Two tables are presented in this section, carried forward
from a previous version of the CIS. The first, CRUDE OILS,
shows general chemical and physical properties of a number
of oils commonly transported through European waters, while
the second table EMULSIONS, gives experimentally determined
properties for water-in-oil emulsions for many of the same
oils.
The purpose of these tables is to provide a reference source
which can be used to predict the behaviour, fate and potential
impact of these oils if spilt at sea. One of the key parameters
is the ease with which an oil forms a water-in-oil emulsion
and the stability of the emulsion once formed. This determines
not only the overall persistence of the oil but also the
degree to which the oil is likely to be amenable to remedial
measures such as dispersion treatment and mechanical recovery.
A brief outline of the importance of each of the parameters
included in the tables in determining the fate or weathering
of an oil once spilled, is set out below.
The crude oils have been grouped according to their anticipated
behaviour once spilt at sea, by pour point (group 1) and
by evaporative loss (group 2-4). Groups 2 to 4 have also
been subdivided according to the pour point of the 200+C°
fraction. This grouping follows the approach taken in the
CONCAWE Report n°8/83, "Characteristics of Petroleum
and its Behaviour at Sea", in which the table of Crude
Oil Properties was originally published.
Group 1 Pour point > 5-10°C
Group 2 Evaporative loss 0-20% (Vol)
Group 3 Evaporative loss 20-40% (Vol)
Group 4 Evaporative loss 40-50% (Vol)
Group 5 Evaporative loss >50% (Vol)
Pour Point: The temperature below which an oil will not
flow. If the ambient temperature is below the pour point,
the oil will essentially behave as a solid and will not
spread over the sea surface as a film but fragments as discrete
patches. Oils falling into Group 1 above have relatively
high pour points and at low ambient temperatures would behave
as solids.
Distillation Characteristics: As the temperature of the
oil is raised, different components reach their boiling
points in turn and are distilled off. The distillation characteristics
are presented as the percentage volumes, which distil off
within set, increment temperature ranges. This determines
the volatility of an oil and controls the rate and extent
of evaporation. Volatile products such as kerosene and gasoline
may evaporate completely within a few hours and light crudes
can lose up to 40% during the first day. In contrast, heavy
fuel oils undergo little, if any, evaporation. In general,
it is found that those oil components with a boiling point
below 200°C evaporate within 24 hours and that evaporation
then proceeds much more slowly.
It is for this reason that Groups 2- 4 above were subdivided
according to the pour point of the 200+°C fraction.
If the pour point of this fraction is greater than 5-10°C
then the oil may reach its pour point during weathering.
Note: The guidelines used by the International Oil Compensation
fund as to whether an oil is considered persistent or non-persistent
under the terms of the Fund Convention, are also determined
by the oil’s distillation characteristics (see references
listed under GENERAL in PART III, Section B).
Density: Apart from determining whether or not an oil will
float, its density can often give a general indication of
other properties of the oil. For example, oils with low
densities tend to have low viscosities and contain a high
proportion of volatile components. However, it is important
to note that some apparently light oils behave more like
heavy oils due to relatively high wax contents.
The density of crude oils is often expressed as API gravity
where
°API = (141.5 / s.g.) - 131.5 where s.g. = specific
gravity : density relative to pure water
API values range from below 10 to over 40 but it should
be appreciated that high API values relate to low density
oils and vice versa.
Viscosity: The viscosity of an oil is its resistance to
flow. High viscosity oils flow with difficulty while oils
with low viscosities are highly mobile and spread quickly
over the sea surface. Viscosities decrease as temperatures
increase and so sea temperatures are important in determining
the fate of an oil. The viscosity of the oil is a key factor
in determining whether an oil is amenable to dispersion
treatment as well as the difficulty with which the oil can
be recovered from the sea.
Wax content: The behaviour of an oil may be influenced
by wax content. Oils with a wax content greater than about
10% tend to have high pour points and if the ambient temperature
is below this, the oil will be either a solid or a highly
viscous liquid. High wax contents can also help to stabilize
water-in-oil emulsions.
Asphaltene content: A good indication of how easily an
oil will form a water-in-oil emulsion is given by the asphaltene
content. Oils with asphaltene contents greater than 0.5%
tend to form stable emulsions. These emulsions can contain
up to 80% water by volume and are generally extremely viscous.
The rate at which emulsification takes place is primarily
a function of sea state, although viscous oils tend to absorb
water more slowly.
Spreading Coefficient: This is a function of the surface
tension of the oil and is used in some theoretical calculations
to determine the rate at which an oil spreads over the sea
surface. In practice, this approach considers only one of
several mechanisms which are involved in the spreading process
but works reasonably well for the early stages of a spill
of a low viscosity, light to medium crude, before emulsification
of the oil becomes the predominant factor.
L = K (ó²t³ / ñ²õ )1/4
Where: L=slick dimension ó=spreading coefficient,
t=time, ñ=density of water, õ=kinematic viscosity
of water, K=10 in the direction of the wind or 1 across
the wind
The emulsion properties listed in this table were measured
for a series of synthetically prepared water-in-oil emulsions
and are therefore only indicative of the properties of emulsions
formed at sea.
Parent Crude Characteristics: This is a summary of the
previous table describing the properties of the crudes from
which the emulsions were prepared.
Dynamic viscosity: The viscosity of the water-in-oil emulsions
is considerably higher than that of the parent crude. The
viscosity is closely related to the water content but also
depends on the degree of dispersion of the water through
the oil. At sea, the viscosity of the emulsion increases
with time as the oil picks up more water that becomes more
evenly dispersed through the oil. The dynamic viscosity
was measured at a low shear rate because water-in-oil emulsions
exhibit non-Newtonian behaviour and are shear-thinning fluies,
i.e. as shear rates are increased, the viscosity falls.
Water content: The water content of the laboratory prepared
emulsions listed here are typically lower than those found
for emulsions formed at sea.
Storage at 10°C: These results show the stability of
the emulsion left to stand at 10°C for one day and for
seven days. The lower the value, the less stable the emulsion
since this indicates loss of water from the emulsion.
Storage at 90°C: These results represent emulsion stability
at elevated temperatures and reflect the ease with which
the emulsion can be broken. This has implications for the
techniques selected for the disposal of emulsions recovered
from the sea.
