The causes of oil
spills can be divided into two groups: accidents and operations.
Accidents:
There is often a large
oil loss in accidents, up to 1/5 have an oil loss of over 700 tonnes.
·
collisions: a common accident with 475
occurring between 1974-1999
·
hull failures: these accidents have
occurred the most between 1974-1999, with 671 hull failures between these two
years
·
fires and explosions: this is the most
uncommon type of accident, only occuring 154 times between 1974-1999
·
groundings: a common accident with 518
occurring between 1974-1999, and the greatest number of oil losses over 700
tonnes
Operations:
Most oil losses occur
when ships are carrying out routine operations at ports or oil terminals, but
the majority of such spills are small, with 93% of them producing a spillage of
less than 7 tonnes.
loading/discharging:
commonest cause of oil spillages (either during routine operations or resulting
from accidents), with 3070 occurring between 1974-1999
bunkering:
the
least common operational oil loss with only 566 occurring between 1974-1999
What
happens to the oil spill?
The oil can end up
anywhere, depending on factors such as wind speed, wind direction, sea
conditions and/or the volume of oil spilled.
When the oil is
spilled, it spreads over the sea surface and as oil has a lower density than
water it stays afloat. The speed at which it spreads depends on how viscous the
oil is. If it has a high viscosity, it will spread more slowly than if it has a
low viscosity. As time passes the oil slick starts to break up and form narrow
strips due to wind, water turbulence and waves. Lighter components of the oil
will evaporate into the atmosphere, an oil with a high percentage of light,
volatile compounds will evaporate more than one with a larger proportion of
heavy compounds. Evaporation increases with high temperatures, high wind speeds
and when the surface area increases. Waves and water turbulence can cause the
oil to break up into little droplets (this is called dispersion), the smaller
droplets will stay suspended in the sea water, whereas the larger tend to rise to
the surface where they either spread out into a thin film of oil or where they
rejoin with other droplets to form another slick.
These are the main
processes involved in the early stages of an oil slick.
Oils tend to react
chemically with the oxygen in the water and air, this is called oxidation. The
rate of oxidation increases as the light intensity increases. Tars form as the
thick layers of oil oxidise, producing tarballs which often wash up on
seashores. The density of crude heavy oils can exceed 1g/cm3, so such would
sink in fresh water. However, very few crude oils have a density greater than
that of sea water (1.025 g/cm3) so rarely sink in marine environments. Sinking
may occur if the oil mixes with sand from the shore and is then washed back into
the sea. Sedimentation may also occur if the oil catches fire - the residues
can have a higher density than sea water and will sink. Another fate of oil is
biodegradation. This is the process by which micro-organisms and microbes feed
off the oil, partially degrading it initially into water soluble compounds and
eventually into carbon dioxide and water. However, not all compounds can be
degraded as some are very resistant to micro-organisms and microbes. The level
of phosphorus (P), nitrogen (N) and oxygen (O) in the water must be high enough
for the microbes and micro-organims to live, because their nutrients are P and
N and they need O to respire.
These processes are
more important later on and determine the ultimate fate of the oil.
Effects
of oil spills:
Oil spills have
affected many people and many industries. They affect both the economy and the
environment. Some of the things affected are:
·
marine life
·
local industries (often tourist
industry)
·
fishing industry
Effect
on marine life:
People immediately
think of birds and fish as being species affected by an oil spill. On the news
you will often see large numbers of dead birds and dead fish washed up on the
beach. Some birds die because they have ingested oil. However, more commonly
the birds drown because they stick to the oil and cannot get out of the water,
starve because there's no food left for them because the fish have been
poisoned by the oil or lose too much body heat because of damage to their
plumage by the oil. Fish die of starvation because of loss of food or from
being poisoned by the oil.
Some marine mammals and
reptiles, such as dolphins, whales and turtles are very vulnerable to oil
spills because they have to be able to surface to breathe and the reptiles also
need to leave the water to breed. The layer of oil makes surfacing difficult
and the animals drown.
Vegetation in the water
can be vulnerable to light crude or light refined oil spills if the oil reaches
the root area of the plants. However, a thick coating of oil on the leaves does
almost no damage if it occurs outside the growing season. Loss of vegetation
can lead to more animal deaths as they lose sources of food. Living coral is
also vulnerable to oil slicks. If the living coral dies, then the reef of coral
can be destroyed by wave erosion. This means many fish and animals lose their
homes.
The time taken for
these damaged populations to recover depends on many factors and is highly
variable.
Effect
on local industries:
Oil, tarballs, dead fish and birds all get washed up
on the shores and the oil slick inteferes in activities such as fishing, sailing,
swimming, ... The local tourist industry suffers because tourists are not
interested in coming to a coastal area where they cannot do the activities as
listed above. Industries that rely on clean seawater for routine operations can
also suffer because operations have to be stopped while the water is cleaned.
Effect
on fishing industry:
The fishing industry suffers badly when an oil spill
occurs. Firstly because the fish are often covered in oil, or have swallowed
oil making them poisonous. Also a large number of fish die, decreasing the
number of fish that could have possibly been caught. It is also difficult for
boats to sail because the oil can damage them and the devices they use to catch
the fish. However there are usually no long-term effects because the normal
over-production of fish eggs means there will be few losses.
Clean-up
techniques:
Three
possible methods of removing the oil:
1.
Dispersant Chemicals:
These are chemicals which enhance the natural
dispersion of oil. This is the most common method used for oil slicks,
especially when mechanical recovery is impossible. They are used to reduce the
extent of damage caused by the floating oil. However, there are some
limitations on chemical dispersants as they can cause damage themselves if they
are not controlled.
How
they work: these chemicals increase the rate of natural dispersion.
When the dispersant is sprayed onto the oil slick the oil forms droplets of
variable size (this process is called dispersion), the larger droplets float
and the smaller droplets remain in suspension. Dispersants have two main
components, a solvent and a surfactant.
Methods
of application: workboats can be used for small spills
in confined areas of water, but for the larger, off-shore oil slicks large
aeroplanes are used and for small oil slicks near the shore small aeroplanes
and helicopters are used. A spray of "rain drop" sized droplets is
used in order not to lose too much in wind drift.
Limitations:
Chemical dispersants have little effect on oil with a high viscosity as the
chemicals run off the oil before the solvent can work. Oils that the
dispersants have an initial effect on become resistant after a while because
the viscosity increases as the more volatile oils evaporate.
2.
Bioremediation:
This is the name for the 2 processes used to promote
the natural biodegradation process. The two processes are bioaugmentation
(application of microbes to degrade oil) and biostimulation (addition of
nutrients).
Bioaugmentation:
This is used in many waters (both sea and fresh) to
degrade waste such as oil as well as raw sewage and industrial discharges.
Microbes are added to the water to feed on these wastes. The species of
microbes added will not compete with the naturally occuring species so that
degradation is as efficient as possible.
Biostimulation:
For the microbes to work efficiently there has to be
enough phosphorus, nitrogen and carbon available. In the case of an oil spill,
the carbon level escalates and the nitrogen and phosphorus levels are too low
to allow the microbes to work at optimal efficiency. Fertilisers containing
these 2 elements are added so the microbes are able to degrade the oil.
Limitations:
Even though these processes seem attractive, they
cannot be taken at face value. If bioremediation is used on oil floating on the
surface, the materials added will quickly dilute and be lost. Oxygen is also
required, which is not present in the oil itself, only at the water/oil
interface. Bioremediation is therefore not suitable for the removal of large
volumes of oil. The processes of bioremediation and bioaugmentation currently
available are too slow to prevent the majority of the oil from reaching the
shoreline. They can be physically and biologically harmful in some habitats
such as salt marshes.
3.
In-situ burning:
This is the name given
to the burning of the oil while it is still at sea. Technically this seems like
a good method which will remove most of the oil, however in reality there are a
number of problems with this technique including: production of a lot of smoke,
formation of residues, ignition of oil, and safety concerns.
Smoke:
when oil burns large clouds of black smoke occur which react with the normal
clouds and causes oily rain which contaminates farm crops and animals. Two oil
accidents were related to on board fires on the Castillo de Bellver (in 1983 on
the coast of South Africa) and the Aegean Sea (in 1992 on the coast of Spain),
both accidents caused problems with local areas: the Castillo de Bellver cause
contamination of sheep and wheat and the Aegean Sea cause temporary evacuation
of the city of La Coruna. These were not intentional in-situ burning however
these can be effects of the smoke produced by burning the oil.
Residue:
the residues formed by in-situ burning are heavily viscous and are extremely
hard to remove from the sea and the shorelines. These residues affect fishing
gear, boats and the shoreline. Some of the residues sink and can poison the
sealife and is almost impossible to recover.
Ignition:
the time it takes for the ignition and fire safety devices to be set up many of
the lighter fractions of the oil will have already evaporated making ignition
more difficult. Ignition can be done by different methods, from simple petrol
or diesel soaked cloths to sophisticated devices such as the helitorch (a
flamethrower attached to a helicopter).
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