Marine Environment Protection

Reporting Ship Sourced Pollution

National Plan

Place of Refuge Guidelines

Educational resources and information

Major Oil Spills in Australia

Prevention of Pollution from Ships

National Maritime Emergency Response Arrangements

AMSAs Role in Maritime Environmental Issues

Register of Local Fuel Oil Suppliers

Torres Strait PSSA

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