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.