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Weathering processes in Arctic oil spills: Meso-scale experiments with different ice conditions
Cold Regions Science and Technology 55 (2009) 160–166
Contents lists available at ScienceDirect
Cold Regions Science and Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c o l d r e g i o n s
Weathering processes in Arctic oil spills: Meso-scale experiments with different ice conditions Per Johan Brandvik a,b,⁎, Liv-Guri Faksness a a b
The University Centre in Svalbard, Pb. 156, N-9171 Longyearbyen, Norway SINTEF Materials and Chemistry, N-7465 Trondheim, Norway
a r t i c l e
i n f o
Article history: Received 29 November 2007 Accepted 17 June 2008 Keywords: Oil spill Weathering Arctic Ice Field experiments
a b s t r a c t The knowledge regarding weathering processes in Arctic oil spills, and especially oil spills in ice, is limited. Experimental studies have been performed in laboratories, but only to a limited degree in the field. This paper presents results from a series of meso-scale field experiments performed on Svalbard, Norway, in 2005. The results from these field experiments performed to study oil behavior (evaporation, emulsification, spreading etc.) with different ice conditions (slush ice, 30% and 90% ice coverage) are presented in this paper. Several weathering properties are strongly influenced by the low temperature, reduced oil spreading and wave action caused by increased ice coverage. Reduced water uptake, viscosity, evaporation, and pour point in dense ice conditions extend the operational time window for several contingency methods compared to treatment of oil spills in open waters. For an oil spill in open ice, this could open up for dispersant treatment and in-situ burning even after an extended period of weathering. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Extensive research has been performed during the last 30 years, including field tests, observations of real oil spills and laboratory studies trying to understand the weathering processes taking place when oil is spilled in ice. However, the majority of this work is now 10 years or older, as also concluded in a recent review on the behaviour of oil in freezing environments (Fingas and Hollebone, 2003). Operationally important weathering processes for oil spill operations like water uptake, emulsion stability and viscosity vary with oil type. Normally they increase relatively fast with increased weathering time in open water. In ice infested water, several studies have indicated that this increase with time (e.g. water content) can be drastically changed depending on ice type, ice coverage and energy conditions in the ice. Little knowledge concerning this is available today, and only for a limited number of oil types and ice regimes, knowledge gained through laboratory and field experiments performed in US, Canada and Norway (NORCOR, 1975; Ross and Dickins, 1987; Payne et al., 1991; Vefsmo and Johannessen, 1994). Compared to the current in-depth knowledge on weathering processes in oil spills in open water and temperate conditions, our knowledge regarding Arctic oil spills is very limited. There is a need for laboratory and field experiments to quantify these weathering ⁎ Corresponding author. SINTEF Materials and Chemistry, Marine Environmental Technology, N-7465 Trondheim, Norway. Tel.: +47 90958576; fax: +47 93070730. E-mail address: [email protected] (P.J. Brandvik). 0165-232X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.coldregions.2008.06.006
processes. These data should be used to improve and validate the algorithms in existing oil weathering models. The objective should be to collect basic research data on evaporation, dispersion, spreading, and other weathering parameters in the marginal ice zone. These data could then be used to enhance and modify or develop new algorithms, and to improve existing models for oil weathering in ice. Increasing oil price and reduced ice coverage have contributed to renewed interest in oil exploration and transport in Arctic areas. Several research programs have been initiated the last years to increase our knowledge on basic processes with oil in ice. The study presented in this paper is part of a joint effort between the University Centre in Svalbard (UNIS) and SINTEF focusing on oil weathering processes in different ice conditions. Another aspect of this project has been to study migration of water soluble components in ice and the toxicity of these components (Faksness and Brandvik, 2008). The work presented in this paper focuses on weathering processes (emulsification, evaporation and others) for one oil type in different ice conditions, and is an important contribution when trying to fill the gap in knowledge regarding fate and behavior of oil spills in ice. 2. Experimental 2.1. Oil type Statfjord crude was used as the oil type for these experiments. This is a representative oil from the group of light and paraffinic crudes. Statfjord is produced in the Southern part of the Norwegian Sea and is
P.J. Brandvik, L.-G. Faksness / Cold Regions Science and Technology 55 (2009) 160–166 Table 1 Main relevant properties for fresh Statfjord crude
161
Table 2 Specifications for the three different experiments
Property
Value
Wax content (wt.%) Viscosity (cP, shear rate 100 s− 1, 13 °C) Asphaltene content (wt.%) Density (g/mL) Flash point (°C) Pour point (°C) Loss at 150 °C (vol.%) Loss at 200 °C (vol.%) Loss at 250 °C (vol.%)
4.3 7.0 0.1 0.834 b−39 −3 25.2 34.2 43.6
by production volume an important oil. Relevant properties for the fresh crude are given in Table 1. 2.2. Circulating flume used for oil weathering A circulating flume was used to study weathering of the oil at different ice conditions. The flume was cut out in the ice in van Mijenfjord close to SINTEF's field research station in Sveagruva at Svalbard (Spitsbergen). The flume dimensions and principal layout are given in Fig. 1. The depth of the flume was 50 cm and it was not cut completely through the fjord ice (total ice thickness 110 cm). Two
Experiment type
Ice coverage (%)
Surface current (cm/s)
Wave height (cm)
Temp air (°C)
Temp water (°C)
Open water + slush Low ice coverage High ice coverage
0
15
15
−15
−1.9
30
9
10
−10
−1.9
90
0
5
−5
−1.9
propellers were used to control circulation, and a wave maker to introduce wave energy. The flume was operated with three different ice conditions; open water (with some slush ice), low and high ice coverage (30 and 90%). The settings and environmental conditions for the three experiments are given in Table 2. After weathering for three days the oil was led to a connected in-situ burning (ISB) chamber and tested for ignitability and ISB effectiveness. 2.3. Oil sampling and analysis A comprehensive sampling program was carried out during the three days each experiment lasted. Initially, 20 L of Statfjord was very
Fig. 1. Principle layout for meso-scale basin made in fjord ice and used for oil weathering and ISB experiments showing the overall dimensions, the position of the current propellers, wave generators, and in-situ burning (ISB) chamber used to burn the weathered oil.
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2.5. In-situ burning (ISB) of weathered oil
Table 3 Physical/chemical properties, units and methods Property
Unit
Method
cP (or mP) at shear rate 10 or 100 s− 1 at Daling et al. 3–6 °C (1990) vol.% Daling et al. Water content of w/oemulsion (1990) Density of water water-free g/mL at 15.5 °C ASTM D4052oil 91 Chemical dispersibility of wt.% Concawe weathered oil (1988) Evaporative loss wt.% Daling et al. (1990) Flash point of water water- °C ASTM D93-90 free oil Pour point of water water- °C ASTM D97-87 free oil Viscosity of weathered oil
The ISB chamber was used for testing of ignitability and burnability of the weathered oil. After each weathering experiment the emulsion was led to the burning chamber by a simple boom arrangement assisted with some shoveling of the surface weathered oil. The film thickness in the 1 m2 burning chamber varied from 15 to 25 mm. The igniter was applied to the middle of the weathered oil and lit by a propane torch. The igniter used consisted of gelled gasoline (500 mL). If no sufficient flame spreading and initial burning of the weathered oil started during the burn time of the igniter (5 min), the weathered oil was classified as “Not ignitable”. In the cases where the weathered oil ignited, the residue was collected with adsorption pads and quantified gravimetrically. 3. Results and discussion
gently applied on to the surface of the flume. Samples were taken from the emulsified surface oil for a series of physical and chemical analyses. Surplus, free sea water in the collected sample was immediately drained off using a separation funnel, before the sample was homogenized and divided into aliquots for further analysis. The analyses of the physical and chemical properties of surface emulsified oil were performed in a laboratory container on the ice close to the flume immediately after sampling (see Table 3). The two last analyses (pour- and flash point) were performed later at the SINTEF laboratories in Trondheim. Details regarding the analysis above are described in Daling et al. (1990). 2.4. Dispersibility testing (field test) The effectiveness of dispersants was tested by the use of a simplified field procedure (Concawe, 1988). Dispersant (5%) was added to the oil in a closed, graded cylinder (100 mL) and the oil was very gently shaken a few times. The following three criteria were used for evaluation of the oil in water dispersion formed and hence the chemical dispersibility of the oil: • Good dispersibility: Brown homogeneous dispersion with only small oil droplets • Reduced dispersibility: Dark brown to black dispersion (with larger oil droplets). • Not dispersable: Black unstable dispersion (very large oil droplets, similar to non-treated oil).
The presentation of the results from these field experiments is divided into two parts. The first part (Section 3.1) presents the properties describing the weathering of the oil in the three different oil-in-ice scenarios. The second part (Section 3.2) presents results from the operationally oriented testing, in-situ burning and dispersibility. 3.1. Physical and chemical properties of weathered oil 3.1.1. Evaporative loss Evaporative loss can be a significant weathering process for many light crude oils and can lead to a loss up to 50–60%. The evaporative loss for these oils in the ice experiments is given in Fig. 2 and shows a clear and significant difference between the three experiments with different ice conditions. The evaporative loss for the open water scenario was 30%, for the 30% ice coverage 25% and finally it was 19% with high ice coverage (90%). The difference in evaporative loss is mainly caused by the different film thicknesses in the three experiments, since evaporation is a surface phenomenon. This is in contradiction to studies by Fingas (1995), which claims that evaporation from oil slicks is independent of film thickness. The presence of the ice limits the spreading of the oil and results in higher film thicknesses. The theoretical film thicknesses, assuming a complete spreading, are 1.3, 2.0 and 13 mm (open water, 30% and 90% ice coverage respectively). The real differences in film thickness between the experiments were probably less, since the oil is not evenly distributed in the flume.
Fig. 2. Evaporative loss (vol.%) from the weathered oil for the meso-scale field experiments with different ice conditions.
P.J. Brandvik, L.-G. Faksness / Cold Regions Science and Technology 55 (2009) 160–166
163
Fig. 3. Water content (vol.%) in emulsified oil for the meso-scale field experiments with different ice conditions. The in-situ measured ignitability for the weathered oil after 2.5 day of weathering is also indicated.
3.1.2. Emulsification (water uptake) Water-in-oil (W/o) emulsification is one of the major oil weathering processes for marine oil spills. The transformation from a fresh crude with low viscosity to an often brown, highly viscous emulsion is of large environmental and operational importance. The increased viscosity and volume (75% water content quadruples the volume) increase the persistence of the oil spill on the surface. From an operational point of view, the increased volume and viscosity is important for selecting appropriate recovery equipment (type of skimmers, pumps and needed tank capacity). Water in oil emulsification for the three experiments is shown in Fig. 3. The difference between “open water” and “30% ice” is small and probably not significant (38–41% water content), while the difference with the “90% ice” experiment is large and significant (about 20% water content). This indicates that the wave energy available for
creating emulsions has not been very different in the two first cases. The small difference between “open water and “30% ice” could probably be explained by the presence of slush ice in the “open water” experiments, due to the low air temperature during the experiments (−15 °C, see Table 2). This slush ice significantly dampens the effect of the waves in the “open water” experiment. Open water weathering of Statfjord, without any slush, was expected to give a water uptake in the range of 75–80% (Moldestad et al., 2001). 3.1.3. W/o-emulsion viscosity The viscosity change due to evaporation alone is very minor (only up to a few hundred cP) compared to the viscosity increase due to w/oemulsification.Theinclusionofwaterassmalldropletsintheemulsifiedoil changes the oil viscosity due to internal friction between the water droplets and the continuous oil phase. Initially, the droplets are large
Fig. 4. Viscosity (cP at shear rate 10 or 100 s− 1, 3–6 °C) for the emulsified oil from the meso-scale field experiments with different ice conditions. Measured chemical dispersibility as a function of weathering is shown as colored areas.
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Fig. 5. Flash point for weathered water-free oil (°C) for the meso-scale field experiments with different ice conditions. The in-situ measured ignitability for the weathered oil after 2.5 day of weathering is also indicated.
(several hundred microns), but as they are stabilized by the naturally occurring surface active components (mainly asphaltenes, waxes and resins) the water droplets are reduced in size. This reduction in mean droplet size diameter down to the range of a few microns drastically increases the interface between water and oil, and also the internal friction between the two phases. This increases the viscosity of the emulsified oil (Pal et al., 1992). The viscosities of the w/o-emulsion from the three experiments are given in Fig. 4. The viscosity is measured at shear rate 100 s− 1 for the less viscous water water-free samples (typically the first 2 h), and at 10 s− 1 for the emulsified samples. The difference between the “open water” and “30% ice” versus the “90% ice” is large and significant. The two first scenarios gave viscosities in the range of 3500–5000 cP while the latter had a viscosity of 1400 cP.
is 6.1% since the high molecular weight wax components do not evaporate. This high wax content is sufficient for the waxes to precipitate and form lattices which change the rheology of the oil and could make it semi-solid. This process is best described by the pour point of the oil, which is defined as the temperatures where the oil does not flow freely anymore (ASTM D97/87). The pour points for these three experiments are shown in Fig. 6. This figure too shows a distinct difference between the “open water” and “30% ice” scenarios versus the “90% ice” scenario. The open water and low ice coverage experiments end with a pour point of 24 °C, while the high ice coverage experiment in the end reaches 19 °C. This is closely connected to the difference in evaporative loss (see Fig. 2). The operational significance of varying pour point is discussed in the next chapter.
3.1.4. Flash point for residual oil Traditionally, evaporative loss has been the main weathering property for evaluating ignitability and the potential for ISB of weathered oils. However, the content of light components initially present in the fresh oil is extremely variable and gives little information regarding the ignitability of the weathered oil. The properties of the weathered residue are more important than the evaporative loss from the original oil. Since the amount of light components is highly variable among crude oils, this means that e.g. the flash point for different residual oil could be very different for two crude oils after the same evaporative loss. For this reason it is more appropriate to use the flash point for the residual oil to estimate if the weathered oils are ignitable for ISB (see 3.2.2). Flash points for the residual oils in the three experiments are shown in Fig. 5. The difference between “open water” and “30% ice” is small (100–110 °C), and probably not significant, while the distance to the “90% ice” experiment is larger and significant (60 °C).
3.2. Testing of operational oil spill options (use of dispersants and ISB)
3.1.5. Pour point of residual oil When the light components in an oil spill evaporate the relative content of wax component will increase. The waxes (straight and naphthenic hydrocarbons with more than 20 carbon atoms) are high in molecular weight and have a very low vapor pressure. Statfjord crude has a wax content of only 4.3%, but in the open water scenario with an evaporative loss of 30% the effective wax content after 3 days
3.2.1. Field testing of chemical dispersibility As the viscosity of the emulsified surface oil increases it becomes increasingly difficult for the dispersant sprayed onto the slick to penetrate into the oil. Due to the reduced molecular diffusion in the viscous oil slick the dispersant droplets need more time to penetrate into the viscous emulsion. If the viscosity of the oil is too high and penetration takes too long time, the dispersant will be washed off by waves flushing over the emulsified oil spill. Also the semi-solidification of the oil spill due to wax precipitation will reduce the effectiveness of dispersant application. It is the same principle; when the oil starts to solidify the dispersant droplets need more time to penetrate into the oil, and the possibility for being flushed into the sea before penetrating into the oil phase increases. For most oil spills at sea these problems start when the oil's pour point is the same as the sea temperature, and the dispersant effectiveness is usually drastically reduced when the oil's pour point is 15 °C above the environmental temperature (Daling et al., 1990; Brandvik et al., 1995). The combined effect of the increasing viscosity and pour point defines the operational “window of opportunity” for dispersant use on an oil spill in ice. Dispersant effectiveness measured with the field test, directly after oil sampling, is given as shaded areas in Figs. 4 and 6. This field testing indicates that for the Statfjord crude these problems occur at 1500 cP, and above 4000 cP the dispersant
P.J. Brandvik, L.-G. Faksness / Cold Regions Science and Technology 55 (2009) 160–166
165
Fig. 6. Pour point of the weathered water-free oil (°C) for the meso-scale field experiments with different ice conditions. Measured chemical dispersibility as a function of weathering is shown as colored.
effectiveness is close to zero. Similarly for the pour point, dispersibility starts to drop at 18 °C above the water temperature and approaches zero at 21 °C above water temperature. For the open water and 30% scenarios this gives a shorter time window for dispersant use compared to similar temperate scenarios (e.g. 15 °C). This is caused by wax precipitation at such low temperatures in the paraffinic Statfjord crude. However, the time window for the 90% scenario is significantly increased compared to an oil spill in temperate waters. 3.2.2. Field testing of in-situ burning The main limitation for igniting a weathered oil slick is the lack of light and volatile components in the residual oil and water emulsified into the oil. It is important to know that the properties or content of light components in the residual oil determine its ignitability, not the amount of light component already evaporated. The evaporative loss has earlier in the literature been used as a rule of thumb to describe the expected ignitability of several oil types (e.g. Bech et al., 1992). However, this parameter is not very useful, since for example a heavy crude oil with limited light ends will even after a low evaporative loss (b5%) be harder to ignite compared to a light crude with a high evaporative loss (N40%). The important property is the amount of residual light ends in the weathered oil, described by the flash point (see Fig. 5). The water content is also important for the ignitability of the weathered oil since the heat from the igniter has to warm the surrounding oil, break the emulsion and evaporate the free water. When the water is removed the oil can be heated to above 100 °C, which often is necessary for weathered oils to create sufficient concentration of hydrocarbons in the air to sustain burning (3–12%). In these experiments the ignition and ISB of the weathered oil as a function of ice coverage was measured with a field test (see Section 2.5 for details). The results from these ISB tests are given in both Fig. 3 (water content) and Fig. 5 (flash point), where the spilled oil after three days of weathering is marked as “Ignitable” or “Not ignitable”. These figures show a correspondence between water content, flash point and ignitability of the weathered oil spills. The residues after “open water” and “30% ice coverage” had a water content of 48 and 42% respectively, and flash points of 98 °C and 91 °C. The residues from both these weathering experiments were characterized as “not ignitable”. The weathered oil from the “90% ice coverage” experiment had both a lower water content and flash point (22% and 59 °C), and
was characterized as “Ignitable” and burned with a high effectiveness (N90%). 4. Conclusions and recommendations The results from these meso-scale field experiments with different ice conditions clearly show that several of the weathering processes of an oil spill in ice are dependent on ice coverage and wave related damping in the ice. Both the evaporative loss and related parameters such as pour- and flash point show significant differences with selected ice scenarios (high low ice coverage). Also the rate of emulsification (water uptake) was different with high (90%) and low ice coverages (0–30%). The main explanation for these observed differences is the increased film thickness due to the limited oil spreading and the reduced energy input because of the wave damping effect of the ice. The increased oil thickness will reduce the evaporative loss and the decreased energy input will reduce the emulsification of water droplets into the oil. The operative consequences of this reduced weathering could open up and widen the operational “time window” for both use of dispersants and in-situ burning with an oil spill in heavy ice coverage compared to in open water. An oil spill which is too viscous for dispersant treatment after only hours of weathering at open sea conditions could still be dispersible after several days of weathering in dense ice. These results also show that ISB could be used for an oil trapped in ice on an extended time scale compared to in open water where high flash points and high water contents limit ignition. Igniters with higher temperatures, e.g. gelled crude oil, and with addition of emulsion breaker might be more effective on weathered oils. Use of such igniters could extend the time window for ISB under these conditions. The results from these limited meso-scale field trials should be further verified with additional work including laboratory and field testing with different oil types, ice conditions and energy levels. The main objective with this research should be to develop algorithms to describe the window of opportunity for use of ISB and dispersants on oil spills in ice, and implement these in current oil weathering models. Acknowledgements This study has been made possible by funding from the Research Council of Norway (project # 157678/S40) and the Norwegian oil
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companies Statoil and Hydro. The four year project has also included funding of PhD and MSc students at UNIS. Thanks to the Governor of Svalbard for permission to release oil for this research program, and also to Store Norske Spitsbergen Grubekompani A/S for logistical support during the field experiments. References ASTM D93-90, 1992. Standard test method for flash point by Pensky-Martens closed tester. ASTM Petroleum products and lubricants (I) D 56-D 1947, vol. 05.01. American Society for Testing and Materials 28–35. ASTM D97-87, 1992. Standard test method for pour point of petroleum oils. ASTM Petroleum products and lubricants (I) D 56 - D 1947, vol. 05.01. American Society for Testing and Materials 57–64. ASTM D4052-91, 1992. Standard test method for density and relative density of liquids by digital density meter. ASTM Petroleum products and lubricants (III) D 3602 — latest; Catalyst, vol. 05.03. American Society for Testing and Materials 226–229. Bech, C., Sveum, P., Buist, I., 1992. In-situ burning of emulsions: the effects of varying water content and degree of evaporation. Proceedings in The 15th Arctic and Marine Oilspill Program (AMOP) Technical Seminar, Edmonton, Alberta, Canada, June 10–12, 1992, pp. 547–559. Brandvik, P.J., Knudsen, O.Ø., Moldestad, M.Ø., Daling, P.S., 1995. Laboratory testing of dispersants under Arctic conditions. In: Lane, P. (Ed.), The use of Chemicals in Oil Spill Response, ASTM STP 1252. American Society for Testing and Materials, Philadelphia, USA. Concawe, 1988. A field guide to the application of dispersants to oil spills. CONCAWE report no 2/88. CONCAWE, Brussel, Belgium. 64 p.
Daling, P.S., Brandvik, P.J., Mackay, D., Johansen, Ø., 1990. Characterisation of crude oils for environmental purposes. Oil & Chemical Pollution 7, 199–224. Faksness, L.G., Brandvik, P.J., 2008. Distribution of water soluble components from oil encapsulated in sea ice. Summary of results from three different field seasons. Cold Region Science and Technology. doi:10.1016/j.coldregions.2008.03.006. Fingas, M., 1995. The evaporation of oil spills. Proceedings of the Eighteenth Arctic and Marine Oilspill Program (AMOP) technical Seminar. June 14–16, 1995 Ottawa, Ontario, Canada, pp. 43–60. Fingas, M.F., Hollebone, B.P., 2003. Review of behaviour of oil in freezing environments. Marine Pollution Bulletin 47, 333–340. Moldestad, M.Ø., Singsaas, I., Resby, J.L.M., Faksness, L.G., Hokstad, J.N., 2001. Statfjord A, B and C. Properties and weathering on sea, characterisation of water soluble components related to contingency. SINTEF report STF66 F00138, SINTEF, Trondheim, Norway, 117p. (In Norwegian). NORCOR. 1975. The interaction of crude oil with Arctic sea ice. Beaufort Sea Project NORCOR Engineering & Research Ltd, technical report no. 27, Canada Department of the Environment, Victoria, British Colombia. Pal, R., Yan, Y., Masliah, J., 1992. Rheology of emulsions. In: Schramn, L.L. (Ed.), Emulsions Fundamentals and Applications in Petroleum Industry. . Advances in Chemistry Series, vol. 231. American Chemical Society, Washington, DC. 1992. Payne, J.R., McNabb Jr., G.D., Clayton Jr., J.R., 1991. Oil-weathering behavior in Arctic environments. Polar Research 10, 631–662. SL Ross Environmental Reseach Ltd., DF Dickins Associated Ltd., 1987. Field research spills to investigate the physical and chemical fate of oil in pack ice. Environmental Studies Revolving Funds Report no. 062. 95 p. Vefsnmo, S., Johannessen, B.O., 1994. Experimental oil spill in the Barents Sea — drift and spread of oil in broken ice. Proceedings 17th Arctic Marine Oil Spill Program Technical Seminar, Vancouver, BC, Canada, pp. 1331–1343.
Contents lists available at ScienceDirect
Cold Regions Science and Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c o l d r e g i o n s
Weathering processes in Arctic oil spills: Meso-scale experiments with different ice conditions Per Johan Brandvik a,b,⁎, Liv-Guri Faksness a a b
The University Centre in Svalbard, Pb. 156, N-9171 Longyearbyen, Norway SINTEF Materials and Chemistry, N-7465 Trondheim, Norway
a r t i c l e
i n f o
Article history: Received 29 November 2007 Accepted 17 June 2008 Keywords: Oil spill Weathering Arctic Ice Field experiments
a b s t r a c t The knowledge regarding weathering processes in Arctic oil spills, and especially oil spills in ice, is limited. Experimental studies have been performed in laboratories, but only to a limited degree in the field. This paper presents results from a series of meso-scale field experiments performed on Svalbard, Norway, in 2005. The results from these field experiments performed to study oil behavior (evaporation, emulsification, spreading etc.) with different ice conditions (slush ice, 30% and 90% ice coverage) are presented in this paper. Several weathering properties are strongly influenced by the low temperature, reduced oil spreading and wave action caused by increased ice coverage. Reduced water uptake, viscosity, evaporation, and pour point in dense ice conditions extend the operational time window for several contingency methods compared to treatment of oil spills in open waters. For an oil spill in open ice, this could open up for dispersant treatment and in-situ burning even after an extended period of weathering. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Extensive research has been performed during the last 30 years, including field tests, observations of real oil spills and laboratory studies trying to understand the weathering processes taking place when oil is spilled in ice. However, the majority of this work is now 10 years or older, as also concluded in a recent review on the behaviour of oil in freezing environments (Fingas and Hollebone, 2003). Operationally important weathering processes for oil spill operations like water uptake, emulsion stability and viscosity vary with oil type. Normally they increase relatively fast with increased weathering time in open water. In ice infested water, several studies have indicated that this increase with time (e.g. water content) can be drastically changed depending on ice type, ice coverage and energy conditions in the ice. Little knowledge concerning this is available today, and only for a limited number of oil types and ice regimes, knowledge gained through laboratory and field experiments performed in US, Canada and Norway (NORCOR, 1975; Ross and Dickins, 1987; Payne et al., 1991; Vefsmo and Johannessen, 1994). Compared to the current in-depth knowledge on weathering processes in oil spills in open water and temperate conditions, our knowledge regarding Arctic oil spills is very limited. There is a need for laboratory and field experiments to quantify these weathering ⁎ Corresponding author. SINTEF Materials and Chemistry, Marine Environmental Technology, N-7465 Trondheim, Norway. Tel.: +47 90958576; fax: +47 93070730. E-mail address: [email protected] (P.J. Brandvik). 0165-232X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.coldregions.2008.06.006
processes. These data should be used to improve and validate the algorithms in existing oil weathering models. The objective should be to collect basic research data on evaporation, dispersion, spreading, and other weathering parameters in the marginal ice zone. These data could then be used to enhance and modify or develop new algorithms, and to improve existing models for oil weathering in ice. Increasing oil price and reduced ice coverage have contributed to renewed interest in oil exploration and transport in Arctic areas. Several research programs have been initiated the last years to increase our knowledge on basic processes with oil in ice. The study presented in this paper is part of a joint effort between the University Centre in Svalbard (UNIS) and SINTEF focusing on oil weathering processes in different ice conditions. Another aspect of this project has been to study migration of water soluble components in ice and the toxicity of these components (Faksness and Brandvik, 2008). The work presented in this paper focuses on weathering processes (emulsification, evaporation and others) for one oil type in different ice conditions, and is an important contribution when trying to fill the gap in knowledge regarding fate and behavior of oil spills in ice. 2. Experimental 2.1. Oil type Statfjord crude was used as the oil type for these experiments. This is a representative oil from the group of light and paraffinic crudes. Statfjord is produced in the Southern part of the Norwegian Sea and is
P.J. Brandvik, L.-G. Faksness / Cold Regions Science and Technology 55 (2009) 160–166 Table 1 Main relevant properties for fresh Statfjord crude
161
Table 2 Specifications for the three different experiments
Property
Value
Wax content (wt.%) Viscosity (cP, shear rate 100 s− 1, 13 °C) Asphaltene content (wt.%) Density (g/mL) Flash point (°C) Pour point (°C) Loss at 150 °C (vol.%) Loss at 200 °C (vol.%) Loss at 250 °C (vol.%)
4.3 7.0 0.1 0.834 b−39 −3 25.2 34.2 43.6
by production volume an important oil. Relevant properties for the fresh crude are given in Table 1. 2.2. Circulating flume used for oil weathering A circulating flume was used to study weathering of the oil at different ice conditions. The flume was cut out in the ice in van Mijenfjord close to SINTEF's field research station in Sveagruva at Svalbard (Spitsbergen). The flume dimensions and principal layout are given in Fig. 1. The depth of the flume was 50 cm and it was not cut completely through the fjord ice (total ice thickness 110 cm). Two
Experiment type
Ice coverage (%)
Surface current (cm/s)
Wave height (cm)
Temp air (°C)
Temp water (°C)
Open water + slush Low ice coverage High ice coverage
0
15
15
−15
−1.9
30
9
10
−10
−1.9
90
0
5
−5
−1.9
propellers were used to control circulation, and a wave maker to introduce wave energy. The flume was operated with three different ice conditions; open water (with some slush ice), low and high ice coverage (30 and 90%). The settings and environmental conditions for the three experiments are given in Table 2. After weathering for three days the oil was led to a connected in-situ burning (ISB) chamber and tested for ignitability and ISB effectiveness. 2.3. Oil sampling and analysis A comprehensive sampling program was carried out during the three days each experiment lasted. Initially, 20 L of Statfjord was very
Fig. 1. Principle layout for meso-scale basin made in fjord ice and used for oil weathering and ISB experiments showing the overall dimensions, the position of the current propellers, wave generators, and in-situ burning (ISB) chamber used to burn the weathered oil.
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2.5. In-situ burning (ISB) of weathered oil
Table 3 Physical/chemical properties, units and methods Property
Unit
Method
cP (or mP) at shear rate 10 or 100 s− 1 at Daling et al. 3–6 °C (1990) vol.% Daling et al. Water content of w/oemulsion (1990) Density of water water-free g/mL at 15.5 °C ASTM D4052oil 91 Chemical dispersibility of wt.% Concawe weathered oil (1988) Evaporative loss wt.% Daling et al. (1990) Flash point of water water- °C ASTM D93-90 free oil Pour point of water water- °C ASTM D97-87 free oil Viscosity of weathered oil
The ISB chamber was used for testing of ignitability and burnability of the weathered oil. After each weathering experiment the emulsion was led to the burning chamber by a simple boom arrangement assisted with some shoveling of the surface weathered oil. The film thickness in the 1 m2 burning chamber varied from 15 to 25 mm. The igniter was applied to the middle of the weathered oil and lit by a propane torch. The igniter used consisted of gelled gasoline (500 mL). If no sufficient flame spreading and initial burning of the weathered oil started during the burn time of the igniter (5 min), the weathered oil was classified as “Not ignitable”. In the cases where the weathered oil ignited, the residue was collected with adsorption pads and quantified gravimetrically. 3. Results and discussion
gently applied on to the surface of the flume. Samples were taken from the emulsified surface oil for a series of physical and chemical analyses. Surplus, free sea water in the collected sample was immediately drained off using a separation funnel, before the sample was homogenized and divided into aliquots for further analysis. The analyses of the physical and chemical properties of surface emulsified oil were performed in a laboratory container on the ice close to the flume immediately after sampling (see Table 3). The two last analyses (pour- and flash point) were performed later at the SINTEF laboratories in Trondheim. Details regarding the analysis above are described in Daling et al. (1990). 2.4. Dispersibility testing (field test) The effectiveness of dispersants was tested by the use of a simplified field procedure (Concawe, 1988). Dispersant (5%) was added to the oil in a closed, graded cylinder (100 mL) and the oil was very gently shaken a few times. The following three criteria were used for evaluation of the oil in water dispersion formed and hence the chemical dispersibility of the oil: • Good dispersibility: Brown homogeneous dispersion with only small oil droplets • Reduced dispersibility: Dark brown to black dispersion (with larger oil droplets). • Not dispersable: Black unstable dispersion (very large oil droplets, similar to non-treated oil).
The presentation of the results from these field experiments is divided into two parts. The first part (Section 3.1) presents the properties describing the weathering of the oil in the three different oil-in-ice scenarios. The second part (Section 3.2) presents results from the operationally oriented testing, in-situ burning and dispersibility. 3.1. Physical and chemical properties of weathered oil 3.1.1. Evaporative loss Evaporative loss can be a significant weathering process for many light crude oils and can lead to a loss up to 50–60%. The evaporative loss for these oils in the ice experiments is given in Fig. 2 and shows a clear and significant difference between the three experiments with different ice conditions. The evaporative loss for the open water scenario was 30%, for the 30% ice coverage 25% and finally it was 19% with high ice coverage (90%). The difference in evaporative loss is mainly caused by the different film thicknesses in the three experiments, since evaporation is a surface phenomenon. This is in contradiction to studies by Fingas (1995), which claims that evaporation from oil slicks is independent of film thickness. The presence of the ice limits the spreading of the oil and results in higher film thicknesses. The theoretical film thicknesses, assuming a complete spreading, are 1.3, 2.0 and 13 mm (open water, 30% and 90% ice coverage respectively). The real differences in film thickness between the experiments were probably less, since the oil is not evenly distributed in the flume.
Fig. 2. Evaporative loss (vol.%) from the weathered oil for the meso-scale field experiments with different ice conditions.
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Fig. 3. Water content (vol.%) in emulsified oil for the meso-scale field experiments with different ice conditions. The in-situ measured ignitability for the weathered oil after 2.5 day of weathering is also indicated.
3.1.2. Emulsification (water uptake) Water-in-oil (W/o) emulsification is one of the major oil weathering processes for marine oil spills. The transformation from a fresh crude with low viscosity to an often brown, highly viscous emulsion is of large environmental and operational importance. The increased viscosity and volume (75% water content quadruples the volume) increase the persistence of the oil spill on the surface. From an operational point of view, the increased volume and viscosity is important for selecting appropriate recovery equipment (type of skimmers, pumps and needed tank capacity). Water in oil emulsification for the three experiments is shown in Fig. 3. The difference between “open water” and “30% ice” is small and probably not significant (38–41% water content), while the difference with the “90% ice” experiment is large and significant (about 20% water content). This indicates that the wave energy available for
creating emulsions has not been very different in the two first cases. The small difference between “open water and “30% ice” could probably be explained by the presence of slush ice in the “open water” experiments, due to the low air temperature during the experiments (−15 °C, see Table 2). This slush ice significantly dampens the effect of the waves in the “open water” experiment. Open water weathering of Statfjord, without any slush, was expected to give a water uptake in the range of 75–80% (Moldestad et al., 2001). 3.1.3. W/o-emulsion viscosity The viscosity change due to evaporation alone is very minor (only up to a few hundred cP) compared to the viscosity increase due to w/oemulsification.Theinclusionofwaterassmalldropletsintheemulsifiedoil changes the oil viscosity due to internal friction between the water droplets and the continuous oil phase. Initially, the droplets are large
Fig. 4. Viscosity (cP at shear rate 10 or 100 s− 1, 3–6 °C) for the emulsified oil from the meso-scale field experiments with different ice conditions. Measured chemical dispersibility as a function of weathering is shown as colored areas.
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Fig. 5. Flash point for weathered water-free oil (°C) for the meso-scale field experiments with different ice conditions. The in-situ measured ignitability for the weathered oil after 2.5 day of weathering is also indicated.
(several hundred microns), but as they are stabilized by the naturally occurring surface active components (mainly asphaltenes, waxes and resins) the water droplets are reduced in size. This reduction in mean droplet size diameter down to the range of a few microns drastically increases the interface between water and oil, and also the internal friction between the two phases. This increases the viscosity of the emulsified oil (Pal et al., 1992). The viscosities of the w/o-emulsion from the three experiments are given in Fig. 4. The viscosity is measured at shear rate 100 s− 1 for the less viscous water water-free samples (typically the first 2 h), and at 10 s− 1 for the emulsified samples. The difference between the “open water” and “30% ice” versus the “90% ice” is large and significant. The two first scenarios gave viscosities in the range of 3500–5000 cP while the latter had a viscosity of 1400 cP.
is 6.1% since the high molecular weight wax components do not evaporate. This high wax content is sufficient for the waxes to precipitate and form lattices which change the rheology of the oil and could make it semi-solid. This process is best described by the pour point of the oil, which is defined as the temperatures where the oil does not flow freely anymore (ASTM D97/87). The pour points for these three experiments are shown in Fig. 6. This figure too shows a distinct difference between the “open water” and “30% ice” scenarios versus the “90% ice” scenario. The open water and low ice coverage experiments end with a pour point of 24 °C, while the high ice coverage experiment in the end reaches 19 °C. This is closely connected to the difference in evaporative loss (see Fig. 2). The operational significance of varying pour point is discussed in the next chapter.
3.1.4. Flash point for residual oil Traditionally, evaporative loss has been the main weathering property for evaluating ignitability and the potential for ISB of weathered oils. However, the content of light components initially present in the fresh oil is extremely variable and gives little information regarding the ignitability of the weathered oil. The properties of the weathered residue are more important than the evaporative loss from the original oil. Since the amount of light components is highly variable among crude oils, this means that e.g. the flash point for different residual oil could be very different for two crude oils after the same evaporative loss. For this reason it is more appropriate to use the flash point for the residual oil to estimate if the weathered oils are ignitable for ISB (see 3.2.2). Flash points for the residual oils in the three experiments are shown in Fig. 5. The difference between “open water” and “30% ice” is small (100–110 °C), and probably not significant, while the distance to the “90% ice” experiment is larger and significant (60 °C).
3.2. Testing of operational oil spill options (use of dispersants and ISB)
3.1.5. Pour point of residual oil When the light components in an oil spill evaporate the relative content of wax component will increase. The waxes (straight and naphthenic hydrocarbons with more than 20 carbon atoms) are high in molecular weight and have a very low vapor pressure. Statfjord crude has a wax content of only 4.3%, but in the open water scenario with an evaporative loss of 30% the effective wax content after 3 days
3.2.1. Field testing of chemical dispersibility As the viscosity of the emulsified surface oil increases it becomes increasingly difficult for the dispersant sprayed onto the slick to penetrate into the oil. Due to the reduced molecular diffusion in the viscous oil slick the dispersant droplets need more time to penetrate into the viscous emulsion. If the viscosity of the oil is too high and penetration takes too long time, the dispersant will be washed off by waves flushing over the emulsified oil spill. Also the semi-solidification of the oil spill due to wax precipitation will reduce the effectiveness of dispersant application. It is the same principle; when the oil starts to solidify the dispersant droplets need more time to penetrate into the oil, and the possibility for being flushed into the sea before penetrating into the oil phase increases. For most oil spills at sea these problems start when the oil's pour point is the same as the sea temperature, and the dispersant effectiveness is usually drastically reduced when the oil's pour point is 15 °C above the environmental temperature (Daling et al., 1990; Brandvik et al., 1995). The combined effect of the increasing viscosity and pour point defines the operational “window of opportunity” for dispersant use on an oil spill in ice. Dispersant effectiveness measured with the field test, directly after oil sampling, is given as shaded areas in Figs. 4 and 6. This field testing indicates that for the Statfjord crude these problems occur at 1500 cP, and above 4000 cP the dispersant
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Fig. 6. Pour point of the weathered water-free oil (°C) for the meso-scale field experiments with different ice conditions. Measured chemical dispersibility as a function of weathering is shown as colored.
effectiveness is close to zero. Similarly for the pour point, dispersibility starts to drop at 18 °C above the water temperature and approaches zero at 21 °C above water temperature. For the open water and 30% scenarios this gives a shorter time window for dispersant use compared to similar temperate scenarios (e.g. 15 °C). This is caused by wax precipitation at such low temperatures in the paraffinic Statfjord crude. However, the time window for the 90% scenario is significantly increased compared to an oil spill in temperate waters. 3.2.2. Field testing of in-situ burning The main limitation for igniting a weathered oil slick is the lack of light and volatile components in the residual oil and water emulsified into the oil. It is important to know that the properties or content of light components in the residual oil determine its ignitability, not the amount of light component already evaporated. The evaporative loss has earlier in the literature been used as a rule of thumb to describe the expected ignitability of several oil types (e.g. Bech et al., 1992). However, this parameter is not very useful, since for example a heavy crude oil with limited light ends will even after a low evaporative loss (b5%) be harder to ignite compared to a light crude with a high evaporative loss (N40%). The important property is the amount of residual light ends in the weathered oil, described by the flash point (see Fig. 5). The water content is also important for the ignitability of the weathered oil since the heat from the igniter has to warm the surrounding oil, break the emulsion and evaporate the free water. When the water is removed the oil can be heated to above 100 °C, which often is necessary for weathered oils to create sufficient concentration of hydrocarbons in the air to sustain burning (3–12%). In these experiments the ignition and ISB of the weathered oil as a function of ice coverage was measured with a field test (see Section 2.5 for details). The results from these ISB tests are given in both Fig. 3 (water content) and Fig. 5 (flash point), where the spilled oil after three days of weathering is marked as “Ignitable” or “Not ignitable”. These figures show a correspondence between water content, flash point and ignitability of the weathered oil spills. The residues after “open water” and “30% ice coverage” had a water content of 48 and 42% respectively, and flash points of 98 °C and 91 °C. The residues from both these weathering experiments were characterized as “not ignitable”. The weathered oil from the “90% ice coverage” experiment had both a lower water content and flash point (22% and 59 °C), and
was characterized as “Ignitable” and burned with a high effectiveness (N90%). 4. Conclusions and recommendations The results from these meso-scale field experiments with different ice conditions clearly show that several of the weathering processes of an oil spill in ice are dependent on ice coverage and wave related damping in the ice. Both the evaporative loss and related parameters such as pour- and flash point show significant differences with selected ice scenarios (high low ice coverage). Also the rate of emulsification (water uptake) was different with high (90%) and low ice coverages (0–30%). The main explanation for these observed differences is the increased film thickness due to the limited oil spreading and the reduced energy input because of the wave damping effect of the ice. The increased oil thickness will reduce the evaporative loss and the decreased energy input will reduce the emulsification of water droplets into the oil. The operative consequences of this reduced weathering could open up and widen the operational “time window” for both use of dispersants and in-situ burning with an oil spill in heavy ice coverage compared to in open water. An oil spill which is too viscous for dispersant treatment after only hours of weathering at open sea conditions could still be dispersible after several days of weathering in dense ice. These results also show that ISB could be used for an oil trapped in ice on an extended time scale compared to in open water where high flash points and high water contents limit ignition. Igniters with higher temperatures, e.g. gelled crude oil, and with addition of emulsion breaker might be more effective on weathered oils. Use of such igniters could extend the time window for ISB under these conditions. The results from these limited meso-scale field trials should be further verified with additional work including laboratory and field testing with different oil types, ice conditions and energy levels. The main objective with this research should be to develop algorithms to describe the window of opportunity for use of ISB and dispersants on oil spills in ice, and implement these in current oil weathering models. Acknowledgements This study has been made possible by funding from the Research Council of Norway (project # 157678/S40) and the Norwegian oil
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companies Statoil and Hydro. The four year project has also included funding of PhD and MSc students at UNIS. Thanks to the Governor of Svalbard for permission to release oil for this research program, and also to Store Norske Spitsbergen Grubekompani A/S for logistical support during the field experiments. References ASTM D93-90, 1992. Standard test method for flash point by Pensky-Martens closed tester. ASTM Petroleum products and lubricants (I) D 56-D 1947, vol. 05.01. American Society for Testing and Materials 28–35. ASTM D97-87, 1992. Standard test method for pour point of petroleum oils. ASTM Petroleum products and lubricants (I) D 56 - D 1947, vol. 05.01. American Society for Testing and Materials 57–64. ASTM D4052-91, 1992. Standard test method for density and relative density of liquids by digital density meter. ASTM Petroleum products and lubricants (III) D 3602 — latest; Catalyst, vol. 05.03. American Society for Testing and Materials 226–229. Bech, C., Sveum, P., Buist, I., 1992. In-situ burning of emulsions: the effects of varying water content and degree of evaporation. Proceedings in The 15th Arctic and Marine Oilspill Program (AMOP) Technical Seminar, Edmonton, Alberta, Canada, June 10–12, 1992, pp. 547–559. Brandvik, P.J., Knudsen, O.Ø., Moldestad, M.Ø., Daling, P.S., 1995. Laboratory testing of dispersants under Arctic conditions. In: Lane, P. (Ed.), The use of Chemicals in Oil Spill Response, ASTM STP 1252. American Society for Testing and Materials, Philadelphia, USA. Concawe, 1988. A field guide to the application of dispersants to oil spills. CONCAWE report no 2/88. CONCAWE, Brussel, Belgium. 64 p.
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