Marine Oil Spills
Transcript of Marine Oil Spills
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Alexandria University
Faculty of Engineering
Dept. of Marine Engineering & Naval Architecture
By/
Basem Elsayed TawfekNo: 21
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Oil pollution is one of the most serious environmental problems in the
marine environment. Episodic pollution events, such as catastrophic oil
spills; in particular, threaten water quality and habitat with a suddenness and
severity rarely matched by other pollutants. Catastrophic spills typically
result from transportation accidents such as collisions or groundings of oil
tankers.
Most oil pollution stems from non-catastrophic events, however, and occurs
most frequently during cargo transfer operations. In fact, of the 3.5 Million
tons of oil that ends up in the ocean every year worldwide, only a small
percent is a consequence of tanker spills. About 70 percent of oil Pollution is
due to chronic pollution from municipal and industrial wastes or run off,
dumping of waste oil, release of oily bilge water, and from other-than-tanker
transportation.
Whats an oil spill?
Oil spills happen when people make mistakes or are careless and cause an
oil tanker to leak oil into the ocean. There are a few more ways an oil spill
can occur. Equipment breaking down may cause an oil spill. If the
equipment breaks down, the tanker may get stuck on shallow land. When
they start to drive the tanker again, they can put a hole in the tanker causing
it to leak oil.
When countries are at war, one country may decide to dump gallons of oil
into the other countrys oceans.
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Terrorists may cause an oil spill because they will dump oil into a countrys
ocean. Many terrorists will do this because they are trying to get the
countrys attention, or they are trying to make a point to a country.
Illegal dumpers are people that will dump crude oil into the oceans because
they do not want to spend money on decomposing their waste oil. Because
they wont spend money on breaking up the oil (decomposing it) they will
dump oil into the oceans, which is illegal.
Natural disasters (like hurricanes) may cause an oil spill, too. If a hurricane
was a couple of miles away, the winds from the hurricane could cause the oil
tanker to flip over, pouring oil out.
Fate of Marine Oil Spills
Oil is a general term used to denote petroleum products which mainly
consist of hydrocarbons. Crude oils are made up of a wide spectrum of
hydrocarbons ranging from very volatile, light materials such as propane and
benzene to more complex heavy compounds such as bitumens, asphaltenes,
resins and waxes. Refined products such as petrol or fuel oil are composed
of smaller and more specific ranges of these hydrocarbons.
Oil, when spilled at sea, will normally break upand be dissipated or scattered into the marine
environment over time. This dissipation is a
result of a number of chemical and physical
processes that change the compounds that make
up oil when it is spilled. The processes are
collectively known as weathering. Oils weather in different ways. Some of
the processes, like natural dispersion of the oil into the water, cause part of
the oil to leave the sea surface, while others, like evaporation or the
formation of water in oil emulsions, cause the oil that remains on the surface
to become more persistent.The way in which an oil slick breaks up and dissipates depends largely on
how persistent the oil is. Light products such as kerosene tend to evaporate
and dissipate quickly and naturally and rarely need cleaning-up. These are
called non-persistent oils. In contrast, persistent oils, such as many crude
oils, break up and dissipate more slowly and usually require a clean-up
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response. Physical properties such as the density, viscosity and pour point of
the oil all affect its behavior.
Dissipation does not occur immediately. The time this takes depends on a
series of factors, including the amount and type of oil spilled, the weather
conditions and whether the oil stays at sea or is washed ashore. Sometimes,
the process is quick and on other occasions it can be slow, especially in
sheltered and calm areas of water.
Behavior of oil at sea
The eight main processes that cause an oil to weather are described below
and summarized in the following diagram.
Fate of oil spilled at sea showing the main weathering processes
Spreading
As soon as oil is spilled, it starts to spread out over
the sea surface, initially as a single slick. The speed
at which this takes place depends to a great extent
upon the viscosity of the oil. Fluid, low viscosity oils
spread more quickly than those with a high viscosity.
Nevertheless, slicks quickly spread to cover
extensive areas of the sea surface. Spreading is rarely uniform and large
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variations in the thickness of the oil are typical. After a few hours the slick
will begin to break up and, because of winds, wave action and water
turbulence, will then form narrow bands or windrows parallel to the wind
direction. The rate at which the oil spreads is also determined by the
prevailing conditions such as temperature, water currents, tidal streams and
wind speeds. The more severe the conditions, the more rapid the spreading
and breaking up of the oil.
Evaporation
Lighter components of the oil will evaporate to the atmosphere. The amount
of evaporation and the speed at which it occurs depend upon the volatility of
the oil. An oil with a large percentage of light and volatile compounds will
evaporate more than one with a larger amount of heavier compounds. For
example, petrol, kerosene and diesel oils, all light products, tend toevaporate almost completely in a few days whilst little evaporation will
occur from a heavy fuel oil. In general, in temperate conditions, those
components of the oil with a boiling point under 200C tend to evaporate
within the first 24 hours. Evaporation can increase as the oil spreads, due to
the increased surface area of the slick. Rougher seas, high wind speeds and
high temperatures also tend to increase the rate of evaporation and the
proportion of an oil lost by this process.
Dispersion
Waves and turbulence at the sea surface can cause all or part of a slick to
break up into fragments and droplets of varying sizes. These become mixed
into the upper levels of the water column. Some of the smaller droplets will
remain suspended in the sea water while the larger ones will tend to rise
back to the surface, where they may either coalesce with other droplets to
reform a slick or spread out to form a very thin film. The oil that remains
suspended in the water has a greater surface area than before dispersion
occurred. This encourages other natural processes such as dissolution,
biodegradation and sedimentation to occur.The speed at which an oil disperses is largely dependent upon the nature of
the oil and the sea state, and occurs most quickly if the oil is light and of low
viscosity and if the sea is very rough. These factors led to the complete
dispersion of the oil spilled from the BRAER (Shetland Islands, United
Kingdom, 1993).
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The addition of chemical dispersants can accelerate this process of natural
dispersion.
Emulsification
An emulsion is formed when two liquids combine, with one
ending up suspended in the other. Emulsification of crude
oils refers to the process whereby sea water droplets become
suspended in the oil. This occurs by physical mixing
promoted by turbulence at the sea surface. The emulsion
thus formed is usually very viscous and more persistent than
the original oil and is often referred to as chocolate mousse
because of its appearance. The formation of these emulsions
causes the volume of pollutant to increase between three and four times.This slows and delays other processes which would allow the oil to
dissipate.
Oils with an asphaltene content greater than 0.5% tend to form stable
emulsions which may persist for many months after the initial spill has
occurred. Those oils containing a lower percentage of asphaltenes are less
likely to form emulsions and are more likely to disperse. Emulsions may
separate into oil and water again if heated by sunlight under calm conditions
or when stranded on shorelines.
Dissolution
Water soluble compounds in an oil may
dissolve into the surrounding water. This
depends on the composition and state of the oil,
and occurs most quickly when the oil is finely
dispersed in the water column. Components that
are most soluble in sea water are the light
aromatic hydrocarbons compounds such asbenzene and toluene. However, these compounds are also those first to be
lost through evaporation, a process which is 10 -100 times faster than
dissolution. Oil contains only small amounts of these compounds making
dissolution one of the less important processes.
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Oxidation
Oils react chemically with oxygen either breaking down into soluble
products or forming persistent compounds called tars. This process is
promoted by sunlight and the extent to which it occurs depends on the type
of oil and the form in which it is exposed to sunlight. However, this process
is very slow and even in strong sunlight, thin films of oil break down at no
more than 0.1% per day. The formation of tars is caused by the oxidation of
thick layers of high viscosity oils or emulsions. This process forms an outer
protective coating of heavy compounds that results in the increased
persistence of the oil as a whole. Tarballs, which are often found on
shorelines and have a solid outer crust surrounding a softer, less weathered
interior, are a typical example of this process.
Sedimentation/Sinking
Some heavy refined products have densities greater than one and so will sink
in fresh or brackish water. However sea water has a density of
approximately 1.025 and very few crudes are dense enough or weather
sufficiently, so that their residues will sink in the marine environment.
Sinking usually occurs due to the adhesion of particles of sediment or
organic matter to the oil. Shallow waters are often laden with suspended
solids providing favourable conditions for sedimentation.
Oil stranded on sandy shorelines often becomes mixed with sand and other
sediments. If this mixture is subsequently washed off the beach back into the
sea it may then sink. In addition, if the oil catches fire after it has been
spilled, the residues that sometimes form can be sufficiently dense to sink.
Biodegradation
Sea water contains a range of micro-organisms or microbes
that can partially or completely degrade oil to water soluble
compounds and eventually to carbon dioxide and water.Many types of microbe exist and each tends to degrade a
particular group of compounds in crude oil. However, some
compounds in oil are very resistant to attack and may not
degrade.
The main factors affecting the efficiency of biodegradation
are the levels of nutrients (nitrogen and phosphorus) in the water, the
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temperature and the level of oxygen present. As biodegradation requires
oxygen, this process can only take place at the oil-water interface since no
oxygen is available within the oil itself. The creation of oil droplets, either
by natural or chemical dispersion, increases the surface area of the oil and
increases the area available for biodegradation to take place.
Combined processes
The processes of spreading, evaporation, dispersion, emulsification and
dissolution are most important during the early stages of a spill whilst
oxidation, sedimentation and biodegradation are more important later on and
determine the ultimate fate of the oil. To understand how different oils
change over time whilst at sea, one needs to know how these weathering
processes interact. To predict this, some simple models have been developed
based on oil type. Oils have been classified into groups roughly according totheir density - generally, oils with a lower density will be less persistent.
However some apparently light oils can behave more like heavy ones due to
the presence of waxes.
One model uses the half-life for a group of oils to describe the persistence
and the time needed for the oil to dissipate. The half life is the time needed
for 50% of the oil to disappear from the sea surface. After six half-lives have
passed, about 1% of the oil will remain. This model, is shown in the
illustration below. Weather and climatic conditions will alter the rates shown
e.g. in rough weather a group 3 oil may dissipate in a timescale similar to a
group 2 oil.
The rate of removal of oil from the sea surface according to type
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Volume of oil and water-in-oil emulsion remaining on the sea surface is
shown as a percentage of the original volume spilled.
Group Density Examples
Group I less than 0.8 Gasoline, Kerosene
Group II 0.8 - 0.85 Gas Oil, Abu Dhabi Crude
Group III 0.85-0.95Arabian Light Crude, North Sea Crude Oils
(e.g. Forties)
Group IV greater than 0.95 Heavy Fuel Oil, Venezuelan Crude Oils
Although simple models such as this cannot predict the changes an oil
undergoes very precisely, they can provide clues about whether an oil is
likely to dissipate naturally or whether it will reach the shoreline. This
information can be used by spill responders to decide upon the most
effective spill response techniques and whether such techniques can be
initiated quickly enough.
Effects of Marine Oil Spills
Oil spills can have a serious economic impact on coastal activities and on
those who exploit the resources of the sea. In most cases such damage is
temporary and is caused primarily by the physical properties of oil creatingnuisance and hazardous conditions. The impact on marine life is
compounded by toxicity and tainting effects resulting from the chemical
composition of oil, as well as by the diversity and variability of biological
systems and their sensitivity to oil pollution.
Impact of oil on coastal activities
The effects of a particular oil spill depend upon many factors, not least the
properties of the oil.
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Contamination of coastal amenity areas is a common feature of many spills
leading to public disquiet and interference with recreational activities such
as bathing, boating, angling and diving. Hotel and restaurant owners, and
others who gain their livelihood from the tourist trade can also be affected.
The disturbance to coastal areas and to recreational pursuits from a single
spill is comparatively short-lived and any effect on tourism is largely a
question of restoring public confidence once clean-up is completed.Industries that rely on a clean supply of seawater for their normal operations
can be adversely affected by oil spills. If substantial quantities of floating or
sub-surface oil are drawn through intakes, contamination of the condenser
tubes may result, requiring a reduction in output or total shutdown whilst
cleaning is carried out.
Biological effects of oil
Simply, the effects of oil on marine life, are caused by either the physical
nature of the oil (physical contamination and smothering) or by its chemical
components (toxic effects and accumulation leading to tainting). Marine life
may also be affected by clean-up operations or indirectly through physical
damage to the habitats in which plants and animals live.
The main threat posed to living resources by the persistent residues of
spilled oils and water-in-oil emulsions ("mousse") is one of physical
smothering.
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The animals and plants most at risk are those that could come into contact
with a contaminated sea surface. Marine mammals and reptiles; birds that
feed by diving or form flocks on the sea; marine life on shorelines; and
animals and plants in mariculture facilities.
The most toxic components in oil tend to be those lost rapidly through
evaporation when oil is spilt. Because of this, lethal concentrations of toxic
components leading to large scale mortalities of marine life are relatively
rare, localised and short-lived. Sub-lethal effects that impair the ability of
individual marine organisms to reproduce, grow, feed or perform other
functions can be caused by prolonged exposure to a concentration of oil or
oil components far lower than will cause death. Sedentary animals in
shallow waters such as oysters, mussels and clams that routinely filter large
volumes of seawater to extract food are especially likely to accumulate oil
components. Whilst these components may not cause any immediate harm,
their presence may render such animals unpalatable if they are consumed byman, due to the presence of an oily taste or smell. This is a temporary
problem since the components causing the taint are lost (depurated) when
normal conditions are restored.
The ability of plants and animals to survive contamination by oil varies. The
effects of an oil spill on a population or habitat must be viewed in relation to
the stresses caused by other pollutants or by any exploitation of the resource.
In view of the natural variability of animal and plant populations, it is
usually extremely difficult to assess the effects of an oil spill and to
determine when a habitat has recovered to its pre-spill state. In recognition
of this problem detailed pre-spill studies are sometimes undertaken to define
the physical, chemical and biological characteristics of a habitat and the
pattern of natural variability. A more fruitful approach is to identify which
specific resources of value might be affected by an oil spill and to restrict the
study to meeting defined and realistic aims, related to such resources.
Impact of oil on specific marine habitats
The following summarizes the impact that oil spills can have on selected
marine habitats. Within each habitat a wide range of environmentalconditions prevail and often there is no clear division between one habitat
and another.
Plankton is a term applied to floating plants and animals carried passively by
water currents in the upper layers of the sea. Their sensitivity to oil pollution
has been demonstrated experimentally. In the open sea, the rapid dilution of
naturally dispersed oil and its soluble components, as well as the high
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natural mortality and patchy, irregular distribution of plankton, make
significant effects unlikely.
In coastal areas some marine mammals and reptiles, such as turtles, may be
particularly vulnerable to adverse effects from oil contamination because of
their need to surface to breathe and to leave the water to breed. Adult fish
living in nearshore waters and juveniles in shallow water nursery grounds
may be at greater risk to exposure from dispersed or dissolved oil.The risk of surface oil slicks affecting the sea bed in offshore waters is
minimal. However, restrictions on the use of dispersants may be necessary
near spawning grounds or in some sheltered, nearshore waters where the
dilution capacity is poor.
The impact of oil on shorelines may be particularly great where large areas
of rocks, sand and mud are uncovered at low tide. The amenity value of
beaches and rocky shores may require the use of rapid and effective clean-up
techniques, which may not be compatible with the survival of plants and
animals.
Marsh vegetation shows greater sensitivity to fresh light crude or light
refined products whilst weathered oils cause relatively little damage. Oiling
of the lower portion of plants and their root systems can be lethal whereas
even a severe coating on leaves may be of little consequence especially if it
occurs outside the growing season. In tropical regions, mangrove forests are
widely distributed and replace salt marshes on sheltered coasts and in
estuaries. Mangrove trees have complex breathing roots above the surface of
the organically rich and oxygen-depleted muds in which they live. Oil may
block the openings of the air breathing roots of mangroves or interfere with
the trees' salt balance, causing leaves to drop and the trees to die. The rootsystems can be damaged by fresh oil entering nearby animal burrows and the
effect may persist for some time inhibiting recolonisation by mangrove
seedlings. Protection of wetlands, by responding to an oil spill at sea, should
be a high priority since physical removal of oil from a marsh or from within
a mangrove forest is extremely difficult.
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Living coral grows on the calcified remains of dead coral colonies which
form overhangs, crevices and other irregularities inhabited by a rich variety
of fish and other animals. If the living coral is destroyed the reef itself may
be subject to wave erosion. The effects of oil on corals and their associated
fauna are largely determined by the proportion of toxic components, the
duration of oil exposure as well as the degree of other stresses. The waters
over most reefs are shallow and turbulent, and few clean-up techniques can
be recommended.
Birds which congregate in large numbers on the sea or shorelines to breed,
feed or moult are particularly vulnerable to oil pollution. Although oil
ingested by birds during preening may be lethal, the most common cause of
death is from drowning, starvation and loss of body heat following damage
to the plumage by oil.
Impact of oil on fisheries and mariculture
An oil spill can directly damage the boats and gear used for catching or
cultivating marine species. Floating equipment and fixed traps extendingabove the sea surface are more likely to become contaminated by floating oil
whereas submerged nets, pots, lines and bottom trawls are usually well
protected, provided they are not lifted through an oily sea surface.
Experience from major spills has shown that the possibility of long-term
effects on wild fish stocks is remote because the normal over-production of
eggs provides a reservoir to compensate for any localised losses.
Cultivated stocks are more at risk from an oil spill: natural avoidance
mechanisms may be prevented in the case of captive species, and the oiling
of cultivation equipment may provide a source for prolonged input of oil
components and contamination of the organisms. The use of dispersants
very close to mariculture facilities is ill-advised since tainting by the
chemical or by the dispersed oil droplets may result.
An oil spill can cause loss of market confidence since the public may be
unwilling to purchase marine products from the region irrespective of
whether the seafood is actually tainted. Bans on the fishing and harvesting of
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marine products may be imposed following a spill, both to maintain market
confidence and to protect fishing gear and catches from contamination.
Oil Spills in Rivers
Here are four reasons why oil spills in rivers differ from spills that occur in
the open ocean:
1. Some oils are denser than river water
Oil usually floats because it is less dense than the water it is floating on.
(Density is the mass, or weight, of a substance divided by its volume.) The
density of river water is usually about 1 gram per cubic centimeter (g/cc).
Water in the open ocean is more dense (usually around 1.02 to 1.03 g/cc)
because it contains more salt (the higher the salinity of water, the moredense it is). Densities of oils range from 0.85 g/cc for a very light oil (like
gasoline) to 1.04 g/cc (for a very, very heavy oil). Most types of oils have
densities between about 0.90 and 0.98 g/cc. These oils will float in either the
river or the ocean. But very heavy oils, which have a density of 1.01 g/cc,
would float in the ocean, but sink in a river. Sometimes the density of an oil
is so close to that of river water that the oil moves along the river partly
underwater. When such oil finally moves into the ocean at the river mouth, it
can refloat! This doesn't happen very often, but it's something we have to
think about whenever oil is spilled in a river.If spilled oil sinks, it can be very difficult to clean up. If something causes it
to pool on the bottom (for example, it may get trapped behind a sunken
vessel), then vacuum devices can be used to try and get the oil off the
bottom. As you might guess, this method may not be very effective because
vacuums may capture a lot of water and sand along with the oil.
2. Movement is usually downstream
Unlike in a bay or the open ocean, currents in a river are generally directed
downstream (except very close to the mouth of the river, where it enters the
ocean; here, a flood tide might actually reverse the flow of the surface
water). The greater predictability of river currents makes it easier to predict
which way the oil will move. Wind blowing across the river also affects
where the oil will come ashore. If the wind is blowing straight down the
river (as might happen on a river with high banks), it often will travel quite
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far down the river before it comes in contact with a beach. In order for the
oil to beach, the wind must blow the oil to one side of the river or the other.
You can see how the wind affects movement of an oil slick by looking at
some predictions from GNOME, our oil spill trajectory model (requires
QuickTime, available at Apple Computer's website).
3. Dams and locks influence oil movement.
Rivers sometimes contain dams or locks that slow or divert water flow.
Dams and locks also slow down the movement of spilled oil. In fact, oil
tends to collect in areas next to dams or locks, where it can be picked up
from the surface of the water by skimmers (devices that "skim" the oil off
the water surface), sorbent pads (big square pads of an absorbing material
that the oil will stick to), or sorbent booms (which work like sorbent pads,
but look more like a string of sausages made out of the inside of disposable
diapers!).
4. Vegetation may grow right at the water's edge
Along many rivers, plants and trees grow right up to the river's edge. Those
rivers don't have the open, sandy shores that you find along the many parts
of the open coast. It's much harder to remove oil from vegetation than from a
hard-packed sand beach. Spill responders try to protect the plants by using
booms, but if the vegetation gets oiled, responders often either cut, burn, or
flush it with water to try to get the oil out.
When we talk about oil spills, how much oil are we talking
about?Quite a lot:
The United States uses about 700 million gallons of oil every day.
The world uses nearly 3 billion gallons each day.
The largest spill in the United States so far was the Exxon Valdez spill
into Prince William Sound, Alaska in March 1989. An oil tanker ran
aground to cause this spill of almost 11 million gallons of crude oil.
While this was a big spill, it was actually only a small fraction--less
than 2 percent--of what the United States uses in 1 day!
These big numbers are hard to relate to everyday life, so let's make some
comparisons. To better understand how much 11 million gallons of oil is,
check the table below. It shows how many familiar rooms or buildings
would be filled up by the approximate amount of oil spilled from the Exxon
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Valdez. For example, that oil would have filled up 9 school gyms or 430
classrooms.
Total Volume Gallons Gyms Houses ClassroomsLiving
Rooms
Exxon Valdez Oil
Spill10,800,000 >9 108 430 797
School Gymnasium
(50' * 50' * 65')1,274,163 1 13 51 94
Average House (40' *
40' * 8')100,365 0.1 1 4 7
Average Classroom(20' * 20' * 8') 25,091 0.02 0.25 1 2
Average Living Room
(12' *18' * 8')13,549 0.01 0.125 0.5 1
Exxon Valdez Oil Spill
Four minutes after midnight on March 24, 1989, the Exxon Valdez hit Bligh
Reef in Alaska's Prince William Sound. Eleven million gallons of North
Slope oil spewed into one of the most bountiful marine ecosystems in the
world, killing birds, marine mammals, and fish, and devastating the
ecosystem in the oil's path. Exxon says that the Sound has recovered.
They're wrong.
In the 1989 spill, crude oil spread across Alaska's coastal seas covering
10,000 square miles, an area the size of Connecticut, Delaware, Rhode
Island, and 25 Washington, D.C.s combined! Within a week, currents and
winds pushed the slick 90 miles from the site of the tanker, out of PrinceWilliam Sound into the Gulf of Alaska. It eventually reached nearly 600
miles away from the wrech, contaminating 1,500 miles of shoreline about
the length of California's coast.
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Before the Exxon Valdez spill, conservationists warned about the potential
impacts of a major spill. In fact, just hours before the disaster, a group of
Valdez residents had gathered at the city council chambers to discuss the
impact of oil on their community. When the conversation turned to response
to a major spill, Riki Ott, a fisherwoman and toxicologist from Cordova
said, "It's not a matter of what if, but when."
Today, conservationists are again sounding the alarm about several risky
Alaskan oil development schemes and practices. First, the vast majority of
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oil shipped from Alaska is still carried in outdated, single-hull tankers.
Second, the Trans-Alaska Pipeline, which carries oil from the North Slope to
the port of Valdez, is aging, mismanaged, and in dire need of repair. In the
last 20 years, dozens of leaks have gone undetected, and workers have been
continually mistreated or intimidated from reporting the company's
environmental abuses to regulators.
Effects of the Spill
The Exxon Valdez disaster killed more wildlife than any other
environmental disaster in the nation's history, including:
More marine mammals and birds died than in any other oil spill, including
an estimated 3,500 to 5,500 sea otters, 300 harbor seals, and 14 to 22 killer
whales. "The Exxon Valdez spill killed nearly ten times as many birds as
any other U.S. or European oil spill," said seabird expert Dr. Michael Fry.As many as half a million birds died, including bald eagles, harlequin ducks,
marbled murrelets and loons.
Fish Critical spawning and rearing habitats, including over 100 salmon
streams, were besieged in oil. In 1993 there was an unprecedented crash of
the sound's Pacific herring population. The spill also caused a noticeable
decline in pink and churn salmon, Dolly Varden cutthroat trout and
rockfish.
Habitat Three national parks, three national wildlife refuges, one national
forest and designated wilderness were oiled.
Toxic Effects Linger
To the naked eye, Prince William Sound may appear "normal." But if you
look just beneath the surface, oil continues to contaminate beaches, national
parks, and designated wilderness. In fact, the Office of Technology
Assessment estimated that beach cleanup and oil skimming only recovered
3-4% of the Exxon Valdez spill.
A decade later, the ecosystem still suffers. Substantial contamination of
mussel beds persists, contributing to the decline of harlequin ducks. Thedepressed population of Pacific herring -- a critical source of food for over
40 predators including seabirds, harbor seals and Stellar sea lions -- is
having severe impacts up the food chain. Recent studies revealed that even
on "cleaned-up" washed beaches, mollusks and other invertebrates were far
less abundant than on comparable unspoiled beaches.
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Wildlife population declines continue for harbor seals, killer whales,
harlequin ducks, common loons, pigeon guillemots, red-faced cormorants,
and double-crested cormorants.
0il is more toxic than thought
Even before the spill, scientists knew a drop of oil could kill a bird's egg.
But after studying the impact of the Valdez spill, they now believe oil
pollution to be at least 100 times more toxic to fish. It is also more persistent
New studies by the National Marine Fisheries Service show that even very
low levels of weathered Exxon Valdez oil are toxic to the early life stages of
salmon and herring. The Exxon Valdez spill resulted in profound
physiological effects to fish and wildlife. These included reproductive
failure, genetic damage, curved spines, lowered growth and body weights,
altered feeding habits, reduced egg volume, liver damage, eye tumors, anddebilitating brain lesions.
CLEAN-UP TECHNIQUES
There are two approaches for responding to marine oil spills at sea: the
enhancement of natural dispersion of the oil by using dispersant chemicals,
and containment and recovery of oil using booms and skimmers. Sorbent
materials may be useful in the final stages of clean up as a polishing tool.Once oil strands on shore, a shoreline clean up will be necessary.
Despite continuing research, there has been little change in the fundamental
technology for dealing with oil spills. Alternative techniques are constantly
being sought and old techniques reassessed. Two techniques currently
receiving fresh attention are in-situ burning and the enhancement of the
natural biodegradation of oil through the application of micro-organisms
and/or nutrients.
Chemical Dispersants
Dispersants are a group of chemicals designed to be
sprayed onto oil slicks, to accelerate the process of
natural dispersion. Spraying dispersants may be the
only means of removing oil from the sea surface,
particularly when mechanical recovery is not possible. Their use is intended
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to minimise the damage caused by floating oil, for example to birds or
sensitive shorelines. However, in common with all spill response options,
the use of dispersants has its limitations and should be carefully controlled.
Dispersant use will be dependent upon national regulations governing the
use of these products.
How chemical dispersion works
Natural dispersion of an oil slick occurs when waves and other turbulence at
the sea surface cause all or part of the slick to break up into droplets and
enter into the water column. The addition of dispersants is intended to
accelerate this process.
Dispersants have two main components, a surfactant and a solvent.
Surfactants are molecules which have an affinity for two distinct liquids
which do not mix, acting as an interface between them. A part of thesurfactant molecule used in dispersants has an attraction to oil (i.e. it is
oleophilic) while another part has an attraction for water (i.e. it is
hydrophilic). Common washing-up liquid is another example of a product
that contains surfactants.
When a dispersant is sprayed onto an oil slick, the interfacial tension
between the oil and water is reduced, promoting the formation of finely
dispersed oil droplets. These droplets will be of varying sizes and although
the larger ones may rise back to the surface some will remain in suspension.
If dispersion is successful, a characteristic plume will spread slowly down
from the water surface a few minutes after treatment. However, the effective
distribution of surfactant throughout the oil is crucial to the success of the
process. To achieve the required distribution, most dispersants contain a
suitable solvent which allows the dispersant to penetrate into the slick and
acts as a carrier for the surfactant.
Figure 1: Mechanism of aerial dispersion.
Limitations
Dispersants have little effect on very viscous, floating oils, as they tend to
run off the oil into the water before the solvent can penetrate. As a general
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rule, dispersants are capable of dispersing most liquid oils and emulsions
with viscosities of less than 2000 centistokes, equivalent to a medium fuel
oil at 10-20C. They are unsuitable for dealing with viscous emulsions
(mousse) or oils which have a pour point near to or above that of the ambient
temperature. Even those oils which can be dispersed initially become
resistant after a period of time as the viscosity increases as a result of
evaporation and emulsification. For a particular oil, the time available before
dispersant stops being effective depends upon such factors as sea state and
temperature but is unlikely to be longer than a day or two. Dispersants can,
however, be more effective with viscous oils on shorelines because the
contact time may be prolonged allowing better penetration of the dispersant
into the oil.
Types of dispersant
There are three main types of dispersants:
Type 1 dispersants are based on hydrocarbon solvents with between15 and 25% surfactant. They are sprayed neat onto the oil as pre-
dilution with sea water renders them ineffective. Typical dose rates
are between 1:1 and 1:3 (dispersant:oil).
Type 2 dispersants are dilutable concentrate dispersants which are
alcohol or glycol (i.e. oxygenated) solvent based with a higher
surfactant concentration. Dilution is normally 1:10 with sea water.
Type 3 dispersants are also concentrate dispersants with a similar
formulation to type 2 products. However, they are designed to be used
neat and typical dose rates are between 1:5 and 1:30 (neat
dispersant:oil).
Type 1 and 2 dispersants require thorough mixing with the oil after
application to produce satisfactory dispersion. With type 3 products, the
natural movement of the sea is usually sufficient to achieve this. The lower
application rates required with concentrates mean that types 2 and 3 have
largely superseded type 1 dispersants for application at sea.
Methods of application at sea
Dispersants can be applied to open water by a variety
of methods. In general workboats are more suitable for
treating minor spills in harbours or confined waters.Large multi-engine planes are best equipped for
handling large off-shore spills. Small, single-engine
aircraft and helicopters are suitable for treating smaller
spills and near shore areas. Regardless of the method used, it must be able to
apply the dispersant effectively. In order to minimise losses due to wind
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drift, a uniform spray pattern of larger droplets, "rain drops", are required
rather than a fog or a mist.
Vessel spraying
Methods for dispersant application from sea going vessels include spraying
through a set of nozzles fixed on outboard booms, and spraying from
modified fire monitors. In a typical boom system, the booms are mounted as
far forward as possible to ensure the dispersant is applied ahead of the bow
wave which helps to mix the dispersant and oil. Spray units can be portable
or permanently installed on a vessel. Systems are available that deliver neat
dispersant or, with a separate water pump, apply dispersant diluted with
water.
Fire monitors can only be used to apply diluted
dispersant. When used, the output must be monitored
to minimise excessive over or under dilution and thereis a tendency to waste dispersant because of the poor
coverage of a strong water jet. Advantages of this
system are the high pump capacity which allows the vessel to travel at a
greater speed and the elimination of the problem of booms striking the water
surface as the vessel pitches and rolls.
Vessel spraying is of limited utility as can only relatively small amounts of
dispersant can be applied and because of the difficulties of locating the oil
from a vessel. Furthermore, when slicks become fragmented or form narrow
windrows, it is inevitable that some dispersant will be sprayed onto unoiled
sea. These problems can be partially overcome by controlling the operation
from a spotter plane but this requires good air to sea communications.
Aerial spraying
The spraying of dispersant from aircraft has the significant advantages of
rapid response, good visibility, high treatment rates and optimum dispersant
use. In addition, aircraft allow treatment of spills at greater distances from
shore than with vessels.
Two categories of aircraft are used: those designed for
agricultural or pest control operations which require
minor modification for dispersant application, andthose which have been specifically adapted for the
application of dispersant. Several types of helicopter
have also been converted although most are able to carry underslung bucket
spray systems without the need for modifications. The ideal aircraft will be
determined primarily by the size and location of the spill, although in reality
local availability will be the crucial factor. The endurance, fuel consumption,
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turn around time, payload and the ability to operate from short or improvised
landing strips are all important. In addition, the aircraft should be capable of
operating at low altitude and relatively low speeds (50-150 knots) and be
highly manoeuvrable.
Only type 3 dispersants are suitable for aerial spraying, since they require no
mixing beyond that provided by the natural movement of the sea. The
relatively low dose rate required also makes the best use of available
payload.
Shoreline application
Dispersants can also be used on some shorelines,
including beaches, rocks and sea walls, particularly
during the final stages of clean-up. However, it is
important to remove the bulk of the stranded oil first,
by other means. Shores subjected to strong wave action
are often cleaned naturally and they should not besprayed unless the oil has to be removed immediately.
All three dispersant types may be used for shore cleaning although those
containing hydrocarbon solvents may be more effective with viscous oils
because of their greater ability to penetrate into the oil.
The most appropriate application equipment depends on the type of
shoreline substrate to be cleaned, the ease of access and the scale of the
operation. For small inaccessible beaches and coves, portable back-pack
sprayers are the most suitable. For large expanses of shoreline, purpose-built
vehicles or tractors can be used.
Environmental considerations
The use of dispersants has in the past tended to provoke controversy since
their application can be seen as a deliberate introduction into the sea of an
additional pollutant in addition to the short term increase in hydrocarbon
concentration in the water. However, there is a wealth of laboratory data
indicating that dispersants and oil/dispersant mixtures exhibit relatively low
toxicity to marine organisms.
The rapid dilution of the dispersed oil, the proximity to sensitive areas aswell as the direction of currents and the mixing depths of surface waters are
all factors which should be considered when deciding upon dispersant use.
In the open sea, concentrations after spraying are unlikely to remain high for
more than a few hours and significant biological effects are therefore
improbable. In shallow waters close to the shore, where water exchange is
poor, higher concentrations may persist for long periods and may give rise to
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adverse effects. However, the controlled application of dispersants may, on
occasions, be beneficial in that it may reduce damage to adjacent
ecologically sensitive shorelines by surface oil.
The decision whether or not to use dispersants rather than other response
options, will need to take into account cost-effectiveness and conflicting
priorities for protecting different resources from pollution damage. On
occasions the benefit gained by using dispersants to protect coastal
amenities, sea birds and intertidal marine life may far outweigh the
disadvantages such as the potential for temporary tainting of fish stocks.
Certain resources such as water intakes, mariculture facilities or fish
spawning areas are difficult to protect from dispersed oil and spraying may
be decided against, near to these resources, even if the risk of damage is low.
Detailed contingency planning can aid in this decision process.
Containment and Recovery of Floating Oil
The use of booms to contain and concentrate floating oil prior to its recovery
by specialised skimmers is often seen as the ideal solution to a spill since, if
effective, it would remove the oil from the marine environment.
Unfortunately, this approach suffers from a number of fundamental
problems, not least of which is the fact that it is in direct opposition to the
natural tendency of the oil to spread, fragment and disperse under the
influence of wind, waves and currents. In rough seas, a large spill of a lowviscosity oil such as a light or medium crude oil can be scattered over many
square kilometres within just a few hours. Oil recovery systems typically
have a swath width of only a few metres and move at slow speeds whilst
recovering oil. Thus, even if they can be operational within a few hours, it
will not be feasible for them to encounter more than a fraction of a widely
spread slick. This is the main reason why containment and recovery at sea
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rarely results in the removal of more than a relatively small proportion of a
large spill, at best only 10 - 15% and often considerably less.
A common difficulty when deploying booms and skimmers to recover oil is
controlling the movements and activities of vessels and directing them to the
thickest areas of oil. This can be overcome by using aircraft equipped with
air to sea communications. Overall, containment and recovery operations at
sea require extensive logistical support, which should not be underestimated.
The limitations that poor weather and rough seas impose on operations at sea
are seldom fully appreciated. Handling wet, oily, slippery equipment on
vessels which are pitching and rolling is difficult and can place personnel at
risk. Winds, currents and wave action seriously reduce the ability of boom to
contain and of skimmers to recover oil. In practice, the most efficient
recovery of oil is achieved only under calm conditions. When containmentand recovery is attempted it is important to select equipment that is suitable
for the type of oil and the prevailing weather and sea conditions. Efforts
should target the heaviest oil concentrations and areas where collection will
reduce the likelihood of oil reaching sensitive resources and shorelines.
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As the oil weathers and increases in viscosity, clean up techniques and
equipment will need to be re-evaluated and modified. For example, the types
of pumps and skimmers may need to be changed.
The first stage of an effective response is to deploy boom to limit further
spreading and concentrate the oil for recovery. Booms vary considerably in
their design, but all normally incorporate the following features:
1. freeboard to prevent or reduce splashover;
2. a sub-surface skirt to prevent or reduce escape of oil under the boom;
3. flotation by air or some buoyant material;
4. longitudinal tension member (chain or wire) to provide strength to
withstand the effects of winds, waves and currents. This is often used to
provide ballast to keep the boom upright in the water.
There are many designs ranging from small, lightweight models designed for
manual deployment in harbours, to large, robust units which usually need
cranes and sizeable vessels to handle them, which are designed for use in theopen sea.
The most important characteristic of a boom is its oil containment or
deflection capability, determined by its behaviour in relation to water
movement. It should be flexible to conform to wave motion yet sufficiently
rigid to retain as much oil as possible. No boom is capable of containing oil
against currents greater than 0.7 knot (0.35 metres per second) at right
angles to the boom, irrespective of boom size or skirt depth. This factor
limits the speed at which booms can be towed to less than 0.5 knots. Oil
patches or water turbulence appearing on the down-current side indicate that
the boom is failing. Other important boom characteristics are strength, ease
and speed of deployment, reliability, weight and cost.
It is essential that a boom is sufficiently robust for its intended purpose and
will tolerate inexpert handling, since trained personnel are not always
available. Strength is required particularly to withstand the forces of water
and wind when being towed. Ease and speed of deployment combined with
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reliability are clearly very important in a rapidly changing situation and may
strongly influence the choice made. Practical limitations of strength, water
drag and weight mean that generally only relatively short lengths (tens to a
few hundred metres) can be deployed and maintained in a working
configuration. Towing booms at sea, for example in U or J configurations, is
a difficult task requiring specialised vessels.
Because of the difficulties of operating multi-ship towed boom systems,
specialised ships have been built which incorporate sweeping arms,
skimming devices and on board oil storage. The limitations posed by sea
conditions still also apply to these vessels, the larger examples of which are
unable to work in shallow inshore waters. The efficiency of a specialised
vessel will mainly be determined by the inbuilt oil recovery system or
skimmer which is deployed. Because of the relatively narrow sweeping
width, they are best suited to recovering oil in ribbons or windrows.
Skimmers which are used to recover oil from the water all incorporate an oil
recovery element and some form of flotation or support. In addition a pump
or vacuum device is necessary to transfer recovered oil and water to storage.
Because skimmers float on the water surface, they experience many of the
operational difficulties which apply to booms, particularly those posed by
wind, waves and currents. Even moderate wave motion greatly reduces the
effectiveness of most skimmer designs. In calm waters more satisfactoryperformance can be achieved provided the skimmer is suited to the viscosity
of the oil in question.
The simplest skimmers are suction devices which remove oil from the water
surface directly or via a weir, although these tend to pick up a lot of water at
the same time. More complex units rely on the adhesion of oil to metal or
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plastic disks, or oleophilic belts or ropes. Yet others employ brush systems
or are designed to generate vortices to concentrate the oil.
It is important to have adequate temporary oil storage facilities available,
otherwise this becomes a bottleneck to successful oil recovery. Temporary
storage needs to be easy to handle, and easy to empty once full so that it can
be used repeatedly. Suitable units include barges and portable tanks which
can be set up on vessels of opportunity. When recovering very viscous oils,
storage tanks may need to be heated to allow them to be emptied.
Many factors should be considered when selecting skimmers. The intended
use and expected operational conditions should first be identified before
criteria such as size, robustness and ease of operation, handling and
maintenance can be weighed up. The most important factors to consider are
the viscosity and adhesive properties of spilled oil, including any change in
these properties over time. At oil terminals and refineries where oil type may
be predictable, specialized units may be selected, but otherwise it ispreferable to retain versatility and select units which can deal with a range of
oils. It is also important to recognize the difficulties posed by floating debris,
both natural (sea weeds, sea grasses, trees and branches) and man made
(plastic, glass, timber). Skimmers may need trash screens and regular
unblocking where debris is common, such as near urban areas or river
mouths. They will also need continuous maintenance by specialist staff and
a supply of spare parts.
Because of the various constraints imposed on skimmers in the field, their
design capacities are rarely realized. Experience from numerous spills has
consistently shown that recovery rates reported under test conditions cannot
be sustained during a spill and so it is important not to have unrealistic
expectations about what can be achieved.
Once oil recovery is completed, booms and skimmers will need to be
cleaned, overhauled and repaired, ready for use in the next spill. It is also
important to inspect and test equipment regularly so that it is in good
working order, and to maintain personnel training standards by regular drills.
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Prevention of oil spills
As hard as people might try, accidents do occur inevitably. However, there
are ways to limit such accidents and spills and avenues to ensure that
response is immediate.
What's Being Done to Make Shipping Safer?
Double hulls or double bottoms are being introduced and, since 1993, are a
requirement for all new tankers. Ships' crews must be well trained and
experienced. Electronic charting is being introduced. It is a computer-based
video display that allows navigators to track the ship's course in relation to
hazards, and warns the navigator of potential danger, both visually and
audibly. All ships must have radar systems to improve navigation. A
technology known as "load-on-top" allows oil and water mixtures fromcleaning to separate, resulting in less pollution. Strict fire safety regulations
apply on board.
Comparison between a conventional and a double hull.
Vessel Design
There are a few common designs for large ships, including double hulls and
double bottoms. Each design has its advantages and drawbacks. Although
structural improvements to ships make tanker traffic safer, recent advances
like double hulls will not eliminate spillage under all circumstances.
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Oil Storage and Handling
As much as 92 percent of all oil spills involving tankers happen at a terminal
when oil is being loaded or discharged. Precautions at terminals include
monitoring oil flows, regular inspections of hoses and connections, and
routine checks of tank levels. Weather conditions are monitored closely.
Dikes around storage tanks prevent oil from escaping if an accident does
occur.
Marine Traffic Control
Marine traffic control systems are in place in many major shipping areas.
The systems can be as simple as traffic lanes in heavily travelled waters or
they can be very sophisticated networks. Governments are introducing
control systems for marine operations similar to those we take for granted atairports. But no matter how simple or complex, traffic control greatly
minimizes collisions and the risk of ships running aground.