, the most efficient recovery of oil is achieved only under calm conditions. When containment and recovery is attempted it is important to select equipment that is suitable for the type of oil and the prevailing weather and sea conditions. Efforts should target the heaviest oil concentrations and areas where collection will reduce the likelihood of oil reaching sensitive resources and shorelines. As the oil weathers and increases in viscosity, clean up techniques and equipment will need to be re-evaluated and modified. For example, the types of pumps and skimmers may need to be changed.
The first stage of an effective response is to deploy boom to limit further spreading and concentrate the oil for recovery. Booms vary considerably in their design, but all normally incorporate the following features:
- freeboard to prevent or reduce splashover;
- a sub-surface skirt to prevent or reduce escape of oil under the boom;
- flotation by air or some buoyant material;
- longitudinal tension member (chain or wire) to provide strength to withstand the effects of winds, waves and currents. This is often used to provide ballast to keep the boom upright in the water.
There are many designs ranging from small, lightweight models designed for manual deployment in harbours, to large, robust units which usually need cranes and sizeable vessels to handle them, which are designed for use in the open sea.
The most important characteristic of a boom is its oil containment or deflection capability, determined by its behaviour in relation to water movement. It should be flexible to conform to wave motion yet sufficiently rigid to retain as much oil as possible. No boom is capable of containing oil against currents greater than 0.7 knot (0.35 metres per second) at right angles to the boom, irrespective of boom size or skirt depth. This factor limits the speed at which booms can be towed to less than 0.5 knots. Oil patches or water turbulence appearing on the down-current side indicate that the boom is failing. Other important boom characteristics are strength, ease and speed of deployment, reliability, weight and cost.
It is essential that a boom is sufficiently robust for its intended purpose and will tolerate inexpert handling, since trained personnel are not always available. Strength is required particularly to withstand the forces of water and wind when being towed. Ease and speed of deployment combined with reliability are clearly very important in a rapidly changing situation and may strongly influence the choice made. Practical limitations of strength, water drag and weight mean that generally only relatively short lengths (tens to a few hundred metres) can be deployed and maintained in a working configuration. Towing booms at sea, for example in U or J configurations, is a difficult task requiring specialised vessels.
Because of the difficulties of operating multi-ship towed boom systems, specialised ships have been built which incorporate sweeping arms, skimming devices and on board oil storage. The limitations posed by sea conditions still also apply to these vessels, the larger examples of which are unable to work in shallow inshore waters. The efficiency of a specialised vessel will mainly be determined by the inbuilt oil recovery system or skimmer which is deployed. Because of the relatively narrow sweeping width, they are best suited to recovering oil in ribbons or windrows.
Skimmers which are used to recover oil from the water all incorporate an oil recovery element and some form of flotation or support. In addition a pump or vacuum device is necessary to transfer recovered oil and water to storage. Because skimmers float on the water surface, they experience many of the operational difficulties which apply to booms, particularly those posed by wind, waves and currents. Even moderate wave motion greatly reduces the effectiveness of most skimmer designs. In calm waters more satisfactory performance can be achieved provided the skimmer is suited to the viscosity of the oil in question.
The simplest skimmers are suction devices which remove oil from the water surface directly or via a weir, although these tend to pick up a lot of water at the same time. More complex units rely on the adhesion of oil to metal or plastic disks, or oleophilic belts or ropes. Yet others employ brush systems or are designed to generate vortices to concentrate the oil.
It is important to have adequate temporary oil storage facilities available, otherwise this becomes a bottleneck to successful oil recovery. Temporary storage needs to be easy to handle, and easy to empty once full so that it can be used repeatedly. Suitable units include barges and portable tanks which can be set up on vessels of opportunity. When recovering very viscous oils, storage tanks may need to be heated to allow them to be emptied.
Many factors should be considered when selecting skimmers. The intended use and expected operational conditions should first be identified before criteria such as size, robustness and ease of operation, handling and maintenance can be weighed up. The most important factors to consider are the viscosity and adhesive properties of spilled oil, including any change in these properties over time. At oil terminals and refineries where oil type may be predictable, specialised units may be selected, but otherwise it is preferable to retain versatility and select units which can deal with a range of oils. It is also important to recognise the difficulties posed by floating debris, both natural (sea weeds, sea grasses, trees and branches) and man made (plastic, glass, timber). Skimmers may need trash screens and regular unblocking where debris is common, such as near urban areas or river mouths. They will also need continuous maintenance by specialist staff and a supply of spare parts.
Because of the various constraints imposed on skimmers in the field, their design capacities are rarely realised. Experience from numerous spills has consistently shown that recovery rates reported under test conditions cannot be sustained during a spill and so it is important not to have unrealistic expectations about what can be achieved.
Once oil recovery is completed, booms and skimmers will need to be cleaned, overhauled and repaired, ready for use in the next spill. It is also important to inspect and test equipment regularly so that it is in good working order, and to maintain personnel training standards by regular drills.