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.