Oil

Characteristics, Behaviour and Effects of Oil

Crude oil is an extremely complex mixture of hydrocarbons together with organic compounds of sulphur,
nitrogen and oxygen. The hydrocarbons can be divided into three major groups based on their chemical
structures: the n-alkanes (paraffins), cycloalkanes (naphthenes) and aromatics. The composition of the
various fractions gives some indication of their likely toxicity. Alkanes are rapidly lost by evaporation, have
a narcotic effect and are the most readily biodegradable group. Aromatics are the most soluble, can be
photodegraded and are the most toxic component of the oil. Naphthenes have intermediate properties. In
general the smaller molecules of each fraction are more toxic than the larger molecules and the low
toxicity of weathered crude (containing high molecular weight compounds) compared to fresh crude has
been demonstrated. North Sea oils are likely to be initially more toxic than other oils because of their
lighter quality, but, where conditions favour evaporation, the toxicity will be quickly reduced. If, however,
oil is dissolved or oil droplets are entrained in the main water body, as may occur in rough weather, a
greater proportion of the toxic compounds will be retained. Weathered crudes containing the heavier
molecules are relatively non-toxic to marine organisms but like other oils will constitute a hazard to birds.

After a spill, oil 'weathers', changing in composition through evaporation, solution in seawater, chemical
oxidation and biological degradation, the relative rates of these processes depending on local conditions.
In the case of oils with fairly low specific gravities such as North Sea crudes (with a boiling point below
250°C), up to 40 %, including some of the components most toxic to marine life, may be lost within one
day. The rate of loss depends on local conditions such as wind, temperature and the turbulence of the
water. The loss of volatile components from North Sea oils is slightly greater than for other crude oils, but
the remaining 'weathered' residue (the proportion boiling above 250°C) will, like other crude oils, degrade
relatively slowly. Less than 2 % of the oil is removed by photo-oxidation and auto-oxidation. Experiments
applicable to temperate waters have shown that after 2-5 months about a quarter of the weathered oil is
broken down by micro-organisms and that this process continues, but at an ever-decreasing rate. The
heavier residues from a spill will either pick up suspended biological and sediment particles until they sink
to the sea bed, or will form tar particles. These particles in turn will eventually sink, or be washed up on
the coast.

There is a tendency for oil to form a water-in-oil emulsion known as 'mousse' which greatly increases the
persistence of slicks. The stability of this emulsion varies depending upon the origin of the oil, although
laboratory tests have shown that some North Sea oils when subjected to emulsification, broke down after
only 12 hours. Small quantities of oil spilled at sea can be expected to break down and disperse naturally
but the larger the spill and the closer it is to the coast the more likely it is to reach the shore. Past
experience suggests that some part of any spill of 1,000 tonnes or more in the seas around the British
Isles, if left untreated, is likely to persist in sufficient quantity to reach some part of the British coast or
that of neighbouring countries. North Sea crude oil toxicity is likely to be reduced in weather and sea
conditions where rapid evaporation of the light toxic compounds can occur. If the oil is distributed in the
water column, as may happen in rough weather or when oil is treated with dispersant, more of the toxic
compounds are retained or dissolved although these will be rapidly diluted with sea water.

Some refined oil products can present a much more serious pollution hazard than crude oils. The 'lighter'
products (those with the lowest boiling point) ranging from petrol to marine diesel, are classed as 'non
persistent', as they are rapidly lost by evaporation and degradation. However they are also readily
dispersed into the body of the sea by wave action and turbulence. Especially in confined situations they
can cause conspicuous and serious initial biological damage (e.g. in some spillages in the United States).
Where they have become trapped in sediments their local effects have been prolonged. Some batches of
marine diesel, a product of variable toxicity, can have a very high content of aromatic fractions (40 % or
more), with other toxic materials such as phenols and sulphur compounds also present. Bunker oils cause
damage chiefly through the smothering action of their heavy residues, but also contain harmful soluble
fractions.

The complex composition and weathering of crude oil, the difference between the modes of presentation
to the organism, and the toxic and sublethal actions of the various components prevent any simple
prediction of the harmful consequences to marine organisms of a given spillage. The effects of exposing an
organism to oil depend on the characteristics of the organism and its habitat, as well as on the quantity
and state of the oil. Oil may act physically to blanket organisms, causing death by preventing vital
processes such as respiration, or in less severe cases hindering other processes such as movement and
feeding. Organisms may be exposed to oil in the form of fine droplets, as components adsorbed onto
particulate material, or as soluble fractions in the water. At high concentrations, these could cause
narcotisation or death while at lower levels they could interfere with physiological or behavioural
processes that ultimately affect the survival or reproduction of the organisms. Most of the literature on
the biological effects of oils or their constituents is concerned with acute toxicity, i.e. with effects rapidly
leading to death. Chronic effects (i.e. long-term) may still result in the death of the individual, and even
sub-lethal effects may reduce the viability of the population, and hence the structure of the biological
community. Within limits, organisms can adapt to changed conditions and increased stresses, especially
those of a temporary nature.

The surface waters of the open sea contain the planktonic plants and animals which form the basis of
most of the marine food chain, as well as the eggs or larvae of important commercial species of fish.
These are particularly sensitive to damaging effects of oil slicks. Plankton exposure to low concentrations
of dispersed oil and its soluble fractions has been shown to affect plant productivity, especially of the
more sensitive species, and the fecundity of copepods (small crustaceans of great importance in the
marine food web). Information that exists on the effects of spilled oil at sea suggests that there have
been plankton kills after dispersal of highly aromatic oils (or of crude oils by the use of dispersants based
on highly aromatic solvents). The effects have been temporary owing to the rapid loss or dilution of toxic
fractions and the mixing of water masses bringing in fresh plankton. The possibility of sub-lethal effects
cannot be ruled out, but they are in any case far less likely in the open sea than in enclosed conditions or
where there is a chronic source of pollution.

After a fresh spill, the oil film gives rise to a shallow layer of hydrocarbons dispersed in the surface water
at relatively high concentrations. The wind, sea state, hydrography of the area and the viscosity of the
oil all influence this process. Sampling directly below oil slicks has shown that the hydrocarbon
concentration decreases progressively with depth and with time and when dispersants were applied there
was initially a much higher concentration of oil droplets in the water; but these rapidly dispersed. Oil spill
dispersants consist of one or more surface active agents (surfactants) mixed with a solvent and
sometimes a stabiliser. The surfactant molecule is composed of two types of group, one type having an
affinity to water (hydrophilic) and one with an affinity for oil (oleophilic). The surfactant molecule will
therefore orient itself between the oil and water layers with the hydrophilic groups in the water and the
oleophilic groups in the oil producing fine oil droplets when the water and oil are mixed. Energy normally
has to be applied to ensure good mixing. The high surface area created in the act of dispersion enhances
the overall rate of degradation of the oil. The concentration of hydrocarbons in sea water samples from
beneath crude oil slicks at sea are so low as to suggest that only sublethal or cumulative effects (rather
than acute toxic effects) might be expected. However in restricted inshore waters, if there is poor
dispersion, the effects of an oil spill could be more serious.

Some oil components have been shown to cause mortality of fish eggs and larvae under laboratory
conditions and it seems likely that some damage could occur to eggs and larvae floating just below the
surface of the water when oil is spilled on it. Similarly, water containing small quantities of dispersed or
dissolved oil is likely to contain components toxic to eggs and larvae. Bottom-living organisms play a large
part in the food chain of the sea as consumers, and themselves often form food for bottom feeders such
as fish and crustaceans. Oil carried to the sea bed could affect the eggs of bottom-spawning fish, such
as herring, which are laid in limited spawning areas of hard gravelly bottoms.

Shallow waters of large sandy bays and many estuaries are valuable fish nursery grounds and important
for their shell fisheries and as a source of bait. Fish appear to be able to tolerate intermittent small spills
but where large or repeated spills occur in restricted waters some mortality may result, depending on the
local circumstances (quantity and type of oil, depths of water etc.). In waters subjected to oil
contamination fish are likely to take up oil by direct contact or through the food chain if they do not move
away from the area. Tainting occurs when a very low level of certain oil components is present in the
flesh and makes the fish inedible. However the taint is lost when ambient hydrocarbon levels return to
normal.

Inshore species of mollusc, such as oysters, cockles and mussels, are very liable to damage from oil
pollution as they are usually found intertidally or in very shallow water and are sedentary filter feeders,
passing large volumes of water over their gills. A large accidental spill or chronic pollution could make the
habitat unsuitable for these species or render them commercially unacceptable because of taint. But the
contamination caused by a single pollution incident does not usually lead to any permanent damage and
shell-fish purify themselves when they are transferred to clean water. Crustacean shellfish such as crabs
and lobsters which live on open coasts and scampi which are found further offshore could also be
temporarily tainted.

Oil pollution generally does little harm to life on exposed shores. On rocky coasts oil may be plastered on
rocks, usually in the upper tidal zone, but much will be removed by surf action. In the splash zone it will
be more gradually reduced in quantity by aerial weathering. For the most part little permanent harm is
done to littoral seaweeds as oil does not cling to mucoid surfaces, and barnacles and mussels can close
themselves off and survive for several days beneath a temporary blanket of oil. Where herbivores such as
limpets and winkles are killed, algal sporelings will grow unchecked on the rocks; but eventually the natural
balance of the community will be restored. Dominant forms may recover quickly, or the area may be
colonised by forms from elsewhere. Rarer species are likely to take a long time to recover. On sandy or
pebble beaches, although exposed to surf action, the process of natural cleansing is normally slower
because oil tends to become buried in the sediments, where it may persist for years. These beaches are
however seldom of fisheries or marine conservation interest and where bird life is unaffected the ecological
effects can be tolerated.

In sheltered bays and estuaries where fine sediments occur, natural cleansing by wave action is poor and
any buried oil is likely to remain unchanged for considerable periods, as biodegradation in the absence of
oxygen is extremely slow. Microscopic algae and diatoms living on the surface of estuarine mud flats, as
well as green seaweeds and sea grass, are important primary producers in the food chain of the area and
are also thought to play a key role in the stabilisation and accretion of intertidal sediments. Oil tends to
interfere with the photosynthetic activity and growth of the micro-algae and to adhere to the grasses. It
could also reduce the potential food supply for birds, fish and bottom-living organisms, provided by the
rich fauna which include worms, bivalves and small crustaceans. Sand and mud flats are important feeding
and roosting grounds for a large variety of birds including ducks, swans, waders and geese. These often
feed in and among the seaweeds and intertidal sea grass which tend to retain oil. Stranded oil can foul
the plumage of these birds and cause heavy mortality. Oil pollution of an estuary may therefore have
particularly far reaching and long lasting effects.

Salt marshes, often found at the uppermost tidal level, are also important feeding and roosting grounds for
birds and may provide grazing for sheep and cattle. After a spill, oil may cling to plants and very little will
be removed by succeeding tides. Unless oil has soaked into the plant base and soil, some plants in these
communities recover quickly by producing new shoots. Oil damage to plants varies with the season, being
less severe in winter. Salt marshes have been shown to recover well from single spillages but chronic
pollution from oily water can cause severe local biological damage and lead to erosion of the marsh.

Concern has been expressed that the accumulation in fish tissues of poly-nuclear aromatic hydrocarbons
(PAH) from oil may be of significance to public health. These components include some compounds, such
as benz-pyrene, which are potentially carcinogenic. Fish taking up significant quantities of PAH from oil are
likely also to have taken up other components, tainting their flesh and rendering them inedible. However,
there is insufficient evidence to rely on the absence of an oil-taint as a guarantee that increased levels of
PAH are not present. There is considerable evidence that the tissues of contaminated fish held in clean
water for a sufficient period lose their oil components, including the PAH. On a global scale, oil is an
insignificant source of such carcinogens in food and in the environment compared with other sources, but
under particular local conditions it could be a major source.

The stranding of oil on amenity beaches generally leads to the greatest outcry against oil pollution,
particularly if it occurs during or just before holiday periods. This is the form of oil pollution which is most
obvious to the public and which affects them directly. The holidays of thousands could be ruined by a
severe pollution incident and the livelihoods of those employed in the holiday industry could be directly
affected. Gross pollution from spills can largely be removed by natural processes and modern cleaning
techniques but oily residues that are buried can persist for years. No less objectionable is the occurrence
of persistent tarry residues. Spills in amenity areas can cause fouling and damage to boats and jetties.
Apart from the problems oil pollution causes for users of the beach and particularly those with young
children, the harm done to birds and to marine life in general has caused considerable public concern.

The speed and direction of movement of oil spills in the seas around the UK is difficult to predict. Although
meteorological data are available to calculate the frequency of various wind speeds and wind directions,
little reliable data on the duration of winds at certain speed and in certain directions are available for
predictions to be made about the ultimate destinations of spills occurring in the North Sea. Two factors
determine movement of a slick: the speed and direction of the wind and the strength and direction of
currents and tides. So far as winds are concerned it is generally accepted that slicks move with the wind
at 3-4 % of wind speed. Currents, unless entirely wind-driven, can have accelerating, countervailing or
other modifying effects upon oil slick movement. Tides are only a significant influence near land;
otherwise, over a 24 hour period, any effect is cancelled out.

Met Office data indicate that in the oil producing areas of the North Sea in winter (September-April) winds
exceed force 5 on the Beaufort scale for about 33 % of the time and in the remaining months for about 11
% of the time. For about 37 % of the time in the winter months and 45 % of the time in the remaining
months winds blow from the North Sea oilfields towards the UK coast. Conversely for about 48 % of the
winter and about 41 % of the time in summer winds blow towards other European coastal states. The
remainder of the time is taken up with calms and south winds. So far as a spill from a tanker is concerned,
with a steady 25 mph on-shore wind and with a tanker travelling some 15 miles from the UK coast, the
slick could reach the coast within 15 hours.

Gulfaks Crude
Gullfaks oil is produced from a large field located in block 34/10 of the Norwegian sector of the North Sea.
Statoil is the operator of the field. Gullfaks is not a typical North Sea crude oil, being more naturally
biodegraded than other North Sea crudes. The biodegradation has removed most of the waxy normal
paraffins, resulting in a heavier, more naphthenic and aromatic crude. The sulphur and metal contents in
spite of the relatively high density, are still low.

Gullfaks Crude - Summary of the weathering properties

Chemical properties
Low content of saturates (a naturally biodegraded oil)
High aromatic and naphthenic content
Very low wax-content (<2 wt.%)
Very low asphaltene content (<0.1 wt.%)

Physical properties with increased weathering
Relatively low evaporative loss (<30% evaporated after 5 days weathering)
Very low pour-point values (no problem with solidification at sea)
Relatively high oil density

Water/oil-emulsification properties
Relatively low water uptake rate (65-80% water content after 5 days weathering)
Form emulsions with relatively low stability and low vicosity
Relatively high effectiveness of demulsifiers (breakers and inhibitors)

Effectiveness of dispersant treatment
Relatively high chemical dispersibility with Finasol OSR-5 (compared with other North Sea crudes)
High upper viscosity limit for dispersant treatment (4000-7000 cP)
Relatively large "time window" for dispersant treatment (36 hours to 5 days)

Mechanical recovery
Due to low water uptake rate it will take 3 hours to 2 days weathering at sea before the w/o-emulsion
passes 1000 cP in viscosity.