sintef report

Transcription

sintef report

SINTEF REPORT
TITLE
SINTEF Materials and Chemistry
Address:
Location:
Telephone:
Fax:
Weathering properties of the Trestakk oil
Final version
NO-7465 Trondheim,
NORWAY
Brattørkaia 17B,
4. etg.
+47 4000 3730
+47 930 70730
AUTHOR(S)
Enterprise No.: NO 948 007 029 MVA
Per Johan Brandvik and Frode Leirvik
CLIENT(S)
StatoilHydro
REPORT NO.
CLASSIFICATION
CLIENTS REF.
SINTEF A6809
Unrestricted
Magnus Eriksen
CLASS. THIS PAGE
ISBN
PROJECT NO.
Unrestricted
978-82-14-04542-0 800849
ELECTRONIC FILE CODE
NO. OF PAGES/APPENDICES
63
PROJECT MANAGER (NAME, SIGN.)
CHECKED BY (NAME, SIGN.)
ForvitringsHåndbok Trestakk final.doc
Per Johan Brandvik
Tove Strøm
FILE CODE
DATE
APPROVED BY (NAME, POSITION, SIGN.)
2008-05-19
Tore Aunaas, Research Director
ABSTRACT
A study of the weathering properties of oil from the Trestakk field has been performed. The data from
the laboratory study is used together with the SINTEF oil weathering modell to predict the behaviour of
Trestakk crude at sea. The weathering properties are discussed in relation to oil spill response both based
on mechanical recovery and chemical dispersion
Use of dispersants:
Laboratory testing has shown a high potential for use of chemical dispersants. The time window for
using dispersants can be as wide as several days depending on weather condition.
Mechanical recovery:
Efficiency of mechanical recovery is expected to be high. Recovery efficiency of offshore weir
skimmers (e.g. NOFOs Transrec Skimmers) will not be limited by viscous emulsions of Trestakk
crude.
Trestakk has a very low pour point and it is not likely for this oil to solidify in scenarios with calm sea
and very high evaporative loss.
KEYWORDS
GROUP 1
GROUP 2
SELECTED BY AUTHOR
ENGLISH
Oil
Weathering
Trestakk
Garn
Spill
NORWEGIAN
Olje
Forvitring
Trestakk
Garn
Søl
2
TABLE OF CONTENTS
1
Introduction ............................................................................................................................ 4
2
Executive summary ................................................................................................................ 5
3
The behaviour of crude oil on the sea surface ..................................................................... 6
3.1 The chemical composition of crude oils ......................................................................... 6
3.1.1 Hydrocarbons ...................................................................................................... 6
3.1.2 Heteroatomic organics ........................................................................................ 7
3.2 Physical properties of crude oils ..................................................................................... 7
3.2.1 Density ................................................................................................................ 7
3.2.2 Rheological properties ........................................................................................ 7
3.2.3 Pour point............................................................................................................ 8
3.2.4 Distillation curve................................................................................................. 8
3.2.5 Flash point........................................................................................................... 9
3.3 The behaviour of crude oil spilt at sea .......................................................................... 10
3.3.1 Evaporation ....................................................................................................... 11
3.3.2 Spreading .......................................................................................................... 12
3.3.3 Drift of an oil slick ............................................................................................ 12
3.3.4 Water-in-oil (w/o) emulsion.............................................................................. 13
3.3.5 Oil-in-water (o/w) dispersion............................................................................ 15
3.3.6 Water solubility................................................................................................. 16
3.3.7 Photo-oxidation................................................................................................. 16
3.3.8 Biodegradation .................................................................................................. 16
3.3.9 Sedimentation.................................................................................................... 16
3.3.10 Submersion........................................................................................................ 16
4
Experimental design of the bench scale testing ................................................................. 17
4.1 Oil Samples................................................................................................................... 17
4.2 Test temperatures .......................................................................................................... 17
4.3 Bench-scale laboratory testing...................................................................................... 17
4.3.1 Evaporation ....................................................................................................... 17
4.3.2 Water-in-oil (w/o) emulsification ..................................................................... 17
4.3.3 Physical and chemical analysis ......................................................................... 19
4.3.4 Chemical dispersibility testing.......................................................................... 19
5
Meso-scale laboratory testing.............................................................................................. 21
5.1 Description of the meso-scale flume............................................................................. 21
5.1.1 Oil weathering in the meso-scale flume............................................................ 21
5.1.2 Sampling of surface oil ..................................................................................... 22
5.1.3 Collection of water samples .............................................................................. 23
5.2 Solar simulation in the meso scale flume ..................................................................... 23
6
Results from the bench scale weathering study................................................................. 25
6.1 Composition and physical properties............................................................................ 25
6.1.1 GC-FID characterization................................................................................... 25
6.1.2 Physical and chemical properties of fresh and weathered residues ..................28
6.2 Emulsifying properties of Trestakk crude oil ............................................................... 29
6.2.1 Water uptake and maximum water content....................................................... 29
6.2.2 Stability and efficiency of emulsion breaker ....................................................29
6.3 Dispersibility testing on Trestakk crude oil .................................................................. 30
6.3.1 Screening of dispersants.................................................................................... 30
6.3.2 Systematic dispersibility testing on Trestakk residues and emulsions ............. 31
3
6.3.3 Chemical dispersibility as a function of viscosity ............................................ 31
7
Results from the Meso-scale laboratory testing................................................................. 33
7.1 Evaporation ................................................................................................................... 33
7.2 W/o-emulsification ....................................................................................................... 34
7.3 In-situ chemical dispersion ........................................................................................... 35
7.4 Mass balance................................................................................................................. 35
7.5 Visual observations....................................................................................................... 37
8
SINTEF Oil Weathering Model – the model and input.................................................... 39
8.1 Input to SINTEFs Oil Weathering Model..................................................................... 40
8.2 Predictions of weathering properties ............................................................................ 42
8.2.1 Prediction charts for Trestakk ........................................................................... 42
8.2.2 How to use the prediction charts, an example .................................................. 42
2.3 Comparison with other oils........................................................................................... 52
9
Weathering properties related to response ........................................................................ 57
9.1 Oil properties ................................................................................................................ 57
9.2 Flash point – fire/explosion hazard............................................................................... 57
9.3 Emulsion formation and stability.................................................................................. 58
9.4 Mechanical recovery..................................................................................................... 58
9.5 Life time at sea – Natural dispersion and evaporation.................................................. 59
9.6 Use of chemical dispersants.......................................................................................... 61
10 References ............................................................................................................................. 62
4
1 Introduction
New oil types are continuously coming into production on the Norwegian Continental Shelf, and
even Barents Sea oil is being prepared (for production). Because of large variations in the physical
and chemical properties of the crude oils, their behaviour and fate if spilled at sea may vary
greatly. The “Braer” accident at the Shetlands and the “Sea Empress” accident in Wales have
shown how important it is to be able to predict the efficiency of different cleanup methods
(mechanical, burning, dispersant treatment etc.). It is therefore important to have good knowledge
about each oil’s expected behaviour at sea in case of an accidental spill. According to regulations
from The Norwegian Pollution Control Authorities (SFT) and the Petroleum Safety Authority
Norway (Ptil) a characterisation of the oil or condensate with respect to weathering properties and
fate in the marine environment should be performed for both exploration drilling and for all oil
coming into production.
An oil sample from the test production at the Trestakk field at Haltenbanken in 2001 (6406/3-2,
DST 2, brønn 6) has been investigated in the laboratory at 5 and 13°C. The difference between the
two temperatures are as expected (linear trends vs. temperature) and only the 13°C is presented in
this report and used to predict the behaviour of Trestakk oil at sea at different weathering
conditions. How the weathering properties influence on oil spill response, both mechanical
recovery and chemical dispersibility are discussed.
5
2 Executive summary
When weathered at the sea surface, the oil will encounter changes in physical properties that will
affect oil spill countermeasures in different ways. This summary gives a brief overview of
changes in the Trestakk oil on the sea surface, and limitations to countermeasures due to the
changes.
Trestakk is light and paraffinic, with a medium vax and asphaltene content. If spilt at sea, the oil
will have a high evaporative loss, which will give rapid change in the physical properties of the oil
spill. Stable emulsions are formed with high viscosity. However, in high sea the combination of
high evaporative loss and natural dispersion will give a short lifetime on the sea surface similar to
other Haltenbank oil.
The flash point of Trestakk will change rapidly initially in a spill and will be above the ambient
sea temperature within 15 minutes after a spill. Some ships have a minimum flash point
requirement at 60°C for storage of liquids. Trestakk will reach this limit within a few hours after
the spill, even at low wind speeds.
Formation of viscous emulsions reduces problems with boom leakage initially in a recovery
operation. Depending on the weather conditions boom leakage, due to low viscosity (< 1000 cP)
will only be expected first 3-6 hours after release.
Laboratory testing has shown a high potential for use of chemical dispersants. The time window
for using dispersants can be as wide as several days depending on weather condition and
emulsification.
Efficiency of mechanical recovery is expected to be high. Recovery efficiency of offshore weir
skimmers (e.g. NOFOs Transrec Skimmers) is not expected to be limited by viscous emulsions
of Trestakk crude.
The emulsion formed could have water content as high as 80%. This means 4 parts of the
recovered fluid will be water. Recovered emulsion can easily be broken and a dosage of 500 ppm
of emulsion breaker is probably sufficient to totally break the emulsion.
6
3 The behaviour of crude oil on the sea surface
3.1 The chemical composition of crude oils
Crude oil is a complex mixture of thousands of chemical components. The relative compositions
vary, giving rise to crude oils with different chemical and physical properties. The components
found in crude oil are classified into two main chemical groups, these are hydrocarbons and
heteroatomic organics, see Figure 3.1.
Figure 3.1: The chemical composition of crude oil.
3.1.1
Hydrocarbons
Hydrocarbons
The majority of compounds in crude oils are hydrocarbons, composed of hydrogen (10-15 wt. %)
and carbon (85-90 wt. %). These range from simple, volatile gases, such as methane with only one
carbon atom, up to large, complex molecules with more than 100 carbon atoms. The hydrocarbons
in crude oils include saturated and unsaturated molecules in linear, branched and cyclic
configurations.
Hydrocarbons are further classified into aliphatic and aromatic compounds. The two main groups
of aliphatics are paraffins and naphthalenes.
Paraffins
Paraffins include n-alkanes and iso-alkanes aliphatic compounds. Waxes are an important subgroup of paraffins, containing more than 20 carbon atoms. The wax components of a crude oil
will be present in solution at elevated temperatures. At low temperatures they may precipitate out
of solution. These are principally n-alkanes. The wax content of crude oils can vary from 0.5 wt.
% up to 40 or 50 wt. % in extreme cases, although the majority of the world's crude oils have wax
contents of 2 - 15 wt. %.
7
Naphthenes
This group includes cycloalkanes containing one or more saturated rings. Each ring may have one
or more paraffinic side chains. They are chiefly 5 and 6 membered rings.
Aromatics
Aromatics are a specific type of unsaturated cyclic hydrocarbons. Benzene, toluene and xylenes
are examples of mono-ring aromatics, naphthalenes are di-ring aromatics and polynuclear
aromatic hydrocarbons (PAH) contain three or more aromatic rings.
3.1.2 Heteroatomic organics
In addition to pure hydrocarbons, some organic compounds in crude oils also contain small
amounts of oxygen, nitrogen or sulphur, and some trace metals such as vanadium and nickel. The
two most important groups of heteroatomic organic compounds are resins and asphaltenes.
Resins
Resins are relatively polar compared to the hydrocarbons, and often have surface active
properties. Resins have molecular weights ranging from 700-1000. Carboxylic acids (naphthenic
acids), sulphoxides and phenol-like compounds can be found in this group.
Asphaltenes
This is a complex group of poorly characterised chemical compounds. They consist of condensed
polycyclic aromatic compounds. They are large molecules with 6 - 20 aromatic rings and side
chains (molecular weight: 1000 - 10 000). Asphaltenes may be classified as "hard" or "soft", on
the basis of the method used to determine the asphaltene content. Crude oils can contain up to 6
wt. % "hard" and 10 wt. % "soft" asphaltenes.
3.2 Physical properties of crude oils
The physical properties of specific oils are a result of their chemical compositions. The most
important physical properties in oil spill scenarios are discussed below.
3.2.1 Density
The density of crude oil normally lies between 0,78 to 0,95 g/mL at 15,5°C. Paraffinic oils have
lower density values, while oils that contain large amounts of high molecular weight aromatic,
naphthenes and asphaltenic compounds usually have higher density values.
3.2.2
Rheological properties
Viscosity
The viscosity of crude oil expresses its resistance to flow and is of special interest when pumping
mechanically collected oil. The viscosity of crude oils can vary from 3 to 2000 cP at 13°C. In
comparison water has a viscosity of 1 mPas and syrup a viscosity of 120000 mPas at 20°C.
The viscosity is temperature dependent. For liquids the viscosity decreases with increasing
temperatures. Viscous crude oils or crude oils that contain wax can exhibit non-Newtonian
behaviour (viscosity varies with shear rate), especially close to or below their pour point.
8
The viscosity of an oil increases with evaporation since the heavier, more viscous components
remain. The difference in viscosity for crude oils is approximately 3 to 2000 mPas for fresh crude
oils and several hundred/thousand mPas for their residues.
Water-in-oil (w/o) emulsions are generally more viscous than the parent crude oil, this is
illustrated in Figure 3.2.
B
Viscosity ratio
1000
100
10
0
0
20
40
60
80
100
Water content (vol. %)
ik41911100\tegner\fig-eng\fig2-5.eps
Figure 3.2: An example of the viscosity ratio as a function of increasing water content
Mackay et al., 1980).
3.2.3 Pour point
The temperature when an oil ceases to flow when cooled without disturbance under standardised
conditions in the laboratory (ASTM-D97) is defined as the oils pour point. In oil spill clean up
situations the pour point provides important information when determining the efficiency of
various skimmers, pumping rates and the use of dispersion agents.
The pour point of an oil with a high wax content will increase dramatically with weathering as the
lower weight molecules that contribute in keeping the wax in solution evaporate. The pour point
of oils with high wax contents can reach 30°C, while low viscous naphthenic oils can have pour
points as low as –40°C.
3.2.4 Distillation curve
Boiling point and boiling range (distillation properties)
The distillation curve shows the relative distribution of volatile and heavier components in the oil.
The distillation curve is obtained by measuring the vapour temperature as a function of amount of
oil distilled. The boiling point of particular chemical component depends on its vapour pressure;
which is a function of molecular weight and chemical structure. Low molecular weight oil
components have higher vapour pressure, and therefore lower boiling points, than higher
molecular weight components of a similar type. Aromatic compounds boils at a higher
temperature than paraffins of the same molecular weight and iso-alkanes boils at lower
temperature than the equivalent n-alkanes. The distillation curve is therefore an indicator of the
relative amount of different chemical components, principally as a function of molecular weight,
but also determined by the chemical composition.
9
100
Midgard condenstate
Kristin
Morvin
Trestakk
Åsgard
Heidrun Export blend
90
80
Evaporated (vol%)
70
60
50
40
30
20
10
0
50
100
150
200
250
300
350
Temperature (°C)
Figure 3.3:
Distillation curves for various crude oils at Haltenbanken.
3.2.5 Flash point
The flash point is the lowest temperature at which the gas or vapour generated by heating an oil
can be ignited by a flame. The flash point depends on the proportion of low molecular weight
components. Fresh crude oils normally have a low flash point (from –40°C to 30°C).
From a safety point of view, flash points are of most significance at or slightly above the
maximum temperature that may be encountered in storage or transport. The flash point is an
approximate indicator of the relative fire and explosion hazard of an oil.
A rule of thumb says that moving in an oil slick where the flash point of the oil is close to or
lower than the sea temperature implies a fire and explosion hazard. Natural weathering processes
like evaporation and emulsion formation contribute in reducing the potential hazard by increasing
the flash point. There is therefore a relatively short fire and/or explosion danger in the initial
stages of an oil spill.
In the laboratory, the flash point is measured in a closed system with an equilibrium between the
components in the oil and gas. In the field, however, the weather situation will influence the
flammability of the air above the slick. For instance the gas concentration will be high just above
the oil film in calm weather and high temperatures, whereas the concentration will be low in cold
and windy weather due to dilution and transport and a lower degree of evaporation.
10
3.3 The behaviour of crude oil spilt at sea
When a crude oil is spilt at sea a number of natural processes take place, which change the
volume and the chemical properties of the oil. These natural processes are evaporation, water-inoil (w/o) emulsification, oil-in-water (o/w) dispersion, release of oil components into the water
column, spreading, sedimentation, oxidation and biodegradation. A common term for all of these
natural processes is weathering. The relative contribution of each process varies during the
duration of the spill.
Figure 3.4 illustrates the various weathering processes and Figure 3.5 shows their relative
importance with time.
Wind
Photolysis
Water-in-oil
emulsion
Drifting
Resurfacing of larger oil droplets
Oil-in-water dispersion
Evaporation
Spreading
Dissolution of water soluble
components
Adsorption to particles
Vertical diffusion
Horizontal diffusion
Microbiological
degradation
Uptake by biota
Sedimentation
Uptake and release from sediment
Figure 3.4: The weathering processes that take place when an oil is spilt on the sea surface.
11
0
1
Hours
10
Day
100
Week
1000
Month
10000
Year
Evaporation
Dissolution
Photo-oxidation
Biodegradation
Sedimentation
Water-in-oil
emulsification
Unstable
emulsion
Oil-in-water
dispersion
Stable "mousse"
Spreading
Drifting
6621\handboker\grafiske\fig-eng\emulsion.eps
Figure 3.5: Weathering processes’ relative importance with time.
The weathering of oil depends on the oil type (chemical and physical properties), the weather
conditions (wind, waves, temperature and sunlight) and the properties of the seawater (salinity,
temperature, bacteria flora etc.).
3.3.1 Evaporation
Evaporation is one of the natural processes that helps removing spilt oil from the sea surface. The
evaporation process starts immediately after the oil is spilt and the evaporation rate decreases
exponentially throughout the duration of the oil spill.
The amount evaporated depends on the chemical composition of the oil in addition to the
prevailing weather conditions, sea temperature and the oil film thickness.
The rate of evaporation will vary for different oil types. Light refinery products (e.g. gasoline and
kerosene) may completely evaporate after a few hours/days on the sea surface. Condensates and
lighter crude oils can loose 50 % or more of their original volume during the first days after an oil
spill.
The most significant difference caused by evaporation is the loss of volatile and semi-volatile
compounds increases the relative amounts of higher molecular weight compounds. The chemical
and physical properties of the remaining oil change for example the density, viscosity, pour point
and the relative wax and asphaltene contents will increase with increased evaporation.
12
3.3.2 Spreading
Oil spilt at sea will spread on the sea surface. Spreading is often the dominating process in the
initial stages of an oil spill.. The spreading decreases as the viscosity and density of the remaining
oil increases. The spreading process is also retarded if the oil’s pour point is 10-15°C below the
sea temperature.
Oceanographic conditions (e. g. wind, waves and current) will affect the spreading process. The
oil slick will be broken into windrows aligned in the wind direction, see Figure 3.6. The thickness
of the oil slick will vary, often differing with a factor of several thousand. Past experience has
shown that 90 vol% of the oil spilt will consist of patches of w/o-emulsion with a film thickness
of 1 to 5 mm that often constitute for less than 10 % of the total oil slick area. The remaining 10
vol% will cover 90 % of the spill area in the form of a sheen (<1 μm oil thickness).
Tick oil and
water-in-oil
emulsion (mm)
Wind
Sheen (< 1μm)
Windrows
ik41961100/tegner/fig-eng/sheen.eps
Figure 3.6: The spreading of oil spilt on the sea surface and the distribution within the oil slick.
3.3.3 Drift of an oil slick
The weather conditions cause the oil slick to drift, see Figure 3.7. The oil slick will drift as the
weathering processes described in this chapter weather the oil. Waves and wind create a current in
the water mass which amounts to approximately 3 % of the wind speed at the sea surface
(Martinsen, 1994).
13
Wind - 20 knots
3%
100 %
ik41961100/tegner/fig-eng./wind.eps
Figure 3.7: An illustration showing how wind and current can influence the drift of an oil slick.
3.3.4 Water-in-oil (w/o) emulsion
The formation of w/o-emulsions is the weathering process that contributes in keeping oil on the
sea surface. A w/o-emulsion has a higher viscosity than the parent crude oil and the emulsification
process will therefore retard/delay the evaporation and the natural dispersion process.
The minimum criteria for the formation of w/o-emulsions is the presence of breaking waves (i. e.
a wind speed of 5 m/s), however, a slow water uptake can also take place during calmer weather.
Surface active compounds present in crude oil will promote the formation of w/o-emulsions and
contribute in stabilising the emulsion. These components contain both hydrophilic and
hydrophobic groups.
The maximum water uptake will vary for different crude oils. Tests performed at SINTEF have
shown that the maximum water uptake is fairly independent of the prevailing weather conditions
as long as the lower energy barrier for the formation of w/o-emulsions is exceeded, however, the
rate depends highly on the weather conditions. In the laboratory the t1/2-value, which is the time in
hours it takes before the oil has taken up half of its maximum water content, is determined.
Previous tests of asphaltenic oils performed at SINTEF show that the water uptake is in inverse
ratio with the viscosity.
The rate of formation of the w/o-emulsion varies for different oil types since it is dependent on the
chemical composition if the oil. A large amount of wax will for instance increase the rate of
formation. Another important factor that influences the rate of formation is the prevailing weather
conditions. Figure 3.8 shows how the wind speed influences the rate of formation.
14
A
100
Windspeed = 10 m/s
Water content (vol.%)
80
60
Windspeed = 5 m/s
40
20
0
0
6
12
18
24
Time (hours)
ik41911100\tegner\fig-eng\fig2-3a.eps
Figure 3.8: An example of how the prevailing weather conditions influence the rate of the
w/o-emulsion formation for a typical crude oil.
The stability of the w/o-emulsion depends on the water droplet size in the emulsion. Not all of the
water droplets in the emulsion are stabile. The largest droplets may coalescence and are squeezed
out of the w/o-emulsion. Larger water droplets may be reduced in size by the flexing, stretching
and compressing motion of a slick due to wave action. After a period of time the emulsion may
only contain small water droplets with a diameter of 1 to 10 μL.
A
B
Figure 3.9: A picture taken with a microscope of the water droplet size in a w/o-emulsion
after (A) 1 hour and (B) 24 hours in a rotating cylinder.
Another factor that influences the w/o-emulsion’s stability is the amount of surface-active
components present in the parent oil. Resins and asphaltenes have hydrophobic and hydrophilic
properties and will concentrate at the interface between the water and oil thereby forming a layer
that stabilises the water droplets. The hydrophobic properties can lead to the concentration of wax
along the water droplets, which further stabilises the interfacial “skin” layer. The interfacial layer
between the oil and water forms a physical barrier hindering coalescence and will stabilise the
w/o-emulsion by hindering the fusion of water droplets. The stabilisation of the water droplets by
asphaltenes and by asphaltenes and wax are shown in Figure 3.10.
15
WATER PHASE
Asphaltene
stabilized
Wax
stabilized
Water droplet ~ 1 μm in diameter
Asphaltene
& wax
stabilized
Unstabilized
OIL PHASE
Asphaltene "particles"
Wax crystals
Resins
ik41961100:tegner\fig_eng\interfac.eps
Figure 3.10: Stabilization of the interfacial layer between the water and oil in a w/o-emulsion
by wax and asphaltenes.
Oils that contain a large amount of wax and little asphaltenes can form w/o-emulsions that may
appear to be stabile. These w/o-emulsions appear to be stabled by the continual phase’s
rheological strength (viscosity and elasticity). This strength is due to the wax structure formed by
participated wax. Wax stabilised emulsions are characterised by large water droplets and are fairly
stabile when stored, however, they may break when stress is applied and/or when the emulsion is
heated to e. g. 40-50°C.
3.3.5 Oil-in-water (o/w) dispersion
Natural o/w dispersion will take place if there is sufficient energy on the sea surface, i. e. if there
are breaking waves present. The waves will break the slick into droplets typically with a diameter
between 1 to1000 μm which are mixed into the water masses. The largest oil droplets will
resurface forming a sheen behind the oil slick (see chapter 3.3.2).
In addition to weather conditions the dispersion rate depends highly on the oil type and can be one
of the main processes that determine the lifetime of an oil slick on the sea surface. The natural o/w
dispersion will gradually decrease since evaporation of the lighter compounds will increase the
viscosity of the remaining oil.
The purpose of applying chemical dispersion agents is to increase the natural o/w dispersion rate.
When effective chemical dispersion is achieved small oil droplets are formed with a diameter of 5
to 50 μm. The dispersion agent reduces the interfacial tension between the water and oil and
promotes dispersion.
16
3.3.6 Water solubility
The water solubility of saturated hydrocarbons (<C4) is very low, while lower molecular weight
aromatic compounds are water-soluble. Within the various types of hydrocarbons the water
solubility decreases from aromatics to naphthenes and from iso-paraffins to n-paraffins. In each
series the water solubility decreases with increasing molecular weight.
Evaporation and the release of oil components in to the water masses are competitive processes
since most of the water-soluble components are also volatile. The evaporation process is
approximately 10 to 100 times faster than the release in the water column. The concentration of
soluble oil components in the water column during an oil spill is quite low (<1 mg/L). The
dissolution of oil components into the water column does not contribute in removing the oil from
the sea surface. However the water-soluble fraction (WSF) is of great interest since it has a high
bioavailability and therefore the potential to cause acute toxic effects on marine organisms.
3.3.7 Photo-oxidation
Under the influence of sunlight some of the oil components will slowly be oxidised to resins and
eventually asphaltenes. This contributes to the stability of w/o-emulsions and therefore has a large
influence on the oils persistence on the sea surface. The photo-oxidised components will stabilise
the w/o-emulsions. After a long period of weathering at sea, tar-balls can be formed. Tar-balls are
broken down very slowly both at sea and on beaches.
3.3.8 Biodegradation
Theoretically, seawater contains microorganisms that can break down all types of oil components.
The various microorganisms prefer specific oil components as an energy source.
Several factors influence the biodegradation rate, among these are temperature, the supply of
nutritive substances that contain nitrogen and phosphor, the oxygen supply, oil type and the
degree of weathering. Bacteria can only degrade oil that is in contact with seawater and is
dependent of the water/oil interface area. The interface area increases as the oil is spread over the
sea surface as a thin layer or by chemical or natural dispersion of oil in the water masses. An area
increase due to chemical and/or natural dispersion will increase the degradation rate in the water
mass to 10 to 100 times the rate at the water/oil interface.
It is difficult to estimate the microbial degradation rate, however rates of 1 to 30 mg/m3 seawater
per day have been reported (FOH, 1984). The degradation rate can reach values of 500 to 600
mg/m3 seawater per day in chronic oil polluted areas (NRC, 1985). The biodegradation of oil
present in sediment is much slower due to the lack of oxygen.
3.3.9 Sedimentation
Crude oil and oil residues rarely sink into the water masses since there are few oils that have a
density higher than water, even after extreme weathering. Oil can sink by sticking to particular
material, which is present in the water masses. W/o-emulsions that have a higher density value
can easier stick to particular material.
3.3.10 Submersion
Highly weathered oils can temporarily submerge from the sea surface. This can greatly influence
the effectiveness of combating oil pollution in the marine environment. The oil density and
viscosity in addition to the weather conditions influence submersion. W/o-emulsions have a
higher density value than the parent oil and can therefore submerge more easily.
17
4 Experimental design of the bench scale testing
4.1 Oil Samples
A 2 x 20 litre crude oil sample of the Garn formation from the Trestakk field was received from
Statoil in July 2007. The sample has got the SINTEF id: 2007-0402 and was originally marked,
“Trestakk Garn formation 6406/3-2 dst2 Brønn 6”.
4.2 Test temperatures
The testing of weathering properties was performed at 5 and 13°C which is regarded as a typical
temperatures in Norskehavet.
4.3 Bench-scale laboratory testing
In order to isolate and map the various weathering processes that take place when an oil is spilled
on the sea surface, the weathering of the oils is carried out using a systematic, stepwise procedure
developed at SINTEF (Daling et al., 1990). The weathering process is illustrated in Figure 3.1
Emulsification with water
Evaporation
WOR = 1
WOR = 3
WOR = max
Figure 4.1: Flow chart for the bench-scale laboratory weathering of a crude oil
WOR=1 (50% water), WOR=3 (75% water).
4.3.1 Evaporation
Evaporation of the lighter compounds from the fresh crude oil is carried out according to a
modified ASTM-D86/82 distillation procedure (Stiver and Mackay, 1984). The fresh crude oil is
distilled, in a simple one step distillation, to a vapour temperature of 150, 200 and 250°C. This
will give oil residues with an evaporation loss typically corresponding to 0,5-1 hour, 0,5-1 day
and 2-5 days of weathering of an oil slick on the sea surface. These residues are referred to as
150°C+, 200°C+ and 250°C+ respectively.
4.3.2 Water-in-oil (w/o) emulsification
The procedures used in the w/o-emulsification studies are described in detail by Hokstad et al.,
1993.
The w/o-emulsification of the fresh crude oil is carried out based on the rotating cylinder method
developed by Mackay and Zagorski, 1982. Oil (30 mL) and seawater (300 mL) are rotated (30
18
rpm) in a separating funnel (0,5 L), see Figure 4.2.The emulsification kinetics are mapped by
measuring the water content at fixed rotation times. The maximum water content is determined
after 24 hours of rotation.
24 hours
mixing
Before mixing
Oil
(30 mL)
WOR
02-
Seawater
(330 mL)
4-
24 hours mixing and
24 hours settling
Axis of
rotation
(30 rpm)
68-
6621/handbøker/grafisk/fig-eng/3skilletrakter.eps
Figure 4.2: Principle of the rotating cylinder method.
To test the effectiveness of the emulsion breaker Alcopol O 60 %, two dosages (500 ppm and
2000 ppm relative to the oil volume) were added drop wise to the w/o-emulsion. After a contact
period of 5 minutes and a rotation time of 5 minutes (30 rpm), the treated emulsion rested for 24
hours before the amount of water drained from the emulsion was determined.
The distilled residues were emulsified with 50 vol% and 75 vol% water in addition to the
maximum water content w/o-emulsion. Four parallel runs were performed to map the w/oemulsion kinetics and two parallel runs were performed with the addition of Alcopol O 60 %.
Several physical and chemical properties of the twelve weathered samples (see Figure 4.1) were
determined. A detailed description of the various analyses is given in chapter 4.3.3.
19
4.3.3 Physical and chemical analysis
The viscosity, density, pour point and flash point of the water free residues and w/o emulsions
will be determined. The analytical procedures that will be used are given in Table 4.1.
Table 4.1: Summary of the analytical methods used in the determination of the physical
properties.
Physical property
Analytical method
Instrument
Viscosity
Density
Pour Point
Flash point
McDonagh and Hokstad, 1995
ASTM method D4052-81
ASTM method D97
ASTM D 56-82
Physica MCR 300
Anton Paar, DMA 4500
Pensky-Martens, PMP1, SUR
The wax content and “hard” asphaltene content will be determined using the analytical procedures
given in Table 4.2
Table 4.2: Summary of the analytical methods used in the determination of the chemical
properties.
Chemical property
Analytical method
Wax content
“hard” asphaltene
Bridiè et al., 1980
IP 143/90
4.3.4 Chemical dispersibility testing
There is a multitude of different tests for evaluating the effect of chemical dispersants. Energy
input will differ in different tests, and the obtained efficiency will be representative for different
wave energies. In the screening of different dispersants the IFP test is used. In the systematic
testing of all residues and emulsions of the Trestakk oil, the IFP test is used. Screening of the
chemical dispersants was performed with the IFP test.
IFP (Institute Francais du Petrole test, Bocard et al, 1984) is a low energy test and is thought to be
representative for low wave energies (2-5m/s wind speed). A ring beating up and down in the test
vessel at a given frequency, gives energy input to the seawater column. The water column is
continuously diluted, which gives a more realistic approach to field conditions compared to other
tests. The test is shown in Figure 4.3.
20
IFP Test
7
6
2
3
8
5
1
1. Experimental beaker
2. Peristaltic pump
3. Storage water
4
4. Sampling bottle
5. Surge beater
6. Electro-magnet
7. Timer
8. Oil containment ring
ik22206200\wp\tegner\ifp-c.eps
Figure 4.3: IFP test apparatus
MNS (Mackay and Szeto 1980) has been estimated to correspond to a medium to high sea-state
condition. The energy input in this system, applied by streaming air across the oil/water surface,
producing a circular wave motion,. The sample of the oily water is taken under dynamic
conditions after a mixing period of 5 min. The test is shown in Figure 4.4.
MNS Test
Air outlet
Thermometer
Air flow
Water sampling tube
Manometer
Air inlet
Oil containment ring
ik22206200\wp\tegner\mns-c.eps
Figure 4.4: MNS test apparatus
Air blower
Cooling coil
21
5 Meso-scale laboratory testing
In an oil spill situation at sea the weathering processes will occur simultaneously and affect each
other. It is therefore important that the oils are weathered under realistic conditions when studying
how the oils behaviour when spilled on the sea surface.
A meso-scale flume basin (Singsaas et al., 1993) located at SINTEF is routinely used to study the
weathering processes simultaneously under controlled conditions. A schematic drawing of the
meso-scale flume basin is given in figure 3. A new flume with updated instrumentation was built
in 2006 at SINTEF Sealab. The flume experiment with Trestakk were performed at 13°C.
5.1 Description of the meso-scale flume
Approximately 4.8 m3 seawater circulated in the 10 meter long flume. The flume is located in a
temperature controlled room (0°C – 20°C). Two fans placed in a covered wind-tunnel allow
control of the wind speed. The wind is calibrated to simulate an evaporation rate corresponding to
a wind speed of 5-10 m/s at the sea surface. A schematic drawing of the flume is given in Figure
5.1.
Wave machine /
breaking board
Wind tunnel
Breaking wave
Fan
0.5m
2m
Water
sampling
Irradiated area
Solar simulator
4m
Figure 5.1: A schematic drawing of the meso-scale flume.
5.1.1 Oil weathering in the meso-scale flume
The oil sample (9 L) was carefully released on the water surface. The oil was weathered for a total
of 3 days in the. Samples of the surface oil where taken frequently in the first hours of the
experiment and then only once a day. Water samples are taken at a few times during the
weathering part of the trial, and at a high frequency during the dispersant application part of the
experiment.
22
Physical properties determined for all emulsion samples during the experiment were:
- evaporative loss
- density
- water content
- viscosity
Analysis performed for a limited amount of samplings was:
- Emulsion stability
- Oil concentration in the water column (droplets and dissolved components)
Methods for each analysis are described below.
5.1.2 Sampling of surface oil
Samples of the surface oil/emulsion was taken by use of an Aluminum tray and transferred to a
0.5 L separating funnel. After settling for 10 minutes in the climate room, free water was
removed. The oil phase was handled further for analysis of rheology, water content in emulsion
and density. One sample is taken from the water surface.
Evaporative loss
As the light end components of the oil evaporates the density increases. The density of the oil was
linearly dependent on the wt% evaporative loss. As the density of the water free residue was
known (method described above) the evaporative loss can be calculated.
Density
Density was measured according to ASTM method D 4052-91 at an Anton Paar DMA 4500
densitometer. The density was measured on the water free sample. The water was removed as
described above under the methodology for determination of water content.
Water Content and stability
The amount of water within the emulsion formed is determined by adding approximately 2000
ppm emulsion breaker to the sample and heating it in a vial. As the emulsion is broken the height
of the water-oil interface and height of the total sample are measured in the vial. The relative
amount of water compared to the total sample volume is calculated.
Rheological measurements
Rheological measurements were performed with a Physica MCR300 rheometer. Analyses done
were stress-sweeps and viscosity measurements. From the stress-sweep elasticity modulus, and
phase angel in the linear visco elastic area were obtained. Yield stress was calculated from the
generated data. Viscosity is measured according to standard methodology described in McDonagh
et.al, 1995.
Experimental setup for viscosity measurements:
Measurement system :
PP50
Gap
:
1mm
Shear rate
:
1s-1, 5s-1, 10s-1, 50s-1,100s-1,200s-1,500s-1, and 1000s-1
All viscosities are reported at 10-1.
23
Experimental setup for stress-sweeps:
Measurement system :
PP50
Gap
:
1mm
Angular velocity
:
10rad/s
Stress interval
:
0.05-1000 Pa (logarithmic increase)
5.1.3 Collection of water samples
Water samples were taken at 50 cm depth through a tap in the basin wall, into a Pyrex glass bottle
(1 L). The water sampled was acidified with some droplets of 10% HCl (pH lower than 2). The
sampling position is shown in Figure 5.1. Samples were extracted by liquid-liquid extraction with
dichloromethane and quantified by UV/spectrophotometry.
5.2 Solar simulation in the meso scale flume
Natural sunlight is simulated with a solar simulator from Gmbh Steuernagel. The 4KW lamp
emits a wavelength spectrum calibrated to fit natural sunlight at high noon and in the absence of
skies. Figure 5.2 shows the measured spectrum from the solar simulator compared to one of the
most widely used standard spectra for solar irradiance (CIE publication 85, 1989).
1000
W/m2
100
CIE publ 85
solarconstant
10
1
280-320
320-360
360-400
400-520
520-640
640-800 800-3000
w avelength interval
Figure 5.2: Measured wavelength spectrum compared to standard spectrum suggested in CIE
publication 85
The exact exposure of the oil in the meso scale flume is somehow hard to calculate. As the oil is
moving it will only be within the irradiated area periodically. The exposure will be highly
dependent on the distribution of the oil in the meso scale flume and the thickness of the emulsion
on the surface. An estimated irradiation pr day is compared with the average irradiation pr day for
some Norwegian cities in Figure 5.3.
24
6000
Trondheim
Arendal
Alta
Solar simulator
5000
Irradiation (Wh/m2/day)
4000
3000
2000
1000
0
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Figure 5.3: Daily Irradiation for some Norwegian cities through the year compared with the
estimated daily irradiation in the meso scale flume.
The simulated irradiance seems to be approximately average to the daily irradiance throughout the
year. It should, however, be noted that simulations are not taking clouds into consideration. The
real irradiation will be lower than the predicted irradiation shown in the figure.
25
6 Results from the bench scale weathering study
The results from the weathering study of the Trestakk crude are compared with similar data for
five other Haltenbanken crudes. These are:
•
•
•
•
•
Trestakk crude
Åsgard crude
Heidrun export blend
Midgard (condensate)
Kristin (light oil)
In figures all five oils are shown (crude/light oil/condensate), but in the tables only values for the
crudes are shown. These oils are all previously tested at SINTEF.
6.1 Composition and physical properties
The physical and chemical properties of the Trestakk crude are shown in Figure 6.1 and Figure
6.2 and listed in Table 6.1 to Table 6.4.
6.1.1 GC-FID characterization
The chemical composition of the Trestakk crude, characterised by gas chromatography (GC), is
shown in Figure 6.2. GC-chromatograms for fresh Trestakk in comparison with Morvin, Åsgard
and Heidrun export blend are shown in Figure 6.2.
Trestakk is a typical light paraffinic oil with a medium content of waxes.
The gas chromatograms show the n-alkanes as systematic narrow peaks. The first peaks represent
components with the lowest boiling points. Some of the more complex components like resins
(NSO compounds) and naphtenes are impossible to separate by this technique and are shown as
broad and poorly defined peak below the sharp peaks. This is often described as UCM
(Unseparated Complex Material).
Gas chromatography is an important tool for identification of an oil spill. A common parameter
used for identification is the nC17/Pristane and nC18/Phytane ratios. Table 6.1 shows these
relations for Trestakk and the oils it is compared to. The values show that the relations vary for
the different oil types. This is one of the parameters used in identification of oil spills. It can also
be useful in determining the degree of biodegradation for weathered samples.
Table 6.1: nC17/Pristane and nC18/Phytane relations for Trestakk compared to the other oils.
Trestakk
Morvin
Åsgard
Midgard condensate
Kristin light oil
nC17/Pristane
1,53
1,72
1,84
1,96
1,91
nC18/Phytane
1,68
2,98
2,32
2,63
2,54
-: Not performed
The Trestakk oil has been evaporated in laboratory to 150°C, 200°C and 250°C and the residues
represent oil evaporative loss corresponding to 0.5-1 hour, 0.5-1 day and 0.5-1 week of
weathering.
26
Fresh
150°C+
200°C+
250°C+
Figure 6.1
GC-FID chromatograms of fresh, 150°C+, 200°C+ and 250°C+ residues of
Trestakk crude oil.
27
Morvin, Fresh
Trestakk, Fresh
FID1 A, (HEIDRUN\03010700.D)
10000
nC-30
nC-25
15000
nC-20
nC-17
nC-15
20000
nC-18
Heidrun export blend, Fresh
nC-13
25000
nC-11
nC-9
counts
5000
5
10
15
20
25
30
35
40
45
min
FID1 A, (I:\PROSJEKT\8016-M~1\HPCHEM\3\DATA\OLJE1105\0256FR.D)
counts
140000
nC13
Kristin light oil, Fresh
nC-17
100000
80000
nC-18
nC-15
120000
nC-20
nC-25
40000
Phytane
Pristane
60000
nC-30
20000
0
0
5
10
15
20
25
30
35
40
45
min
12500
nC-20
nC-17
nC-13
15000
nC-18
Åsgard, Fresh
17500
nC-15
counts
20000
nC-11
nC-9
FID1 A, (AASGARDA\FERSK000.D)
7500
nC-30
nC-25
10000
5000
2500
5
10
15
20
25
30
35
40
45
min
Figure 6.2: GC chromatogram of fresh Trestakk in comparison with Morvin, Heidrun Export
blend, Kristin light oil and Åsgard.
28
6.1.2 Physical and chemical properties of fresh and weathered residues
Results from chemical analysis performed on the water free residues are shown in
Table 6.2. Calculates and measured physical properties for the residues are shown in
Table 6.3. Water content and viscosity for the emulsions tested are shown in Table 6.4.
Table 6.2:
Oil type
Trestakk
Morvin
-:
Wax and asphaltene content for the Trestakk and Morvin crudes.
Residue
Fresh
150°C+
200°C+
250°C+
Ph.Ox.
Fresh
150°C+
200°C+
250°C+
Ph.Ox.
Asphaltenes
(wt. %)
0,13
0,16
0,19
0,22
0,05
0,06
0,07
0,09
-
Wax
(wt. %)
2,6
3,2
3,7
4,4
5,4
6,9
8,3
10,4
-
Not performed
Table 6.3: Physical parameters for different Haltenbanken crudes at 13°C. Viscosities are
reported at a shear rate of 10s-1.
Oiltype
Trestakk
Morvin
Åsgard
Heidrun
Export
bland
-:
Residue
Fresh
150°C+
200°C+
250°C+
Ph.Ox.
Fresh
150°C+
200°C+
250°C+
Ph.Ox.
Fresh
150°C+
200°C+
250°C+
Ph.Ox.
Fresh
150°C+
200°C+
250°C+
Ph.Ox.
Evaporated
(vol.%)
0
10,7
34,1
45,9
0
23,5
37,0
51,0
0,0
23.0
36.3
46.7
0,0
7.0
14.3
23.6
-
Not performed
Residue
(wt. %)
100
80,5
68,8
58,9
100
78,5
37,0
52,0
100,0
78.2
65.5
54.7
100,0
94.4
87.8
79.2
-
Density
(g/mL)
0,8314
0,8624
0,8770
0,8902
0,8174
0,8440
0,8575
0,8710
0.814
0.848
0,864
0,875
0.892
0.905
0.914
0.924
-
Flash
Point
(°C)
46
85
127
45
83
125
35
75
354
46
89
130
-
Pour
Point
(°C)
<-39
<-39
<-39
-15
-27
12
18
27
-36
-3
18
27
-48
-42
-39
-24
-
Viscosity
(mPas)
5
14
29
84
15
377
1456
10276
26
189
354
977
37
63
119
266
-
29
Table 6.4:
Viscosity of Trestakk oil and water in oil emulsions at 13°C.
Residue
Water content
Viscosity (cP)
(vol.%)
10 s-1
100 s-1
Fresh
0
5
5
150°C+
0
14
13
200°C+
0
29
29
250°C+
0
84
72
150°C+
50
446
144
200°C+
50
464
254
250°C+
50
1050
580
150°C+
75
200°C+
75
2290
815
250°C+
75
4430
1450
150°C+
71
764
241
200°C+
90
3800
668
250°C+
85
9020
2010
-: ikke utført p.g.a maksimalt vannopptak lavere enn 75 vol.
6.2 Emulsifying properties of Trestakk crude oil
6.2.1 Water uptake and maximum water content
The water uptake rate and maximum water uptake have been studied in rotating flasks. The water
content in the emulsions as a function of time is shown in Table 6.5. The parameter reported as t1/2
is the halftime for the formation of a maximum-water emulsion. The parameters are derived from
the data in Table 6.5, and are used as input to the Oil Weathering Model.
The data from the 5°C experiments are presented and used for the evaluation of the water uptake
levels and rates. Several replicate experiments were performed at 13 °C, but the results from these
experiments were not conclusive and showed no systematic trends.
Table 6.5:
Water uptake and water uptake rate of 150°C+, 200°C+ and 250°C+ residues of
Trestakk crude in rotating flasks at 5°C.
Mixing time:
Start
5 min
10min
15 min
30 min
1 hour
2 hours
4 hours
6 hours
24 hours (max water)
(t1/2)
150°C+
(Vol. % water)
200°C+
(Vol. % water)
250°C+
(Vol. % water)
0
41
55
65
76
91
91
90
91
91
0,02
0
49
60
69
91
91
91
91
91
88
0,19
0
33
79
83
85
85
86
86
86
85
0,45
6.2.2 Stability and efficiency of emulsion breaker
Stability testing of emulsions formed from weathered residues of Trestakk crude and the
efficiency of emulsion breaker (Alcopol O 60 %) has been evaluated. The results are shown in
Table 6.6.
30
Table 6.6:
Residue
150ºC+
200ºC+
250ºC+
150ºC+
200ºC+
250ºC+
150ºC+
200ºC+
250ºC+
Stability of emulsions (No emulsion breaker) formed of weathered Trestakk crude
and efficiency of emulsion breaker (Alcopol O 60 %) at 13°C.
Water in emulsion,13°C (vol. %)
Reference
24 hours
91
91
88
87
85
81
91
25
88
40
85
43
91
14
88
21
85
51
Emulsion breaker
No emulsion breaker
No emulsion breaker
No emulsion breaker
Alc. O 60 % 500 ppm
Alc. O 60 % 500 ppm
Alc. O 60 % 500 ppm
Alc. O 60 % 2000 ppm
Alc. O 60 % 2000 ppm
Alc. O 60 % 2000 ppm
The Trestakk oil forms very stable emulsions. However, addition of emulsion breaker (500 or
2000 ppm) partly breaks and reduces the water content in the emulsions.
6.3 Dispersibility testing on Trestakk crude oil
6.3.1 Screening of dispersants
A screening study was performed, to find the best dispersant for the Trestakk crude. The
screening was performed on an emulsion made of a weathered sample of the Trestakk crude
(200°C+/50%). Results from the screening and dosage study are listed in Table 6.7.
Results show that the dispersants Dasic EW, Dassic NS and Corexit 9500 were the most
promising in the test. Given the uncertainty in the test (3*StDev=7) it is hard to distinguish the
effectiveness of between these dispersants. As Dasic NS is the dispersant most widely used in
Norwegian oil spill contingency, Dasic NS is chosen for the continued testing on Trestakk to
define the time window for using dispersants.
Table 6.7:
Screening and dosage testing of dispersants on the Trestakk crude.
Dispersant
Efficiency of
dispersant on
200°C+/50% (%)
Viscosity
(mPas)
shear rate
10s-1
Dasic NS (1:25)
86
426
Corexit 9500 (1:25)
88
426
Dasic EW (1:25)
87
426
Superdispersant 25 (1:25)
70
426
To study the dispersant effectiveness dependence on dosage, the two dispersants Dasic NS and
Corexit 9500 were tested at different dosages (Figure 6.3). The figure shows that the effectiveness
at ordinary dosage 1:25 (or 4%) and half dosage 1:50 is not very different. At lower dosages the
effectiveness drops.
A dosage rate of 1:25 (or 4%) is usually the operational target when applying dispersant on a
marine oil spill (vessel or arial application). This testing indicates that this dosage is sufficient for
a slightly weathered oil spill of the Trestakk crude.
31
100
Dasic NS
90
Corexit 9500
Dispersant effectiveness (IFP%)
80
70
60
50
40
30
20
10
0
1:25
1:50
1:100
1:200
Dispersant dosage compared to oil emulsion
Figure 6.3: Dispersant dosage testing on the Trestakk Crude
6.3.2 Systematic dispersibility testing on Trestakk residues and emulsions
Results from the systematic dispersibility study are listed in Table 6.8. All the tests are performed
with a dosage of 1:25 and with the dispersant Dasic NS.
Table 6.8:
Residue
150+
200+
Efficiency of Dasic NS on weathered oil/emulsions of Trestakk crude at 13°C.
IFP
MNS
Volume % Viscosity Efficiency Efficiency
water
(mPas)
(%)
(%)
shear rate
10s-1
102
102
0
14
99
0
29
104
87
250+
0
84
113
150+
50
446
104
86
200+
50
464
106
67
250+
50
1050
104
150+
75
76
200+
75
2290
91
36
250+
75
4430
85
100
150+
81,1
1490
100
200+
90,9
3800
77
63
250+
84,8
9020
-: Not performed due to maximum water content lower than 50 or 75 %.
6.3.3 Chemical dispersibility as a function of viscosity
The potential for use of chemical dispersants will fall as the viscosity of the weathered oil
increases. Dispersant effectiveness for the two tests (IFP and MNS) is plotted versus viscosity in
32
Figure 6.4. The figure shows no significant decrease in effectiveness for the MNS test, which has
the highest energy input of the two. In the IFP test a decrease in effectiveness can be shown.
120
Dispersant effectiveness (MNS%)
100
80
60 Limit between dispersible and reduced dispersibiliy
40
IFP
MNS
20
Limit between reduced dispersibility and poorly/slowly chemical dispersible
0
1
10
100
1000
10000
Viscosity (cP)
Figure 6.4:
Efficiency of dispersant on Trestakk crude and its weathered fractions.
Different efficiency criteria are used in categorising the potential for use of chemical dispersants
in the two tests. The criteria are listed in Table 6.9.
Table 6.9:
Criteria for definition of time window for effective use of dispersant.
Disperibility
Chemical dispersible
Reduced chemical dispersibility
Poorly/slowly chemical
dispersible
Criteria
IFP-efficiency > 50 %
MNS- efficiency > 70 to 80 %
IFP- efficiency < 50 %
MNS- efficiency > 5 %
MNS- efficiency < 5 %
In Figure 6.4 the results from the MNS test never reach any of the limits, and dispersibility is
defined as good for all the tested emulsions.
The tested emulsions had a viscosity range (up to 9000 cP) which were similar to the emulsions
observed in the meso-scale testing (see Table 7.1 and Figure 7.3). A limit of 3000 and 10000 cP
are used in the predictions to indicate that there is an upper limit for dispersant application (see
Figure 8.6).
33
7 Results from the Meso-scale laboratory testing
In this chapter the experimental results obtained for Trestakk in the meso-scale laboratory testing
are presented. The experimental test temperature was 13°C.
Table 7.1: Sampling and chemical analysis from the meso-scale weathering experiment.
Sample no
Time
(Hours)
0
1
2
3
4
5
6
7
8
9
10
0
0,25
0,5
1
2
4
6
12
24
48
72
11
12
13
5
10
30
Water
Content
(vol%)
Evaporative
Loss
(vol%)
Viscosity
(mPas)
12
0,0
10
14,3
46
19,6
79
23,5
84
26,8
87
29,7
86
31,6
85
34,1
83
36,8
88
39,5
88
41,5
Application of 60g Dasic NS
78
41,2
69
41,2
-
0
10
76
73
146
498
521
1857
2195
5649
7259
899
722
-
Dispersed oil in
water
(ppm)
(% of init)
63
3,8
29
1,8
24
1,4
78
264
488
4,7
15,8
29,3
7.1 Evaporation
The evaporation results obtained in the meso-scale laboratory testing of Trestakk are presented
and compared with the predicted evaporation results from the SINTEF OWM in Figure 7.1.
Property: EVAPORATIVE LOSS
15 m/s
10 m/s
5 m/s
2 m/s
flume data
3.1b © 2008
Pred: febr. 10. 2008
Summer Conditions (13°C)
60
Evaporation (vol%)
50
40
30
20
10
0
0,25
0,5
1
2
Hours
3
6
9
12
1
2
3
4 5
Days
Figure 7.1: The evaporation results obtained for Trestakk in the meso-scale laboratory testing
and the predicted results from the SINTEF OWM.
34
7.2 W/o-emulsification
The w/o-emulsification results obtained in the meso-scale laboratory testing are plotted with the
predicted results from the SINTEF OWM in Figure 7.2 and Figure 7.3.
Property: WATER CONTENT
15 m/s
10 m/s
5 m/s
2 m/s
flume data
3.1b © 2008
100
90
Water Content (vol%)
80
70
60
50
40
30
20
10
0
0,25
0,5
1
2
3
6
9
12
Hours
1
2
3
4 5
Days
Figure 7.2: Predicted water uptake for Trestakk and the results obtained from the meso-scale
laboratory testing.
Property: EMULSION VISCOSITY
15 m/s
10 m/s
5 m/s
2 m/s
flume data
3.1b © 2008
100000
Viscosity (mPas)
10000
1000
100
10
0,25
0,5
1
2
Hours
3
6
9
12
1
2
3
4 5
Days
Figure 7.3: Predicted viscosity of the Trestakk w/o-emulsions and the results obtained from
the meso-scale laboratory testing.
35
7.3 In-situ chemical dispersion
After 72 hours of weathering in the flume the dispersion agent Corexit 9500 was sprayed onto the
w/o-emulsion, see Table 7.2, where:
- DOR is the dispersion agent to oil ratio
- DER is the dispersion to w/o-emulsion ratio
Table 7.2: The application time and amount of the dispersion agent Corexit 9500.
Weathering
[h]
72
Amount of Corexit
9500 applied [mL]
75
Cumulative DOR
[%]
1.8
Cumulative DER
[%]
0.3
Samples are taken 3, 10 and 30 minutes after the dispersant application. The residual surface
emulsion was given a second treatment with Corexit 9500.
7.4 Mass balance
The main elements in a mass balance for a crude oil spilt at sea are:
- Evaporative loss
- Surface oil
- Dispersed oil
However, the initial sample and water volume in the flume is reduced throughout the test,
therefore the following parameters must be taken into consideration:
- Amount of oil sampled
- Amount of water sampled
- Amount of oil clinging to the flume walls
The amount of oil evaporated, dispersed and sampled was calculated, and the oil adsorbed to the
flume walls was estimated. Table 7.3 shows the mass balance for Trestakk during weathering in
the meso-scale flume at 13°C.
Table 7.3: Mass balance (%) for Trestakk during the meso-scale laboratory test at 13°C.
Evaporated
Oil on water surface
Dispersed
Sampled amount of oil
Amount of oil adsorbed to
the flume walls
After 72 hours of
weathering
42
40
1.4
1.8
After application of
dispersant (30 min)
42
29
29
1.8
15
15
36
Mass balance 13°C
100 %
90 %
80 %
70 %
evap %
surface %
sampled %
wall
disp %
60 %
50 %
40 %
30 %
20 %
10 %
0%
0,25
0,5
1
2
4
6
12
24
48
72
time (hours)
Figure 7.4: Predicted mass balance for Trestakk in the meso-scale laboratory test at 13°C.
100 %
90 %
80 %
% of available
70 %
60 %
surface
% dispersed
50 %
40 %
30 %
20 %
10 %
0%
before disp
3 min after disp
10 min after
disp
30 min after
disp
Figure 7.5: Predicted mass balance for Trestakk in the meso-scale laboratory test at 13°C.
37
7.5 Visual observations
1hour
The emulsion has started to emulsify
and has taken a light brown color.
The fresh emulsion is foamy and
unstable.
Sampled emulsion is quantitatively
broken within 30 minutes when
stored.
6 hours
The emulsion has reached its
maximum water content and has
turned brown or orange. Due to the
volume increase following water
uptake the slick thickness has
increased.
1 day
An orange thick slick is formed
overnight. The viscous oil forms a
thick an uniform slick.
The image on next page is a close-up
of the slick and shows the thickness
(4-5mm).
38
3 days
The area of the slick appears to be
considerably smaller compared to the
1 day image. This is partly because
the slick thickness has increased
during the experiment.
Underwater image shows the
considerable increase in thickness of
the emulsion, partly due to the high
viscosity and the constrained
environment.
At sea the emulsion is expected to
spread out an form thinner slicks
After dispersion
The image is taken 10 minutes after
the second application of 75ml of the
dispersant Dasic NS.
Most of the surface oil (70%) is
dispersed into the water.
10 minutes after 2nd dispersant application
39
8 SINTEF Oil Weathering Model – the model and input
A laboratory study of the weathering properties of Trestakk at 13°C has been performed. The data
is used as input to SINTEFs Oil Weathering Model (version 3.1beta). The SINTEF OWM relates
oil properties to a chosen set of conditions (oil/emulsion film thickness, sea state and sea
temperature) and predicts the rate in change of an oil’s properties and behaviour at the sea surface.
The SINTEF OWM is schematically shown in Figure 8.1.
SINTEF Oil
Weathering Model
Predicted oil properties by
time at chosen environmental
conditions:
Laboratory data of fresh and
weathered oil samples:
Distillation curve (TBP)
Densities
Viscosities
Flash points
Pour points
Water uptake rates (t0.5-values)
Maximum water uptake ability
Viscosity ratios
(w/o-emulsion/parent oil)
Viscosity limits for chemical
dispersion
Evaporative loss
Density
Viscosity
Flash point
Pour point
Water content
Viscosity of w/o-emulsion
Natural dispersion
Total oil mass-balance
"Time window" for use of
dispersants
Criteria used in the model
Environmental
conditions
(Wind speed, sea temperature,
oil film thickness)
6621/håndbøker/grafisk/fig-eng/model-col.eps
Figure 8.1: Schematic diagram of the input data to the SINTEF OWM and the predicted
output oil properties.
The predictions obtained from the SINTEF OWM are useful tools in Environmental Impact
Assessment studies and for determining the most effective response. In this report the presented
predictions span over a period of time from 15 minutes to 5 days after an oil spill has occurred.
This covers all potential spill situations from where the response time is short (e. g. close to
terminals) to offshore spills where the response time can be several days. The SINTEF OWM is
described in more detail in Johansen, 1991 and in the user’s guide for the model.
In the laboratory testing, a systematic stepwise procedure developed at SINTEF (Daling et al.,
1990) is used to isolate and map the various weathering processes that take place when an oil is
spilled on the sea surface. The experimental design for the study of Trestakk is described in
chapter 4 and the results are presented in detail in chapter 6. The input data to the SINTEF OWM
is given in chapter 8.1. The experimental weathering data are processed and used as input for the
SINTEF OWM. The following physical and emulsification properties obtained in the testing are
used in the model:
• specific gravity
• pour point
• flash point
• viscosities of fresh and the water-free residues (150°C+, 200°C+ and 250°C+)
• viscosities of the 50% and 75 % w/o-emulsions
• water uptake (maximum water content, stability and emulsification rate)
• dispersibility limits
40
Spill scenario
The spill and release rate chosen when using the SINTEF OWM is of importance. In this project a
surface release at a rate of 1,33 metric tons per minute was chosen as the spill scenario.
Oil film thickness
In the SINTEF OWM the oils are categorised into condensates, emulsifying crudes, low
emulsifying crudes, heavy bunker fuels or refined distillates based on experimental results
obtained in the bench-scale testing. The terminal film thickness varies among these categories
based on experimental field experience.
Sea temperature
The prevailing weather conditions greatly influence the weathering rate of oil on the sea surface.
Two sets of predictions are given in this report, one at the average summer temperature the other
at the average winter temperature for the area of interest. The temperatures chosen for Trestakk
are 5 and 15°C.
Wind speed
The relationship between the wind speed and the significant wave heights used in the prediction
charts obtained from the SINTEF OWM are shown in Table 8.1.
Table 8.1: The relationship between the wind speed and the significant wave heights used in the
SINTEF OWM.
Wind speed
[m/s]
2
5
10
15
Beaufort
wind
2
3
5
6-7
Wind type
Light breeze
Gentle to moderate breeze
Fresh breeze
Strong breeze
Wave height
[m]
0,1-0,3
0,5-0,8
1,5-2,5
3-4
8.1 Input to SINTEFs Oil Weathering Model
Geographical area:
Initial oil film thickness:
Terminal oil film thickness:
Release rate:
Norskehavet
20 mm
1 mm
1,33 metric tons/minute, totally 200 metric tons
Sea temperature:
Wind speed:
5°C and 15°C
2 m/s, 5 m/s, 10 m/s and 15 m/s
The data used as input to the SINTEF OWM for Trestakk are given in Table 8.2 to Table 8.4.
41
Table 8.2: Physical and chemical data
Properties of fresh oil:
Specific Gravity (60 F/60 F)
Pour Point (°C)
Reference temperature #1 (°C)
Viscosity at ref. temp.#1 (cP)
Asphaltenes (wt. %)
Flash Point (°C)
Wax Content (wt. %)
Dispersable for visc. <
Not dispersable for visc. >
Maximum water uptake (%)
0.8314
-39
13
6
0.13
<20
2.6
2000
9000
80
Table 8.3: The true boiling point (TBP) curve used for Trestakk
Temperature
(°C)
50
69
99
175
217
303
317
331
450
Volume
(%)
8
12
18
34
43
58
61
63
83
Table 8.4: Lab weathering data at 13°C
PROPERTY
Fresh
150°C+
200°C+
250°C+
Boiling temp. (°C)
193
252
305
Volume topped (%)
0
10.7
34.1
45.9
Residue (wt. %)
100
80.5
68.8
58.9
Specific gravity (g/l)
0.8314
0.8624
0.8777
0.8902
Pour point (°C)
-39
-39
-39
-15
Flash point (°C)
<20
46
85
127
Viscosity at 13°C (cP)
6
31
64
188
Viscosity of 50% emulsion (cP)
446
464
1050
Viscosity of 75% emulsion (cP)
2290
4430
Viscosity of max water (cP)
1490
3800
9020
Max. water cont. (%) (Lab. data)
71
91
85
Halftime for water uptake (hrs)
0.02
0.19
0.45
Stability ratio
0
0.06
0.24
* Emulsion viscosities of the 150°C+ residue are not listed for 75% water content, due to a
maximum water content of 71%.
42
8.2 Predictions of weathering properties
8.2.1 Prediction charts for Trestakk
The predictions shown are:
Figure 8.2: Evaporation
Figure 8.3: Flash point
Figure 8.4: Pour point
Figure 8.5: Water content
Figure 8.6: Viscosity of emulsion
Figure 8.7 to Figure 8.10 : Mass balances
8.2.2 How to use the prediction charts, an example
If Trestakk has drifted for a period of time on the sea surface the prediction charts can be used to
determine the remaining oil’s chemical, physical and emulsifying properties. Table 8.5 shows
examples for the following scenario:
• Drift time:
24 hours
• Sea temperature:
5°C/15°C
• Wind speed: 10 m/s
Table 8.5: Weathering properties for Morvin obtained from the prediction charts.
Property
Evaporation
Pour point
Water content
Viscosity of the emulsion
Winter temperature
[5°C]
41
-24
80
8700
Summer temperature
[15°C]
43
-25
80
6600
43
Property: EVAPORATIVE LOSS
Oil Type: TRESTAKK 2007
Description: Trestakk Garn formation 6406/3-2 dst2
Data Source: SINTEF Applied Chemistry (2007), Weathering data
OWModel 3.1beta Nov 30 200
© 2008
Surface release - Terminal Oil film thickness: 1 mm
Release rate/duration: 20.00 metric tons/minute for 5 minute(s)
Pred. date: Mar. 05, 2008
Wind Speed (m/s): 15
Wind Speed (m/s): 5
Wind Speed (m/s): 2
Winter Conditions (5 °C)
50
Evaporated (%)
40
30
20
10
0
0.25
0.5
1
2
3
6
9
12
Hours
1
2
3
4 5
2
3
4 5
Days
Summer Conditions (15 °C)
60
50
Evaporated (%)
40
30
20
10
0
0.25
0.5
1
2
Hours
3
6
9
12
1
Days
Figure 8.2: Evaporation of Trestakk crude oil at sea temperatures 5°C and 15°C.
44
Property: FLASH POINT FOR WATER-FREE OIL
© 2008
Wind Speed (m/s): 5
Wind Speed (m/s): 2
No fire hazard
Fire hazard in tankage (<60 °C)
Fire hazard at sea surface (below sea temperature)
200
Flash Point (°C)
150
100
50
0
0.25
0.5
1
2
3
6
9
12
Hours
1
2
3
4 5
2
3
4 5
Days
200
Flash Point (°C)
150
100
50
0
0.25
0.5
1
2
3
6
9
12
Hours
1
Days
Based on flash point measurements of weathered, water-free oil residues.
Figure 8.3: Flash point of Trestakk crude oil at sea temperatures 5°C and 15°C.
45
Property: POUR POINT FOR WATER-FREE OIL
© 2008
Wind Speed (m/s): 5
Wind Speed (m/s): 2
Chemically dispersible
Poorly / slowly chemically dispersible
40
Pour Point (°C)
20
0
-20
-40
-60
0.25
0.5
1
2
3
6
9
12
Hours
1
2
3
4 5
2
3
4 5
Days
40
Pour Point (°C)
20
0
-20
-40
-60
0.25
0.5
1
2
3
6
9
12
Hours
1
Days
Based on pour point measurements of weathered, water-free oil residues.
Figure 8.4: Pour point of water free Trestakk crude oil at sea temperatures 5°C and 15°C. The
operational window for chemical dispersability is larger than indicated on this
figure due to low emulsion viscosity and stability.
46
Property: WATER CONTENT
© 2008
Wind Speed (m/s): 5
Wind Speed (m/s): 2
80
Water content (%)
60
40
20
0
0.25
0.5
1
2
3
6
9
12
Hours
1
2
3
4 5
2
3
4 5
Days
80
Water content (%)
60
40
20
0
0.25
0.5
1
2
Hours
3
6
9
12
1
Days
Figure 8.5: Water content of Trestakk crude oil at sea temperatures 5°C and 15°C.
47
Property: VISCOSITY OF EMULSION
© 2008
Wind Speed (m/s): 5
Wind Speed (m/s): 2
Chemically dispersible (<3000 cP)
Poorly / slowly chemically dispersible (>10000 cP)
100000
Viscosity (cP)
10000
1000
100
10
1
0.25
0.5
1
2
3
6
9 12
Hours
1
2
3
4 5
2
3
4 5
Days
100000
Viscosity (cP)
10000
1000
100
10
1
0.25
0.5
1
2
Hours
3
6
9 12
1
Days
Based on viscosity measurements carried out at a shear rate of 10 reciprocal seconds.
Chemical dispersability information based on experiments under standard laboratory conditions.
Figure 8.6: Viscosity of emulsion of Trestakk crude oil at sea temperatures 5°C and 15°C.
Viscosity is predicted based on measurements performed at shear rate 10s-1.
48
Property: MASS BALANCE
© 2008
Evaporated
Surface
Naturally dispersed
Temperature: 5 °C
Wind speed: 2 m/s
100
Mass (%)
80
60
40
20
0
0.25
0.5
1
2
3
6
9
12
Hours
1
2
3
4 5
2
3
4 5
Days
Temperature: 5 °C
Wind speed: 5 m/s
100
Mass (%)
80
60
40
20
0
0.25
0.5
1
2
Hours
3
6
9
12
1
Days
Figure 8.7: Predicted mass balance for Trestakk at 5°C and wind speeds of 2 and 5 m/s.
49
© 2008
Evaporated
Surface
Naturally dispersed
Temperature: 5 °C
Wind speed: 10 m/s
100
Mass (%)
80
60
40
20
0
0.25
0.5
1
2
3
6
9
12
Hours
1
2
3
4 5
2
3
4 5
Days
Temperature: 5 °C
Wind speed: 15 m/s
100
Mass (%)
80
60
40
20
0
0.25
0.5
1
2
Hours
3
6
9
12
1
Days
50
© 2008
Evaporated
Surface
Naturally dispersed
Temperature: 15 °C
Wind speed: 2 m/s
100
Mass (%)
80
60
40
20
0
0.25
0.5
1
2
3
6
9
12
Hours
1
2
3
4 5
2
3
4 5
Days
Temperature: 15 °C
Wind speed: 5 m/s
100
Mass (%)
80
60
40
20
0
0.25
0.5
1
2
Hours
3
6
9
12
1
Days
51
© 2008
Evaporated
Surface
Naturally dispersed
Temperature: 15 °C
Wind speed: 10 m/s
100
Mass (%)
80
60
40
20
0
0.25
0.5
1
2
3
6
9
12
Hours
1
2
3
4 5
2
3
4 5
Days
Temperature: 15 °C
Wind speed: 15 m/s
100
Mass (%)
80
60
40
20
0
0.25
0.5
1
2
Hours
3
6
9
12
1
Days
The algorithm for prediction of natural dispersion is preliminary and is currently under improvement.Model
predictions have been field-verified up to 4-5 days.
52
2.3 Comparison with other oils
The weathering predictions of Trestakk are compared with predictions of several other
Haltenbanken crudes Trestakk, Kristin (lettolje), Heidrun export blend, Midgard (condensate) and
Åsgard at 15°C and 10 m/s in Figure 8.11 to Figure 8.15
Evaporative Loss
Trestakk is a relatively light crude resulting in an evaporative loss only slightly lower than most of
the other Haltenbanken oils in the comparison (Kristin, Morvin and Åsgard). The Midgard
condensate has the highest evaporative loss and the biodegraded, napthenic Heidrun has a lower
evaporative loss.
Property: Evaporative loss
3.1b © 2006
Pred: Febr. 10, 2008
Trestakk
100
Morvin
90
Kristin
80
Evaporated (%)
70
Midgard
60
Åsgard
50
40
30
20
10
0
0,25
0,5
1
2
Hours
3
6
9
12
1
2
3
4 5
Days
Figure 8.11: The predicted evaporative loss at 15°C and 10 m/s for Trestakk, Morvin, Kristin
(lettolje), Heidrun export blend, Midgard (condensate) and Åsgard.
53
Flash point
Due to the relatively high evaporative loss the weathered Trestakk has a medium flash point.
Explosion hazard is only a problem during less than the first half hour at sea (Flash point similar
to water temperature).
Property: Flash Point
3.1b © 2006
Trestakk
180
Morvin
160
Kristin
140
Flash Point (°C)
120
Midgard
100
Åsgard
80
60
40
20
0
0,25
0,5
1
2
Hours
3
6
9
12
1
2
3
4 5
Days
Figure 8.12: The predicted flash point at 15°C and 10 m/s for Trestakk, Morvin, Kristin
54
Pour point
The pour point is highly dependent on the wax content of the oil and the amount of light
components able to keep the waxes dissolved in the oil. Despite the high evaporative loss the pour
point of the Trestakk crude stays low due to the low initial high vax content and the medium
content of asphaltenes helping to keep the waxes dissolved.
There is no problem with solidification of the spilled oil on the surface, even at low temperature
scenarios, with the Trestakk crude (pour point > sea temperature + 15).
Property: Pour Point
3.1b © 2006
Trestakk
Pour Point (°C)
50
Morvin
40
Kristin
30
20
Midgard
10
Åsgard
0
-10
-20
-30
-40
-50
0,25
0,5
1
2
Hours
3
6
9
12
1
2
3
4 5
Days
Figure 8.13: The predicted pour point at 15°C and 10 m/s for Trestakk, Morvin, Kristin
55
Water Content
Trestakk and Morvin oils show both very rapid water uptakes. However while Morvin (high wax,
very low asphaltenes) forms very unstable emulsions, Trestakk forms stable viscouse emulsion
due to the balance between the vax and asphaltene content stabilising the water droplets in the oil
phase. The midgard condensate doesn’t contain any natural components (e.g. waxes, asphaltenes
or resins) stabilising the water droplets and show no water uptake (emulsification).
Property: Water content
3.1b © 2006
Trestakk
Water content (%)
90
Morvin
80
Kristin
70
60
Midgard
50
Åsgard
40
30
20
10
0
-10
0,25
0,5
1
2
Hours
3
6
9
12
1
2
3
4 5
Days
Figure 8.14: The predicted water uptake at 15°C and 10 m/s for Trestakk, Morvin, Kristin
56
Emulsion viscosity
Due to the balanced content of vaxes and asphaltenes Trestakk forms very stable water-in-oil
emulsions with small droplets and high viscosity. Figure 8.15 shows that Trestakk forms
emulsions with very high viscosity compared to most of the other Haltenbanken crudes.
Property: Viscosity of Emulsion
3.1b © 2006
Trestakk
100000
Morvin
Kristin
Viscosity (cP)
10000
Midgard
1000
Åsgard
100
10
1
0,25
0,5
1
2
Hours
3
6
9
12
1
2
3
4
5
Days
Figure 8.15: The predicted emulsion viscosity at 15°C and 10 m/s for Trestakk, Morvin,
Kristin (lettolje), Heidrun export blend, Midgard (condensate) and Åsgard.
57
9 Weathering properties related to response
9.1 Oil properties
Trestakk is a light crude (0,831 mg/ml) with a low asphaltene content (0,13 wt%) and a medium
wax content (2.6 wt%) compared to other Norwegian crudes. As most light paraffinic oils the
initial evaporative loss is high. This high evaporation causes a rapid increase in the relative
amount of wax and asphaltenes in the early stages of weathering. As the relative concentration of
heavy end components increase the physical properties of the oil will change rapidly.
9.2 Flash point – fire/explosion hazard
As oil is spilt on the sea surface the temperature of the oil will be cooled to ambient water
temperature within a short time. The fire hazard will be at its greatest as long as the Flash Point of
the oil is below the sea temperature. For the Trestakk crude the flash point will be above the sea
temperature within 15 minutes, even at low sea states (2m/s wind).
Some vessels have a Flash Point limit of 60°C for liquids to be stored onboard. At low wind
speeds (2m/s) this limit will be reached in approximately 3 hours, and considerably faster at
higher winds.
Property: FLASH POINT
15 m/s
10 m/s
5 m/s
3.1b © 2007
Pred. Dato: Febr 10, 2008
2 m/s
180
160
Flash Point (°C)
140
120
100
80
Tank storage limit
60
40
20
Fire/explosion hazard
0
0,25
0,5
1
2
Hours
3
6
9
12
1
2
3
4 5
Days
Figure 9.1: Flash point for different sea states of the Trestakk crude.
58
9.3 Emulsion formation and stability
The Trestakk crude emulsifies rapidly on the sea surface and forms stable emulsion with relatively
high water content (80%). The high stability is due to a balanced wax and asphaltenes content that
help stabilising the emulsions. The water mixed into the oil makes the volume of the slick larger.
As the total amount of oil on the sea surface is lowered due to evaporation and natural dispersion,
the amount of emulsion is actually increasing in the initial stages of weathering. This is shown in
Figure 9.2
Property: SURFACE OIL/EMULSION
3.1b © 2007
Pred. Dato: febr 10, 2008
Surface emulsion
Surface oil
180
Surface oil or emulsion (vol%)
160
140
120
100
80
60
40
20
0
0,25
0,5
1
2
Hours
3
6
9
12
1
2
3
4 5
Days
Figure 9.2: Changes in total slick volume due to emulsification. Decrease in total oil volume
due to evaporation and natural volume (predicted at 10m/s wind, and 15°C).
9.4 Mechanical recovery
Past experiences from Norwegian field trials have shown that the effectiveness of many
mechanical clean up operations is reduced due to a high degree of leakage of the confined oil or
w/o-emulsion from the oil spill boom (especially in high current). This leakage is especially
pronounced if the viscosity of the oil or the w/o-emulsion is lower than 1000 cP (Nordvik et al.,
1992). The lower viscosity limit for an optimal mechanical clean up operation has therefore been
set to 1000 cP. As shown in Figure 9.3 the viscosity of Trestakk remain beneath this limit for only
5-12 hours in 5-10 m/s wind.
Efficiency of mechanical recovery is expected to be high and is not reduced by highly viscous
emulsions (10000 cP after 5 days 10 m/s). Offshore weir skimmers operated by NOFO
(Transrec) have a high capacity within this viscosity range.
Due to the low pour point, solidification on the sea surface would not be a likely scenario with
the Trestakk oil.
59
Property: EMULSION VISCOSITY
15 m/s
10 m/s
5 m/s
2 m/s
3.1b © 2007
Pred. Dato: Febr 10, 2008
100000
Viscosity (mPas)
10000
1000
Boom leakage
100
10
1
0,25
0,5
1
2
Hours
3
6
9
12
1
2
3
4 5
Days
Figure 9.3: Predicted emulsion Viscosity for Trestakk at 15°C compared with expected viscosity
limits for extensive boom leakage and poor flow to weir skimmers.
9.5 Life time at sea – Natural dispersion and evaporation
Natural dispersion and evaporation are the main weathering processes removing an oil spill from
the surface. This life time of an oil spill at sea is very dependant on the oil composition, the
release conditions (e.g. at surface, underwater) and environmental conditions (temperature, wind,
waves).
Trestakk is a light crude with a relatively high evaporative loss, but it forms very stable emulsions
when spilt at sea. Figure 9.4 shows the remaining surface oil as a function of time at different sea
states. Trestakk has a similar life time at sea compared to several of the other Haltenbank crudes,
see Figure 9.5.
The graphs in these life-time figures are based on an instant surface release of 200 m3.
Considerably larger spills (>1000 m3) is expected to give longer life times and individual
modelling for such spills are needed.
60
Property: Remaining surface oil
2 m/s
5 m/s
10 m/s
15 m/s
3.1b © 2007
Pred. Dato: Febr. 10, 2008
100
Surface oil (% of initial volume)
90
80
70
60
50
40
30
20
10
0
0,25
0,5
1
2
3
6
9
12
Hours
1
2
3
4 5
Days
Figure 9.4: Predicted remaining surface oil for Trestakk at 15°C for different sea states or wind
strengths. Predictions are for an instant surface release of 200 m3.
Property: Remaining surface oil
3.1b © 2007
Pred. Dato: Febr. 10, 2008
Trestakk
Surface oil (% of initial volume)
100
Morvin
90
Kristin
80
70
Midgard kondensat
60
Åsgard
50
40
30
20
10
0
0,25
0,5
1
2
Hours
3
6
9
12
1
2
3
4 5
Days
Figure 9.5: Predicted remaining surface oil for Trestakk and other Haltenbank oils at 15°C and
10 m/s wind. Predictions are for an instant surface release of 200 m3.
61
9.6 Use of chemical dispersants
Trestakk has a very good potential for use of chemical dispersants even after several days of
weathering. This is illustrated in Figure 9.6. The figure shows that dispersants can be used with
expected good effectiveness to at least five days of weathering.
In very rare cases, with very limited emulsification (very calm sea) and high evaporation,
dispersant efficiency could be limited by solidification of the oil (high pour point of water free
oil).
Property: VISCOSITY OF EMULSION
© 2008
Wind Speed (m/s): 5
Wind Speed (m/s): 2
Chemically dispersible (<3000 cP)
Poorly / slowly chemically dispersible (>10000 cP)
100000
Viscosity (cP)
10000
1000
100
10
1
0.25
0.5
1
2
3
6
9 12
Hours
Figure 9.6: Time window for use of chemical dispersants.
1
Days
2
3
4 5
62
10 References
Bocard, C., Castaing, C. G. and Gatellier, C. 1984. Chemical oil dispersion in trials at sea and in laboratory tests:
The key role of the dilution process. In: Oil spill chemical dispersants: Research Experience and
recommendations, ASTM STP 840. (T. E. Allen, ed), Philadelphia, USA, pp. 125-142.
Bridié A.L., T.H. Wanders and W.V. Zegweld, H.B. den Heijde. 1980. Formation, Prevention and Breaking of
Seawater in Crude Oil Emulsions, Chocolate Mousse. Marine Poll. Bull., vol 11, pp. 343-348.
Daling, P. S., Brandvik, P. J., Mackay, D., Johansen, Ø. (1990): Characterisation of crude oils for environmental
purposes. Oil & Chemical Pollution 7, 1990, pp.199-224.
Daling, P. S., O. M. Aamo, A. Lewis, and T. Strøm-Kristiansen, 1997: SINTEF/IKU Oil-Weathering Model:
Predicting Oil Properties at Sea. Proceedings 1997 Oil Spill Conference. API publication No. 4651, Washington
D. C., pp 297 – 307.
Daling, P. S., Aamo, O.M., Lewis, A., Strøm-Kristiansen, T. IKU Oil Weathering Model - predicting oil’s properties
at sea. 1997 International Oil Spill Conference, Fort Lauderdale, Florida. 2 - 10 April, pp 297-307.
Daling, P.S., Brandvik, P.J., Mackay, D. and Johansen, Ø. 1990. Characterisation of crude oils for environmental
purposes. Paper at the 13th AMOP seminar, Edmonton, Canada 1990. DIWO-report no. 8. IKU report
02.0786.00/08/90. 22 p. Open.
FOH, 1984, Oljers skjebne og effekter i havet, Avslutningsrapport, Miljøverndepartementet, Norge.
Hokstad, J. N., Daling, P. S., Lewis, A., Strøm-Kristiansen, T. 1993: Methodology for testing water-in-oil emulsions
and demulsifiers Description of laboratory procedures. In:Proceedings Workshop on Formation and Breaking of
W/O Emulsions. MSRC, Alberta June 14-15 24p.
ITOPF 1986. Fate of Marine Oil Spills. Technical information paper no. 11/86. The InternationalTankers Owners
Pollution Federation Ltd., London, England.
Johansen, Ø. 1991. Numerical modelling of physical properties of weathered North Sea crude oils. DIWO-report no.
15. IKU-report 02.0786.00/15/91. Open.
Leirvik, F., Moldestad, M., Johansen, Ø.,2001. Kartlegging av voksrike råoljers tilflytsevne til skimmere.
Mackay, D. and Zagorski, W. 1982. "Studies of W/o Emulsions". Report EE-34: Environment Canada, Ottawa,
Ontario.
Mackay, D., Buist, I., Mascarenhas, R.and Paterson, S. 1980. "Oil Spill Processes and Models". Report EE-8,
Environment Canada, Ottawa, Ontario.
Mackay, D.and Szeto, F. 1980. Effectiveness of oil spill dispersants - development of a laboratory method and
results for selected commercial products. Institute of Environmental Studies, University of Toronto, Publ. no. EE16.
McDonagh, M, J.N. Hokstad and A.B. Nordvik. 1995. ”Standard procedure for viscosity measurement of water-inoil emulsions”. Marine Spill Response Corporation, Washington, D.C. MRSC Technical Report Series 95-030,
36 p
National Research Council (NRC): Oil in the sea. Input, fates, and effects. National Academy Press, Washington,
D.C., 1985.
Nordvik, A.B., Daling, P. and Engelhardt, F.R. 1992. Problems in the interpretation of spill response technology
studies. In: Proceedings of the 15th AMOP Tecnical Seminar, June 10-12, Edmonton, Alberta, Canada, pp. 211217.
Reed, M., C. Turner, and A. Odulo (1994): The role of wind and emulsification in modelling oil spill and surface
drifter trajectories. Spill Science and Technology, Pergamon Press (2): .143-157.
Ross, S.L., 1986: An experimental study of the oil spill treating agents that inhibit emulsification and promote
dispersion. Report EEø87, Environmen Canada, Ottawa, Canada.
Stiver, W. and D. Mackay. 1984. Evaporation rate of spills of hydrocarbons and petroleum mixtures. Environ, Sci.
Technol., vol. 18 (11), pp. 834-840.
63
Aamo, O. M., M. Reed, and K. Downing (1996): Calibration, verification, and sensitivity analysis of the SINTEF oil
spill contingency and response (OSCAR) model system (in Norwegian). Report No. 42.4048.00/01/96. 87 p.
Aamo, O. M., Reed, M., Daling, P. S. (1995): Evaluation of environmental consequences and effectiveness of oil
spill response operations with a possible change in first line response at the Veslefrikk field. (In Norwegian).
SINTEF Report No.. 95.006, SINTEF Petroleum Research 1995.
Aamo, O. M., Reed, M., Daling, P. S., Johansen, Ø. (1993): A Laboratory-Based Weathering Model: PC Version for
Coupling to Transport Models. Proceedings of the 1993 Arctic and Marine Oil Spill Program (AMOP) Technical
Seminar, pp.617-626

sintef report

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