In the past, criteria of advantages and disadvantages for the use of a skimmer was used. These criteria were very subjective, what could be an advantage for one user might by a disadvantage for another and vice versa.
The photo left is an "golden oldie" from my photo collection which shows the different sizes of wier skimmer produced by this manufacturer.
For each type of skimmer the manufacturers have different sizes and models of the equipment. Different manufacturers produce similar devices, each one with its own design and standard of quality and name. In many places response personnel are familiar with the equipment they have in their bases.
These specific names will confuse another base operators if they have not heard of the name. Dependent on the incident there could be a need to call in extra skimmers from other sources which the operator on site has never seen before but knowing the principle of the new skimmer will help with its efficient operation.
Skimmers may be:
Self-propelled – Moved, with thrusters for forward and reverse movement operated by joysticks or levers. This makes it possible to position the skimmer in the place of greatest oil concentration during an operation.
Dynamic – Generally they are positioned on the bow or on side of a vessel. It makes it possible for recovery of the oil and the movement of a vessel simultaneously.
Stationary – Placed in the location with the greatest concentration of oil. They are moved and retained by ropes.
For many years the response industry has been making efforts with the development of methods and equipment for the recovery of oil, some of the problems to overcome are mentioned below:
Following the containment of oil, we now have to remove it from the place of the spill and transport it to where it may be disposed of in a secure and acceptable way.
In this section we will see several types of equipment and operations that may be used in the recovery of spilled oil in different places, evaluating the advantages and disadvantages of the different types of skimmers.
The efficiency of a skimmer will depend on several parameters, such as;
The response crew should be aware of four important points:
The principal application of a skimmer is in sheltered waters. Some may be used in the open sea; however it is generally considered that
when the waves are higher than 2 meters, their efficiency will be very low.
After the first year of the Exxon Valdez spill 89 the US Coast Guard having carried out various studies, issued a report on skimmer efficiency versus the manufacturer’s quotes. The result was that the quantity of recovery by any skimmer quoted by the manufacturer should be reduced by 80%.
This is important as any test done during R&D is done in a test tank with the best conditions and a large quantity of oil, even in a boom at sea, with good quantities of oil in the apex it is very difficult to recover the amount quoted.
This photo right from Deepwater Horizon explains dramatically that a skimmer which the manfacturers say's can recover 100m3 per hour (the smoke is from the rig before it sank).
The evaluation section shows you that 100m3 of fresh crude oil can cover 1km2 this photo shows much more the 50km2.
Containing that amount of oil at sea and recovering it in an hour or a day would be practically impossible.
This is why the figure of 15% is used for offshore recover and the reason there are other strategies used during a response
In many cases manufactures use 6'' connections on skimmer pumps to increase the recovery rate, unfortunately all supply vessels have 4'' connections to access their tanks. This means that the recovery rate stated by the manufacture is much more than is actually attainable.
Supply vessels have small tanks constructed for drinking water, diesel and chemicals used during drilling operations. This can cause big problems in many cases because the tanks will be filled very quickly, if the captain allows them to be used at all. The vessel will then have to leave to discharge. In Europe all vessels used for this type of work have to have an oil recovery certificate (Oilrec) before they can be used.
On the other hand in a calm area with a good thickness it is much easier, as was found during and after the Gulf war 91 when oil thicknesses in some places were as much a metre.
Unfortunately in various countries, government departments have written formulars for recovering the manufacturer figures.
So now to recover the manufacturers hourly rate, companies have to buy 5 skimmers to recover 20% each of the 100% hourly recovery rate.
In many cases, people in the oil industry will tell you the API° of the oil. API° was designed to show the market value of oils. The higher the number the more value it has on the market.
A responder needs to know the Specific Gravity of the oil. How close to 0.0 or the density of water is it. Will it sink or will it completely evaporate. Now we know what type of skimmer to use.
To get you off to a good start copy this photo and then you won't need to remember the formular SG = 141.5
API + 131.5
I thank these idiots on the internet who publish photo's of oiled hands. I would use gloves! but they do explain the difference in viscosity of the oils we have to recover.
Fresh medium crude oil This could be Heavy Crude or Fuel oil Emulsified Crude oil
As can be seen from the table below the different types of skimmer are only useful for certain viscosities of oil. Over time oil due to the weathering process viscosity changes so the coordinator needs to be able to acquire the type of skimmer that will work the best for the incident in hand and for the time it is expected to last for.
Skimmers may be classified in four main categories, based on the the way they recover oil
Asking for a specific type will speed the process up. It is important that the operators know the principles of all the different skimmers.
Dependent on the type of oil there will be a necessity to ask for a specific type of skimmer to best recover the oil.
Skimmers are usually classified in following four groups based on their principles of operation Wier, Oleophilic, mechanical and Vacuum.
The recovery rate is the function of the pump and the encounter rate of the skimmer.
The weir operates at the interface of the oil with the water allowing the oil to flow into the skimmer. A simple adjustment of a weir in some case greatly minimises the quantity of water recovered.
The oil passes over the weir and in some cases water exits through orifices located below the weir plate. Some models have pumps coupled to the skimmer itself and others use external pumps.
These pumps vary according to the type of oil they were designed to recover.
Adjusting the height of the weir improving the efficiency of oil/water ratio. Beware of single acting hydraulic rams as they invariably tend to creep either down so collecting more water of up out of the water causing over heating of the pump due to lack fluid which acts as cooling. They cannot be left alone.
The system consists of a weir-type skimmer fitted into a boom. It has a chamber filled with air for buoyancy. Some systems also have a chamber filled with water that serves as ballast for the boom.
The system allows the adjustment of the height of the boom and consequently the height of the weir. This equipment also permits the recovery of large volumes of water. There are various models available some have pumps which are very susceptible to trash others use screw pumps which lend themselves to a much wider range of viscosities.
This RoSkim system consists of a screw pump built into a wier section which is connected between 2 booms sections the hydraulic hoses and discharge hose are tied to the ballast chain.
This is the original wier boom where the pumps are built into the boom, the oil wiers through into the pumps and is pumped to storage through the boom.
The photograph on the left is the original 10 wier trail at the Ixtox spill in 1979.
The 10 pumps recovered 500m3/hr and as can be seen was pumped back into the sea. Dependent on sea conditions this can be 80% water.
Unfortunately this photograph which is still used by the manufacturer allows the purchaser to forget the need for huge storage tank capacities.
The photograph on the right is a 3 wier boom used during the Rose Bay spill England in the late 1980's.
In this case a tanker was used as storage vessels as recovery rates are now 180m3 or 210m3/hr and supply boats do not and never will have the tankage space for this type of equipment. Tankers can separate oil from water and transfer it within the ship, supply vessels cannot.
The imaginative name of this weir skimmer is SLURP (Self-Leveling Unit for Removing Pollution), originally developed by the Esso Research Centre,
It is remotely controlled from the pump; there is nothing to adjust at the skimmer.
There are no moving parts meaning low maintenance and can be operated in water depths of 5 inches for the aluminum model and 10 inches for the steel version.
The word is Latin for having an afinity with oil as with the hand oil adheres but water runs off. These skimmers include drums, ropes, disks, brushes and belt type skimmers.
The rotating movement through the oil allows it adhere to the surface and is then scraped or squeezed from the surface into the collection area. Skimmers that use oleophilic properties often achieve a 90% recovery in relation to water. They function best with oil having an averageviscosity between 100 and 2000 cSt centistokes. Oils with low viscosity such as diesel oil or kerosene, do not accumulate well on oleophilic surfaces.
Materials with high viscosity, such as heavy oils are excessively adhesive and difficult to remove; however viscous emulsions of water in oil can be almost non-adhesive.
Oleophilic drums rotate using a hydraulic system. The oil adheres to the drum surface which passes by scrapers, the oil then being pumped away. The drums are also floatation for the skimmer allowing this system to be used in very shallow water like on a shoreline.
Some systems allow the depth of the drum to be adjusted.
Some models have pumps coupled to the system itself and other types are connected to external pumps.
The speed of the rotation of the drum varies in accordance with the type of oil and oil thickness. This type of skimmer functions well in calm seas and with low, medium and high viscosities. It functions in the presence of trash and its oil/water recovery ratio is good, reaching 50% with Diesel oil and 90% for Bunker C oil. Medium units may be operated by two people. Their use is limited to calm waters.
Victoria Broje with her professor Arturo Keller at the Bren school of environmental science and management at the University of California with a major manufacturer developed a grooved drum version (Photo right) which doubled the recovery rate of the smooth drum version, this has been on the market since 2006.
The disk skimmers have a good recovery rate for hydrocarbons with medium and low viscosities. Dependent on the pump type and weight some are very large and heavy. Modern models are very light and may be operated by just two people. The oil recovery rate is high. In calm waters the system functions well. The models with four disk banks recover oil from 360°. These types of skimmers do not recover solidified oil and oils or has been treated with dispersants. This skimmer does not function well in areas that have trash, being easily blocked.
Break throughts do happen after Deepwater Horizon the Wendy Schmidt Oil Cleanup Challenge came along to encourage people to think of new ideas for recovering larger quantities of oil it was thanks to the competition off the back of the grooved drum this new Elastec patented grooved disk was born and won first place.
It uses the meniscus effect for the liquid to adhere to the now much greater surface area to fill each of the grooved so that the discs can be run fasted. Recovery rates in comparison with a smooth disc increased 3 to 4 times.
Diagram left show the different effects, dependent on the type of liquid used:
A = concave
B = convex
The results were impressive using 4 banks of 16 discs the achievement of 4670 gpm (at 89.5% efficiency), reached more than 3 times the industry’s previous best oil recovery rate tested in controlled conditions recovery results are below.
Continuous ropes with polyethylene fibers woven through the lay of continuous loop ropes. The ropes are rotated by the same rollers that squeeze the oil into a discharge pump allowing a cleaner surface to return to the oil.
These systems can be used vertically or horizontally in calm and rough waters, tanks, drains.
The come in various sizes from one rope up to 6, rope diametres differ too. The vertical system is used preferentially offshore, because this type of skimmer is not directly affected by the waves. This is one type of the equipment that shows best recovery rates of sea conditions above 4 of the Beaufort scale.
The vertical type has a lot less problem with debris as gravity takes over and the oil goes up and the rubbis falls down.
It is not a standard but many companies manufacture white soft ropes for light and medium oils and black stiffer ropes for heavy oils. In very cold locations, the ropes may freeze. The lower the velocity of rotation, the more oil adheres to the cords. If the oil is emulsified and very viscous, the strands of the ropes stick to themselves and lose a little of their efficiency. The ropes may wear out, if the oil is impregnated with sand due to abrasion.
This type of skimmer is effective in any type of sea; it has a high rate of oil /water recovery and allows for dynamic recovery. It is effective over a broad range of oil types. This type of operation requires a trained response personnel. The useful life of the belt may be short dependent on oil type and conditions of use.
The oil adheres to the surface of the continuous band of brushes and is removed by a comb scraper. Generally skimmers are composed of a number of bands of brushes running side by side.
This equipment may be installed on the bow of a vessel. Generally these vessels are connected with flow diverters, which make a dynamic recovery possible. They can also be used on one of the sides of the vessel normally with short boom to divert the oil into the skimmer.
With brushes of soft fibers for light to medium viscosity oil and stiff brushes for heavy or emulsions. They are fairly tolerant of trash. Generally the system has a tray that the trash falls onto and is sorted manually. The oil recovery rate is good. Soft brushes usually have diaphragm pumps where as sfiff brushes are fitted with screw pumps to deal with the different oil types.
For recovery to occur, the vessels should be in movement or should have a current bringing the oil on to the brush. Wave movement makes the recovery work more difficult.
The changing of the brushes can be difficult as the chains they are attached to normally do not have a split link (commonly found on bycicle chains. I have mentioned over the years that a gap be left and a split pin used in this gap would be a major help during maintenance but to date nothing has changed. I also asked for a mixture of stiff and soft brushes as many people who buy skimmers do not know the difference and unfortunately many manufactures agents do not know either so this would make life easier as the skimmer would work across the whole range of oil types. This has also fallen on deaf ears.
Oleophilic skimmers should never be cleaned with detergent as this removes their ability to recover oil
This equipment uses vacuum pumps as well as air venturi system in order to suck oil from the surface of the water directly into a storage unit. Specially-designed skimmers may be coupled to the ends of suction hoses to increase their efficiency. Vacuum's should never be used for Petrol or other highly inflammable products due to the high risk of explosion.
The pumping of viscous material is possible, allowing the induction of water to act as a fluid medium. The diameter of the suction hose determines the size of solids that may be picked up. Any blockage will result from the partial or total loss of vacuum.
When 200 liter drums are used for recovery, they should be re-enforced, as they can collapse.
The vacuum systems are efficient for the collection of oily residues and high viscosity products. However, a relatively thick layer of oil is needed and should be used in calm water to reduce quantities of recovered water.
The use of vacuum systems is common for the recovery of oil from beaches, for the removal of oil on land or in confined areas, with good thickness. In contrast with the usual pumps, the vacuum systems are efficient for the collection in locations with trash, because they are no rotating mechanical parts in contact with the residues.
The vacuum truck allows recovery, storage and for the transportation of the recovered product to a disposal site.
Beware tanker trucks hired for an incident which do not normally work with hydrocarbons as they can have a serious safety problem. The exhaust from the vacuum pump can be very close to the exhaust of the engine. Thus warm gases can be close to an ignition source. Other problems with the gas being sucked into the engine through the air filter can cause engine over running.
These include belts with paddles, metallic and plastic toothed disks, collection buckets and drums. They all depend on physical action to bring the oil into the skimmer; the more viscose oils will not flow towards the collection area. Combined usually with a screw or lobe pump, they should be capable of accepting and transferring viscous oil, stable water in oil emulsions and solid waste. For the recovery of these types of oil some manufactures combine a water injection system with the pump which increases the recovery rate dramatically.
After the spill from Erika off France the need for more development of mechanical skimmers was noted and some models were available 2 years later for the Prestige spill off Spain.
Belts with paddles
A series of paddles mounted on a belt rotates taking the water into a tank. The quantity of water recovered is high. Generally the recovery vessel has a separator. In some locations around the world it is not possible to return the water to the sea and so the recovered water has to be disposed of as well as the oil.
This type of skimmer functions very well for oils with high or medium viscosities and they work well in the presence of trash. If there is a thick layer of oil, they will recover large quantities of oil. Waves and strong currents interfere with the recovery operations. They recover all types of oil but are very efficient for the recovery of oils with medium to high viscosity.
This system often functions with a directed flow device to push the oil to the skimmer. Experienced boat handlers are needed to execute this type of operation. The vessels on the market that possess this type of system operate at slow speeds.
The viscous oil adheres to the coarse brush, and is removed by a scraper. Generally skimmers are composed of a number of lengths of brushes running side by side.
This equipment may be installed on the bow of a vessel. Generally these vessels are connected with flow diverters, which make a dynamic recovery possible. It can also be used on one of the sides of the vessel normally with short boom to divert the oil into the skimmer.
The drum rotates using a hydraulic system. The oil is forced inside the drum which is similar to a snail shell the oil is force ever closer to the centre with ever revolution of the drum until it reaches the centre, there is an Archimedes screw which moves the oil to a screw pump on the outside from which the oil is pumped away.
The speed of the drum can be adjusted but needs to be controlled with the discharge pump both are on separate hydraulic circuits but the pump should never be run dry.
With heavy oils the drum can be run faster to allow water to be picked up which is necessary to increase the flow through the discharge hose. Without water to assist the recovery rate will be much lower and there is a likelihood of hoses bursting under the pressure.
These disks cut into the viscous oil clawing it into the skimmer. This type of skimmer also incorporates a screw or lobe type pump due to the high viscosity of the oil..
It may be used with very heavy oil and even with solidified oil.
They are sensitive to waves and very sensitive to trash and sediment.
As stainless steel has oleophilic properties this skimmer will also recover light to medium oils.
With events like Erika 1999 and Prestige 2002 the recovery of highly viscose oil has been necessary when the oil from Prestige entered the sea it's viscocity was 45,000 cts, 2 months later emulsified with 60% water the viscocity was 200,000 cts. This Hi Wax skimmer was used at both events succesfully. It consists of 2 rotating vaned drums which physically drag the oil into the centre of the skimmer where it is transfered to the vessel using a water injection system on the pump to assist with the flow in the hose.
The oil spreads rapidly on the surface of the water and a rapid response is necessary for the problem to be minimized. We have to choose the correct type of skimmer for the type of oil spilled.
The greater the thickness the layer of oil, the more chance we have to use a recover high quantities. We should also concern ourselves with the rates of efficiency of oil and water.
When pumping highly viscous oils the injection of water into the pump or discharge hose greatly reduces the friction and reduces the power required to pump the material. Below is a test carried out by the USCG with a screw pump pumping 200,000 cst oil through a 92 meter 6" inch hose shows the major differences.
Inlet lube water Outlet lube water Capacity Pump pressure
No water No water 4.5 m3/h 11.9 bar
4 % cold water 14C 4% cold water 14C 44.8 m3/h 2.8 bar
4% hot water 99C 4% cold water 14C 45.7 m3/h 0.6 bar
The self-propelled skimmers cannot deal with currents of more than three knots recovery above this velocity is also very difficult.
The oleophilic skimmers have their rotational velocities adjusted in accordance with the quantity of oil that adheres to the oleophilic surface. Generally, lighter oils require slower rotation of the system.
The operational conditions expected should be identified first, before other criteria, such as size, robustness, ease of operation, maintenance of the skimmers can be evaluated.
The most important factors to be taken into consideration are the adhesive and viscosity properties of the spilled oil, including any changes to their properties with the passage of time.
In predictable situations such as maritime terminals and refineries, the type of oil is usually known and a specific skimmer may be chosen.
The need to know the type of oil involved at the early stages. As some oils will emulsify and become much more viscose and more difficult to pump. This information is imperative as to which type of skimmers are chosen. During large tanker incidents the equipment can be flown in from across the world.
Choosing the correct skimmer type for the oil reduces cost and makes the response more efficient.
Skimmers may need to work at an incident for weeks or in some cases months, maintenance during this time will be necessary.
There is also a need at this point to deal with the separation of waste. Liquid oil is much easier to dispose of than oil full of trash and sand. The disposal of the waste is one of the most difficult parts of the response so assisting with the separation helps to diminish this problem.
All skimmers suffer from debris ingress especially with spills in ports, lakes, and rivers near towns. In this case a cheap modification has been used to stop debris reaching the wier edge. The orange material is from a temporary fence normally used to protect excavations. It is pulled tight around the skimmer floats and fastened with electrical tie raps. This can be removed and disposed of at the end of the operation. It is a useful thing to have in certain circumstances and is cheap.
The importance of not knowing the oil type where the spill may occur may result in equipment being purchased that will not work in the event of an oil spill;
Here are three examples:
The above cases show that during the planning stage sometimes lack of knowledge of the whole pipeline system and what influence other oils or chemicals may have on the structure of the oil that is spilled. This knowledge and advice from response personnel can result in the correct and efficient equipment being purchaced before an event happens.
Since the early nineties there have been increasingly more spills involving heavy fuel oils. The venuzelian orimulsion started a move towards a type of recovery device that would work in the case of a spill. Orimulsion comes to the surface with aid of a viscocity reducing chemical, as with the Natuna Sea incident if there was a spill this chemical would readily disolve into the water leaving a very heavy oil behind. This oil dependent on the salinity of sea water could sink or become negativly buoyant.
Early during the research and developement stage it was decided that the skimmers available at the time would be of little use.
A system using the same technology as for fishing was developed, trawl nets with removable back end where the cod end would be, were produced. Finding photographs of these early versions has been difficult but this is one from the late eighties this is a throw away version as there is no removable part. Having said that when a newer version is used the oil funnels into the bag like structure at the back end of the system contaminating all the material, making it extremely difficult to clean thoroughly.
In the wake of Erika and Prestige two of the largest heavy oil spill incidents to date I find it difficult to believe that apart from the Norwegians there is nothing happening with this type of recovery system.
The strange thing I find with Norwegian booms is there passion to continue with the fishing theme. All of their booms have rope nets and rope connections which are used in the fishing industry. To me a piece of wire rope would do the same job as the net, it is there to stop the opening being opened to wide. During decontamination it could be thrown away and replaced by another. Net is nearly impossible to clean and the need for training with the same equipment is necessary while waiting for the next inevitable oil spill to happen, this can only be done with clean equipment.
Storage of recovered oil
This is one question that seems to wait until the problem arrives before it is addressed. In the case of supply vessels which are commonly used especially during offshore spill due to their availability. These vessels are used basically as trucks to transport the tools, chemicals, fuels and equipment required for the offshore activies. Their tank space is commonly being used for water, diesel and drilling mud, if there is any available only 900M3 can be used as the regulations will change the supply vessel into a tanker. With skimmers recovering 200m3 per hour it will not take long to remove a vessel from the operation while it goes to discharge.
With this now in mind the use of removable ISO tanks on the decks of vessels make the logistics easier, the vessels continue working and a shuttle service to change full ones with empty ones using smaller faster vessels makes for a smoother operation. This also deals with the temporary storage problem while a final destination is found for the waste. The frame around the tank is very important as offshore crane operators are renown for banging equipment against other equipment before landing it.
In some cases as seen in the Weir boom photo above small tankers can be used if available. In some countries these vessels will have to be certificated for oil recovery work.
Also bear in mind that the connections need to be compatible ( Size and type).
High skimming rates are achieved by manufactures with 6" hoses and connections most tank connections are 4" of diverse types.
This 30,000ltr tank belongs to an oil company working in the Gulf of Mexico who have obviously been caught out in the past and are now prepared for the future.
Note the bracings welded to the tank and vessel. Welding container twist locks or their shoes to the deck makes it a safe cargo and easy to change out while at sea.
Not much can be said when public opinion over rules common sense
The "A Whale" or probably a better name would have been the White Elephant was a wonderful scam dreamt up by Taiwanese ship owner Mr. Nobu Su who had the ship delivered in January and had no work for it. It was sailed to
It was finally decided that the oil was not thick enough and the sea was to rough for the vessel to work.
If this photo above right shows a rough sea then she would really have a problem in the North Sea or in fact anywhere else in the world.
It was impressive to see such a scam taking up so much of the response team’s time. Just because a politician, Louisiana Governor (Bobby Jindal) though it was marvelous.