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Content:
Joe Mullin
Pagemasters:
OEMM Web Team
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Mechanical
Containment and Recovery
Mechanical
containment and recovery is the most commonly used and most environmentally
acceptable response technique to cleanup oil spills in the United States.
Mechanical spill response uses physical barriers (containment booms) to
contain and concentrate floating oil, mechanical devices (skimmers) as well
as natural and synthetic sorbent materials to remove oil from the water’s
surface, and temporary storage devices to store the recovered oil and water
until it can be disposed of properly. Where feasible and effective, this
technique is preferable to other methods (use of dispersants or in situ
burning), since spilled oil is removed from the environment to be recycled
or properly disposed. In most instances, the containment and recovery phase
of an oil spill proceed simultaneously.
Spilled oil floating on the water’s surface is
affected by wind, currents, and gravity, all of which cause it to spread,
fragment and disperse. The first stage of an effective response is to deploy
containment boom to limit further spreading and concentrate the oil for
recovery. Containment of an oil spill is the process of preventing its
spread by confining the oil to the area where it has been discharged.
Containment not only localizes the spill but also facilitates the removal of
the oil by causing it to concentrate in thicker layers on the surface of the
water.
Oil
containment booms are
generally the first equipment mobilized at the scene of a spill and the last
to be removed. They are used for concentrating oil so that it is thick
enough to be skimmed, for keeping oil out of sensitive areas, or for
diverting oil into collection areas. When deploying booms and skimmers to
recover spilled oil a common difficulty is controlling the movements and
activities of vessels and directing them to the thickest areas of the oil
slick. This can be overcome by using aircraft equipped with air to sea
communications.
Booms come in many different shapes, sizes, and styles
ranging from small, lightweight models intended for manual deployment in
harbors, to large, robust units which usually need cranes and sizeable
vessels designed for the open seas to handle them. Booms vary considerably
in their design, but all normally incorporate the following features:
- Freeboard to prevent or reduce
splash-over;
- Sub-surface skirt to prevent or reduce
escape of oil under the boom;
- Flotation by air or some buoyant
material; and
- 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.
The
most important characteristic of a boom is its oil containment or deflection
capability, determined by its behavior in relation to water movement. The
boom should be flexible enough to conform to wave motion yet sufficiently
rigid to retain as much oil as possible. Most booms are not capable of
containing oil in currents greater than 0.7 knot (0.35 meter/second) that
flow 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 (0.26 m/s). The success of containment booming is dependent on
currents, wind, and waves. Even minor currents can draw oil under the booms;
waves may cause splash-over, and wind and currents may cause the boom to
sink or plane. Oil patches or water turbulence appearing on the down-current
side indicates that the boom is failing. New open ocean boom designs capable
of containing oil as tow speeds greater than 3 knots (15.4 m/s) are becoming
commercially available. In Arctic conditions even very low concentrations of
ice can seriously affect the performance of most booms. Containment booms
will quickly collect ice and subsequently lose oil as flotation chambers are
submerged or lifted out of the water. Other important boom characteristics
are strength, ease and speed of deployment, reliability, weight and cost.
It is essential that a boom be sufficiently robust for
its intended purpose and tolerate inexpert handling, since trained personnel
are not always available. Strength is required to withstand the forces of
water and wind when being towed. Ease and speed of deployment combined with
reliability are very important in a rapidly changing situation and may
strongly influence the selection of equipment. Practical limitations of
strength, water drag and weight mean that generally only relatively short
lengths (tens to a few hundred meters) can be deployed and maintained in a
working configuration. Towing booms at sea in “U” or ”J” configurations, is
a difficult task requiring specialized vessels. Because of the difficulties
of operating multi-ship towed boom systems, specialized 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 larger
versions of these vessels of which are unable to work in shallow inshore
waters. The efficiency of a specialized vessel is mainly determined by the
built-in oil recovery system or skimmer which is deployed. Because of the
relatively narrow sweep width, these specialized vessels are best suited to
recovering oil in ribbons or windrows. Following containment of the oil, the
next step in the cleanup operation is physical recovery of the oil from the
water’s surface.
Skimmers
that 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 some sort of temporary storage device.
Because skimmers float on the water surface, they experience many of the
same operational difficulties that 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 better performance can
be achieved if 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. Still others
employ brush systems or are designed to generate vortices to concentrate the
oil. The effectiveness of a skimmer is determined by how quickly it can
collect the oil, and how well it minimizes the water to oil ratio collected.
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 evaluated. 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, specialized units may be
selected. Otherwise it is preferable to retain versatility and select units
which can deal with a range of oils. A wide variety of skimmers are
available that use different methods for separating oil from water.
Vessel-based skimming systems remove oil from open water, while vacuum
trucks are often used to remove oil that has collected near the shoreline.
It is also important to recognize the difficulties
posed by floating debris, both natural (e.g. sea weeds, sea grasses, trees
and branches) and man made (e.g. plastic, glass, timber). Skimmers may need
trash screens and regular unblocking where debris is common, such as near
urban areas or the mouths of river. In Arctic conditions, even very low
concentrations of ice seriously affect the performance of most skimmer
systems through plugging and bridging. The skimmers will need continuous
maintenance by specially trained staff with a supply of spare parts. Because
of the various constraints imposed on skimmers in the field, their design
capacities are rarely realized. Experience from numerous spills has
consistently shown that recovery rates reported under test conditions cannot
be sustained during a spill. It is important not to have unrealistic
expectations about what can be achieved.
Oil collected by the skimmer is stored in a
containment tank or temporary storage device. It is important to have
adequate temporary oil storage facilities available otherwise this becomes a
bottleneck to successful oil recovery operations. 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 must be heated to allow them to be emptied.
Once oil recovery is completed, booms and skimmers
will need to be cleaned, overhauled and repaired and made 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.
Overall, containment and recovery operations at sea
require extensive logistical support. In rough seas, a large spill of low
viscosity oil such as a light or medium crude oil can be scattered over many
square kilometers within just a few hours. Oil recovery systems typically
have a swath width of only a few meters and move at slow speeds (1 knot)
while recovering oil. Thus, even if response personnel can be operational
within a few hours, it will not be feasible for them to encounter more than
a fraction of a widely dispersed 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% of the
spilled oil and often considerably less.
Sorbents
are insoluble materials or mixtures of
materials used to recover liquids through the mechanism of absorption, or
adsorption, or both. Absorbents are materials that pick up and retain liquid
distributed throughout its molecular structure causing the solid to swell in
volume by 50 percent or more. Adsorbents are insoluble materials that become
coated by oil on the surface, including associated pores and capillaries,
without the solid swelling by more than 50 percent. To be useful in
combating oil spills, sorbents need to be both oleophilic (oil-attracting)
and hydrophobic (water-repellent). Although they may be used as the sole
cleanup method in small spills, sorbents are most often used to remove final
traces of oil, or in areas that cannot be reached by skimmers. Sorbent
materials used to recover oil must be disposed of in accordance with
approved local, state, and federal regulations. Any oil that is removed from
sorbent materials must also be properly disposed of or recycled. Sorbents
can be divided into three basic categories: natural organic, natural
inorganic, and synthetic.
Natural organic sorbents
include peat moss, straw, hay, sawdust, ground corncobs, feathers, and other
readily available carbon-based products. Organic sorbents can adsorb between
3 and 15 times their weight in oil, but there are disadvantages to their
use. Some organic sorbents tend to adsorb water as well as oil, causing the
sorbents to sink. Many organic sorbents are loose particles such as sawdust,
and are difficult to collect after they are spread on the water. These
problems can be counterbalanced by adding flotation devices, such as empty
drums attached to sorbent bales of hay to keep them afloat, and wrapping
loose particles in mesh to aid in collection.
Natural inorganic sorbents
consist of materials like clay, perlite, vermiculite, glass wool, sand, or
volcanic ash. They can adsorb from 4 to 20 times their weight in oil.
Inorganic sorbents, like organic sorbents, are inexpensive and readily
available in large quantities. These types of sorbents are not used on the
water’s surface.
Synthetic sorbents
include man-made materials that are similar to plastics, such as
polyurethane, polyethylene, and polypropylene designed to adsorb liquids
onto their surfaces like a sponge. Other synthetic sorbents include
cross-linked polymers and rubber materials, which absorb liquids into their
solid structure, causing the sorbent material to swell. Most synthetic
sorbents can absorb up 70 times their own weight in oil.
Desired Outcomes
- Improve the
operational capability of existing spill response equipment to respond to
oil spills in the marine environment.
- Expand the
capability of existing spill response equipment to enable oil recovery in
cold water/broken ice conditions.
- Develop
capabilities to deflect or redirect oil in a broken ice field. Separate
oil and ice on the waters surface to increase encounter rates for possible
mechanical recovery or in situ burning in firebooms.
- Improve the
capabilities to process and transfer viscous oil and oil/water emulsions
and integrate all processing phases (collection, pumping, storage and
offloading).
Tactical Plan (2005-2010)
- Continue
technology assessments of prototype and commercially available offshore
and fast-water booms and skimming devices, oil water separation and
emulsion-breaking systems, and modular, easily transported temporary
storage devices required to respond to open water oil spills.
- Test and
evaluate equipment and technologies required to respond to oil spills in
cold water/broken ice conditions. Increase the encounter rates for
mechanical response in broken ice.
- Extend
recent work on viscous oil pumping to understand the effects pieces of ice
contained in the oil. The impact of slush ice and small ice pieces on the
ability to pump recovered cold oil, oil-water emulsions and oily waste is
a continuing problem.
- Test and
evaluate new and innovative technologies that will facilitate the
separation of oil, ice and water when recovering spilled oil in cold water
and broken ice conditions.
- Test and
evaluate oil spill containment systems capable of operating in high
current environments.
- Test and
evaluate equipment and technologies required to handle and treat oily
liquid and oily solid waste collected during response operations.
- Test and
evaluate the performance of sorbent materials.
- Take
advantage of planned full-scale field trials to validate and prove
response technologies and strategies developed in laboratory and meso-scale
experiments and to develop operational guidelines for particular response
technologies.
For more information on Mechanical Containment and
Recovery Projects, contact
Joseph Mullin
at 703-787-1556 or via email.
Mechanical Containment and
Recovery Projects |
004 |
Cavitating
Water Jet Cleaning Nozzle |
032 |
Recapture of
Oil from Blowing Wells |
084 |
Surface Oil
Spill Containment and Cleanup |
085 |
Subsea
Collection of Blowing Oil and Gas |
109 |
Oil Spill
Response Equipment Performance Verification |
113 |
Open Ocean
Boom Test |
121 |
Water Jet
Barrier Containment of Oil in the Presence of Broken Ice |
152 |
Recovery
Methods for High Viscosity Oils |
153 |
Alaska Arctic
Workshop |
155 |
Identification
of Substitute Test Facilities for OHMSETT |
156 |
World Catalog
of Oil Spill Response Products |
158 |
Development
and Evaluation of Shoreline Cleanup Techniques |
159 |
Evaluation of
Skimmers for Offshore and Ice-Infested Waters |
163 |
Preparation a
Test Protocol for Offshore Oil Skimmers and Containment Booms |
180 |
Testing and
Evaluation of Sorbents |
244 |
Testing of
Fire Resistant Booms in Waves and Flames |
247 |
Numerical
Modeling of Oil Boom Behavior and Rapid Current Boom Development |
287 |
Fate and
Behavior of Deepwater Subsea Oil Well Blowouts in the Gulf of Mexico |
289 |
Re-Engineering
of a Stainless Steel Fireproof Boom for Using in Conjunction with
Conventional Firebooms |
291 |
Technology
Assessment and Concept Evaluation for Alternative Approaches to In
Situ Burning of Oil in the Marine Environment |
295 |
In situ Clean
up of Oiled Shorelines; Svalvard Shoreline Project |
297 |
Comprehensive
Spill Response Tactics for the Alaska North Slope-Oil in Broken Ice
Spill Response Scenarios |
298 |
Testing at
Ohmsett to Determine Optimum Times to Decant to Temporary Storage
Devices |
299 |
Estimation of
Towing Forces on Oil Spill Containment Booms |
309 |
Development of
an OHMSETT Activity Summary Report |
310 |
Mechanical Oil
Recovery in Ice Infested Waters (MORICE) - Phase III |
311 |
Oil Spill
Containment, Remote Sensing, and Tracking from Deep Water Blowouts:
Status of Existing and Emerging Technologies |
324 |
Experimental
and Analytical Study of Multi-phase Plumes in a Stratified Ocean with
Application to Deep Ocean Spills |
330 |
Oil Spill
Response - Performance Review of Booms |
333 |
Field
Experiments at the Ohmsett Facility, Especially for a Newly Designed
Boom System |
348 |
Detection and
Tracking of Oil Under Ice |
353 |
The Use of Ice
Booms for the Recovery of Oil Spills from Ice Infested Waters |
354 |
International
Oil and Ice Workshop |
377 |
Project "Deep
Spill" |
391 |
Fire Boom
Testing at Ohmsett |
395 |
Extending
Temporary Storage Capacity Offshore With Emulsion Breakers |
428 |
Procedures for
Reporting Tests of Oil Spill Containment Booms and Skimmers |
457 |
Effect of Oil
Spill Containment Boom Characteristics on Boom Performance |
478 |
Development of a Standard Method for Measuring the Buoyancy-to-Weight
Ratio for Oil Spill Containment Boom |
511 |
Tailored
Polymetric Materials for Oil Spill Recovery in Marine Environments |
512 |
Transfer
of Decanting Technology Research to Oil Spill Response Organizations
and Regulators |
515 |
Wave Field
Characterization at the Ohmsett Wave Test Basin |
516 |
Development of a Method to Produce Large Quantities of Realistic
Water-In-Oil Emulsions for use in Evaluating Oil Spill Response
Equipment and Methods |
520 |
Summary
Report of Activities at the Minerals Management Service Ohmsett
Facility |
528 |
Optimization of Oleophilic Skimmer Recovery |
555 |
Partnering
in a Workshop to Determine the Scope of an Experimental Oil Spill in
Pack Ice in Canada |
573 |
Oil
Recovery with Novel Skimmer Surfaces under Cold Climate Conditions |
586 |
Planning
Support for an Experimental Oil Spill in Pack Ice |
587 |
International Oil in Ice Workshop 2007 |
589 |
Investigation of the Ability to Effectively Recover Oil Following Dispersant
Application |
625 |
Oil Spill Training and
Response (STAR) Calculator Program |
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Last Updated:
08/08/2008,
01:29 PM
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