Oil Spill Cleanup Concepts
Posted by Jeff Id on May 31, 2010
Dr. Weinstein sent me an email on oil spill cleanup which has some interesting history. I don’t think he will mind if I publish the email he sent which contains some clues as to the history of these concepts. I remember seeing similar things on TV and in print way back when the Exxon Valdez incident happened. Anyway here is the important part of the email. Feel free to discuss any suggestions or to add further knowledge. From the sign in link above, there are a few lurking resume’s around here.
I developed the enclosed technology after the Exxon Valdez spill, while I was still at NASA. Exxon and others were not interested. Some of the ideas were published as a NASA Tech Brief, and that was the end of that. I later had additional ideas and updated my notes. When the spill in the gulf happened, I sent the enclosed writeup to BP. No interest shown. I know they were mainly concerned by the continuing leak, so I guess they did not want to be distracted by the cleanup details.
The following material contains conceptual subject matter that is being made available due to the present oil spill emergency
OIL SPILL CONFINEMENT AND CLEAN UP SYSTEM
Leonard M. Weinstein, ScD
May 2, 2010
The purposes of the current ideas are to contain and clean up large oil spills in water. This includes preventing spilled oil from contaminating selected areas. The invention would use barrier skirt oil confinement and clean up devices, combined with methods to deploy and use these devices effectively, and with different variations for protective barriers and storage tanks.
Oil spills can cause considerable environmental damage. In addition, the clean up cost and legal penalties can be huge. For the Exxon Valdez leak, the cost was over 1 billion dollars, and damage to portions of the Alaskan coast was extensive.
There are thousands of oil spills every year, and many of these are not reported. In recent times, there have been about 100 reported oil spills a year that exceed 10,000 gal, with the average of these close to 100,000 gal. In addition, there has been an average of one spill a year near 1,000,000 gal, and about one spill in 5 years close to 10,000,000 gal. Significant spills are bound to occur in the future.
There are currently only partially effective confinement and clean-up devices and methods for oil spills in water. The oil spreads to a very large area, and pollutes the sea and land around the leak. Conventional “boom” systems can give some confinement and protection, but are difficult to carry and deploy over large areas, and tend to be less effective in strong wave action. Large quantities of oil require a large depth of skirt, or a very large enclosure area. There are several versions of “deep skirt” systems that have previously been suggested, that are deployed from a leaking ship, and that place a deep confinement enclosure to hold a large quantity of oil from spreading. These are complex to carry and deploy. In addition, none of these have been found to be very effective. Other techniques try to recover or clean up the slick by trying to scoop the slick up, or by dispersing or sinking the oil. It should be noted that the oil that sinks may do long term damage to the environment. Relatively small slicks are often absorbed with special materials, and the absorbed oil soaked waste collected. All of the current methods are either too complex or not very effective, and none can handle large leaks well. If the oil is recovered (as with skimmer boats) the oil has to be stored when the boat is full, to allow the skimmer to continue. There are some versions of deployable storage barges able to store reasonable quantities of recovered oil. They are bulky, and very expensive, and are difficult to transfer oil.
All variations of the current invention have in common a lightweight flexible skirt that is able to confine moderate to deep oil thicknesses. The skirt is designed to remain nearly vertical when deployed. The skirt is braced vertically at fixed locations, but is able to flex along its deployed length. It is weighed at the bottom, and may use variable geometry floatation devices (which will be described later). Tension carrying cables are positioned to minimize tendencies to overturn when confining oil, even in a modest water current. The variable geometry versions of skirts can be stored flattened and rolled up. They can be quickly and easily deployed.
There are four major parts described, which allow a wide range of uses. The first part describes enclosures, and methods to deploy them, able to confine large quantities of oil leaking into water (from tankers, oil platforms, oil loading facilities, etc.) and prevent most of the oil from spreading, so that it can be more easily recovered. The second part includes a single boat deployable adjustable area enclosure to clean up oil slicks that have already spread away from the source. The third part includes a two-boat skimmer to collect very large areas of slick in a short time. The fourth part includes storage containers to hold and transport large quantities of oil confined and collected by the first three parts of the current invention.
1) Deep confinement skirt and method of use:
A confinement skirt is deployed over a moderately sized area to completely enclose a source of leaking oil. Since the oil is lighter than the water, the height of the oil in the enclosure will be greater than the height of the water outside the enclosure. As the oil fills the enclosed space, pressure due to this excess height forces the skirt outward. This height difference can also result in a force moment tending to overturn a skirt unless the restoring moment is made large enough, or the tension cable is located near the center of force. The deployed length of skirt, and enclosed area can be moderate, since large quantities of oil can be contained in a fairly small area. This allows a relative small auxiliary boat (or other means of deployment) to carry and release the skirt. The auxiliary boats may be carried by the oil tanker, oil platform, etc., or they could be stationed at strategic fixed locations to be available for protection of an area. A sketch of one version of a basic variable geometry skirt design is shown in figure 1. The bladder version is suggested due to its ability to be compactly rolled up for storage.
A design with the dimensions shown in the figure could confine 9,000,000 gal of oil with a skirt enclosing 1,000 ft diameter, and with the oil about 1.5 ft deep. The small diameter and moderately large oil thickness also minimize evaporation and mixing with seawater, so recovery could be more complete. The large oil depth also makes it easier to pump the oil without ingesting water. The design of figure 1 consists of a flexible skirt material that encloses an air pocket (bladder) along the height of the skirt. Vertical stiffeners are attached every few feet to prevent the skirt from bending. Weights are distributed along the bottom of the skirt, and the tension carrying cable is located at a height to minimize overturning due to oil pressure and water currents (this may vary depending on expected use). The skirt is stored rolled on a spool, so that a large length can be easily stored and deployed. When the skirt is deployed, the water pressure pushes the air to the top, and the skirt is near vertical.
A possible means to deploy these skirts is shown in figure 2.
A spool holding the skirt is held on a platform partially submerged, and a sea anchor, buoy, or any other way to secure one end of the skirt is used to hold that end in place while the boat is moved so as to unroll the spool around the desired area. The area is completely enclosed, and the skirt is sealed to prevent oil leakage. The deployed skirt will be held against a ship or other object, as shown in figure 3.
If the oil is leaking slowly, and if the oil is recovered continuously during the leak (and pumped into storage containers), smaller confinement depths and diameters can be used. These conditions would allow a smaller confinement skirt, which can be more easily stored and deployed, and which would have lower drag forces.
2) Single boat method to capture and clean up modest sized areas of free floating slicks:
When the oil has escaped from the source and spread to a larger slick area, a different tactic is employed. A relatively small depth and strength skirt can be used, since the oil thickness will be much smaller than the confinement version, and current forces are small (due to the skirt moving with the current). The design shown in figure 1, but with a total height of only about 18” (12” below the water) could be used. Since the skirt is much smaller, far greater lengths can be easily held and deployed with a boat such as shown in figure 2. The example in figure 4 shows a skirt enclosing an area 4,000 foot in diameter, which would capture a large portion of a slick at a time.
An important feature in the current approach is to then steadily reduce the enclosed and sealed area by spooling the skirt back in, which increases the oil thickness. This is made practical by the ability of the air filled bladder to flatten when pulled between rollers, to keep the oil from leaking while reducing the area. When the area is reduced enough, the oil thickness can be raised to any desired thickness, which allowed direct pumping of the oil into storage containers without including much water. The storage containers would be carried by the recovery boat, and deployed to hold the recovered oil. The design of the storage containers will be discussed more in following sections. Since the skirt and boat are now drifting with the current, no significant secondary flows of water or oil in the enclosure would tend to cause leakage from the skirt. The same auxiliary boat that can be used to confine the leaks can also be used as a slick clean up boat. This, along with the deployable storage tanks, allows the entire spill management system to be in one location.
3) Two boat skimmer method to capture and clean up very large areas of free floating slick:
The variation has three features that overcome the limitations of present fast skimming approaches.
* The skimmer skirt is used at a highly swept angle to the flow direction, which reduces the normal velocity to the skirt. This cuts the normal (to the skirt) flow speed to a fraction of the full speed. Tension supports are used as needed to hold the sweep back angle nearly constant.
* The skirt may include a vertically descending partially open portion at the bottom. This puts a lower speed barrier between the deep flow and the surface, and allows slightly higher normal current speeds to be confined than without the partially open portion.
* The main tension carrying cable is separated from the skirt by several feet, and small connectors attach the two every few feet. The connectors are designed to prevent overturn of the skirt, and to uncouple the local section of the skirt from the overall high tension. This allows large local vertical motion of the skirt relative to the main tension cable. An example of separate tension and skirt design is shown in figure 5 (but not showing the lower decelerator net used in the two-boat skimmer).
Figure 5. Separated tension cable from skirt for independent wave following
The two boat skimmer design is shown next in figure 6.
Figure 6. Two boat skimmer with oil storage tanks
With the above system towed by two small boats at about 2 knots, the oil will be collected over a large area and moved to the back. An underskirt horizontally placed, and passing progressively less under flow would decelerate the oil so that it can be pumped into trailing storage tanks. The tank detail and a sketch of the process are shown in the figure. More detail can be obtained from the author.
4) Portable storage containers used to hold large quantities of oil:
The need for large oil storage containers was described in the first three parts. Different tasks would result in needing a greatly different capacity for each one, so more than one version is probably needed. The basic design for these containers is shown in figure 7.
Figure 7. Temporary storage containment for large quantities of oil
The main differences between the storage containers and the skirt versions used for the first three tasks is that generally significantly larger depths of oil are held in a much smaller diameter, and that a bottom partially permeable partition prevents currents from sweeping the oil from the enclosure. These skirts would be stronger and heavier (per length) than other versions, but still have small total weight due to small total length. Deployment consists of unrolling and enclosing a skirt into the water, and attaching the bottom enclosure (or using pre-attached enclosures and bottoms). The bottom enclosures would be close mesh net or have small holes to allow slow water flow but be tight enough to block underflow from removing oil. As oil is added, the water escapes out the bottom. The location of two tension members prevents overturn instability at all oil depths, and the double attachment cable minimizes overturn tendencies when the container is towed. The height of the skirt and the enclosed area determine the quantity of oil that can be held. Two examples are given that cover a typical desired range of sizes:
Diameter Total height Max. Oil height Capacity Dry weight
20’ 3.5’ 2’ 5,000 gal 150 lb.
60’ 6’ 4.5’ 100,000 gal 1,000 lb.
These configurations give considerable holding capacity in an easily stored and handled container size. Even larger container sizes can be used, but they become progressively more difficult to handle, so a number of the 100,000 gal sized ones are recommended for large spills, rather than larger containers.
The present spilling oil and slick in the Gulf of Mexico are a major problem to manage with existing technology. The proposed ideas in this paper are not tested and ready to go ideas with existing equipment, but some existing skirt equipment could probably be modified to do some of the required cleanup using one or more of these ideas in a reasonable time if no better solution is found. The two-boat skimmer version and storage tanks are most probably the best chance for rapid adaptation. The author of this paper is willing to assist if more detail is required.