OIL SPILL SCIENCE chapter 16 – a practical guide to chemical dispersion for oil spills

OIL SPILL SCIENCE chapter 16 – a practical guide to chemical dispersion for oil spills OIL SPILL SCIENCE chapter 16 – a practical guide to chemical dispersion for oil spills OIL SPILL SCIENCE chapter 16 – a practical guide to chemical dispersion for oil spills OIL SPILL SCIENCE chapter 16 – a practical guide to chemical dispersion for oil spills OIL SPILL SCIENCE chapter 16 – a practical guide to chemical dispersion for oil spills OIL SPILL SCIENCE chapter 16 – a practical guide to chemical dispersion for oil spills

A Practical Guide to Chemical Dispersion for Oil Spills

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Appendix B Nomograms toCalculate Spreading andViscosity with Time

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16.1 INTRODUCTION AND DECISION MAKING

Dispersants are recognized as an alternative countermeasure for controlling oilspills on water When used properly, with certain oils, and under the rightconditions, chemical dispersants can rapidly reduce the oil on the surface ordivert surface oil from following undesirable trajectories Chemical dispersioncan shorten the response time to an oil spill, thus reducing the chances that theoil will move further on the water surface and thereby protecting sensitiveareas Rapid dispersion of oil can prevent the oil from reaching shorelines,which are difficult to clean and where the greatest environmental damagecaused by oil spills occurs

Dispersants are surfactant formulations that create small oil droplets thatmove into the water column They are applied to achieve an approximatedispersant-to-oil ratio of 1:15 to 1:25, although slick thickness is hard to judgeand no measurement technique is available Although boats and ships havehistorically been used for this purpose, their ability to treat large areas is veryrestricted Today, dispersants are usually applied from low-flying aircraft.Spray units have been designed for many platforms and are available as eitherpermanent or temporary installations on whatever platform is available As

Oil Spill Science and Technology DOI: 10.1016/B978-1-85617-943-0.10016-4

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dispersants are applied undiluted from aircraft and are usually diluted withseawater when sprayed from boats, quite different equipment is required for thetwo application platforms.

Effectiveness is a primary concern when using dispersants It has beenfound that their effectiveness depends primarily on the type of oil beingdispersed and then on the amount of sea energy at the time of the dispersaloperation The primary factor affecting the ability of an oil to disperse is itscomposition The lighter the oil, the better it disperses Heavier componentssuch as resins, asphaltenes, and waxes do not disperse Heavy residual oils such

as Bunker C, which consist mostly of these heavier components, disperse little,

if at all A minimum of sea energy is required before dispersants function Thehigher the sea energy, the more effective is the dispersant As oil weathers, itsmore dispersible content decreases and its viscosity increases, making it moredifficult for the dispersants to mix with the oil Heavy oils and highly weatheredoils may not disperse at all under certain conditions Light oils will dispersewell and, if left untreated, may also disperse naturally It is important torecognize that not all oil that is treated will disperse; there will always besome residual slick This certainly must be weighed in considering responseoptions

It is also very important to recognize that the dispersants are a temporarymeasure Much of the oil dispersed will resurface within 12 hours Thus if thetrajectory of the surface slick is the same as the subsurface slick, the overallbenefits of dispersant treatment will be nullified

The use of dispersants is a trade-off between a number of factors, such asprotecting shorelines and birds versus possibly adversely affecting fish In mostcountries, the use of dispersants is tightly regulated by government agencies.The decision process involves assessing the current situation and the protectionpriorities such as fish, shoreline types, and special habitats Other factors thatmust be assessed include the probable effectiveness of the dispersant on thetype of oil spilled, the temperature and salinity of the waters, and the ability todisperse significant amounts of oil with the equipment and dispersant available.The steps involved in chemical dispersion are shown in Figure 16.1 Howdispersion decision making proceeds is outlined inFigure 16.2 These figureswill be highlighted in the discussion that follows

16.1.1 An OverviewdHow, When, and Where Dispersants Are Used

Dispersants are considered an alternative countermeasure for oil spills, alongwith the many other countermeasures available Often, several countermea-sures should be applied at one time Dispersants are a control tool and do notremove oil from the environment Thus they are used when oil spills areapproaching sensitive areas that require quick protection When consideringthe use of dispersants, the proximity of the dispersant application to sensitive

marine environments, such as mangrove forests, sea grasses, marshes, andcorals, is an important factor.

Decisions about the use of dispersants must be based on a net environmentalbenefit analysis of use versus nonuse of dispersants as discussed in Section16.1.2 The potential environmental impact of the trajectory of the dispersed oilplume should be considered in addition to the environmental impact of theundispersed oil plume

Oil spill occurs

 Sea energy is sufficient

 Plans are in place

 Equipment is obtainable

Implement equipment

deployment plans

Implement health and safety plans

Brief all personnel on deployment

and health and safety plans

Transport supplies and personnel

Begin dispersant operation

Conclude operation

Return equipment

Assess and report results

Monitor dispersant operation

FIGURE 16.1 Steps in chemical dispersion.

Many jurisdictions impose regulatory limitations on the water depth to whichdispersants can be applied (3 to 30 m) Dispersants are best applied in deepwaters and not close to sensitive resources Applying dispersants to preventoil from entering the sensitive habitats should be considered to minimizeenvironmental impact Dispersants should not be used, however, to remove oil

Yes

Yes

wind <20 m/s (40 knots), wave height <3 m , but > 0.6 m and no other conditions unsuitable for dispersant operations

emulsification does not occur thickness > 1mm , and viscosity < 5000 mPa.s

No

No

Begin the implementation of the dispersant operation according to plan

FIGURE 16.2 Decision flowchart for chemical dispersion.

adhering to mangroves or shorelines Instead, shoreline cleaners or washing agents should be considered.

surface-It is important to carry out the preplanning noted in this document beforeapplying dispersants The following are the minimum conditions that should bemet when using dispersants:

1 Fundamental conditions

a) There must be a positive potential benefit to using dispersants

b) There must not be sensitive environments that could be impacted.c) The operation should be feasible, with enough equipment and dispersant

to produce a beneficial result

d) The oil should be dispersible and in dispersible condition, for example,not highly weathered or in the form of a stable emulsion

e) The oil should be heading toward an area or environmental habitat thatneeds to be protected

f) The water should be of sufficient salinity (>25%) to allow for effectivedispersion

g) The effectiveness should be considered, as well as the effects of theremaining oil on the surface, which will always be present

h) The inevitable resurfacing of the dispersed oil must also be considered

c) The dispersion will result in a net environmental benefit as described inSection 16.1.2

Dispersants are generally applied when the above conditions exist They areapplied directly to the oil using application equipment either from boats, ships,aircraft, or helicopter buckets Plans for the application must be developed inadvance to ensure safety and the efficiency/effectiveness of the operation Theitems to be considered in this preplanning stage are listed inTable 16.1

16.1.2 Net Environmental Benefit Analysis

A Net Environmental Benefit Analysis or NEBA is used to evaluate the sion to use oil spill dispersants The concept is that the total benefit of applyingdispersants is evaluated compared to the potential damage that would occur

deci-if they were not applied It is important to stress that at this time NEBA is more

of a concept than a developed tool The NEEBA, or Net Environmental andEconomics Benefit Analysis, is a variation on this theme and adds the concept

of economics to the vector It should be stressed that this concept has been evenless developed as a tool than the basic NEBA

The sequence of analysis is intended to proceed from the concentrations ofoil expected under the slick, the toxicity of these diluted fractions to the localflora and fauna, and a comparison of this with the distribution and fate of the oil

if not treated It is currently proposed that models be used to generate this type

of information and sometimes even the economic analysis that flows with it.The following is the scenario for a full NEBA/NEEBA analysis

1 Generate the dispersant application and countermeasures scenarios This istypically carried out using computer analysis and includes:

l Spread, thickness, and geographical prediction of oil deposition,including weathering and chemical composition;

l Application scenario, including dosage of the various amounts, tion of effectiveness, and deposition of unburned fractions;

predic-l Very importantlydconcentrations of the oil under the slicks and theirfate and movement;

l Effectiveness of mechanical and other countermeasure scenarios; anddeposition and movement of the oil if no countermeasures were carried out

A NEEBA analysis would include the economics of the three componentsddispersant application, no-countermeasures, and the normal mechanical removalefforts The dilution of dissolved and dispersed hydrocarbons can be calculated.Dispersant scenarios should be calculated with the variances in the dispersibility

of the oil It is typical to calculate all the dispersant scenarios, with effectivenesshalf of the predicted amount and some percentage above to yield three scenarios

2 Generate ecological-impact scenarios for mechanical countermeasuresonly, other forms of countermeasures that may be considered orapplieddin suites or separatelyddispersant-only, and no countermeasures

TABLE 16.1 Preplanning for Dispersant Application

1 Environmental Resources

Sensitivity mapping coral reefs mangroves shallow areas endangered species species at risk

3 Oil Types Tests with oil types Effectiveness scenarios Weathering profiles Time windows

2 Dispersants and Equipment

Dispersant stocks Equipment for this Dispersion planning Dispersion scenarios

4 Preplanning Preplanned NEBA Preplanned conditions

Again, economics can be included to yield a cost analysis The impactscenarios will include the toxic effects on key species in the affected areas,the variances in trajectories, and a sensitivity analysis of the predicted oilconcentrations under the slick The predicted recovery of species is alsomodeled.

3 The outputs of the total process include information on the effects for eachcountermeasure scenario, including the effects of undispersed oil or oil thatwas not subject to the dispersant process These effects can then be evalu-ated in terms of their ecological value The social costs and benefits are alsoevaluated at this stage

A NEEBA analysis would include an evaluation of the economic costs andthe benefits of each countermeasure scenario

The sequence of NEBA planning is listed inTable 16.2

16.1.3 Scenarios For Which Dispersants Might Be Used

It is important that scenarios for dispersant application be developed for theregions of concern Typical scenarios for situations in which dispersants might

be used are listed in Appendix A

16.1.4 Planning Process and Checklists

Planning begins with assessing the environmental resources in the area as listed

inTable 16.1 After the sensitive areas are mapped, focus is then on the availabledispersants and application equipment, which will dictate the capability tochemically disperse oil slicks If the equipment is available, planning thenproceeds to scenarios such as illustrated in Appendix A This is tied together withthe NEBA analysis as summarized in Table 16.2 It is important to set thepriorities for chemical dispersion in line with the priorities for environmentalprotection

Planning then continues with examination of the oil types that may requiredispersion in the region Effectiveness scenarios are set for the oils, along withthe weathering profiles, and the resulting time windows for effective chemicaldispersion This is readily accomplished by referring to Appendix B Thisappendix provides tables and nomograms that give values or estimations ofthe spreading area, thickness, oil viscosity, dispersant needed, and the hours ofoperation for several dispersant application platforms for 100, 1000, and 10,000ton scenarios and for 12, 24, and 48 hours The final step is to compile all theinformation into preplanned NEBA and preplanned scenarios so that decisionscan be made quickly

Oil spilled on water undergoes several changes with time, and this should betaken into account when planning dispersant operations The processes thatcause these changes include emulsification, evaporation, oxidation, spreading,and natural dispersion In order to determine the effectiveness of a dispersant

operation for a particular oil slick, it is important to understand how theseprocesses change the properties of spilled oil and ultimately affect the oil’sability to disperse.

As time progresses, the slick becomes increasingly thinner on the water Alight crude oil will be an average of 0.5 mm thinner after about 48 hours,

a thickness that is nearly impossible to treat with dispersants For practicalpurposes, 1 mm will be taken as the practical limit in this guide

TABLE 16.2 Sequence of Analysis in NEBA

Direction of surface slick

Amount Fate modelling Direction of plume/

dispersed oil Location

Oil type Weather and predicted weather Oceanographic information

available

Dispersant modelling

Dispersed oil concentrations Dispersant operation

information

Countermeasure modelling

Social analysis Social costs and benefits

of each scenario

The formation of emulsions significantly changes the properties and acteristics of oil spills Stable emulsions contain from 60 to 80% water, thusexpanding the original volume of spilled material by two to five times Mostsignificantly, the dynamic viscosity of the oil typically changes from a fewhundred mPa.s to about 100,000 mPa.s, a typical increase of 1000 A liquidproduct is thus changed to a heavy, semisolid material For practical purposes,once an emulsion has formed, the oil is considered to be nondispersible.

char-16.2 HOW DISPERSANTS ARE USED

The primary method for applying dispersants is to spray the dispersant onto theslick as soon as possible after the spill The dispersant must be applied ontoslicks before they become too thin and before the oil weathers excessively.Studies have been done on the application of dispersants directly into leakingtanks This has been largely abandoned because analysis of case histories showsthat very little opportunity for such application actually exists Furthermore,implementing such a method with a stricken tanker would probably be moredifficult than to conduct a spray operation

Dispersants were first applied on oil spills using boat-mounted spraysystems In the early 1970s, it was realized that small boats with a spray width

of about 10 m could not deal with a very large amount of oil Both large systemsfor larger boats or small shipsdthe size of supply boatsdand spray systems foraircraft were developed Smaller vessels are rarely used today for application.While there are spray systems for applying pesticides, spray systems fordispersants must be designed quite differently as spray volume is generally 10

to 50 times greater Most pesticide systems are designed to apply pesticide as

a fine spray or mist with droplet sizes from about 50 to 200 mm, whereasdispersants are best applied at 400 to 700mm These droplets are large enough

to result in good deposition rather than being blown away and small enough thatthe dispersant droplets do not break through the oil slick

The physics of the system is such that dispersants must be diluted in order to

be sprayed from a slow-moving ship, whereas they are applied “neat” fromaircraft Applying dispersants in neat form is thought to be best because, whendiluted with water, dispersants may not repartition to the oil phase and could belost to the water column Mixing seawater on vessels requires apportioningpumps or devices to ensure a consistent mixture of oil and water There areseveral practical references and standards on the design and calibration of suchsystems

The advantage of aerial spray systems is that, in theory, they can cover

a large area A concern is that of the desired droplet size All past work wasbased on the premise that a 400 to 700mm droplet size was best for deposition.The effect of droplets in this size range on spills has not been determined Spillsspread rapidly and are often at this size range or thinner within hours It isknown that slicks will often spread down to a 100mm (0.1 mm) thickness

within hours Researchers at earlier aerial trials have suggested that the meandroplet diameter should be about half of the slick thickness.

16.2.1 Dispersion Spray Equipment

Dispersants are best applied either “neat” (undiluted or diluted with water).Aerial spraying, which is done from small and large fixed-wing aircraft, as well

as from helicopters, is the most popular application method Spray systems onsmall aircraft used to spray pesticides on crops can be modified to spraydispersants Such aircraft can carry about 250 to 1000 L of dispersant and canperform many flights in one day in diverse conditions Some spray systems andtheir aerial coverages are listed in Table B2 in Appendix B As can be seen inthe table, large spray systems on large aircraft are attractive from the aerialcoverage point of view

Spray systems are available for boats, which vary in size from 10‑ to wide spray booms to tanks from 1000 to 10,000 L As dispersant is almostalways diluted with seawater to maintain a proper flow through the nozzle,extra equipment is required on the vessel to control dilution and applicationrates About 10,000 to 100,000 L of dispersant can be applied in a day, whichwould cover an area of 1 million m2or 1 km2 As this is substantially less thancould be sprayed from a single aircraft, spray boats are rarely used for a largespill A spray system operating from a vessel is shown inFigure 16.3.When spraying dispersant, it is important to deliver fine droplets (400 to 700Fm) to the slick at sufficient dosage to produce results The dispersant-to-oilratio is generally taken to range from 1:15 to 1:25 It is also essential to ensurethat the dispersant comes into direct contact with the oil Droplets larger than

30‑m-1000 Fm will break through the oil slick and cause the oil to collect in smallribbons, which is referred to as herding This can be detected by the rapidclearance of the oil in the dispersant drop zone without the formation of theusual white-to-coffee-colored plume in the water column This is very

FIGURE 16.3 A spray system from a boat in operation (note the underlapping spray pattern).

detrimental and wastes the dispersant Herding can also occur on thinner slickswhen the droplets of dispersant are smaller.

Dispersants must always be applied with a system designed specifically forthe purpose If pesticide spray equipment is used, small droplets form that mayblow away and not enough dispersant is deposited onto the oil slick Thedistribution of smaller droplets of dispersant is not desirable, especially whenspraying from the air as small droplets will blow away with the wind andprobably not land on the intended oil slick

Unless suitably modified, fire monitors or regular hoses from ships may notproduce correct droplet sizes or quantities of dispersant per unit area Further-more, the high velocity of the water/dispersant mixture can herd the oil away,resulting in loss of dispersant to the water column, where it has little effect on oilfloating on top of the water New single-point application nozzles have beendeveloped that provide relatively good distribution of correctly sized droplets

It is very difficult with aerial equipment to spray enough dispersant on

a given area to yield a dispersant-to-oil ratio of 1:15 to 1:25 The rate at whichthe dispersant is pumped and the resulting droplet size are critical, and a slickmust often be underdosed with dispersant rather than creating very smalldroplets Tests have shown that reapplying dispersant to the same area severaltimes is one way of ensuring that enough dispersant is applied to the oil.The nozzle and flow calibration procedure is done in four steps First, theequipment is inspected, and any defects are corrected before further calibration.The second step is the calibration of the flowmeter, and third is calibration ofthe unit by catching water spray from each nozzle The fourth step is thepreparation of a calibration curve Spray equipment should be maintained andperiodically calibrated Procedures and standards for design, maintenance, andcalibration are given in the literature listed in Section 16.4

16.2.2 Spray Aircraft

The required dispersant load and coverage obtained by the various dispersantdelivery platforms are given in Appendix B As discussed, aerial spraying isdone from both small and large fixed-wing aircraft as well as fromhelicopters

Transport aircraft with internal tanks can carry from 4000 to 12,000 L ofdispersant Large transport aircraft such as Hercules fitted with portable spraysystems can carry about 20,000 L, which could treat 400,000 L of oil at

a dispersant-to-oil ratio of 1:20 At a thickness of 0.5 mm, this oil would coverabout 400,000 m2 or 0.4 km2 This treatment could be applied in as little as anhour after loading the dispersant and as many as eight flights could be flown in

a day, depending on the distance from the airport to the spill Figure 16.4illustrates a Hercules spraying dispersant in a test over land

When using large aircraft, however, it can be difficult to obtain the requiredamount of dispersant A co-op typically stores 100 drums or about 20,000 L of

dispersant, which could be sprayed in one flyover Further flights would have towait for the arrival of more dispersant from other co-ops or production sources.

An entire country’s supply of dispersant can easily be used up in one day ifspraying with large aircraft

Flying at an altitude of 15 to 30 m, the optimal spray altitude, the pilot of thespray aircraft cannot see the slick When dispersant is being sprayed from anaircraft, a spotter aircraft must therefore precede the spray aircraft to provideinstructions for the setup of lines, when to turn the spray on and off, and smalldirectional corrections

When using helicopters, spray buckets are available in many sizes fromabout 500 to 2000 L If applied at a dispersant-to-oil ratio of 1:20, 10,000 to40,000 L of oil could be treated If the slick is 0.5 mm thick, this would coverabout 10,000 to 40,000 km2(or about 0.01 to 0.04 km2) It would take about 1

to 2 hours to fill and spray each bucket over the oil As a spill countermeasure,this rapid coverage of such a large area is appealing Figure 16.5 shows ahelicopter applying dispersants from a bucket

16.2.3 Spray Nomograms and Calculations

It is important to calculate the feasibility of performing the dispersant operation

To this end, a series of simple nomograms have been created and are provided inAppendix B Figures B1 to B3 shows the calculated areas, slick thickness, andviscosity with weathering for spills of 100, 1000, and 10,000 tons

Figure B1 can be used to estimate the area of slicks at the three sizes of spills.Figure B2 can be similarly used to calculate the thickness of the oil slick Oilslicks less than about 1 mm will be hard to treat with dispersants Figure B3 showsthe increase in viscosity over time for a very light crude oil After 48 hours, theviscosity of this light oil is such that it may not be treatable with dispersants Thenomogram for the increase in viscosity for a medium crude oil is shown in

FIGURE 16.4 A large aircraft applying dispersant.

Figure B3 It can be seen that, after about 24 hours, the oil will not be amenable todispersants Similarly, the nomogram for treating a heavy crude oil is shown inFigure B5 This figure shows that heavy oils are only amenable to treatment bydispersants in the first few hours After the viscosity of the oil reaches 1000 mPa.s

or cSt, the oil is poorly dispersible and the upper limit at which any dispersionoccurs is about 5000 mPa.s

Figure 16.6 shows the dispersant runoff that occurs after application ofdispersant to viscous oil In such a situation, almost none of the oil would bedispersed

FIGURE 16.6 Dispersant running off a heavy oil patch.

FIGURE 16.5 A helicopter spray system in operation.

16.2.4 Monitoring, Sampling, and Analytical Equipment

The purpose of monitoring protocols is first to determine whether or notdispersant applications are effective and second to estimate their relativeeffectiveness Dispersant effectiveness is defined as the amount of oil that thedispersant puts into the water column compared to the amount of oil that wasspilled In the field, effectiveness is visually indicated by the formation of

a yellow-to-coffee-colored plume of dispersed oil in the water column whichmay be visible from ships and aircraft This is shown inFigure 16.7

Dispersant effectiveness is primarily monitored by visual surveillance or situ measurements of oil concentrations When testing dispersant effectiveness

in-in the field, it is very difficult to measure the concentration of oil in-in the watercolumn over large areas and at frequent enough time periods It is also difficult

to determine how much oil is left on the water surface as there are no methodsavailable for measuring the thickness of an oil slick and the oil at the subsurfaceoften moves differently than the oil on the surface

The quantitative method is not used in modern monitoring practices.Instead, a relative measure of dispersant effectiveness is made Quantitativemeasures are difficult because effectiveness values depend on establishing

a mass balance between oil in the water column and that left on the surface situ fluorometry can be used to give an indication of the relative concentration

In-of the oil in the water column Some protocols to do this have been developed,e.g., SMART (Special Monitoring of Advanced Response Technologies)protocol