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Weathering of Oil at Sea

and the Implications for the Estimation of the Window of Opportunity for Use of Oil Spill Dispersants

Introduction

The weathering of spilled oil on the water is determined by the:

oil composition,

oil slick thickness,

temperature of seawater and air, and

wind speed and sea state.

The relationship of the chemistry and physical properties of oils and refined products is complex and not easily explained, but now it is possible to use computer software to assist in predicting the changing properties of oil as it weathers at sea.

Computer Modelling of Oil Weathering

The software Automated Data Inquiry for Oil Spills (ADIOS), or similar software (eg IKU), is designed to assess the changes spilled oil undergoes during weathering.

Information required to run the model includes:

oil name/product type (approximately 100 oils are in the database),

amount of oil spilled,

instantaneous or continuous spill,

wind speed (constant or time dependent),

wave heights (constant or time dependent),

water temperature,

water salinity, and

emulsification constant (if known).

Three type of output are provided:

time dependent graphs of
- viscosity
- density
- water content
- evaporation
- natural dispersion

Oil budget graph
- pie chart at selected time intervals

Oil budget table
- listing of oil mass budget in tabulation form.

To demonstrate the effect of these conditions on the physical properties of a selection of oils, the ADIOS computer software was run by AMSA on three oils: a Gippsland crude, an Arabian Light, and a refined Jet JP-1 fuel oil.

In the following figures (1-5) a Gippsland oil was modelled by the ADIOS software for a spill of 1000 tonnes on a sea temperature of 20ºC, with 2 metre seas and 10 knot winds.

Figure (1) Natural dispersion of spilled Gippsland crude for conditions stated.

Fig 1 Natural disperion of spilled oil

Figure 1 plots natural dispersion of the oil under these conditions over 120 hours of the spill.

Figure (2) Evaporation of spilled Gippsland crude for conditions stated.

Fig 2 Evaporation of spilled oil

Figure (3) Kinematic Viscosity (cSt) of spilled Gippsland crude for conditions stated.

Fig 3 Kinematic Viscosity

Figure (4) Density (g/cc) of spilled Gippsland crude for conditions stated.

Fig 4 Density

Figure (5) Water content (%) of spilled Gippsland crude for conditions stated.

Fig 5 Water content

ADIOS Oil Budget Output

Figure 6 shows an alternative output from ADIOS, providing an indication of the mass balance of the oil spilled for the Gippsland crude under the conditions stated previously.

Figure (6) Oil budget for Gippsland crude after 12 hours from the spill time.

Fig 6 Oil budget

ADIOS Oil Budget Table

Another form of output from ADIOS is a mass balance table over a period of 120 hours.

In Table (1) the ADIOS prediction for the mass balance of the spilled Gippsland crude is presented.

Table (1) Oil Budget Table for Gippsland Oil for conditions stated previously

Time (hours)	Total Released (metric tons)	Evaporated (percent)	Dispersed (percent)	Floating (percent)
0	1,000	0	0	100
3	1,000	26	2	72
6	1,000	32	5	63
9	1,000	35	8	57
12	1,000	37	10	53
15	1,000	39	11	50
18	1,000	40	12	48
21	1,000	41	13	46
24	1,000	42	13	45
30	1,000	44	14	42
36	1,000	45	15	40
42	1,000	46	16	38
48	1,000	47	17	36
60	1,000	49	19	32
72	1,000	51	20	29
84	1,000	52	22	26
96	1,000	53	23	24
108	1,000	54	24	22
120	1,000	54	25	21

Effect of Wind Speed on Spilled Oil Properties

With increasing wind speed and wave action the loss of light oil components increases (evaporation becomes more significant causing an increase in viscosity and density of the remaining oil and forming emulsions with water).

It is generally accepted that over 2000 cSt (Centistokes - a measurement of the mobility of oil) viscosity the effectiveness of oil dispersants decreases significantly.

Time/viscosity graphs were obtained from the ADIOS software of the weathering of a Arabian Light crude oil spilled on the sea at a surface temperature of 20ºC over three different wind speed conditions: 5, 10 and 25 knots.

It can be seen from Table 2 that at 5 knot sea winds, the limit of dispersant effectiveness is reached (viscosity at 2000 cSt.) in around 40 hours after the spill, but at 25 knot sea winds the threshold is exceeded within 3 hours of the spill.

Table (2) Arabian Light Oil

Calculation of kinematic viscosity (cSt) at various sea surface wind speeds; water temperature 20ºC.

Wind Speed at spill site (knots)	Hours after spill when dispersant effectiveness is markedly reduced
5	40
10	14
25	3

In comparison, time/viscosity graphs of the weathering of a Gippsland crude oil mix were obtained for sea surface wind speed of 10 and 25 knots.

It can be seen from Table 3 that the window of opportunity for the use of chemical dispersants is greater for Gippsland crude than the Arabian Light crude at equivalent wind speed conditions.

Table (3) Gippsland Crude Oil Mix

Calculation of kinematic viscosity (cSt) at various wind speeds.

Wind Speed at spill site (knots)	Hours after spill when dispersant effectiveness is markedly reduced
10	120
25	60
(water temperature 20ºC)

Effect of Sea Waves and Mixing Energy on Oil Dispersion

Time/viscosity profiles were obtained from ADIOS for the weathering of the Gippsland and Arabian light crude oils under high sea waves (3 metre seas), and at 30 knot winds and at a sea temperature of 20ºC.

Using the viscosity limit threshold of 2000 cSt for dispersants the Light Arabian crude exceeded this level in only 3 hours of weathering at sea, compared to the Gippsland Crude of 30 hours (Table 4).

Table (4) Weathering of Gippsland Crude Oil Mix & Light Arabian Crude

Calculation Dispersant Use Window at High Sea States.

Crude oil Weathered at:

Oil Type	Hours after spill when dispersant effectiveness is markedly reduced
Gippsland	30
Light Arabian	3
(water temperature 20ºC, 3m seas, 30 knot winds)

The primary factor for the increase in viscosity for the weathering of the Light Arabian crude oil, under these sea conditions, is the formation of an emulsion after only a few hours at sea.

After the initial evaporation of the light components from this oil the high asphaltenes and aromatics enhance emulsion formation. The Gippsland crude has a high saturate content and will undergo greater evaporative loss prior to emulsion formation.

Effect of water temperature on oil properties

In Table 5 the effect of wind speed and water temperature is calculated for the spill of a Jet JP-1 oil. Two water temperature extremes are given for temperate water at 12ºC and 28ºC for tropical waters.

Table (5) Effect of Wind Speed and Water Surface Temperature on Spilled JP-1 Oil

Seawater Temperature (ºC)	Wind Speed (knots)	Hours to achieve 40% evaporation of the JP-1 oil
12	5	30
12	10	18
12	25	9
28	5	18
28	10	10
28	25	5

From Table 5, it can be seen that the evaporative effect of water temperature on the jet fuel increases with increase it temperature but is not as significant as the effect of increasing wind velocity on the slick.

Recommendation

It is recommended that if any oil weathering is suspected in an oil spill incident, then the modelling of the changing oil properties using the software Automated Data Inquiry for Oil Spills (ADIOS), or similar software (eg IKU), is required to assess the increase in viscosity, natural dispersion, degree of evaporation, increase in specific gravity, potential to emulsify etc., and their implications for determining the window of opportunity for use of chemical dispersants.

This service can be provided by AMSA on request as part of the National Plan arrangements.

For the Sixth Scientific Support Coordinators Workshop
Launceston, Tasmania
2-6 December 1996.

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last updated: 26 April 2005