6. OFFSHORE RESPONSE. 6.1 Response Option Decision Process

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1 6. The prime focus of oil spill countermeasures activities is in prevention and planning. This is achieved through well-designed equipment, good maintenance and operating procedures, sound training techniques, and a high degree of awareness and concern at all levels by employees and management. Prevention and mitigation measures planned for the Project include: 24-hour manned automated monitoring, leak detection systems, visual monitoring, and emergency shut down systems for storage and transportation systems. Spill response equipment will be stockpiled at the FSO and other key locations. Despite best management practices an incident may occur. In the event of an incident the objective of the oil spill response is to assure that actions are efficient and compatible with the balanced environmental, social, and economic needs of the community. The response strategy includes all viable techniques to reduce damage from a spill. No oil spill response option would be ruled out or limited in advance. Spills of oil to the marine environment require immediate response action to stop the source of the discharge and to limit the spread of oil. Immediate response actions and notification procedures are outlined in Section 2. Attention must be paid to fire and safety hazards. This section provides an overview of response options used on the open water for a spill of oil into the offshore marine environment. A discussion of each response option is presented below. Shoreline protection and cleanup techniques are discussed in Sections 9 and 10, respectively. All protection and countermeasures options will be considered to ensure quick and efficient response in concert with NEBA guidelines. 6.1 Response Option Decision Process Offshore response options include: surveillance and tracking (see Section 5), mechanical containment and recovery (Sections 6.2 and 6.3), chemical dispersion (Section 6.4), and in-situ burning (Section 6.5). Decisions on the appropriate response options to use in open water will be based on an analysis of the rate and duration of the spill event, wind and sea conditions, weather, limitations of equipment and supplies, and the expected behavior of the spilled oil (see Figure 6-1). Each option has a specific function and is used for different spill situations. The NEBA process (see Section 1.3.1) will be applied to offshore spill response, nearshore response (Section 9), and shoreline cleanup (Section 10). For example, response to an offshore spill that has the potential to come ashore in a productive marsh or wetlands area would take into consideration options to prevent or reduce the impact to the marsh. Mechanical containment and recovery, dispersants, and in-situ burning are the options that would be evaluated in determining the response strategy that would result in the greatest net environmental benefit. These options may be considered individually, or in combination with each other to yield the maximum benefit. For example, to minimize impact to the marsh, dispersant use may be the preferred response option. Mechanical recovery, while it may be the least intrusive, is inherently inefficient and likely to allow oil to impact the marsh. Dispersants on the other hand, while they may have a short-lived negative effect on some surface-floating marine organisms, are much more efficient at removing the oil thus minimizing the impact to the marsh and resulting in maximum net environmental benefit. Section 06 Page 6-1 of 42 September 1999

2 Figure 6-1. Oil Spill Response Decision Tree Oil Spill Occurs Gather Spill Information Gather Environmental Information Evaluate Oil Characteristics and Behavior Gather Weather and Wave Forecasts Evaluate Spill location and Oil Movement Is Shoreline Contamination Expected? Implement Shoreline Protection Undertake Surveillance and Monitoring No Yes Perform Net Environmental Benefit Analysis Can Oil Type and Condition Be Chemically Dispersed? No Is Mechanical Containment Possible? No Yes Yes Is a Dispersion Operation Possible? No Is an In-Situ Burning Operation Possible? Is Mechanical Recovery Possible? Yes Yes Yes No No Implement Use of Dispersants Implement In Situ Burning Implement Mechanical and Containment Recovery Are Wildlife Contacted by Oil? No Yes Implement Wildlife Rescue and Rehabilitation No Are Shorelines Contacted by Oil? Yes Complete Actions Transfer and Storage of Oil Debris Perform Net Environmental Benefit Analysis Dispose of oil and oily Waste Document actions and Demobilize Implement Shoreline Cleanup Section 06 Page 6-2 of 42 September 1999

3 Level of Response The tiered response system is described in Section 2. Table 6-1 summarizes the offshore response options to be considered for Tier 1, 2, and 3 spills. Table 6-1. Offshore Response Option Matrix (Exxon, 1992) Response Spill Type Option Tier 1 Tier 2 Tier 3 Surveillance and Tracking Mechanical Containment & Recovery Dispersant Application In-situ Burning (naturally contained) In-situ Burning (contained in fire boom) (Oil Spill Response Vessel) (Oil Spill Response Vessel + Vessels of Opportunity) (local + national + international response equipment) (use fire monitor or spray booms to spray dispersant) (use aircraft to spray dispersant) Section 06 Page 6-3 of 42 September 1999

4 Table 6-2 summarizes practical guidelines, or rules of thumb to consider when implementing a response. Table 6-2. Response Options Rules of Thumb Booms Booms work for most oil types and large or small oil volumes. Containment is most effective when the booms can be accurately directed towards the oil; a boat is not a good place from which to locate the oil. Booms almost always leak, even under the best of circumstances. A boom is only as good as the crew that deploys and controls it. Booms are not a static piece of equipment; they require constant attention. Offshore containment costs can be high per barrel (but still considerably less than shoreline cleanup and resource damages). Skimmers Different skimmers work for different oil types. Skimmers are inefficient in rough waters. Oil recovery rate equals total volume recovered less the amount of water. Oil recovery cannot exceed storage capacity. Offshore recovery costs can be high per barrel. Dispersants Disperse when you can. Dispersion can be a very effective method for oil removal from a water surface. Environmental effects of chemical dispersion at sea are much lower than the effects of oil in the coastal zone. Chemical dispersion can be effective with minimal environmental effects in nearshore zone with good water (tidal) circulation. Dispersion does not preclude other actions taking place concurrently (i.e., mechanical recovery, in-situ burning). Aerial spraying can cover very large areas (tens of hectares/acres per minute). In-situ Burning Burn when you can: burning has a limited window of opportunity. Know which way the wind is blowing and find out about the forecasted winds. Air emissions (usually) are not a health and safety issue; fire and smoke only look bad. Burning does not preclude other actions taking place concurrently (i.e., containment and recovery). Burning is the simplest method and can be the most effective method for oil removal from a water surface: may be as much as 90% oil removal. Be safe; think what will happen as soon as the oil is ignited it will happen fast! Section 06 Page 6-4 of 42 September 1999

5 6.1.1 Site-Specific Conditions Weather and sea conditions in the offshore area near the FSO are normally relatively calm: currents are typically less than 0.16 m/s (0.3 knots), wave heights are less than 1.0 m (3.3 ft.), and wind conditions are light. Under these conditions, the application of conventional containment and recovery techniques may be possible, depending on the size of the spill. Chemical dispersion, however, will be the first response option considered. In rougher conditions, the application of dispersants might be the only response option that can be expected to protect the Cameroon shore since the use of booms and skimmers may be impractical. Figure 6-2 illustrates the appropriate ranges of oil thickness and sea conditions that each different response option can generally be employed (note that deviations from these guidelines are often possible). Figure 6-2. Range of Sea Conditions and Oil Thickness Versus Spill Response Options (Allen, 1988) Sea Conditions Wave Height (feet) (meters) mechanical recovery dispersant application in-situ burning Natural Degradation and Dispersion (Monitor and Wait) millimeters inches Average Oil Thickness Assessment and Monitoring The volume of oil spilled should be estimated using as many methods as possible, and then combining the results. These include: the rate of flow through a pipeline and the duration of spill before shutoff the size and number of cargo tanks breached on a tanker the color and size of slick Estimating oil volume based on the color and size of slick is often complicated by the complexity of slick(s) and its geometry and other factors. Observer experience will also have a Section 06 Page 6-5 of 42 September 1999

6 marked effect on the accuracy of estimates. The following rules should be considered when estimating slick volumes: Slick thickness varies considerably, especially if it appears dark brown or black; most of the oil is located in the darker areas of the slick. Colored or silvery bands indicate very thin slicks. Figure 5-1 (Section 5) can be used to estimate slick volumes based on their estimated area and appearance (color and sheen). Section 06 Page 6-6 of 42 September 1999

7 6.2 Containment Booms are typically the first mechanical response equipment taken to a spill site. They are used to contain or deflect slicks for removal by skimmers or burning, and/or to protect sensitive shorelines and amenities. Booms are manufactured in a wide variety of designs, sizes and materials for different applications. Table 6-3 compares four basic boom types and rates them under various operational criteria. Additional boom evaluation and selection criteria are provided in the Exxon Oil Spill Response Field Manual (see Section 5, Exxon 1992). Booms are stored in a variety of ways that include reels, containers, vessel decks and racks. In tropical climates as experienced in Cameroon, storage methods will be used that minimize the risk of mildew and other forms of fabric deterioration that can affect the use of booms. Storage options will be chosen that facilitate keeping boom clean and dry, out of direct exposure to sunlight, and away from activities that might damage it. Quick deployment will also be a criterion used in selecting storage sites and methods. For example, a boom reel placed on the aft deck of a vessel or on the FSO are possible boom storage options. Table 6-3. Boom Selection Matrix (after Exxon, 1992) Environmental Conditions Performance Characteristics Convenience Characteristics Legend internal foam flotation Type of Boom self-inflating pressureinflatable open water/offshore* protected water* fence calm water* high current (>0.5 m/s, or 1 knot) 2** shallow water (<0.3m, or 1 foot) operation in debris excess buoyancy wave response strength ease of handling ease of cleaning compactability good 2 fair 3 poor * open water/offshore wave height >1 meter (3 ft.) and current velocity <0.5 m/s (1 kt) protected waterwave height meters (1 3 ft.) and current velocity <0.5 m/s (1 kt) calm water wave height <0.3 meters (1 ft.) and current velocity <0.25 m/s (0.5 kt) Not all the booms of a particular type have the rating shown, but at least one or more commercially available booms of the type in question have the rating shown. ** Specially designed high-current river booms may be available. Section 06 Page 6-7 of 42 September 1999

8 6.2.1 Booming Floating Oil Boom containment of floating oil requires consideration of on-water conditions, weather conditions, available equipment, and booming strategy. The following discusses several considerations for boom lengths, applications, and configurations. Boom Lengths Before boom is deployed, the approximate length of the boom required should be assembled as completely as possible either on land or on the deck of a boat. Suggested lengths of booms for various applications are presented in Table 6-4. It is important to ensure that all boom connectors are compatible, especially when boom from multiple manufacturers are used. Table 6-4. Suggested Boom Lengths (after Exxon, 1992) Application Type of Boom Quantity circle a stricken vessel contain leakage from terminal operations use with an ocean skimmer protect entrance to estuary, stream, river, etc. bays, harbors, marshland offshore or harbor, depending on sea conditions calm water or harbor, depending on sea conditions offshore calm water calm water or harbor, depending on sea conditions 3 x (ship s length) 1.5 x (ship s length) m (2,000 3,000 ft) per skimmer 3 to 4 x (width of the body of water) (1.5 + current in knots) x (width of the body of water) Once the boom is ready, it can be launched and towed into position by a boat. Its final configuration can be arranged by setting suitable anchors or securing it to permanent anchor points. Where a boom is being used to collect oil or to protect a sensitive area, care should be taken to seal the shore end of the boom so that no oil can escape (see Section 8, Shoreline Protection). This is particularly difficult in tidal waters and at sites where the shore is rocky or strewn with boulders and crevices. Boom length may have to be modified after the boom has been deployed. This can be difficult to do from a vessel, particularly in strong currents, high winds, or low temperatures as loose shackles, bolts, and tools can be lost over the side. Often, boom length cannot be changed once the boom is in the water, and the boom must be retrieved, reconfigured, and redeployed. Section 06 Page 6-8 of 42 September 1999

9 Booming Configurations Figure 6-3. Catenary Booming (Chen, 1998) width = 1/3 length tow boat current Booms are used in various configurations to contain and recover slicks. Two vessels can tow a boom in a U-configuration to collect oil by drifting downstream, holding in a stationary position, or by moving upstream toward the spill source. tow boat Figure 6-4. V-Booming (Chen, 1998) tow boat Booms can be deployed in a V-configuration using three vessels and a skimmer. trailing boat/skimmer current tow boat Figure 6-5. J-Booming (Chen, 1998) tow boat Booms can be towed in a J-configuration that will divert the oil to a skimmer to allow simultaneous containment and recovery. current tow boat/skimmer Section 06 Page 6-9 of 42 September 1999

10 Table 6-5 shows each of the four basic boom types and their applicability in calm, protected and open water in various configurations. Table 6-5. Boom Type Versus Configuration (after Exxon, 1992) Boom Use Internal flotation Pressure Inflatable Selfinflating Fence calm water U/V J protected U/V water J open water U/V J Legend 1 good 2 fair 3 poor Boom lengths of m (1,500 2,000 ft) are typically used when towing boom in a U-, V- or J-configuration to maximize the oil encounter rate. Maneuverability also is improved with shorter boom lengths. The use of proper towing bridles or paravanes will minimize damage during towing by efficiently transferring the point load tension from the line to the connector. Towing devices also prevent the boom from twisting when being towed at high speeds. Lines between the boom ends and the vessels should be of sufficient length to avoid sharp stress or snatching on a towed boom. Approximately 60 m (200 ft) for 460 m (1,500 ft) length of boom is typical. When feasible, an odd number of sections of boom should be used to avoid having a connector at the apex from which oil more readily escapes. Oil concentration by towed booms can be slow in thin slicks. Boom performance can be judged at the apex of the U or J by eye. Oil lost under the boom will appear as globules or droplets rising behind the boom. Eddies behind the boom are also an indication that towing is too fast; however, sheens are usually present even when the boom is functioning well. Note that the apex of the boom is often unobservable from the wheelhouse of a towing vessel. Aircraft equipped with suitable air-to-sea communications can assist in controlling the movements and activities of vessels to ensure that they are operating in the heaviest concentrations of floating oil. Oil slicks can be more easily located from the air than from the water surface. The thickness and volume of a slick can also be estimated from an over-flying aircraft. Good communication between two towing vessels is required for them to maintain proper station (or position) relative to one another. It is an operation that improves with practice. Towing in a J-configuration is difficult with untrained crews. For maximum maneuverability at low speeds the ideal towing point aboard the vessel must be determined by trial-and-error and may need to be altered according to the wind and course direction. A towing point well forward of the stern is best. Boom Limitations When booms are placed at right angles to relatively high current (>0.4 m/s or 0.7 knots), oil can become entrained in the water passing under the boom. Oil losses will result (see Figure 6-6). Section 06 Page 6-10 of 42 September 1999

11 Figure 6-6. Entrainment (Chen, 1998) current > 0.7 knots In high currents, however, when placed at an angle to the slick travel, containment booms can be used to divert oil away from sensitive areas, or toward sacrificial areas for collection and recovery. This method is useful in currents up to approximately 1 m/s (2 knots). Table 6-6 shows boom angles and additional lengths of boom required to reduce the relative velocities of five different current velocities to operational levels. Table 6-6. Boom Requirements in High Currents (Chen, 1998) Current Velocity (m/s) (knots) Required Angle Extra Boom Required o 0% o 33% o 67% o 100% o 167% In some cases, it may be possible to divert oil using a single boom. In the example below (Figure 6-7), the diversion angle is approximately 60. Figure 6-7. Diversionary Booming (Chen, 1998) diversion angle m/s current recovery area Section 06 Page 6-11 of 42 September 1999

12 6.2.2 Booming Submerged Oil A number of subsurface barriers may be used to intercept, contain and/or recover oil that is below the water surface. These barriers generally are used to recover heavy oils, such as weathered, emulsified crude or tar balls; however, they will not recover light, low viscosity oils that can flow through the boom s mesh. Netting boom can usually be towed at higher speeds than conventional boom, i.e. at speeds greater than 0.5 m/s (1 knot). This may allow higher recovery rates, better steering control and continuous operation of the tow vessel (i.e., the vessel can maintain its speed without stopping and starting). Netting boom is also suitable for collecting oil along beaches and shorelines with significant wave action. This type of boom may be set up just below the high tide line to catch any weathered oil or emulsion that floats ashore, preventing the oil from penetrating into the soil below. The netting can be retrieved, cleaned and reapplied before the next tidal cycle, or it can be incinerated. Note that retrieval and cleaning of oiled netting is a messy task that usually requires the subsequent cleaning of vessels and crew. In the event that there is sufficient open water to deploy a boom, an oil trawl can be used to attempt collection of the submerged oil (Figure 6-8). Figure 6-8. Trawl Boom for Surface and Submerged Oil V-Sweep At a towing speed of 4 8 km/hour (2 4 knots), submerged and floating oil is forced into the net tunnels (one on each side), which extend 4 meters (13 feet) below the conventional containment boom. Oil then moves along the tunnels into a funnel located behind the apex of the boom. A series of up to 8 funnels, each containing 2 4 tonnes of oil, can be tied off, and removed when full. Other designs include a V-boom configuration with a mesh slung from the bottom connecting the two boom sides. 6.3 Mechanical Recovery Mechanical recovery, or the physical removal of oil from the environment, is the method that is usually perceived as the least harmful to the environment. However, mechanical recovery usually is able to recover only a small fraction of the spilled oil. Experience has indicated that recovery of more than 20% of the original spill volume is seldom achieved in marine spills. In fact, in open water under strong current and wind conditions, recovery of only 5 to 10% is not uncommon. Therefore, mechanical recovery is normally used in conjunction with other methods. In its simplest form, mechanical recovery relies on a skimmer capable of removing oil from the surface of the water and pumping it to a storage vessel for subsequent treatment and Section 06 Page 6-12 of 42 September 1999

13 disposal. Numerous devices are commercially available that employ a variety of oil pickup mechanisms. Oil spilled on water tends to spread rapidly to a thin sheen, thus reducing the potential skimmer-to-oil ratio (encounter rate), and makes mechanical recovery inefficient. To maximize the encounter rate, oil spill containment booms are deployed to concentrate the oil. Specific types of boom and skimmers are selected on a case-by-case basis, depending on the location of the spill, type of oil, environmental conditions, etc. In all cases, efficiency depends on oil/skimmer encounter rates, weather conditions, currents, sea state, and operator efficiency. Sufficient temporary storage for recovered liquid should also be planned. Decanted water will be put back inside the boom to maximize the use of available temporary storage Skimmers Skimmers can be grouped into four categories based on oil recovery principle. Each category contains various skimmer types that are distinguished by specific oil collection mechanisms (see Table 6-7). Most manufacturers produce a range of skimmer models or sizes designed around a single oil collection mechanism. Table 6-7. Skimmer Types (Exxon, 1992) Skimmer Category Examples weirs oleophilic surface skimmers simple, self-leveling, screw auger-assisted, stationary and advancing, and boom/weir systems drums, discs, ropes, belts and brushes; deployed independently, mounted on a vessel, or used with a boom hydrodynamic skimmers water jet, submersion plane and rotating vane other devices vacuum systems, air conveyor and paddle belt There are a number of skimmers in each of the four skimmer categories that might be applicable to the oil properties and sea conditions likely to be encountered in the event of a spill at the FSO. Table 6-8 is a simple matrix that can be used to select a skimmer best suited for a particular cleanup need. The matrix indicates the expected performance of various generic types of skimmers according to the operating environment, oil viscosity, and skimmer characteristics. Section 06 Page 6-13 of 42 September 1999

14 Table 6-8. Skimmer Selection Matrix (after Exxon, 1992) Generic Type of Skimmer Oleophilic Surfaces Weir Vacuum Units Hydrodynamic Devices Other Methods Brush Disc Rope Rope/Belt (catamaran mounted) Sorbent Belt (downward moving) Sorbent Belt (upward moving) Advancing Combination Weir/Boom Saucer Screw/Auger Evaluation Criteria Self-Leveling Vortex Vacuum System with Skimmer Head Hydrocyclone Submersion Plane Water Jet Combination Trawl /Boom Paddle Belt Available as VOSS Operating Environment Oil Viscosity Skimmer Characteristics (Vessel of Opportunity Skimming System) Open Seas Hs>3ft: V<1kt Harbors And Bays Hs<3ft; V<0.7kt Protected Inshore Hs<1ft; V<0.5kt High Currents <2kt Shallow Water (<1ft) Debris High Viscosity (>1000 cst) Medium Viscosity ( cst) Low Viscosity (<100 cst) O/W Pickup Ratio* Pickup Rate Ease of Deployment X X X X X X X X X X X Available as an Advancing Skimmer X X X X X X X X X X Available with Storage X X X X X X X X X X *O/W Pickup Ratio = % Oil in Skimmed Products Legend: 1- Good, 2- Fair, 3- Poor Hs- Significant Wave HeightV- current speed Section 06 Page 6-14 of 42 September 1999

15 Weir Skimmers A simple weir (Figure 6-9) skims the top layer of fluid into a hopper from which it is transferred by a reversible screw auger pump. Newer models use a self-leveling weir. The screw pump drives, and is cleared by, a rotating scraper. The tightly fitted scraper seals the screw and creates a positive head at the pump discharge. Figure 6-9. Screw Auger Weir (Exxon, 1992) float weir lip auger scraper screw auger opening floating weir discharge hose Expected Performance low viscosity oil medium viscosity oil high viscosity oil mode sea state debris tolerance % Oil recovery fair good good stationary calm good fair Skimmers outfitted with screw auger pumps can recover heavy, viscous oils; however, oil must flow readily for these skimmers to function well. They are also suitable for processing debris. Some heavy (viscous) oils may not flow freely over the weir. High percentages of water pickup (70 90%) should be expected and temporary storage/separation equipment must then be considered. Advantages can operate in shallow water capable of pumping highly viscous oils and debris, e.g., seaweed, wood chips, etc. screw auger pump neither requires priming nor forms oil/water emulsions easily deployed and operated Disadvantages limited to calm and protected water with heavy, viscous oils and calm water with light oils manual push of heavy oils sometimes required over the weir lip if they do not flow over it can develop high back-pressure in the discharge line high water recovery Section 06 Page 6-15 of 42 September 1999

16 Oleophilic Drum One or more oleophilic drums (Figure 6-10) are rotated downward into the oil, driven by hydraulic, pneumatic, or electric motors. The recovered oil is then scraped off the drum(s) into a trough and, in some models, a sump. Either external or onboard discharge pumps are available. Some, newer skimmers also feature a mechanism to allow the operator to adjust the drum submersion depth. Figure Drum Skimmer (Exxon, 1992) scraper oil water scraper suction hose rotating drums frame low viscosity oil medium viscosity oil high viscosity oil Expected Performance mode sea state debris tolerance % Oil recovery fair good fair stationary calm/protected fair good Rotational speeds of approximately 40 rpm result in the maximum recovery rate of medium viscosity oils; however, reducing the rotational speed to 20 rpm generally improves the oil content in the recovered liquid by 10% (but lowers the recovery rate by 50%). Waves often significantly decrease the recovery rate of self-contained (i.e., not vesselmounted) drum skimmers. Calm water operation is recommended. Advantages small units can be operated in shallow water simple design/good reliability often compact including onboard pump many models can be lifted by 2 people high oil/water pickup ratio can tolerate some debris can be used in calm, harbor and some offshore applications Disadvantages limited to calm and protected water will not recover solidified/ highly viscous oils due to pumping, oil flow problems Will not recover dispersant-treated oil Reduced recovery rate in thin slicks can occur if drum water wets limited capacity sump large units can be relatively expensive Section 06 Page 6-16 of 42 September 1999

17 Vertical Oleophilic Rope Mop Single or multiple polyethylene fiber ropes (Figure 6-11) are pulled through a slick by wringerrollers. The wrung rope mop is continuously returned to the slick, repeating the cycle. Recovered oil is collected below the wringer assembly or pumped via a suction hose. Some models require a return pulley while those operated vertically are simply suspended above the slick so that the rope mop contacts the oil. Figure Vertical Rope Mop Skimmer (Chen, 1998) rope wringer suction hose oily rope clean rope low viscosity oil medium viscosity oil high viscosity oil Expected Performance mode sea state debris tolerance % Oil recovery fair good poor stationary all good good Rope mop skimmers generally function best in warm weather (>20 o C or >68 o F) in medium viscosity oils that adhere to the mop. The oil content in the collected liquid reduces as mop speed exceeds about 0.4 m/s (1.25 ft/s). Recovery in low waves of cm (1 2 feet) should be possible. If oil is emulsified and viscous, rope mop strands can mat together and jam the wringer assembly, e.g., Bunker C, aged crude. Low pickup rates of light oils, e.g., diesel, are also common. Advantages effective in calm, protected and open water can operate in any water depth (and over dry patches) good pickup rate wide, effective reach can tolerate most debris rope can recover oil in low currents Disadvantages ropes and wringer-rollers will wear if oil is mixed with sand inefficient unless oil is confined or pooled attachment points/lines required for tail (return) pulley (horizontal models only) tail pulley may have to be repositioned in tide changes not effective in highly viscous oil Section 06 Page 6-17 of 42 September 1999

18 Oleophilic Brush Closely spaced brushes pick up oil (Figure 6-12) that is then removed by a comb-like scraper before being conveyed to storage. On some smaller models, the brushes are mounted on a drum; most larger models employ multiple linear chains deployed from the side (as shown below) or bow of a dedicated vessel or a vessel of opportunity. Figure Oleophilic Brush Skimmer (Exxon, 1992) brushes scraper support vessel recovered oil recovered oil water low viscosity oil medium viscosity oil high viscosity oil Expected Performance mode sea state debris tolerance % Oil recovery poor good good advancing all good fair Brush skimmers function optimally in medium and, particularly, high viscosity oils provided that a suitable pump is used to transfer the latter to storage. Brush drums operated at approximately 20 rpm can achieve highest oil recovery rates in viscous products. However, the percent oil recovered in the collected liquid is highest at 5 to 10 rpm. The higher oil content is achieved at the expense of oil recovery rate, which decreases in direct relation to the decrease in rpm. Linear brushes work best at 0.3 m/s (1 fps). Their wave tolerance is generally good since water flows through the brushes. However, response of the entire skimming system depends on the sea-worthiness of the working platform. Excessive movement of the brush packs in waves due to vessel pitch and roll reduces oil recovery rate, particularly if they are bowmounted. The recovery of light oils, e.g., diesel, is not effective with most standard brushes due to low recovery rate and percent oil recovered. Advantages suitable for weathered or emulsified oil effective oil removal comb/scraper system relatively simple mechanical design flow of oil into brushes not affected by waves if units are mounted on stable vessels can tolerate some debris Disadvantages low pickup rate of light oil oil losses under booms of side collectors possible bow-mounted collectors affected by vessel movement/ bow wave interference require relative movement between skimmer and oil Section 06 Page 6-18 of 42 September 1999

19 6.3.2 Pumps Pumps are used during oil spill response to transfer oil, water, emulsions and dispersants. Recovered liquids typically need to be transferred from: (1) a skimmer to an interim storage device (2) interim storage to a larger storage/separation or transportation vessel (3) a transportation vessel to a final storage/disposal facility Preplanning recovered oil transfer is critical to ensure the continuity of OSR operations. Transfer equipment must be selected to suit the quantities and types of liquids being moved. Although a wide range of pumps can be used for fresh, unemulsified oils, as transfer conditions become more difficult, pump options can become limited. Careful consideration must therefore be given to each specific transfer situation, particularly in the case of long-term mechanical recovery operations when, over time, oil weathers, viscosity increases, and debris is collected. Pumps may also be used to offload oil from stricken vessels (called lightering ) and to transfer dispersants from drums and other containers to dispersant application systems. Generally, spill cleanup does not require pumps with extreme capabilities. The hydrostatic head through which the pump must push liquid is usually about 2 to 6 m (6 to 20 ft) and suction lift from skimmer to pump is often much less than that (i.e., only a few feet or about a meter). In some cases a large hydrostatic head is required, especially when oil is pumped from a skimmer to a large, unballasted barge or storage vessel. In this case the head required may be 10 m (30 ft) or more. Some pumps are not suitable for oil spill work for the following reasons: They neither self-prime nor maintain prime when the skimmer rolls. Suction capacity is limited. Pumping capacity decreases even with slight increases in oil viscosity. Cavitation occurs in warm or high viscosity oil. Emulsification of oil and water occurs. Debris blocks the pumping mechanism. Damaged is caused by running dry. Four pumps that are both suitable and commonly used for spill cleanup are: centrifugal peristaltic screw/auger reciprocating (diaphragm) Section 06 Page 6-19 of 42 September 1999

20 Centrifugal Pumps Figure Centrifugal Pump (Exxon, 1992) suction port discharge port Operating Principle Liquid enters the centrifugal pump (Figure 6-13) at the center of a rapidly rotating fan-shaped impeller. Centrifugal force then accelerates the fluid toward the impeller s outer edge. From there the fluid exits through a nozzle on the periphery of the impeller housing. The pressure generated by the pump results from the kinetic energy imparted to the fluid by the impeller. Suitable Uses pumping low viscosity fluids at high rates for short distances supplying water to dispersant spray booms or fire nozzles flooding shoreline with seawater to prevent oil from sticking to soil unloading drums of chemicals, fuels, etc. mixing demulsifier chemical into emulsified oil Advantages small, lightweight, easy to handle high capacity with low viscosity fluids inexpensive considering output capability mechanically simple (one moving part) tolerant of most debris easily repaired in the field Disadvantages output decreases markedly with increasing viscosity of fluid many models are not self-priming oil/water can be emulsified pump performance impaired by debris Section 06 Page 6-20 of 42 September 1999

21 Peristaltic Pumps Figure Peristaltic Pump (Exxon, 1992) Operating Principle The pumping action of a peristaltic pump (also called a hose pump ; Figure 6-14) results from alternate compression and relaxation of a specially-designed resilient hose. The hose is compressed between the inner wall of the housing and the compression shoes of a rotor. A liquid lubricant in the housing minimizes sliding friction. The fluid being pumped is in contact only with the inner wall of the hose. During compression, abrasive particles in the fluid are cushioned in the thick inner hose wall returning to the fluid stream after compression. The pump has no seats, seals or valves. It is self-priming and can be run dry without damage. Suction, even for medium viscosity materials, is generally excellent. Suitable Uses offloading emulsions from skimming vessels or oil storage barges moving low and medium viscosity products. Advantages can be run dry without damage can process low to medium viscosity oils self-priming can pass most debris up to 1 in. (25 mm) easily repaired in field flow reversal can be used to clear blocked hose Disadvantages have pulsating flow internal hose wears and can require frequent replacement speed control is overly simple vacuum on suction line can be lost on medium viscosity oils Section 06 Page 6-21 of 42 September 1999

22 Screw/Auger Pumps Figure Screw/Auger Pump (Exxon, 1992) Operating Principle Oily material is gravity fed to the screw through a large hopper (Figure 6-15). As the screw rotates, it carries the oily mixture forward until a special rotary lobe scrapes the oil from the groove and forces it out the front of the pump. Debris that fits between the threads of the screw is processed. A special cutter at the edge of the hopper can chop up long stringy debris so it can also be pumped. Because of its low rotational speed and the relatively loose clearance between screw, housing, and lobe, the pump has very little self-priming or suction capability. Suitable Uses pumping weathered crude or mousse offloading recovered oil from storage barges transferring contents of earthen storage pits to incinerator Advantages can pump highly viscous or semi-solid materials processes most debris does not emulsify oil/water some models can be run dry some models are integral with a weir skimmer Disadvantages inefficient with light fluids (water) at high discharge heads not self-priming poor suction/lift capacity relatively low pumping rate for its power relatively expensive for capacity can develop high back pressure Section 06 Page 6-22 of 42 September 1999

23 Reciprocating (diaphragm) Pump Figure Reciprocating Pump (Exxon, 1992) Operating Principle A drive rod moves a diaphragm to one side of its housing, creating a vacuum in one chamber of the pump and pressure in the other (Figure 6-16). The vacuum opens one inlet valve and closes an outlet valve. When the rod reverses and moves a second diaphragm to the other side of the housing, the action of all valves is reversed. Since liquid moves to the discharge port on both forward and reverse strokes of the rod at the same speed as the pump motor, a continuous discharge results. Suitable Uses offloading low and medium viscosity oils from skimming vessels or barges injecting picked-up oil into incinerator devices Advantages can pump low to medium viscosity fluids at high pressures strong self-priming capability processes debris can be run dry for extended periods Disadvantages some models tend to walk if not tethered rubber diaphragms wear when exposed to abrasives is not equipped with quick-connect fittings Section 06 Page 6-23 of 42 September 1999

24 6.3.3 Temporary Storage Since oil recovery operations may not be conducted near permanent waste storage facilities, temporary storage options must be considered as part of the response planning process. The type and quantity of storage devices required will depend on the size of the spill and the expected performance of mechanical recovery systems. Storage needs may be met by various resources (barges, vessels, tanks, etc.) already available or by purpose-built storage units procured to meet specified technical requirements. Generally, temporary storage will be required to facilitate the ongoing collection, containment and transfer of oily wastes. Common applications of temporary storage include: 1) Medium-capacity marine storage (e.g., towable tank) to contain liquid recovered by a skimmer. 2) High-capacity storage (e.g., barge) to store liquid collected by multiple skimming systems and transfer to land-based separation or disposal facilities. 3) Land-based storage (e.g., open stationary tanks) for collecting wastes generated from shoreline cleanup and shore-based oil recovery operations. Types of Temporary Storage Devices There are many temporary storage options. Commercial products specifically designed for oil spill response are detailed in the World Catalog of Oil Spill Response Products. One should also consider other general purpose devices to meet temporary storage requirements. Examples of both categories of storage devices are indicated below, and are described in detail in the Exxon Oil Spill Response Field Manual (Exxon, 1992, 1999). Barges Barges offer the best temporary storage capability during open water spill response but are sometimes difficult to procure within the first few hours of spill response. Operations may consider the pre-arranged use of: tank barges deck barges with deck tanks hopper barges supply boats with deck tanks Towable Tanks Flexible, towable tanks (Figure 6-17, right) are purpose-built storage units designed for small and medium size spills. They provide rapid deployment and superior maneuverability at a lower cost than barges. Basic designs include: tubular tanks open tank barges flat tanks Section 06 Page 6-24 of 42 September 1999

25 Stationary Tanks Stationary strorage tanks (Figure 6-17, left) are typically used for on-shore temporary storage of both oily liquid and solid wastes; some stationary tanks can also be mounted on vessel decks for water-based cleanup operations. There is a wide range of options to choose from although commonly used devices include: Purpose-Built open, (frame-based) pools open, inflatable pools collapsible, (pillow-type) tanks General Purpose deck barge with deck tanks 55-gal oil drums vacuum or air conveyor truck tank truck pick-up or dump truck containers of opportunity (e.g., livestock tanks, fish boxes, garbage skips, etc.) plastic swimming pools plastic trash bags or super sacs plastic-lined earthen pit or dike Figure Examples of Land-based Storage (below left) and Floating Storage (below right) (Chen, 1998) brass grommets provided for lashing line corner hinges cover tank drain tube side hinges towing line hull/flotation chamber Section 06 Page 6-25 of 42 September 1999

26 Factors Affecting Storage Selection The following factors will be considered when determining temporary storage needs: Storage Location All likely offshore, nearshore and onshore storage requirements will be determined in relation to likely cleanup sites. Intertidal areas will not be designated for interim storage. Storage devices may be swept away or storage areas flooded during high tides. Precipitation Storage facilities will be planned so that rainwater and runoff do not affect either the initial or longer-term placement of materials. For example, open oily waste storage will be covered during rainfall to avoid an increase in waste volume. Required Capacity The size and location of possible spills will be considered where mechanical recovery will be likely. The selected skimming rate of the system, expected water uptake (due to emulsification) and free water will then be considered. Decanting water and offloading turn-around times must also be considered. Type of Material Generally, open-topped metal containers are used for solids while both open and closed tanks are used for storing liquids. Storage Duration Service time will be estimated that will be required for temporary storage facilities (hours, days, weeks, or months). Disposal The disposal method will be considered, e.g., incineration, landfill, landfarm, reprocessing/recycling, biodegradation and associated need to separate wastes. Transport Temporary storage devices will also serve as transport containers to reduce waste handling and the risk of further spills due to transfer operations. However, towable tanks are less likely candidates because they can flex when towed at excessive speeds resulting in spills. Loading/Unloading Ease of loading and unloading will be considered. For example, towable bags can be difficult to unload if there is debris mixed in with the recovered fluid. Safety The possibility of a buildup of flammable gases in an enclosed tank will be examined. Biogenic gas can also be generated from recovered fluid if not vented properly. Sulfur is not a constituent of the crude oil that will be produced. Legal Local, regional and national regulations pertaining to hazardous waste storage and disposal will be considered. Certification When selecting an oil spill response barge, the vessel will be certifiable for petroleum storage and transport, and will be in a safe, sound and seaworthy condition. Section 06 Page 6-26 of 42 September 1999

27 6.4 Chemical Dispersion Chemical dispersants are used to break oil slicks into fine droplets that then disperse into the water column. This prevents oil from being driven by wind and currents toward shore and promotes its biodegradation at sea. Key aspects of dispersant use are listed below. Dispersants should be considered for use with other potential spill response methods and equipment, and not as a last resort. For maximum effectiveness, dispersants should be applied as soon as possible after a spill. During the early stages of a spill, the oil is unweathered and less spread out, making it easier to disperse. The decision on whether or not to use dispersants should be made after considering the potential effects of dispersed oil versus undispersed slicks. The objective should be to minimize ecological impacts and maximize net environmental benefit. If possible, the on-scene-commander should consult with technical advisors who can provide insights into the area s ecology and the advantages and disadvantages of using dispersants there. The use of dispersants requires logistics planning including aircraft and/or vessels, application gear, the re-supply of dispersants, refueling, and food and accommodation for crews Suggested Dispersants Three dispersants manufactured by Nalco/Exxon Energy Chemicals (NEEC) are candidates for use on oil spills. They are COREXIT 9527, COREXIT 9500, and COREXIT All products are solvent-based concentrate dispersants, which may either be applied in their undiluted (neat) or diluted form. All dispersants are effective in the earlier stages of response. COREXIT 9500 is more effective on viscous, weathered spills. If comparable brands of dispersants are used, information concerning applicability, toxicity and effectiveness should be obtained from the manufacturer Dispersant Use Guidelines Dispersants can minimize the amount of oil reaching sensitive and biologically productive habitats such as: mangroves salt marshes coral reefs kelp beds Dispersants can also reduce impacts by lowering the adhesive properties of the oil. Whatever net environmental benefit is achieved, it is best to disperse oil some distance before it approaches important ecological habitats. For situations where this is not possible, consult the guidelines for using dispersants near specific ecological habitats discussed below. In all cases, care should be taken to avoid overdosing. Dispersant use for cleanup of oiled shorelines is discussed in Section Section 06 Page 6-27 of 42 September 1999

28 Kelp Beds Dispersant use is recommended for protecting kelp beds, which occur most often on rocky subtidal habitats. Dispersants should be used only where there is sufficient water circulation and flushing for dilution. Seagrass Beds Dispersant application can protect seagrass beds in the intertidal zone. The use of dispersants over shallow, submerged seagrass beds or in areas of restricted flushing is generally not recommended but should be weighed against the consequences of untreated oil stranding on shore. Coral Reefs Dispersant use should be considered in waters near coral reefs to prevent oil from contacting the reefs. Applying dispersants to oil directly over shallow, submerged reefs is generally not recommended, particularly if the water exchange rate is low such as in lagoons and atolls. Nearshore Subtidal For nearshore zones of sandy and gravel/cobble beaches and for enclosed bay habitats, dispersant application is a possible option only if there is sufficient water circulation and flushing capacity for dilution. Tidal Flats Dispersant use can be considered near shore in shallow water since little dispersed oil is incorporated into the sediments and, therefore, little biological impact can be expected. Dispersants should not be applied to oil that is stranded on tidal flats. Salt Marshes Salt marshes can be protected by applying dispersants offshore or to open water and channel areas in the marsh, but not to the marsh surface itself. If the oil slick is approaching the marsh, dispersants should be applied as the tide starts to rise, if possible. This will maximize dispersion of oil. Dispersants should not be used in marshes that cannot be flushed. Mangroves Dispersant use is a preferred technique for protecting mangroves because of the persistence and lethal effects of non-dispersed oil on the roots. If a mangrove forest with little tidal range is impacted by dispersed oil, attempts should be made to flush it out as soon as practical. Marine Birds and Mammals Dispersant use is recommended to prevent oiling of marine birds and mammals by oil slicks. Direct spraying of dispersants on the animals should be avoided Dispersant Application There are two basic methods used to apply dispersants: workboat application aerial application Dispersant treatment of spills should proceed in a planned manner (see Figure 6-18). The objective of treating a spill with dispersants should be to prevent the intact slick from reaching a shoreline or sensitive area. Normally treatment should begin at the outer edges of the thicker parts of the slick rather than through the middle or on very thin sheen surrounding a slick. If the Section 06 Page 6-28 of 42 September 1999