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OCR for page 215
5
How Dispersants Are Used:
Techniques, Logistics, Monitoring, and
Application Strategies
DESIGN OF DISPERSANT APPLICATION SYSTEMS
Application systems are designed to meet, within practical limits,
the following basic criteria that were discussed in Chanter 2:
The dispersant must be sprayed on the oil.
-sea
- The dispersant must mix with the of} and move to the oil-
water interface.
· The dispersant must attain the proper concentration at the
oil-water interface, ideally causing maximum reduction of interfacial
tension.
· Sufficient energy must be supplied from natural or artificial
sources to disperse the slick into droplets.
How these four goals are accomplished in practice is the subject of
this chapter.
Another factor that influences the application of dispersants is
the surface area of the spill. Most oils spilled on water attain an
average thickness of 0.1 mm or less within a few hours. Therefore, the
surface area that must be sprayed to control the spill is approximately
proportional to the volume of the of} spin.
Spray Systems
Systems must be able to apply dispersant uniformly to the slick;
ideally the droplets will be small enough to descend to the of! surface
215
OCR for page 216
216
USING OIL SPILL DISPERSANTS ON THE SEA
at a relatively low velocity without penetrating through the of} into
the water below, yet large enough that they will not be carried away
by the wind (SmedIey, 1981~. Thus, the primary objective in de-
signing a spray apparatus is to achieve relatively uniform application
without undue wind drift losses. Oil thickness or other factors do not
influence the design of the apparatus (Exxon Chemical Company,
1985~. Application rate per unit area is usually varied by changing
pump rate, boat or aircraft speed, or number of spray nozzles, or by
repeat application (Before, 1985; Chau et al., 1986~. A new approach
uses an air jet called a "dispersant spraying gun" which is capable
of treating a 0.~- to 0.2-mm-thick of! slick from a distance of 30 m
(Barbouteau et al., 1987~.
Spray systems can be mounted on a boat, a fixed-wing aircraft,
or a helicopter. They usually consist of spray nozzles mounted on
a manifold (the spray boom), a reservoir for the dispersant, pumps,
meters, valves, and other controls (API Task Force, 1986; Exxon
Chemical Company, 1985; Lindblom, 1979~.
Small-scale applications on offshore structures, along shorelines,
and near piers and bulkheads can rely on hand spraying. Typical
equipment ranges from a 5-gal (19-liter) portable tank with spray
wand (ITOPF, 1982) to gasoline or electric pumps with reel-mounted
hoses and sprayers.
Spray Droplet Size
Droplet size strongly influences dispersant effectiveness, but it
is not easily controlled. Several studies have shown that if droplet
size is larger than the of} film thickness, the dispersant tends to
penetrate the film without mixing and is diluted by the water. If
larger droplets predominate, effectiveness is diminished (Gill and
Ross, 1980; Smediey, 1981~.
Gill and Ross (1980) argue that the advantage of spraying a
larger-sized droplet, which improves the probability of hitting the
target, outweighs other considerations in successfully applying dis-
persant via aerial spraying. Little quantitative data exist on how
droplet size distribution affects the dispersant fraction that actuary
reaches and interacts with of] on the water surface. Although exact
droplet size can be measured in the laboratory by electromechanical
methods, spray patterns from aircraft are normally studied over land,
using Kromekote cards to preserve the droplet pattern (used in the
APT 1979 southern California field trials; McAuliffe et al., 1981~.
OCR for page 217
HO W DISPERSA NTS A RE USED
217
Smedley (1981) found reduced effectiveness with larger droplet
sizes, although not as much as would be expected based on film pen-
etration. Penetration is dependent on ail viscosity: higher viscosity
requires greater kinetic energy for the falling droplet to penetrate
the slick. Smedley also pointed out that given the same droplet size
distribution, dispersants tend to beless effective on thinner slicks,
possibly because there is greater penetration. He found, however,
that all droplets smaller than 400 I'm in diameter have about the
same effectiveness. This is similar to the typical range of slick th~ck-
nesses, 20 to 200 I'm (Chapter 2~. There is some evidence that
hydrophobic dispersant droplets that penetrate a film are not lost;
they resurface on the underside of the film and hence are particularly
effective (McAuliffe, private communication).
Airborne test data show that 100-pm droplets are not effectively
deposited from aircraft because of windage loss, whereas 500-pm
droplets are effectively deposited (Figures 5-l and 5-2; Smediey,
1981~. Therefore 350 to 500 I'm appears to be a reasonable size range.
This range is produced naturally with more viscous formulations
under typical air shear and is practical, since the droplets will hit the
target and not drift away on the wind (Lindblom and Cashion, 1983~.
In any event, only limited control over droplet size is possible. Fine
droplets can easily be achieved in the air, but windage loss defines the
lower usable droplet size. The upper size is controlled within limits
by aircraft speed, pump capacity, and nozzle size, but large drops are
broken up by air turbulence (Hornstein, 1973~.
Distance between drops also plays an important role in deter-
mining efficiency. To achieve good coverage (i.e., acceptably small
drop-to-drop distances) requires a droplet diameter smaller than 700
I'm (Smediey, 1981~. For an of] film 0.02 to 0.2 mm thick, generally
acceptable for dispersant application (Exxon Chemical Company,
1985), a dosage between 20 and 70 liters/ha (2 and 7 gal/acre) is
recommended.
Mechanical system design factors that strongly affect droplet
size distribution are nozzle orifice diameter, number of nozzles, fluid
pumping rate, and aircraft air speed (Lindblom and Cashion, 1983~.
These factors affect the shear acting on the fluid as it passes through
the nozzle and enters the air stream behind the aircraft. Mechanical
shear at the nozzle outlet, under laminar flow, varies inversely with
the cube of the orifice diameter:
Shear rate (sec~ ~ ~ = 8V/d = 1.7 x 105Q/d3,
OCR for page 218
218
USING OIL SPILL DISPERSANTS ON THE SEA
| ~ CENTER LINE OF FLIGHT PATH
Gin ~ ~ ELEVATION OF AIRCRAFT
T 46 m ABOVE GROUND
2.0
LL
IL
G
3 1.5
IL
o
-
cn
~ 1.0
o
a:
0.5
o
·\
CROSSWIND OF 8.33 m/s
(18.6 mph MEASURED AT 10 m ABOVE GROUND)
a.
/ ~91%
.
NO VORTEX EFFECT
500 MICRON DROPLETS
/ 'act
/ OF VOLUME ~
DEPOSIT AT '—
105 SECONDS \
~_\
i
1 `97% \
I OF \
VOLUME\
I DEPOSIT ~
I AT 105 \
SECONDS
FULL VORTEX EFFECT
500 MICRON DROPLETS
0 100 200 300 400 500 600 700
DOWNWIND DISTANCE FROM RELEASE POINT (METERS)
FIGURE 5-1 Droplet (500 microns) deposition with a crosswind. Source: Smedley,
1981.
where V = linear velocity of fluid at the orifice in mm/see, Q = flow
rate in liters/min (or 3.785 x gal/min), and d = orifice diameter in
mm.
Orifice, flow rate, and the number of nozzles used are chosen
to keep the mechanical shear below 10,000 sec~i (Exxon Chemical
Company, 1985~. Fortunately, the range of droplet sizes naturally
produced with more viscous formulations under typical air shear is
in the range recommended above, 350 to 500 am (Lindblom and
Cashion, 1983~.
An even stronger effect results from air shear, which increases
with aircraft speed or decreasing fluid exit velocity from each nozzle:
Fluid exit velocity (m/sec) = 21 L/nd2,
where L = total system flow in liters/min, n = number of nozzles,
OCR for page 219
HOW DISPERSANTS ARE USED
CENTER LINE OF FLIGHT PATH
ELEVATION OF AIRCRAFT, 46 m ABOVE GROUND
2.0
LL]
A) 1 5
CD
LL
o
-
cn
J 1.0
o
C)
0.5
o
219
CROSSWIND OF 8.33 m/s
(18.6 mph MEASURED AT 10 m ABOVE GROUND)
NO VORTEX EFFECT
100 MICRON DROPLETS
0.2% DEPOSIT AT
105 SECONDS
\
FULL VORTEX EFFECT
100 MICRON
DROPLETS
./ A: 79% -
~ OF VOLUME DEPOSIT
AT 105 SECONDS
0 100 200 300 400 500 600 700
DOWNWIND DISTANCE FROM RELEASE POINT (METERS)
FIGURE 5-2 Droplet (100 microns) deposition with a crosswind. Source: Smedley,
1981.
and d = inside diameter of nozzle (mm). Subtracting the fuid exit
velocity from the air speed of the aircraft (also in m/see) yields the
differential velocity. Differential velocity is the single most important
factor affecting droplet-size distribution and therefore depositional
efficiency. Extensive testing by API and the Exxon Chemical Com-
pany has shown that depositional efficiency drops off markedly if
clifferential velocity is more than about 64 m/see (Lindblom, 1987;
Smediey, 1981~. Anything that causes a decrease in median droplet
size, such as a decrease in dispersant viscosity, increase in differen-
tial velocity, or increase in nozzle shear, will decrease depositional
efficiency and accuracy.
For a given pump rate, a decrease in the number of nozzles in-
creases mechanical shear but decreases differential velocity. However
OCR for page 220
220
USING OIL SPILL DISPERSANTS ON THE SEA
since the air shear determines ultimate (deposited) droplet size dis-
tribution, it is only necessary that the nozzle orifice be large enough
that it will not produce very small droplets. Extremely high pres-
sure drop through the nozzle, which increases flow rate and thus
shear rate at the nozzle, should be avoided. Spray-boom pressures
on aircraft-mounted systems rarely exceed 2.7 atm (Lindblom and
Cashion, 1983~.
The preceding effects have been assumed to be independent
of aircraft type (Meyers, private communication). Vortex fields,
however, may vary from aircraft to aircraft and are known to have a
substantial effect (Smedley, 1981~.
Dispersant Type
Four physical properties of a dispersant formulation affect drop-
let size during aerial dispersant spraying (I.indblom and Cashion,
1983~:
· Viscosity is by far the most important.
Volatility is not likely to be important for any concentrate
dispersant formulations, but can be significant for hydrocarbon-based
formulations.
Density has an essentially negligible effect, except when drop-
lets penetrate the slick.
· Surface tension has a small effect on droplet size, but is of
minimal concern for aerial spraying of dispersant.
As the viscosity of sprayed fluid decreases, the median diameter
of the droplet distribution also decreases and can become very small,
producing a mist or fog that drifts far from the target, which can
lower depositional efficiency to less than 50 percent (Figures 5-1, 5-2,
and 5-3~.
On the basis of viscosity at 60°F, dispersants can be grouped
into three classes (Lindblom and Cashion, 1983~:
Five to twenty-five centistokes, typical of most hydrocarbon-
based products, generally is not recommended for aerial application.
· Thirty to sixty centistokes including hydrocarbon-based prod-
ucts with high surfactant concentration as well as less-active formu-
lations in other solvents, can be effectively sprayed from helicopters
or small airplanes.
· Higher than 60 cSt, including concentrates with high surfac-
tant content, can be applied using all types and speed of aircraft.
OCR for page 221
HO W DISPERSA NTS A RE USED
350
300
_ 250
o
.O
-
a, 200
a)
E
~ 150
a,
E
o
> 100
50
o
221
_
"
_/ ;"
-I''
In
o
o
O m
_
z ~
, 0
z C3
O I
(1)
,/
CO
C) O
O
C
a:
30
z o
o ~
O I
(2)
, .
.K
O ~
z
UJ
O I'
'a
Weiss and Worsham (II)
_~~
_ ,~Mugele
,~ C,,
_..
Inaebo
Thai
Mayer
a'
1 l
Weiss and Worsham (I)
Density - 1.0 g/cm3
Relative Velocity -120 knots
Nozzle Diameter - 2 cm
Surface Tension - 30 dye/cm
I I .
0 0.2 0.4 0.6 0.8 1 1.2
Dynamic Viscosity (Poise)
1.4 1.6 1.8 2
FIGURE 5-3 Comparison of various droplet- - ze models. volume mean diameter
versus dynamic viscosity. Weiss and Worsham presented two possible equations to
correlate available data, as noted by (I) and (II). Source: Lindblom and Cashion,
1983.
In most cases, glyco} ether- or water-based dispersants are used
at sea, but hydrocarbon-based dispersants appear (from laboratory
tests) to be more effective on mousse and highly viscous of! (Chapter
2~. Undiluted chemicals require a lower flow rate, and nozzles must
be properly chosen and sized accordingly (Becker and Lindblom,
1983~.
Dosage Control
Dosages recommended by manufacturers are determined by ex-
perience and limited in range by viscosity and other properties of the
formulation as well as the application system itself. Dosages can also
be based on laboratory determinations of optimum dispersant-oi} ra-
tios; 1:20 is typical (Cormack, 1983c, and private communication).
This calculation requires an assumption of slick thickness to con-
vert observed area to of} volume. Typically a thickness of 0.1 mm
OCR for page 222
222
USING OIL SPILL DISPERSANTS ON THE SEA
is chosen, equivalent to 940 liters/ha (approximately 100 gal/acre)
of oil. A dispersant-oil ratio of 1:20 would then require application
of 50 liters/ha (5 gal/acre) of dispersant, in agreement with the 20
to 70 liters/ha (2 to 7 gal/acre) guideline quoted above. Since oil
slicks are far from uniform in thickness, a constant dose per unit
area wiD produce a correspondingly nonuniform dispersant-oil ratio.
Unfortunately, there is no practical way to vary application rate to
achieve a constant dispersant-oil ratio.
Another reason dosage is regulated by area sprayed is that the
amount of oil being treated is usually not accurately known (Horn-
stein, 1973~. At the time of spraying, the amount of oil the disper-
sant contacts depends on slick thickness, which is only approximately
known, and on the accuracy with which the slick is hit by the spray.
Deliberately aiming for the thickest part of the spin, for example,
near a wrecked tanker or a blowout, has been recommended as an
efficient strategy (Mackay, private communication). Injecting disper-
sant directly into a blowing well through a previously installed port
below the wellhead has been proposed, and some field tests have been
encouraging (Audunson et al., 1987; Kolnes, 1986~.
Recent tests of depositional accuracy were made using Krome-
kote cards to obtain total amount deposited per unit area and the
percentage of total fluid pumped that reached the target (Lindblom,
1987~. Data from these tests indicate that 45 to 90 percent depo-
sitional efficiency can be expected (most of the data were in the 65
to 85 percent range), with spray altitude on the order of 15 m and
differential velocity (air-speed minus exit velocity of fluid from spray
nozzle) of 64 m/see (210 ft/sec) or less.
Boats
Workboats, barges, fireboats, and tugboats have been adapted to
spray dispersants, but the use of surface vessels is complicated by slow
speed and vessel motion caused by sea conditions. Only relatively
small spins (less than 1,000 bbl) can be treated by boats, even with a
nearby chemical reloading point. For example, a boat operating at 5
kn, spraying a 12-m (40-ft) swath can only treat about 1.3 km2 (0.5
sq mi) in 12 hr. If slick thickness averages 0.l mm, the boat could
treat about 830 bbl per day assuming that the boat has dispersant
storage and fuel capacity to operate the entire day without having to
interrupt spraying operations (APT Task Force, 1986; Belore, 1985;
Chau et al., 1986; McAuliffe, 1986~. Use of multiple craft or vessels
OCR for page 223
HO W DISPERSA NTS A RE USED
TABLE 5-1 Amount of Oil Spray Boats Could Disperse in 10-hr
_ray Width
System Meters Feet
Amount Dispereeda
at,
Meters" Barrels
One boat 5 16 56 350
One boat 10 23 111 8700
One boat 80 100 333 2,100
a Calculation based on boat traveling at 6 kn, an average click
thickness of 0.1 mm, and 100 percent efficiency.
223
with greater swath widths increases treatment area proportionately
(Table 5-1~.
Many improvements have been made since the early Warren
Spring Laboratory work (Chapter 1) in the design of dispersant
spraying equipment for boats. Examples include spray booms that do
not require overhead rigging, low-drift nozzles, electric-start pumps,
and flow meters (Frank Ayles & Associates, 1983, 1984~.
Most systems now use water-compatible concentrate dispersants
diluted with seawater during application. A seawater pump allows
the dispersant to be added to the water stream where it is mixed with
the water and applied through the nozzles of a spray boom mounted
as far forward as possible. This is done to avoid the bow wave, which
may push of} out of reach of the spray at typical boat speeds of 2 to 10
kn (Exxon Chemical Company, 1985~. High-pressure water jets have
been tested in the laboratory as a means of improving dispersant
electiveness by applying mixing energy to the dispersant and water
system. In the laboratory, at a dispersant-oi} ratio of 1:100, 80 to
100 percent of the slick was dispersed at the equivalent of low boat
speeds (e.g., 2 kn), compared to 10 percent using the Warren Spring
breaker-board system (Before, 1987~.
If the dispersant is to be added from a positive displacement
metering pump, only the minimum flow rate of water necessary to
fill and maintain pressure in the spray system is used, thus keeping
water dilution to a minimum.
Booms should be rigged with multiple nozzles arranged to pro-
duce a fan-shaped spray pattern, with sufficient pressure to maintain
this pattern until it reaches the water level. The required pressure de-
pends on the distance between the nozzles and the water. The spray
pattern should be flat and perpendicular to the direction the boat is
. ~
OCR for page 224
224
USING OIL SPILL DISPERSANTS ON THE SEA
traveling, and the spray should strike the water in droplets like fine
rain. The fan-shaped sprays from adjacent nozzles should overlap
just above the oil-water surface and the inboard spray strike the hub
just above the water line (APT Task Force, 1986; Exxon Chemical
Company, 1985; I.indblom, 1979; Lindblom and Barker, 1978; Spray-
ing Systems Co., 1984, 1985~. Cone-shaped spray patterns are less
efficient because they concentrate the dispersant unevenly along the
edges rather than in the middle (Spraying Systems Co., 1984, 1985~.
In a properly designed system, the effect of wind on the spray
pattern, spray-boom location relative to the bow wave and distance
above water, vessel speed, nozzle design, pump rate, and pressure
are considered (Exxon Chemical Company, l9SS). However, even
with a properly designed system, there are disadvantages to using
water as the carrier for the dispersant. Field tests with boats have
indicated much lower effectiveness rates with diluted dispersants
when compared to neat applications (McAuliffe et al., 1981~. In
heavy seas, application becomes progressively less effective due to
pitch and roll, which alters the spray pattern on the water (Cormack,
private communication). Direction of spraying operations from the
boat deck is limited by poor visibility.
In an emergency, water-dilutable (glycol ether-based but not
hydrocarbon-based) dispersant concentrate can be educted into a
boat's firefighting system, but dosage control and distribution is
difficult (Exxon Chemical Company, 1985~. Eduction of dispersant
depends on the flow rate of the water stream. The fire system
normally provides much more water than is required so its flow must
be reduced by valve control or by providing a "bleed" in the stream.
For either of these procedures, flow calibration is required.
Aircraft
Aircraft can be equipped with chemical tanks, pumps capable
of spraying 20 to 100 liters/ha (2 to 10 gal/acre), digital readout
flow meters, spraybooms (generally airfoil in shape attached to the
aircraft), and nozzles. Unless the dispersant viscosity is higher than
approximately 60 cSt, aircraft speed should not exceed 100 kn (~85
km/hr or 115 mph). At faster than about 100 kn, spray nozzle tips
are not necessary because the wind shear at the nozzles is sufficient
to break the stream into small droplets, and the droplet size tends to
be less than 350 ,um if the dispersant viscosity is less than 60 cSt. The
spray-boom altitude should not be over 9 m (30 ft) for helicopters
OCR for page 225
HO W DISPERSANTS ARE USED
TABLE 5-2 Dispersant Spray Capabilities of Various Aircraft
225
.
Aircraft
Tank Volume
Metered Barrels
Amount Dispersed
per Flighty
Meters Barrels
Various helicopters
Agriculture spray planes
Piper Pawnee, Cesena
Agtruck, and Ayres Thrush 0.4-1.1
Turbo Thrush 1.5-2.6
DC-3, Fokker F-27, and
Canadair CL-215
Four-engine aircraft
(DC-4,
DC-6)
Hercules C-130, ADDS
System
0.4-2.3 2.5-15
3.0-4.5
8-46 50-290
2.5-7.3 8-22 50-140
9.5-16 30-52 190-330
19-28
5.7-11 36-72
21 13
60-90 380-570
115-230 730-1,400
420 2,600
aAssumes dispersant-oil ratio is 1:20 and 100 percent efficiency of
dispersion.
SOURCE: Adapted from McAuliffe (1987b).
and small aircraft, although the pilots of larger aircraft prefer to fly
at 15 m (50 ft) or higher (Lindblom and Cashion, 1983~. Table 5-2
shows the dispersant spray capabilities of various aircraft.
Small airplanes used in agricultural spraying generally have very
small load capacity, often less than 380 liters (100 gal), although
some newer planes stave I,500- to 2,300-liter (400- to 600-gal) tanks,
as shown in Table 5-2. For dispersant spraying, agricultural spray
aircraft need to be fitted with adequate pumps, meters, and aft-facing
nozzles. Most pumps and nozzles used for agricultural spraying
produce too fine a spray, although the airplanes may be quickly
converted if the correct equipment is included in a contingency plan.
Small aircraft are best used for rapid response to small spills
and test applications while larger equipment is being readied and
brought to the spill site. I.arge two- and four-engine airplanes (i.e.,
DC-3, DC-4, DC-6, Cl'-215, and C-130) are most useful for large
spins because of their greater range, capacity, speed, and potential
for great areal coverage (Lindblom and Barker, 1978~.
The Airborne Dispersant Delivery System (ADDS, also known
as ADDSPACK) unit developed for the Lockheed C-130 is the only
spray system currently available that does not require permanent
OCR for page 228
228
USING OIL SPILL DISPERSANrTS ON THE SEA
1978). The Warren Spring Laboratory has employed sidewall hov-
ercraft on an experimental basis, but they are more vulnerable to
rough seas than most workboats (Nichols, private communication).
Another problem specific to hovercraft is that spray is blown away
from the craft by the propulsion system's strong air currents (Jones,
private communication). It also appears that turbulence caused by
the hovercraft may reduce the dispersant contact time enough to
lessen effectiveness.
Boats Versus Aircraft
Boats, aircraft, and helicopters Al have a role to play in the
proper application of dispersants. For applying dispersant to large
spins in the open ocean, large aircraft appear to be the only practical
means. This is indicated by Table 5-2, which shows the area typically
covered by an oil spill as a function of the size of the spin. For some
small spills, for example, less than 38,000 liters (10,000 gal), the
most practical and cost-effective method of dispersant application is
a helicopter with a spray bucket. Boats can spray dispersants on
small spills, but their effectiveness may be diminished by limited
range and coverage area, weather and sea state (spray-boom length
is limited by vessel roll), and the difficulty of determining where to
spray. Spotter aircraft generally should be used to direct aircraft and
boat spraying operations.
The necessity (in most boat-mounted systems) to spray with
diluted dispersant generally diminishes the overall effectiveness of
the dispersant. In some situations, however, such as a spill caused- by
a shipwreck where there is an ongoing salvage operation, boats wig
not only be available but may be best suited to spray close to the
wreck. This may also be true for of! weD platforms where a minor
spin occurs.
Calibration
Both boat and aircraft application systems require calibration to
provide proper dosage control, allow optimum dosage, vary dosage
as conditions change, and provide accurate documentation. Dosage
from boat-mounted systems is determined by swath width, boat
speed, and water or chemical pump rates (Exxon Chemical Com-
pany, 1985~. Published pump curves, tables of nozzle output versus
pressure, eductor percentage settings, and even calculated swaths
OCR for page 229
HO W DISPERSA NTS A RE USED
229
provide only general guidance, not true calibration. Manufacturers
of spray units generally do not provide the necessary calibration
(Lindblom, private communication). Water can be used to calibrate
units designed for diluted dispersants and for aerial application sys-
tems (Exxon Chemical Company, 1985~.
For calibration of eductors and chemical metering pumps, the
dispersant to be used should be employed if possible. It is especially
important that eductors be properly cleaned and maintained and
frequently recalibrated (Onstad and Lindblom, 1987~. Calibration of
commercial equipment at U.S. EPA's Oil and Hazardous Materials
Simulated Environmental Test Tank (OHMSETT) facility showed
that flow rates among nozzles varied significantly, some as a result
of plugged or defective internal parts. The flow meter integral to the
system was also inaccurate. Calibration procedures and development
of operating charts are described by Onstad and Lindblom (1987~.
The results of calibration tests should be made available to equip-
ment operators and response coordinators in easy-to-use form, such
as tables, charts, and curves. This format allows for rapid actions
in dosage control that do not depend on numerical calculations or
interpolations (Onstad and Lindblom, 1987~.
System evaluation should be conducted under realistic operating
conditions and data recorded during the tests so that a basis for
calibrating and adjusting the system can be formulated. Field trials
can help quantify the effects of pumping rates, swath width, and
speed (Strum and Nash, 1987~. Recommended eductor settings and
pump curves, or calculated swath widths, provide a starting point
for calibration, but do not substitute for field tests.
Aerial swath width can be estimated as the width of the spray
boom at 9-m (30-ft) altitude, or I.2 to 1.5 times the distance between
terminal nozzles for an aircraft operating into the wind at 15 m (50
ft) (Exxon Chemical Company, 1985; Lindblom, 1987; Lindblom and
Barker, 1978~. Crosswind swaths can only be estimated roughly by
guidance aircraft.
MONITORING EFFECTIVENESS OF DISPERSANTS
Directly monitoring the fraction of oil removed from the surface
and dispersed in the water is the best method of determining ef-
fectiveness if it can be done reliably. This concept of effectiveness,
equivalent to obtaining a mass balance on the amount of of! dispersed
in the water column, has given good estimates in a few elaborate field
OCR for page 230
230
USING OIL SPILL DISPERSANTS ON THE SEA
tests; but it is complicated, requires set-up time, and is not practical
in real spins. However, the concentrations of dispersed of! in the
water are also relevant to assessing potential toxicity.
Water sampling programs have been useful for scientific purposes
and have been proposed as a regulatory requirement by local agencies;
for example, in California (API Task Force, 1986; Boehm and Fiest,
1982; McAuliffe et al., 1980, 1981~. But water sa~npling programs are
expensive and of limited value unless carefully planned and executed.
Even when the protocols, mechanisms, and equipment for such a
program are set up beforehand, as in research field tests, the logistics
of getting the appropriate people and equipment to the scene and
carrying out the program satisfactorily are difficult (Meyers and
Onstad, 1986~.
Distribution of oil in the water column during oil spill experi-
ments off the coast of Norway in June 1985 was determined by use
of instruments that detected light scattering by of} droplets in water
and the ultraviolet fluorescence of dissolved of! components (Gen-
ders, 1986~. Fluorescence in water is the sum of natural fluorescent
materials as well as oil. Backgrounds in the North and Baltic seas
varied from 0.5 to 10 ppb and occasionally up to 50 ppb. Attempts
have been made to separate backgrounds by using different excita-
tion and reception wavelengths. Background is usually a factor of
at least 10 less than the signal in the case of significant of] pollution
(Hundah] and JojersTev, 1986~.
Remote Sensing
Other real-time monitoring methods that might lead to prac-
tical measures of effectiveness have been proposed. Ideally a spiD-
detection and monitoring system should be able to
~ provide continuous, day and night, ad weather, wide area
real-time surveillance;
J7
~ detect any of! spin that occurs in the marine environment
both on and below the surface;
confirm that the detected substance is in fact oil;
map the areal extent of the spill;
obtain the thickness distribution and quantify the amount of
oil spired;
identify the source and type of pollutant being discharged;
~ provide precise navigation information for spill source loca-
tion and for positioning vessels;
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HOW DISPERSANTS ARE USED
231
· provide objective data on dispersant effectiveness and on the
presence and behavior of threatened organisms (especially birds and
mammals) in the area; and
.
document Al collected data.
Since no one sensor can do all these things, a multisensor system
is required. A good sensor wig detect oil, identify it as oil, and
determine the of} type (Garrett et al., 1986; Intera Environmental
Consultants, 1984; Moniteq, 1985~. Multispectral devices and active
laser fluorosensors are now used for more sophisticated detection
systems. The laser fluorosensor is an active device that gives basic
of} type and operates in any weather, day or night (Hoge and Swift,
1980).
Aircraft provide adequate spatial coverage and can be dispatched
to a spill with ah the necessary systems operative. They are often
the platform of choice for monitoring (Fast, 1985~.
Aerial or remote sensing has a distinct advantage over shipboard
observations because of the rapid and extensive coverage possible.
Oil can be detected from surface vessels by a variety of methods
(e.g., visible color, fluorescence, and radar), but the limited area
visible from a boat makes these techniques less than ideal.
visual and Near-Visible Observation
Reflection, absorption, and scattering from water with and with-
out spiked oil, and the ability of oil to affect the interference and
polarization of reflected light allow visual and near-visible obser-
vation of oil. During experiments in the North Sea, for example,
observers aboard aircraft noted that ~ day after treatment a chemi-
cally dispersed slick had disappeared, but an untreated control slick
was still clearly visible (Lichtenthaler and Dating, 1985~. Videotap-
ing is one simple, inexpensive, and available technique that can be
used from airplanes or boats to document the extent and configura-
tion of a spill. It can also record how of} responds to dispersants, the
effects of wind, waves, and currents, and the presence and behavior
of birds, marine mammals, and other organisms. Using a polarizing
filter to enhance the reflectance differences, even thin oil films can
sometimes be observed (Burns and Herz, 1976~.
The primary drawback of videotape, like visual or photographic
observation, is dependence on natural lighting. Sunlight reflecting off
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232
USING OIL SPILL DISPERSANTS ON THE SEA
water can reveal the shape of very thin slicks, but lack of contrast be-
tween of} and water on overcast days greatly reduces the effectiveness
of visual observations.
Although there are many examples of qualitative observations,
obtaining ground (or sea) truth to calibrate a quantitative measure-
ment system is a difficult challenge. One day after the 1986 Beaufort
Sea tests, various observers could not agree about how much surface
area was covered by the thick portions of the slick. The two observers
who were in aircraft estimated 20 percent of the slick was thick oil,
but the observer on the water estimated 80 percent. In addition,
observers on the water saw numerous of] particles or flakes, or small
tar balls, detail that could not be discerned in the remote-sensing
imagery (Fingas and Jones, private communications).
Under relatively calm conditions, experimental slicks tend to
spread more extensively when treated with dispersant than do un-
treated controls (Lichtenthaler and Daling 1985; Sergy, 1985~. Be-
cause of spreading, herding, and the irregular shape and thickness of
a large slick, it is difficult to tell visually how much of the of] has been
removed from the surface, and, therefore, it is not always possible to
determine effectiveness accurately by this method (Cormack et al.,
1986/87; Fingas, 1985~.
Infirared Sensing
Because oil absorbs and retains more heat (from solar infrared ra-
diation) than does water, it is possible with the use of infrared-sensing
devices to detect of} on the water surface by means of temperature
differences. Sensitivity is greatest when the of} is warmer (in sunlight,
early afternoon); but when the of} cools to water temperature (in the
evening or morning), sensitivity may drop to zero.
High-resolution devices, including television-type cameras, are
now available that detect the thicker parts of of} slicks. The suc-
cessfu} use of infrared image intensifiers for night vision in combat
suggests that night flights might track a slick with infrared monitors
in darkness or low visibility, which would allow response forces to
position themselves for the next day's activities. Such a procedure
was tested at night and detected of] at the natural of} seeps of the
Santa Barbara Channel (Onstad, private communication). However,
the 1986 Beaufort Sea tests, using a different camera, cast doubt on
the value of night flights in tracking slicks (Jones, private communi-
cation).
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HO W DISPERSANTS ARE USED
233
The most commonly used sensors are infrared plus ultraviolet.
Remote-sensing airplanes are used in Canada, France, the Nether-
lands, Norway, Sweden, the United Kingdom, and the United States
(Cormack et al., 1987; Fast, 1987; McCoD et al., 1987; Lavache,
private communication). The ideal system is portable from plane
to plane, fitting into window openings, screens, or camera hatches
(Fingas, private communication).
Microwave Sensors
Both microwave emissivity and reflectance vary with of} spin
presence. Dual-channel-imaging microwave radiometers have mapped
of] spills thicker than 100 am (HoDinger and Menelia, 1973~. Active
microwave sensors have been used successfully, especially in Europe
(Croswel1 et al., 1983; I,oostrom, 1986/87~.
Radar
Side-Iooking airborne radar (SLAR) is used as a means of de-
tecting of! slicks at sea. SLAR can operate through a cloud cover
and outline the slick, but cannot distinguish a thick slick from a thin
sheen. It is a key element in remote-sensing packages used to detect
of] spills in the Netherlands, Norway, Sweden, the United Kingdom,
and the United States (Cormack et al., 1987; Fast, 1987; Schriel,
1987; White et al., 1979; McAuliffe, private communication).
Detection of slicks by SLAR depends on damping of capillary
waves by the floating oil. Reliable detection depends on a variety
of factors such as the size of the slick and environmental conditions,
but slicks have been detected at distances up to 20 km (Cormack et
al., 1987~. In the context of monitoring dispersant effectiveness, it
may be that the primary use of SLAR would be the important job of
locating a slick rather than measuring the subsequent disappearance
of the of! from the water surface when treated by dispersants.
Summary of Monitoring Techniques
The best method of surveying appears to be from an aircraft
using SLAR for initial mapping, followed by infrared line scans to
determine relative slick thickness and ultraviolet line scans to produce
a picture that can be correlated with visual observation (Fast, 1985,
1987~. With such data, aircraft can direct spraying units to the
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234
USING OIL SPILL DISPERSANTS ON THE SEA
correct portion of the slicks (Cormack et al., 1986/87; Fingas, private
communication).
Remote-sensing information on very thin slicks has produced evi-
dence for the enhanced spread of dispersant-treated oils (Cormack, et
al., 1986/87; Lichtenthaler and Ding, 1985; Cormack, private com-
munication). Remote sensing is partially successful in experimental
situations, and appears to be at the stage of commercialization of
the needed measurement devices. Equipment has been developed
for the U.S. Coast Guard (Kim and Hickman, 1975), the Swedish
Coast Guard (Fast, 1985), the Canadian Environmental Protection
Service (Intera Environmental Consultants, 1984), and the British
government (Nichols and Cormack, private communications). This
equipment can measure from aircraft some of the parameters nec-
essary to determine slick thickness and configuration, but it is very
expensive compared to other response equipment and considerable
development is required before remote sensing becomes routine. One
or more backup systems have been suggested because there are many
opportunities for sensitive instruments to malfunction (Fingas, pri-
vate communication).
Regulatory Requirement
The U.S. National Contingency Plan does not require or regulate
documentation of cleanup, and subjective visual evaluation is nor-
maDy sufficient. The Region {X contingency plan (Pavia and Smith,
1984) requires documentation, as do some local policies, but does
not specify detailed methodology. Canadian regulations imply that
effectiveness of cleanup should be monitored, but no techniques are
specified (Fingas, private communication).
STRATEGY OF DISPERSANT APPLICATIONS
Development of contingency plans for spill response requires a
great deal of thought in advance of any incident. The necessary
equipment, personnel, and chemical dispersant must be available to
go to work immediately, whenever the need arises. Contingency plan-
ning strategy should consider different spin sizes, weather conditions,
and ah available control measures (see Chapter 6~.
Since timely application is essential for success, it is necessary
that ah personnel involved in decisions be educated in the scientific
and technical information and be briefed on their role and the con-
sequences of failure to act promptly or decisively. It would also be
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HO W DISPERSA NTS A RE USED
235
desirable for them to have participated in field trials, spraying real
dispersants on real oil.
Dispersibility of Oil
Dispersibility of the oil is the logical place to start in the planning
process. Many important characteristics of an oil from a known
source can be obtained from existing data. Laboratory or field testing
of dispersant on a sample of the spired oil can give even more
information, but it is difficult and time consuming to obtain an
adequate oil sample from an actual stick. An exception would be if
the source were known ahead of time, as with a production rig or a
port where a well-defined type of of! is handled. Spraying a portion
of the slick is probably the best method of determining effectiveness
(Smith and Pavia, 1983; Stacey, 1983~.
SpiD Size and Configuration
Spill size and configuration must be known for effective disper-
sant application. Generally, the goal of dispersant use is to protect
sensitive marine areas or coastline by controlling the most threaten-
ing portions of the slick. This can be done by treating portions of the
slick, such as its leading edge or selected windrows of oil. The por--
tions of the slick most effectively treated with dispersant are about
0.02- to 0.2-mm thick (Exxon Chemical Company, 1985~.
The state of California Contingency Plans require clear evidence
of a leading edge (an unusual configuration for a large spill) and
threat to marine mammals or shoreline before permission for disper-
sant application can be granted. By the time a clear threat can be
demonstrated, however, it may be too late to apply dispersant.
The Exxon Chemical Company (1985) suggests that boat spray-
ing operations generally proceed from the spill's outside edge and
work gradually toward the center, so that the slick is not disrupted
by the boat's wake. However, most of the oil volume is in a relatively
small thick portion, while the outlying portions consist of thinner
sheen layers. Circling from the outside in results in much of the
dispersant being sprayed on the sheen, rather than on the bulk of the
oil.
Belore (1985) and Chau and Mackay (1985) have suggested re
peated application of dispersant to thicker portions of the stick
This has been verified in field tests (LichtenthaTer and Daling, 1985
McAuTiffe et al., 1981~.
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236
USING OIL SPILL DISPERSANTS ON THE SEA
Lindblom, Meyers, and Onstad (private communications) rec-
ommend that small spills generally be treated through their thickest
part and large ones treated across a leading edge threatening shore-
lines or environmentally sensitive areas. They recommend that sheen
be allowed to disperse naturally. Often the thick areas consist of a
number of separate patches or windrows, and must be identified and
located by aerial reconnaissance. Visual observations must be in-
terpreted cautiously (as discussed earlier)—after treatment, of! can
spread more thinly over a larger area and the slick may appear to be
about the same size as before, even though much of the of} has been
dispersed.
Large spills cannot be treated in their entirety, but dispersants
can be used tactically under favorable conditions to protect sensitive
shoreline areas. Even without dispersant application, the concentra-
tions of oil in a relatively self-conta~ned body of water can remain
high for months after a spill. For example, 2 months after the Argo
Merchant spin on Georges Bank in December 1976 the dissolved
hydrocarbon levels were as high as 14 to 60 ~g/liter; and 1 to 25
g/liter 5 months afterward. The "normal" unpolluted level for that
area appears to be 5 ~g/liter (Farrington and Bochum, 1987~. Would
the use of dispersants have increased or decreased the amount of hy-
drocarbons retained in the water column? That question cannot be
answered without a great deal of additional knowledge about water
circulation, exchange with the open ocean, and sedimentation and
resuspension rates in that area.
Aerial Spraying Strategy
When flying directly into the wind, the effective swath width
is roughly T.2 to 1.5 times the overall length of the spray boom.
Crosswind application, which is common in agricultural spraying,
may be useful when very large spills are treated by large aircraft
with high-volume pumps (Smedley, 1981~. This technique gives a
much greater swath width than upwind spraying but has a potential
drawback for smaller spills unacceptable and off-target dispersant
drift. For crosswind application, increased pump rate and nozzle
adjustment are necessary to allow for a wider swath.
Other Strategies
It is logical to be prepared, especially for small operational spills,
that is, on onshore exploration or production platforms, or loading
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HOW DISPERSANTS ARE USED
237
platforms. Here of} and dispersant can be pretested. This is a
condition for "maximum effectiveness."
During the Chevron Main Pass Block 41 Platform Cspill, a water
spray system consisting of a large number of fire monitors mounted
on a barge (used for fire control), applied dispersant on the blowing
weds and on the of} slick in the immediate vicinity of the platform
(McAuli~e et al., 1975~. The same type of system was also used
at other platform blowouts. The barge was kept upwind to prevent
the spray from covering the barge and to produce the most effective
downwind coverage of the platform and surrounding water.
During the early stages of an offshore continuous discharge, the
oil can be treated initially, at least during daylight hours, by aircraft
and boat spraying. Oil that is released during the night and moves
as a slick away from the platform can be sprayed the next day. This
technique may be sufficient to control smaller of} releases, but it may
not control large spins.
An offshore platform blowout, either at the surface or subsea,
provides a special opportunity for developing techniques to apply
chemical dispersants (Audunson et al., 1987~. There is time to as-
semble a spray system at the source of the oil release, and the
platform or surrounding support vessels allow for 24-hr operation of
spraying.
Applying dispersant near the of} discharge point has the added
advantage of spraying fresh of! and generally thicker oil, thereby
more efficiently using the chemical dispersant and better controlling
the slick. As discussed in other sections of this report, chemical
dispersion becomes more difficult when the of! has weathered and as
the slick thins and breaks into patches and particles.
One proposal is to add dispersant or emulsion inhibitor to the
cargo of a tanker before the of! is released to the water (Gordon
and Milgram, 1986; Ross et al., 1985~. This approach does not
seem practical it would require enormous amounts of dispersants,
could affect the chemistry of of} refining and combustion, and the
dispersant would have to be mixed with the of! before it was loaded
in tankers.
Command and Control
An aircraft, given a cloud base of more than 60 m (200 ft) and an
indication of where to look, will usually be able to find of! Boating on
the water. A boat or ship with dispersant spray equipment may find
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238
USING OIL SPILL DISPERSANTS ON THE SEA
some oil, but has little hope of locating major patches in the area
without spotter aircraft. Radio contact between aerial spotters and
spraying boats is therefore essential (API Task Force, 1986; ITOPF,
1982; McAuliffe et al., 1981~.
Spotter aircraft are also necessary when aerial spraying is con-
ducted. The spraying aircraft flies adjacent tracks and turns the
spray on and off in order to hit the slick without wasting chemical on
open water. This requires coordination between spotter and spray
units (ITOPF, 1982~. The potential use of remote sensing by spotter
aircraft as discussed earlier can possibly assist in locating the thicker
portions of the slick and provide an indication of effectiveness of the
dispersant.
Weather
Wind and waves not only affect the physicochemical processes of
(lispersion, they also affect spraying operations. High winds may blow
dispersant spray off target. Heavy seas, however, can be advantageous
since breaking waves more rapidly disperse the of} (Bonwmeester and
Wallace, 1986a; Raj and Griffith, 1979~. Fog and low clouds cause the
most difficulty because they obscure visibility and can stop aircraft
operations. Application techniques in adverse weather conditions are
included in some contingency plans. (EPA Regions OX and X plans
allow consideration of dispersant application when mechanical means
are impossible.)
Representative terms from entire chapter:
exxon chemical
